Hub flange failure - diagnosis, engineering chums?

Donnington 31st October......another new drive flange failure !!!!!!

 

Would you fancy sending it to me?

EDIT: The bits of it? I should have said!

 

Genuinely interesting reading.

For new flanges to be failing so quickly some force must be getting applied that wants to pull the flange away from its spigots natural line of rotation surely unless they really are made of pooh and giving up immediately under the loads of a track car?

If the spigots natural line of rotation is defined by the weight of the car acting through the hub on its spigot into the bearings then the only thing that i can see trying to move the flange from this line, would be if the brake pads (and therefore calipers, mountings etc) are not at 90 degrees to the shaft line, so trying to prise the flange from its centre shaft on every rotation of the wheel under the stresses of heavy braking?

I presume the pads would just wear quickly to compenate though?

Trying to remember when I've heard reference to really shocking chineese materials coming into this country

 

Thanks Mexicorich thats an interesting hypothosis I will check the pads - another theory is that the pawl type LSD (which is set very tight - I need and have big biceps)) is creating some sort of problem at the outer end of the drive shaft/cv exerting extraordinary forces on the flange - It is always the nearside hub (i.e the outside on right handers) and is always followed (or maybe beforehand) by huge gearbox leak caused by a seal failure.
John

 

Info from a US book which I've recently picked up again, due to other topics around the forum:

Source: Volkswagen Water-cooled, Front-drive Performance Handbook, Greg Raven, 1987.

 

Nice find BBB.

Firstly:

Looking at the date - 1987 - these will be Mk1 or Mk2 hubs using 34mm and 35mm bearings respectively?

One wonders if the post 1988 Mk2s, with 40mm bearings, suffer in the same way?

Of course, VWMS used Audi 80 hubs, with 39mm bearings.

Secondly:

One wonders what steel the respective hubs are made from? Did it differ for the 8V 112bhp cars to to that used for the 16V 150 bhp cars?

IE. Was 191 407 615 made from inferior material to 357 407 615?

Were Audi 80 hubs from better material?

Thirdly:

Do the smaller bearings, and hence hubs, have a smaller root radius? The smaller hubs will be more highly stressed at this point anyway, and if there is a smaller fillet radius, it will be a worse stress raiser for fatigue!

Fourthly:

The through spline, and large thread, do not lend themselves to stretching and breathing. Over torquing them as described will hardly stretch them at all, as the vast majority of the stretch will end up occuring it the short length of thread between nut and end of spline. But, this will probably lead to plastic deformation of the steel at that point. It will stay stretched and won't breath as a proper stretch bolt does. This is in effect not much better than using the standard torque unless you are lucky enough to stop tightening just before plastic deformation occurs!

The Audi 80 stretch bolt is far more effective from this point of view.

 

What's the root dia Dave? The 34 / 35 / 39 / 40mm bearing ID, except being the opposing part of the hub flange?

 

I know the root diameter of a bolt/screw is the diameter at the `troughs` of the thread, ie; an M12 bolt if measured on the threads/shank will be 12mm. Less the (supposed) 1.5mm thread pitch, the root diameter of the bolt is therefore 10.5mm.

I`m not sure how to relate that to a bearing however. Dave! We need an engineering drawing!

 

Sorry about that folks. I meant to type radius not diameter!

I have just edited my post!

For some, unknown, reason, I have always called the radius between the inside diameter of a bearing, and the end face, a 'Root Radius'.

So. Bearings have a radius, at the corner, where the end face meets the inside diameter. This radius governs the radius on the hub where the outside diameter of the spigot, where the bearing fits, meets the flange face.

If the bearing has a root radius of, say, 2mm, then the maximum radius on the shaft has to be less than 2mm, so that the end face of the bearing will sit up to the face of the flange.

Now, to allow for machining tolerances, the radius on the shaft, in this case the hub, will be, say, 1.75mm/1.5mm.

OK. Smaller bearings usually have a smaller root radius, so the radius on the shaft is smaller.

Small radii are massive stress raisers. Avoided like the plague!

Thing is though. This is a flange, not a continuous shaft.

For a continuous shaft, the two diameters, where the bearing sits, are easily allowed for in the fatigue calcs.

With a flange, the small diameter, where the inside diameter of the bearing sits, is easily defined. What is harder to define, is the effective diameter of the flanged part. It is a lot less than the outside diameter of the flange!

But. The loads in the hub are a pain in the ar5e to define as they are being generated by circular bending, and torsion, where the flange meets the spigot that the bearing sits on.

Putting it simply: A wheels hub is not the easiest thing to stress out!

 

Thanks for the explanation, Dave, makes sense to me.

 

Ta so, StuMc.

ps. If you are currying favour, for some reason, it's a Keema Vindaloo, in the Kash, in Bradford!

pps. I told her you live over the border. If I'd told her you live in the Red part of Manc. you'd have been quids in!

 

There`s no ulterior motive here! But I do live in the Red part of Manc-land...

 

You'd better send her a photo then.

A couple of others have!

A prawn sandwich box, at the 'Theatre of Dreams', would probably seal the deal!

 

more discussions here http://www.vask.org.nz/index.php/topic,4531.0.html