Advice: MAX Wheel/Tire Weight

[Christstopher]

Hello All,

Been digging around looking for info pertaining to the effects of rotating mass and the impact on and limitations by base vehicle. Basically, I'm wondering is there where the 'Too Heavy' line is when it comes to wheel tire combo.

Here's the set-up: Outfitting my Dodge Ram 2500, 4x4, 6.7 Diesel as the base vehicle for my camper. Looking to keep the camper weight under 3000lbs (wet) (the suspension is getting an upgrade and the tires will be rated appropriately). I want to go with 20" rims in order to use the MPT-81 (275/80) and I'm about to pull the trigger on a set of 20"x10", 50/50 offset, two-piece rims off an MRAP. They are vey stout, rated somewhere around 8000Lbs per rim which also makes them 120lbs each. This is about 3x heavier than a stock rim. I'm not quite sure how much the MPTs weigh but I'm guessing it's in the 110lb range bringing the combo in at ~240lb.

How heavy is too heavy? And are there any drawbacks or dangers other than the obvious effects on steering response (not a race car), fuel mileage (who cares), acceleration and added engine load (it's a large diesel)? How significant is the added stopping load given that these trucks were designed with moderately heavy towing in mind? Slotted and cross-drilled rotors are already installed. Also looking to upgrade the front axle 'Unit Bearing' that has a tendency to fail. I'm assuming the added unsprung weight can be accounted for with shock tuning and a quality steering stabilizer? Missing anything?

Just being prudent and I haven't seen much discussion of this topic.

Thoughts and comments are much appreciated. Thank you.

Christopher

 

[JakeH]

The differential internals will be under some serious strain with 1000lbs of wheel & Tire under there. I know those trucks are built tough, but that's a serious amount of mass.

 

[rayra]

Unsprung weight is a factor just like carrying capacity. Ball joints. drivetrain. You're talking about greatly increasing the weights / loads / moment arms in all your drivetrain and suspension components. Are they engineered to cover so much deviation from factory components?
Your solution has to be part of a system, you can't 'solve' with just one component in system.

And what about spares. Carrying at least one? If you need a second, where are you getting a tire that size while you are on a trip?

 

[Mundo4x4Casa]

Christopher,
What year RAM 2500 are we talking about? Once I know that, I'll be able to see where the actual capabilities lie and where the edges are. I wish it were so simple. Before you do anything you might check out my recent article about upgrading wheels/tires/axles/gears/suspension on a Dodge 2500 4WD HO Cummins, expressly to carry a truck camper. jefe
http://truckcamperadventure.com/2017/01/extreme-truck-camper-drivetrain-build/

 

[Regcabguy]

How about some of these?
They make them in 17 x 8.5 x 17 or 17 x 10 x 17. 6.0" backspacing which is perfect for the Rams.
3650# rated with 40's.

http://www.aev-conversions.com/vehicles/prospector#anchor-whl

 

[Christstopher]

Thanks so much for the feedback, all.

Unsprung weight is a factor just like carrying capacity. Ball joints. drivetrain. You're talking about greatly increasing the weights / loads / moment arms in all your drivetrain and suspension components. Are they engineered to cover so much deviation from factory components?
Your solution has to be part of a system, you can't 'solve' with just one component in system.

And what about spares. Carrying at least one? If you need a second, where are you getting a tire that size while you are on a trip?

I will carry at least one spare. The 20" rim size is fairly common in off-road circles (not the MOST common perhaps, but ample options) and most travel will be throughout the US, Canada and Baja, so I won't be straying too far from any global supply chain. The MPT-81 275/80s are the smallest of the bunch coming in at 37", so not TOO over or rare-sized.

Already have bulletproof, easily rebuildable DynaTrac ball joints and as mentioned am seriously considering a much stouter hub/stub shaft/bearing option for the AAM 9.25. Suspension components (control arms, track bars, steering components, shocks, and all the rest) will be either Thuren or Carli, and I have plenty of confidence that their parts are up to the task of keeping everything tracking and steady. I figured that if their components can take the shock load of an 8000lb truck getting airborne and sucking up whoops at 50mph over and over, and then driving home, then they should be able to take the relatively minor abuse my set-up will impose. Please do tear down my assumptions.

