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Alloy fins brake drums: advise needed

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I own a Valencia blue Tr250 for 28 years now. It has  a Chris Witor modified  25/65 65/25 camshaft with SU carbs. The car runs very good and also the brakes work well. It has the standard rear brake drums with 0,875” brake cylinder. By accident I found and bought a pair of alloy fin rings  to fit over the brake drums.

It is very similar to the Racetorations part:


The inside diameter of the ring is 23,8 cm. This is the same size as the (lateral)outside of the brake drum. But the brake drum is tapered with a bigger diameter towards the inside of the car of 24,5mm.

So there has to be removed some material from the brake drum or from the new alloy fin.

Because the drum is tapered it is practically impossible to  remove material from the alloy fin ring to get the same angle on the alloy fin ring to achieve an interference fit.

The other possibility is to remove material from the drum on a lathe. Towards the inside there has to be removed more material to a max of 3,5 mm. Than drum will  have a equal thickness, instead of original increasing thickness towards  the inside. Also a part of the lip has to be removed.

I presume that with an interference fit of the ring with the drum, the ring will increase the strength. With 3 or 4 small screws the ring can be fastened (just like the racetorations ones (first picture)

Is this the way to go? Will the brake drums be strong (and safe!) enough? Or is there another better solution?


Regards Henk




Edited by henktr
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Hi Henk,

 the finned brake drums do look neat but are of little use under normal operation.

If you want a quick fix then look at the TRShop offering for Alfins  https://www.trshop.co.uk/special.html

If you want to machine the drums then why not shim the exiting oD of the drums so you know the angle then machine the Fins to match.

As they will be on a slight taper you can calculate the dimensions so that when heated and then screwed down they would shrink to fitted well and good.



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Thanks for your reactions!

@Harry: The freeze and heat approach was also my initial intention, but because the drum is tapered more than  7 mm ( 2x 3,5) and therefore to big in total,  this won’t work.

@ Roger : The TR-shop would be the most sensible option, but I want it to give it a try. (However an nice option if it fails…)The outside of the drum isn’t machined and the tolerance is not that good. With a micrometer i found differences of 0,4-0,5 mm. And the lathe of my friend can not make a stable angle.


regards Henk



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3 hours ago, henktr said:

I presume that with an interference fit of the ring with the drum, the ring will increase the strength. With 3 or 4 small screws the ring can be fastened (just like the racetorations ones (first picture)

Hello Henk, I fear that, particularly with the temperatures involved, the thermal expansion of alloy (ring) will be greater than that of the steel (drum). And that unless, during your fitting it as an interference fit, you exceed the maximum temperatures likely to be encountered - then in practice the ring (save the face which is screw fastened) will be loose when it is hot.  Naturally any gap inbetween the two components will negate any strength that may have come from banding, and also compromise the heat transfer.

Perhaps it may be of interest..  Not so many years ago I restored a 1973 850cc Norton Commando motorcycle.  The cast-steel rear-brake-drum on that bike incorporates the final-drive-chain's teeth. ie., it served as both the brake drum and the sprocket.  Originally, rough cast (..and pretty ugly) I had mine (a new replacement part bought from a reputable Norton motorcycle specialist) machined and balanced.  It was surprising to the machinist, who also happened to be an owner of a similar model, just how imbalanced the new (but supposedly standard spec) brake drum had been. 


^ While at it, I had the machine shop cut, on their lathe, shallow U-section grooves around  the drum. This in turn left the brake drum with shallow cooling fins.  Although they didn't look quite as impressive as deep fins, I'd calculated the surface area for heat dissipation to the air-flow was three times that of the original.  Suitably painted the brake drum looked especially neat and, in part thanks to its balance - the ride of the bike was very smooth. 

Food for thought perhaps.?





Edited by Bfg
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I would wait until you can find one piece drums. Mine are aluminium that have a steel band friction surface cast into the ali drum. 
please with them 

but can only find this picture. 


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