Tyre Width And Grip
Discussion
Anh said:
Mr Whippy said:
I don't disagree with that 
Optimising the temp of the tyre by choosing the right size tyre for application, pressure, and mounting for it (wheel size), means you optimise mu in the last place it can be optimised, and so optimise grip.
Ie, as you kinda mentioned, the narrowest, smallest tyre, at possibly the lowest pressure, mounted ideally, that doesn't overheat for intended purposes, is all you need.
So assuming you are doing everything else right, heat is the concern imho. If any given tyre is mounted and inflated appropriately for the job at hand, and it overheats doing intended job, then it's not right for the job, and you might go wider/bigger (assuming you have optimised as best you can alignment etc)
If we do silly things like run a really wide tyre on a light car at high pressure, we are in for trouble to start with, with mu moving around because of things we have full control over and can change easily.
Tis a complex subject, I don't believe I understand it amazingly well, I want to buy Pacejka's book shortly to try improve my understanding more, but I do think that if you are managing heat within the bounds of your tyre and what you want it to do, you are 99% of the way there
So heat is important. Doesn't really answer the question though I suppose, oops
Dave
Unfortunately tyre makers do not publish pretty simple to read graphs for us average Joe's to work out which one is best and at what conditions are required to get the grip we want and at what point does the tyre's co-effiecient of friction starts to deteriorate. Optimising the temp of the tyre by choosing the right size tyre for application, pressure, and mounting for it (wheel size), means you optimise mu in the last place it can be optimised, and so optimise grip.
Ie, as you kinda mentioned, the narrowest, smallest tyre, at possibly the lowest pressure, mounted ideally, that doesn't overheat for intended purposes, is all you need.
So assuming you are doing everything else right, heat is the concern imho. If any given tyre is mounted and inflated appropriately for the job at hand, and it overheats doing intended job, then it's not right for the job, and you might go wider/bigger (assuming you have optimised as best you can alignment etc)
If we do silly things like run a really wide tyre on a light car at high pressure, we are in for trouble to start with, with mu moving around because of things we have full control over and can change easily.
Tis a complex subject, I don't believe I understand it amazingly well, I want to buy Pacejka's book shortly to try improve my understanding more, but I do think that if you are managing heat within the bounds of your tyre and what you want it to do, you are 99% of the way there
So heat is important. Doesn't really answer the question though I suppose, oops
Dave
I am not saying that temperature is of no importance, after all the tyre compound must reach its optimal temperature to deliver the potential cornering force over a given load - what I strongly disagree on is the notion that a wider tyre provides more grip because it overheats less - which is what one of the earlier posts made by saab93 was asserting.
Wider tyres can grip more if it generates a larger contact patch area, assuming equal tyre pressures. And to do this more tyre load is needed from the car itself, be this from more weight transfer, downforce or more weight.
Another very important and often very overlooked aspect of tyre widths is the slip angles they run at to hold the car's mass during a corner. A narrow tyre often enough must run at a higher slip angle to grip the same as moderately wider tyre, so assuming 50:50 weight distribution, placing different width tyres front and rear will change the balance of the car, i.e it will create understeer or oversteer.
Porsche is an extreme example, huge rear weight bias + RWD on the rear tyres. Very wide tyres are required not because they grip more but because they provide the grip needed at a sensible slip angle compared to the sensibly sized front tyres. You'd hear alot of horror stories of 911's oversteering at high speed corners, this is due to the reasons mentioned above but made worse at high speed when 1) strong rear wing provides too much load to the rear wheels and increases slip angle even more, 2) you need to apply more power at high speeds to maintain the speed and thus increase slip angle even more, hence more oversteer.
I'm quite interested in the slip angle tuning. I'd always assumed that a larger optimum slip angle at the rear would be ideal, so you didn't breach the optimum slip angle at the back before the front and fall off the peak later grip point (ie, as you get slipping past a certain point it's a point of no return in a sense)
However, your point made with the 911 is quite interesting, and in simulation terms is one I've had difficulty with, but I have always been tuning the curves for higher optimum slip angles at the back... maybe I need to try them lower... hmmm... what would be ideal then would you say? Would a wider lower profile rear tyre at appropriate pressure be thus setup for a higher maximum stiffness, to provide the sharper optimum slip ratio?
