Are the rear leaf u-bolts one time use? - removing bump stops.

I ordered a set of airbags for the rear for when I am pulling a trailer. The instructions say to shave the bump stops down to be level with the u-bolts. I saw a couple videos where they just loosened the u-bolts and removed the bump stop and mount. I thought the u-bolts are one time use only because od stretching when they are torqued down.

Is there an easier way to remove them other than trying to wedge a sawzall in there and cutting them?

 

I never change it when I install my timbren

 

Folks do reuse them, buy they are in fact one time use as they stretch a little when torqued. I personally have never heard of anyone having an issue reusing them but that does not mean it is the best thing to do.

 

I didn't reuse mine and I only had about 15k miles on them. But being that I offroad and usually drive heavy and with the family, I didn't want to take any chances. Cheap insurance imo.

 

They are technically one-time use, but they don't stretch all that much when torqued - at least not enough to deform threads or permanently deform the material like torque-to-yield head bolts. I ended up reusing my factory ones when I installed airbags; I did inspect the threads and u bolts for signs of stress or deformation. That was 100k miles ago and zero issues. However, if you have access to another set, I would most likely go that route.

Edit: I should have noted that I just removed my bumpstops when installing the airbags rather than deal with the mess and fuss of cutting rubber bumpstops with a sawzall.

 

Followup - the plate the bump stop sits on has two small lips on each end. Will the u-bolts stay in place or any chance of slipping without those?

 

Very little, if any.

 

Kept mine when I got SumoSprings.

Sourcing new metric ones is tough. Pretty much have to go with 9/16ths to buy new ones off the shelf.

 

When I go to Sumos I intend to replace mine. My last two sets were from Alcan and will be again when I remove and install.

 

So, I’m not trying to argue that u-bolts are intended to be a one use item, but why?

what’s the torque spec and diameter? About 9/16” and torque to about 70#?

Why can we reuse our whee studs over and over? About the same diameter and torque to about 100#.

Before you say simply they’re different material, please make sure you can provide data to back up the claim.

 

14mm from the factory. Commonly replaced with 9/16".
The explanation I have always come across is that they stretch when torqued.
I don't know why U bolts are that way and lug studs are not. We know the length is longer, thread pitch? Material?

 

The best I found is that they are torqued to 75#. That is less than wheel studs and how much could they stretch at that size and torque. I reused mine after removing the bump stops for airbags and I there was a lot of threads left. I don't think I have ever heard of them failing except in off roading, racing and abusive situations.

I wonder if it is a Ralph Nader thing ( you younger members look that up) or engineers being overly cautious.

 

From my understanding, they are made from different material and the loading scenario is also different. Wheel studs are typically a chromoly steel making them tougher, stronger, and more rigid; when the lug is torqued, it holds the wheel right to the hub in order to keep the majority of the load forces vertically across the hub face, which puts the fastener in shear instead of bending, compression, or tension (the small torque value of even 200 lb ft on the steel wheels is in tension, but it is minuscule compared to the shear forces that can be placed on the lugs).

U bolts, on the other hand, are typically cold rolled low or medium carbon steel (usually low, but I haven’t looked in to exhaustively). That means it is still strong but more elastic and not as tough or rigid as chromoly steel. It needs to be more elastic to form the tight bend over the axle without compromising the structure, as well as endure the abuse of constantly changing loads. They see tension, bending, and some shear. And in different directions as opposed to the mostly vertical force shear seen by a lug nut. Cornering does induce higher tension loads on the lugs, but still nowhere near the shear and impact force from potholes and bumps placed in the u bolts.

But you are right - the torque value is smaller on the u bolts and I suspect it had more to do with safety factors and the simple fact that u bolt threads are exposed to the elements and corrode, where most wheel lugs are not. I’ve had vehicles with zero issues with the U bolts until I tried to remove them and they literally spun in place. The threads above and below were so corroded that what I attempted to spin the nut off, it bound up on the corrosion. So I pulled out a breaker bar or impact and the threads that were engaged by the bold were also corroded enough to strip entirely. So, really, that’s my bet - the u bolts threads are exposed to the elements and corrode.

 

Very well thought out response; thank you. I’m sure you’re correct on most, if not all of your points, and I concur.

U-Bolts are almost certainly softer material than whee studs, or they couldn’t be formed.

From personal experience, I’ve abused unbolts way more than the average driver. We’ve raced, jumped, and generally beat the shit out of u bolts by dragging them over rocks and too many other types of terrain. We’ve pulled them to swap axles and reused more times than I can count, and have never had a failure. None broken, and not a single stripped thread on either a u bolt or hi-nut.

Im not saying it’s a bad idea to replace with new after removal, just that I’ve yet to have one Peter out…

 

Good post, but some confusing terms.

Chromoly is stronger. If we are talking about yield strength, this means it will take a higher load before it permanently deforms (which you don't want to happen). It is more "elastic", meaning it will stretch farther before it yields. Even 8.8 steel 14mm bolts (like grade 5 9/16) should be torqued to 110 ft-lb, but as you mentioned, the U bolts have a very un-ideal bend, and are usually torqued to ~90 ft-lb.

Rigidity (or stiffness) for both types is basically the same in the operating zone (less than yield).

Toughness is a measure of how much energy can be absorbed before complete failure, and the lower strength steel will win here. Even though it will take less stress to make it permanently deform, it will yield long past that point before it completely breaks.

Regarding whether to chuck perfectly good U-bolts because you've removed them... I reused mine. If you check the torque occasionally, you should notice the tension dropping before they fail.