Mid Corner Grip/tyre Judder Issue

[djr81]

An odd question perhaps, but does anyone else have issues with tyre judder fron the front of their cars- particularly GT-R's.

I have recently changed a couple things on my Gt-R

1. Tyre are now the S2 versions of the RE55's.

2. New 1.5 way Nismo rear diff.

Which meant a change in the way the car needs to be driven, basically turn in under brakes, let the thing briefly understeer across the apex until you get on the power for the exit. Problem being in the mid corner phase (ie no brake or throttle input) there is a bad tyre judder in the 2nd gear corners.

So a couple questions:

1. Has anyone else had this.

2. Any recommendations other than more -ve camber on the front, more toe out at the front?

Oh the weight distribution from leaning into the corner is worth 2/10ths per lap, so please save your sledges.

[Roy]

Well, most NSW GTRs i had been in as a passenger at the track did it, especially at Eastern Creek. It happened when they were tryign to carry too much corner speed and the thing started to understeer and the fwd woudl start to kick in making the vibration and understeer even worse.

Strangley have not noticed it in one single Vic GTR, so either its a track surface:car thing or its the fact that most Vic GTR get setup by the same workshop and has the alignment all set up.

So i would suggest it has something to do with ride heights and the car willingness to pitch onto its outside tyre.

[djr81]

Thanks Roy, I guess it must be a lattitude thing.

I guess I will keep looking for more mid corner grip.

[Sydneykid]

Problem being in the mid corner phase (ie no brake or throttle input) there is a bad tyre judder in the 2nd gear corners.

That's the problem, you should be either on the brakes or on the throttle If you can't get on the throttle, then maybe you are coming in too hot, have a go at decreasing the entry speed slightly and getting on the throttle earlier for a faster exit. The small amount of time you loose by a slightly slower entry will come back 3 times over in the faster exit.

Some things to check;

Make sure it isn't running out of travel on the front, the old cable tie on the shock shaft trick will tell you

Try some more front toe out, since it is the slower (2nd gear) corners, it may not have enough Ackerman, in which case some static toe out will help

Since Barbagallo is mostly right handers, you may want to try reducing the camber on the RHS front. That will put some more tyre contact patch onto the inside tyre which always helps with mid corner understeer. The tyre temperatures will tell you how much camber to remove.

You have probably already tried backing the front swaybar off to the softest setting, if not that will tell you if it's too much anti roll causing the understeer. Try the reverse, put the front swaybar onto the highest setting, that will tell you if it's too much weight transfer onto the outside front tyre. I assume you already have the rear swaybar on its highest setting, if not try that.

Cheers

Gary

[djr81]

That's the problem, you should be either on the brakes or on the throttle If you can't get on the throttle, then maybe you are coming in too hot, have a go at decreasing the entry speed slightly and getting on the throttle earlier for a faster exit. The small amount of time you loose by a slightly slower entry will come back 3 times over in the faster exit.

Some things to check;

Make sure it isn't running out of travel on the front, the old cable tie on the shock shaft trick will tell you

Try some more front toe out, since it is the slower (2nd gear) corners, it may not have enough Ackerman, in which case some static toe out will help

Since Barbagallo is mostly right handers, you may want to try reducing the camber on the RHS front. That will put some more tyre contact patch onto the inside tyre which always helps with mid corner understeer. The tyre temperatures will tell you how much camber to remove.

You have probably already tried backing the front swaybar off to the softest setting, if not that will tell you if it's too much anti roll causing the understeer. Try the reverse, put the front swaybar onto the highest setting, that will tell you if it's too much weight transfer onto the outside front tyre. I assume you already have the rear swaybar on its highest setting, if not try that.

Cheers

Gary

Thanks Gary.

Just to explain basically the problem is when I am getting off the brakes & then moving my foot over to the accelerator. The ATTESSA controller I have ordered hasn't turned up yet so the car is still suffering from a lack of traction in second gear (& third) - hence the need to try & carry some speed across the apex.

Anyway, from your list:

Front is high enough that it should not run out of travel. I will give the cable ties a shot but thew Collie circuit is pretty bumpy so the bump stops tend to get used anyway.

I have zero toe presently at the front. Is there a X turns of the thread approx equals Ymm toe?

Wanneroo is all third gear except for the short circuit which has only one second gear corner. Problem isn't so bad on third gear corners. I have to get it pretty badly wrong to get a good amount of shudder whereas it is nearly always present on the tighter stuff.

I was thinking about chucking a harder front sway bar setting ont he front in the off chance the anti roll will keep the camber more correct & help front grip. I love the way stuff works completely backwards sometimes. The front bar is presently full soft. Rear bar was a Whitleine on full hard but I put a Cusco item in which isn't adjustable until I get some bits made.

As much as anything I guess I was curious to see who else may have come across this.

[GTS4WD]

maybe Im wrong..

But why not decrease tyre pressure in the front? Then you have a larger contact patch?

[Sydneykid]

Thanks Gary.

Just to explain basically the problem is when I am getting off the brakes & then moving my foot over to the accelerator. The ATTESSA controller I have ordered hasn't turned up yet so the car is still suffering from a lack of traction in second gear (& third) - hence the need to try & carry some speed across the apex.

Anyway, from your list:

Front is high enough that it should not run out of travel. I will give the cable ties a shot but thew Collie circuit is pretty bumpy so the bump stops tend to get used anyway.

I have zero toe presently at the front. Is there a X turns of the thread approx equals Ymm toe?

Wanneroo is all third gear except for the short circuit which has only one second gear corner. Problem isn't so bad on third gear corners. I have to get it pretty badly wrong to get a good amount of shudder whereas it is nearly always present on the tighter stuff.

I was thinking about chucking a harder front sway bar setting ont he front in the off chance the anti roll will keep the camber more correct & help front grip. I love the way stuff works completely backwards sometimes. The front bar is presently full soft. Rear bar was a Whitleine on full hard but I put a Cusco item in which isn't adjustable until I get some bits made.

As much as anything I guess I was curious to see who else may have come across this.

I always try and not use the bump stops, except for the larger ripple strip bashing. For toe adjustment, around 1 turn = 2 mm, close enough to test anyway. From memory, the off the shelf Cusco rear bar is too soft as it designed to run with the high (Jap style) rear spring rates.

The race team drivers tell me that there should be zero time between brakes and accelerator, I'm not that good though. The reasoning is when you take your foot off the brake it removes some weight off the front wheels, hence reduces their grip. With judicious use of the accelerator you can balance the front grip, Plus, in a GTR, some torque transmission to the front wheels increases their steering effect in comparison to free wheeling (on overrun/accelerator off). They seem to have some success with that technique and it is technically correct, I'm personally not good enough to achieve that sort of control, so I settle for a much simpler slow in fast out.

Cheers

gary

[djr81]

The bar I got was stiffer than the Whitleine one I had. It can be made stiffer again when I finish the drawing/machining/welding. When I say I will finish it, I mean when I get someone else to.

I can't do the left foot braking thing in a Skyline. All I get is a massive cramp in my leg anytime I try it.

So the time it just the gap between bleeding the last of the retardation off the brakes & feeding power in. Not long but you notice it when the car starts to carry on. Driving a race car is supposed to be about maximising acceleration in the chosen direction. Unfortunately for me it is mostly about not fking up.