Topic: Guide to Wheel Offset (Read 2882 times)

ClubCalibra · « **on:** May 09, 2006, 20:17:15 PM »

A simple guide to Wheel Offset

Wheel Offset is the measurement between the centreline of the wheel and the mounting face. It is important to maintain the correct offset when fitting aftermarket wheels to avoid upsetting the handling of the car.

Most Vauxhall cars use an unusually high offset of 49mm. If you ignore this when fitting aftermarket wheels, you can run the risk of upsetting the handling/stability of your car and/or causing the wheels to rub on the bodywork when going over bumps etc.

On the subject of wheel offset, Courtnay Turbo say this:
"When GM introduced their first front wheel drive car in 1981 they introduced their innovative 'Negative Scrub Geometry' which gives excellent stability in the event of uneven braking due to eg. gravel in the verge or a blow-out. At the core of the design was the adoption of a unique offset of 49mm negative. Aftermarket wheel manufacturers and retailers almost universally ignore the requirement for GM fwd cars to run on this offset alone and normally supply the more common 38-45mm ranges. Because we are specialists and aware of the importance, we will not supply a wheel which is not ET49."

It is worth mentioning that Irmscher, the suppliers of the Vauxhall approved aftermarket accessories, initially specified an offset of 42 - 49mm for their wheels for the 16 valve Calibra. However, on speaking to Courtnay about this, they explained that Irmscher, being German, have different requirements for offset. This is because they drive on the other side of the road and the camber affects the car differently. For the UK market, they have redefined the requirement to be an offset of 49mm only.

Note: If fitting wider tyres than standard, offset is particularly important. Wheels with a lesser negative offset than recommended will 'stick out' more than standard wheels. This will make a wider tyre more likely to rub on the wheel arches than if a wheel of the correct offset is fitted.

Courtnay say that they never have trouble with 215 tyres fitted to a Calibra if the correct offset is used. It has been known for engineers to machine 1mm off the mounting face of a wheel to increase the offset by 1mm in order to get a 48mm offset wheel to the correct offset for GM cars. We would not recommend that you machine any more than this else you run the risk of over-weakening the wheel.

lee · « **Reply #1 on:** May 30, 2006, 17:19:50 PM »

if i fit 10''wide wheels on the back and 7.5 ''on the front with an offset of 12mm [to bring the wheels out in the body kit] how will it effect the wheel bearings?

215/35/18 on the front

255/35/18 on the back

many thanks

Rodders · « **Reply #2 on:** March 18, 2007, 19:47:40 PM »

If you fit wheels that wide on your calibra you are likely to nackker the cv joints and will also put alot of stress on the wheel bearings.

buchanan_vaux · « **Reply #3 on:** April 30, 2007, 08:59:29 AM »

I have a set of 17" wheels and wondered if I could fit a set of 225/35/17's from 215/40/17? If I could would there be any wheel rubbing on the shocks or body work

J@zV6 · « **Reply #4 on:** April 30, 2007, 09:34:58 AM »

Quote from: buchanan_vaux on April 30, 2007, 08:59:29 AM

I have a set of 17" wheels and wondered if I could fit a set of 225/35/17's from 215/40/17? If I could would there be any wheel rubbing on the shocks or body work

The overall diameter of a 225/35/17 tyre is 589mm.
The overall diameter of a 215/40/17 tyre is 604mm i.e. 15mm or 2.55% larger
So the 225/35/17s are actually smaller overall than the 215/40/17s - this will reduce your overall gearing slightly, which will marginally improve acceleration and reduce top speed slightly.

p.m.adams · « **Reply #5 on:** September 28, 2007, 22:35:18 PM »

Anyone know what offset I should be looking for a 5 stud 4x4 turbo and what size tyre I should go for on a 18" rim?

J@zV6 · « **Reply #6 on:** September 29, 2007, 21:11:44 PM »

Quote from: p.m.adams on September 28, 2007, 22:35:18 PM

Anyone know what offset I should be looking for a 5 stud 4x4 turbo and what size tyre I should go for on a 18" rim?

