• Scam Alert. Members are reminded to NOT send money to buy anything. Don't buy things remote and have it shipped - go get it yourself, pay in person, and take your equipment with you. Scammers have burned people on this forum. Urgency, secrecy, excuses, selling for friend, newish members, FUD, are RED FLAGS. A video conference call is not adequate assurance. Face to face interactions are required. Please report suspicions to the forum admins. Stay Safe - anyone can get scammed.

Wheel stud fatigue failure from insufficient torque

Dabbler

ersatz engineer
Thanks John I was wondering how tight they should be. I'll break out the torque wrench.
CORRECTION

For those not used to my shorthand... The metric equivalent of the 3/8 grd8 bolt is the 10mm bolt. The ANSI ratings for a grd 8 bolt are less than the ISO ratings for a metric cap head screw, and David has not given me the benefit of which fastener he used. A 10mm cap head bolt is equivalent to a grd 10 bolt, or almost 25% stronger for its size and the 10mm is larger. Hence if he used 10mm cap head bolts, the number isn't 37 foot lbs (gotta love mixed units!) but about 46, or 25% more.

The precise number isn't all that important. the principle of torquing a fastener to 70% if is max working load is really important. It provides the optimal fastening strength - and it is why we torque our wheel nuts to these obscure values. If the tire shop fails to tighten them into this range, the nuts fall off over time.
 
If the tire shop fails to tighten them into this range, the nuts fall off over time.

^^^ this! +++

When they are tight but not tight enough, the fatigue cycle kicks in, and the lug fails. Most people see the broken stud and blame the shop for overtightening them. They don't know that it's the opposite.

Or swear a lot if they use the uggadugga gun a bit to much. :mad:

We just can't win can we!

When my wheels get torqued by someone else for whatever reason, the first thing I do is retorque them properly.
 
^^^ this! +++

When they loosen up too much, the fatigue cycle kicks in, and the lug fails. Most people see the broken stud and blame the shop for overtightening them. They don't know that it's the opposite.



We just can't win can we!

When my wheels get torqued by someone else for whatever reason, the first thing I do is retorque them properly.
Most lug nuts are over tightened beyond the friction range and tensile strength to point of yielding and stretching. This is what cause bolt failure. This is from the retired head of engineering at Alcona wheels.

Under tightening does not create enough tension for sufficient friction to prevent the nut from backing off under vibration.
 
Most lug nuts are over tightened beyond the friction range and tensile strength to point of yielding and stretching. This is what cause bolt failure. This is from the retired head of engineering at Alcona wheels.

Under tightening does not create enough tension for sufficient friction to prevent the nut from backing off under vibration.


I think you misunderstood him because I am certain he didn't mean or say exactly that. Google "Fatigue failure of wheel studs". You will find countless engineering and technical papers and articles that support what I said.

Here is just one good resource.

The overwhelming evidence, facts, and analysis indicate that the vast majority of studs fail in fatigue caused by inadequate torque.

Furthermore, I have personally analysed dozens of broken wheel studs where someone claimed something else happened. I have never even once seen any other failure mode.

If you give me the name of your expert, I'll call him for you and see what he really meant.
 
Im not gonna argue your theory that "broken lug bolts are from loose wheels cuz I think your right...the lub bolts do break as a result of the wobble torque exerted by the wheel as it moves on the hub after coming loose....BUT....loose wheels are often caused by over tightening rather than under tightening. Over tightening causes a premature thread failure - not bolt breakage.
It would be pointless to argue this much because we didnt see any particular wheel leave the tire shop loose or over tightened but I have seen many tire changers at a heavy tire shop too damn lazy to grab a torque wrench and just turn the dial to max on the 1" air gun and let-er-buck until it slows down from lack of air.
The Transport Safety Council of Canada changed that a few years ago so that not "finish tightening transport lugs" with a torque wrench was an indictable offence.
 
