RE: Ford Focus RS at the 'ring: Time For Coffee
Discussion
havoc said:
I'm not a racer, but from my knowledge of car dynamics you'd achieve it mainly through:-
- Differential roll stiffness - make the rear-end more resistent to roll and as you corner harder (so transfer more weight to the outside wheels), the front outside wheel will proportionately (i.e. adjusted for static weight distribution) receive more weight increase than the rear outside wheel...which will lead to a change in the grip proportion front/rear in favour of the front wheel.
- Geometry, in particular how rear toe changes under load.*
- Playing with tyre pressures.
Also bear in mind that, once you've commenced acceleration, the weight transfer f/r has already happened (weight transfer is affected by pitch of the car, which is affected by acceleration, so the second-order differential of speed).
* Hot-Hondas typically have dynamic rear toe - the more the suspension compresses, the more rear toe-in you get, increasing stability under cornering, which SHOULD contradict my comment above, and in 95% of road driving it does - it gives you lift off oversteer if you're rough with the car - more progressive than e.g. a 306GTi-6, but still fun.
But that dynamic toe must have a limiting point, and (my theory - poke holes in it if I'm wrong) at extreme loads it's already reached a peak toe-in, so what's previously been a gradual increase in the ability of the rear to support cornering reaches a limit. Conversely the front ATB-diff actually INCREASES the turning torque as rpm increases (revs seem to increase the locking effect). So if you find a long, accelerating corner, you've got an outside rear which reaches peak supportable load e.g. mid-corner, while the front-end may be cornering harder, leading to the rear feeling a little loose.
In reality I suspect the car in those dynamic conditions is actually set-up to be very neutral rather than genuinely oversteery, so any change in outside forces (e.g. tarmac grip levels or camber) could pitch that balance one way or another, and the times I've experienced it have been due to outside forces rather than innate dynamics. But it wouldn't take MUCH change in the ARB/geometry set-up to alter that towards power-oversteer, as the BTCC boys (really do) have it.
Good post!- Differential roll stiffness - make the rear-end more resistent to roll and as you corner harder (so transfer more weight to the outside wheels), the front outside wheel will proportionately (i.e. adjusted for static weight distribution) receive more weight increase than the rear outside wheel...which will lead to a change in the grip proportion front/rear in favour of the front wheel.
- Geometry, in particular how rear toe changes under load.*
- Playing with tyre pressures.
Also bear in mind that, once you've commenced acceleration, the weight transfer f/r has already happened (weight transfer is affected by pitch of the car, which is affected by acceleration, so the second-order differential of speed).
* Hot-Hondas typically have dynamic rear toe - the more the suspension compresses, the more rear toe-in you get, increasing stability under cornering, which SHOULD contradict my comment above, and in 95% of road driving it does - it gives you lift off oversteer if you're rough with the car - more progressive than e.g. a 306GTi-6, but still fun.
But that dynamic toe must have a limiting point, and (my theory - poke holes in it if I'm wrong) at extreme loads it's already reached a peak toe-in, so what's previously been a gradual increase in the ability of the rear to support cornering reaches a limit. Conversely the front ATB-diff actually INCREASES the turning torque as rpm increases (revs seem to increase the locking effect). So if you find a long, accelerating corner, you've got an outside rear which reaches peak supportable load e.g. mid-corner, while the front-end may be cornering harder, leading to the rear feeling a little loose.
In reality I suspect the car in those dynamic conditions is actually set-up to be very neutral rather than genuinely oversteery, so any change in outside forces (e.g. tarmac grip levels or camber) could pitch that balance one way or another, and the times I've experienced it have been due to outside forces rather than innate dynamics. But it wouldn't take MUCH change in the ARB/geometry set-up to alter that towards power-oversteer, as the BTCC boys (really do) have it.
Ok, the dynamic rear toe subject is interesting, every car with a twist beam suspension tends to have increasing toe out on the outside rear tyre with increasing cornering force. Multi link rear suspensions are usually set up the other way around, but there is no reason a race engineer couldn't replicate a simple twist beam suspension characteristic. The twist beam solution is usually less stable, but can feel more yaw adjustable/agile/unstable than multi link set ups.
