Studs can break if the lug nuts were not properly tightened. If a lug nut is loose or if they were unevely tightened, they can break.

If the wheel rim's lug nut mating surface has become egg-shaped or oblong (by loose lug nuts), the wheel could never be properly tightened and would need replacement. This can be seen with a quick visual inspection.

The proper lug nut torque value is somewhere around 70 fl-lbs with a clean, dry fastener. Use a torque wrench to be sure.

Lou Braun

 

Strange . . . I've read about broken studs, but not in this repeating fashion.

So, since the tires were replaced, the right rear bearing/hub has been replaced three times. And over that same period, there's been four instances of broken lugs. Presuming that the repairs were done by different shops and/or technicians, there doesn't seem to be much to suggest that over-torquing is the cause in this case, at least not so many times.

I'm not sure how oblong-worn wheel mating surfaces will cause the lug to break (I can appreciate the possibility of it causing the lug nut to loosen) but I guess it's possible if all five nuts loosen there could be abnormal stresses on the lugs. A problem with the wheel itself would explain the repeated incidents with new bearings and lugs.

Has it always been the same wheel on the RR, at least since the tires were replaced? Alloy or steel, and original or aftermarket? How long (miles) has it been since the tire replacement and how long between incidents?

p.s. Lug nut torque spec is in the Owners Manual, sub-section on In Case of Emergency, section Flat Tire. My 07 is 74-89 ft-lbs.

 

I had several wheel studs break on a 2001 Forester. I never figured out why, but a related problem on that car and on my 2008 Impreza is that lug nuts work loose. This happens any time a wheel is off. With the Subaru aluminum wheels, you must -- must -- torque them down after driving 25 or 50 miles. Otherwise they work loose to the point of falling off, but you will feel instability before that happens. I wonder now if on one of the loosening events (it happened several times, usually to my son or daughter), the wheel got damaged and that later led to the stud breaking off (again and again). It was remarkably expensive to replace the studs.

 

I noticed this with my 2001 Legacy also after doing a full 4 wheel bearing replacement. Snugged em down to 70lbs then had to pull over and do it again about 40 miles up the road.

If you stress all the nuts at one time you may break one but have the others stressed which may break later due to the first instance. So its possible you had one or two instances that actually caused the failure even though they failed later after some use.

 

Indeed the wheel nuts should be retorqued after some driving -- this is also in the Owners Manual.

Like many here, I rotate the tires myself, and use a clicker torque wrench when I do. I've always gone back after a while and rechecked, and I can't recall when there was any noticeable turning of the torque wrench before it "clicked". This, I believe, is as it should be.

The idea of elongated holes, was raised. In this regard we'll have to wait for PhilB to respond.

But looking at the issue more generally, dirty/corroded wheel mounting surfaces might be a factor (perhaps not in the OPs case because of the triple hub replacement, but others). The Subaru wheels are hub-centric -- the center hole of the wheel fits snugly on a mating round protrusion on the hub. Very often the protrusion becomes pitted or rusted -- this sometimes makes it more difficult to remove a wheel, as the wheel's centering hole binds on the protrusion -- as does the surface behind against which the back of the wheel is held. It's possible, when re-installing a wheel, that it isn't seated properly because of binding on the hub-centering protrusion, or there's dirt or rust on the vertical flat surface of the hub. In this case, the nuts will initially appear to properly torqued, but the wheel isn't actually tight against the hub all the way around. After driving a short distance, it moves ever so slightly, and the nuts are now no longer properly torqued.

There was a discussion here, or on another Subaru forum, quite some time ago about putting a small smear of high temperature grease on the mating surface of the hub where it comes in contact with the back of the wheel and around the centering protrusion, to keep these from corroding. This helps keep the centering protrusion and the flat surfaces of the hub clean and clear of dirt or rust. The mechanics at my Subaru dealer do this, and so do I.

I guess another good idea is to regularly glance at the four wheels to make sure no nuts, or studs, are missing.

 

Thank you all for your posts. Here are some short answers to further questions.

------How much mileage? About 2500 occured after installation of the new tires, and approximately the same between each of the broken stud incidents. The car is driven about double the previous mileages, after a job change. High speed travel accounts for the additional mileage. The same wheel and the same location. When the dealer replaced the RR hub, he also replaced the LR hub. Those lugs/nuts seem ok.

After the first incident, I had been watching for broken studs, and I found several, leading to my visit to the dealer which resulted in the middle hub. But in the last incident, all five studs failed without warning.

I was unaware that the nuts might loosen after being removed and retightened. I think these are the aluminum hubs. I will get a torque wrench. What if the break pads are a little tight, perhaps excessive heat would be produced. Then differential expansion of the steel hub- aluminum wheel might stretch the lug.

Subaru has extended the warrantee on the bearing/hub assembly. They say the reason is excessive noise. Is it possible that a new hub assembly masks a vibration problem inside the axle? If that is the case, I could guess that the axle problem could get to the point that replacing the hub assembly no longer helps.

