Facebook Fontana Rear Hub

[Graham Smith]

I found this on the Club's Facebook page.

I thought some on our forum website might find it interesting.

Advice needed. I have a JMC Egli. Fontana rear hub. On Sunday the rear wheel studs snapped.
I can't comment on whether they worked loose and snapped or the lack of cush drive caused it.
Either way I need to sort this out.
Can anyone give me some insight on to how to sort this out so it doesn't happen again.
I have the skills to repair it just not sure whether to stick with the rigid mount or adapt some cush bushes into the sprocket on studs fitted to the hub.
My main concern with doing this is I will lose my Speedo drive.
Has anyone out there had this happen to their bike?

 

[Peter Holmes]

I would be interested to hear what folks from an engineering background think of this, from a purely instinctive point of view, I can’t help but think this is an accident waiting to happen, with a countersunk screw the threaded section goes right up to the countersunk head, which seems to provide a perfect shear line when fore and aft inertia is applied, but I am not an engineer, and concede in advance that I could be talking gobbledegook!

 

[bmetcalf]

Pre-broken, in other words?

 

[greg brillus]

Awesome brake assembly half destroyed by pathetic engineering practice........countersunk screws should never be used in high torque areas like this.......Not going to be an easy one to fix, as the damage is quite bad.........It needs to have some decent sized dowels installed to take the loads and do away with the countersunk screws all together.

 

[vibrac]

I can only speak of the sprocket carrier I made for Bens racer
Accuracy of the bolts is paramount. I used an intermediate jig to accurately associate the five hub bolts to the carrier bore and to ensure concentricity (see below) Because of the large engine sprocket we needed to get smaller rear sprockets than 45-6 so we used Honda sprocket blanks using the same 5 bolt holes (allen cap bolts) and I reamed the bolt holes to ensure a snug fit in the carrier
I made the carrier from a large lump of aluminium, the swarf was a wonder to behold!

 

[davidd]

This same problem did occur to someone here in the States about 15 years ago. His Egli came from Slater, but it contained many original Fritz Egli parts at the customer's request. Unfortunately, Fritz did not have an original rear hub.

The original hub was a Campagnolo item, which had a cush drive. The hub was modified by Egli to fit the Vincent by making an adapter that was fitted to the outside plate of the cush drive that provided the correct chain line for the Vincent.

I do not know why Fritz came to the conclusion that a cush drive was necessary, but I think it was a good idea. I suggested to the US Egli owner that he use a Ducati 450 rear hub, which had a cush drive. The owner purchased the Ducati hub and made an adapter plate to mount the Vincent sprocket. Sprocket makers will often make adapters for customers.

Above is the modified Ducati 450 rear hub on the Egli. The speedo drive on the early Eglis was located on the left side and driven by two small holes in the disc brake, in which the speedo drive tabs fit. This was discussed on the Forum years ago when Jerry was building up an Egli 500 he purchased from Beno Rodi. Jerry did not have a rear wheel for his bike but was worried about driving the speedo. He later called me and asked what he should use as the Fontana rear hub was so expensive to buy. I suggested he use a stock Vincent rear hub as it was inexpensive and easy to get. Although it did not have a cush drive, it works well for decades with few reliability problems.

I use a sprocket adapter on the non-brake side for the racer.

So, there are two reasonably good solutions.

David

 

[timetraveller]

Without access to the parts, it is difficult to assess the damage and I do not understand the function of the part with three countersunk holes as opposed to the seven countersunk holes. However, if the damage to the seven holes which had the countersunk screws in could be overcome by going up one, or even two, sizes of thread then could the hub be recovered by going for larger socket headed screws and, if all else fails, a GPS speedo?

 

[vibrac]

Memory fade the sprocket had separate bolts using the standard Honda (countersunk) screws (ex Enduro bike). Chain is of course 5/8" X1/4" (well worth doing) and sharks fin shield

 

[ClassicBiker]

I would be interested to hear what folks from an engineering background think of this, from a purely instinctive point of view, I can’t help but think this is an accident waiting to happen, with a countersunk screw the threaded section goes right up to the countersunk head, which seems to provide a perfect shear line when fore and aft inertia is applied, but I am not an engineer, and concede in advance that I could be talking gobbledegook!

That perfect shear line is referred to as a stress riser. It concentrates an infinitely high load on an infinitely small point. It is the whole point, pun intended, behind how shears, scissors, tin snips, anything that cuts works. The blades are at an angle to each other creating an infinitely small point. Screws should never be used in shear, they are for clamping only. Bolts for shear. I might be preaching to the choir but the difference being screws have threads that run all the way to the head, bolts have a non threaded portion under the head and can take a shear load on that portion as they don't have a stress riser.

Incidentally I am an engineer.

 

[Robert Watson]

Just for clarification.

In North America an engineer is a person who has gone to school to learn all the theory of his particular discipline, be it mechanical, electrical, structural etc.

A person who runs machining equipment, such as a lathe, milling machine, grinder, etc. is a machinist.

I believe in Europe the term engineer is often applied to machinists!