where do your wheels go

[SteveJJH]

Ive got a question about wheel widths for you to all have a go at. (i dont have an answer, just theories in my head), its quite difficult to explain, so bear with me:

Suspension geometry on a car (for arguments sake, lets use a valver as an example) is set up to run with the original size tyres and wheels (i guess this is right?)

When you buy wider wheels with wider tyres on, the centre of the wheel widthways (looking onto the tread, not the rim) is moved inwards, are you still with me?

What I am trying to say is if you went from 185 section to 215 section rubber, the outside edge of the tyre stays where it is and the inside edge moves inwards by 30mm, is this how it works?

Assuming im still talking something close to sense, wouldnt it be a good idea to use a 15mm wheel spacer in this situation to keep the centre of the wheel in the same place?

 

[RobFenn]

 

[BenR]

yes, thats correct..........

you are effectively (but crudely) affecting the scrub radius and it has alot to do with the actual FEEL of the steering, how light the steering is, the amount of fight.

a reduced scrub radius will feel more dead.

 

[Nick Read]

Well AFAIK if you keep the same offset, the symmetry of the wheel is the same as originally. Therefore if you go to a 205 tyre from the factory 185 then you have an extra 20mm of width theoretically (in practice this varies a little). If you have kept the same offset then your 20mm is spread evenly between the inner and outer edge, 10mm each way.

This sounds all good because the balance of the wheel will be correct as a load on the hub, but it brings the inner edge of the wheel/tyre closer to the inner arch. The outer edge isnt so much of a problem, the wheelarches can always be cut or rolled to give more clearance, but the inner arches cant. Hence the Euro Look with 10" wide wheels where on the inner edge of the wheel it is in the same place as it should be. But the outer edge is sticking out so far you knock over small children as you drive past. To get this effect you just choose how wide you want the wheel/tyre then lessen the offset as much as you need to.

Therefore on a standard valver with a 36 offset, if you change from a standard 185 tyre to a 215 on a 36 offset wheel, but dont want to bring the tyre any closer inboard, youve got to get rid of 10mm on the inner edge. So put in a 10mm spacer to bring the effective offset down to 26mm instead of 36. Of course another way to do it is just to buy a 26 offset wheel. If you want to bring the wheels inwards because you dont plan on doing much steering and need to keep the arches untouched, then youd do the opposite, increase the offset of the wheel either by buying one with the offset you need or machining back the wheel mounting face.

Altering the factory offset can tidy up arch clearance problems but will alter the weight distribution of the rim in relation to the suspension.