Christopher,
What year RAM 2500 are we talking about? Once I know that, I'll be able to see where the actual capabilities lie and where the edges are. I wish it were so simple. Before you do anything you might check out my recent article about upgrading wheels/tires/axles/gears/suspension on a Dodge 2500 4WD HO Cummins, expressly to carry a truck camper. jefe
http://truckcamperadventure.com/2017/01/extreme-truck-camper-drivetrain-build/

Great article, jefe!

And sorry about that. It's a 2008 2500 Long-Bed, quad-cab, 4x4, manual trans. It has the 11.5 AAM rear axle and the 9.25 in the front. I know the front is know for weak 'Unit Bearing' hence, the aforementioned hub/bearing upgrade from SpynTech. Planning on running Carli's HD spring pack plus a set of airbags. Truck weighs 7000lbs with a nearly empty tank.

Curious......what did the Stockton rims weigh? And have you noticed any adverse effects from the moderate step up in rotating mass? Do you have any brake upgrades? Suspension? Sorry if you've posted in other threads, I haven't yet run through your history. I'll do so soon.

How about some of these?
They make them in 17 x 8.5 x 17 or 17 x 10 x 17. 6.0" backspacing which is perfect for the Rams.
3650# rated with 40's.

http://www.aev-conversions.com/vehicles/prospector#anchor-whl

I DO really like these things except, I feel the 3650lb load rating doesn't give me enough safety cushion considering the rear carries the load of about 3000lbs now and I'm shooting for a camper weight of ~3000lbs. I know these things are designed with a safety factor in mind, but I'm not an engineer, nor do i have experience at running rims near their max capacity. My mind is open to evidence and experience that shows I'm being a paranoid



The reason I'm going with the 20" rims and MPTs is that it really is the only combo that meets my desire to have a true off-road tire with the stability and durability (wearability) of a commercial truck tire. (if you know of another, please let me know) They are load rated at around 4800lbs per tire and are reputed to run many tens of thousands of miles longer than off-road radials. I'm mulling these MRAP wheels because they really are the strongest, cheapest option. And I can machine the adapter plate myself even ensuring that they mount as hub-centric. Rickson is an option at 5000Lbs capacity and I currently have a call into them to get some pricing on a set of custom 20"ers.

Thanks!

Christopher

 

[Buliwyf]

The AEV's are GTG IMO. 3650 is plenty. It's very hard to get anyone to rate pickup wheels higher. The highest rated wheels I ever find are Factory.

Sounds like you'll actually be close to needing a dually.

 

[Darwin]

Seems like a bad idea. Personally I would go for 37x17 Toyos for the close to same load capacity but with more sidewall for a better ride offroad, not to mention a lot let weigh than using those heavy rims. But I bet the MRAP wheels look good, just doesn't seem to make a lot of sense given the weight. IF you had a ram 5500 I would consider it.

 

[Ozrockrat]

If you can fit them over the brakes another option is the HMMV wheels. With the 24 bolt rims and something like the Goodyear Wrangler MTR (E) you are looking at 4540# @ 65 PSI and rated for 87 mph. Not as heavy as the MRAP option and easier to get them in your bolt pattern. Easily available everywhere surplus at the moment and Stazworks can sort out any stud pattern and offset you need.

I run the 20" MRAP wheels (365/80R20) and they are a pig to move around. Just the rim itself sucks and even with the freightliner I do not carry a spare complete wheel but just a tire as the combination aired up is over 230#.

 

[cummins-overlander]

I have a 2011 Ram Power Wagon (3/4 ton) and I'll throw my thoughts in.

