Wheel offset/track width changes and thier effect on handling?

[BMWTurbo]

Hi Guys,

Probably a rudimentry question, but what effect would reducing the track on the front, by changing wheel offset have on the steering?

The question has stemmed from the fact I've currently got some factory 16's on the car 16x7.5's with 215-60-16 rubber and the car has much better turn in and sharper steering response (I'm not meaning steering weight here) than it did with the 18x8 with 235's.

Not certain on the 16's offset, but will confirm tonight the 18's were ET20.

In choosing wheel offsets what are teh effects on handling by reducing and increasing track width on the front?

So this is due to the 'scrub radius'. Assuming the same offset on the wheels the scrub radius should be the same?

TIA

[GTV]

Scrub radius is important to steering feel and the factory offset wheels are likely getting you in the sweet spot compared to the reduced offset aftermarket. You can gain a bit of grip by having a wider track, but it may not be worthwhile if the steering feel goes to hell. You are also likely to get more tramlining and loading up with smaller offets.

Also, the narrower rubber is giving you a longer contact patch, which gives a more progressive feel at the limit.

[BMWTurbo]

Scrub radius is important to steering feel and the factory offset wheels are likely getting you in the sweet spot compared to the reduced offset aftermarket. You can gain a bit of grip by having a wider track, but it may not be worthwhile if the steering feel goes to hell. You are also likely to get more tramlining and loading up with smaller offets.

Also, the narrower rubber is giving you a longer contact patch, which gives a more progressive feel at the limit.

Ta mate. I should have mentioned that the overall diameter increased with the std wheels/tyres also, 645mm to 664mm

So if I was to use teh same offset wheels, even if they are wider, I shouldn't notice too much of a difference?

[Uncle Arthur]

Fairly sure that there was a thread on this topic not that long ago, not BMW centric though.

[GTV]

Yes, offset makes the most difference. A wider rim and wheel will have an impact on steering feel, but is less pronounced (unless we are talking extremes). Overall diameter will have an impact on the shape of the contact patch, with taller usually being better, but will also add more gyroscopic effect, which is bad (light wheels do make a difference). Its all a compromise in the end.

[BMWTurbo]

So put simply, by changing from a 7.5 ET20 to a 8.5 ET20 rim the scrub radius hasn't changed, and all being equal, ie tyres same diameter etc, I shouldn't notice too much of a difference? I'm trying to work out what I need to fit to get teh same response as I'm getting with the std wheels when I put some larger rims/tyres on it in the future.

Richard, I had forgotten about the 'stance' thread.

[GTV]

^ for all practical purposes, that is correct. Pedants could debate the impact of the wider rim and or wider tyres but they are much less orders of magnitude.

[Momus]

The goodness of the rubber alone could easily account for the turn in and feel difference.
It's not impossible that the likely more rounded profile of the 60 series suits the struts geometry better than a wider/flatter tyre.

Going from a 645 to a 664 diameter tyre would give a scrub radius change (reduction) of only a couple of mm with a typical 10 to 12 degree king pin inclination.

[dynoryder]

Was a wheel alignment performed after the change ?

[BMWTurbo]

Ta Guys... I hope I'm not going to need to run a '215' on the front to retain the current turn-in...

Ideally I'd change to a ET15 for clearance, but can't find a wheel style I'm happy with in 8.5 ET15's.

No alignment, literally changed wheels only.

[dynoryder]

What is the car used for ?

[styler]

As far as I know for legalities its +26mm increase in track in qld but no decrease allowed due to stability and inside wheel clearance,
but say that aside its also a combination of toe, caster, camber, steering axis inclination and scrub radius that affect steering
so its only a part of it but yeah more offset from what I have experienced seems to equal heavier steering in most cases.

But that being said if you run toe at 0 or run toe out the car will tend to steer more easily as such but its not so simple as the
car will also not self correct to straight and will tend to track ie follow lines in the road which can be annoying. Thats as far as
I understand it anyway, someone else may have a better understanding of this which may be more enlightening.

Turn is is affected by sidewalls on tyres as well, eg dot r semi slicks have much stiffer sidewalls and better turning response.

[turbosurfer]

Yep I got toasted about this for not mentioning type of car.
It wasnt about stance by the way if you care to read it fully.