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2018 Subaru BRZ
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Discussion Starter · #1 ·
On power rotation is something that always puzzled me in RWD cars. I could never figure out if the rotation is caused by oversteer, with the engine overpowering the tires and causing them to slip, or is there some other mechanism at play? I'm talking about the feeling of the rear of the car pushing the car into the corner when you apply extra throttle during a turn, tightening the turning radius.
The question becomes even more interesting to me for mid engine cars with sticky tires - does the extra weight on the rear tires coupled with the traction of the tires themselves mean that this feeling of rotation won't be present? Or am I completely mistaken here?

Note - I'm coming from an FR car with not sticky tires, and this rotation is very easy to feel.
 

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I'll hazard that with a mid engine rear wheel drive car you'll encounter a few unexpected "feeling of rotation" moments.

A lot of times you brake ahead of a turn- the right conditions will most certainly result in the rear end starting to come around. For stock elise/exige, throttle oversteer isn't much of an condition unless your tires are all seasons, or you have some other low grip situation.

If you do encounter this situation, often times the reaction to counter unwanted oversteer is the opposite of a FR engine car. That is, you add more gas.
 

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I've tracked my car for many years, and also front engined cars, and in all cases, putting power down in a turn transfers weight backwards and induces understeer - it doesn't make it tuck in. My Focus RS tucks in on power, but that's because it's got active systems to do that, the passive response is to increase understeer. I think what you're feeling is that our cars have so much more grip than you are accustomed to, that adding that gas presses you harder into the seat before the understeer comes on, and you're not actually pushing it hard enough to change the trajectory through a turn.

The rear end coming around due to power-on oversteer is very different, it's like the back end of the car "letting go", not tucking into a turn.
 

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2006 Exige
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On power rotation is something that always puzzled me in RWD cars. I could never figure out if the rotation is caused by oversteer, with the engine overpowering the tires and causing them to slip, or is there some other mechanism at play? I'm talking about the feeling of the rear of the car pushing the car into the corner when you apply extra throttle during a turn, tightening the turning radius.
The question becomes even more interesting to me for mid engine cars with sticky tires - does the extra weight on the rear tires coupled with the traction of the tires themselves mean that this feeling of rotation won't be present? Or am I completely mistaken here?

Note - I'm coming from an FR car with not sticky tires, and this rotation is very easy to feel.
Be very careful of “tightening the turning radius”situations I usually end up spinning out if I do that.
 

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Be very careful of “tightening the turning radius”situations I usually end up spinning out if I do that.
Another term for that is "pinching off the exit" - a sure fire way to spin... I did exactly that at 95mph at Pacific Raceways three weeks ago.....

A stock Elise/Exige is difficult for me to "rotate" under trail braking because the brakes are heavily biased towards the front wheels. My Formula Ford has adjustable brake bias which makes it much easier to rotate the car at corner entry.
 

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I don't quite understand your description, and a few here have touched on different aspects. Also "which" car you are using and what speed can change all the dynamics.
Assuming a "stock" 05 Elise, on a tighter autocross course it is easy to feel what feels like understeer if you don't slow enough for the turn. If you trail brake and don't get it right, the tail will start to rotate. If you can't catch it in time with throttle, it will continue to come about. (weight transfer too late), if you managed it right, you "could" have the tail out under throttle and the front would "tuck in", but it is more of an oversteer/inertia situation. Stock Elises do not have enough to do that low in the the power band, and if done at cam changeover, you could have it power oversteer... but even then, it's an inertia/pendulum more than raw power sidestepping.

A year of autocrossing my Elise in stock class has shown me how many handling characteristics they have, at different points of the course or the powerband!
 
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I don't quite understand your description, and a few here have touched on different aspects. Also "which" car you are using and what speed can change all the dynamics.
Assuming a "stock" 05 Elise, on a tighter autocross course it is easy to feel what feels like understeer if you don't slow enough for the turn. If you trail brake and don't get it right, the tail will start to rotate. If you can't catch it in time with throttle, it will continue to come about. (weight transfer too late), if you managed it right, you "could" have the tail out under throttle and the front would "tuck in", but it is more of an oversteer/inertia situation. Stock Elises do not have enough to do that low in the the power band, and if done at cam changeover, you could have it power oversteer... but even then, it's an inertia/pendulum more than raw power sidestepping.

