Does RWD understeer
Discussion
Yes, absolutely it does.
In theory RWD is far better at not understeering as the power and steering is done by different tyres. Practically however you have a number of things that get in the way, not least of these is that road cars are not setup for ultimate handling (despite what their marketing departments may say) and with very few exceptions are setup to understeer.
Both layouts will understeer if you go in too hot or there isn't enough grip. Both can tend to understeer on throttle, the difference is that with RWD will tend towards oversteer with more power while FWD will understeer more.
Practically I've found in a decent RWD you can pick which you want, balancing between mild under and oversteer just on the throttle.
A FWD in comparison can understeer less for the same low power because there is more weight and more weight gives more grip, with a decent setup it can even pull the nose in tighter under power but then with more power it understeers more and more and more. Backing off will both transfer weight as well as removing the "power" load on the tyres and they'll then grip. Weight transfer plus that sudden turn in will cause it to tend towards oversteer.
Practically I've found in a decent FWD you can pick which you want, balancing between mild under and oversteer just on the throttle.
You will find that many people *prefer* the way a RWD feels, even if it's not as good round the corners. There is an old top gear where they took a bunch of 2l cars round the track. A fwd Mondeo (Mk3) was quickest, the (more powerful) 3-series (E46) was something like 2s slower. Yet people would say the 3-series handles better even though objectively it's slower round the corners.
I will say however that on balance in this day and age that RWD layouts tend towards being better purely because the only people who have retained them tend towards the sporty end while FWD tends to be on cost and "packaging".
Personally I love both for different reasons, I wouldn't have either *just* for their layout. I'd far rather a decent FWD to a crap RWD and equally I'd rather a decent RWD to most FWD.
Petrolsmasher said:
To the same extent a FWD car understeers?
No. You have however here to distinguish between the theoretical and the practical. In theory RWD is far better at not understeering as the power and steering is done by different tyres. Practically however you have a number of things that get in the way, not least of these is that road cars are not setup for ultimate handling (despite what their marketing departments may say) and with very few exceptions are setup to understeer.
Both layouts will understeer if you go in too hot or there isn't enough grip. Both can tend to understeer on throttle, the difference is that with RWD will tend towards oversteer with more power while FWD will understeer more.
Petrolsmasher said:
Does the RWD affect understeer in any way, does it make it worse or better or no different?
If you've already got some understeer going on then an RWD with *some* power will tend to "push" and keep it understeering whereas if you were off throttle it would scrub speed plus some weight transfer and then grip. Equally applying *more* power will tend towards oversteer and tighten it up. Practically I've found in a decent RWD you can pick which you want, balancing between mild under and oversteer just on the throttle.
A FWD in comparison can understeer less for the same low power because there is more weight and more weight gives more grip, with a decent setup it can even pull the nose in tighter under power but then with more power it understeers more and more and more. Backing off will both transfer weight as well as removing the "power" load on the tyres and they'll then grip. Weight transfer plus that sudden turn in will cause it to tend towards oversteer.
Practically I've found in a decent FWD you can pick which you want, balancing between mild under and oversteer just on the throttle.
Petrolsmasher said:
Which is faster round a corner hypothetically? A rwd or fwd car?
For a theoretical pair of cars with identical weight perfectly setup then the RWD would turn in quicker and have better drive out of the corner but static state cornering would be *identical*. In short the RWD would be quicker. You will find that many people *prefer* the way a RWD feels, even if it's not as good round the corners. There is an old top gear where they took a bunch of 2l cars round the track. A fwd Mondeo (Mk3) was quickest, the (more powerful) 3-series (E46) was something like 2s slower. Yet people would say the 3-series handles better even though objectively it's slower round the corners.
I will say however that on balance in this day and age that RWD layouts tend towards being better purely because the only people who have retained them tend towards the sporty end while FWD tends to be on cost and "packaging".
Personally I love both for different reasons, I wouldn't have either *just* for their layout. I'd far rather a decent FWD to a crap RWD and equally I'd rather a decent RWD to most FWD.
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?
Plough-on understeer does make FWD cars more predictable and you could say "safer". With RWD it becomes a question of how much power you've got and how you use it. It also depends how good a limited slip differential you have, or haven't, got. (Some LSDs tend to promote understeer, some are slow, some are electronic).
In any high power RWD car it's a bad idea to just stamp on the throttle. YouTube is filled with videos of people doing exactly that - and proceeding straight to the scene of the accident when the back of their car decides to pull out and overtake the front....
Really depends on the car and the weighting, but yes, you can easilly understeer most RWD cars with an aggressive turn. Back wheels are shoving you forward, and if you turn the fronts hard, they just are not going to be able to grip forever.
Good examples are two cars I had:
Westfield Narrow 1.6. RWD - rarely, in fact, never, understeered - including competative hill climbs and track days. I'm sure I could have made it understeer with bad driving if I wanted to. Light weight, well distributed. I know it "never" understeered because I remember being quite shocked when the one below first did it which I owned after.
Westfield Seight - V8 3.4 RWD - would easily understeer if pushed over agressively into a 90 degree corner. Had it corner weighted but it's still not great - far too heavy an engine for the car. Made it very front heavy, which I guess made the front more inclined to plough on. However..... very easy to get back, or, just stab the throttle and the back comes round - but it's too late for that on public roads with no gravel traps... :-)
Good examples are two cars I had:
Westfield Narrow 1.6. RWD - rarely, in fact, never, understeered - including competative hill climbs and track days. I'm sure I could have made it understeer with bad driving if I wanted to. Light weight, well distributed. I know it "never" understeered because I remember being quite shocked when the one below first did it which I owned after.
