Tyre Grip and Weight Transfer
Discussion
I recently stumbled across an old thread on Piston Heads regarding weight transfer and roll centres. This is an area I am eager to learn more about and so I became engrossed in it. It was a fancinating read and I want to thank the contributers for furthering my knowledge and clearing up some of my own mis-interrpretations on the subject!
From this thread I am happy with the basics of weight transfer and that adding an anti roll bar to the front of a car will increase the proportion of the cars weight carried by the front wheels during cornering. Please correct me if I am wrong with this so far!!!! Also written in this thread was that this resultant increase in vertical force carried by the front wheels has the effect of reducing grip and causing understeer. I have also read this in books with the classic example being a passenger car will be fitted with a front ARB to induce understeer for us non Lewis Hamilton types.
This seems strange to me as I thought increased vertical load multiplied by the co-efficient of friction of the tyre would result with increased lateral grip of the tyre not less. I am not questioning the practice of fitting an ARB to front causing understeer but just don't follow the science as to why this happens.
Can anyone expand on this any further? As I said the last thread explained load transfer and the effect of anti roll bars and spirng stiffness very well. I am not grasping how this then relates to lateral force generated by the tyres, ie grip.
I look forward to the responses and no doubt can of worms I have just opened for many a reader!!!
From this thread I am happy with the basics of weight transfer and that adding an anti roll bar to the front of a car will increase the proportion of the cars weight carried by the front wheels during cornering. Please correct me if I am wrong with this so far!!!! Also written in this thread was that this resultant increase in vertical force carried by the front wheels has the effect of reducing grip and causing understeer. I have also read this in books with the classic example being a passenger car will be fitted with a front ARB to induce understeer for us non Lewis Hamilton types.
This seems strange to me as I thought increased vertical load multiplied by the co-efficient of friction of the tyre would result with increased lateral grip of the tyre not less. I am not questioning the practice of fitting an ARB to front causing understeer but just don't follow the science as to why this happens.
Can anyone expand on this any further? As I said the last thread explained load transfer and the effect of anti roll bars and spirng stiffness very well. I am not grasping how this then relates to lateral force generated by the tyres, ie grip.
I look forward to the responses and no doubt can of worms I have just opened for many a reader!!!
Well there's quite a few things wrong (but it is a very complicated subject), so let's start by trying to explain one part.
Because a wheel rotates, friction does not work the same way as a sold, fixed object (like a box across the floor). As the tread goes through the contact patch (as the wheel rotates), the rubber needs to distort to generate the friction. Through the contact patch, the rubber travels in a direction that is neither the direction the wheel is pointing, nor is it the direction the car is pointing. This creates a Slip Angle . Read this and get an idea of the concept.
You are correct that, in principle, lateral force (grip) generated should be related to the vertical force (weight of body). However, there are several effects that mess this up.
First, lets work with a stable vertical force. In that condition, a tyre can generate a certain amount of grip in any single direction. That could be totally lateral (cornering), but also braking or acceleration. Draw this out and it would broadly be a circle. But you will note that if you need to generate any longitudinal force (accelerating, braking, or even rolling resistance), you lose lateral grip - less cornering force.
Secondly, suspension geometry. If you brake, accelerate or corner, the vehicle body is going to want to move around - patching or rolling. Weight transfer means you increase or decrease the vertical force. As a result the suspension moves about. Depending on the suspension geometry (and loads, and spring/damper stiffness), you will induce some change in the tyre's attitude - camber change particularly. The vertical load across the tyre constact patch will change - if negative camber is induced, the inner shoulder is taking a lot more of the vertical load than the outer shoulder. Meanwhile, the tyre carcass itself is distorting. So if you considered the slip angle that is being generated in strips across the contact patch, it will vary greatly.
Third, how the tread blocks and rubber in the contact patch act under these different loads varies. Overstress the rubber, and too much heat is generated in the rubber. This changes the instantaneous coefficient of friction that it can generate.
So, going back to that circle of forces (with a fixed vertical load), we could now generate a three-dimensional graph where the third dimension is vertical load. This would be a funnel/cone shape, but importantly with curved sides rather than straight. Even then, we haven't allowed for the fourth dimension of camber (and I'm not particularly good at explaining four-dimensional shapes).
So, whilst more vertical load generally means more friction/lateral grip, the relationship is far from linear.
Now, let's talk about weight transfer. There are two types to consider:
Pitch is created as you brake before or into a corner, or accelerating out.
Roll is created as you corner. As the car rolls, the outer wheel goes into bump (into the wheelarch), inner wheel rebound (out of the wheelarch. The roll bar gets twisted. The effect is that the roll bar tries to push the outer wheel harder into the ground than just the natural weight transfer of the vehicle. The net effect is that the outer tyre (which is more important as it is carrying more of the load) is actually getting more overloaded than it should, and this alos happens faster than it should, if there were no arb.