Im not gonna argue your theory that "broken lug bolts are from loose wheels cuz I think your right...the lub bolts do break as a result of the wobble torque exerted by the wheel as it moves on the hub after coming loose....BUT....loose wheels are often caused by over tightening rather than under tightening.

I am glad you commented. It gives me a chance to provide more detailed information. Your insight is not actually correct. That is exactly why I could not let Degen's post stand.

I worked with Transport Canada for quite a while when I was responsible for vehicle safety at my employer. I can, assure you that the regulation was developed at industries request. Industry doesn't like fatalities any more than Transport Canada does.

The first thing to understand is that wheels flex with load changes as they roll down the road. As a result they experience millions of load cycles. When the stud is properly torqued ABOVE the normal tension stress load, but below its tensile strength, the stud does not lengthen or shorten. It simply holds the wheel on and the stress changes do not "work" the stud. If the stud is overtightened, it almost always snaps when tightened and does not fail on the road. It does take a gorilla to get the nuts or studs off though.

The problem with undertightening (which is not the same as loose at all) is that the load cycles can cycle the actual length of the stud whenever its working load exceeds its clamping load. This works the stud over the course of millions of cycles and causes it to fail in fatigue over time.

A loose stud will usually be obvious because the wheel starts flopping around and makes a loud racket. Most drivers notice it long before the nuts or studs fail. This is NOT the more common fatigue failure mode.

The problem with fatigue failures is that the wheel is not loose. It is tight, but not tight enough. So there is no flopping around or wheel noise and the driver does not know it's happening until its too late.

As Transport Canada noted, it is very important to tighten wheel studs to the recommended torque and the reason is to avoid fatigue failure in a stud that is tight, but not tight enough.

Wheel studs are a Goldilocks situation. They shouldn't be too tight or not tight enough. They must be just right. It's very hard to do that without a torque wrench.

That already takes too much away from @David_R8's thread so the fatigue issue has been moved to its own new thread here where we can discuss it to our hearts content.

Edited - Anyone wanting more info should feel free to do their own research or contact me by PM or post in this new thread. I can provide as much detail on the relevant engineering theory and material science as anyone might so desire.
 
When my wheels get torqued by someone else for whatever reason, the first thing I do is retorque them properly.
I do the same, twice in my life, once at Firestone and once at Canadian Tire (35 or so years ago) the "tire guy" grossly over tightened them.

I was having snow tires installed at CT, I was watching from the window, young guy has the impact set for tighten instead of loosen and goes at it until one of the wheel bolts snaps off (my Mazda GLC actually had wheel bolts), he looks around and spots a Mazda 626? on another hoist and takes one off that customers car and puts it on mine.

The other episode (Firestone) was worse, I went for an oil change just before heading out for a lengthy trip on the transCanada. They did a "free" inspection. I payed and they gave me the keys. Jumped in the car an immediately noticed a terrible sound as soon as the wheel did a couple of turns. I jumped out to look and see what was going on. One of the four bolts was missing and they managed to drive another one half way through the wheel such that it was hitting something perhaps the caliper every time the wheel did a revolution. I was minutes from taking the car on the highway. They ended up replacing the wheel bolts, the hub etc. Those clowns should be locked up, they could have killed me.
 
Agree with @Susquatch & @Dabbler

Lots of studies have been done, broken bolts are most often caused by under torquing, rather than over torquing

the studs/bolts actually see higher cyclic stresses when under torqued than when over torqued, eventually it causes the stud/bolt to shear off, the highs of the cyclic loads are actually lower with an over torqued bolt than with an under torqued one, shorter leash kinda thing
 
I read that really critical fasteners are not actually torqued, instead they measure bolt stretch.

That is the best way to do it. But it's very difficult to do. I've seen studies that use torque to get rid of play and then count turns. The results were quite good as I recall. It also requires quality fasteners and quality fastened material. But it's hard enough to get people to use a torque wrench!