A year of autocrossing my Elise in stock class has shown me how many handling characteristics they have, at different points of the course or the powerband!
I think we were talking about road racing vs you referring to auto cross.We are generally on the second cam and often have modified cars and going at higher speeds.We tune out under steer and body roll that a stock Lotus has. Drifting around a turn at high speed takes more skill and risk to my car than I’m willing to take.A mistake could mean going into Armco.Also at speed if the rear comes out more than 15deg your not going to save it. I learned that from an instructor that races a Elise on a regular basis.
 
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Discussion Starter · #8 ·
It's obvious I'm very poor in explaining myself :). I'm talking more about street driving at 6/10, not track/autocross driving at the limit, so I'm not sure if considerations like power on understeer (due to weight transfer on acceleration in a turn) or trail braking are applicable. The tires are pretty far from their limits of traction.

I've tried paying more attention to the what the car is doing during the morning drive. I think that I'm talking about the very beginning of oversteer - before you need to do any correction, where the rear is just slightly stepping out. In low speed turns, if I add even more throttle it will oversteer, requiring a flick of counter steer to correct. In higher speeds (say 60 MPH) letting out a little steering angle is more than enough, even with full throttle (presumably due to the car lacking the power to fully overcome rear traction).

So I would imagine this like so, where the X axis is throttle input and the Y axis is additional car angle vs. steering angle (i.e. oversteer angle).
In the graph, until 30% throttle the car is neutral, going exactly the angle the steering wheel is pointed at. After 60% throttle the car oversteers, requiring countersteering or the car would spin.
I think I'm asking about the zone in the middle, between 30% and 60% throttle.
1292752


I'm sure that graph is gross simplification, and probably different cars/tires/road surface combinations will have wildly different graphs, but I hope that helps explain what I'm asking about.

BTW, if this imaginary graph is completely wrong, I would be more than happy to learn about that relation in real life.

For stock elise/exige, throttle oversteer isn't much of an condition unless your tires are all seasons, or you have some other low grip situation.
Low grip situations (wet or even snow/ice) are a great point - I think that in these cases the gap between "have traction" and "need countersteer" is too narrow to feel.

I think what you're feeling is that our cars have so much more grip than you are accustomed to, that adding that gas presses you harder into the seat before the understeer comes on, and you're not actually pushing it hard enough to change the trajectory through a turn.
That's actually a good point, and I think it does explain what I sense partially. I tried adding the same amount of throttle going at the same speed in a straight as I would in a turn, and it does have a somewhat similar feel, but in the turn the angle of the car definitely increases without changing the steering wheel position.

The rear end coming around due to power-on oversteer is very different, it's like the back end of the car "letting go", not tucking into a turn.
I'm not sure I agree that it's completely different. The feeling I'm trying to describe is pretty similar to lift-off-oversteer on FWD cars - you take a turn at a steady speed, then let go of the gas pedal. The rear of the car gets unloaded and you can feel the car tucking into the corner. You don't have the feeling of the back letting go or sliding, rather it's a little similar to a a sharp but smooth increase in steering lock. The main difference between this FWD scenario and what I'm talking about in a RWD car is that in the FWD car the feeling goes away in a second or so, while in a RWD it continues as long as you give it throttle. To get the rear to let go in a RWD car I'd need to add more throttle.
 

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Stock front engine cars (and indeed most street cars) generally understeer on pavement all the way to 8/10 or 9/10. Then they can get unpredictable depending on tires, suspension setup, etc. That's by design thanks to Ralph Nader convincing people that understeer is somehow safer than oversteer or 4w drift.

An Elise without the wider front rims (no LSS) is generally prone to understeer by design as well because of limited front grip. The difference is that the car has a very low polar moment of inertia (engine and driver center of mass close to each other and near center of car), has near 50/50 weight distribution, and has low-compliance suspension from the factory so it's easy to quickly shift the mass of the car from one axle to the other. When you unload an axle, you reduce its traction. Want to spin an Elise? Trail brake into a turn and then apply a big steering input while still braking. You've unloaded the rear tires by braking and it's pretty easy to exceed their grip in this condition.