That overload means that (because of what I discussed at the top) the total lateral grip available will be actually less than if both tyres were doing the work they were supposed to do. So an arb on the front will sacrifice some grip at the front, adding a tendency to understeer. You have actually lost some total grip, but what you might gain in stability and feel may be worth it.
Remember that there are all sorts of elements (roll centres, CoG position in all axes, suspension geometry at each end, and more) that will create their own tendencies to under or oversteer in any particular situation. A front arb will not create understeer, it will only add a tendency to it. If a car is inherently an understeerer, adding a front arb may lead you to terminal understeer. Some (normally older) vehicles had a tendency to understeer at low speeds, moving to oversteer as you approached the limit of adhesion - great fun if it's progressive, but snap oversteer is considered the most dangerous of all.
You might also now be able to see how fore-aft weight transfer can affect under/oversteer. Accelerating mid-corner (in a rear wheel drive) has two effects:
Because a wheel rotates, friction does not work the same way as a sold, fixed object (like a box across the floor). As the tread goes through the contact patch (as the wheel rotates), the rubber needs to distort to generate the friction. Through the contact patch, the rubber travels in a direction that is neither the direction the wheel is pointing, nor is it the direction the car is pointing. This creates a Slip Angle . Read this and get an idea of the concept.
You are correct that, in principle, lateral force (grip) generated should be related to the vertical force (weight of body). However, there are several effects that mess this up.
First, lets work with a stable vertical force. In that condition, a tyre can generate a certain amount of grip in any single direction. That could be totally lateral (cornering), but also braking or acceleration. Draw this out and it would broadly be a circle. But you will note that if you need to generate any longitudinal force (accelerating, braking, or even rolling resistance), you lose lateral grip - less cornering force.
Secondly, suspension geometry. If you brake, accelerate or corner, the vehicle body is going to want to move around - patching or rolling. Weight transfer means you increase or decrease the vertical force. As a result the suspension moves about. Depending on the suspension geometry (and loads, and spring/damper stiffness), you will induce some change in the tyre's attitude - camber change particularly. The vertical load across the tyre constact patch will change - if negative camber is induced, the inner shoulder is taking a lot more of the vertical load than the outer shoulder. Meanwhile, the tyre carcass itself is distorting. So if you considered the slip angle that is being generated in strips across the contact patch, it will vary greatly.
Third, how the tread blocks and rubber in the contact patch act under these different loads varies. Overstress the rubber, and too much heat is generated in the rubber. This changes the instantaneous coefficient of friction that it can generate.
So, going back to that circle of forces (with a fixed vertical load), we could now generate a three-dimensional graph where the third dimension is vertical load. This would be a funnel/cone shape, but importantly with curved sides rather than straight. Even then, we haven't allowed for the fourth dimension of camber (and I'm not particularly good at explaining four-dimensional shapes).
So, whilst more vertical load generally means more friction/lateral grip, the relationship is far from linear.
Now, let's talk about weight transfer. There are two types to consider:
- Pitch: fore/aft, caused by braking/accelerating
- Roll: left/right, caused by cornering
FrazM said:
From this thread I am happy with the basics of weight transfer and that adding an anti roll bar to the front of a car will increase the proportion of the cars weight carried by the front wheels during cornering.
No, an anti-roll bar deals with roll, seeking to modify weight transfer left-right.Pitch is created as you brake before or into a corner, or accelerating out.
Roll is created as you corner. As the car rolls, the outer wheel goes into bump (into the wheelarch), inner wheel rebound (out of the wheelarch. The roll bar gets twisted. The effect is that the roll bar tries to push the outer wheel harder into the ground than just the natural weight transfer of the vehicle. The net effect is that the outer tyre (which is more important as it is carrying more of the load) is actually getting more overloaded than it should, and this alos happens faster than it should, if there were no arb.
That overload means that (because of what I discussed at the top) the total lateral grip available will be actually less than if both tyres were doing the work they were supposed to do. So an arb on the front will sacrifice some grip at the front, adding a tendency to understeer. You have actually lost some total grip, but what you might gain in stability and feel may be worth it.
Remember that there are all sorts of elements (roll centres, CoG position in all axes, suspension geometry at each end, and more) that will create their own tendencies to under or oversteer in any particular situation. A front arb will not create understeer, it will only add a tendency to it. If a car is inherently an understeerer, adding a front arb may lead you to terminal understeer. Some (normally older) vehicles had a tendency to understeer at low speeds, moving to oversteer as you approached the limit of adhesion - great fun if it's progressive, but snap oversteer is considered the most dangerous of all.
You might also now be able to see how fore-aft weight transfer can affect under/oversteer. Accelerating mid-corner (in a rear wheel drive) has two effects:
- As the rear tyres are using some of their available grip for acceleration, they have less for cornering, so the rear tyres will start to slide, giving you power oversteer.
