Some things I have learned about Spridget front wheel bearings.

 
 

by

Tom Smith

tom's e-mail

 
 

Many people have been involved in this research.

My thanks to those on The MG Experience and the MG Enthusiasts forums

And for the help and guidance of

Peter Caldwell and Bill Young

Peter has been very patient during all the visits to his shop and has helped in so many other ways.

Bill provided feedback during the early days when it was needed the most.

The entire Spridget crowd thanks you all.

 
 

See "STUB AXLE MODULUS STUDY" at bottom of this page.

A special thanks to Norm Kerr for his work on this.

 
 

All material Copyright 03/04/2010


 
 

Disclaimer Notice - Please Read before proceeding

Warning, any attempt at the mechanical repairs or modifications,
or following the procedures in this article, is done entirely at your own risk.

 No liability will attach to either myself, those that have provided guidance,

any company, or individual that performed work on my car.

 
 
 
 

Not terribly important, but for those that might be interested

A little about me.

 
 

Early in the summer of 2009 I purchased a 1974 Midget in kit form with “some” assembly required. Body parts had been replaced and it had been painted. The engine had been professionally rebuilt. The only thing was that nothing had been put back. The bonnet was loose, nothing was in the engine bay and there were about 15 large boxes of parts. The left front suspension had the spring removed and I found the wheel nut on but no cotter key to retain it. It seemed that when the previous owner got to this point where he needed to start working on the car that he just gave up. It was not until later that I realized what might well have been the reason he stopped. He apparently could not find wheel bearings that would fit correctly.

 

I began working on the car. I rebuilt the front suspension and repainted the engine bay. Installed rebuilt shocks from Peter Caldwell and generally completely restored the front suspension. It wasn’t until I began installing my freshly rebuilt calipers that I found a problem. The new wheel bearings caused the rotors to rub on the calipers. This led to an extensive search for solution not only for my needs but also for the best answer to keep these cars on the road and do it safely.

 

Before going further I will give a little of my background. I started working on cars with my Dad nearly 60 years ago. During the summers while I was in high school I worked for a road construction company which included a considerable amount of time repairing heavy equipment. For several years I was a full time mechanic for the Department of Defense as a civilian at the same time I was the maintenance sergeant for an Army Reserve ammo company and in charge of the motor pool. While farming about 300 acres I worked as a millwright in a large gray iron foundry. Later I ran a machine shop repairing industrial food processing equipment. Add to that I am a machinist and taught machine tool courses at the local Tech college. The courses included tool design, metallurgy, and CAD/CAM.

 

During all those years I replaced thousands of bearings and learned more than a little about them in the process. And after reading many posts about the search for bearings for these Spridgets I felt that there was too much confusion on the subject. Being retired and having some knowledge I decided to do some investigating. From the start Peter Caldwell from World Wide Auto Parts has been involved and soon after Bill Young became part of the team. Both gentlemen highly respected in the world of Sprites and Midgets.

 
 
 
 

Before there is any talk of bearings there are some details of the cars themselves that need to be discussed. That would be some findings about the spindles, spacers, and hubs. Before looking at them individually it should be pointed out what they have in common. That being they are old and many are considerably worn. This wear effects how different bearings fit or probably more accurately, don’t fit. My Midget has less than 40,000 miles and has very little wear. Couple that with the fact that the new taper roller bearings, rotors, and break pads, added to the rotor/caliper contact condition. A car with more wear in those parts might not have the problem I had attempting to fit taper roller bearings but they still would not fit correctly. Additionally, wear of the hub and/or spacer can make it difficult or even impossible to have properly adjusted ball bearings.

 

As this project progressed we found something disturbing. It seems that the included radius between the shaft the bearing fits on and the face it fits against is not the same on all spindles. We found that it varies from 1mm to 2mm. around 2mm seems to be the norm but Peter has found close to 3mm on spindles from a '79 Midget. At this time we are not sure if the radius changed with the year of manufacture or if it was a product of dimension tolerance and a production decision of the shop or batch being manufactured. With the age of our vehicles, changes of ownership, maintenance, and who knows what other reasons we may never know the answer.

 

So why is this a problem? Looking through Peter's Ransome & Marles bearing books. It seems that the 58-61 Sprite, Sprite MKll 61 and on and the Midget 61 and on (these books are old so I do not know the definition of “on”) were provided a ball bearing with a 1mm radius. Which would suggest that the 07097 (1.5mm radius) and the 30205M (1mm radius) would fit. The fact is that these bearings do not fit many and possibly only a few Spridget spindles. We only found one spindle of those in Peter’s shop with a 1mm radius and most were closer to the 2mm dimension. On the other hand Peter had a pair of original face adjusted ball bearings that had a 2mm radius and fit my car perfectly.

