Understeering car.....

[Julian Thompson]

Somebody once told me that once I'd cured the idle problem on my car the next thing i'd have is the "dreaded SLR understeer" and went on to explain how he'd cured it. Sadly, I took little notice because I was too busy with the idle and hadn't had any chance to really drive the thing....

 

Well,

 

Could anyone help with the best remedy for the following symptoms: Into a fast corner, brake/change down - even trail the brake if the speed is high - and turn in. And some more lock, and (shhhhhiiiiitttttt) some more lock and I hope backing off tucks the bloody nose in - and it does. But it feels awful and looks worse! The tail doesn't wag when I back off the throttle, it just snaps the nose back on track.

 

The car is running the Bilstein shocks and R500 wheels with CR500's. It has Watts link and roll bars F+R.

 

 

[murph7355]

Flat floor it then tweak the rear ride height to suit would be my first port of call. PC documented a flat floor technique on here some time back.

 

Once you have the floor to a suitable height, raising the rear will induce oversteer, lowering it understeer. More or less (good start point is probably 10-15mm of ride height difference between front and rear).

 

If you can't get it balanced from there (without it looking like a top fuel dragster) then you'll need to start think about ARBs, springs etc. But it should be possible to get it nicely set up.

 

Tyre pressures can also help but mine seems OK with evens all round (I run about 16psi on the road, often less on hot track days).

[R500 CYA]

Agree with Andy as the best place to start.

 

Check your tyre pressures, are the fronts getting warm enough? Maybe they are too hard?

 

If you increase your rear ARB you will put more force on the front of the car, that should help.

 

However before you start experimenting make sure you have good foundations. Get the car flat floored, the ride height sorted and go from there.

 

Nick

[Miraz]

Changing the rear ride height will not affect oversteer/understeer characteristics to the same extent if the car is fitted with a Watts linkage.

[scooby dooby doo]

soften the front ARB / stiffen the rear ARB? adjustable ARBs are good for experimentation here.

 

HOOPY CYCLE WINGS CUCUMBER

[Julian Thompson]

Thanks guys. I don't have an adjustable front bar, I'm sure, but the rear on is adjustable.

 

Am I right in thinking that I want the front a touch softer and the back a touch harder, generally speaking?

[Peter Carmichael]

This is getting a bit too simplistic, chaps.

 

For a start, the front is double wishbone and the rear is de dion. ANY ROLL WHATSOEVER changes the angles at the front with the possibility of reducing grip.

 

If the car is soft in roll, you should run more static negative camber on the front so that the car rolls onto its outdside tyre, but this means that the inside tyre is doing bugger all.

 

What this means is that up to a point, a stiffer front ARB increases grip just by reducing roll.

 

The effects of the rear ARB are more noticeable in high speed bends.

 

The Watts linkage removes almost all the rear steer effect although there are still roll axis effects. My car ran at Curborough with the wrong length springs meaning that it was over an inch too low at the back. This will have the effect of making it break away quicker over bumps, but it didn't understeer.

 

The other big issue with an SLR is the ZF diff which needs to be modified (Road and Race Transmissions) to less preload and 30 degree ramps.

 

After that, everything else is in the detail of camber castor and toe at the front end. Probably worth getting a four wheel alignment check or a competent flat floor to baseline your current situation before making tweaks.

[V7 SLR]

ZF diff? I don't have one of those. Is this a new addition to the SLR spec?

 

Incidentally, I reckon that most people don't go for the "quick" checks before experimenting with suspension setups... TYRES. I've found quite dramatically different handling can be achieved by altering tyre pressures WHEN HOT until I found a setup that worked. I still think mine has room for improvement, but now I'm definitely in the realms of suspension setups. I've got a reasonable flat floored setup although the last time I checked it, it had changed (possibly due to settling after the initial build). Worth re-checking an a reasonably regular basis... although that's easy for me to say as I am perfectly happy with PC's well-documented "trolley jack" method.

 

I originally thought that Caterham had setup the top wishbones with too much neg camber because my car is predominantly a road car and it wears the inside shoulders of the ACB10s far more quickly that the outer edges, but now I'm not so sure. If I only had time and a pyro-gun to play with camber settings and tyre pressures I'd know for sure. Anyone lend me a pyro-gun for a weekend?

 

Worcs L7 club joint AO.//Membership No. 4379//Azure Blue SLR No. 0077//Se7ens List Tours

 

[J.R.]

If you still have problem when next on track try tweaking your driving style. It can be done during the session and is a lot quicker than fiddling with suspension.

 

Brake later right up to the turn point and turn in quite aggressively whilst still hard on the brakes, you should find that the weight transfer on to the front will make it dart in the direction you want to go.

 

You have to be very fast getting off the brakes and on to the throttle whilst simultaneously catching the snap oversteer.

 

Lots of fun but arguably no quicker than slow in fast out.

 

PLEASE TRY THIS ON SLOW BENDS FIRST!!!

 

Blame someone else if you come to grief.

[Mike Bees]

Nige,

 

The ZF LSD is the old plate-type. Caterham reverted to these because (a) the race cars were lifting their inside wheel when kerb-hopping which causes the AP Suretrak to go "open" (putting big shock loads into the transmission when the wheel lands again), and (b) the supplies of the Suretrak dried up after AP dropped it (passed it over to another company IIRC). For all-round use I still reckon the Suretrak is a nicer bit of kit, barring the clunks & clanks.

 

Mike

[V7 SLR]

Ah, thanks Mike.

 

Apart from the aforementioned breakages particular to kerb hopping, I prefer the "fit and forget" nature of the AP type.

 

Worcs L7 club joint AO.//Membership No. 4379//Azure Blue SLR No. 0077//Se7ens List Tours

 

[jonhill]

Hi Julian,

 

I told you so

 

I 'cured' (term used advisably, as these things are very much a personal preference, and the work in never really finished anyway) the problem, by putting 170lbs springs on the back, and 250s on the front. I replaced the front roll bar (18mm) with a Juno/Freestyle adjustable, which I run fairly soft, and I have the std rear arb, set on the second hole from the back at both sides. This transformed my car from one that behaved like FWD into one that can have its rear steered nicely on the throttle.

 

I find this a decent compromise between track and road. If I was to use the car exclusively on the road, I'd probably go 150s back and 200s front, as it's a tad bouncy on really bumpy roads.

 

Alternatively leave the rear progressive springs in place and soften the front end. I tried this by combining the front 250s with the 13mm front arb. This was very well balanced, and great for back roads, but too soft for my liking on the track.

 

I also tried the stiffer at the front to reduce roll and increase grip theory, but it didn't work for me.

 

Cheers,

 

Jon

[felix.klauser]

Jon, out of curiousity, what year is your chassis and what engine is in your car?

[Julian Thompson]

Peter, Nige, Jon:

 

Thanks!

 

It was actually Jon who mailed me his solution in the first place! I remember now!

 

I think Nige's theory or checking pressures hot is a good idea too; certainly it is the easiest thing for me to do "in a flash"! I have never been one to do too much tyre pressure checking; god knows what they are at now!

 

I'll try the 16psi and report back!

 

It is a total bugger to try and set the car up properly on the road, too, because I really seem to struggle stringing the corners together into anything meaningful - the only reason I know it's wrong is a few snapshot experiences where I've thought "NO - it shouldn't do that!"

 

 

[jonhill]

Felix,

 

It's a 2001 SLR.

 

Jon