Wheel Width for 12.5" tire

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Glenn-BJ74

Glenn-BJ74

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I have 33X12.5R15LT on 8” wheels and notice my tires are wearing more in the middle and less on the outside and inside edges.

After reviewing tire manufactures’ wheel width recommendations for a 33X12.5R15LT tire they list 'approved wheel' width range of 8.5” to 10”. Well they don’t make a 15x 8.5” wheel, so does that mean the 8” wheel is actually too narrow and 'not approved' and I should be using 10” wide wheels. The LT 315/75/R16, which has a cross-setion of 12.4" - they recommend 8.0" to 10" wide wheel, which are available.

I know many run 8” wheels and 10” with 12.5” tires, and I was wondering what wear and performance differences are showing up on the tires mounted on these two different wheel widths.
 
measured 8"
 
Glenn-BJ74 said:
measured 8"
Click to expand...
...from bead seat to bead seat?

(width inside the rim, the inner surfaces where tyre sits against the rim, less than total outer width of rim)
 
i have found that different brands have different heights and widths even though claiming the same.
i like 10" rims for the 12.5 but you should not be seeing undue wear.
do you do a lot of highway driving with this rig? higher speeds equate to heat and heat to quicker wear...
 
The two reasons i know that cause tire wear in the center like that are over inflation and excessively large tire size on a smaller wheel. I don't think your tire/wheel package should cause problems.

Something I've wanted to do, is take my rig to get to get weighed. Take the weight and divide it by the load rating of you tires, say all four tires combined at max psi can carry 8000 or so. Say your cruiser weighs 4000. Our tire pressure should be about half the max tire pressure. If you daily drive take the weight of what you carry, when you go out on the trail take that weight of that load. Might want to add some weight for passengers.

I would take a guess that running at 30-34 PSI is still to much. I currently run 35PSI on 285/75's, I might take it to the Flying J's next week and see what's up, I'm curious.:cheers:
 
proper way to set tire pressure, draw a number of chauk lines across the tire, drive down the road. if the center wears first then too much air pressure, if the outside wears first too little. when you have the chauk wear evenly then you have the proper tire pressure...
easy,
 
I have always wondered how to determine the correct air pressure with oversized tires – so I will try that chalk test.

My BJ74 loaded on a long trip, 40 litres spare fuel, food, gear, ARB fridge weights just under 6,000 lbs, about 51 % and 49% front (each axles weighed on DOT scale). The 33X12.5R15 has a maximum weight rating of 2,205 lbs. So 6,000/4 = 1,500 per wheel load, which is 1500/2205 = 68 % tire capacity. So at maximum load I am using 68% of the tire weight capacity . So what air pressure should I run at full load ? That is the question – and I bet many others would like to know as well.
 
you can sit with your calculator and play away but you will not end up with the proper result since each brand have stiffer or softer sidewalls, tread patterns etc

<but i love to watch the geeks at work>
 
I like the chalk line. Good idea! I've also heard using a business card. At proper inflation the business card should be able to slip under the outside and inside of the tire a 1/4 inch.

you can sit with your calculator and play away but you will not end up with the proper result since each brand have stiffer or softer sidewalls, tread patterns etc
Click to expand...

While i do agree there is practical and theoretical applications, and they all have there place. If your tires are the same make and model follow the manufactures ratings, the manufacture works this into there rating. For Example:
  • Truxus tires 33 x 12.50 - 2250LBS @ 35 PSI
  • BFG KM2's 285/75 16 - 3750 @ 80 psi


Glenn - 68% of the max PSI of 35PSI is 24 psi.

Seems a bit low, but I bet if you do it and check with one of the other methods you tire will last a bit longer. Worst case you'll wear out the outside tread! Keep us posted!
 
yah, yah, that is why i get along with engineers so well...
<surprisingly, some of my closest aquaintences are engineers>
 
Why don't the tire manufacturers tell us the best tire pressures for the axle load by tire size? I guess they assume we too stupid to know what our vehicles weigh. Vehicle manufactures do that, but for the only OEM size tire. I guess a liability issue.

I will try the non-geek chauk method then
 
After reading about tires, wheel width, and their effects on sidewall flexibility I have come to the following conclusion. The reason my tires are wearing in the middles is that I am over-inflating the tire to compensate for the excessive sidewall flexibility that is partially result of running 8” rims, which are a little narrow for a 33X12.5.

“A rim that is too narrow in relation to the tyre width will allow the tire to distort excessively sideways under fast cornering. On the other hand, unduly wide rims on an ordinary car tend to give rather a harsh ride because the sidewalls have not got enough curvature to make them flex over bumps and potholes.”

Car Bibles : The Wheel and Tyre Bible Page 3 of 4

So I need a slightly stiffer sidewall, which I hope to get in a 35X12.5 which has a higher load rating than the the 33s I have now.
 
The answer is to use a 16" rim and the stiffer/stronger tires that come with using 16" rims... 285 75R16 or 315 75 R16 is a good bet for what you need. I prefer 255 85 R16, however.

8" wide rims for your purpose is correct to protect the rim from rock and terrain damage. 10" wide rims are better for floatation and where the rims won't see bead edge damage (snow).


~John
 
I would if I could - but I have looked for alloy 16" x 8" rim with 3.5" backspacing with no luck.
 
so then your post begs the question... why are you going so fast around the corners?

i agree with John, the 06 rims do allow for stiffer sidewalls but also lead to a harsher ride due to less flex.
for me, it is what ever is in the pile at the time. i have 35X12.5X15 on 10" rims, tracks beautifully, rides very smooth and corners well. i have run the same tires on 8" rims with the same results.

maybe you need different tires?
 
Glenn-BJ74 said:
I would if I could - but I have looked for alloy 16" x 8" rim with 3.5" backspacing with no luck.
Click to expand...

I know you want alloys for overall weight and balancing, but with a better tire you won't have the same balance issues as you and before. A rim and tires is un-sprung weight - and it is not necessarily a bad thing.

There are lots and lots of people out there who won't touch an alloy for off-roading off the beaten track because once you've damaged the bead edge, you're pretty much done. At least with a steel wheel you can beat it back into place.

There are some alloys out there that work... I see them appearing on Cruisers around here from time to time. Also, backspacing is not as much of an issue (tie rod end clearance) with 16" rims. The only real spacing issue that comes up is inner wheel well clearance.

If I had my choice: 255 85R 16 Toyo Muds. If you need wider, then 285s or 315s.

Just some more thoughts....

~John
 
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