 

Here is the problem. When the bearing radius is smaller than the spindle radius the bearing does not fit against the flange. On my spindle a bearing with a 1.5mm radius left a .020” gap behind the bearing and a bearing with a 1mm radius left a .023” gap. The contact between the bearing and the flange face is designed to help support the axle load. Hence, a gap would provide a flex point which could easily lead to failure of the spindle shaft. I have come to believe that this is a likely cause of the majority of spindles breaking, not the lack of using the original bearing spacer that has been commonly blamed. I also wonder how many cars may be on the road with bearings that have a gap in this location without owners even being aware. BTW, I have been in contact with an engineer at Timken and he agrees a gap in that location is a bad thing.

 

Machining the bearing radius. I have had several say why not just grind or machine the radius on the new bearing. I did ask the Timken engineer if that could be done. He flatly recommended against it. He said that it would likely create stress fractures in the bearing race. By that time I had chased this dog around the bush so many times that I was getting a little cynical and although I respected him I did not believe it could be true.

Then I had the opportunity to ask specific questions of Jeff Weiss of LMS Bearings, Inc. LMS makes custom bearings. Jeff was very patient and explained that bearing races can only be machined before the bearings are assembled. In fact that is how and when the radii on bearings are applied. Finally the question became clear and the answer was soon to follow. The machining does not damage the radius area of the race. The machining causes the balls or rollers to bounce around and produce a brinelling effect on the inside of the race. Even though the marks may be invisible to the naked eye these small dimples or creators caused by this brinelling effect on the race surface from the movement of the balls or rollers during machining can dramatically decrease bearing life.

You can Google "brinelling effect" to better understand.

 
 

 

 
 

The pictures below show the actual gap behind a 07097 taper bearing installed on my car while the second picture below shows a .020” feeler gauge in the gap.

 
 

 

 
 

 
 

 

 
 

 

 
 

The bearing races in these drawings are accurate in width, diameter, and bore.

The balls and rollers in the drawings are only added for visual effect and are not accurate in size or numbers.

 
     
 

First we look at the original bearings and how they fit.

 

 

I measured the original ball bearings from my 1974 Midget. Here is the result. Peter had a set of original replacements and they also fit my spindles.

These drawings make a good control reference for comparing other bearing fits.

I have heard several folks complaining of their hubs being loose after installing new bearings.

Take note of the shim notes in this drawing.

 

 
 
 

 

 
 

Now let us look at the 7205/7303 ball bearing combination.

It is worth noting that on 03/03/2010 I was told by Motion Industries that bearings NSK no’s 7303BEAT85SUN and 7205BEAT85SUN have been discontinued.

 

More on this later in this article, but it is important that the shim is thick enough so the bearing tightens against it and the stub face rather than against the radius of the shaft. In other words it would be be better to be a little thicker than needed than to be too thin.

Although the 7205 bearing is the same thickness as the original bearings it forces the hub away from the stub face because of the radius mismatch with the stub axle. Although the seal still should contact the seal surface of the stub it should be verified.

 

 

 
 

 

 
 

Here is a look at the 07097/30303 taper bearing combination.

 

 

For the taper bearing person this is a possible solution. Not perfect, but possible.

First, whether or not the spacer between the bearings is used it would be important to shim between the inner bearing and the stub face. As discussed earlier, thicker is better than too thin.

Because these bearings also push the hub away from the spindle face, something to be aware of would be the seal contact on the stub seal surface. Make certain it is OK.

If the bearing spacer is to be used it would require an additional spacer or shims of about 3mm. If the retainer nut was to be torqued  without the required spacing it would be disastrous.

The option would be to use the traditional adjustment process for taper bearings.

More on the spacer issue later in the article.

There is also the issue with the retainer nut. It will require .050" to .080" to be removed so the key can be used.

There is also the possibility to machine the surface in the hub that the inner bearing seats against. While it would not help with some of the issues it would help with the seal issue and with the nut/key issue. The down sides include some degree of machining and mechanical design. It would also alter the hubs so returning to an original bearing would not be easy.

 

 

 
 

 

 
 

Here is a look at the 30205M/30303 taper bearing combination.

 

 

For the taper bearing person this is a possible solution. But far from perfect, but still possible.

It has all the concerns of the 07097/30303 combination only more so. Notice the seal location.

It is possible this combination would also cause the brake rotors to rub in the calipers.

This bearing combination will require machining the hub where the inner bearing seats.

The down sides include some degree of machining and mechanical design. It would also alter the hubs so returning to an original bearing would not be easy.

And what if the 30205M bearing becomes discontinued?

Even though the hub alteration will change the bearing spacer issue, that is if it is to be used, it will still almost certainly be too long or too short.