-I would highly suggest ditching the MRAP rims and going with good Method or Trail Ready wheels. They are very well made, come in the exact sizes and backspace/offset you need, very strong, easy to balance, come in beadlocks. Way lighter. One HUGE reason I would go with those wheels is that you should really stick with 17" wheels. Going to 20" wheels is like taking your ride quality out back and murdering it. Probably the first thing Don (Thuren) or Radio (Carli) is going to recommend is getting rid of those stupid big and heavy wheels. But call Don, he will set you straight. Carli tends to give biased answers to the OC bling truck guys i my experience... I would trust what Don says and go with that. Also, I would worry far more about a tire failure than a wheel failure. If you have a blowout on your single rear wheel, you're in a world of crap, especially since you have the Titanic strapped on top of your truck (3000#? Are you sure?) Aluminum wheels tend to be damaged less than steel. Steel bends quite easily, though I have no idea on those MRAP wheels. Look up "2wagons1driveway" on Dodge Trucks Xtreme - that dude breaks wheels fairly often, but you need to see exactly what he is doing that does and does not break wheels. It's amazing that he doesn't rip suspension mounts off of his frame with what it takes to break his wheels.

-Check out the forum called Dodge Trucks Xtreme. Those guys are crazy, they are the go-fast Ram guys and they are super helpful. Guys there have made your truck into Raptor killing machines. They will help you. Most of them have your same truck and run the 37"x13.5R17 Toyo MT, and love them. Super hard to kill those tires, great traction in most conditions. Those are as heavy as I would ever want to go.

-Your Dynatracs will die under those MRAP wheels and big Continental tires. They are an improvement - they are not invincible.

-TRUSS YOUR AXLE. I know you aren't looking to jump your truck, but most of the time guys kill that AAM 9.25 is from an accidental hit. When I was driving my buddies FJ Cruiser in Escalante, UT, we were just scooting along on a smooth dirt road and hit a drainage ditch at 35mph. I'm pretty sure that if the control arms weren't the nice FJ ones and it had 1st gen Tacoma arms, we would have ripped a wheel off. Just didn't see it. You will find yourself in the same boat, and with the Cummins on the front axle you need to help that axle. Thuren makes an excellent truss that doesn't hurt ground clearance much and works much better than it looks like it will. Carli also makes a truss that costs more than double, but works quite well if you like to use your truck as a Main Battle Tank.

-Put real shocks on that thing. I have King 3.0's up front and 2.5 Bypasses in back. This truck now has less sway with no swaybar on than it did when I had the factory swaybar and Bilstein shocks. Shocks are super important, especially if you have a camper. Your front axle is open to running 3.0's as well, and are the best bang for the buck, but 2.5's rock also. I highly suggest bypasses in the back so you can tune your shocks for the weight of the camper with hand tools, and it gives you a bump zone to help from bottoming out (which will help to save your wheels). Talk to Don (Thuren) about shocks as well, nobody can touch his shock valving. He will set you up, and his shocks are made specially for the Rams so the dimensions are dialed.

-Boogie Bumps from Thuren are great too. They replace the factory bump stops and are a night and day improvement. These go a loooong way in saving your axle from bending and wheels from braking when you take a big hit. It's like landing on a pillow.


Marcus

 

[rruff]

How heavy is too heavy? And are there any drawbacks or dangers other than the obvious effects on steering response (not a race car), fuel mileage (who cares), acceleration and added engine load (it's a large diesel)? How significant is the added stopping load given that these trucks were designed with moderately heavy towing in mind?

The rotational momentum of your wheel/tire is 1x its static weight at the tread, and drops down to 0x at the hub center. In other words it will be a trivial effect on acceleration and deceleration compared to the weight of your entire truck and load. Same for fuel mileage. Frictional losses in your tire suck down your MPG and low speed acceleration. Heavy duty tires with gnarly tread are the worst, but what can you do? Taller gearing will hurt your low speed acceleration also.

I'm going to break from the herd and tell you that 37s on 20" rims are fine. But a 325/70r18 would be better. And 275/80 seems too narrow for a 10" rim also. And there is no reason to go nuts with heavy rims on your rig.

 

[cummins-overlander]

I need to throw this in here haha, this is incorrect.

- Inertia (Not rotational momentum) increases exponentially based on the distance from the hub to the tread. Going from a 31" tire to a 37" tire about doubles how difficult it is to accelerate. Momentum does not change based on any distance, and momentum has little to do with this. Momentum would be used if we were calculating the difference in acceleration after throwing the 4 tires into the bed of the truck, not spinning on the axles. Turning those tires, even with the 6.7 Cummins, is going to friggin suck. Guys that just go up to 37's normally regear their axles to 4.56 or 4.88 on these trucks so the acceleration doesn't totally suck. You will also lose a noticable amount of fuel efficiency, it will make a big difference. Like a really big difference, think going from 16mpg to 11mpg, just with that tire/wheel combo.

