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Wheel width and offset

Charles Johns

Donation Time
I just checked my 14" wheels before removing the front suspension, and found they are too wide. The original steel wheels have 4" offset but mine have 3.5" offset and are 6" wide. I do not want to flair the fenders but do not want 13" tires either. I see 15" on this site but do not know what the offset is. I would prefer custom wheels but one must take what one can get sometimes. 15" with 4" offset look like they would work but a 4.25" bolt pattern with 4 lugs seems rare in custom rear wheel drive wheels. I'll check the FAQ section but last I looked it was BIG, BIGGER, and BIGGEST for racing. Any ideas?
 

Bill Blue

Platinum Level Sponsor
I used Saab 9000 wheels. I used 1 5/16" offset, 6" wide 15" wheels. 5.5" wheels are much more common and have the offset adjusted so the 5.5" and 6" wheels mount the same. Saab wheels are 4 on 4 1/4", the only problem is the Alpine hub is about sixteenth of an inch larger than the hub opening. Saab used a lot of different wheels. You can get an excellent rundown on styles and fit here. You can find some very nice used Saab wheels.
http://jpowell.tripod.com/saab-wheels/

Bill
 
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Barry

Diamond Level Sponsor
I just checked my 14" wheels before removing the front suspension, and found they are too wide. The original steel wheels have 4" offset but mine have 3.5" offset and are 6" wide. I do not want to flair the fenders but do not want 13" tires either. I see 15" on this site but do not know what the offset is. I would prefer custom wheels but one must take what one can get sometimes. 15" with 4" offset look like they would work but a 4.25" bolt pattern with 4 lugs seems rare in custom rear wheel drive wheels. I'll check the FAQ section but last I looked it was BIG, BIGGER, and BIGGEST for racing. Any ideas?



The original steel wheels do not have anywhere close to 4" offset. Backspace maybe, but certainly not offset.
 
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Charles Johns

Donation Time
Barry, my use of "offset" may be wrong but the distance from the outer rim to the mounting surface is what I reference. In other words, how far inward does the rim set? My steel wheels measure 4" from the inner mounting surface to the inner rim. BACKSPACE or OFFSET? With open-wheel Hot Rods it was not a problem, but simply determined the width of the footprint. With a sportscar and limited area under the fender, it makes a difference. THAT is why I am on this site...so you guys can learn me something. Learning never ends.
 

Fergusonic

Donation Time
I used VTO Performance, 15 x 6 4 x 108 15 mm + offset with 195/55R15 Tires. No rubbing / no trimming.
 

Barry

Diamond Level Sponsor
I have posted this before, but the forum search function is apparently not getting a lot of use.

Backspace and offset are not the same thing. Zero offset means the wheel mounting flange is centered in the rim width. Positive offset means the wheel mounting flange is moved toward the outside of the wheel from the center line and negative offset is just the opposite. Backspace is the distance from the wheel mounting flange to the innermost part of the wheel and is equal to one-half the overall wheel width (not rim width) plus any positive offset and minus any negative offset. A typical 6" rim width wheel has an overall width of about 7" and with zero offset will have a backspace of about 3-1/2".

At the front of a Series Alpine, the limiting factor on backspace is clearance between the wheel / tire and the outer end of the upper A-arm. On my S-V, it is 4-5/8" from the plane of the front wheel mounting flange to the outer end of the upper A-arm and the tip of the A-arm is 8" above the wheel spindle center line.

For a typical 13" wheel on a Series Alpine, the safe backspace is about 4" and the maximum backspace is about 4-1/8".

For a typical 14" wheel on a Series Alpine, the safe backspace is about 4-1/4" and the maximum backspace is about 4-3/8".

For a typical 15" wheel on a Series Alpine, the safe backspace is about 4-1/2" and the maximum backspace is about 4-5/8".

It's an Alpine, so you might (or might not) get away with an additional 1/8" of backspace.

For typical 16" or larger wheels, the inner wheel flange is probably above the outer end of the upper A-arm and the backspace can be considerably greater.
For typical 6" rim width wheels:

A backspace of 4" means an offset of about +12.5 mm (+15 mm should be OK).

A backspace of 4-1/4" means an offset of abut +20 mm.

A backspace of 4-1/2" means an offset of abut +25 mm.

It's an Alpine, so your measurements may vary.
 
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Charles Johns

Donation Time
Barry, THANKS for the clarification. I am now up-to-speed on wheel terms. This is where sportscars and street rods differ in wheel terms. The terms are the same but their usage is not as troublesome on a fender less street rod or a fat-fendered old car. My forty Fords had plenty of room under those fenders, and our worry was loading the outer bearings on daily drivers.
 

alpine_64

Donation Time
Changing the offset on a wheel changes the scrub radius of the suspension and also the leverage of the vertical travel on the suspension and its geometry.

While on a street rod it may not have interference issues with bodywork if it runs open or woth cycle guards it would certainly impact the suspension and handling the same as it does im any other situation.
 

Bill Blue

Platinum Level Sponsor
Barry, if used as presented, the site search function is worthless. You may know of a way to make it useful. I sure don't.

Bill
 

65beam

Donation Time
FYI, VTO wheels has a chart on their site showing the wheel offset of their wheels for all Alpines from 1959 thru 1967. They also have a chart for Tigers. They do not show the back set on this chart.
13X5.5 19mm
13X6 13mm
14X5.5 22mm
15X6 24mm
 

Charles Johns

Donation Time
One of the reasons I am on this site is the volume of information available. I am not an engineer but I have read their books and have several on suspension, brakes, steering, wiring, etc. With limited knowledge I am not embarrassed to ask NOW while building. The number of cars I have seen with Ackerman problems, scrub-line troubles, bump steer, unsafe mounting issues, and more, tells me it is better to ask and expose one's lack of knowledge on a subject, than not ask and expose one's lack of wisdom. For that reason and others, I tend to stay close to what the factory built but with proven known improvements. Barry's OFFSET-BACKSPACE explanation taught me something I had never truly studied. Now I know, and with the Sunbeam and maybe clearance issues, I will research more before making a wheel-tire choice.
 

Barry

Diamond Level Sponsor
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Charles Johns

Donation Time
I went to the search forums, clicked on...looked for wheel width and offset...got many answers. Unfortunately I first looked for FAQ and did not find any. NOW I KNOW! Thanks Barry. My research continues.
 
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