Does RWD understeer

[Petrolsmasher]

To the same extent a FWD car understeers?

Does the RWD affect understeer in any way, does it make it worse or better or no different?

Just curious really, i drive a FWD hot hatch and it understeers like a pig on damp roads with good tyres but im guessing most cars do? And in the dry it grips hard but if it understeers i can just lift off and the rear of the car will straighten my line up. Just wondering how a RWD car compares.

Obviously you can purposefully oversteer but thats a bit risky as it can let go a lot easier if your not careful.

Which is faster round a corner hypothetically? A rwd or fwd car?

[Tannedbaldhead]

RWDs can and do understeer. I've had them do it with a bit of rear end slide giving a gorgeous balanced drift and I've had terrifying moments where the front has completely wiped out.
It's a lot harder to balance understeer on the throttle in a RWD car as the rear can flick out powering on and on lifting off.

[MikeM6]

Yes, any car will understeer in the wrong conditions.

My basic understanding of this is:

A FWD car will push wide under throttle as the tyre is coping with lateral as well as forward forces. There is only finite adhesion to cover both forces.

If you lift on a FWD car and it tucks back in, it's because you are reducing the demand on the tyre to go forwards and so there is more adhesion left over to turn.

A RWD car won't be putting power through the steering wheels so can use more of that adhesion to turn.

It won't have pushed wide as soon as the FWD did, but once the adhesion is gone, it will push wide the same as a FWD car.

The other factor is the weight of the engine and it positioning. In a FWD car it is over the wheels pushing them down into the road, increasing their adhesion. As you accelerate, the weight shifts rearwards and off the driven wheels, reducing the level of adhesion.

On a RWD car, the opposite happens. Under acceleration the weight shifts onto the driven wheels.

[DoubleD]

I think most RWDs are set up for understeer as its safer.

[kambites]

Regardless of what wheels are driven, road cars are invariably set up to understeer at neutral throttle. If you apply a small amount of power, they will understeer more but the effect is more pronounced on FWD cars than RWD because the longitudinal forces being applied to the front contact patches reduce their effective lateral grip levels.

Of course if you apply a lot of power, a FWD car will understeer more and more where a RWD one will eventually be putting so much longitudinal force through its rear contact patches that they break grip and you'll get a power slide (although without a limited slip differential what you usually get in practice is the unloaded inside wheel spinning up).


As for which is faster 'round a corner, the driven wheels only make up a small aprt of the equation but all things being roughly equal the answer is that it makes no significant difference until you get to the apex but the RWD car will be able to accelerate faster out of the corner because it will be putting its power through the rear wheels where the weight is shifted under acceleration, leaving the relatively unloaded front wheels which are usually the limit on corner exit to just handle lateral loads.

Edited by kambites on Tuesday 18th February 08:17

[Alex_225]

I'm pretty sure if I shoved my S Class into a corner, in the wet hard enough it would want to go forwards not sideways haha. Partly as it weighs as much as the moon!

[Grrbang]

There are some open corners in my area which I enjoy pushing hard around in my fwds. On a damp day it doesn’t take much to start drifting wide.

When I move to rwd I worry that I won’t have as much fun, because even though they are biased towards understeer and have more grip, I will always want to leave more headroom to avoid oversteer. Has this been anyone else’s experience moving to rwd?

[kambites]

Grrbang said:

When I move to rwd I worry that I won’t have as much fun, because even though they are biased towards understeer and have more grip, I will always want to leave more headroom to avoid oversteer. Has this been anyone else’s experience moving to rwd?

You have to employ (and enjoy) a different driving style. Where FWD cars can often thrive on being thrown past their limits then recovered from the ensuing understeer, RWD cars tend to reward more with a smoother driving style, feeling the chassis work as you approach the limits of grip but never actually exceeding them. If you try and drive a RWD car, especially one with a short wheelbase, like a hot hatch it will probably spit you off the road.

This is why in terns of fun RWD isn't really "better" than FWD, it's just different.

[samoht]

Yes, RWD cars can and do easily understeer, especially on corner entry if you come in too fast. Even on power, the rearwards weight transfer can reduce grip at the front, causing some understeer.

Others have noted the advantage of RWD that the front wheels aren't taking drive loads as you accelerate out, so all their grip is available for cornering. There's also another advantage, of weight distribution. A FWD car needs to have the majority of its weight over the front wheels, in order to provide good traction for acceleration (when weight transfer is working against you), so the engine is generally mounted ahead of the front wheels to achieve this. A RWD car can have its engine set back giving 50/50 or even slightly rear-biased weight distribution. What this means is that even on corner entry when the engine isn't driving the wheels forward, the front tyres still have less work to do, reducing understeer - if you have equal tyres and equal weight at each end, both ends will tend to grip or slip together. It's worth noting that many RWD cars don't actually take advantage of this and are still nose-heavy, but those that do are worth seeking out.

