Rim Width for 35x12.5 tires

[Steelrides]

I'm looking to change from Load E 315/75R16 tires to a Load C tire and looking for help with all the variables. What size rims are you running on a TJ with Load C 35's?

These 16" Load E tires came on it when I purchased it. Seems there aren't many (if any) 16" Load C rated tires, so I'm looking at new rims, too. 15" seem like the most practical since there are a lot of Load C tires in that size. I read somewhere I have to watch out for 15's rubbing the brake calipers on the Rubicon (I believe it came with 16" rims, so does anyone have more info on that?), but 17" could also work for a few tires. My 2006 Rubicon has a 5.5 long-arm and is a play vehicle, not a DD, used on road in the summer and rock crawling in PA or WV 5-10 times per year. My 315/75R16 Nitto Trail Grapplers just barely clear everywhere when fully articulated, so I don't think I'll be going to 37's anytime soon.

Looking at the Cooper STT Pro, KO2, and KM3 and all have specs for their 35x12.5R15LT Load C tires for a Min Rim Width 8.5-11". Are these specs relative to the nominal dimension of the rim (i.e. 15x9 or 15x10) or is it a measured dimension between the beads? There aren't that many 15x9 rims, so I'd be looking at 15x10, but I've mostly read people run 15x8 for better bead retention when aired-down, so I'm confused.

Thanks.

 

[L J]

15x8

 

[Steel City 06]

315/75R16 are very close in size to 35x12.5R15 (except rim diameter). A lot of companies use the same molds, so the tread width and outer diameter are the same.

For either, an 8" wide wheel with 3.75" backspacing is ideal. A 9" wide wheel with 4" of backspacing would also be okay.

Many users run 12.5 wide wheels on 8" rims without any issues. At the lower loads we use them at, it generally does not cause any wear issues, but provides extra resistance to breaking a bead and allows lower tire pressures (as low as 8 psi without beadlocks).

Personally I would choose 15X8 wheels with 35X12.5R15.

I currently run 315/75R16 Duratracs, which are load range E, but I have no issues with the ride, nor do I plan to swap them. They have the same diameter and tread width as a 35X12.5.

The only other consideration you might look into is a 15" rim will limit how large of a brake kit you can install. There are some load range C in a 17" rim, which would allow a much bigger brake kit. However, if you live somewhere flat, it might not be necessary.

15" rims will fit the stock Rubicon brakes and even a 15" big brake kit. If the vehicle already has a big brake kit on it, the smaller rims might not fit.

 

[Mr. Bills]

You can use wheel widths of 8-10" with 12.50" wide tires. The majority advocate 8" in the belief that it helps reduce the chance of a "popped bead" vs. the 10" width, but the evidence of whether it really matters for all but the highly technical trails is purely anecdotal. I have run both widths on various jeeps through the years and have never had any issues with either.

Terrain and locale may impact your wheel width decision. Not everyone is a Southern California rockcrawler or Midwestern mudder or Southeastern root and rut buster. What works best in one situation may not be the best in another. Where I live now, snow wheeling in winter is as popular as summertime rockcrawling. The guys who have been snow wheeling all their lives seem to have no issues with 10" rims at extremely low pressures, i.e. 2-4 psi, and run the same 10" wheels in good weather on the rocks albeit at slightly greater pressures.

Bottom line is that the "8 vs. 10" debate is overblown. It all boils down to your particular needs and your personal preferences. You won't be making a mistake regardless of which width you choose

If I were in your position I would seriously consider 17x9 wheels (a common size) and 17" tires. Appropriate backspacing would be 3.75" or 4". There are many good choices for 35x12.50R17 and 315/70R17 ("metric 35") tires with both C and D load ratings. In many ways, 17's are becoming the new 15's.

If you go to 17" rims you may have trouble finding a wheel you like with the appropriate backspacing. Most seem to have 4.75", 5" or 5.25" backspacing. No worries - just use 1" or 1.25" spacers to net your desired final backspacing. Here is one source among several: https://www.titanwheelaccessories.c...=yr_2006~mk_jeep~md_wrangler~sz_wheel-spacers

 

[Jerry Bransford]

I'm looking to change from Load E 315/75R16 tires to a Load C tire and looking for help with all the variables. What size rims are you running on a TJ with Load C 35's?

These 16" Load E tires came on it when I purchased it. Seems there aren't many (if any) 16" Load C rated tires, so I'm looking at new rims, too. 15" seem like the most practical since there are a lot of Load C tires in that size. I read somewhere I have to watch out for 15's rubbing the brake calipers on the Rubicon (I believe it came with 16" rims, so does anyone have more info on that?), but 17" could also work for a few tires. My 2006 Rubicon has a 5.5 long-arm and is a play vehicle, not a DD, used on road in the summer and rock crawling in PA or WV 5-10 times per year. My 315/75R16 Nitto Trail Grapplers just barely clear everywhere when fully articulated, so I don't think I'll be going to 37's anytime soon.