 

I doubt the brake pads would cause this. Certainly the rotor and hub can get exceptionally hot, but binding calipers/pads is a common issue in brake-related posts here and elsewhere, and I don't recall anyone also indicating broken studs. In addition, when the hub is replaced, the caliper and caliper mount have to be removed, and if there was excessive overheating it would (should) have been noticed (metal color, worn out/burned pads etc) and either corrected or brought to your attention.

Ditto on the axle. There's many posts here about various axle problems, and vibration is one symptom, but no mention of loose lug nuts or broken studs. Of course, this is not to say it's not possible.

Perhaps have the wheel taken off and inspected closely for cracks, or wear.

Also, check that the lug nuts are the correct Subaru ones, assuming the wheels are the originals from the factory. Odd, but not impossible that the tire shop interchanged the lug nuts with another car's. Different lug nuts might not have the correct angle where the nut holds against the wheel, and this is critical to proper torqueing and retention.

 

Today, with a torque wrench set at 10ft-lbs I twisted off a lug on the new bearing unit -- about 1000mi after replacement. The nut stamp showed 12x1.25, the same as originals I removed in favor of locking nuts, after the first stud failure, when I thought this might be an aborted theft attempt.

I suppose that the next step here is to replace the rotor assembly and the wheel. And I further suppose this must be done in a symmetric fashion both on the LR and RR? Obviously I do not want to do this unless we can confirm a defect. Does anyone have a good feel for what measurement to make? --flatness of the wheel and rotor seating surface? Roundness of the nut mounting holes on the wheel?

What intensity of noise should I experience if things aren't right? This car feels like an off the road vehicle in the vibration/road noise that I hear. But I put it off on the deep tread of the 225/60R16 97H Bridgestone Potenza RE 92 M+S tires which I matched on replacing the originals.

 

Not to worry you, but is there any chance someone is doing this to you intentionally? Loosening/tightening things when you aren't around, to try to cause you to have an accident?

 

I was worried about vandalism, but the last repair (new bearing unit and studs) was 1000mi from home, and the car arrived home with a broken stud. It literally came off in my hand with less than 10ft-lbs of torque. So the car was completely under our control and never unattended.

 

Same shop handling the fix each time? Broken studs are pretty odd multiple studs in your case makes me wonder if the shop has faulty equipment and they haven't noticed yet as a result they keep damaging the studs when they mount the tires?

Very strange issue given it takes quite a bit to break a stud.

 

Did you say the hub is ALUMINUM?


Dave

 

Today, with a torque wrench set at 10ft-lbs I twisted off a lug on the new bearing unit -- about 1000mi after replacement.

So we seem to be dealing with either lug nuts that weren't torqued up to begin with, or that loosened over time (in this case, though, it didn't break the stud, yet).

The nut stamp showed 12x1.25, the same as originals I removed in favor of locking nuts, after the first stud failure, when I thought this might be an aborted theft attempt.

I have to do some searching but I believe there's more to it than the diameter and threads per unit distance. The end of the lug nut is tapered, and this has to mate with the corresponding angled "seat" surrounding the hole in the wheel. I believe there are some standard automotive "angles", but even though the diameter (12) and thread count (1.25) are the same, the contact angle can differ. (Any others can confirm or correct me on this?)

Have you tried swapping the tire/wheel to another corner? With your own torque wrench, you can set the nuts where they should be, then drive for a while, and check again to see if the torque is the same, and if loose, if the loose nuts moved with the wheel, or are still at the same location. (Might have to also move the lug nuts and then after swap them with a set from a third wheel to eliminate the nuts themselves.)

 

Four different shops, four different bearing units, the last lug breakage happened after one day and about 1000 miles.

The wheels are aluminum.

I think the angle on the nuts looks is the same -- +- a couple degrees.

I have not tried swapping tires around. They are unidirectional so they would have to be swapped back to front and, frankly, if I have to lose a wheel, I want it to be a rear wheel.

 

I did a search of several other Subaru forums and didn't find anything like this. The odd broken stud, but no repeating problem.

Have you had the car since it was new? What, if any work was done in the RR wheel area (other than the tire and subsequent bearing/hub replacement)? Any brake or suspension work? Was the rotor ever replaced?

When that last stud virtually came off in your hand, were the other nuts still reasonably tight on their studs?

 

I have not tried swapping tires around. They are unidirectional so they would have to be swapped back to front and, frankly, if I have to lose a wheel, I want it to be a rear wheel.

Appreciate not wanting to move the risk to the front.

I'm sort of leaning toward a faulty wheel. It appears to be the aluminum alloy one and they're susceptible to corrosion and can develop cracks.

Are the wheels the original Subaru factory ones? Are you located in an area prone to corrosion, e.g. by the sea or a winter-wonderland where salt is used on the roads? If the latter, do you change wheels for the winter months?

