Tuning out understeer without loosing grip

[Tam Lin]

I've got a low-cost car which is both transport and track-toy, a 924s.

Its purpose in life is to teach me about setup. To this end, it has dampers adjustable for bump/rebound, coil-over suspension which is height adjustable, adjustable ARBs, and it has camber plates so I can try out various degrees of negative camber. The wheel/tyre combination is optimised for minimum unsprung weight and balance, rather than out and out grip (195/55x15 on 5.5 forged rims front, 205/55x16 on 6 forged rims rear). Weight distribution is 52/48 F/R.

Now my question..I am trying to get better turn-in to medium/high speed corners, and generally reduce traces of understeer. I don't want to change the wheels/tyres.

What is the correct way of tuning the suspension (i.e. achieves better turn-in/less understeer without loosing grip)?
1) Tyre pressure variation F/R
2) ARB variation F/R
3) Damper stiffness variation F/R

Edited by Tam Lin on Friday 17th August 22:57

[ELAN+2]

If you've got the adjustment, increase the front camber slightly, and possibly the toe in

Not sure about the damper settings though

[GreenV8S]

Are you trying to change the overall steady-state balance from understeer towards oversteer, or just avoid transient understeer in this one situation? That's to say, are you trying to make the car more tail happy generally?

[grahambell]

I'd suggest adjusting the dampers to either make it softer on the front or harder on the rear (or maybe bit of both) and see how that feels.

[Tam Lin]

GreenV8S said:

Are you trying to change the overall steady-state balance from understeer towards oversteer, or just avoid transient understeer in this one situation? That's to say, are you trying to make the car more tail happy generally?

Having thought about it: Both
1) I haven't got enough power to get in trouble (160 bhp/155ftlb per ton), so slight oversteer might be more fun
2) I *hate* with a vengeance not being able to turn in cleanly, even when I get the braking and gear changing correct (for once). The steering is pretty low geared, and having to throw in an armful is just uncool

What I don't want is that fake (well, slow) rolly-poly oversteer you can achieve easily by low rear tyre pressures, or a similar feel, i.e. I want both ends to grip, but ultimately the rear less so.

[Tam Lin]

grahambell said:

I'd suggest adjusting the dampers to either make it softer on the front or harder on the rear (or maybe bit of both) and see how that feels.

Thanks, why dampers over the other two options? (Not being difficult, just trying to learn)

Edited by Tam Lin on Friday 17th August 22:54

[Tam Lin]

ELAN+2 said:

If you've got the adjustment, increase the front camber slightly, and possibly the toe in

Not sure about the damper settings though

Thanks, I'll try the camber first (just because it's easier to adjust on my setup). At -2Deg front, I havent seen any unequal wear, so I might try 1/2 deg more.

[GreenV8S]

I would say the first things to check are the tyre pressures and the camber on the outside wheels. Ideally both outside wheels will be nearly vertical under heavy cornering. This won't necessary cure the problems during turn-in, but it may help. Tyre temperatures are the way to see whether the pressures are right, and they will also give you an indication of the camber, but if you can get any action photos of the car cornering hard this will give you a much clearer, um, picture, of what's happening.

Once those are about right look at toe-in and roll stiffness. For steady state cornering you're mainly interested in roll springing. For transient response roll damping is also significant. Basically the more roll stiffness you have at one end of the car, the more of the weight transfer will happen there and the less grip it will generate as a result. Less front toe-in (or even slight toe-out) will tend to make the car more pointy but less stable. More toe-in at the rear will tend to reduce power oversteer. Tyre pressures are not really the way to tune the handling, you want them right. You can get minor handling changes with the dampers but these will also have an optimum setting to suit the springs and mass of the car and moving away from there will compromise other aspects of the car setup. Roll stiffness front/rear, camber and toe-in are your main tools here. I'd also suggest you buy two or three books on vehicle dynamics and handling. Fred Puhn's book "how to make your car handle" is a little dated now but is still an excellent primer.

[grahambell]

Tam Lin said:

grahambell said:

I'd suggest adjusting the dampers to either make it softer on the front or harder on the rear (or maybe bit of both) and see how that feels.

