Alrite chaps,

Just after a bit of advice really for all you track day goers. Looking for some advice on setting up the suspension on the track car, and to also highlight an issue I had the other day.

Basically, I'm still not 100% confident with the rear of the car, it can be fun but at times I dont trust it to not go wayward and try and turn me around.

The other day I was out in it late at night returning home, and going through an empty industrial estate. As I went round a roundabout (under the speed limit, about 25-30 mph) I lifted slightly to tempt the back end out to see how progressive it is. Now I will state I was running on R888's on the road, and had no heat in the rear tyres, but as soon as I lifted, it was like pulling the handbrake. Instantly turned me round 180 degrees which was a bit of a shock to be honest

It was a small roundabout, and I have noticed previously if you dont lift, just turn in and accelerate, you can feel the back end kinda slide as its dragged round, but not a problem as the front is pulling it through, therefore keeping it in check.

To be honest though, I would much rather have the rear end a lot more planted, would give me a lot more confidence.

The spec of the car is as follows:

Front-
KW v2 coilovers (unsure on spring rate, just what they came with and damping was set to softest setting)
roller bearing wishbone bushes
OMP upper strut brace

Rear-
Rebuilt rear beam
30mm ARB
23.5mm Torsion bars
Bilstein dampers (not adjustable, cant remember the model, but think they are 20% stiffer than standard)
solid rear beam mounts

The car is fitted with a rollcage, and there is basically no weight over the rear end.

I will admit that the rear dampers need to be changed for stiffer units, with the option of adjusting the damping, but has anyone else had any of these issues? Is there anything that you can suggest to get the rear end more planted?

Any help or advice will be greatly appreciated

Thanks,
Chris

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Chris
1999 China Gti6 Supercharged [Fun car] >>Project Thread<<
2000 Bianca 306 Diesel [Work runabout]

Front spring rate is very important and as you already realise your dampers will be doing next to nothing with those bars in. Was the roundabout rough?

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Would it not pay to fit 21 tb's that are very fresh and a hybrid arb as the current set up sounds to be far too stiff.

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See the sheer power and might of the lesser known burrowing owl.

Couldn't agree more. Then sell the ARB and TB's to me

You've highlighted a few things yourself, cold tyres, not the best shocks on the back etc.

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205 GTi Red - G939 RTU - Died!
306 GTi 6 Diablo - Being resurrected! Project Thread

Looking for:
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PR1.2 16"

I'd put some smaller TB in, but keep the 30mm ARB. Decent damping is obviously very important, and I would personally have it too soft than too stiff, although I think you'll find 21mm ideally suited to fast road. You can then alter the amount of roll you get depending on conditions, ie dry and fast stick a 30mm in, wet and slippy, stick the standard bar back in...

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Oulton Park in a BTCC 306 vid 1
Oulton Park in a BTCC 306 vid 2
Lap of the 'ring

Just allows the car to roll more before the tyres start to lose their grip. Body roll isn't as important in the wet as it is in the dry as corner speeds are lower so you can run a smaller ARB. Watch some of the really wet BTCC races and you'll see some of the cars seem to run no ARB at all!

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Oulton Park in a BTCC 306 vid 1
Oulton Park in a BTCC 306 vid 2
Lap of the 'ring

If your rear shocks are 20% stiffer than standard then they are B4s, not sure how soft your coilovers can be set to much even the softest setting may be mismatched.
Also, front spring rate is important as the rear is quite stiff so the car may also have unbalanced spring rates as well as damping rates.

Stiffer springs lead to higher variance in contact patch loads, which lead to decreased levels of grip. This effect is magnified on bumpier surfaces (possibly like your roundabout).
On a track car, you actually want as soft a set of springs as you can get away with (contrary to what a lot of people think), to minimise the variance in contact patch loads. However, as always it is a trade-off between minimising this, preventing the car from grounding out, having an acceptable amount of body roll as opposed to loads which would destabilise the vehicle, and in road cars, keeping the suspension geometry in alignment, as generally road car suspension isn't great, doesn't have optimum pick-up points etc. etc. whereas open-wheel single-seaters have a lot more freedom.

