Torque for Lug Nuts

I thought overtightened lugs causing warped rotors was BS until I had my Volvo 960. The hub said to tighten to 60 ft/lbs right on it, and they meant it. I had two tire shops ruin rotors before I caught on - and actually it was volvochat (as exciting as it sounds, unfortunately) that made me realize it. I use a torque wrench for lugs probably 90% of the time after owning that car.

 

Guess I need to go snug mine up! Just bled my brakes on all 4 corners over the weekend, and I know I didn't pull 'em all the way to 75.

 

Yes Chris, absolutely right - the manufacturer's torque spec accounts for the CoF. That is, when the lug bolt was new and free of any corrosion or surface wear to the threads or the mating surface to the wheel. My 30 year old lug bolts for my 308 are far from new, having a good dose of the usual type of wear and surface corrosion you would expect with that age. Hence, the anti-seize....

Incidentally, anyone who says putting a small dab of anti-seize on a lug bolt is a mistake hasn't spent 30+ years restoring old cars with their own hands.

 

Yes my fingers are impressive. Ok but putting the lugs in finger tight, then snugging with the breaker bar, wheel in the air and free to spin, results in nearly 75 ft-lbs. Just seems awfully light to me, having been a track rat for so many years.

 

75 foot pounds on a 14mm bolt is about 16,000 lbs of tension load in the bolt. At 95 that goes to 20,500 pounds (assumption is clean, dry threads.)

A 14mm bolt is good for about 25,000 pounds in tension before it deforms. The bolts are intended to act as springs, at high loads they will stretch and spring back. If you torque them up close to the yeild strength at where they will no longer spring back... they will permanently deform under the load and they will appear to be loose. A common misconception is that if you find a loose bolt you should torque it even higher but going over the yeild strength of the bolt is counter productive, you are making the problem worse. Normal design bolt torque ranges are to 80% of the yield strength of the bolt. For some things like head bolts that goes to 90%. Overtorquing by just 20% can permanently deform the bolt not to mention the parts its holding together.

Use a torque wrench ALL the time. Overtorquing is just as bad as undertorquing. Torque ranges that come from the manufacturer are calculated, not a good guess and will provide the best possible connection.

I work in aviation. We use certified torque wrenches for everything... even screws and non structural parts. Doing it right the first time prevents warranty claims at the low end and prevents major accidents for the critical stuff.

IMHO, hand tightening critical parts on your car is stupid.

 

I'm kind of stunned this is a thread, that there's any question of torquing. I must be naive, but for 20 years I've been torquing my wheel nuts to the recommended specs. They taught us to do it in public high school automechanics class.

 

"Snugging" is not really a scientific measure or highly repeatable. And how long is the breaker bar? 10 inches? 2 feet? And... how do you know you're nearly at 75 foot pounds?

 

I bet I am the only one here who regularly has his torque wrench tested (it has consistently tested within spec and not needed adjustment). Anyone else?

 

Must just be the northwest wet air... mine has a cert sticker on it as well.

 

Yep...Every year on all four of my torque wrenches.

 

I am, as well. Then again, with over 40 stripped threads/pulled studs to contend with in a 308QV engine that I'm currently overhauling for a client, the argument that proper torque is optional is somewhat comical...and somewhat indicative on why just so many DIY maintained cars are rapidly turn to crap.

 

Yep...Exactly what this gentleman said!

 

Ever use a beam-type torque wrench? Easy to see initial torque when using them. FWIW I have three different torque wrenches, check them all the time, store them fully open, and would never put a wheel on without proper torque - as many years as I've spent on the track, anything else would be unthinkable. I've changed thousands of wheels over the years and torqued them all, so I do have some experience in it. After getting the bolts snug with the 2' breaker bar, at 70-ish the beam wrench is just starting to turn. So yeah, if i can spin on the lugs and then get to 75 ft-lbs without the car on the ground (and without trying hard), it seems light to me. But maybe I'm wrong in this application since my experience is with Porsches and Hondas on the track.