- Weight is transferred to the rear, which gives back some grip to the rear (at the expense of the front), creating a tendency to understeer (unless other effects like geometry changes piss that away)
HiRich said:
So, whilst more vertical load generally means more friction/lateral grip, the relationship is far from linear.
That's the important bit to remember.HiRich said:
I think...[you misunderstand]... because you said:
No, I think FrazM is perfectly correct, with one minor caveat highlighted in bold, below. FrazM said:
From this thread I am happy with the basics of weight transfer and that adding an anti roll bar to the front of a car will increase the proportion of the cars weight carried by the front wheels during cornering.
No, an anti-roll bar deals with roll, seeking to modify weight transfer left-right....Certainly, an anti-roll bar deals with (limits) roll, but its other (arguably more important) function is to manage diagonal weight transfer. Put in simple, plain English, fitting an ARB to the front (or stiffening the front in proportion to the back by any means - widening the front track or fitting stiffer road springs, for example) will make the car 'lean' diagonally on the front corner, so the OP is quite correct when he says that fitting an ARB will increase the proportion of weight (or load, if we're being incredibly pedantic, which is where the other thread became entertaining
) carried by the outside front wheel during cornering.As HiRich said, in pure cornering, total weight only transfers from one side to another... only acceleraton or braking will cause a net transfer of weight fore-and-aft, but the roll stiffness can be used to manage a proportion of that lateral weight transfer so that it is fed diagonally across from one wheel to another and hence can increase the load (and therefore increase the slip angle) on one outside (loaded) corner of the car compared to the other.
...and if you're cornering hard, it's only the outside (loaded) tyres that you're worried about, because the insides will be very lightly loaded indeed and contribute very little to the total available grip.
Edited by Sam_68 on Tuesday 29th July 21:00
So am I correct in saying and do we all agree that increasing the front roll stiffness of a car (be it ARB, springs, track whatever) we increase the vertical load supported by the front outside tyre? This comes about by reducing the weight carried by the diagonally opposite corner?? That second part I am not too sure on yet so please confirm or deny this!!
I understand that the characteristics of a trye are not linear and in fact I guess you would be hard pushed to find anything on a race car that does behave practically linear. I also understand that a tyre cannot corner to it's maximum ability while also accelrating/braking to it's maximum abilty at the same time because it simply cannot do two different tasks at the same time. I still thought that increasing the vertical load on a tyre would yield more grip and I don't understand how this can be over loading a tyre. How does downforce work? The tyres are pushed harder into the road and give more grip. Therefore will increasing the front roll stiffness and therefore increasing the front outside tyre vertical load not generate more grip on that tyre?
For many other aspects of the car could be sent to pot by fitting an ARB and obviously the camber change through the suspension geometry will be different because the roll angle is reduced. Surely we could minimise all these other changes such in a given car so that the main change from increased front roll stiffness would be the change in weight transfer distribution. Would this car still have an increased understeer tendancy even though it's outside tyre is support more vertical load?
In terms of slip angle I have always understood it to be a result of the tyre grip. The tyre grips the road and the mass of the car wishes to travel straight on. This loading of the flexible tyre carcass results in the wheels not travelling in the direction the are pointing because the tyre twists under load. The angular difference between the wheel heading and it's actual direction of travel being the slip angle. There is then an optimum slip angle to produce maximum lateral grip for any tyre. This optimum also varies with vertical load which is perhaps what you are meaning with saying the tyres then operate at a higher slip angle and thus reduces the grip they have, more understeer? Perhaps this is another area in which my thinking is wrong? I still would think a higher lateral force is possible when the tyre is more heavily loaded but it will occur at a different slip angle.
Thanks for the responses so far and keep them coming! I am determined to understand all of this one day and it has been bugging me for years so far!!!!
PS I am not trying to tell anyone to suck eggs here just explaining my own understandings so that someone can point any serious flaws in my logic so far!
I understand that the characteristics of a trye are not linear and in fact I guess you would be hard pushed to find anything on a race car that does behave practically linear. I also understand that a tyre cannot corner to it's maximum ability while also accelrating/braking to it's maximum abilty at the same time because it simply cannot do two different tasks at the same time. I still thought that increasing the vertical load on a tyre would yield more grip and I don't understand how this can be over loading a tyre. How does downforce work? The tyres are pushed harder into the road and give more grip. Therefore will increasing the front roll stiffness and therefore increasing the front outside tyre vertical load not generate more grip on that tyre?
For many other aspects of the car could be sent to pot by fitting an ARB and obviously the camber change through the suspension geometry will be different because the roll angle is reduced. Surely we could minimise all these other changes such in a given car so that the main change from increased front roll stiffness would be the change in weight transfer distribution. Would this car still have an increased understeer tendancy even though it's outside tyre is support more vertical load?