Thanks, why dampers over the other two options? (Not being difficult, just trying to learn)

Edited by Tam Lin on Friday 17th August 22:54

Simply because they - along with tyre pressures - are the quickest and easiest things to adjust.

Might not do the job if your spring and anti-roll bar rates are a mile out, but if they're about right then tweaking damper settings and tyre pressures can provide enough fine tuning to adjust the handlng balance as you want.

[Tam Lin]

Thanks all for the hints where to start.

Got the Puhn book about a week ago following a thread on here, now need a less mickey-mouse pyrometer (the one I have seems to produce pretty random numbers). And lots of track time.

Edited by Tam Lin on Saturday 18th August 12:32

[NJH]

Tam Lin said:

Thanks all for the hints where to start.

Got the Puhn book about a week ago following a thread on here, now need a less mickey-mouse pyrometer (the one I have seems to produce pretty random numbers). And lots of track time.

Edited by Tam Lin on Saturday 18th August 12:32

I have been doing my basic setup and tuning this year on a very similar car, and have recently reached a sort of nirvana, although I would like a LSD with a high deceleration locking factor and various brake mods but that is a different story (car oversteers trail braking more then I would like). I have the Allan Staniforth book on race car suspension which is proving a good read.

If you have fresh tyres then one can measure the wear across the tyre after their first track day. I did this with my 944 S2 and realised that I needed loads more camber, I had 1 degree on one wheel and 1.5 degrees -ve on the other. I now have about 2.25 degrees -ve on both front wheels and the front grips much much better.

If you also monitor how much the tyre pressures increase during each session it also gives a nice indication of how evenly or not the 4 corners are sharing the work. I tend to see the same increase front to rear, same increase across the 2 rear tyres and usually slightly less increase on the inside front tyre i.e. a bit less then the other 3.

Myself and others that I have talked to with these cars like a lot more caster then the standard settings. Some ppl don't agree so much with this but I have previously set the caster eccentric bolt on the 944 to the position that gives as much caster as possible, however I didn't like the feeling of falling into the corner so I backed it of just a touch.

Some of the things I have done on my car are a bit unusual compared to what most guys have done. For example I started by changing the track widths because the 944 S2 has a trait of wanting to push on under throttle i.e. gently understeer, this scrubs the front tyres heavilly under hard cornering with the bizarre result that I was seeing heavier tyre wear on the front. OK so you can play with the rear toe setting but I like plenty of rear toe in such that the car remains stable when coming on the throttle. I wanted the car to naturally and gently follow the bend under throttle so I run 9mm extra of spacer on the rear axle compared to the front (6mm front spacers 15mm rear spacers). Now even with the rear squatting right to the floor I don't get understeer out of bends but unfortunately this made the car more of an oversteerer in the low speed corners especially.

To dial out the oversteer I run a 968 M030 front ARB and use a high rebound setting on the shocks to keep the front down in the turns. This has the dual effect of getting a more neutral balance whilst holding the front down at the point where the rear is starting to squat a lot under throttle.

Does it all work?

Well if you keep an eye on the PCGB forum I started a thread about how I broke 2 Porsche alloy wheel nuts on the Haynes test track last week on the front outside wheel! The one that didn't fly off is heavilly distorted and has the top 2 threads torn off! For safety sake I will most likely run steel nuts as per the PCGB race rules in the future. I don't want another scare like that.

I also run spring rates that are effectively 10% higher wheel rate on the rear compared to the front (190 lbish/inch rising rate H&R spring on the front, 225 lb/inch torsion bars on the rear(27mm)). Again this is against what a lot of ppl seem to suggest who go for even spring rates or more on the front but AFAIK Porsche themselves ran much harder springing on the rear of the 968 CS and rising rate springs on the front, something like 175 lb front and 240 lb rear. I also have the 968 M030 rear ARB but I always have it on the lowest setting.

Lastly I aim for a totally level ride height front to rear but this sometimes looks like the rear is lower when it isn't i.e. the sill is parrallel with the ground is my definition of level on these cars.
Unfortunately I don't have height adjustable spring platforms on the front so each corner is at a slightly different height to try and keep it even i.e. you adjust the rear adjusters to counter the effect of the heavilly canted over engine in the front causing the front RHS to be lower.

