Trying to get my bearings, so to speak
November 28, 2011 8:24 AM   Subscribe

What do I need to know about "bearings" in my car? I don't know anything about "bearings" in my car.

Clueless car owner asking: A friend observed that I need to have my bearings looked at. He noticed a low "wum wum wum wum" sound when I drive, a sound I've been hearing for months and never thought anything of it. He told me the bearings are part of the wheel, and mentioned that it's not really something that should be delayed. That's all I know and I'm asking here because I'm rather embarrassed to ask ignorant questions in real life.

So on a basic level, what are these bearings for? What could have gone wrong with them? Can they be repaired, or would a full replacement be needed? How much money should I expect to pay for the work? What are the risks if I do nothing right now (money being very tight)? What symptoms should I look for to know if the situation is worsening or becoming dangerous? Also, could anything else besides bearings make a sound like that (in case my friend is wrong)? The noise appears to be coming from the rear of the vehicle.

No idea if this makes a difference, but the car is an AWD Subaru Forester and I'm in the USA.
posted by The Winsome Parker Lewis to Grab Bag (18 answers total)
The risk of doing nothing now: much, much more expensive repairs later.

(Wheel) bearings are what allow the axles to spin freely at the point where they're connected to the strut assemblies (in a very basic manner of speaking). When bearing start going out, it puts more stress on the entire drivetrain (CV joints, axles themselves, transmission).

It is far less expensive to get this done now than to wait. Waiting will simply pile on additional cost and parts that will need to be replaced, especially in an AWD vehicle.
posted by chrisfromthelc at 8:29 AM on November 28, 2011

That kind of sound can be caused by lots of different things. Most likely is your tires are either out of alignment, or unbalanced. Both of these issues are easy to check for and fix at any tire place.

Alignment refers to whether the wheels both point straight ahead if you turn the wheel straight. A wheel out of alignment would point left or right while the other wheel is pointing straight.

Balance refers to whether the weight of the tire is perfectly circular. It's adjusted by addition or removal of tiny weights clipped into the wheel. An out of balance tire is heavier on one side.

There are some other things that could cause the problem, but those two are quick to rule out and cheap to fix.

Does the sound increase or decrease in pitch as you increase or decrease speed? Can you hear it only at certain speeds and not others?
posted by odinsdream at 8:29 AM on November 28, 2011

Wheel bearings are most likely what your friend was referring to although there are all manner of bearings in a car, especially in the engine and transmission.

In a Subaru, there are several possible sources for the thrumming sound including a wheel bearing, a tire or a CV joint. A mechanic versed in Subaruisms should be able to isolate the source of the vibration fairly quickly.
posted by bz at 8:32 AM on November 28, 2011

If it is something like wheel bearings or CV joints they are replaced, rather than repaired although it may not be necessary to replace all of them if only one wheel has a problem. Tires can usually be re-balanced if there's no physical issue with the tire itself.

Basically, any sound coming from the drivetrain is a cause for concern and will cost more if you put off getting things checked out.
posted by tommasz at 8:47 AM on November 28, 2011

A bearing allows something to spin in something that doesn't - ie the wheel to turn while the car stays still. They are important and can only be replaced, although this isn't really a big deal or rare on a higher mileage car - they are a serviceable component and replacement is normal. They just wear out over time.

Wheel bearings can make the noise that you describe, as has been mentioned however, so can lots of other things. Wheel bearings usually make noise from one side (although it can be hard to isolate in some cars) but also usually make more noise when you load them up - ie on a left hand bend the rh wheel bearing may make a louder or more pronounced noise.

However, tyres can also make this change in pitch too. So it isn't all that easy to give a layman's guide to diagnosing a wheel bearing issue. You do need some experience to be accurate.

If you have been hearing this for months and it hasn't changed my suspicion would be for tyres rather than wheel bearings.

If it turns out to be wheel bearings, though, you should allow a decent chunk of money to look at this - maybe up to $200 for both the front wheel bearings for parts alone plus a couple of hours labour each side at the minimum. You probably can get the bearings cheaper (and the rears may be less) but don't scrimp on the cheapest bearings if you intend to do a lot more miles. Cheap bearings are generally crap.

