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Front wheel bearing - end float

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Hi Steve.
I have heard that story too, for well over 20 years.

When I first heard it, I may have remembered enough of my college "structures" classes to explain why the story was wrong. Today, who knows what I remember, but here is my best shot.

Working backwards, the weight of the car (load) is transferred from the tires to the wheels and then to the hub. The load is then transferred from the hub to the front axle through two points of contact, each made via the inner and outer wheel bearings, with the primary load being transferred through the inner bearing, which is why it is larger. It does not make any difference what is between these two points of contact, unless whatever it is, in this case the distance piece, becomes part of the axle. In the case of the distance piece, just because it is tight between the two bearings, it is not adhered to the axle and therefor is not part of the axle. There is still an air space between the distance piece and the face of the axle. The load being transferred through the wheel bearings to the axle dos not pass through the distance piece. Think how plywood or laminated veneer lumber works. Without each layer of veneer being bonded to the layers that it contacts, each layer of would act independently, like the loose pages of a phone book. But when bonded, the structural capability dramatically increases.
So, if it is not transferring load, and not an inherent part of the axle, what does it do? It performs the function Steve described.

 
But could the distance piece, if properly installed with zero end play, serve to stiffen the axle and thereby make it less prone to submit to the bending moment during use. The load path for that would be axle nut to inner race of outer bearing to distance piece to inner race of inner bearing to axle. Doesn't that "effectively" make it part of the axle?
 
The process of transmitting the weight of the car to the ground is the load path. It is much like a building. Floor and roof loads are transmitted through horizontal joists and beams, to vertical walls or columns, that eventually support the loads by contact with the ground through a foundation system.

For a car, the tries are the vertical walls or columns, and the car’s frame, lower links, kingpins, and axles make up a complicated cantilever system of transferring the loads to the tires.

Once the load reaches the front axle, the only part that makes contact with the front axle and can carry vertical loads are the two wheel bearings. From there, the only thing that has vertical contact with the wheel bearings are the two races pressed into the hub. The hub transfers the loads to the wheel, which transfers the loads to the tires.

The securing nut has nothing to do with that load path. It has no contact with the hub, and cannot resist vertical loads. In fact, it can be removed under a full load (tires on the ground) demonstrating it has nothing to do with load transfer. It simply holds the hub onto the axle and is a mechanism for preloading the wheel bearings, as Steve stated.

I suspect zero end play would prohibit the wheel bearings from turning.

Tightening the securing nut places the wheel bearings, distance piece, and shims in compression against the back of the axle. If this had any effect on the strength of the cantilevered portion of the axle, it would a negative effect since it would be introducing additional loads unrelated to the weight of the car. That may be one reason the distance piece does not make any contact with the axle, to isolate the axle from these horizontal compressive loads.

Perhaps if there is a structural engineer on this forum, they could clarify what I have stated.
 
Well, RonR, I do happen to be a (now retired) structural engineer with a 30-year career in U.S. Naval aircraft. I thought you explained it very well.
There is no doubt that the torquing of the nut puts the distance piece under some compression, but in order to compress the distance piece the nut has to "stretch" the axle (i.e., put it under tension by an equal amount in the opposite direction). Any tensile stresses thus introduced into the axle are added to the normal bending stresses experienced by the bottom of the axle from the weight of the car and the suspension loads. This argues against the distance piece being there to "strengthen" the axle.
Since, as you explained, the "column" including the distance piece is not an integral part of the stub axle (not welded or bolted), it cannot transmit loads to the axle to relieve the bending stress.
 
Interesting discussion. I, too, have always read that the importance of the distance piece and shims was to strengthen the stub axle. That has been the rationale of why one should always install all the pieces and not just tighten the bearing and slack off a flat. If the "strengthen the axle" idea isn't true, what is the necessity of the distance piece and shims? The design engineers must have had a reason for them, or they could've same some money in production by leaving them out.
 
I suppose the designers thought they had a more elegant solution to applying the proper amount of pre-load to the bearings than just leaving it to the mechanic to estimate. If the bearings are too loose there is play in the wheel. If too tight, the bearings won't last long. With the shims and distance piece, and especially with a dial indicator to measure end float, the bearing pre-load is "just right".
 
Great discussion on this subject,but what I really like is the picture with captions in post #20.
Thanks for that.
 
Y'all can debate all you want, but I'll continue to set up all the ones I work on with .001" - .002" end float.
 
Randy Forbes said:
Y'all can debate all you want, but I'll continue to set up all the ones I work on with .001" - .002" end float.
Click to expand...

I'm really getting into this "reply with quote" stuff. I just wanted to note how well Randy has integrated with his new surroundings way down there in the 'deep south'. Even if it is Florida instead of Alabama or Mississippi. That Y'all really sounds good. But I'll have to say, Youne's will note that a little looseness works just fine. :smile:
 
Thanks, Steve B.
I am still trying to get my car set up properly.

With the recommended end float, the hub will not rotate one complete revolution when I spin it. That is with no grease and the wheel off the car. This seems too tight to me.

How freely should the hub spin?
 
RonR -- if the hub doesn't spin freely when you have zero end-float, it may be that you have overtightened the nut in an effort to line up a nut castellation with the cotter pin hole. Once I have the end float established, if the cotter pin hole doesn't line up within the 70 lb-ft limit, I put a distance piece shim under the nut and try again. You can usually get this right within a couple tries with the shims.
 
RonR said:
Thanks, Steve B.
I am still trying to get my car set up properly.

With the recommended end float, the hub will not rotate one complete revolution when I spin it. That is with no grease and the wheel off the car. This seems too tight to me.

How freely should the hub spin?
Click to expand...

Too tight. I'm with Randy - mine have had barely perceptible float for years with no problems.
 
With the recommended end float, the hub will not rotate one complete revolution when I spin it. That is with no grease and the wheel off the car. This seems too tight to me.

How freely should the hub spin?​
Click to expand...

If you have the grease seal in place it will cause some friction drag. Without the seal and with dry bearings and .001-.002 float, it should spin freely.
Dave
 
steveg said:
Is that at the end of the hub before you reinstall the wheel?
Click to expand...
As good a place as any.

Like any piece of metal machinery, I expect it to expand slightly when it heats up to operating temperature. I'm currently in New Orleans, so I don't have my usual library of reference material at hand, but I am certain that the specification was rewritten for the MGB models, and it may be up to .003".

Is anyone with a Complete Official MGB Bentley manual able to confirm or disprove?
 
Re: Front wheel bearing - end float

RonR -- if the hub doesn't spin freely when you have zero end-float, it may be that you have overtightened the nut in an effort to line up a nut castellation with the cotter pin


Yep Prolly the case
​
​
 
Last edited by a moderator:
Don't forget with no grease and without the weight of the wheel, the hub won't spin as freely as it should. Even with this set up, I would think the hub should spin 1-2 full revolutions with a light spin. Also as said earlier, if the grease seal is in it will have some drag.
 
A reminder - there are 2 cotter holes at right angles:
screenshot.1272.jpg


The threads are 16 per inch. 1 full turn = .0625".
The relationship between the castellated nut and holes is such that you're never more than 30 degrees away from a hole.
That equals 1/12 of .0625" or .0052" - roughly 5 thousandths.

...FWIW
 
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