In terms of slip angle I have always understood it to be a result of the tyre grip. The tyre grips the road and the mass of the car wishes to travel straight on. This loading of the flexible tyre carcass results in the wheels not travelling in the direction the are pointing because the tyre twists under load. The angular difference between the wheel heading and it's actual direction of travel being the slip angle. There is then an optimum slip angle to produce maximum lateral grip for any tyre. This optimum also varies with vertical load which is perhaps what you are meaning with saying the tyres then operate at a higher slip angle and thus reduces the grip they have, more understeer? Perhaps this is another area in which my thinking is wrong? I still would think a higher lateral force is possible when the tyre is more heavily loaded but it will occur at a different slip angle.
Thanks for the responses so far and keep them coming! I am determined to understand all of this one day and it has been bugging me for years so far!!!!
PS I am not trying to tell anyone to suck eggs here just explaining my own understandings so that someone can point any serious flaws in my logic so far!
FrazM said:
So am I correct in saying and do we all agree that increasing the front roll stiffness of a car (be it ARB, springs, track whatever) we increase the vertical load supported by the front outside tyre? This comes about by reducing the weight carried by the diagonally opposite corner?? That second part I am not too sure on yet so please confirm or deny this!!
Yes, essentially. Get a copy of Allan Staniforth's book 'Competition Car Suspension'. This has a chapter on weight transfer, written by David Gould (Staniforth himself is a bit hazy on the concepts involved, from my conversations with him!). The sums are not complicated, and once you've worked through them a couple of times, it all starts to make sense.FrazM said:
I still thought that increasing the vertical load on a tyre would yield more grip
Yes, it does, but not in proportion to the increase in load. For example, if you double the load on the tyre, you might find that it only generates, say, 1.5x the grip.Also, understeer and oversteer aren't strictly about ultimate grip, it's about slip angles. A car can be oversteering (rear tyres operating at a higher slip angle thand the front tyres) while the tyres are still well within their ultimate grip capability. Indeed, if you exceed the tyre's ultimate grip, you're not ovwersteering, you're spinning.
FrazM said:
How does downforce work? The tyres are pushed harder into the road and give more grip. Therefore will increasing the front roll stiffness and therefore increasing the front outside tyre vertical load not generate more grip on that tyre?
Yes, both increase the grip generated by the tyre... but not by as much as the increase in load. 200lbs of downforce will not result in 200lbs or extra cornering force.FrazM said:
In terms of slip angle I have always understood it to be a result of the tyre grip.
Again, you're not far off, but the real situation is slightly different because of the non-linear behaviour of the tyre. The curve for grip (cornering force) vs. slip angle will typically climb steeply, then flatten off to a peak before dropping off (though not far past the peak it will cut off sharply, at the point where the tyre lets go altogether and starts to slide). The optimum load/grip/slip relationship would keep the tyre balanced at the peak of this curve and if your front and rear tyres are identical(ie. they peak at the same cornering force/slip angle), then the optimum would see identical slip angles front and rear... ie. a car that with neutral balance that is neither understeering nor oversteering.The thing with downforce is that unlike adding "weight" to increase the tyre's grip, the downforce only works downwards. When you add weight to the car, it's a bit self-defeating because the extra grip has to work harder to pull the extra weight round the corner. I understand that you're not quite talking about that, you're talking about taking "weight" from another part of the car and adding it to the outside corner (by stiffening an anti-roll bar), but downforce is nice because it can add weight to both tyres on an axle without any creating any extra "centrifugal" force for the tyres to overcome.
Ok so I am not barking up the wrong tree then with what I am thinking. Increasing vertical load on a tyre will yield more grip albeit not in a linear fashion.
So why therefore does increasing front roll stiffness give an increase in understeering tendacy if we are infact increasing the grip available at the front outside tyre? I know there are a myriad of other consequences to stinffening roll stiffness but if in principle we achieve more grip at the front outside tyre how can the net effect be understeer? My brain is telling me the opposite and that more front grip will dial out understeer and introduce oversteer? This is against the rules of thumb I have read and heard but I still can't understand the science of it!
I actually do have that Staniforth's book and must admit I have been a bit lazy because I was half way through that chapter when I got emblroiled in the old entertaining thread I found on here which started answering a lot of my questions!!! I will get back onto it and work the numbers as you suggest! Thanks!
Cheers for the downforce input! I was hoping I wasn't going crazy although I think that suspension can do that to people!!! Maybe I will come back with some more indepth questions on that later when I fully get my head around the "simple" mechancics of suspension and handling first!
