Suspension / drivetrain question

[Jez_h-dev]

I run a saloon car, front engined rear wheel drive, in the Time Attack series. I have a problem with medium speed tight corners. The inside rear wheel lifts off the ground and due to torsen diff I current have, I end up with no drive on the corner exit until the wheel regains contact with the ground. Annoying.

Otherwise the car turns in and handles very well at the moment.

My question is: would it be better to change the suspension setup to try to keep the inside rear wheel on the ground or should I change the diff to a plate type, so if the wheel does leave the ground I still have drive out of the corner.

I'm not sure what other information would be relevant, so if you can help and require more info, let me know

[ss_collins]

Sounds a good subject for Mark Ortiz, mind if I forward on to him?

You got any more details and perhaps some pics of the machine?

[Jez_h-dev]

Feel free to forward it on. Who is Mark Ortiz? Excuse my ignorance.

Heres a picture of the car without some of the aero bits on (splitter and rear wing):

We just rebuilt the engine and had it on the dyno last week. Its a Nissan SR20DET engine - an inline four turbo charged engine. Managed to get 471bhp from it at 1.4 bar boost (with 7k rpm rev limit), which isn't too bad.

The car weighs 1108kg inc fluids ready to run, excluding driver. We have to run Toyo R888 tyres (a road legal semi slick). Suspension is MacPherson strut at the front and multi link rear. Current spring rates at 8kg/mm front and 6kg/mm rear, with front and rear anti roll bars (I don't know the rate of them, although they do have leverage adjustment)

[coln72]

Is welding the diff legal? Cheap but nasty way out.

[1200Datto27]

Have you tried softening or even disconnecting the anti roll bars and seen if it makes a difference? The rear one may be to stiff for the weight of the car.

[Al Weyman]

Maybe as the last poster said too stiff a rear bar but maybe not stiff enough on the front. Are they a solid axle on those? Also have you checked to see if you have enough travel in your shockers, I had this once on my IROC-Z, the guys who helped me build it modified a couple of adjustable shocks for the rear, only trouble was they did'nt extend enough causing the rear wheel to left and go into oversteer. Another thing I found on the IROC and I am talking live axles here, was when we first built it we used non complient (i.e. no give) poly bushes and I think it was getting into a bind situation and would also tend to not allow the suspension to droop as it should. Would have been better off with rubber bushes if thats what the rules require.

BTW 471bhp from a four cylinder not too bad! Thats some serious bhp there, I dont get that from 5.8 litres.

[Jez_h-dev]

I've not done any experimentation yet. We have got a trackday booked on the 8th Aug, so we'll try a few things then.

The anti roll bar rate is low comapred to the rate of the springs. We will try disconnecting the rear ARB and see if that helps.

The rear coilovers have enough travel, the problem being that there is little compression of the rear springs due to the cars weight - I can't reduce the pre-load on the spring any further as the spring would come loose when the wheel lifts.

[tristancliffe]

Shouldn't a TORSEN diff provide torque to the wheel that can take the most - i.e. the one on the road - and therefore still give you some accelerative effort? It won't be as good as with two wheels, so curing the wheel lift is more important, but you shouldn't end up with no drive at all unless your diff is knackered.

[tristancliffe]

Shouldn't a TORSEN diff provide torque to the wheel that can take the most - i.e. the one on the road - and therefore still give you some accelerative effort? It won't be as good as with two wheels, so curing the wheel lift is more important, but you shouldn't end up with no drive at all unless your diff is knackered.

Edit: Sounds a bit like your rear springs are too stiff if the car is compressing them. Perhaps softer springs with more preload the maintain ride height would be a better option, so that in roll there is enough droop travel to keep both wheels on the track.

[1200Datto27]

Tristan, Torsen's need a load being applied to both sides of the diff to work, if they have no load through one side, they just go into an open state until load comes back on them.

Jez, I also spoke to a couple of mates who run Silvia's like your's, and they all think that your main issue is that the spring rates are way to high for the weight of the car. They believe that you should be looking at max rates of 6Kg/mm front and 4Kg/mm rears.

[tristancliffe]

Originally Posted by 1200Datto27

Tristan, Torsen's need a load being applied to both sides of the diff to work, if they have no load through one side, they just go into an open state until load comes back on them.

Oh, I see. Learn something new every day At least I redeemed myself with sensible suggestions on spring rates.

[Jez_h-dev]

Originally Posted by 1200Datto27

Jez, I also spoke to a couple of mates who run Silvia's like your's, and they all think that your main issue is that the spring rates are way to high for the weight of the car. They believe that you should be looking at max rates of 6Kg/mm front and 4Kg/mm rears.