[kambites]

The flip side of that is that is that (front-engined-) RWD cars tend to be heavier in absolute terms for a given usable interior space and almost invariably have a higher moment on inertia.

[Blanchimont]

Any car, regardless of it's drivetrain can understeer.
FWD is easiest when applying power, obviously.
RWD if you're slowly adding power in will push at the front, before the rear. Clog it, and it'll oversteer.
AWD tend to be more front biased so will push at the front first.

[Fastdruid]

samoht said:

A RWD car can have its engine set back giving 50/50 or even slightly rear-biased weight distribution. What this means is that even on corner entry when the engine isn't driving the wheels forward, the front tyres still have less work to do, reducing understeer - if you have equal tyres and equal weight at each end, both ends will tend to grip or slip together. It's worth noting that many RWD cars don't actually take advantage of this and are still nose-heavy, but those that do are worth seeking out.

50:50 being ideal is a myth spread by BMW marketing.

[kambites]

Fastdruid said:

50:50 being ideal is a myth spread by BMW marketing.

True but the optimal, at least to my mind, is to have the weight further back than that not further forward like most FWD cars.

[PartsMonkey]

I wrote my Sierra off by understeering into a kerb so yes RWD can do it. Especially if you're eighteen and stupid.

[AmosMoses]

Yes RWD understeers, as can any drivetrain layout.

Chassis setup is all it comes down to.

I've ran neutral RWD cars which is in my opinion the best overall setup. You can play with the car at any point of the corner including a nice slide on exit.

But there is also a lot to be said for a well setup FWD, barrelling into a corner and feeling the rear follow is good fun but on corner exit you can only get under steer.

[Fastdruid]

kambites said:

Fastdruid said:

50:50 being ideal is a myth spread by BMW marketing.

True but the optimal, at least to my mind, is to have the weight further back than that not further forward like most FWD cars.

That's optimal for a RWD car. Not having any weight over the wheels is just as much of an issue as having too much.

The optimal is about 60:40 or 40:60 depending on if it's FWD or RWD. MR (Mid engined RWD) having an a particular advantage there as well as having lower weight (and hence inertia) at the front aids turn in while the FF (Front engined FWD) has to get that hefty lump of weight moving in a different direction.

<troll>Arguably a 50:50 RWD is worse than a 60:40 FWD car...</troll>

[NGRhodes]

kambites said:

Fastdruid said:

50:50 being ideal is a myth spread by BMW marketing.

True but the optimal, at least to my mind, is to have the weight further back than that not further forward like most FWD cars.

I like to think of weight balance as the best compromise (road or race car).
Even within a single car, everything else being the same on the car, there are different optimal weight balances for braking, accelerating, corning, stability over undulating surfaces, wet, dry, comfort, performance, varying loads/passengers carried etc

[coppice]

If , like me you are old - and unfortunate enough - to have driven a Morris Marina (all rwd , of course ) you would know that oversteer was not a word in its very limited vocabulary .... Go to a club race meeting like a CSCC event and you will see first hand that rwd can understeer like a pig , fwd oversteer and 4wd both . It depends on a lot more than which wheels are driven.

That said I have driven nothing which understeered more than the Hertz Panda I drove on the Futa Pass in the Apennines . 155/80 tyres didn't help - not exactly Q3 spec ... A hoot .

[kambites]

NGRhodes said:

I like to think of weight balance as the best compromise (road or race car).

I guess it's a matter of personal preference. I think for me the optimal compromise for a RWD road car is about 45:55. I'd prefer 40:60 to 50:50, though.

[NGRhodes]

Fastdruid said:

kambites said:

Fastdruid said:

50:50 being ideal is a myth spread by BMW marketing.

True but the optimal, at least to my mind, is to have the weight further back than that not further forward like most FWD cars.

That's optimal for a RWD car. Not having any weight over the wheels is just as much of an issue as having too much.

The optimal is about 60:40 or 40:60 depending on if it's FWD or RWD. MR (Mid engined RWD) having an a particular advantage there as well as having lower weight (and hence inertia) at the front aids turn in while the FF (Front engined FWD) has to get that hefty lump of weight moving in a different direction.

<troll>Arguably a 50:50 RWD is worse than a 60:40 FWD car...</troll>

Are those optimums you claim allowing or disallowing staggered tyres (a question to provoke thought into how many factors there are to consider as to what optimum actually is/means in this topic) ?