- You are going to kill your steering gearbox quickly with that tire/wheel combo. Make sure your box is already upgraded to the Mopar "Big Box". The smaller sector-shaft boxes have a tendency to snap sector shafts and kill people.

- Your factory steering linkage wasn't mentioned, but it needs to be addressed. If you haven't had the tie rod end recalled, you need to do that for safety reasons. I would highly suggest an EMF steering setup, or at least an EMF drag link on top of a 4th gen Tie Rod. Way stronger and will give you a much better feel than the stock setup. The factory setup of that year is known to fail catastrophically, and people have died from it ( The tie rod ends shear off and the two tires go different directions).

- Engine load: Doesn't really matter with what you're doing. It will increase load, but that is a good thing with your engine. You might only strain it if you try to keep cruise control on going up the Eisenhower Grade, otherwise you are good. There are guys towing 25,000lb trailers out there with your truck, you won't touch that.


I really apologize if I'm coming off strong, but you are talking about doing some serious no-no's and you are not talking about some things that really need adressing. I would highly suggest reading through Dodge Trucks Xtreme to see some of the things guys have done to make their trucks reliable and fun. With your year of truck I would highly suggest making a longer list of upgrades or ditching this monster-wheel idea.

 

[rruff]

Inertia (Not rotational momentum) increases exponentially based on the distance from the hub to the tread. Going from a 31" tire to a 37" tire about doubles how difficult it is to accelerate. Momentum does not change based on any distance, and momentum has little to do with this. Momentum would be used if we were calculating the difference in acceleration after throwing the 4 tires into the bed of the truck, not spinning on the axles.

No. Radius is cancelled out by the reduction in angular velocity. He may experience a 2x increase in how difficult it is to accelerate the wheel and tire by itself because of his weight increase, but it is a small fraction of the total inertia of the truck. If anyone would like to figure it themselves, https://en.wikipedia.org/wiki/List_of_moments_of_inertia https://en.wikipedia.org/wiki/Rotational_energy

If you assume that the tire/wheel is a solid cylinder for purposes of mass distribution, then I=m*r^2/2. The rotational energy E=I*w^2/2 = m*r^2*w^2/2. Note that r is the radius of the tire, so will increase a substantial amount, but w is the angular velocity, which will decrease in the same proportion if the vehicle velocity is the same.

I don't know anything about Dodges but lots of people go from 32s to 37s on Tundras and don't bother to regear and have no issues at all. And that's a gas motor. Fuel economy is no worse than trucks with smaller tires of the same design. Of course you will feel a difference in acceleration with taller gearing and tires with higher rolling resistance.

 

[rruff]

I DO really like these things except, I feel the 3650lb load rating doesn't give me enough safety cushion considering the rear carries the load of about 3000lbs now and I'm shooting for a camper weight of ~3000lbs. I know these things are designed with a safety factor in mind, but I'm not an engineer, nor do i have experience at running rims near their max capacity. My mind is open to evidence and experience that shows I'm being a paranoid

So long as you are under the load rating I wouldn't worry.

Are you sure you have 3000 lb over the rear axle now? Should be substantially less than the front. Plus, won't some of your camper weight be on the front axle?

 

[cummins-overlander]

I do want to make it clear that rruff's math is correct, but the assumption is incorrect. I'm speaking from experience, that you would have a noticable, but still livable difference if you went up to 37" tires, with the Cummins. I did make a mistake and was thinking that those tires you are looking at were 41" tires, and that would make a huge difference. You would be fine then running 37's on 20" wheels in terms of acceleration. Still, that is a very heavy tire/wheel combo and you don't have all that much sidewall. Going with a skinnier wheel of smaller diameter goes a long way for ride quality. You will feel a lot of the smaller, choppier stuff with that setup. Big stuff will feel the same.