Nice to find some new ideas on this because as always the 911 often makes points really well due to it's really different configuration.
Dave
Edited by Mr Whippy on Sunday 1st November 13:15
Anh said:
I am not saying that temperature is of no importance, after all the tyre compound must reach its optimal temperature to deliver the potential cornering force over a given load - what I strongly disagree on is the notion that a wider tyre provides more grip because it overheats less - which is what one of the earlier posts made by saab93 was asserting.
I didnt 
I said opposite to that
A wider tyre will only provide more grip if its up to temperature, and as its generally lower profile and have less sidewall flexing, its more difficult.
Something like that
Anyone tried narrower tyres and found benefits?
Having read all this...
The obvious question is:
Why do Supercars run 345's etc when they aren't that heavy, is it just fashion ?
And surely drag racers would opt for the best size available as every 100th counts and they tend to be pretty wide !
The obvious question is:
Why do Supercars run 345's etc when they aren't that heavy, is it just fashion ?
And surely drag racers would opt for the best size available as every 100th counts and they tend to be pretty wide !
Edited by redgriff500 on Sunday 1st November 14:08
redgriff500 said:
I am aware that by increasing tyre width from say 195 to 225 will change the shape of the contact patch on the road.
But is there any quick and easy formula to work out what it will actually do regarding grip levels - both whilst cornering and on acceleration.
(Assuming the same type of tyre)
IIRC it won't improve cornering speeds but will improve grip upon acceleration thus reducing wheelspin but by how much ?
Setting aside any changes in rubber compound and tyre pressure (hence contact area), changing the shape of the contact patch affects the way the tyre distorts sideways under cornering. This can have very significant effects on the vehicle handling in cornering. It would have minimal effect on the behaviour in straight line acceleration or braking.But is there any quick and easy formula to work out what it will actually do regarding grip levels - both whilst cornering and on acceleration.
(Assuming the same type of tyre)
IIRC it won't improve cornering speeds but will improve grip upon acceleration thus reducing wheelspin but by how much ?
Under cornering loads the sideways deflection of the tread increases towards the trailing edge of the contact patch. This means the trailing edge reaches the limit of grip sooner than the leading edge. The longer the contact patch, the more variation there is in lateral stress between the front/back of the contact patch and the greater this effect is.
A long narrow contact patch will have a very high variation, so the trailing edge will tend to break away relatively early while the leading edge is well below the limit of grip. This produces a very soft and progressive break away as the tyre starts to slide. It also means the center of lateral resistance moves a long way, which means you get a big change in the self-aligning torque around the limit (in the case of front wheels this provides valuable steering feedback).
A shorter wider contact patch has less variation which means more of the contact patch tends to be around peak grip at the same time. This means that the tyre produces more ultimate grip, the grip is available at lower slip angles, but the grip falls away more sharply (than for the narrower tyre) when the slip angle goes past the maximum. In other words there's more grip, less steering feedback (front wheels) and the car breaks away more suddenly if you push past the available grip.
There's a difference between a nice clean track and a typical leafy lane.
A 911 has engine weight and weight transferance under power to the rear wheels so can take wider rubber.
More here
http://www.youtube.com/watch?v=8nGiZKQmODA
A 911 has engine weight and weight transferance under power to the rear wheels so can take wider rubber.
More here
http://www.youtube.com/watch?v=8nGiZKQmODA
redgriff500 said:
redgriff500 said:
Having read all this...
The obvious question is:
Why do Supercars run 345's etc when they aren't that heavy, is it just fashion ?
And surely drag racers would opt for the best size available as every 100th counts and they tend to be pretty wide !
Anyone ?The obvious question is:
Why do Supercars run 345's etc when they aren't that heavy, is it just fashion ?
And surely drag racers would opt for the best size available as every 100th counts and they tend to be pretty wide !
Wide tyres are also used to balance the slip angles - when you consider how much torque/power a super car can deliver to the rear wheels, the slip angle becomes much larger than the front tyre slip angle leading to too much on-power oversteer.