 

So true about corrosion, since torque doesn't really account for not being able to remove a rusted bolt or nut. And of course, once we are talking about how much of an effect rust has on actual clamping force vs. anti-seize, well, just make sure there is an equal clamping force on the dang wheel lol. At some point there has to be a decision that we are fixing cars, not space shuttles or labratory equipement. There is a margin of error available by design. I guess making an informed decision is the point here.

 

I think you should do what ever you feel comfortable doing. But to throw some fuel on the fire here, I once called a "very very" well known and respected west coast Ferrari mechanic and asked for some input on torque specifications for a Vintage Ferrari. He told me that after as many years in the business as he has been, he no longer uses a torque wrench, and wouldn't know without looking it up. I don't know if he was just jerking the chain, but that was a comment I got.

 

That would scare the crap out of me. Like someone else said, if it's worth doing then it's worth doing properly.

 

75 foot pounds seems light to you compared to what? If it's other brands of cars you can't really compare unless the wheels are made of the same material, the bolt head design is the same, the thread pitch is the same, the surface finish of the bolts are the same etc. etc.

 

I have worked on commercial aircraft for 25 years. Always torque everything that you work on to manufacturers specs. Wheel bolts are no exception. Torquing is a good habit and always ensures that you have completed the job properly. If you think that 75 ft/lbs is a lot, a Boeing 737 main wheel is initially torqued to 500ft/lbs backed off and final torque is 150ft/lbs (useless trivia). I treat my 308 GTB the same way I treat our jets.

 

Several years ago I ran into a guy who was into older 911s. He was a budding young DIY guy and keen on doing things by the book and learning all he could learn about his early 911. He was methodically trying to pull off a nut and bolt restoration on the car, but doing so while still driving it....one weekend rebuild a brake caliper, next weekend replace the headliner, etc. He told me that he liked the car except that he thought it was unsafe because the brake calipers would periodically come off and leave him fighting the wheel to stop it without running into anything. "huh?" I wisely said. He pulled a mangled bolt out of the glove box to show me and indeed the bolt had obviously sheered after what looked like the thread being chewed up. He had found a torque spec for the caliper bolts in some WSM and insisted that he always torqued bolts to the manufacturer's spec including the caliper bolts.

I looked at the bolt and it was some super cheesy grade 1 type chinese bolt. The guy knew nothing about bolt specifications and was buying the cheapest shiney new bolts he could find at the local general store. Torquing the bolt to spec was destroying the threads and then the bolt would bend or sheer as it worked its way out (no lock nut either). The car no doubt originally came with 10.9 or 12.9 type spec bolts for that application.

The moral of the story is that the torque spec is certainly helpful, but it's less than half the story. If you're doing DIY work and you don't have a complete understanding of bolt specifications (and corresponding locking mechanisms) then you're really only playing with half a deck.

There's some good information available on the web covering the (quite extensive) specs for bolts of different grades and types.

 

It may be useless relative to our cars, but it is interesting. What is the purpose of this procedure?

 

What's the purpose of torqueing to a very high value and then backing off? Boeing's engineers know what they're doing, what do they know that I don't?

What size is this bolt that can stand 500 ft/lbs?

 

Interesting thread. Can anyone suggest a proper torque for spinners on a 250gt.

 

Glassman, I think this is my favorite post of yours that I've ever read.

Do you need me to sell you a torque hammer?

 

i started using a torque wrench back in'74 on my 2002.
the dealership scared the begeezus out of me, so i followed there advice.

i've used that same wrench on every car since. the germans are very anal
about such things as proper wheel tightness.

borranis require a 2lb. brass mallet lifted no more than 10" above the spinner ears
and struck smartly twice.

but first your arm must be properly calibrated for maximum torqueage as specified
by caltech engineers and nasa high energy physisists.

 

Wow. That's like a surgeon saying he no longer uses a scalpel; just a sissors.