I have a 1998 Subaru outback wagon auto trans. 2.5l engine. 145,000 miles. The wheels were rotated by tire shop I have been using for 20 years about 5,000 miles ago. I was traveling this weekend and noticed what I thought was low tire pressure about 130 miles into the trip so I pulled off and checked and the right rear was missing 2 lug nuts/bolts sheered off and the third basically fell off when I checked with my hand. I had not noticed anything unusual until this trip with most of my driving being 20-30 miles per drive or less. What would cause the bolts to sheer that way....I understand loose bolts could sheer with wobble but what caused the nuts to loosen? I have heard that with an all wheel drive there could be a problem with the transmission when it is malfunctioning and keeps trying to transfer power to a side or a wheel thinking it is slipping such as in snow etc. which could cause the sheering?

 

I did have another instance of the front right nuts coming loose a couple years ago but did not have anything sheer off as this was noticeable everytime I would take my foot off the gas and found this right away.

 

I have had a problem with the transmission where sometimes it does not want to shift into drive right away and I have to wait or rev engine for it to catch and this happens with no real pattern hot, cold weather or sitting overnight or just driven.

 

Thanks for any advice you could give me to solve the problem

 

 

John

[...]the right rear was missing 2 lug nuts/bolts sheered off and the third basically fell off when I checked with my hand. I had not noticed anything unusual until this trip with most of my driving being 20-30 miles per drive or less. What would cause the bolts to sheer that way....I understand loose bolts could sheer with wobble but what caused the nuts to loosen?

Either under- or over-tightening of the lug nuts could lead to what you've described. Too loose, and they can back off as you drive, then the lugs could flex/fatigue/snap. Too tight, and the lugs could be stressed beyond their elastic limit, no longer "clamping", and again the nuts could loosen as the lugs fail. All lug nuts should be tightened in a manner that the correct torque is applied, whether by using a torque wrench by hand or torque "sticks" with an air wrench. The threads should be clean, so that the torque properly represents tension in the lug, and not significantly friction of the threads. Also, to be on the safe side with alloy wheels, the torque should be rechecked after a hundred or so miles.

 

 

I have heard that with an all wheel drive there could be a problem with the transmission when it is malfunctioning and keeps trying to transfer power to a side or a wheel thinking it is slipping such as in snow etc. which could cause the sheering?

Highly unlikely.

 

 

I did have another instance of the front right nuts coming loose a couple years ago but did not have anything sheer off as this was noticeable everytime I would take my foot off the gas and found this right away.

In which case the lug nuts were probably not sufficiently tightened initially, and you were lucky that you noticed the problem.

 

 

I have had a problem with the transmission where sometimes it does not want to shift into drive right away and I have to wait or rev engine for it to catch and this happens with no real pattern hot, cold weather or sitting overnight or just driven.

You might want to start a separate thread on this topic, or use the search function first. It certainly doesn't relate to the lug/nut issue.

 

Oh, and welcome to the forum.

by chance is this the same wheel you had

the lug nuts come loose on before?

 

I ask because there is a possibility the holes are

now ellipsed.

This allowed movement that loosen the lugs

and or wore thru the studs causing them to shear.

before putting the wheel on make sure that the mating surfaces are clean..I think almost any honest mechanic will tell you that they have had a wheel or two come off a customers care...alloy wheels may look nice, but also involve more work and awareness to be safe for yourself and to others...invest in a torque wrench

before putting the wheel on make sure that the mating surfaces are clean..I think almost any honest mechanic will tell you that they have had a wheel or two come off a customers care...alloy wheels may look nice, but also involve more work and awareness to be safe for yourself and to others...invest in a torque wrench

 

Japanese carmakers calibrate their impact wrenches every day.

 

That's what makes them different from a tire shop where the kid likes the sound of a impact wrench banging an unlubricated lug nut home at 200 ft lbs.

 

That's what broke your lugs off. Thanks for reminding me. I have to retorque the truck wheels which I had rotated.

I think almost any honest mechanic will tell you that they have had a wheel or two come off a customers car

 

Ah, no. After a period of citizens taking exception to getting killed/maimed by flying truck wheels (and the law suits and wholesale lifting of mechanics licences) I *think* you will find a LOT of mechanics almost anal about wheel mounting.

Basically what has happened was at some point the lug nuts were over tightened. This fatigued them, and set them up for future failure. My shop tightens lugs by hand (thats why i like them).

 

I hope you had the rest of the nuts replaced.

 

 

nipper