Contrary to most ppl my 944 S2 with this setup more or less never understeers into corners unless of course you are mad and keep youre foot planted. If I aim it at a bend to fast the car gently oversteers and if way to fast spins round in the middle of the track.

[Tam Lin]

NJH said:

Lots of excellent stuff

Many thanks for the food-for-thought.

I must explore the caster a bit more, first in theory & then in practice.

I'll also try a quarter or half a degree more negative camber on the front from the current 2.0.

Interesting that I've arrived at the same conclusion with the wider rear track, but by a different route. Originally, I ran 4x195/65 15 all round on the road, and 4x205/55 16 on D90s on circuit. However, I found that replacing the rears with Fuchs 16s reduced understeer ever so slightly, presumably due to the offset being ET 36 rather than ET 55.

Thanks for the hint about softer front springs too, I was going to have to look at the whole spring rate/damper thing anyway, as I'm replacing the bonnet with GF & the sunroof with a sheet of CF as the next mod.

After that, it may well be 3.0L engine time...

BTW, what do you use between suspension and body, Powerflex bushes, solid, or standard?

Edited by Tam Lin on Tuesday 21st August 14:12

[NJH]

Tam Lin said:

NJH said:

Lots of excellent stuff

Many thanks for the food-for-thought.

I must explore the caster a bit more, first in theory & then in practice.

I'll also try a quarter or half a degree more negative camber on the front from the current 2.0.

Interesting that I've arrived at the same conclusion with the wider rear track, but by a different route. Originally, I ran 4x195/65 15 all round on the road, and 4x205/55 16 on D90s on circuit. However, I found that replacing the rears with Fuchs 16s reduced understeer ever so slightly, presumably due to the offset being ET 36 rather than ET 55.

Thanks for the hint about softer front springs too, I was going to have to look at the whole spring rate/damper thing anyway, as I'm replacing the bonnet with GF & the sunroof with a sheet of CF as the next mod.

After that, it may well be 3.0L engine time...

BTW, what do you use between suspension and body, Powerflex bushes, solid, or standard?

Edited by Tam Lin on Tuesday 21st August 14:12

Yes it amazes me how ppl spend so much time fiddling with springs and dampers yet ignore what can be done by altering the front and rear track widths relative to each other.

Softer front springs on these cars IMHO work really well with thicker front ARB, and more rebound on the front dampers to keep the front down when it rolls. The reason why this is IMHO key on the 924/944 series cars is because the rear end squats like mad. You could fit 400 Lb/inch rear springs and on the circuit the rear will still squat like mad (there is a pic of Andrew S on the PCGB forum running slicks which shows just how much squat). I made the decision about 2 years ago to just accept it and find ways round it such as trying to get the front to 'squat' down mid and after mid corner with the front ARB and high damper rebound.

For bushes I have KLA solid top mounts, 968 caster blocks and Bert Gears delrin type wishdone bushes all on the front and Weltmeister delrin type torsion bar end cap bushes on the back. The Weltmeister bushes bind and squek like mad so I will be taking them out at some point and putting in Elephant racing phosphur bronze weather sealed bushes (far to much money but look like a superb product). I already have the bannana arm bushes waiting to go on.

[nsa]

Tam, if you manage to find a carbon fibre bonnet for the 924, could you let me know please? I want to replace my heavy steel bonnet with either aluminium or carbon fibre.

[Tam Lin]

nsa said:

Tam, if you manage to find a carbon fibre bonnet for the 924, could you let me know please? I want to replace my heavy steel bonnet with either aluminium or carbon fibre.

Hi Nick,

Simon is your man. He produces both GF and CF bonnets. I went for the glass fibre, as in the end the difference in price was about £200 per Kilo between Carbon and Glass. The good news is that he supplies "street" bonnets in both materials, i.e. reinforced with mountings, that do not require bonnet pins to attach/stabilise.

Simon Davidson
Essential styling ltd
PH: 01296 66 8888
www.essentialstyling.com

[fergus]

To set camber accurately, you NEED a tyre pyrometer. This is the only way you can tell how much of the tyre is being worked, and how hard. Get a probe typ, not an infrared one as these are unreliable in my experience.

Corner weighting the car may also help the transition when turning in.

As a good handy reference guide, I'd recommend Carrol Smith's 'Engineer in your pocket' series on setup.