So why therefore does increasing front roll stiffness give an increase in understeering tendacy if we are infact increasing the grip available at the front outside tyre? I know there are a myriad of other consequences to stinffening roll stiffness but if in principle we achieve more grip at the front outside tyre how can the net effect be understeer? My brain is telling me the opposite and that more front grip will dial out understeer and introduce oversteer? This is against the rules of thumb I have read and heard but I still can't understand the science of it!
I actually do have that Staniforth's book and must admit I have been a bit lazy because I was half way through that chapter when I got emblroiled in the old entertaining thread I found on here which started answering a lot of my questions!!! I will get back onto it and work the numbers as you suggest! Thanks!
Cheers for the downforce input! I was hoping I wasn't going crazy although I think that suspension can do that to people!!! Maybe I will come back with some more indepth questions on that later when I fully get my head around the "simple" mechancics of suspension and handling first!
Again, you need to remember that understeer does not necessarily mean that the limits of grip have been exceeded (unless we're talking complete carry-on-in-a-straight-line-despite-2-turns-of-steering-lock front-end washout).
Understeer simply means that the front tyres are operating at higher slip angle than the rear ones.
If you refer again to a graph of slip angle versus cornering force for a tyre, you'll see that the slip angle naturally increases as the cornering forces rise.
So, if you want to make the front tyres generate more grip (cornering effort), you are automatically going to be expecting them to operate at higher slip angles... in other words, if you want more grip from the front you are automatically going to be getting more understeer.
But because of the 'diminishing returns' rule on load vs. grip, of course, eventually you'll reach a point where the increasing cornering force overtakes the increasing grip and the car will slide gracefully off the road in a display of terminal understeer, but that's just basic physics... if you push beyond the ultlimate limits of grip, an understeering car will slide off forwards, an oversteering car will spin off backwards and a perfectly neutral car will slide off sideways, but they'll all slide off eventually!
Understeer simply means that the front tyres are operating at higher slip angle than the rear ones.
If you refer again to a graph of slip angle versus cornering force for a tyre, you'll see that the slip angle naturally increases as the cornering forces rise.
So, if you want to make the front tyres generate more grip (cornering effort), you are automatically going to be expecting them to operate at higher slip angles... in other words, if you want more grip from the front you are automatically going to be getting more understeer.
But because of the 'diminishing returns' rule on load vs. grip, of course, eventually you'll reach a point where the increasing cornering force overtakes the increasing grip and the car will slide gracefully off the road in a display of terminal understeer, but that's just basic physics... if you push beyond the ultlimate limits of grip, an understeering car will slide off forwards, an oversteering car will spin off backwards and a perfectly neutral car will slide off sideways, but they'll all slide off eventually!
Ok you might be getting somewhere with me now!! ;-)!!!! I always took understeer / oversteer to be a description of when one end the car exceeds it's grip limitations before the other. You are now descrbing this as terminal under / overteer.
So are you saying that if we have a car and we corner it just short of the limit so that there is no sliding we will achieve certain slip angles at all corners and if the car is designed to perfection the slip angles on the outside tyres will be at the peak of their slip angle bs lateral force curve for the given vertical loads they are experiencing. Now if we increase the front roll stiffness we achieve more vertical load on the front outside tyre and less on the rear outside trye during steady state cornering. We now have more lateral grip available at the front tyre due to the increase in vertical load however to achieve the increased lateral force (grip) the result is a higher slip angle seen by the driver as increased steering input and therefore understeer?? Albeit faster cornering speed?? Is this correct?
So what happens at the rear outside tyre that is surely equally as important during steady state cornering? The vertical load has been reduced therefore the available lateral tyre grip has been reduced. So although the front tyre has the ability to drive faster around the corner and achieve higher grip and therefore slip angles the rear cannot match this and surely the increase in corner speed will become more than can be taken by the rear tyre and we will have terminal oversteer?
Slightly related question. Is the rear slip angle constant for a given toe setting and corner radius regardless of how fast we corner?
Thanks for your time and patience on this. I really want to get my head around it!
So are you saying that if we have a car and we corner it just short of the limit so that there is no sliding we will achieve certain slip angles at all corners and if the car is designed to perfection the slip angles on the outside tyres will be at the peak of their slip angle bs lateral force curve for the given vertical loads they are experiencing. Now if we increase the front roll stiffness we achieve more vertical load on the front outside tyre and less on the rear outside trye during steady state cornering. We now have more lateral grip available at the front tyre due to the increase in vertical load however to achieve the increased lateral force (grip) the result is a higher slip angle seen by the driver as increased steering input and therefore understeer?? Albeit faster cornering speed?? Is this correct?
So what happens at the rear outside tyre that is surely equally as important during steady state cornering? The vertical load has been reduced therefore the available lateral tyre grip has been reduced. So although the front tyre has the ability to drive faster around the corner and achieve higher grip and therefore slip angles the rear cannot match this and surely the increase in corner speed will become more than can be taken by the rear tyre and we will have terminal oversteer?