ET49 is the optimum for most Vx. You can get away with less, down to around ET40, but if you go much below that, it starts to affect handling - you tend to get an effect called 'tramlining'. It may also put greater stress on other components.

s.kumaran · « **Reply #7 on:** May 16, 2008, 21:15:58 PM »

hi how r u all out there..
can i put this tyre on my alu rims?
i got now 215 X 35 X 18 , im going to change tyre so its possible i can change the new tyre to 225 x 35 X 18? or can use this one.. 215 X 40/45 X 18? will it reduce my speedo meter? or i will get any trouble? pls help..

J@zV6 · « **Reply #8 on:** May 16, 2008, 21:40:51 PM »

Quote from: s.kumaran on May 16, 2008, 21:15:58 PM

hi how r u all out there..
can i put this tyre on my alu rims?
i got now 215 X 35 X 18 , im going to change tyre so its possible i can change the new tyre to 225 x 35 X 18? or can use this one.. 215 X 40/45 X 18? will it reduce my speedo meter? or i will get any trouble? pls help..

The rolling radius of a standard 205/55 R15 tyre is 303.2mm, giving 525revs/km
The rolling radius of a 215/35 R18 tyre is 303.8mm, giving 524 revs/km i.e. very similar (0.2% difference)
The rolling radius of a 215/40 R18 tyre is 314.6, giving only 506 revs/km (3.6% difference) this is a significant difference, but you might still get away with it, at the expense of an under-reading speedo
The rolling radius of a 215/45 R18 tyre is 325.4mm, giving only 489 revs/km (6.9% difference), which is far worse
The rolling radius of a 225/35 R18 tyre is 307.4mm, giving 518 revs/km (1.3% difference) which is better, but still a little bit out

P.S. the rolling radius doesn't have much to do with wheel offset - unless you've got a problem with clearance.

euphorbia · « **Reply #9 on:** August 16, 2008, 14:32:32 PM »

Hi, everybody

I need some help! I don't choose 215/45 R17 or 215/40 R17
I know /45 is more comfortable but I affraid it will too big.
HELP PLEASE!!!

Richie86 · « **Reply #10 on:** August 16, 2008, 14:35:32 PM »

i had 215/45's on mine

please add a location to your profile and welcome.

J@zV6 · « **Reply #11 on:** August 16, 2008, 23:56:37 PM »

Quote from: euphorbia on August 16, 2008, 14:32:32 PM

I need some help! I don't choose 215/45 R17 or 215/40 R17
I know /45 is more comfortable but I affraid it will too big.
HELP PLEASE!!!

The rolling radii of the OEM tyres were:
195/60 R14 - 294.8mm
205/55 R15 - 303.2mm
205/55 ZR16 - 316mm* std option on Turbo only
The rolling radius of a 215/45 R17 tyre is 312.6mm
The rolling radius of a 215/40 R17 tyre is 301.9mm
So the 215/40 R17 is closer to the OEM standards, but the 215/45 R17 is still smaller than the OEM R16 option, so it should still fit easily enough.
The rolling radius of a 215/50 R17 would be 323.4mm, which is probably too big and will affect gearing, although I doubt if you would have clearance problems unless the car is lowered.

gorytus · « **Reply #12 on:** August 17, 2008, 06:26:25 AM »

Quote from: J@zV6 on August 16, 2008, 23:56:37 PM

The rolling radii of the OEM tyres were:

205/55 R15 - 303.2mm
205/55 ZR16 - 316mm* std option on Turbo only

The Turbo was fitted with 205/50 not 205/55 profile tyres. Thus rolling radius of 305.7mm

Here is a handy checklist of potential Calibra tyre profiles showing profile, rimsize, rolling diameter, circumference and the all important % difference from OE fit. I used 205/55/15 as a base, keeping within +/- 2.5% of rolling diameter.

