Under/oversteer/grip
Discussion
Can someone explain this?
Track day in my Griff on a hot dry day:
Going round relatively slow corners on light/balanced throttle the car runs wide i.e. understeers.
Applying more throttle in the corner noticeably shifts weight to the rear and you have the sensation that the outside tyres, particularly the rear, really “dig in” and grip.
The car now flies round the corner with no sensation of understeer but also no oversteer – unless you apply even more throttle and then oversteer and steering adjustments come into play.
What I don’t understand is why applying more throttle, which takes weight off the front tyres, should improve the cornering.
I have a couple of theories but would be very interested in other people’s views.
(Oh... and Happy New Year!
)
Track day in my Griff on a hot dry day:
Going round relatively slow corners on light/balanced throttle the car runs wide i.e. understeers.
Applying more throttle in the corner noticeably shifts weight to the rear and you have the sensation that the outside tyres, particularly the rear, really “dig in” and grip.
The car now flies round the corner with no sensation of understeer but also no oversteer – unless you apply even more throttle and then oversteer and steering adjustments come into play.
What I don’t understand is why applying more throttle, which takes weight off the front tyres, should improve the cornering.
I have a couple of theories but would be very interested in other people’s views. (Oh... and Happy New Year!
) I seem to remember reading something on this in the bible. I think it goes something along the lines of driving through the understeer? I'm sure shpub will be along shortly to correct me or fill in more details. I'll ledit this to post a page number in the bible if I can find it [the page, that is].
ed to add: The Bible, Second Ed., p357.
also, p358 -- 4 bullet points:
>> Edited by kdd on Saturday 3rd January 12:10
ed to add: The Bible, Second Ed., p357.
also, p358 -- 4 bullet points:
The Bible said:
Basically, if you are in a corner and find the car is understeering your choices are:
1. Ease off the power and wait for the corner to open up (this is the safe option)
2. Floor it and hope you are in a low enough gear to get over the understeer hump (this is an act of faith, also be prepared to catch a big powerslide the first few times until you learn how much power you need).
3. Ease off the steering and then 'flick' the car abruptly nto the corner. This will often un-settle the car enough to kill the understeer.
4. Abruptly lift off the power and then get straight back on it. This will unsettle the car big time! You have been warned!
Get any of these last three wrong and you will probably end up in bigger trouble than you started with.
>> Edited by kdd on Saturday 3rd January 12:10
The car is naturally setup to understeer, as most cars are, so the front will lose grip first.
When you apply a lot of throttle, the back will come round and you would need corrective lock. However, there is a balancing point where the back is coming round ever so slightly which will cancel out the understeer at the front.
Also, as you said, the weight is transfered towards the rear, and so there is less weight for the fronts to carry round the corner (think centripetal motion).
I've only had my Chimaera for about 2 months and have been playing with a bit of throttle in slow corners and have found the same - a little bit of power helps the car round the corner and feels like its on rails.
When you apply a lot of throttle, the back will come round and you would need corrective lock. However, there is a balancing point where the back is coming round ever so slightly which will cancel out the understeer at the front.
Also, as you said, the weight is transfered towards the rear, and so there is less weight for the fronts to carry round the corner (think centripetal motion).
I've only had my Chimaera for about 2 months and have been playing with a bit of throttle in slow corners and have found the same - a little bit of power helps the car round the corner and feels like its on rails.
GarryM said:
Can someone explain this?
Track day in my Griff on a hot dry day:
Going round relatively slow corners on light/balanced throttle the car runs wide i.e. understeers.
Applying more throttle in the corner noticeably shifts weight to the rear and you have the sensation that the outside tyres, particularly the rear, really “dig in” and grip.
The car now flies round the corner with no sensation of understeer but also no oversteer – unless you apply even more throttle and then oversteer and steering adjustments come into play.
What I don’t understand is why applying more throttle, which takes weight off the front tyres, should improve the cornering.
(Oh... and Happy New Year!
That quote from the bible was taken from a fairly long post here on Pistonheads about the mechanics of understeer and oversteer. I'm not going to try to reproduce it again (took me bloomin ages the first time) but no doubt a search will turn up the original post.
GreenV8S said:
That quote from the bible was taken from a fairly long post here on Pistonheads about the mechanics of understeer and oversteer.
nah, I just typed it in while reading the relevant pages
if I'd realised/remembered there was an older post about it I'd have done a search and saved my fingers 
>> Edited by kdd on Saturday 3rd January 12:38
Thanks for the replies and to kdd for taking the time to type out that quote (did you have permission?