Damping rates should be selected one spring rates have been selected, but it is a relatively complex process compared to selecting spring rates, and optimisation goals aren't always clear (contact patch loads, body pitching under braking/acceleration, ride comfort etc.).


Anyway, the variance in contact patch load decreases the lateral acceleration provided, which according to an equation I won't show decreases the effective cornering stiffness of the tyre.
Looking at this equation, where Wf and Wr are weights of the car front and rear, g is acceleration due to gravity (9.81 metres per second squared), and Caf and Car are cornering stiffnesses front and rear:

K = ( Wf / g*Caf ) - ( Wr / g*Car )

Assuming the weight proportion remains the same front to rear, having a very low cornering stiffness at the rear due to having the rear very stiffly sprung relative to the front makes the 2nd half of the equation a larger number, which means K is a larger, negative number.

K is the understeer gradient, and a negative understeer gradient = oversteer.


The problem is probably either that the front is severely mismatched with the rear, with the rear far stiffer, combined with the fact that you have taken a car with an extremely light and fairly stiff rear end with a tendency to not hold grip at the rear over bumpy roads and have made it stiffer, thus even worse at holding grip over bumpy roads.

On a road car that you are modifying for track, unless you are completely changing the suspension design and are having it optimised on a rig or with computer simulations etc., you want to go a bi lower and a bit stiffer (springs and dampers) at BOTH ends, by the same amount!

If you or anyone else is interested, I wrote a paper at university on damper optimisation with a colleague/coursemate last year which explains the maths behind damper optimisation (and selecting springs) that I touched upon here and I'd be happy to email it to you as long as it stays with you. It isn't as comprehensive as we'd have liked as we had a page limit, sadly, and we had to cut out a lot of text which made it clearer and added other nice bits of extra information, however this does mean it is quite concise.

Apologies for the huge and probably incoherent essay.

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61k miles Ph.1 Diablo '6 Project Thread

I run the Ksport setup on my car, with 280lb front springs and relatively soft damping, 22mm tb's at the back with a hybrid arb and the ksport rear dampers set to soft.

Negative camber on the front, a few degrees of toe in and solid rear beam mounts.

Run a Quaife diff and Yokohama A048r's and it's TOTALLY planted on the road, and on bumpy circuits like the Nurburgring. If I go on a smooth circuit, I can always up the damping a bit.

For me, it's perfect.

Key I found for setting it up was:

1) Corner weighting. It's perfect 50/50 side to side and 65/35 front to rear.
2) Ride height. A lot of people run really low, which is s**t for the road. Mine isn't low at all, thus:

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306 GTi-6 Track Car - 200+ hp all N/A!
306 DTurbo Track beasty - 209.9hp - RIP

way too low indeed, needs to come up about 30mm.

that'll be on the bumpstops already

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Bring on the Trumpets.

I disagree that outright handling will improve. I've ran both and the grip is the same it's the characteristics that alter slightly. I prefer the OE mounts, they corner tighter and are much more fun.

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See the sheer power and might of the lesser known burrowing owl.

I didn't run solid mounts on my track 306 because I liked the adjustability of 90k mile old standard mounts, quite entertaining in the wet! But when you chuck it into a corner and the whole car wants to swap ends until it reaches the limit of the movement offered by the rubber mounts, there is no way in this world that is quicker or better for the tyres than having a solid rear end, even if it is more fun!

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Oulton Park in a BTCC 306 vid 1
Oulton Park in a BTCC 306 vid 2
Lap of the 'ring

Exactly, they are fun and they make the car more controllable when you're sliding about but there's no way they are quicker than solid mounts! I remember taking the double apex right before Flugplatz flat and the whole of the rear felt like it was going to tear itself off the rubber mounts, quite scary but you have to put trust in the car! Without them the rear would have just stayed in check, the downside been like I said before that you don't get that warning that the tyres are going to break away.

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Oulton Park in a BTCC 306 vid 1
Oulton Park in a BTCC 306 vid 2
Lap of the 'ring

The thing is 'feels better' and actually producing the most lateral acceleration are not always the same thing.

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61k miles Ph.1 Diablo '6 Project Thread