Porsche 924 Rally Car set-up advice

[rallyimp]

Hi all
As this is my first post I guess I should start with a short intro
I've got a degree in Automotive engineering, and have worked for a WRC rally team followed by a tyre company in their motorsport and then aircraft departments.
I'm into rallying having previously played with rear engined cars before moving onto FWD stuff.
I've now moved onto a Porsche 924, Website

https://sites.google.com/site/valeracing09/home/porsche-924/motorsport

, but am struggling with a bit of under-steer and was hoping you might be able to help me out, in understanding the best way to tackle fixing it as I'm struggling to know what is the best option from spring rate or ARB to dial out the under-steer for rallying and Autotests (Auto-x for the Americans).
I had a good read through these forum posts, but I'm still not sure I'm any wiser
Forum thread ARB’s

http://www.eng-tips.com/viewthread.cfm?qid=414221

Forum Thread MR2

http://www.eng-tips.com/viewthread.cfm?qid=392647

I'll give you a few basic details of the suspension set up and hopefully you can point me in the right direction
Weight distribution Front 49% / Rear 51% this is STD from Porsche and confirmed on a friends car (yet to weigh mine)
Spring rates 140lbs front through McPherson Strut and 126lbs wheel rate rear from rear torsion bars
I've found this table of options useful

http://www.clarks-garage.com/shop-manual/susp-15.htm

my car is a 1981 with out the options
ARB's I've removed the 21mm front which has helped slightly to reduce the under-steer, I've also bought a 14mm rear, which is next to be tested

I understand that I need to make the front softer or the rear harder to adjust the balance, and I feel the car is currently a little too soft, but my question to you is, is it better to do it with springs or ARB's?
Will stiffening the rear springs to match the weight distribution sacrifice grip such that i'm better using a rear ARB?

Thanks
Chris

 

[GregLocock]

With a RWD non aero car the conventional wisdom is to soften the rear springs as much as possible and remove the R ARB, to improve traction, and then to dial up whatever roll stiffness you need at the front, using the front wheels to supply a stable platform for the rear axle.

Therefore you seem to be taking an interesting course.

Last time I counted there are at least 7 different forms of understeer, I wonder which one you don't like?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[rallyimp]

Its mostly low speed tight turns on the Autotests, and over 90° bends on gravel or dusty tarmac
the car is very stable at high speed, and initial turn in is ok, but the car starts to push from the mid turn, wont hold a tight line and wants to run very wide as you try to exit the turn and apply power
I've spent a season getting out of my FWD habit of rushing into turns and using either lift off over steer or the hand brake to get round.
But I still feel the car has been made overly safe for road use by designing in understeer which I want to dial out to make it quicker for competition use

 

[GregLocock]

Yes that's a very vw trait. I don't think springs and arbs are the best way to cure throttle on understeer, given you are happy with the first half of the corner. I'd have a look at the tuning chart in Milliken. Do you have an LSD? Traction control?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[gruntguru]

Surely the understeer characteristics are different on gravel vs tarmac? Even dusty tarmac.

Also your driving style should be different for those two surfaces - more throttle earlier when on gravel. I am not an expert but I know that controlling a RWD through gravel turns is all about throttle control - when you get it right the steering wheel isn't doing much.

1. No throttle = oversteer.
2. A bit more throttle = understeer.
3. Still more throttle = just right. The car rotates through the turn with the steering close to straight.
4. Even more = oversteer.

If you can only get 1 and 2 or only 2, you are in the wrong gear - go down a gear to get closer to the power band.

je suis charlie

 

[rallyimp]

gruntguru as much fun as power oversteer may be, its not always the fastest option, and with 125bhp and an open diff not always possible, so I want to be able to drive the car as well.

Greg, I cant find my copy of Milliken at the moment, Not sure I've used it since I graduated, so will have to have a route round in the loft. What do you suggest as a better way to correct on throttle under-steer if not springs or ARB's

Cheers
Chris

 

[gruntguru]

For a RWD car on gravel, power oversteer is always the fastest option. (Gravel/dirt only)

125 bhp is plenty. but the open diff will make it more difficult.

je suis charlie

 

[cibachrome]

It's possible your Ackermann is unfavorable at large turn angles. You probably can't alter the mechanism too much, so toe is your only crutch. In or out will depend on the tire construction's characteristics, especiially if your reserve is very high.

Alternatively, intermix of front and rear tire sizes will help, but if that's not possible, then mixing brands of the same size is always a good possibility. Make sure you swap the pairs front and rear during each evaluation. If one config is worse, then the swap will be better.

 

[GregLocock]

The tuning matrix said.... not much directly.

So, LSD should help. Narrower rear tires. Higher rear RCH. Toe out in traction. Change rear tire pressures. Change driving style - higher apex speed, delay acceleration (yeah, right).

As ever you should consider instrumenting your car rather than just buying bits for it. For instance, DIY Correvit

https://pixhawk.org/modules/px4flow

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[FennLane]

Why do you think understeer is slow on gravel - If you drive an car with understeer in the correct manner it can still be effective.

Look at the old videos of Blomquist in a SAAB 96 V4.

If you have been using an Imp for gravel rallies then you should know how to avoid issues with making the car turn in and many of the problems can be cured with setting the front to rear ride heights.

As the 924 has an almost 50/50 weight distribution you will need to take positive action to make it turn in and spring rates won't really do much to change the balance.

If the car is too soft then it is just too soft.

You need to consider the wheel rates compared to the overall weight of the vehicle.

My last Imp rally car won its class on the BHRC in 2005 and ran standard front spring rates and uprated rear rates with dampers set to suit. we used top win the 1 litre class by around 10 minutes on a typical gravel event and on Epynt our best result was a class win of more than 25 minutes.

When we switched to a 911 we used standard front bars and uprated the rear. This car won the BHRC outright with a 3.0 litre engine.

I would consider looking at some slightly stiffer rear bars and you should be able to find some 23.5mm diameter which is about a 30% upgrade.

You also should think about disconnecting the rear sway bar on gravel.

I don't know what you are doing about dampers but you will need increase rebound and probably reduce bump settings all round.

Depending on the weight of the sump guard you my need to uprate front springs but possibly only up to around 175lb/in

If you can set the front and rear ride heights independently I would try it with it slightly nose down - say 1.5 - 2 degrees.

 

[rallyimp]

Hi FennLane
I'd take a guess at that being Chris F rather than Jeff,
I remember yours and Johns Rally Imp, I thought about buying the shell off you, but was already too far committed to rebuilding mine at the time

I've mostly been doing Historic Road Rallies and its the tight turns round cones 180° where the car suffers, so I've been using autotests to tune it,
I've just done an event this weekend with F&R ARBs fitted and although it removed roll, it did not feel as good. More understeer into snap oversteer compared to running without

Having also rallied the Imp on STD 195LBS front springs and 600LBS rears (can't remember the wheel rate conversion) I cant get my head round the 924 which runs 140LBS front and 126LBS effective (23.5mm rear bars). I think I'm heading for 175Lbs both ends to match the weight distribution. I had the car setup, rear high when I first got it and it seemed worse than the current level setup.

I only live just down the road from you so might drop round and pick your brains next week if you don't mind



Cheers
Chris

 

[gruntguru]

"Look at the old videos of Blomquist in a SAAB 96 V4."

Rear wheel drive???

je suis charlie

 

[rallyimp]

Saab is FWD with lots of left foot braking
As for Stig if only I had that much talent

Cheers
Chris

 

[gruntguru]

Understeer on gravel is slow - unless you are driving a FWD or possibly AWD. This topic is about a RWD car.

je suis charlie