)
You may well be right. Trouble is though, taking weight off the front should make that tyre slip more so although you are introducing some slip into the rear I can't quite see it catching up - unless you unsettle the rear completely as referred to in kdd's quote and introduce real oversteer. In fact kdd's quote from the Bible sums up how it "should" be. The trouble is it doesn't seem to do that i.e. you don't have to a) slow up or, b) create oversteer by excess throttle etc. All you do is increase throttle slightly and it goes round on rails.
It's the weight on the front tyre that stops it sliding away completely. The understeer is caused by the tyre's lack of grip rather than the centripetal force. Reduce the weight and it will slide more.
I've read quite a bit about the physics of over and understeer (I think I remember your PH post Peter and I certainly remember your Sprint article). I've also experienced both on the track. What I'm experiencing on the face of it seems to contradict received wisdom.
My other theory is that the outer front tyre is deforming under the load and so its available grip is reduced. Reducing the load on it (by accelerating) enables the tyre to "reform" and retain max grip which combined with the slight increase in slip angle of the rear tyre helps the car to corner effectively. I'm not entirely happy with this though as it's easy to create this effect and I doubt it is that easy to deform the front tyre.
) jsr said:
... there is a balancing point where the back is coming round ever so slightly which will cancel out the understeer at the front.
You may well be right. Trouble is though, taking weight off the front should make that tyre slip more so although you are introducing some slip into the rear I can't quite see it catching up - unless you unsettle the rear completely as referred to in kdd's quote and introduce real oversteer. In fact kdd's quote from the Bible sums up how it "should" be. The trouble is it doesn't seem to do that i.e. you don't have to a) slow up or, b) create oversteer by excess throttle etc. All you do is increase throttle slightly and it goes round on rails.
jsr said:
... the weight is transfered towards the rear, and so there is less weight for the fronts to carry round the corner (think centripetal motion).
It's the weight on the front tyre that stops it sliding away completely. The understeer is caused by the tyre's lack of grip rather than the centripetal force. Reduce the weight and it will slide more.
I've read quite a bit about the physics of over and understeer (I think I remember your PH post Peter and I certainly remember your Sprint article). I've also experienced both on the track. What I'm experiencing on the face of it seems to contradict received wisdom.
My other theory is that the outer front tyre is deforming under the load and so its available grip is reduced. Reducing the load on it (by accelerating) enables the tyre to "reform" and retain max grip which combined with the slight increase in slip angle of the rear tyre helps the car to corner effectively. I'm not entirely happy with this though as it's easy to create this effect and I doubt it is that easy to deform the front tyre.
When you apply power you are pushing the car round the corner. Same when driving a kart go round the corner with no throttle and it will understeer/drift round. Apply a little throttle and it will corner tighter and grip more. (Kart physics are a little different though due to no diff).
This is why you should always apply power through a turn. Sow in Fast out and all that, the trick is to get the power back on again sooner than you think usually before the Apex.
This is why you should always apply power through a turn. Sow in Fast out and all that, the trick is to get the power back on again sooner than you think usually before the Apex.
Garry, speaking from my experience I see nothing unusual or unexpected in it what you describe. In slow to medium speed corners (sub 90mph ish) if I have a problem with understeer I cancel it out with throttle. I disagree that moving the apparent weight off the front tyres will make grip worse in this case as understeer exists from the tyre being over worked surely? I only ever come off the throttle sharply to cancel understeer in a front drive car. I'd not dare try that in a powerful rear drive car.
Washy
Washy
GarryM said:
What I don’t understand is why applying more throttle, which takes weight off the front tyres, should improve the cornering.
IMHO by applying more torque to the rear tyres you are increasing their propensity to break traction - in this case they just slip a little which balances the movement of the front, hence giving you more neutral cornering. You can also call it steering with the throttle!
Try it on a skid pan - surprising understeer at low speed, then (violent) oversteer as you boot it to get the back out.
It's silly but this problem has been really bugging me!
Reading the replies I think I'm beginning to understand it. Thank you.
My confusion, I think, comes down to how much throttle is required to balance the car whilst cornering. Light throttle is not enough. In this state the front tyres are also being asked to slow the car (not by much as I'm not off the throttle completely) - they are already at their limit of traction so they start to gently slip. Applying more throttle removes the braking component and allows the front tyres to get on with turning.
This makes sense as I'm not experiencing oversteer unless I push harder on the throttle. (Which I like to do of course!)
It's an interesting point (to me at least!) though because it took a couple of track days for me to get the feel of this and whilst I been happily doing it ever since I couldn't explain the physics as I thought I was having to apply more than a "balanced" throttle - it feels like you are accelerating (which, of course, it would due to the centripetal force - doh!)