I think I've mentioned this before, but there is a fellow in England who is a threaded fastener expert that industry used for unusual applications who was very good. I used him once after I retired and he helped me without charging me. I was wanting to mix an imperial male to metric female that would thread together and was concerned about the stack up over many threads. I'll have to look him up again and see if he still does it.
 
When my wheels get torqued by someone else for whatever reason, the first thing I do is retorque them properly.
This. Few things bug me more than watching some idiot w/ an impact gun rattle away interminably on each lug nut, then throw a torque wrench at 'em, wait for the click & proudly announce, "They're done!". Not even close, Sherlock.
 
Just gonna drop this here:

decals.jpg


From the U-haul website. With the millions of trailers they have, i'm sure they know what they are doing, but that sticker has really ruffled the feathers of some of my fellow mechanics.

I use neverseize on studs, always have. I reduce torque accordingly. Never had a problem in over 30 years.

@JustaDB - i do that everyday. But, on cars and light suv's where the spec is 100 ft/lbs I use my 3/8 gun which usually torques them to around 60 ft/lbs after a few impacts. On bigger trucks i use my 1/2" gun which gets them to 100. Then I finish up with a torque wrench, which always moves the nut 1/4 turn or so. When you do it all day, everyday, you can know your guns and get pretty damn close.
 
Years ago I worked as a mechanic for a Kenworth truck dealer. When Kenworth switched from fine thread imperial frame bolts to a coarser thread metric bolt they started having problems with bolt failures. They determined that this was a result of improperly torqued bolts from the factory. They never said if it was too high or too low a torque causing the failures.
The trainer came to our location and did a couple hour course on how to properly install frame bolts as well as a few other things. He brought two two foot lengths of frame rail with matching holes drilled in them and a strain gauge or load cell I think he called it that had a digital readout. He got two of us mechanics to bolt the frame segments together with the load cell between them and then torque the four 16mm bolts to the required spec. The readout showed the clamping force which was in the thousand of pounds.
Next he got us to loosen the bolts completely and re-tighten them to the same torque. This time the clamping force dropped by over 15% after reusing the bolts. Increasing the torque a bit more did not increase the clamping force. We were told to never reuse a frame bolt.
 
I read that really critical fasteners are not actually torqued, instead they measure bolt stretch.
Yes ! I drag raced for many years, did my own engine assembly, bought a rod bolt stretch gauge and learned to use it. IIRC, the stretch spec for the rod bolts I used (ARP) was about .006" ( there was an acceptable range). I tried for ~ .0055", rather eye-opening to watch the dial on the DTI rotate as you tightened the bolt . They wanted you to record a pre-install length, stretch the required amount, then on tear-down / rebuild, any bolts that did not return to within .001" of original were to be replaced because they had been taken past their yield point.
Their tech info also addressed using torque, they had a chart that showed different torque specs for dry threads, lubed with motor oil and their proprietary moly lube - the torque spec went down as the lube quality went up.
If using torque only, they suggested you take a fastener to torque value, back it off and re-torque 3 times (IIRC) to burnish the threads and any mating shoulders.
 
It is so nice to see so many posts from guys who actually do know how to do it right. If I had a nickel for every mechanic or wrench jockey who figured they knew better, I could buy that shaper in Quebec and have it shipped here.

@Darren - I think @JustaDB meant over torque it with a gun, then put a clicker on it to show it was perfect. It's a really stupid stunt but it easy to fool a fool. The last time I saw that, I actually heard the guy tell his buddy that's why torque wrenches are stupid!
 
@Darren - I think @JustaDB meant over torque it with a gun, then put a clicker on it to show it was perfect. It's a really stupid stunt but it easy to fool a fool. The last time I saw that, I actually heard the guy tell his buddy that's why torque wrenches are stupid!
I totally got that, and I have seen it 1000's of times. That's what separates a good mechanic who cares about his work, and a hack. These days there are more hacks...
 
Back
Top