1973T Coupe, wheel wobble at 70 mph...

[911hillclimber]

Hello all, I have a 73T which I've run for nearly 29 years, road and hill climb.
It is modified but my question here is close to the standard car, hope some can help me.

The car has 7" Cookie Cutters and Dunlop Blu Response sport tyres, 195x15x65 91V, currently 3000 miles old and 28/32 psi pressures Fr/Rr.
Front suspension is Bilstein struts, RS anti roll bar, stock torsion bars, stock rubber bushes, Turbo track control arms, normal modified stuff.
I can find no wear etc anywhere, all joints are new.

Issue is a steering wheel wobble at 70 mph (actually 68-73mph) there the rim will wobble +/- 5mm in your hands.
Perfect above and below this speed.
The car had this problem before fitting the tyres and now.
The wheels have just been balanced and needed a lot of weight added, 2 x strips of weights, each about 100mm long in 2 places.

Question:
Has anyone had this wobble at this speed on their car and found a way of correcting it?
Hope someone can help me, Graham.

 

[911hillclimber]

As an additional thought, can too much caster generate this kind of wobble?

 

[38F3E1]

Hi Graham.

Have you tried swapping the wheels front to back or borrowing a set off someone to try and eliminate it being wheel and tire related? If I still lived up your way I would lend you a set but Cornwall too far away unfortunately.

Ian.

 

[911hillclimber]

Thanks Ian, yes, tried every combination last year inc using the spare with a brand new never used tyre (Dunlop like those on the car)

 

[38F3E1]

What about the moulded on rubber bushes in the front A-arms? It is very difficult to feel any wear in them by hand but they can cause vibration issues. They also tend to cause tire wear issues from memory on inside edge.

 

[911hillclimber]

Good call.
they look perfect, but will get a pty bar to them to check tomorrow along with checking the geo overall.
thanks!
Graham.

 

[911hillclimber]

Quick check:

Jacked the front up, tyres 25mm or so off the ground.

Tyre variation

Concentricity 0.5m for 50% of the circumference on the NSF, 1mm or less on the OSF, probably much the same as each other.
Laterally, both tyres are true as can be.

I used a wax chalk in yellow to mark up the lateral on the centre rain groove and concentricity about 10mm to one side on the road surface, both central to the contact patch as possible.

Caster:

Rough check using my magnetic gauge. Steel part of the centre caps just strong enough to hold the weight of the gauge.
Car back on the floor and rolled forwards/back a few time to help settle the suspension.

Zero'd the gauge and then gave the steering one complete turn which looks like 20 degrees to me.
Reading was 3.5 degrees positive. on the OSF, wheels turning 'right' as you sit in the car., thus the gauge is pointing back to the back of the car @ 20 deg.
Same method, same result on the other side.
Thus this is roughly the Caster with the car at static ride height.

My garage floor is really quite level luckily.

Ian: If the A arm bush(s) were tired then the body would move? The wobble I have is only in the steering whell.
I'll check them tomorrow anyway when the car is all jacked up.

 

[911hillclimber]

Having done all that, I am now confused!

How do you measure the caster?

Went on the Gunsun site and it seems I've been measuring it incorrectly....

They state to turn the OSF wheel to the right by 20 deg and zero the Caster gauge, then turn the wheel to the left by 20 deg and see where the bubble lands.

Just done this and it reads 5 deg +, much closer to the 6 deg required.
Assuming this procedure is correct, I need to find another deg on both sides and still have room to get 1 deg neg camber.
Will try this tomorrow and track the front for Zero Toe using my fishing line approach
At least the front is easy to work on once I have the track rod end locking 1/2 nuts loose, always a hard bit.

 

[Motorhead]

Graham,

Just looking at this out of interest.

From your description of the problem I'd think that it's more than likely either due to wheel imbalance and/or play in the suspension rather than a geometry issue.

I'm sure you'll get a result using gauges and bits of string but I'd be inclined to get a comprehensive electronic geometry measurement and set-up if you've got a reliable specialist in your area. Center Gravity in Atherstone, Warwickshire provide an excellent - albeit expensive - service, although there are many others.

On the subject of wheel balance, it might be worth seeing if you can locate a local company who have a Hunter Road Force Balancer http://www.balancemycar.co.uk/

Jeff

 

[911hillclimber]

Thank you Jeff,
The check on the alignment is really just to be sure nothing is wildly out/loose, the fact it drives really straight and well indicates my DIY job last year was ok, except that I've been told zero toe is right and I have 2mm of toe in overall.

Just had the 2 front wheels balanced by my local tyre place who have never failed me before, and both wheels were out by yhe same amount!

Now weighted to 'Zero' so perfect on the machine.

Finding a place with a Hunter has proven tricky despite being in the heart of the West Midlands, but worth trying as you say.

I have a specialist in mind for a laser geo set if all these smaller checks fail me, MCE down in Banbury. Have already spoken to Mike about it.
I'm going to hve a good prod around the rubber bushes tomorrow, something I've assumed were ok.....

 

[JohnCRS]

Graham. I assume you have ruled out the wheel bearings (play in just one would do it)?

 

[911hillclimber]

All brand new 12 months back and nipped a bit tight TBH.
Lower ball joints new at the same time, track rod ends new etc!

Track control arms are Turbo, not a noise from the front end at all on any surface.
Checking all the moving parts tomorrow and tracking to Zero instead of toe in of 2mm.

 

[911hillclimber]

Today was check everything day.

Checked for loose parts on the front, all tight.
Checked the toe, and it was Zero, so must have set it there 10 months ago. Thought I had toe-in.
Checked camber, both sides 1 degree Negative
Checked caster, the 'new to me way' and this was 5.25 degrees both sides.

Checked and set the tyre pressures to my usual 28/32psi.

Took it for a drive, down to the local motorway, my usual private test track.

No wobble.
The only thing changed is getting the cheap tyres balanced to spot-on by the local 2 men and a cat tyre bay.

Now, will they stay this way? Time will tell.
Along side all this, I know all it set well as can be, my DIY alignment is good using fishing line, poles, camber guage and a 6" rule.

Self Maintenance Rules!

 

[38F3E1]

You can get very good results with the traditional pole and string method as long as you have a decent flat floor and take your time :- )

Glad it looks to be solved Graham.

 

[911hillclimber]

Indeed you can Ian.
There is a spot in my garage floor which is really good and I've used these spots many times for this car and the Lola-Porsche hill climb car.
The Lola is very sensitive to geo so I'm used to taking my time!

A whole different world...