Slightly related question. Is the rear slip angle constant for a given toe setting and corner radius regardless of how fast we corner?
Thanks for your time and patience on this. I really want to get my head around it!
FrazM said:
...if the car is designed to perfection the slip angles on the outside tyres will be at the peak of their slip angle vs lateral force curve for the given vertical loads they are experiencing.
Broadly speaking, yes.FrazM said:
...Now if we increase the front roll stiffness we achieve more vertical load on the front outside tyre and less on the rear inside tyre during steady state cornering.
EFA but basically OK so far.
FrazM said:
We now have more lateral grip available at the front tyre due to the increase in vertical load...
No. You're forgetting the law of diminishing returns, again. The load on the tyre has increased, so the 'grip' (cornering resistance) it is generating increases, but not by as much as the increase in load. Remember that the extra weight that's pushing the tyre into the ground is also the extra weight that it's trying to drag around the corner, when the weight really wants to carry on going in a straight line. Since the extra cornering force being generated by the tyre isn't quite as much as the extra weight it's trying to drag round the corner, if the tyre was already operating at the peak of its slip angle vs. lateral force curve, then it will be pushed past that peak by the extra load. FrazM said:
… increased lateral force (grip) the result is a higher slip angle seen by the driver as increased steering input and therefore understeer?? Albeit faster cornering speed?? Is this correct?
The driver will certainly see increased steering input/understeer, but if the front and rear tyres were in perfect balance before, adding an anti-roll bar (or otherwise increasing load onto the outside front) will simply create a situation where the outside front tyre is overworked and the outside rear tyre is underworked. Net result will be a reduction in ultimate grip/cornering speed.FrazM said:
So what happens at the rear inside tyre that is surely equally as important during steady state cornering?
EFA again, but, no, it doesn’t work like that, unfortunately. The lightly loaded inside tyre isn’t being worked very much at all, so it’s not operating at a big slip angle and it’s not generating much grip.
The ultimate example is when a front-wheel drive car picks up the inside rear wheel completely clear of the tarmac… clearly, in this instance the outside front is being worked very hard indeed, whereas the inside rear – dangling in fresh air – is generating no grip at all and is operating as zero slip angle.
Remember that total lateral weight transfer when cornering is purely a function of CG heigh, track and cornering force, so the inside pair of tyres are always going to be grossly underworked and so are relatively unimportant; the best you can hope to do is to balance the heavily loaded outside tyres so that they are both close to optimum (though most designers favour a fair margin of built-in understeer, simply for safety).
FrazM said:
Slightly related question. Is the rear slip angle constant for a given toe setting and corner radius regardless of how fast we corner?
No, slip angle will increase depending on how much the tyre is loaded, so the rear slip angle will increase with speed/tighter corner radius. Toe settings – although a fairly minor component in the overall picture – will result in a directly equivalent slip angle… if you think about it, if you run with, say half a degree of rear toe-in, then you are forcing the tyres to run at half a degree slip angle, when you’re driving in a straight line!You can, of course, do clever things with the toe settings so that they change under load or roll and so help control slip angles/handling balance, but that’s advanced stuff and best left to the Lotus and Porsche engineers, IMO!
Edited by Sam_68 on Friday 1st August 17:12
Sam you are a genius!!! I understand!!!!! Just had one of those Eureka moments and the room filled with bright lights and godly music!!!!!
I actually was just missing the point that even though we increase the grip at one tyre it is still then the job of that particular tyre to take the weight around the corner!!!! Sounds silly not to have understood that in the first place now I say it out loud!!!!!
I said inside tyre in my last post when I meant outside but you have answered the outside in your first couple of paragraphs anyway.
I've been reading my Miliken and Miliken book on tyre characteristics and your description of the rear tyre is making a lot of sense to me too! We corner faster, lean on the tyre more, it flexes more and therefore works at a higher slip angle to generate the extra grip!!
Cheers for the input!! I will keep reading my books and no doubt have more questions soon if you have the time to answer them?!?
Fraz
I actually was just missing the point that even though we increase the grip at one tyre it is still then the job of that particular tyre to take the weight around the corner!!!! Sounds silly not to have understood that in the first place now I say it out loud!!!!!
I said inside tyre in my last post when I meant outside but you have answered the outside in your first couple of paragraphs anyway.
I've been reading my Miliken and Miliken book on tyre characteristics and your description of the rear tyre is making a lot of sense to me too! We corner faster, lean on the tyre more, it flexes more and therefore works at a higher slip angle to generate the extra grip!!
Cheers for the input!! I will keep reading my books and no doubt have more questions soon if you have the time to answer them?!?