I feel better now
Reading the replies I think I'm beginning to understand it. Thank you. My confusion, I think, comes down to how much throttle is required to balance the car whilst cornering. Light throttle is not enough. In this state the front tyres are also being asked to slow the car (not by much as I'm not off the throttle completely) - they are already at their limit of traction so they start to gently slip. Applying more throttle removes the braking component and allows the front tyres to get on with turning.
This makes sense as I'm not experiencing oversteer unless I push harder on the throttle. (Which I like to do of course!
) It's an interesting point (to me at least!) though because it took a couple of track days for me to get the feel of this and whilst I been happily doing it ever since I couldn't explain the physics as I thought I was having to apply more than a "balanced" throttle - it feels like you are accelerating (which, of course, it would due to the centripetal force - doh!)
I feel better now
Time for the physics 
...
Understeer is caused by carrying too much speed into a corner, which is a result of the front tyres not being able to carry the weight of the front of the car through the corner.
GarryM, whilst you are correct in thinking that taking weight away from the front will make the tyres slip more, this is only the vertical component of force. Dont forget that there is also a horizontal component of force which gets greater the faster you corner. This horizontal component of force acts away from the centre of the corner.
Think about a Formula 1 racing car. It is very light but has a lot of downforce. The downforce only adds to the vertical component of force. Cornering at say x mph will generate a centripetal (horizontal) force of say y, purely due to the weight of the car (not the downforce).
Now imagine you remover the aerodynamic aids and increased the weight of the car to give the same vertical force as that of the downforce enabled car.
Cornering again at x mph will now produce a centripetal force of z and this will be greater than y.
It may even be so great that the car cannot stick to its path, the tyres lose grip and hence cause understeer (or oversteer or even both).
Getting back to GarryM's situation, by reducing the weight at the front (by appying the throttle) the vertical and horizontal components of force are reduced, so even though the front tyres will lose some of their ability to grip, the centripetal force has also reduced, freeing up some of the available grip. This, hopefully, explains why the shift in weight affects the ability to corner.
(obviously it does get a bit more complicated than this but i think this illustrates the point)
What i've never really understood though, is that the centre of gravity of the car under braking or accelerating will probably barely move - maybe a 1cm or something, yet it has so much affect on the car
... Understeer is caused by carrying too much speed into a corner, which is a result of the front tyres not being able to carry the weight of the front of the car through the corner.
GarryM, whilst you are correct in thinking that taking weight away from the front will make the tyres slip more, this is only the vertical component of force. Dont forget that there is also a horizontal component of force which gets greater the faster you corner. This horizontal component of force acts away from the centre of the corner.
Think about a Formula 1 racing car. It is very light but has a lot of downforce. The downforce only adds to the vertical component of force. Cornering at say x mph will generate a centripetal (horizontal) force of say y, purely due to the weight of the car (not the downforce).
Now imagine you remover the aerodynamic aids and increased the weight of the car to give the same vertical force as that of the downforce enabled car.
Cornering again at x mph will now produce a centripetal force of z and this will be greater than y.
It may even be so great that the car cannot stick to its path, the tyres lose grip and hence cause understeer (or oversteer or even both).
Getting back to GarryM's situation, by reducing the weight at the front (by appying the throttle) the vertical and horizontal components of force are reduced, so even though the front tyres will lose some of their ability to grip, the centripetal force has also reduced, freeing up some of the available grip. This, hopefully, explains why the shift in weight affects the ability to corner.
(obviously it does get a bit more complicated than this but i think this illustrates the point)
What i've never really understood though, is that the centre of gravity of the car under braking or accelerating will probably barely move - maybe a 1cm or something, yet it has so much affect on the car
Thus, assuming you don't want to kill yourself going backwards through a hedge at 90 mph, there are two choices,
The cheaper option is to try better tyres.
The real solution is to buy a better handling car.
Most cars that are both powerful and handle well are viciously expensive, ie welcome to the £100k plus club! Us mere mortals generally have to choose either a small light car with good handling or a powerful car that's a bit of a handful in the corners.
The cheaper option is to try better tyres.
The real solution is to buy a better handling car.
Most cars that are both powerful and handle well are viciously expensive, ie welcome to the £100k plus club! Us mere mortals generally have to choose either a small light car with good handling or a powerful car that's a bit of a handful in the corners.
You're on the right lines but you need to make a distinction between mass and weight.
There are lots of factors that effect the way a car understeers or oversteers, but the two most important factors are weight transfer, and the trade-off between lateral and longitudinal grip generated by the tyre (usually referred to as the elipse of friction). But the interaction between these two factors is surprisingly complicated because the weight transfer is a linear effect while the grip trade-off is decidely non-linear.