If it gets louder, it is getting worse, but I wouldn't wait too long to get this diagnosed. If you can hear zero difference in noise side to side then you may be able to leave it a bit longer or it may be tyres. But getting this checked is pretty much a case of 20 minutes with a mechanic. You can jack the wheel up and wiggle the wheel (top and bottom of the wheel in and out) and any movement or play will suggest a definite requirement for prompt attention.
posted by Brockles at 9:14 AM on November 28, 2011 [1 favorite]

I just replaced a bearing. At least for my make and model it is fairly straight forward. Its an easy repair and will prevent issues down the line. Took me about 1a hour if that. Just remember to grease your bearing and use lock tight gel (dont make the same mistakes I did...loosing 2 bolts outta 3
posted by handbanana at 9:15 AM on November 28, 2011

Historically, a bearing was a metal bit set into the bottom of a wagon axle that prevented the wheel from grinding away at the wooden axle. It bore the load. This was only a so-so solution to the problem and by time terms like miles per hour and rotations per minute entered the lexicon, better solutions were needed.

There are a lot of different types of bearings depending on the application, how much of a load, whether the load is longitudinal or axial relative to the rotation, etc. The part in your case probably looks something like this, but may be sealed (so you don't have to wallow around in bearing grease like the guy in the picture I linked) and may be press fit into a larger assembly.

If you allow them to get too bad it's more strain on everything and parts that shoulding be chewing on one another can start to which results in repairs that make wheel bearings look like an oil change.

Here's a discussion of (maybe) your particular issue on a Subaru owner's forum. I've found this sort of forum to be a tremendous help figuring things out with my Volvo and whether or not I could tackle a problem myself.
posted by Kid Charlemagne at 9:53 AM on November 28, 2011

What bearings are and what they do

Short version: if two smooth pieces of metal slide along each other there'd be a lot of friction between them. That means heat, noise, wasted power and most importantly wear. A bearing is something round, a ball or a cylinder, that's placed between the two surfaces. Imagine pyramid-building slaves pushing a giant stone block by putting it on top of a bunch of logs and rolling it. That's what bearings are on the most basic level.

Here's a first order approximation of what's happening to your car. The wheels are spinning but the car is standing still, right? The wheel bearings are what connects the spinning wheel's axle to the car. There's a collar full of metal balls wrapped around the axle holding it in place. If those balls are smooth then the axle can spin freely inside the collar -- no noise. If those balls are damaged it's like trying to push that stone block over rectangular logs. The first thing that happens is that you'll hear slaves groaning and whips cracking because they have to push a lot harder to keep the log moving. Next the slaves start dying of exhaustion. It's a lot cheaper to replace the logs than to replace the slaves.
posted by justsomebodythatyouusedtoknow at 10:00 AM on November 28, 2011 [1 favorite]

handbanana has it right. Anybody with the most minimal of lughead/greasemonkey skills can disassemble a wheel easily enough to inspect/repack with grease/replace an axle bearing without affecting the alignment, suspension or braking systems.
posted by No Shmoobles at 10:03 AM on November 28, 2011 [1 favorite]

Check it now.

I failed to do so once (didn't have the privilege of someone with a clue suggesting it might be the bearings and I had never heard of them before either) in your exact same position and when they finally failed, the entire back right side of my car was messed up completely. Axel problems, wheel had to be replaced.

It was very expensive.
posted by HopStopDon'tShop at 10:13 AM on November 28, 2011

If it's making noticeable noise already, the wheel bearing will have to be replaced. IME, it's around $200-$300 per wheel at a dealer, so it'll probably less at an independent shop. Unless you've driven through high water or something and submerged all the wheels past the hub, chances are only one is going bad.

Generally, they get very loud before they fully fail. First with something of a low buzzing noise, then later with the characteristic noise that I can't quite explain. Not long after that, the wheel will seize up and you'll have a really bad day if you're on the highway at the time. Especially if it's a front wheel.