Fraz
This is just a tiny part of an incredible, fascinating and totally immense subject (handling and grip), I too only have a limited understanding but from what I've learnt over the last 4 years or so being involved in motorsport has taught me that oversteer and understeer are not necessarily good or bad but not only have to be controlled but communicated to the driver in such a way that he "feels" more confident in the cars ability.
There is also great fun to be had exploring these effects for yourself (safety and the law permitting of course).
For example there is a large, fairly quite round about near where I live used by Motorcyclists getting their knee down etc. In the wet though when it's quite I've known car drivers to use it for their own "experiments" in both over and understeer - I actually quite enjoy a bit of understeer which can be used quite effectively on the exit of a well executed corner
Good examples of grip, handling and slip angle can be seen in photo's and video's from F1 pre aerodynamics, truly a man and machine era - often the tyres peak grip was found at various angles of attack - beautiful
A word of caution though, a friend and work colleague (who now is a team manager in F3) of mine who had spent a lot of time in the states told me to steer clear of American books on the subject as there are many bods out "there" who have distorted "theories" on handling and set up, many of which are based on a misunderstood knowledge of science and physics!
There is also great fun to be had exploring these effects for yourself (safety and the law permitting of course).
For example there is a large, fairly quite round about near where I live used by Motorcyclists getting their knee down etc. In the wet though when it's quite I've known car drivers to use it for their own "experiments" in both over and understeer - I actually quite enjoy a bit of understeer which can be used quite effectively on the exit of a well executed corner
Good examples of grip, handling and slip angle can be seen in photo's and video's from F1 pre aerodynamics, truly a man and machine era - often the tyres peak grip was found at various angles of attack - beautiful
A word of caution though, a friend and work colleague (who now is a team manager in F3) of mine who had spent a lot of time in the states told me to steer clear of American books on the subject as there are many bods out "there" who have distorted "theories" on handling and set up, many of which are based on a misunderstood knowledge of science and physics!
Ben Magoo said:
A word of caution though... steer clear of American books on the subject
Dunno about that. To be honest the best books I've come across are Carroll Smith's series and the aforementioned Milliken & Milliken, both of which are american (though you're a brave man if you read the latter from cover to cover - it's better used as a reference text, to dip in and out of).
Allan Staniforth and Colin Campbell and Jeff Daniels (British) are OK, but Staniforth's knowledge is pretty patchy and Campbell and Colins are more production (passenger car) oriented in their treatment of the subject.
Put the following on another topic, just thought one of you might be good enough to pass an opinion?
Reading some of the above replies and articles, it appears some of you are really knowledgable about suspension set up, so going on from "steering loading up". I have a similar trait with my TVR S1. I have assumed it is set up like this on purpose with a lot of castor, for stability for road use, but as the author questions, the stering loads up too much, and it makes the car difficult to manouvere at high cornering speeds (trackdays etc), which is not desirable! Unfortunately the castor is not adjustable, not at least without modiying the wishbone mounts to allow horizontal adjstment.
The other question I have concerns roll. The suspension uses double wishbones at the front, and trailing arms at the rear. The rear spring/shock mounting is forward of the wheel centre, so given my limited way of thinking, does not work well in resisting roll. I have changed spring rates progressively, and gianed considerable improvement, and infact for road use, is now excellent. However, I want to reduce roll even further, so I have had a brainwave of fitting a rear anti roll bar. At present, the car enters a corner, steering and lateral forces build up, followed by a small amount of body roll. If you push it further, you have the distinct feeling the back end will snap. In practice, soemtimes it does, and sometimes it progressive. So in otherwords, its a challenge (some people like this who apparently have lots of hair on their chest, I for one really want a car that drives like a go cart).
The logic behind the rear antiroll bar, is that as the lateral forces build up, the loads will be transfered sooner to the outside tyre, making it break sooner and more progressively, and also reducing further body roll. I am not looking for ultimate cornering speed, just controllable fun.
Am I barking up the wrong tree?
Thanks
Mark
Reading some of the above replies and articles, it appears some of you are really knowledgable about suspension set up, so going on from "steering loading up". I have a similar trait with my TVR S1. I have assumed it is set up like this on purpose with a lot of castor, for stability for road use, but as the author questions, the stering loads up too much, and it makes the car difficult to manouvere at high cornering speeds (trackdays etc), which is not desirable! Unfortunately the castor is not adjustable, not at least without modiying the wishbone mounts to allow horizontal adjstment.
The other question I have concerns roll. The suspension uses double wishbones at the front, and trailing arms at the rear. The rear spring/shock mounting is forward of the wheel centre, so given my limited way of thinking, does not work well in resisting roll. I have changed spring rates progressively, and gianed considerable improvement, and infact for road use, is now excellent. However, I want to reduce roll even further, so I have had a brainwave of fitting a rear anti roll bar. At present, the car enters a corner, steering and lateral forces build up, followed by a small amount of body roll. If you push it further, you have the distinct feeling the back end will snap. In practice, soemtimes it does, and sometimes it progressive. So in otherwords, its a challenge (some people like this who apparently have lots of hair on their chest, I for one really want a car that drives like a go cart).
