Why the small rim?

Hello all,

I been reading everyones mods and noticed that nearly everyone who runs a big tire (31-33)runs a "narrow" rim with it(7-8") why? The reason that this seems odd to me is because before I purchased my money pit, I mean xj, I ran my toyota with 33's and 10" rims. And everyone that I know prefered this combination because of the wider footprint, not to mention the fact that it is a hell of a lot easier to mount the sensor from the alignment rack on a wheel that isn't submerged in rubber, and the fact that I could run a lower psi with this set-up on the trail. Is there a mechanical reason why the narrow rim is necessary or is it just preference? Please enlighten this young XJ Rookie.

In most situations a narrower rim will hold a bead better at lower PSI than a relatively wider rim. So typically people running up to 12.5" wide tires will use 8" wide wheels. Wider tires will be more common on 10" wheels.

Of course, there are always exceptions, but generally speaking this is the case.

Actually, you can run lower psi with a narrower wheel. Also the more sidewall bulge created by the narrow wheel protects the wheel better than a wider wheel combo. Another reason is with the wheel wells designed like they are the there is greater clearance with a more narrow wheel ( with proper backspacing). To get the proper backspacing on a 10" wheel would cause severe rubbing on the springs, LCA's and wheel well compartment.


Everything but the PSI issue isn't a problem with Toy's. ( I had one before as well and ran 33's on 15x8 w/ 3.25 BS.)

Does thaT mean i can run 31x10.50's on a stock 1990 cherokee's rims?

Sure. I have 31 x 10.50 on a spoke stock rim and am abaout to go to a 33 x 12.50.

I run 36x12.5x15 tsl on a 7" wheel. I have had them as low as 8psi with all the weight on the side wall, and no lost bead! Once I do pop a bead I will buy bead locks. But until then i like the skinny wheel and fat tire combo. If you street drive alot you will have problems with the tire wearing evenly across the tread with this set-up.

Also the narrow rim keeps the edge of the rim bead protected from rocks.

I have always noticed that the Yota guys around here like to run wide rims. It always looked to me like the tire was about to blow off the rim. I couldn't imagine airing down with that setup.

Plus the narrower/more offset rims help to keep the front axle's unitized hub bearings alive much longer than wider/less offset rims will.

I run 31x10.50x15's on my stock 93 sport steel rims, no probs but a little LCA rub and that can be cured (so i hear) with WJ LCA's cuz they are slighty bent in and more of a tubular shape so they seem a little stronger and dont rub like factory XJ arms
- Aaron

Thanks for the education guys. Ill keep it in mind when I buy my new blackees.

Just FYI not starting a debate.
When I shopped for 33x12.50 some shops refused to mount them on anything narrower then a 10" rim as this was the mfg specs for that size tire. I'd make sure if you get tires with a warranty (lots of mud tires have no mileage warrenty) that putting them on a 8" rim won't void that warranty.
I personally went with 10" rims as I liked the wider footprint and more stability I still have another year or so with the XJ as a daily driver and I don't want to burn the tires up to fast.
Barger

Stability I can possibly see, but wider footprint? That is twice that has come up from people running wider rims. A properly inflated 12.5" wide tire will have the same contact area on a 10" rim as it would on an 8" rim. Yes, if you overinflate your tires, you will see only contact in the center, thus a smaller footprint.

But if you keep and check your tire pressure regularly, they are exactly the same exact tire with the same footprint.

my 35's wouldnt ride flat on the 8" rims even at 18lbs on the street. Started to feel wiggle and wobble like that. Went to 10" rims and have a real good contact patch now and can keep a good amount of air in them. but.... havent tried airing down yet. My friend had his tj with 35"bfgmt's on 10" and now 9" wide rims and regularly pushes them off the bead when aired down with both sizes.

As with all tire discussions keep in mind almost every single different tire will be different and that even then what wheel they are on will make them even more different. I run my 36x12.50 TSL's on my CJ5 at about 8lbs. on the trail on custom made 2" BS 10" Stockton Explorer steel wheels http://members.aol.com/realoldjeep/newpics5.html and have yet to pop a bead even when down to 6 lbs., now the fact that the stockton's have a very deep bead lip and the TSL's have a very stiff bead are the major factors here. Some aluminum and steel rims have shallow bead lips and the stiffness & thickness of the bead varies from brand & even model within a brand.... so some combo's may give you problems even when running upwards of 12-15 psi.
Just something to think about,
Mike B.

I work as a tire sales guy for a living. All the big manufacturers (Michelin, Goodyear, BFGoodrich) recommend a wheel with a width 70% of the section width of the tire. The section width of a flotation-sized is easy to calculate since it is just the second dimension of the tire size. For example:

31 x 10.50 R 15 LT

This tire is 31" tall and 10.5" wide. Therefore the optimum rim width for this tire would be 10.5" x .7 = 7.35" or roughly 7.5 inches wide. For a 12.5" wide tire the rim width would be 12.5" x .7 = 8.75".

This should give the maximum benefits for tread life. However, a tire will have less deflection (tighter high-speed cornering) on a wider rim, and better sidewall flex on a narrower rim.

You should never vary from the 70% rule by much. So a 10.5" tire would fit on say a 6.5"-9" rim, and a 12.5" tire gets a 7.5"-11" rim. Narrower is just flat out better for airing down and fender clearance, but wider looks cool and handles nicer on the road.

As far as the quality of balance you will get out of different rim widths, it depends on how the wheels are balanced. There are basically 2 types of balancing methods used: static and 2-plane.

Static balance involves placing one weight group in one spot on the rim. This corrects only straight up-and-down imbalance of the tire-rim assembly. This is fine for the rear, because the tire cannot move wobble side to side as it is held straight by the solid axle.

2-plane balance is placing two weight groups, one on each side of the rim. This balance corrects not only the up-and-down imbalance, but also the side-to-side imbalance (wobble).

If a static balance is used, the wider wheels will balance worse, because the weight is applied only to the inside edge of the rim. This means that the outside of the rim is fairly far away from the wheel-weights and typically, this will cause an even worse wobble. Lots of physics involved in explaining it any deeper, so I'll spare us all.

But the bottom line is: If you use wide wheels, have weights put on both sides of the rim. And beware, you will lose the bead easier at low pressures.

Anyways, enough rambling. Just remember the 70% thing and my life's purpose is complete.:angel:

The stock rim on a 2000 xj is 15x7 right? So would i be able to run 32x11.50-15s on them??
please help

im running 31x11.50 on stock 7"wide rims, with no problems