Looking at the Cooper STT Pro, KO2, and KM3 and all have specs for their 35x12.5R15LT Load C tires for a Min Rim Width 8.5-11". Are these specs relative to the nominal dimension of the rim (i.e. 15x9 or 15x10) or is it a measured dimension between the beads? There aren't that many 15x9 rims, so I'd be looking at 15x10, but I've mostly read people run 15x8 for better bead retention when aired-down, so I'm confused.

Thanks.

I'm running 15x8 wheels with my 35x12.50 tires and for those who air down significantly (not all do) that's the width I recommend. The wider the wheel the more air pressure the tire needs to stay seated between the wheel's seating edges. Keep in mind that a tire's advertised width is neither its tread width nor its bead seating width, its advertised width is measured at its widest point which is half-way up the sidewall. Not many understand that well enough to think the wheel width makes much difference once you're aired down significantly for a tough trail... it does.

 

[Mumblewood]

I’d look into 15x8’s, although I used to run 15x10’s on a 12.5” tires. I could air them down to 8psi no problem but I didn’t like how wide they made my Jeep.

 

[Steelrides]

Keep in mind that a tire's advertised width is neither its tread width nor its bead seating width, its advertised width is measured at its widest point which is half-way up the sidewall.

If BFG advertises a min rim width of 8.5-11”, isn’t that the bead seating width which is the 8 in 15x8? Is that spec to make installation easier and not a recommendation for bead retention?

 

[Steelrides]

Thanks for all the responses. I can find 15x8 +4BS -12OS and 15x10 +4BS -38OS and 17x9 +5BS 0OS, which are all going to keep my tire geometry in the same place. All things being equal, (for the KM3 at least) the 35x12.5R15 is $233 while the 315/70R17 is $328, so I think the 15” may win on that front.

 

[Steel City 06]

Thanks for all the responses. I can find 15x8 +4BS -12OS and 15x10 +4BS -38OS and 17x9 +5BS 0OS, which are all going to keep my tire geometry in the same place. All things being equal, (for the KM3 at least) the 35x12.5R15 is $233 while the 315/70R17 is $328, so I think the 15” may win on that front.

Offset, not backspacing, determines where the tire will be. We use backspacing as a guide because it's easier to understand for most people. You want 0mm offset (ideally -6mm to -13mm) or less for 12.5 wide tires.

For example, a 15x8 wheel with 4" backspacing (0 mm offset) will center the tire in the same location as a 15x10 wheel with 5" of backspacing (0 mm offset). The noticeable differences between the two will be the rim itself is less tucked into the side of the tire on a 10" wide rim vs an 8" wide, and the section width of the tire will increase somewhat, which could cause rubbing issues during steering or flex.

Offset is measured in millimeters, backspacing in inches. (1 inch = 25.4 mm)

Anything beyond about -19 mm of offset (3.25" backspacing on a 15x8 or 4.25" on a 15x10) risks significantly reducing the lifespan of ball joints and front wheel bearings.

 

[Steelrides]

Offset, not backspacing, determines where the tire will be. We use backspacing as a guide because it's easier to understand for most people. You want 0mm offset (ideally -6mm to -13mm) or less for 12.5 wide tires.

For example, a 15x8 wheel with 4" backspacing (0 mm offset) will center the tire in the same location as a 15x10 wheel with 5" of backspacing (0 mm offset). The noticeable differences between the two will be the rim itself is less tucked into the side of the tire on a 10" wide rim vs an 8" wide, and the section width of the tire will increase somewhat, which could cause rubbing issues during steering or flex.

Offset is measured in millimeters, backspacing in inches. (1 inch = 25.4 mm)

Anything beyond about -19 mm of offset (3.25" backspacing on a 15x8 or 4.25" on a 15x10) risks significantly reducing the lifespan of ball joints and front wheel bearings.

Thanks. Good explanation. i was thinking about it wrong. So if my current 16x8 rim has -12 OS, i’ll try to aim around there so the tire stays in roughly the same place.

 

[Steel City 06]

Thanks. Good explanation. i was thinking about it wrong. So if my current 16x8 rim has -12 OS, i’ll try to aim around there so the tire stays in roughly the same place.

Correct. Just note that the section width of the tire increases with rim width. So if the sidewall is close to rubbing, you might run into an issue with a wider rim. The section width on the tire websites usually specifies a "measured" rim size.

The tread width will be unchanged, unless the tire itself has a wider tread width.

Here's a diagram from google that describes offset and backspacing:

The tire will almost always center itself on the wheel centerline, so you want to maintain the centerline location to maintain tire location.

 

[Mr. Bills]

For Jeep TJ's it is best to focus on backspacing and ignore offset.

For a 35x12.50 or a "metric 35" tire you will want 3.75" or 4.0" of backspacing. 4.25" may work, depending on the particular tire and the air pressures you use when aired down.

This backspacing range will work for both 8" and 10" wide wheels.

If you want to play with theoretical spatial relationships when altering wheel width and backspacing, try this calculator:

http://www.rimsntires.com/specs0.js...&et2=42&text1=Stock&text2=Sparco Assetta Gara

 

[Plumber1]

15x8 wheels with 15X12.5x35 BFG KM2's no problems at all and I air down to 10-12 psi and have not lost a bead yet, knock on wood. Don't let those pretty wheels in the first picture fool you.