The logic behind the rear antiroll bar, is that as the lateral forces build up, the loads will be transfered sooner to the outside tyre, making it break sooner and more progressively, and also reducing further body roll. I am not looking for ultimate cornering speed, just controllable fun.
Am I barking up the wrong tree?
Thanks
Mark
I suspect your main problem here is the open diff, which in my S2 used to result in a slightly uncomfortable mix of power on oversteer and understeer depending whether it managed to spin the inner rear wheel. Increasing the rear roll stiffness might make this worse since it would be more inclined to spin the inner wheel but also more prone to oversteer if the inner wheel didn't spin.
Are you still using the original rubber bushed drop links on the front ARB? Replacing these with rod-end bearings tightens the handling up significantly with no particular compromise in ride comfort. What spring rates are you running now? I've gradually increased the spring/damper rates over the years and currently running 600/400 lb/in springs which is about as far as I'd want to go for road use but gives gokart like handling on the track. I'd like to increase the roll stiffness further but this will have to be done on the anti-roll bars since I can't afford to increase the road spring stiffness any further.
The seats by the way are Cobra Monacos fitted on seat brace bars under the floor. The seat brace bars stop the seat wobbling around.
The steering does suffer a little from kickback and tramlining on the road due to the combination of wide tyres, stiff suspension and unassisted steering. On the track the steering lightens up as it approaches the limit of grip. At the limit in the wet, the steering is almost weightless allowing fingertip control.
The seats by the way are Cobra Monacos fitted on seat brace bars under the floor. The seat brace bars stop the seat wobbling around.
The steering does suffer a little from kickback and tramlining on the road due to the combination of wide tyres, stiff suspension and unassisted steering. On the track the steering lightens up as it approaches the limit of grip. At the limit in the wet, the steering is almost weightless allowing fingertip control.
Edited by GreenV8S on Wednesday 20th August 13:44
Peter, I have fitted some Steve Heath solid drop links like you mention some time ago.....I have to admit I did not really notice any quantifiable difference, but it would also be fair to say that a number of other things were far from right at that time.
Like you I have messed around over the last couple of years with sring rates...I have 550lbins at the rear, and about 300lbins at the front....I must admit I cannot remember what the last front spring rate was, its been changed that much... I have boxes of spare springs now! Maybe the front spring rate is too soft now, but it does feel a descent compromise.
You mention you are only left to play with anti roll bars....barsss...have you one on the rear already? I have spent a long time working out how to fit one, and have only recently found a solution which will still give adequate road clearance. I have a bespoke exhaust, which allows a bar to fit across the chassis, underneath.
I am running Toyo Proxi`s with standard profiles. I used to run Bridgestones (can`t remeber what exactly, but were sticky and lower profile), but think the standard profile looks better, and works better, except on the limit on a track. I have found that lowering the pressure to around 20/21 psi works really well with these tyres(although I have yet to try this set up on a track). I am thinking about getting some "R" spec tyres though! I do get some tramlinig and kick back, but nothing unacceptable considering the nature of the car.
I think I really want something the car can never be, but then its fairly straightforward to play with. I have GAZ adjustable shocks, which are value for money, and I am very pleased with them, but I know if I fit some fancy nitrons then the ride/handling can be improved significantly, but then the shocks will be worth more than the car, and I cannot come to terms with that type of expenditure.
Like you I have messed around over the last couple of years with sring rates...I have 550lbins at the rear, and about 300lbins at the front....I must admit I cannot remember what the last front spring rate was, its been changed that much... I have boxes of spare springs now! Maybe the front spring rate is too soft now, but it does feel a descent compromise.
You mention you are only left to play with anti roll bars....barsss...have you one on the rear already? I have spent a long time working out how to fit one, and have only recently found a solution which will still give adequate road clearance. I have a bespoke exhaust, which allows a bar to fit across the chassis, underneath.
I am running Toyo Proxi`s with standard profiles. I used to run Bridgestones (can`t remeber what exactly, but were sticky and lower profile), but think the standard profile looks better, and works better, except on the limit on a track. I have found that lowering the pressure to around 20/21 psi works really well with these tyres(although I have yet to try this set up on a track). I am thinking about getting some "R" spec tyres though! I do get some tramlinig and kick back, but nothing unacceptable considering the nature of the car.
I think I really want something the car can never be, but then its fairly straightforward to play with. I have GAZ adjustable shocks, which are value for money, and I am very pleased with them, but I know if I fit some fancy nitrons then the ride/handling can be improved significantly, but then the shocks will be worth more than the car, and I cannot come to terms with that type of expenditure.
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