Planning class 4/5 rig. Am I nuts to use LT tires (dually) ?

[klahanie]

F4/550, 7 1/2 x7 1/2 x 12' box with full king overcab. est 15K lbs. Don't really want to go military SS for a bunch of reasons. Willing to start with a dually set up. Application: x-country RV (lots of hwy), with off highway capability (gravel for camp spots)

When I learned about these 20x8.25" DBO rims

Wheels – Thorson's Extreme Off-Road

www.thorsonsoffroad.com

I thought this would be my solution, used with something like the Nitto 37x11.5R20 LT tire. 37s on 20s provide decent sidewall. Load index is same as oem 225/70R19.5 tires but are only an "E" LR not the "G" the oem light commercial tires have. Plus the 37's have a lot more sidewall and I'm unsure how they will handle sway from the load. Smaller, 35s on 20s are out because I can't find a narrow, high LI size to work in a DRW.

Camper box has grown some from my original concept (mostly taller) and who knows what the final loaded weight will be. Now I'm questioning if the LTs are a non starter and I should go back to looking for a commercial tire solution (265/70R19.5 or ??)

I know folks run 1 ton duallys with heavy, large, tall campers (Host, Arctic Fox etc) but not so much with 37s.

What do y'all figure ? Even if the tire load index covers the weight would LTs be just wrong for this application ?

 

[rruff]

Sounds good to me! I would expect a sway problem since you should be well under the weight rating of the tires.

You can fit 12.5s as well, I assume?

 

[klahanie]

Sounds good to me! I would expect a sway problem since you should be well under the weight rating of the tires.

You can fit 12.5s as well, I assume?

Thanks.

Haven't looked into 12.5 w tires on account of dually spacing. One of the delicious ironys of LT vs commercial tires is yes, you can air down the LTs but you'll need the extra space between the rear tires. And any aftermarket rim/tire pushes the oaw of a dually into bro-dozer status. 🤘🤘

I figure the Thorsens dual spacing for a 1 steel/1AL rim set is ~323mm (~12.72") and figure the Nitto 37x11.5R20 are ~11.2" section width on a 8.25" rim. So call it 1.5" between tires. But at what psi ?? Even that is prob borderline for oem specs - which I can't get for the Nittos anyway...

Looking at ~100" oaw with the above set up IIRC. Already limiting where can drive.

12.5s would need spacers (for me anyways) in which case I midaswell go with semi adapters and 19.5 or 22.5 Alcoas or similar.

 

[LikeABoss]

I briefly had a 4500 on those 37x11.5 nittos. Worked great with my camper setup at about 16k. I had stazworks wheels. 98” wide in the rear.

 

[klahanie]

I briefly had a 4500 on those 37x11.5 nittos. Worked great with my camper setup at about 16k. I had stazworks wheels. 98” wide in the rear.

Nice ! Thank you.

 

[tirod3]

Does the truck have a locking differential?

 

[klahanie]

Does the truck have a locking differential?

No.
LSD.

 

[tirod3]

An LSD uses friction clutches similar to the ones in automatic, allowing some different axle speeds side to side when cornering. Under a lot of power they stay locked up in a straight line. I have a ratchet locker which is much less refined, it does the same, keeps one axle from rotating a lot faster which results in a spinning wheel.

Airing down can help some when the going is real greasy but if its gravel roads wont be needed, much less airing two extra tires up later. And, airing down reduces the load rating, and also leads to a small increase in popping the bead, or contact with a dual as was suggested. The LSD is helping more than airing down, other than ride "quality" low pressure on dry gravel roads doesn't contribute much.

YMMV, try both back to back, you can only go so far with duals, tho as the combined weight capacity isnt additive, it is typically less. IIRC its heat that lowers it when in close proximity. Aired down I dont think the load rating would be too little but that should be looked up to know.

 

[klahanie]

Airing down can help some when the going is real greasy but if its gravel roads wont be needed, much less airing two extra tires up later. And, airing down reduces the load rating, and also leads to a small increase in popping the bead, or contact with a dual as was suggested. The LSD is helping more than airing down, other than ride "quality" low pressure on dry gravel roads doesn't contribute much.

Pretty much the only reason we've aired down - ride comfort on gravel. (altho, noted, if overinflated the front tires will wander on highway and will be adjusted).

Comfort needed because our rig the spring rates are too high (my bad). F350 SRW >12K lbs. 10 psi out of the tires makes a big difference. Traction on soft surfaces hasn't really been an issue.

I expect duallies will be a pain to adjust pressures. Ideally I'd find a tire that correct hwy pressures would also work on FSRs. The Nittos I mentioned, I figure I would run ~45 front / ~55 rear. Not too shabby. Wouldn't be looking to air those down out of the gate.

Commercial truck tires in the .5 sizes seem to be min recommended psi of 70, 75, 80 etc. From my limited experience and expected use, that's a lot for off highway. For me tho it's not that I want to "air down" to 20 psi or something. "Airing down" is for comfort at 50, 65 kph etc.

Michelin has an exception with their XDE2+ 265/70R19.5 size (34.4 OD). That one, the mfr states 65 psi min. Still enough load rating for me and could maybe live with that pressure off highway (or cheat a bit).

LT tires OTOH can be driven with lower pressure (load rating provided). It's whether they'd be right for the application, IDK ?

Commercial tires are reputedly tough, which would be a plus (I'd be happier with a single spare, for eg), but drivability might not be right for me ??

 

[HMT]

It’s a shame these tires aren’t available in a 20” wheel.

Super Single — Founders MT Radial

www.foundersmtradial.com

 

[klahanie]

It’s a shame these tires aren’t available in a 20” wheel.

Super Single — Founders MT Radial

www.foundersmtradial.com

Yes I thought they looked pretty good, and would run their 19.5 rim but the 68 max speed gave me pause. And tho the max load is good (6610 lbs) it's rated at 120 psi. Midas well run commercial tires.

 

[klahanie]

I'm now thinking of turfing the 37" LT dually tire idea and going to commercial, ss set up with adapters and Alcoa rims. Say a 295/70R19.5 (40").

No sign of 2025 Superduty order bank opening so plenty of time to percolate.

Currently thinking 7.3 gas but 40s with a large camper is gonna drink fuel.

 

[rruff]

Say a 295/70R19.5 (40").

Those are < 36".

If you are worried about fuel consumption, stick with your original idea of LT... and get tires with low rolling resistance. Size has nothing to do with it.

 

[klahanie]

Those are < 36".

If you are worried about fuel consumption, stick with your original idea of LT... and get tires with low rolling resistance. Size has nothing to do with it.

Sorry, I meant 295/75R22.5

So, if I put on a tall pizza cutter with a long haul highway tread I shouldn't see much of a mileage hit.

Good to know. I may be relying on that, thanks.

 

[rruff]

So, if I put on a tall pizza cutter with a long haul highway tread I shouldn't see much of a mileage hit.

I expect a heavy duty tire with high psi and load, and low speed rating, to have high rolling resistance and high fuel consumption. That's why I thought the LT would be better, not worse! The commercial tires are made to haul a lot more weight than you need... that's the issue. Maybe that is something you can find data on... ?

Contrary to popular belief, larger tires with the same construction and compounds tend to have lower, not higher Crr. Larger diameter, greater width, lower profile are good for Crr. Hysteresis is the problem; how much internal friction is generated as the casing distorts and springs back. Speed ratings are temperature limited so that can give you an indication.