 

 

 
 

 

 
 

 

 
 

The spacer or no-spacer question and a fact based answer.

STUB AXLE MODULUS STUDY

BY

Norm Kerr

you are right, I should have been more clear in my email about the need for a shim, or, ideally, the correct 2mm radius on the bearing inner race, so that the inner bearing surely has full face contact with the face at the base of the stub axle. We are both totally in agreement about that. Thank you for providing those dimensions, with them I was able to calculate the section modulus of the two options:

1) no spacer, "let the stub axle flex" - section modulus at the inboard end = 0.09 cubic inches (section modulus is represented in units of volume). 0.985^3 * pi / 32

2) with spacer - section modulus of the hollow pipe (spacer, plus the inner races of the inner and outer bearings), I roughly calculated its section modulus to be 0.154 cu in.

 

(1.31^4 - 0.985^4) * pi / (32 * 1.31) (for easy reference, these formulas are also in Wikipedia Section Modulus, option 1 is "solid circle", option 2 is "pipe").

 

In structural engineering, the section modulus is used to calculate the strength of a beam, and the area of highest stress will be where the beam is rigidly supported (in our case, at the base of the stub axle, inboard of where the inner bearing face contacts, the 1.31" diameter solid portion.

 

Using the spacer should provide 1.7x the strength (0.154 vs. 0.09), compared to just the stub axle alone.

 

Honestly, I was a little bit surprised that the benefit of the spacer was only 1.7x, I was expecting it to be more than that.

 

On second thought, there is another important thing to strive for in structural engineering: avoiding sudden changes in section, as these can be stress risers (places where stress will concentrate). Even though there is "only" a 1.7x improvement of section modulus between option 1 or 2, they both terminate in to the same, solid base of the stub axle which is 1.31" dia. Calculating the section modulus of this solid circle I get 0.22 cu in. The transition of 0.154 to 0.22 is much less of a change (1.4x) than the transition of 0.09 to 0.22 (2.4x). Even if this is not more important than simply comparing the stress modulus of each option, this reduction of sudden section change further supports the idea that using the spacer improves stub axle stress management.

Norm, thank you for the contribution to this project, Tom


A couple things before going on.

The dimensions I gave Norm almost certainly will differ some on individual cars. The principle would remain the same but the actual numbers might be slightly different.

I show ball bearings but the principle would as readily apply to taper bearings.

You might take notice of the position of the seal contact point in the drawings. The drawings above also demonstrate this, especially when using the 30205M bearing.

Most important, even though this study shows the strength or weakness of different bearing assembly configurations. It in no way says your axle will break if you have one of the non-original configurations.


Original Bearing configuration.

This first example is of the original bearing configuration. Includes proper fitting bearing and spacer between the bearings.

The shaded area shows where the strength of the assembly is derived from.

This is the strongest of the assemblies due to three facts;

1)  It has an improved section modulus. (1.7x)

2)  A reduced section change. (1.4x vs 2.4x)

3)  The section change is more outboard to an area of less stress.

The correct bearing provides the best fit of the seal contact position on the stub seal surface. It also provides the best fit of the rotor in the caliper.

And because the correct fitting bearing allows the hub in the proper position closer to the stub face it causes less stress on the axle.


Improper bearing fit

This is an example of a bearing that does not fit correctly because it has a smaller radius than the stub/face radius.

Like the first example it has the spacer between the bearings.

The shaded area shows where the strength of the assembly is derived from.

Consider this; The lack of a shim behind the bearing causes a big stress concentration in that area.

That notch can change stress concentration even at small input loads.

Additionally the hub is moved outward a small amount adding to the already considerable increase in stress.

This configuration would be the most likely to lead to the stub axle breaking at the gap position.

You might also notice the position of the seal contact is moved outward.


Correct bearing fit without spacer between bearings.

This shows a correct fitting bearing without a spacer between the bearings.

This and the following example would be more likely used with taper bearings.

By not having the spacer between the bearings there is a big section change where the stub axle meets the stub face.

That big section change would create a high stress concentration.


Incorrect bearing fit without spacer between bearings.

This shows an incorrect fitting bearing without a spacer between the bearings.

This and the previous example would be more likely used with taper bearings.

By not having the spacer between the bearings there is a big section change where the stub axle meets the stub face.

That big section change and the addition of the gap behind the bearing creates an even higher stress concentration.

Again, notice the position of the seal contact point on the stub seal surface.

 
 

 

 
 

I believe I have found a source for the correct bearings and ordered a set for my car. If they are correct I am working to make them available through a supplier. I will report my findings.

McMaster Carr has shims.

 
 

 

 
 

 

 
     
 

 

 
     
 

 

 
 

Peter at World Wide Auto. www.nosimport.com

 
 

All material Copyright 11/25/2009