If you look at the Top Fuel Drag cars, they adopt alot of tricks including 100% anti-squat, and even downforce to load the wide tyres to get the traction out of big rubber.
Edited by Anh on Tuesday 3rd November 11:04
redgriff500 said:
redgriff500 said:
Having read all this...
The obvious question is:
Why do Supercars run 345's etc when they aren't that heavy, is it just fashion ?
And surely drag racers would opt for the best size available as every 100th counts and they tend to be pretty wide !
Anyone ?The obvious question is:
Why do Supercars run 345's etc when they aren't that heavy, is it just fashion ?
And surely drag racers would opt for the best size available as every 100th counts and they tend to be pretty wide !
You use the smallest tyre you can to do the job, but a smaller tyre will deform more under the tractive and lateral loads these kinds of cars will generate.
BUT, there will be an element of style as well, big wheels look cool these days, so a low profile seems popular, and that is also maybe forced by HUGE brakes these days too (but then they are mainly needed to haul the weights of huge cars down from speed, F1 cars have very small and thick discs and high profile tyres for example)
A big wide tyre or a narrow one have the same potential for grip, but the wide one will run smaller slip angles or slip ratios to do it's work, deforming less, and staying cooler, and keeping within optimum operating parameters for longer!
I think
Dave
Mr Whippy said:
I'm quite interested in the slip angle tuning. I'd always assumed that a larger optimum slip angle at the rear would be ideal, so you didn't breach the optimum slip angle at the back before the front and fall off the peak later grip point (ie, as you get slipping past a certain point it's a point of no return in a sense)
However, your point made with the 911 is quite interesting, and in simulation terms is one I've had difficulty with, but I have always been tuning the curves for higher optimum slip angles at the back... maybe I need to try them lower... hmmm... what would be ideal then would you say? Would a wider lower profile rear tyre at appropriate pressure be thus setup for a higher maximum stiffness, to provide the sharper optimum slip ratio?
Nice to find some new ideas on this because as always the 911 often makes points really well due to it's really different configuration.
Dave
Ok, 2 things, firstly you need to work out what you are trying to ultimately acheive with the car, is it more ultimate grip, or to balance the car for more/less oversteer? There really isn't an ideal slip angle, much like there really isn't an ideal spring rate or weight distribution. However, your point made with the 911 is quite interesting, and in simulation terms is one I've had difficulty with, but I have always been tuning the curves for higher optimum slip angles at the back... maybe I need to try them lower... hmmm... what would be ideal then would you say? Would a wider lower profile rear tyre at appropriate pressure be thus setup for a higher maximum stiffness, to provide the sharper optimum slip ratio?
Nice to find some new ideas on this because as always the 911 often makes points really well due to it's really different configuration.
Dave
Edited by Mr Whippy on Sunday 1st November 13:15
This leads to the second point, altering weight distribution or roll stiffness will indeed alter slip angle as the loads on each tyre will change, which means the balance of the car is altered during grip - so as with suspension and chassis tuning, you need to treat tyre size selection in the same manner, consider the car's corner mass and the loads the car is capable of.
Just a side note: Slip angle doesn't mean the angle of slide or loss of adhesion, slip angle is just the difference between the direction the wheel is pointing to compared to the actual direction the tyre is rotating.
Edited by Anh on Tuesday 3rd November 11:38
Anh said:
Mr Whippy said:
I'm quite interested in the slip angle tuning. I'd always assumed that a larger optimum slip angle at the rear would be ideal, so you didn't breach the optimum slip angle at the back before the front and fall off the peak later grip point (ie, as you get slipping past a certain point it's a point of no return in a sense)
However, your point made with the 911 is quite interesting, and in simulation terms is one I've had difficulty with, but I have always been tuning the curves for higher optimum slip angles at the back... maybe I need to try them lower... hmmm... what would be ideal then would you say? Would a wider lower profile rear tyre at appropriate pressure be thus setup for a higher maximum stiffness, to provide the sharper optimum slip ratio?
Nice to find some new ideas on this because as always the 911 often makes points really well due to it's really different configuration.