I think you're right - although I don't want to go too soft and increase body roll as the aero etc will scrub the ground. I'm going to try things in this order:
Remove the rear ARB
Try softer rear springs
Try softer rear and front springs

If I need to raise the ride height to avoid scrubbing then I'll have to do it I guess.

[Al Weyman]

How would softer front springs help?

[Jez_h-dev]

Not sure - maybe to balance up softer rears??

[Al Weyman]

Originally Posted by Jez_h-dev

Not sure - maybe to balance up softer rears??

I would have thought more likely to allow the front to dive exasperating the problem. Would it be worth trying running the rear of the car a little lower?

[tristancliffe]

I would try softening front and rear springs together THEN change the rear anti-roll bar. If your rear bar is too strong now, reducing the ride spring rate will also reduce their roll stiffness contribution, so you'll automatically be closer to where you want to be.

I think the problem isn't "too stiff in roll" more than "insufficient droop travel" because your springs are too stiff.

[Jez_h-dev]

I'll try some different configurations and see how I get on.

BTW - Tristan, I see you're in the Monoposto club - one of my friends used to race in that, a chap called Tom Brown - long shot, but maybe you know him?

[tristancliffe]

I certainly do. He was racing a Formula Ireland last year, and we had some close races until he dropped a valve at Silverstone. This year he is in the ex-Russ Giles FVL, and isn't fully up to speed in it just yet (but only had a few outings). Nice guy, very friendly, and very approachable. Fair on the track too.

[Jez_h-dev]

Small world isn't it. He's a good guy. My workshop is part of the farm he owns. We were at Combe last weekend in a Jade he's just bought - it was the cars first time out in years, went very well.

Anway, just ordered some softer springs for the Nissan

[phoenix]

Lifting an inside wheel at either end of the car is generally down to the opposite end of the car being too soft in roll - or the end of the car where the wheel is lifting being too stiff in roll.

If it were my car I would first disconnect/remove the rear anti-roll bar. If that didn't cure the problem then it would be a case of either softening the rear springs (which should also give more droop travel) or stiffening the front - by either changing to stiffer springs or adding a stiffer anti roll bar.

Any of these options will change the balance of the car. In theory you will adding understeer by softening the rear or stiffening the front, because the front will end up carrying more cornering load, particularly the outside front tyre.

[Supercar]

Jez, you were less confused before you asked this question, weren't you?

The car handles well, but it spins the inside rear. What do you do? Well, let's see why it does that...

1) Torsen does not work with one wheel in the air.
2) The wheel lifts because of a too high CG.
3) Too much font weight distribution.
4) Too high rear roll stiffness / too low front roll stiffness.
5) Too grippy tires that allow a lot of weight transfer.

Well, assuming you do not want to make your car slower by installing street tires on it, you can change items 1, 2, 3, and 4.

If you do not want to change the balance, then do not change you springs and bars. Change items 1,2, or 3.

I assume the car is already as low as it can be. Then you have only items 1 and 3 to play with.

Item #3: Have you already moved everything you can into the trunk, like the battery?

Item #1: You have an option to install a plate-type diff, or weld the stock diff.

I am actually in the same situation with my AWD car. I have a clutch-type diff ready to go it. But I will first weld the diff without removing it to see how the car feels.

[Al Weyman]

Wont a welded diff tend to make the car nose on? I have often thought about running a locked spool instead of a lsd but have always bottled out but was interested to note with the NASCAR derived IROC Camaro I have in my garage that they run an LSD on the ovals and a locked spool on the road courses which I would have thought would be the other way around but with tire stagger etc on the ovals they need an LSD yet when I used to do 1/4 mile ovals the done thing was to lock your diff up which may not have been the right way to go. Has anyone here had any experience running a locked diff on the road race circuits?

[Notso Swift]

Yes it will, but Lightweight and lots of power will help fix it!
You just need commitment.
Oversteer in (trailing throttle) and understeer out

[Al Weyman]

The nascars weigh in at 3800lbs I believe so lightweight is not on their criteria, plenty of grunt and torque though. Maybe I will look out for a spare axle and build one up with a spool and give it a try, not sure about welding though, sounds a bit mickey mouse.

[AU N EGL]

I have seen a few FWD cars do the lift of the inside rear wheel through corners.

Turned out the cars were did not have a good roll cage in to stiffent the chassie up, the cars were not corner balanced and the front springs were way too soft.

and these drivers were wondering why the out side front tires were waring faster then the other tires ?