wheel offset?

getting ready to build a new chassis. I am currently using a 15x11 wheel with 3" back spacing. I was told to center the axle flange in the center of the rim, in other words... ex..15x10 w/ 5"bs. 15x12 w/ 6' bs and so on. some thing about axle deflection, the cars weight is not centered on the rear wheels or some thing like that. What is the proper way? what I was told or personal preference?

The old school thinking was to center the axle flange-to-wheel surface in the center of the wheel/tire. The thinking behind this did involve worry about axle/flange deflection, bearing life, etc, etc.

But now days with 35 & 40 spline axles, housing back/top/bottom bracing, and most everybody using at least a 3.150" OD Ford/Dana/Olds/Poniac sized axle bearing, (and even the super wiz-bang dual bearings) the thinking now is to narrow the housing as much as possible & use a wheel that has very little back spacing. This makes for the lightest housing & axles possible, and the narrower the housing the less likely it is to bend/flex/deflect under loading.

You do however want to leave yourself some future fudge room on the back spacing numbers if a bigger rim/tire might also be used down the road. So lets say your thinking you might be needing both 15x14 & 16x16 rims in the future (for whatever reason). And the rim company you want to use offers back spacing in 1" jumps from 3.5" bs up to 6.5" bs. In this example you would build the rear housing to fit the 15x14 rim with the 3.5 back spacing and use a little deeper back spacing for the 16x16 so when mounted on the car both tire/wheel sets will both have the same sidewall-to-body clearance on the street side.

If done the other way around & the least back space is used on the bigger rim, the smaller wheel/tire set will sit way too far under the body.

Like what was said above, that was the "old" way of thinking. With the wheels getting better and huge axles I think it boils down to just personal preference. We run 15x12's with a 6 5/8 bs on our ride. One thing about backspacing is if you want double beadlocks then you need to talk to the wheel manufacturer that you prefer and decide what they are willing to make. Some wheel companies will only go to 3.5" on dbl beadlocks. If beadlocks are not an option then you are limited by your imagination or what the company is willing to make.

Thanks guys for setting me straight. I run 35 spline Moser axles. When I build the new chassis I will make sure the frame is narrow enough for the future growth in wheel size. I can fit a 29x15.50 sportsman pro under it on a 12 rim with 3" bs. 30" dia tire is all the bigger I can go ..unless I stretch the openings, which wasn't an option a few years ago <----fresh paint.

Go with as short a backspace as possible..........less unsprung weight, and less rotating weight.

feetfirst wrote:Go with as short a backspace as possible..........less unsprung weight, and less rotating weight.

explain that more please. I am still learning.

Shorter axles shorter axles tubes less moving weight.

Northwest outlaw wrote:Shorter axles shorter axles tubes less moving weight.

Yes,

I am running a 8.8 which measures 46" housing flange/housing flange, ABSOLUTE shortest possible axle assembly with stock suspension in stock location.
This is 8.5" shorter than factory, not sure on the weight savings......

that makes since. these heavy ass peterbilt style 40 spline axles are pretty heavy even gun drilled. I was also limited on my rear width due to the stock mounting points. Thanks for clearing that up. never really thought of it that way.

whatbumper wrote:these heavy ass peterbilt style 40 spline axles are pretty heavy even gun drilled.

That's not always true these days.

If you go with MW's stronger 300 M material for the 40 spline axles, you get to machine an even larger gun drill I.D. I swapped out a set of regular MW 35 spline axles for their lightest 300 M gun drilled 40 spline set on a guy's car & the 40 spline axles were surprisingy lighter than the old 35 spline set.