Dave
Ok, 2 things, firstly you need to work out what you are trying to ultimately acheive with the car, is it more ultimate grip, or to balance the car for more/less oversteer? There really isn't an ideal slip angle, much like there really isn't an ideal spring rate or weight distribution. However, your point made with the 911 is quite interesting, and in simulation terms is one I've had difficulty with, but I have always been tuning the curves for higher optimum slip angles at the back... maybe I need to try them lower... hmmm... what would be ideal then would you say? Would a wider lower profile rear tyre at appropriate pressure be thus setup for a higher maximum stiffness, to provide the sharper optimum slip ratio?
Nice to find some new ideas on this because as always the 911 often makes points really well due to it's really different configuration.
Dave
This leads to the second point, altering weight distribution or roll stiffness will indeed alter slip angle as the loads on each tyre will change, which means the balance of the car is altered during grip - so as with suspension and chassis tuning, you need to treat tyre size selection in the same manner, consider the car's corner mass and the loads the car is capable of.
Just a side note: Slip angle doesn't mean the angle of slide or loss of adhesion, slip angle is just the difference between the direction the wheel is pointing to compared to the actual direction the tyre is rotating.
Haha, the second point, the feedback loop of eternal tuning and compromise removal, chasing your tail forever. It seems there is no hard fast rule, except tune for what you want and expect to have to compromise because of it.
No, sorry, when I say slip angle in the former post, I mean optimum slip ratio, as in the angle of slip where the most force per unit load is generated.
May I ask about your background in tyres, as it's nice to learn more from people who have an interest (be it hobby or professional) and can shed light on interesting subjects like this
Dave
Tyres don't just work on coulomb friction (F=muN) they also work by shear friction, so increasing the contact patch does to some extent increase grip. It also increases the available area to do the work, so it should reduce tyre temperature/wear for a given load, allowing you to run softer tyres. But as discussed, it's a bit more complicated than that.
In reality, once you start taking into account contact patch behaviour it's such a bloody complex topic that unless you've got access to serious simulation tools it's best to just work with common knowledge and the seat of your pants.
By my experience, the setup you're suggesting doesn't sound unreasonable.
In reality, once you start taking into account contact patch behaviour it's such a bloody complex topic that unless you've got access to serious simulation tools it's best to just work with common knowledge and the seat of your pants.
By my experience, the setup you're suggesting doesn't sound unreasonable.
Mr Whippy said:
Yeah, I think in my simulated example, it was stability, and adjusting the optimum slip angle to be reached sooner for the rear tyres, has improved the high-speed stability markedly!
Haha, the second point, the feedback loop of eternal tuning and compromise removal, chasing your tail forever. It seems there is no hard fast rule, except tune for what you want and expect to have to compromise because of it.
No, sorry, when I say slip angle in the former post, I mean optimum slip ratio, as in the angle of slip where the most force per unit load is generated.
May I ask about your background in tyres, as it's nice to learn more from people who have an interest (be it hobby or professional) and can shed light on interesting subjects like this
Dave
Dave,Haha, the second point, the feedback loop of eternal tuning and compromise removal, chasing your tail forever. It seems there is no hard fast rule, except tune for what you want and expect to have to compromise because of it.
No, sorry, when I say slip angle in the former post, I mean optimum slip ratio, as in the angle of slip where the most force per unit load is generated.
May I ask about your background in tyres, as it's nice to learn more from people who have an interest (be it hobby or professional) and can shed light on interesting subjects like this
Dave
I'm just a hobbyist, have owned driven a variety of fast and fast-ish cars, FWD, RWD, AWD, RWD->AWD, hot hatches, roadsters, barges, GTs, supercars, karts, quads, experimented about with tyres and suspension.
There is alot information out there, published and on the web, however some information tends to be wrong and some right, from both the amatuers and the professionals! So it is a frustrating learning curve.
Most people approach a problem backwards without realising, prime example is minimumising weight - however unfortunately no matter how light a car is, if the tyre isnt in firm contact with the surface and is loaded, the car becomes a hovering paper weight.
Maximising tyre contact is the key to all handling and performance of a car.
Edited by Anh on Wednesday 4th November 16:47
Edited by Anh on Wednesday 4th November 16:48
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