Technical Wheel Offset For Dummies Like Me (Up Date)

As I said on earlier posts, offset and backspace together give useful information. Either one on its own doesn't give you the full picture and is "practically useless".

 

Well all I can say , My rims / wheels where custom made , had to pay full price in avance No refund No return
"" All On Me "" & I had to supply all measurement , I think @ Time each wheel was $700 each
My self I only used a measure tape ,
& one Square ,,, No card board & No tire
"" I did not or do not have the Tool ""

 

All so to add to post above ,
I also was dealing with Fenders & odd
Shape inner well

 

That's how I figured mine out too, even though I referred to the positive/negative thing backwards

 

The only mistake I made was assuming everyone knew that you use the total wheel width when measuring backspace.
I didn't realize this was such a mystery to so many people. The difference between advertised wheel width and actual measured physical wheel width seems to be where most get lost. This became obvious as the thread progressed.

I have corrected my comments in post #44. Thank you for bringing it to my attention....

 

@swade41
That Cragar & tire looks perfect stance to my eyes

 

I'd like a little more rubber and have bought Carpenter's tub so I might have to go to a custom offset.

 

The Micky Eyes 50 , where like
12 wide ? &
I believe the Pro Trac's
275/60/15 was the biggest of 275 compared to other manufacturers,
Not all manufactures of the 275 are the same size, advertised sized as 275 /60s

 

Yep. 275mm is the sectional width, so 275mm divided by 25.4 (mm in an inch) = 10.827"
Every brand is slightly different, so 10.70" is likely correct for that brand of tire.
There can be a lot of variance between companies, and sometimes things are mixed up between sectional width and tread width. These are two different things.

 

No idea. The first set of tires I purchased were metric so that’s what I cut my teeth on.

 

We used to run Pro Trac L60s on the old truck...this was in 1979. They seemed huge at the time. And it was amazing how fast they wore out with teenage drivers!

 

most factory wheels shoot for 0 scrub radius

 

No rear steer on this one.

 

These were a 275/60/15 on an 8.5 wide rim, my understanding was it was the same as the old L-60

 

I just remembered these were L-60's, definitely look bigger but they were on a 10 inch rim too

 

I guess I could go measure the L60 Formula 1 Super Stock tires hanging in my shop

 

They don't, really. Most front ends have a little bit of scrub radius designed in.

 

To further confuse the issue....

Is there a technical reason to center the wheel? I have always shot for center, assuming that it would help spread the load. It was backwoods math, so I dunno if there is any merit at all. Just one of those things I have always done.

 

Some people claim that it's easier on the bearings, whereas something with a large negative offset (lots of wheel lip to the outside) increases leverage and is hard on bearings. In my experience, I've found that my bearings survive just fine regardless of wheel offset.

 

I've always wondered. Never tried it.

 

The theory says that bearing stress would be increased, but it is too hard to measure in a real world driving situation because there are just to many variables at play.

 

Pushing a tire/wheel centerline out beyond the stock track width also increases the leverage against the shocks possibly making them less effective.
Don't ask me how much, though. I'm no rocket lever surgeon.

 

It wouldn't on a solid axle. You could have the track twelve feet wide and the relationship of spring base, spring rate, damping rate, sprung mass, motion ratios, roll axis location, roll moment arm, etc. would be exactly the same as before. Think of the axle as attached to the ground and it should be obvious.

On independent suspension it'd influence the roll centre, on most types slightly. Only on a swing-axle setup would it affect motion ratios.

Increasing the track width by whatever means would reduce load transfer for any given lateral acceleration, all else equal.

 

If you're considering both wheels moving exactly the same up and down travel then you're right for a solid axle. When one wheel hits something, there would absolutely be a longer lever arm acting on the shock and axle. However, were talking about added inches and not feet.

Wheel bearings and suspensions routinely have to withstand loads much higher than regular operational loads due to bumps, potholes, etc. to a degree, that is known when they're designed. The idea that slightly moving the wheel centre will significantly impact operation is not supported by fact.

 

If you can notice a significant change while driving, then you should be a full-time race car driver.

 

On a 60's Impala forum I used to frequent, a guy was adamant that any change in tire and wheel meant near-imminent bearing failure for your car.

This included changing from bias to radials, 8" wheels with centered wheel mounting surface, basically anything that wasn't stock.

Obviously that claim isn't supported by evidence. Sure, if your system was old and worn out and you put big tires on it that increased steering effort and bearing load you're much more likely to have a failure. In reality I'd have to postulate that the number of bearing failures on vintage cars related to your wheel/tire combo is insignificant compared to bearing failures due to manufacturing defects, improper installation, or insufficient maintenance.

 

Come to think of it: yes. Insofar as the sprung mass is great and the springs are soft we can think of the sprung mass as rigidly attached to the sky (!) rather than the axle attached to the ground. That way a given single wheel bump registers as smaller with a wider track than with a narrower track: a way effectively to improve ride quality as determined by single wheel bumps without any loss in roll stiffness. But ride quality isn't determined solely by single wheel bumps.

 

Damnit Johnny....look what you started

 

Like Monty Python and the swallow debate....