Wheel scrub on full lock

[MikeParker]

Hi All,

Don't know if anyone has come across this. My car has always, as far as I can remember, tended to scrub when on full lock. The outer front wheel seems to have a very high positive camber and this appears to result in an effective toe-in.. You would think the outer front tyre should, if anything, have negative camber on full lock. You can hear the scrubbing sound and it takes considerable force to move the car through the turn, particularly on a grippy surface such as tarmac. On a shingle surface it is fine.

This is clearly a geometry issue of some sort but I am at a loss as to where to start. Any ideas?

Thanks, Mike

[Goldstar]

Could it just be the Ackerman principle Mike. This is the answer I got when I asked the same question about my wife’s Boxster which does it (they all do). I can’t explain the principle to you but Wikipedia can. I’ve never noticed it as obviously in a stag tbh but maybe it is part of the answer.

https://en.m.wikipedia.org/wiki/Ackermann_steering_geometry

[MikeParker]

Thanks Goldstar. That's a fascinating explanation. I have always wondered what defined the "ideal" geometry and that wikipedia article explains it really well. Are we saying that all Stags do this. i.e. have excessive positive camber on the outer front wheel at full lock? It seems like a pretty major flaw. No car from the last 20 years that I have driven has shown this effect. I am surprised that a modern car like a Boxster does this too. If you are saying your Stag is not too bad in this respect that does imply there is something I could do to improve things.

[Goldstar]

Hi Mike. It’s possibly more that I never noticed it in the stag compared to the Boxster where it is quite pronounced on cold tyres on a “grippy” surface.

[trunt]

Isn’t change in camber at full lock down to the castor? Is that OK on your car?

Found this:-

Caster angle affects the camber of the wheel during steering. ... If a car is set up with positive caster, the outside wheel will gain negative camber during cornering. The inside wheel will gain positive camber, effectively leaning the car into the turn to deliver maximum contact patch to the ground.


Terry

[MikeParker]

Terry, I hadn't thought of that, but you are right. If the front wheel on the outside of the curve has a positive camber (i.e. leans out at the top of the wheel) that does imply a negative caster angle which is definitely not right. Is there an easy way to check castor angle. Is that simply the angle to the vertical of the MacPherson strut, "viewed" from the side of the car?

[trunt]

I was reading up on that..

Turn wheels 20 degrees one way, measure camber. Turn them 20 degrees the other way measure camber again. Find the difference between the two and multiply by 1.5 that’s your castor angle.

I did a quick dirty check on mine and suspect my castor is negative… plan to do it properly later.. then make a decision, probably the super flex castor increase kit. If it is off I’m a bit miffed at the alignment place not picking that up

[MikeParker]

Terry, I will try and measure the castor angle as you suggest. I've done some more tests on my steering and the scrub is definitely worse on right lock than left lock. See the pictures of the wheels on a tarmac surface on right lock. It takes some force to drive the car through the turn.

The positive camber on the outside wheel can easily be seen. It looks more of a problem than a few shims could correct so what's going on here? Having looked on the web at pictures of a number of Stags, there are some that have negative camber but most have some positive camber but none quite as pronounced as mine!







DSC_0237.JPG DSC_0235.JPG

[trunt]

Wow, that looks a lot! Is it the same on the other side at full lock the other way or just that side. I would make sure the drag struts are ok, not bent maybe measure them? and check the control arm bushings..

I just did mine and it worked out that I had:-

Left
-0.5 camber 0 castor
Right
- 1.2 camber +0.2 castor

Im not overly bothered about the rhs camber but as near as dammit I have no castor. I always felt the self centring was weak, but brushed it off as many report this.. I have ordered the castor increasing bushes so give me a few weeks (Months?) and I’ll report back


Terry

[sujitroy]

Maybe disconnect the track rod ends and rotate the wheels by hand. Is the height 14.5" from bottom of wing to center of tire? Could the ride height be contributing to the angle. Is the shock absorber straight as it looks like as you rotate from lock to lock, the travel is in an arch shape pushing to top out.
Sujit

[MikeParker]

Not as bad on left lock and car does not scrub. Also on a less grippy surface, the camber does not seem as pronounced. I did have the axle stub snap on the right side, so that one is new, so may be the left stub is on the way out and flexing. A scary thought!





DSC_0233.JPG

[MikeParker]

Sujit,
I will check that measurement. Certainly the camber increases dramatically as the wheel drops. With the steering straight ahead all looks normal though with only a marginally larger camber on the "problem" left hand wheel. I need some turntables to put under the tyres so I can rotate the wheels with no lateral force on the tyre and see what is going on. Thanks both for your input.
Mike

[trunt]

Looking at those pics it seems that your wheels turn out further than mine… anyone agree?

if so there should be limiting blocks on the rack to reduce the lock, maybe someone left them out?

[SteveD]

Could it just be the Ackerman principle Mike. This is the answer I got when I asked the same question about my wife’s Boxster which does it (they all do). I can’t explain the principle to you but Wikipedia can. I’ve never noticed it as obviously in a stag tbh but maybe it is part of the answer.
https://en.m.wikipedia.org/wiki/Acke...ering_geometry

My 2017 Alfa Giulia does the same, they all do, and I was advised it was due to the Ackerman steering system also.

"Looking at those pics it seems that your wheels turn out further than mine… anyone agree?" I agree

Steve.

[MikeParker]

Admission here. I did rebuild the rack about 30 years ago, so might well have left something out! Are those limiting blocks actually on the main rack shaft? I do remember some white nylon cylinders but it was a long time ago!