Because of the timing of the first event, and the consistent RR symptom (and nothing similar on any other wheel) I'm wondering if the RR wheel might have been damaged when the tires were replaced. Can you inspect it closely, or bring it to a tire/wheel shop? There should be no corrosion or cracks. The back side of the wheel should be smooth (not pitted) in the area around the stud holes and where the back of the wheel comes in contact with the rotor/hub. While it can be difficult to detect cracks, the wheel should be checked closely, perhaps with a light and magnifier, especially around the stud holes, for either single cracks radiating out from the opening, or the appearance of spider-web type patterns anywhere in the area.

I'd at least want to eliminate the wheel as a possible cause before getting into changing other components.

Incidentally if it is the wheel itself, and you can find another original (matching) one from an 05, there's no need to change them in pairs.

 

I'm sort of leaning toward a faulty wheel. It appears to be the aluminum alloy one and they're susceptible to corrosion and can develop cracks.

I'm wondering if the RR wheel might have been damaged when the tires were replaced.

I'd at least want to eliminate the wheel as a possible cause before getting into changing other components.

This sounds like a likely scenerio -- Particularly since aluminum against iron is not a good combination due to electrochemical corrosion. I will pull the wheel and have a close look at it.

I think this is probably not a result of damage prior to my ownership, since I drove it for 60k+ miles without a problem before I replaced the tires.

 

The dealer decided it was a warped rotor assembly. The face of it was not flat, so the wheel / rotor / bearing hub could not be pulled together. The wheel would therefore rock -- probably every revoluiton. We will see. This scenerio suggests that the rotor was damaged during mounting of the new set of tires. Not really sure how this might happen, so I'm going to keep a really close eye on the lug nut torque during the next few 1000 miles.

 

The dealer decided it was a warped rotor assembly. The face of it was not flat, so the wheel / rotor / bearing hub could not be pulled together. The wheel would therefore rock -- probably every revoluiton.

do you know if both sides were uneven or if it was the "wheel side" or "body side" of the rotor mating surface? did they actually measure/mic it?

 

Interesting. Wonder what could have damaged the central part of the rotor (the part that is squeezed between the wheel on the outside and the hub on the inside).

In any event, this could be a new one for the knowledge base here so please do let us know if it works out.

 

After 2500mi, the nuts are still torque properly. So the warped rotor assembly appears to have been the problem. I'm glad to put this one behind me.

 

Thanks for the update. Glad that the problem seems to be resolved (2500 miles - that's further than you went trouble free before).

It shows how we have to watch what happens to our cars when someone else is working on them, and if possible, check them over afterwards.

 

I have 2003 Baja on which this has happened SEVERAL times. 2 years ago, lost RR wheel doing left turn at stoplight. Luckily, I was only going 10 MPH. $400 repair bill. More broken studs off and on since. Then, 2 nights ago, noticed same banging noise coming from rear of car. Pulled over and found 2 more broken studs, and another broke when repair shop went to take off wheel. I have now saved this latest evidence and intend to pursue with Subaru.

 

I just had my second or possibly third instance of this on my 05 Outback. I know it has happened at least once before on the rear wheel, I think it was the right one. I just found another when I was putting on my Snows today. It actually broke as I was taking off the wheel. I don't ever remember seeing broken studs on anyother car before.

I know I have never overtorqued them and I don't think anyone else has in the 45k I have had the car. I use a battery impact to remove them and I also lightly snug them up that way, but then torque them when the car is sitting on the ground.

 

>8000 miles now and the studs are fine. In retrospect, road noise was a clue that the wheel was not running true. The noise was much reduced with the new wheel/rotor/hub. I have not had the rear wheels off since the trouble and will be very nervous when it needs to be done.

 

I have an 05 XT with a similar problem. I just made my third trip to the shop with broken lug nuts on the rear passenger side. This time it was three that snapped off and the fourth was ready to go. Any ideas besides a warped rotor assembly?

 

I'm also curious if replacing the brake rotor has still fixed the problem? I'm having the same broken studs problem on my 00' Outback.

Thanks, Josh

 

I'm also curious if replacing the brake rotor has still fixed the problem? I'm having the same broken studs problem on my 00' Outback.

PhilB last reported at the beginning of April that all was still good. That's about 8 months and some 8000 miles since the rotor was replaced. Given that the studs were previously breaking within a far shorter time and mileage, at this point it's hard to think the new rotor wasn't the solution, at least for his 2005.

There's no way to know for sure if this is your, or sailing's, problem, but if nothing else seems to be at fault, I would think closely examining, or replacing, the relevant rotor would be the way to go.

 

Given how the rotors slide over the lug bolts - it would make sense that every time you hit the brakes a warped rotor will stress the bolts. Never had a broken lug before on any car, but after doing a full wheel bearing job only way I can see lugs being stressed like that is from a damaged or bent wheel or the rotor is warped creating issues with how the wheel seats causing damage to the lugs when you hit the brakes.