205/40 R17 596 mm 1872 mm -1.76 %
215/40 R17 604 mm 1898 mm -0.45 %
225/40 R17 612 mm 1923 mm 0.87 %
205/45 R17 616 mm 1935 mm 1.62 %
225/30 R18 592 mm 1860 mm -2.36 %
205/35 R18 601 mm 1888 mm -0.96 %
215/35 R18 608 mm 1910 mm 0.2 %
225/35 R18 615 mm 1932 mm 1.35 %
205/40 R18 621 mm 1951 mm 2.42 %
205/30 R19 606 mm 1904 mm -0.15 %
215/30 R19 612 mm 1923 mm 0.84 %
225/30 R19 618 mm 1942 mm 1.83 %

I used max width of 225 and min height profile of 30% to maintain Calibra/ human compatibilty & comfort.

J@zV6 · « **Reply #13 on:** August 18, 2008, 08:26:27 AM »

Quote from: garyb on August 17, 2008, 06:26:25 AM

The Turbo was fitted with 205/50 not 205/55 profile tyres. Thus rolling radius of 305.7mm

Sorry you're right, that makes more sense. The optional 16" rims on the Turbo were fitted with /50 profile tyres. Doh!

C20 LET
205/50 ZR 16
205/55 R15-87
195/60 R15-87

Quote

Here is a handy checklist of potential Calibra tyre profiles showing profile, rimsize, rolling diameter, circumference and the all important % difference from OE fit. I used 205/55/15 as a base, keeping within +/- 2.5% of rolling diameter.

205/40 R17 596 mm 1872 mm -1.76 %
215/40 R17 604 mm 1898 mm -0.45 %
225/40 R17 612 mm 1923 mm 0.87 %
205/45 R17 616 mm 1935 mm 1.62 %
225/30 R18 592 mm 1860 mm -2.36 %
205/35 R18 601 mm 1888 mm -0.96 %
215/35 R18 608 mm 1910 mm 0.2 %
225/35 R18 615 mm 1932 mm 1.35 %
205/40 R18 621 mm 1951 mm 2.42 %
205/30 R19 606 mm 1904 mm -0.15 %
215/30 R19 612 mm 1923 mm 0.84 %
225/30 R19 618 mm 1942 mm 1.83 %

I used max width of 225 and min height profile of 30% to maintain Calibra/ human compatibilty & comfort.

silky07 · « **Reply #14 on:** August 18, 2008, 08:44:51 AM »

ok im sorry but im totally lost with all of this lol .....

ok i want to put 17" on my calibra 8v is this off set to do with the rims or the tyres

thanks .

gorytus · « **Reply #15 on:** August 18, 2008, 09:42:11 AM »

The offset is relating to the wheels - read the first post in the thread as from the second post onwards, all posts are discussing the correct tyre profiles to use.

To briefly summarise look for rims that ideally have an offset of ET49, failing that try and ensure at least in the 40s. ( Personally I wouldn't drop below 42).

silky07 · « **Reply #16 on:** August 18, 2008, 09:45:27 AM »

thanks alot dude. much apreaciated.

gorytus · « **Reply #17 on:** August 18, 2008, 09:59:36 AM »

Quote from: silky07 on August 18, 2008, 09:45:27 AM

thanks alot dude. much apreaciated.

No worries, remember to check PCD ( the spacing on your wheel bolts) too.
Yours will be 4x100, a relatively rare spacing. I think some Honda & Subaru models have that spacing.

silky07 · « **Reply #18 on:** August 18, 2008, 10:12:33 AM »

thats just great lol where on earth am i going to find alloys if everything is rare. lol

gorytus · « **Reply #19 on:** August 18, 2008, 10:43:53 AM »

Flip through online Calibra galleries, those showing club events are a good source.

Visually, my favourite 17s on CC are angelmouses. They are 4 stud; obviously your tastes may differ though.

Author Topic: Guide to Wheel Offset (Read 2882 times)

Rodders

buchanan_vaux

p.m.adams

s.kumaran

euphorbia

gorytus

gorytus

gorytus

gorytus