30 replies to this topic

[CoupedUpSubie]

Chris, on 16 February 2012 - 09:49 AM, said:

I put my big-ass pry-bar between the control arm and the knuckle. And jump on it. That usually pops them out.

Jump on it? It takes me just leaning on it. Now if they have been apart recently I can usually smack the contol arm with a hammer and its apart.

[Chris]

CoupedUpSubie, on 16 February 2012 - 03:25 PM, said:

Jump on it? It takes me just leaning on it. Now if they have been apart recently I can usually smack the contol arm with a hammer and its apart.

Your car is 6 years newer than mine. I assume on the Legacy it wouldn't fight as bad.

[IBratmanI]

I just had to pop the control arm off the ball joint last night on the 98 Forester. I had to use a maul to give it a good whack to pop off. Rust sucks

[Mighty Subie]

IBratmanI, on 13 February 2012 - 05:58 PM, said:

I have NEVER heard of anything about not using an impact on an axle nut. In fact, it's the way I have always done it. It can't really do anything to the bearing unless it wasn't tight enough. I have never had an issue from tightening an axle nut and I have tightened hundreds at this point. Never had any pulling, and no failed bearings. The hub tightens against the bearing race, so unless you are using an impact meant for big trucks or something, you should never have to worry about that.

Sorry for being a little late on this, but dude.... Overtightening a bearing is bad bad bad. It will overheat it, burn up the grease and burn up the bearing. Also it will pull to that side and compromise not only handling but mileage too. This is coming from someone who has worked on plenty of cars, and tractor trailers. There is a torque spec for a reason. Live by it.

If you want a little lesson on overtightening a wheel bearing, take a small trailer. Tighten the heck out of one wheel on it. Tug it down the road. Don't just go a few feet but check it out. If it's empty and nice and cranked on, you will have an awesome flat spot on the tire and it would have been hopping all over the place. Or if it's forced to turn because there is a load on it, I am betting that it will be very unhappy. This is the most simple explanation and something that I am sure everyone can understand. If you don't understand it, do it.

[IBratmanI]

Trailer bearings and tractor trailer bearings are quite a bit different than Subaru wheel bearings. Again, by design, they really can't be overtightened! The axle nut can only be tightened so much. I know all about trailer bearings and bearings in general. Of course if you use a tool that has enough energy, you could do damage, but an everyday impact gun will not hurt a Subaru wheel bearing, it just isn't going to happen.

[CoupedUpSubie]

Does anyone have a cutaway of a wheel bearing for these? The pictures online dont really show a conical bearing(which is used on a trailer) which is affected directly by the torque. Instead it looks like it may be a needle bearing.

Edited by CoupedUpSubie, 16 February 2012 - 08:36 PM.

[Chris]

I just went on google and looked up buying a bearing for a legacy. Here is the picture of the part.


Yes, it is a taper roller bearing. And you can see the inner races of each bearing touch each other in the middle. They're machined such that the correct preload of the bearing is when the races are seated up against each other.
Because of this, you can't tighten the bearing too much. You'll squeeze the races together tighter, but you won't change the bearing preload.
You'd probably strip the axle nut out before you got it so tight you deformed the bearings enough to cause a problem.

This is completely different than a trailer wheel bearing, or the bearing on an un-driven wheel of anything, for that matter.
You can certainly over tighten those.

[CoupedUpSubie]

It is not the squeezing of the bearing that will ruin it. It is the axial force pushing in one direction that will ruin it. This is why they wear out over time. No matter what there is a slight amount of axial force shoving one race against the rollers. This breaks down the grease and allows metal to metal contact. They would last 10x longer if they had no real force applied to them. From what I can see from drawings the hub mates to the axle. The hub and axle have a small area that rides on the bearing just to keep the wheel where it should be.

Chris, on 16 February 2012 - 08:44 PM, said:

You'd probably strip the axle nut out before you got it so tight you deformed the bearings enough to cause a problem.

Chris is completely right. The axle nut would give way before the bearing was over tightened.

The reason there is a torque spec is so that when Joe Shmo fixes his car and doesn't secure the axle nut via the cotter pin or peening the flange but tightens the nut to spec the chances of the nut coming off is a lot smaller. I go with torque specs to cover myself. If you go in and tell a parts store the axle nut you put on your car failed due to threads stripping they will ask what you used to tighten it, if you say an impact then its on you.

Edited by CoupedUpSubie, 16 February 2012 - 09:23 PM.

[Chris]

The torque spec is so you don't under tighten the bearing. It's also there to make people who don't know how to work on cars happy, because they have a number to follow.
There are very few things I actually torque to spec, for the most part you get a feel of how tight things should be.

Pressure plate is important, if it's too tight, or not evenly tight it'll deform and be a problem. Water pump is important, same deal with deformation there.
Heads, definitely probably the most important to torque properly.

And unless you're rebuilding an engine or transmission, that's about all that's super important.

[Mighty Subie]

they dual taper bearings. Kind of like this::



This would just be for the bottom half. The hub would be below this and then the axle above this. Then invert that and you'll get the top.

[Chris]

So you're changing your tune on how tight the bearing should be?
Took me a sec to understand that pic, but yes. That would be a longitudinal cutaway.
Since the force of the axle and hub are applied to the inner race, the preload on the bearing will never change no matter how much force you put on it.
Assuming no deformation in the metal, you could put an infinite amount of torque on the axle nut, and it would still have the same preload.

I do not personally have a picture of a Legacy front bearing, but I do have pictures of a GL rear bearing. And it's more or less the same bearing design.

Here you can clearly see how the bearings look when they're out of the race.


And again, here. Same rear bearing going into the trailing arm. But you can see the inside of the race. For those that may not totally get how it all works.

Edited by Chris, 16 February 2012 - 10:18 PM.