Wheel/Tire recommendations for Isuzu NPR?

[Psyched]

I'm seeking recommendations for wheel and tire size recommendations for the family adventure rig I'm building. The vehicle will take our family of six on a one-year adventure exploring North America, then we'll use it as a work/farm truck. We'll do lots of highway driving so comfort will be important. We will try to drive to the end of every dirt road we find, but I don't plan on doing heavy rock crawling, as I've never really wanted to go anywhere that my stock Tundra couldn't take me (although I've beat it up a fair bit in doing so).

Vehicle details: 2021 Isuzu NPR crew cab that started life as a fence install truck; 4x4 is currently being installed by RidgeRock Expedition Vehicles in Florida, and I live in Nevada. As part of the 4x4 system it will get a 5" lift and 5.13 gears, as well as have the wheelbase shortened to 138". Next it will get an 11' aluminum flatbed, then I'll build a removable camper using composite panels from Boreas. I'm waiting to get the truck in hand before finalizing the design of the camper, but I'm guessing it will weigh no more than 4000 lbs, which would put the weight of the vehicle ~10k lbs.

I won't know the weight on each axle until the camper is installed, so I probably won't choose a final wheel/tire combo until the, but I want to decide on tire size before the bed is installed because I want the bed to sit as low as possible while still allowing for tire clearance. I don't want to set the bed for 37" tires and then later decide to install 33's and have the bed 2" higher than needed; or worse would be to do the opposite!

I'm considering having super single wheels built by Wheels Now, and running Founders AT 36" tires. But that's probably overkill for my use case. I am open to keeping my stock 16" rims and running dual AT tires or even single wheel light truck tires, depending on axle loads. The key right now is deciding on tire diameter to facilitate the bed install.

I'd love all opinions!

 

[rruff]

Cool project! Will be good to see it come together.

Is there a reason you aren't getting super singles in a common size so you can use standard tires? 20" are pretty common, and you could use 37-40" tires. 4k lb per should be plenty. IME in the west, ground clearance is most important (even vs 4wd), as I prefer dry and pleasant conditions, making traction less of an issue. I wouldn't skimp on the tire size.

You will need an articulating subframe of some kind... have you thought about that?

I recall Boreas saying their panels were 60" wide, max... or are you getting them custom? If so, what does that cost?

 

[Psyched]

Cool project! Will be good to see it come together.

Is there a reason you aren't getting super singles in a common size so you can use standard tires? 20" are pretty common, and you could use 37-40" tires. 4k lb per should be plenty. IME in the west, ground clearance is most important (even vs 4wd), as I prefer dry and pleasant conditions, making traction less of an issue. I wouldn't skimp on the tire size.

You will need an articulating subframe of some kind... have you thought about that?

I recall Boreas saying their panels were 60" wide, max... or are you getting them custom? If so, what does that cost?

Thanks for the thoughts. I can order the super single rim in any size. 3800 lbs is about the max capacity of LT tires I've seen, but 19.5 tires can carry 6k+. I wonder if a 20" wheel/tire at max capacity would ride better than a 19.5" at 1/2 capacity and lower psi?

Boreas stocks panels in roughly 5'6" x13", they are ordering 7'4" x 14' for me at the same $/sf. They can order larger, but the cost jumps up due to less efficient packing in the shipping container. I think they place an order 2-3x each year to ship panels up from Australia.

I am definitely open to suggestions on an articulating subframe. At what level of off-roading do you need one? Will a little freedom of movement between the slide-in camper and the flatbed provide enough flex? Teach me!

 

[mog]

and 5.13 gears

You might also want to factor your final drive ratio into your tire diameter search. My 2002 Fuso FG had 5.13 and going to 37" tires helped a lot with lowering the RPM, with the benefit of better MPG and greatly reduced cabin noise (diesel, so maybe an apple to orange comparison with your NPR?). My current Ram (gas), has 40s with 4.88 gearing and the freeway/around town RPM is perfect (IMO), so perhaps run some RPM/tire diameter numbers?
BTW- looks like an awesome project.

 

[rruff]

You are correct, in a 37 an e-load will usually be 3750 lbs at 65 psi. But it sounded like you'd be well under that. And 38 or 40" have a higher rating. The Founders tires you linked are 68mph, 6600 lb @120 psi. I'm pretty certain the 37-40s will ride better and give you more options.

The articulating subframe allows the chassis to flex torsionally, without stressing the box. This shows how much that frame twists.


I articulated my Tundra even though most people don't. I used Energy Suspension soft poly isolators, with the front ones wide, and the rear ones mounted on the centerline, to make a pseudo 3 point. They seem to work great. If you are planning on a passthrough, then 3 point with the front "fixed" is best I think. I like the poly mounts because they allow some movement in all directions, unlike solid bushings, plus they spread the load over the frame much better. Even though the guy from EC told me each one was good for a static load of 800 lbs, I'd stick to about half that. They are ~$30/pair so 14 of them would be $210 and good for 5,600 lbs IMO. I only used 6 of them. We can go over details on that...

Are those panels from StyroMax I wonder?... Are you assembling them yourself?

 

[Korey H]

I met with Rolly a while back and looked at that NRR he has. Good stuff. Seems he has wheel options for you. Curious how much travel you’ll have with your lift. We really want to go to a cab over crew (we are a fam of six plus dog) currently in a c5500 self built expedition truck (@wehartstravel). Always interested to meet and follow other travel fams. Do you have a social and or build thread? Excited for your rig.