You can get an idea of how far along the failure is by jacking up the car and spinning the wheel with the failed bearing. The farther gone it is the harder it will be to spin and the more noise it will make while doing so. If it makes a lot of noise when the wheel is spinning at slow speed without the weight of the car on it, it'll be done rather soon. If you can barely hear it and the wheel still spins rather freely, you've probably got a thousand miles.

Another test, which only works if you don't have wheel covers, is to feel how hot the wheel is getting. An imminently failing bearing will (usually) generate a lot of heat. Do note that at this point you're operating on borrowed time and the bearing could fail literally any time, even if it's not making a whole lot of noise or getting really hot yet. There's a decent chance it won't fail right this second, though, if it's not already really far gone.

For future reference, cars shouldn't suddenly start making new noises. If they do, it's a warning that something is either loose or failing.
posted by wierdo at 10:24 AM on November 28, 2011

Chiming in with my experience of not replacing the wheel bearing until way too late. Was driving along and all of a sudden the car lurched 3 feet to the right and almost went onto the sidewalk as the driver's right wheel bearing basically exploded. As for the financial aftermath, the bearing cage fused itself onto the spindle and had to be cut off with a torch. Would have been a lot cheaper to just bite the bullet and replace them before that happened. Won't make that mistake twice...
posted by barc0001 at 10:33 AM on November 28, 2011

wum wum wum is not necessarily a wheel bearing.Bearings are a surface to surface type of thing and tend to make a growling grinding type of noise, now a wum wum. It could be a bad tire (separation of the belt from the body of the tire), it could be a warped rotor, warped wheel ( like if you hit a curb). To diagnose you can do a few things.
1. rotate the tires and see if the noise comes from a different corner of the car.
2. apply the brakes slowly and see if you feel a pulsing. N.B. if it is the bearings applying the brakes could make the sound go away as it changes the pressures on the bearings. applying brakes could make the sound worse too for the same reason.

posted by Gungho at 11:50 AM on November 28, 2011

Simple check:

1. Jack up car to get wheel off ground.
2. Spin wheel by hand, listen.
3. Grip wheel (hands 180 degrees apart), try to move wheel to check for play about the axle shaft - try HARD, then shift position 180 degrees and try again.
4. Repeat with other wheels as necessary.

The wheel should spin silently, and there should be no movement. A bit of noise, and little movement, you have time (but maybe not much); lots of noise, and/or lots of movement, time has run out, get to a workshop fast.
posted by GeeEmm at 1:46 PM on November 28, 2011

then shift position 180 degrees and try again.

That should read 'shift 90 degrees and try again' (Cue GeeEmm going "doh!" for his typo).
posted by Brockles at 2:14 PM on November 28, 2011

The Forester is based on the Impreza (as far as running gear are concerned) and they are made in Japan on AFAICT the same production line. Imprezas are infamous for rear wheel bearing failures. The bearings are dual-row, integrally-preloaded ball bearings that are both sealed and easily mis-handled on installation. Subaru -just has this problem- with rear wheel bearings. It goes back at least to the SVX (mid '90s) and it's worse in Imprezas than in Legacys, but both my Legacys have needed RWB replacement, one of them under extended warranty and one not.

You are going to have to have the bearings replaced, and it is one job I would not try in the garage. Take it to a well-known, independent Subaru mechanic and just have it done.

These failures are -random-. Replace only the bearing that is failing; don't be talked into doing all of them.
posted by jet_silver at 7:13 PM on November 28, 2011 [1 favorite]

Grip wheel (hands 180 degrees apart), try to move wheel to check for play about the axle shaft

For the uninitiated, what you need to do here is pull away from the car body with one hand and push in the opposite direction with the other, then reverse directions. A worn bearing will have a small amount of play, a good bearing won't have any.
posted by HiroProtagonist at 7:55 PM on November 28, 2011

then shift position 180 degrees and try again.

That should read 'shift 90 degrees and try again' (Cue GeeEmm going "doh!" for his typo).
posted by Brockles at 2:14 PM on November 28 [+] [!]


Thanks Brockles, and Hiroprotagonist for your elaboration of my too-cryptic description.
posted by GeeEmm at 9:39 PM on November 28, 2011

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