Wheels Offsets

[Churchy]

Another back art I am sure.

We ordered the front wheels today from Simmons Wheels and he asked about wheel offsets. We are not interested in deep dish, and are going with the standard Jag XJ6 wheel stud pattern on the front and the ford pattern on the rear.

Ian has mentioned that the distance from the hub face to the rim extremity was 2.5" on both front and rear. Is this the offset ? Ian mentioned that the 2.5" front and rear means that the tyre wall lines up with the guard on both front and rear. We are going to be running 18" x 8" on the front and 18" x 10" on the rear.

The front brake assembly is 330 x 28, sloted and grovved, four pot Brembo. Simmons says that the internal clearance is 435 mm, so the 18's will clear the calipers.

All feedback greatly appreciated.

Cheers Churchy

PS, Craig (400TT) thanks heaps for the feedback

[pneall]

Churchy,

I'll bring the spare hallibrand centres and wheels over next week. We can get confused togrther.

Phil

[Gav]

I have just ordered my wheels and have ordered the fronts in 8" with 5.5" back spacing and the rears in 9.5" with 7.5" back spacing.
I will receive a set of dummy wheels to try but hopefully this should be fine.

Gav

[Luke Gillett]

Hi Churchy,
My understanding of wheel offset is that its the measurement between the inside mounting face of the rim and the centre line of the rim.
There's a better explanation on the vintage wheels website(http://www.vintagewheelsus.com/tech.html)

What type of wheels did you end up going for??? I am also not too fused about a deep dished wheel and am currently exploring brake/wheel options. I'm also still debating wether or not I should change the front wheel stud pattern to Ford while I'm trying to work all this out. Has anyone else done this and what issues should I be considering??

Luke....

[Churchy]

Ian has confirmed that the 2.5" spacing from mounting face to the outside of the rims remains constant for both front and rear.

So that would mean that being that I am going with 18" x 8" on the front, then offset for the front would be 1.5", and if using 18" x 10" rears, that the rear offset would be 2.5". I think I may even understand this.

We have gone for the 18" OM wheels with the charcoal grey (gunmetal) centres with polished rims. They looked quite nice at the showroom, just hope they go well with the impulse metallic (the current SS Commodore dark blue) colour we are painting the car.

Cheers

[400TT]

Offset is measured from the bolt up face of wheel/hub face to the centre of the wheel.

Backspace is measured from bolt up face of wheel/hub face to back edge of rim.

Plenty of good diagrams on the web, just search for "wheel offset backspace"

Remember with backspace measurements you need total edge to edge measurement of rim which in most cases is 1" wider. So 10" rims is measured at tyre bead and will actually be approx. 11" from edge to edge.

With caliper clearance you have to check that calipers will clear the rear of the spokes. A lot of wheel suppliers will provide a template or measurements for this. Also brake suppliers will also provide templates to help with this. They create brake caliper clearance by both rear spoke design & by building up the wheel hub. The more they build up the wheel hub, the less outside rim you end up with.

I know with wide deep dish Simmons wheels, like 12" wide wheels, Simmons make both outside & inside rims the same size and make up the dish by supplying a really deep centre hub. It's not unusal to see tops of wheel nuts 50mm or more below surface of the hub. So what you thought should be really deep dish wheels or not supplied as such.

And with Simmons and other 3-piece wheels you have to also take into account clearance on the nuts that hold rims together. In saying that you should be fine. Those brakes won't be big enough to cause problems with bolts.

Are Simmon's still using the same size centre on 17 & 18" rims?

I'm actually a bit more of a fan of US Billet Wheels these days. Both inner and outer rim are polished, centre caps are also billet and centres are generally made of higher quality alloy so hold a superior polished finish. And most are supplied with true size centre so you get that nice big centre/flat rim look. The irony of it is, most of the rims are sourced separately to the centres...so for example a Aus BonSpeed is not the same as US BonSpeed. Even look can be different as they skimp on wheel centre size.

Simmons are about the cheapest 3-piece wheels on the planet though and have a proven track record. I have had good success with Simmons in the past, just hope they have fixed up their custom paint offerings and we had to re-paint a set of Simmons wheel centres as paint finish was visually poor & also did not bind with the primer, as in you could scratch it off with your finger nail.

[400TT]

Originally Posted by Churchy

Ian has confirmed that the 2.5" spacing from mounting face to the outside of the rims remains constant for both front and rear.

Cheers

That's what you would call front space.

So if I'm understanding you correctly, the backspace would be:
Front: 9" - 2.5" = 6.5" (165mm)
Rear: 11" - 2.5" = 8.5" (216mm)

Offset would be:
Front: 2" (+50mm)
Rear: 3" (+76mm)

I'll draw a diagram and check those measurements when I get a chance.

You would have to confirm with Simmons the total edge to edge width of 8" & 10" rim.

Or just give Simmons the front space and they can work it out for you.

[Churchy]

Hi Craig,

Mate thanks for all your input, you have really helped.

I measured the calipers last night, and the calipers only hang about 40mm past the rotor, which brings it out to about 370mm, so even if it was 80 mm past it still clears. The clearance with 18" at Simmons is now 435 mm, and the 17" are 395 mm, so I am assuming that they using different rims sizes.

Going with the following rims sizes, I think the offset would be

Front: 8" - 2.5" = 5.5" (140 mm)
Rear: 10" - 2.5" = 7.5" (190.5 mm)

Offset would be:
Front: 1.5" (+38mm)
Rear: 2.5" (+63.5mm)

I may actually understand this. Now that would be a first

[400TT]

Churchy, not sure if you are quite following my rims sizes.

Your 8" & 10" rims are measured at the tyre bead. So measured on the inside edges of the rim.

If you measured your 8" rim from edge to edge, it would actually measure 9"
Same with your 10" rim, it would measure 11".

Since front space & back space are measured from outside edge of rim, you need to calculate this using 9" & 11" rim measurements.

This is why wheel manufacturers use offset, it's measured from wheel centre line to bolt up face. Don't have to worry about true width of rim.

Hooray, sounds like Simmons have true 18" rims now. Next time I'm near some 17" Simmons I'll measure the centre. I might have to put Simmons back on my shopping list. I checked 12 months ago and they were definitely only 17" centres and the only 18" Simmons I saw had 17" centres. Glad that's fixed. Maybe they offer better rim to centre offsets as well.

With those size brakes you won't have any problems with true 18" wheels. 17" wheels would be close.

For everyone elses benefit. one of the clearances I was actually talking about is if the caliper sticks out proud of the the bolt up face. Those pesky centre-rim bolts can cause fouling in that case if the wheels are small enough.

From memory my front brakes are just over 400mm in diameter - 355mm x 36mm. The problem is the 6 piston calipers that really stick up proud of the bolt up face and cause clearance problems with centre.

[Churchy]

Dooh. Got it now !!

I had a good chat with Ian at Simmons and he has confirmed exactly what you are saying.

The DBA rotors I am using mount on the back of the Jag hub, so the calipers should have no issue with sticking out proud. I will also measure that tonight.

Simmons have also said that they will give me a dummy wheel to ensure all clearances are spot on.

Cheers

[400TT]

Sounds good.

Getting a dummy wheel is a great idea. One of the places I was checking out for billet wheels actually sent up mock wheels to test offsets & clearances before producing set of rims. Gives great peace of mind.