Westfield Seight - V8 3.4 RWD - would easily understeer if pushed over agressively into a 90 degree corner. Had it corner weighted but it's still not great - far too heavy an engine for the car. Made it very front heavy, which I guess made the front more inclined to plough on. However..... very easy to get back, or, just stab the throttle and the back comes round - but it's too late for that on public roads with no gravel traps... :-)
RogerDodger said:
Really depends on the car and the weighting, but yes, you can easilly understeer most RWD cars with an aggressive turn. Back wheels are shoving you forward, and if you turn the fronts hard, they just are not going to be able to grip forever.
Good examples are two cars I had:
Westfield Narrow 1.6. RWD - rarely, in fact, never, understeered - including competative hill climbs and track days. I'm sure I could have made it understeer with bad driving if I wanted to. Light weight, well distributed. I know it "never" understeered because I remember being quite shocked when the one below first did it which I owned after.
Westfield Seight - V8 3.4 RWD - would easily understeer if pushed over agressively into a 90 degree corner. Had it corner weighted but it's still not great - far too heavy an engine for the car. Made it very front heavy, which I guess made the front more inclined to plough on. However..... very easy to get back, or, just stab the throttle and the back comes round - but it's too late for that on public roads with no gravel traps... :-)
You need to increase the weight towards the rear to balance the engine out, as you point out currently the weight is all at the front. Good examples are two cars I had:
Westfield Narrow 1.6. RWD - rarely, in fact, never, understeered - including competative hill climbs and track days. I'm sure I could have made it understeer with bad driving if I wanted to. Light weight, well distributed. I know it "never" understeered because I remember being quite shocked when the one below first did it which I owned after.
Westfield Seight - V8 3.4 RWD - would easily understeer if pushed over agressively into a 90 degree corner. Had it corner weighted but it's still not great - far too heavy an engine for the car. Made it very front heavy, which I guess made the front more inclined to plough on. However..... very easy to get back, or, just stab the throttle and the back comes round - but it's too late for that on public roads with no gravel traps... :-)
Looking closely at the Westfield chassis layout I suspect the most enjoyable way to do this is to eat lots of pies and drink more beer.
Fastdruid said:
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. I think 50:50 for a ROAD car is optimum. Road cars are NOT about absolute performance but about having the widest operating zone where the car remains linear in its response, and having mass split evenly front to back achieves exactly that......
On track for absolute max performance then yes, a more rewards distribution is going to be faster, but it will also be harder to drive (ie have a narrower window where that abs max performance is accessable)
kambites said:
Max_Torque said:
I think ...
That's the key bit. It is purely personal preference. If you sell Formula 1 cars to world class drivers, sure, 50:50 is not "best", but BMW don't....
Max_Torque said:
kambites said:
Max_Torque said:
I think ...
That's the key bit. It is purely personal preference. If you sell Formula 1 cars to world class drivers, sure, 50:50 is not "best", but BMW don't....
Desiderata said:
Anyone ever driven an eighties Skoda Estelle? All the handling disadvantages of an early 911 without any of the power to help you play with it.
I loved mine.
Yes and I had one for 2 years as my first car...good memories for sure. I loved mine.
It also worked well with the ladies in the winter...by keeping them nice and warm with the engine being behind the backseat!
The steering was crazy light though and the gearbox was an absolute pig to use...apart from that though I loved it and I'm pleased I had one looking back now. That was 30 years ago this year...where does the time go!
I also think a 50:50 weight distribution is a very good starting point.
Tail heavy cars need a lot of vehicle dynamic tricks to make them predictable to drive. Front heavy cars have naturally more understeer...but when they do let go at the back they need counterintuitive driver actions to stabilise quickly...
Tail heavy cars need a lot of vehicle dynamic tricks to make them predictable to drive. Front heavy cars have naturally more understeer...but when they do let go at the back they need counterintuitive driver actions to stabilise quickly...
I bought my 1st car a few months after I was 17. It was a MK2 Cortina so RWD, and it taught me about understeer on a wet road with 155 x 13 tyres, as well as oversteer when I got too exuberant leaving a damp roundabout!
I've had at least 30 cars since then, mostly RWD, and they were pretty much all set up for a degree of understeer - even including the 8 BMWs I've had in the last 15 years.
But at least with RWD you have a decent chance of overcoming the understeer with your right foot!
I've had at least 30 cars since then, mostly RWD, and they were pretty much all set up for a degree of understeer - even including the 8 BMWs I've had in the last 15 years.
But at least with RWD you have a decent chance of overcoming the understeer with your right foot!
Max_Torque said:
indeed. But i'm talking about optimising a car to suit it's market and users. And here, on average, BMW are correct to say 50:50 is "best"
I don't see how you can come to that (or any other) conclusion on the matter, really. To be honest if you put a load of PHers in otherwise well set up cars with 55:45, 50:50 and 45:55 weight distributions I doubt more than one in twenty of them could tell you which was which. I think moment of inertia is far easier to feel that static weight distribution. I guess one approach which would yield interesting results would be to see what weight distribution yeilds the most even tyre wear uner "enthusiastic road driving" type conditions. Rather difficult to test though.
You're probably right that if you could come up with a way to measure it, the average personal preference isn't far off 50:50 though. But then most people buy silver or grey cars, so most people have poor personal preference.
Edited by kambites on Wednesday 19th February 08:03
Gassing Station | General Gassing | Top of Page | What's New | My Stuff