Aftermarket Alloys: Wheel Offset & Backspace Qstn.

[M@H]

Chaps (and 'esses)

If one was to change the wheel rim width when putting different wheels onto the Chim what is it most critical to keep the same.. the Wheel Offset or the Backspace ?

If for arguments sake one went from a 7J wheel to an 8J wheel with exactly the same offset then the backspace is going to change by 1/2" (12.7 mm). So is it "better" to go for a wheel with a lower offset, eg ET 20 instead of ET 35 so that the Backspace is closer to the original setting, or better to presereve the offset ?

All thoughts comments etc. much appreciated.

Cheers,
Matt

[antonyj]

Take a look at this calculator

[url]http://toy4two.home.mindspring.com/offset.html[/url]

>> Edited by antonyj on Wednesday 4th May 18:11

[19560]

In a straight choice keep the offset the same to preserve the coincidence of the suspension angles. However an increased track also helps, within reason, so in the example given the difference would typically be split with a slight bias towards the ET number, say 30 ET. Purely from observation. Jonathan

[M@H]

19560 said:

In a straight choice keep the offset the same to preserve the coincidence of the suspension angles. However an increased track also helps, within reason, so in the example given the difference would typically be split with a slight bias towards the ET number, say 30 ET. Purely from observation. Jonathan

Thanks for that..

..of course there is another limiting factor that if one went from a 7" to an 8" wheel (as per the example) and an ET of 30, then outer rim of the wheel would be 71mm from the hub, rather than 54mm from the hub. Accordingly I think on a Chim (at the front) that the extra 17mm track each side would push the tyre "outside" the wheelarch so perhaps there would be a problem at MOT time.. Hmmm..

I can see its going to be a level of compromise all round if one changes the wheels.

[nztrev]

I must find out my offsets,but I insisted on the widest track possible without fouling,8"fr,9r.
I can't see how wider wheels will affect geometery?

[19560]

nztrev said:

I can't see how wider wheels will affect geometery?

I'll try later to post a link to a diagram that Ive got at home. It isn't wider wheels that affect the geometry it's the offset.

[19560]

M@H said:

If one went from a 7" to an 8" wheel (as per the example) and an ET of 30, then outer rim of the wheel would be 71mm from the hub, rather than 54mm from the hub.

It depends upon exacly what you do though because TVR were notoriously narrow on rims. For example the rear of a lot of Chims have a 7.5" rim as standard with a 245 tyre. You can easily fit an 8.5" or 9" rim with the same tyre width. At the front TVR typically fitted 225s onto 7" rims so you could switch to 8" rims with the same tyres. In both of these examples the tyre (as opposed to the wheel) would only be 5mm further out.

[M@H]

19560 said:

M@H said: If one went from a 7" to an 8" wheel (as per the example) and an ET of 30, then outer rim of the wheel would be 71mm from the hub, rather than 54mm from the hub. It depends upon exacly what you do though because TVR were notoriously narrow on rims. For example the rear of a lot of Chims have a 7.5" rim as standard with a 245 tyre. You can easily fit an 8.5" or 9" rim with the same tyre width. At the front TVR typically fitted 225s onto 7" rims so you could switch to 8" rims with the same tyres. In both of these examples the tyre (as opposed to the wheel) would only be 5mm further out.

Bloody Good Point ! jeez I am thick at times !!

So when I'm considering my backspace issues I really should try and factor in the change in tyre width at the same time..

Matt

[M@H]

19560 said:

nztrev said: I can't see how wider wheels will affect geometery? I'll try later to post a link to a diagram that Ive got at home. It isn't wider wheels that affect the geometry it's the offset.

Would appreciate seeing that...

[19560]

M@H said:

Would appreciate seeing that...

Found it, never mind the make, www.miata.net/garage/offset.htm
Consider a 5or6mm shift drawn to scale. I have never heard an engineer worry about that amount.
Jonathan

[M@H]

Excellent, thanks for that.. As the wheels are probably going to be machined to my own offset requirements I'd bet get my crayons out and do some doodling.

Cheers
Matt

[19560]

As a footnote the natural Griff size appears to me to be 8 x 17 ET30 and run 3mm spacers at the front. (Front original 25mm new 27mm, rear 32mm, new 30mm, all from memory.) Tyres 215/40/17 and 245/40/17. Anywhere close to your musings?

[M@H]

Musings are currently leaning towards sticking with a 7J wheel width due to clearances at the front.. still thinking

>> Edited by M@H on Friday 20th May 10:26

[19560]

M@H said:

Musings are currently leaning towards sticking with a 7J wheel width due to clearances at the front.. still thinking

IMHO a lot depends upon whether you are trying to order the same wheel for front and rear but either way I would have thought that 7.5" was a possibility esp. for 215/225 tyres.