Wheel Torque

[Ron and Cathy Stein]

I have 15″ Panasport wheels on my Tiger. For years I have torqued the wheel nuts to 75 ft-lbs. This was based on internet research for a 7/16ths wheel stud, which it still has. I needed to take off the rear wheels recently and two of the acorn wheel nuts on one side would not let go. I put my bigger bar on it and the wheel studs started to turn. I didn’t want to get into the grief of drilling and other stuff to get the nuts off, so took the car down to my local garage who run into that problem once in a while. They got the wheel off and ended up drilling slightly larger holes in the hub (I have Wilwood rear discs) and put in new 7/16th studs, but with a larger head. They think the studs were over torqued in the past. Maybe there was some cross threading. Looking in the Tiger shop manual they recommend 48 ft-lbs. The depth of the acorn nuts contain many more threads than the original nuts for the stock wheels. That seems pretty light, and a number of web sites seem to confirm the 75 ft-lbs and some sites even have a range of of 70-80 ft-lbs and 75 -85 ft-lbs. They could have been over torqued by the tire shop a number of years ago, but the wheels have been off a few times since, with no issues. I normally turn the nuts with my fingers to as close to seating as I can. The studs are not original and I assume they were installed by the previous owner with the brake conversion, so they weren’t stretched tired original Tiger studs. The shop has now set them at 60 ft-lbs. I want to feel secure in my car. The only other thing I can think of is I have some spacers behind the wheels to keep the fatter tires out a bit away from the leaf springs. The previous owner had a bit of rubbing so put them in. That might have stressed the studs. I might be able to slightly reduce the spacer width without rubbing. I have about 365 hp which is also a bit of added stress. I’m not sure what to do to stop this problem in the future. I find it hard to believe 75 ft-lbs is too much. Any thoughts/comments?

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[Joe and Kerch McConlogue]

Ron, the Alpine studs are also 7/16-20 and the manual specifies 48 ft-lb. I have heard that over-torquing could stretch the studs, weakening them. I think this could also lead to thinning at the root where the stud grips the axle plate, so that may explain your experience.

I have a table that shows the torque for dry threads on a 7/16-20 bolt at 55 ft-lb for grade 5 and 80 ft-lb for grade 8. Another website I saw calls out 41 and 58 ft-lb (with tension at 70% of yield), logically then higher torque puts the stud closer to yield, but I don’t know how to figure how much. I don’t know what grade the studs used by Rootes may be, but I doubt that they are grade 8.

You might want to consider a stronger stud material if you can find it. I could be wrong, but I think the critical item is the clamping force, so as long as the spacer used is incompressible, the longer stud shouldn’t change that.

FWIW my Z4 is specified at 130 nm or 96 ft-lb.

I’m sure there’s a mechanical engineer out there that can chime in on this topic with a better discussion of wheel stud torque, clamping force and the impact of axle torque on the studs. I’d like to learn a bit more too.

[warren-g]

There’s a spline on the wheel stud. Let’s just go with this.. how often have you seen a guy who puts on your tires and wheels change the pressure or setting on his torque wrench..
Not once I would say.
But I’ve seen Tech at Uniteds exceed the manual stated amount.

[Robert Rehman]

Ron I have the stretched studs from my Tiger which I will show you when we meet. Somewhere in a tech article it said to use studs for a 68 – 82 corvette. I bought Moser studs for that application and installed. When I looked up the torque spec for the corvette ut said 65 lbs and that’s what I have been going by. OK so far.

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