Geometry checking - depth of tread important?

[John]

Obviously it makes sense that you have alignment / geometry checked before changing tyres so the new ones track right and the car drives as it should.

However, is it necessary for a car to have all new tyres for a full geometry check to be accurate?

Or is it the case a geometry check and correct can be done on older tyres (I'm wondering if the difference in tread depth might alter the readings)?

I've had a geometry check and correct done on my E55K previously and once done, the worn out fronts started getting really screwed up as the geometry was quite "out" when I first had it done. However it didn't matter because I went on to change them and the new tyres were fine for ages.

It's never been pointed out before to me prior to a geometry check and correct hence the doubt.

 

[Atavus]

They are not affected by tyre wear.

 

[clk320x]

As above tyre wear won’t affect it.

 

[markjay]

I am no expert, Tony at WIM told me to always bring the car to him AFTER the new tyres were fitted, not before...

 

[markjay]

In fact I think he even said that if the tyres were badly worn, he wouldn't do the alignment at all.

 

[johnsco]

I am no expert, Tony at WIM told me to always bring the car to him AFTER the new tyres were fitted, not before...

That certainly makes sense to me.
The rolling relationship between the tyre (and tyres) and the road will clearly be affected by the tread depth and the effective-diameter of the
wheel(s).
..... Maybe not much - But enough to affect handling and tyre-wear.

 

[markjay]

As I said, I am no expert... maybe Tony @wheels-inmotion will see this thread and comment directly.

 

[John]

The rolling relationship between the tyre (and tyres) and the road will clearly be affected by the tread depth and the effective-diameter of the
wheel(s).

This is what has been passing through my mind hence the post really.

New tyres are 8mm and the difference between that and potentially 1.6mm is quite a lot.

To my mind, the tyre and therefore wheel and suspension sits differently through this range, albeit slightly.

It might be the same car with 8mm tyres is in the "same tolerance i.e. range" as one with 1.6mm.

What I don't know is whether it a) matters and b) makes any difference.

I know Tony is the guru on this.

 

[Deleted member 149005]

Surely uneven wear is more of an issue? Eg 4mm on inner edge but 6mm on outer edge?

 

[johnsco]

Uneven wear is even more of an issue ... Especially, as the uneven wear may vary from side to side and front to back.
I'm no expert, either, but I certainly see it as an issue.

 

[John]

Surely uneven wear is more of an issue? Eg 4mm on inner edge but 6mm on outer edge?

Well yes, except that I already know my alignment is out on the fronts.

The question is when the check and correct should be done in terms of tyre tread depth e.g. only on new tyres or makes no odds.

 

[rf065]

The guy I use says fit new tyres first or he cannot do it accurately, he knows his stuff too.

 

[wheels-inmotion]

Good question and often asked over many forums.

First point is the chassis can influence the tyres but the tyres cannot influence the chassis. In the event a car comes to us and it needs tyres then I'll measure/ adjust before the tyres are fitted? This has nothing to do with the tyres, it's the suspension I want in a virgin position.

Due to our new AI cars that are more and more self-aware I have to measure within 1-10th of a mm. Even air temperature is accounted for. Jacking a car before the chassis measurement is not wise.

 

[John]

So a geometry check and correct can be done on worn-out tyres and will still be accurate once new rubber is fitted after (once the chassis has settled-down from any stress of jacking)...