 

[Psyched]

I met with Rolly a while back and looked at that NRR he has. Good stuff. Seems he has wheel options for you. Curious how much travel you’ll have with your lift. We really want to go to a cab over crew (we are a fam of six plus dog) currently in a c5500 self built expedition truck (@wehartstravel). Always interested to meet and follow other travel fams. Do you have a social and or build thread? Excited for your rig.

@Korey H , we live a pretty analog life. I tried to post on IG (@TandemFamily) and Facebook for our first sabbatical (bicycle touring with six kids). I learned that social media isn't my thing, but I did enjoy making an short film that played around the world.


However, I do love to connect with people in real life. So if we're ever in the same state lets get together!

I don't think the lift on the NPR will give us any more travel, as it Rolly doesn't do any other suspension mods.

 

[Korey H]

@Korey H , we live a pretty analog life. I tried to post on IG (@TandemFamily) and Facebook for our first sabbatical (bicycle touring with six kids). I learned that social media isn't my thing, but I did enjoy making an short film that played around the world.


However, I do love to connect with people in real life. So if we're ever in the same state lets get together!

I don't think the lift on the NPR will give us any more travel, as it Rolly doesn't do any other suspension mods.

Followed your IG. Ever get to the Midwest we’re always interested to meet other travel families. And I’d love to see your rig.

 

[Psyched]

... I used Energy Suspension soft poly isolators, with the front ones wide, and the rear ones mounted on the centerline, to make a pseudo 3 point. They seem to work great. If you are planning on a passthrough, then 3 point with the front "fixed" is best I think. I like the poly mounts because they allow some movement in all directions, unlike solid bushings, plus they spread the load over the frame much better. Even though the guy from EC told me each one was good for a static load of 800 lbs, I'd stick to about half that. They are ~$30/pair so 14 of them would be $210 and good for 5,600 lbs IMO. I only used 6 of them. We can go over details on that...

Are those panels from StyroMax I wonder?... Are you assembling them yourself?

@rruff I don't think Boreas gets their panels from StyroMax, or at least when I asked them if I could order a different thickness they said no. I plan to assemble everything myself following StyroMax's techniques. I'd love to do a passthrough to the cab using a custom Breezer window, but the rear window on the crewcab is only 12" tall, so the useable opening on the Breezer window would only be 7" and only my 10 & 13 year-old kids could fit through that! And if we enlarge the window any lower it will drop below the seatback of the rear bench, and we want all 7 seats available in the truck.

Thanks for the prompt about articulating subframes, I had no idea there was so much flex in the frame! There's so much I don't know. I'd love to learn more about how you envision using the poly isolators for a flatbed mount. Since your post I dove down the rabbit hole of subframes, the Globe Trekker ISO subframe looks very similar to we want to do. I had also considered putting a dump hoist on our flatbed for use as a work truck when not exploring, that would be like a 3-point mount but I don't know how the torsion would affect the lift mechanism.

 

[Korey H]

@rruff I don't think Boreas gets their panels from StyroMax, or at least when I asked them if I could order a different thickness they said no. I plan to assemble everything myself following StyroMax's techniques. I'd love to do a passthrough to the cab using a custom Breezer window, but the rear window on the crewcab is only 12" tall, so the useable opening on the Breezer window would only be 7" and only my 10 & 13 year-old kids could fit through that! And if we enlarge the window any lower it will drop below the seatback of the rear bench, and we want all 7 seats available in the truck.

Thanks for the prompt about articulating subframes, I had no idea there was so much flex in the frame! There's so much I don't know. I'd love to learn more about how you envision using the poly isolators for a flatbed mount. Since your post I dove down the rabbit hole of subframes, the Globe Trekker ISO subframe looks very similar to we want to do. I had also considered putting a dump hoist on our flatbed for use as a work truck when not exploring, that would be like a 3-point mount but I don't know how the torsion would affect the lift mechanism.

In my c5500 build I deleted my rear glass and bonded in an aluminum panel. Then cut the crawl through as large as I could. To get around the seat back issue I sourced Ford super duty jump seats (which I installed 4 across), they fold individually and have built in should restraints.

 

[rruff]

A couple issues with subframes that use fixed pivot points are a) they don't match the natural twist of the chassis perfectly, introducing stresses, and b) these stresses in addition to the ones you get from just carrying the load, are concentrated on a small portion of the chassis. The spring loaded subframes spread out the load better, but have other issues.

These are called "Energy Suspension Universal Body Mounts". I used the soft ones. They go between the subframe and chassis. If you wish to have a pass-through (and I think you can just cut a hole above the rear seats for that) then you will want the front of your habitat to follow the rear of your cab pretty well... which means have the front mounts spaced wide, and the rear ones on the centerline so they can tilt. They do this with very little resistance. You'll need to bridge across your frame rails in the rear with beams.

The frame rails are surely a standard 34" width, so if you position the front ones on the rails that should be plenty of width. It takes a good amount of force to squish these 1/8", so even 1/4" (1/8" up and down) over 34" won't end up being significant at your pass-through opening.

 

[rruff]

Thought this might explain a little better. It's simple... just need to drill holes and insert bolts. The weight is distributed on each of the 8 bed mount locations. The beams are 6061 Al. All of it weighs ~100 lbs. I made the floor/base strong and stiff so didn't need a regular subframe.

You would need more of the isolators with a heavier camper. The ones up front that are wide apart will need to be pretty close together, but the rest can be distributed along the length on the mid-point between the rails.