There are lots of factors that effect the way a car understeers or oversteers, but the two most important factors are weight transfer, and the trade-off between lateral and longitudinal grip generated by the tyre (usually referred to as the elipse of friction). But the interaction between these two factors is surprisingly complicated because the weight transfer is a linear effect while the grip trade-off is decidely non-linear.
jsr said:
Time for the physics
This is a long one…
Well I admit what I know about mechanics is derived from A levels (a long time ago!) and a general interest in the subject. I’m unsure whether you have a better grasp of the subject than me (but I wouldn’t be surprised
) but there appears to be a fundamental flaw in your reasoning. You appear to be ignoring the fact that the forces act through the centre of gravity of the car. To take your parting comment first – the shift of weight when braking or accelerating (or cornering) is a consequence of the CofG being above the level of the tyres’ contact with the ground. As a result, when braking, for example, the CofG tries to continue when the front tyres are trying to stop. The car is trying to rotate about its CofG. The only reason it doesn’t is that the tarmac is, in effect, pushing up harder on the front tyres. It is this rotational force, or torque, that explains the significant weight shift during accelerating braking and cornering and not just the fact that the CofG will move by an inch or so due to suspension movement. To work it out all you need to know is the position of the CofG, the wheelbase of the car (or track), the car’s weight and static weight distribution and the braking/accelerating force being applied (or cornering force). As an example the weight shift in a typical sports car during a 1G braking manoeuvre (hard but realistic braking) is in the order of 20%. That is 20% of the car’s total weight shifts from the rear on to the front.
The same thing applies with cornering. The centripetal force required to stop a car sliding off the track has to be within the tyres limits of adhesion. If that limit has already been exceeded when cornering in a balanced state, reducing the weight acting on that tyre simply makes the matter worse. (edited to add: remember the weight of the car hasn't changed.) This is not debateable unless you wish to doubt Isaac Newton!
The understeer I have been pondering over (I am now fairly sure) is simply due to the car not being in a state of balance when I had previously thought it was.
Hope it was worth reading!
>> Edited by GarryM on Sunday 4th January 22:52
gaston said:
The real solution is to buy a better handling car.
I like to think a solution is to better understand what is happening to the car so that you can adapt your driving style. When you think what drivers like Stirling Moss had to put up with, skinny tyres etc. etc. and yet they could drive at simply outrageous speeds. He averaged nearly 100mph on Italian roads for the 10 hour Mille Miglia (that is incredible).
You need also to think about slip angles of the tyres. a tyre is happiest at zero slip- ie. straight ahead. there are slip angles which work best for tyres but exceed these and the tyres will slide.
by adding power when the car is understeering, you have to unwind lock, hence bring the tyre back towards its preferred slip angle and hence its 'grippiest' angle. so less understeer and the back of the car is also helping push the nose into the turn. the rear tyres will now have some slip angle in them.
you need to also consider castor and camber angles and also spring and damper rates. tyre compund and tread also play a part. but the biggest influence on a car's handling is the driver.
try a copy of competition car suspension by alan staniforth.
by adding power when the car is understeering, you have to unwind lock, hence bring the tyre back towards its preferred slip angle and hence its 'grippiest' angle. so less understeer and the back of the car is also helping push the nose into the turn. the rear tyres will now have some slip angle in them.
you need to also consider castor and camber angles and also spring and damper rates. tyre compund and tread also play a part. but the biggest influence on a car's handling is the driver.
try a copy of competition car suspension by alan staniforth.
RichB said:
Garry, I'm not going to pretend to try to offer advice because there others far better qualified than me to do that but have you veer driven a decent kart? It's quite enlightening, and really does open you mind to the effect the power on/off has on understeer/oversteer. Rich...
Hi Rich. There’s no doubt I have a lot to learn about driving (don’t we all?
) but my question was really only about the physics to explain that moment when my Griff gets past slight understeer into a state of balance. I agree driving and just feeling it is more fun. Indeed all you need to know about car control can be learnt from driving - I just have a perverse interest in trying to understand why you experience what you experience. From my point of view, trying to understand it helps as I don’t have the time or opportunity to do more track driving than I already do. I don’t claim to fully understand it either, as tuscan thunder points out there is the whole business of chassis tuning and tyre characteristics which complicate matters to a mind boggling degree. The useful point from this discussion though is that the standard response of "reduce throttle (gently) and unwind lock when experiencing understeer" is not always correct (but is in most circumstances) because in the situation I described at the beginning of this thread, the understeer can be reduced if a little more throttle is applied to bring the car into a balanced state. It’s not new and I’m sure you do this instinctively once you’ve experienced it. I’m not saying I don’t know what under/oversteer is either. It’s just a small point of detail!! (can you see my anorak from here?
)
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