The grip of the front versus the grip of the rear axle is also interesting. Imagine fitting super narrow rear tyres and super wide front tyres and driving in a constant circle, with big oversteer. As soon as you attempt to accelerate the car will tend to oversteer less/understeer more. You pick up speed, and then drive at a new higher constant speed with even more oversteer. Each time you accelerate you tend to oversteer less, irrespective of the speed.
A fwd car could be set up to have a oversteering typical body slip angle on the way out of a corner, though it would probably be a pretty tricky/slow car to drive through fast direction changes, like a chicane, because it would always develop the oversteering body slip angle under lateral acceleration and changing this body slip angle from one side to the other would be slow, awkward and cumbersome.
I see fwd race cars as being very similar to fwd road cars in their setup, just with additional lift off oversteer, to get the car to turn in.
I've never been shown 1 video of a fwd car where the car tended to oversteer more with acceleration. Even with Macdonald's trays under the rear tyres a fwd car will tend to oversteer less with acceleration.
blade7 said:
Robert-lhcbq said:
Can't remember a fwd hatch ever oversteering on power.
My old series one Escort RS Turbo used to do that quite often at the limit. Kawasicki said:
I see fwd race cars as being very similar to fwd road cars in their setup, just with additional lift off oversteer, to get the car to turn in.
I was chatting to Mat Jackson* at the Silverstone Classic two or three years back, he described the set-up I've tried to communicate to you.If it's a more extreme version of my Honda set-up (do BTCC cars have diffs? As that is what gives the front end enough tenacity to force the rear unstuck in my experience - cannot see it working in a FWD car without a strong diff effect), then at lower speeds you wouldn't notice anything, but if it's more related to toe (I THINK he was talking about toe angles but can't be sure), then it could be as you describe - but then if you watch those guys race they're either on the power or on the brakes, so they're probably largely letting the back sort itself out...
* His Dad's garage serviced our Fords when we had them...
s m said:
blade7 said:
Robert-lhcbq said:
Can't remember a fwd hatch ever oversteering on power.
My old series one Escort RS Turbo used to do that quite often at the limit. blade7 said:
They did, and with the tie bar front suspension the front did stick well. The series two Escort RS Turbo was much softer.
Yes but they don't power oversteer, ie you cant provoke oversteer with the throttle......you have to back the car into a corner and control the oversteer with the throttle, which is a little different.......M-SportMatt said:
blade7 said:
They did, and with the tie bar front suspension the front did stick well. The series two Escort RS Turbo was much softer.
Yes but they don't power oversteer, ie you cant provoke oversteer with the throttle......you have to back the car into a corner and control the oversteer with the throttle, which is a little different.......Edited by blade7 on Friday 28th July 12:09
blade7 said:
There was no playing with the throttle required to provoke oversteer on my 86 RS Turbo, ultimately the rear let go before the front. Though that was at silly speeds. The Fiesta ST front sticks the same but the back end is tied down better.
Classic LSD lift off oversteer.....not power oversteer.Edited by blade7 on Friday 28th July 12:09
I ran several Mk3 escorts of various guises over the years and love them, so not being a pedant :-)
My favourite being my RS1600i, mega front end on that thing
M-SportMatt said:
Classic LSD lift off oversteer.....not power oversteer.
The power was full on, before, during and after FFS. An RS1600i not having an LSD and being 50+bhp down probably couldn't go fast enough to do it. Anyway I didn't like the characteristic because it was tricky to bring it back into line. I sold it for an 87 Cosworth after a few months. Edited by blade7 on Friday 28th July 14:29
Edited by blade7 on Friday 28th July 14:32
blade7 said:
M-SportMatt said:
Classic LSD lift off oversteer.....not power oversteer.
The power was full on, before, during and after FFS. An RS1600i not having an LSD and being 50+bhp down probably couldn't go fast enough to do it. Anyway I didn't like the characteristic because it was tricky to bring it back into line. I sold it for an 87 Cosworth after a few months. Edited by blade7 on Friday 28th July 14:29
Edited by blade7 on Friday 28th July 14:32
I wish itt was true but am also struggling with a fwd car power oversteering. I have a friend with an S2 RSTurbo and its not something we have experienced yet.
Will out on track at Ford Fair at Silverstone next weekend to find out for sure.
Will out on track at Ford Fair at Silverstone next weekend to find out for sure.
Edited by Ahbefive on Saturday 29th July 09:29
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