The fix is easy - put some weight on the rear axle before turning through the corner, either by getting off the brake early enough to allow the car to settle, or by applying throttle to actively take weight off the front axle and put it on the rear. Throttle will induce understeer (even on my LSS car) if aggressively done.

Front engine cars have a heavy lump of metal over the front axle, so getting lots of weight transfer with braking or accelerating is harder. Ditto a true rear engine car, although braking weight transfer in combination with rear suspension jacking in a turn can have some entertaining results above 8/10 as any Corvair/old Porsche/old Fiat, etc driver can tell you.
 

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Another term for that is "pinching off the exit" - a sure fire way to spin... I did exactly that at 95mph at Pacific Raceways three weeks ago.....

A stock Elise/Exige is difficult for me to "rotate" under trail braking because the brakes are heavily biased towards the front wheels. My Formula Ford has adjustable brake bias which makes it much easier to rotate the car at corner entry.
Indeed. Which turn? I had a nasty wreck there years ago in a Spec Miata.
 

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I think this large old thread covers all points well...

 

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Stock front engine cars (and indeed most street cars) generally understeer on pavement all the way to 8/10 or 9/10. Then they can get unpredictable depending on tires, suspension setup, etc. That's by design thanks to Ralph Nader convincing people that understeer is somehow safer than oversteer or 4w drift.

An Elise without the wider front rims (no LSS) is generally prone to understeer by design as well because of limited front grip. The difference is that the car has a very low polar moment of inertia (engine and driver center of mass close to each other and near center of car), has near 50/50 weight distribution, and has low-compliance suspension from the factory so it's easy to quickly shift the mass of the car from one axle to the other. When you unload an axle, you reduce its traction. Want to spin an Elise? Trail brake into a turn and then apply a big steering input while still braking. You've unloaded the rear tires by braking and it's pretty easy to exceed their grip in this condition.

The fix is easy - put some weight on the rear axle before turning through the corner, either by getting off the brake early enough to allow the car to settle, or by applying throttle to actively take weight off the front axle and put it on the rear. Throttle will induce understeer (even on my LSS car) if aggressively done.

Front engine cars have a heavy lump of metal over the front axle, so getting lots of weight transfer with braking or accelerating is harder. Ditto a true rear engine car, although braking weight transfer in combination with rear suspension jacking in a turn can have some entertaining results above 8/10 as any Corvair/old Porsche/old Fiat, etc driver can tell you.
50/50 weight distribution? Mines closer to 40/60
 
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Discussion Starter · #15 ·
OP, are you talking about the tire traction circle?
Since I'm not taking about 10/10 driving, I don't think the traction circle is relevant, unless what I'm asking about is just plain oversteer, in which case it would be related for the rear tires.
 

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Since I'm not taking about 10/10 driving, I don't think the traction circle is relevant, unless what I'm asking about is just plain oversteer, in which case it would be related for the rear tires.
Even if you are not at the limit, the tire would compensate with more or less slip angle, depending on the weight transfer. So if you were at the limit, you would loop it around. Below the limit, the car would just rotate more, making you release some steering. I think OP might be referring to that feeling.
 

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Yeah, I almost called that out.. but let it go. IIRC its close to 37/63
You are correct,are cars have 63% rear bias.This is why they are easy to spin.
 
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Thankfully Turn 8. Lots of room. I have video, will have to make a clip.
Yes, that's a common one and I've done the same more than a few times. Trickly to add throttle there or even modulate slip angle.

Try not to spin going down the hill toward 3a, particularly if its wet. Not a good place to do that! :)
 

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Since I'm not taking about 10/10 driving, I don't think the traction circle is relevant, unless what I'm asking about is just plain oversteer, in which case it would be related for the rear tires.
I think you are talking about what many call the traction circle, it doesn't matter if at 6/10s or 10/10s.

Do you have any autoX or track experience? If not, I recommend at least an autoX school to have a safe environment to play with the car and an instructor to explain what your inputs are doing to the car. I don't think trying to induce oversteer is really safe on the road, maybe an empty parking lot.

At 6/10s on the street I don't think you're swinging the rear of the car out causing oversteer unless you're on really slick concrete roads, wet roads, bad alignment or just overpowering the rear tires grip by standing on the throttle. It's also possible you've dialed in more steering input than you think you have and as you increase the speed it feels like the car is rotating more. You don't mention what car your driving, the BRZ?
 
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