H: Hubs, Wheels and Tyres Seven Inch is in the Regulations

Peter Holmes

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Experimenting with brakes like this would terrify me, locking up the front wheel at any speed can cause serious injury, it was brought home to me when one of my vans failed an MOT test by the local main Vauxhall dealer, Pedestal garage, High Wycombe, they talked me into letting them replace the rear brake shoes, I think they must have let a lad on work experience do the work as when I collected the van it soon became apparent that something was very seriously wrong, driving along there would suddenly be a loud bang followed by the rear wheels locking up solid and rear of the van squatting down momentarily before sorting itself out and then doing it again soon after, several times, it turns out the wrong length pull off springs had been fitted, and they were not pulling the brake shoes off, this experience made me aware of the power of the humble single leading brake shoe when left to servo uncontrolled, now I have no idea how this correlates to fiddling with the standard Vincent brakes, but fully floating sounds a bit like self servoing to me, not sure that I would want to risk it, I think for me it is either standard brakes, Vincent Speet brakes, or a disc conversion.
 

Black Flash

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The brake David Dunfey described is what we call a servo brake in Germany. In 60s they were fitted to the front axle of MAN trucks.
During my apprenticeship I learned in a company where we still had a customer using one of the old trucks. He said the brakes were great when the truck was fully loaded, but very snatchy when empty .
I first designed a servo brake in 8 inch diameter but then decided against it for two reasons.
First I want a brake with feel. I am therefore willing to sacrify some of the outright braking power for feel.
Second you have almost no brake action [in the best case one trailing shoe per side] when it comes to stopping a bike rolling backwards on a hill.
I know this is unimportant on a racer, but I intend to use my brake on the road.
Also do not underestimate the forces produced in a servo setup. For ultimate braking the secondary shoe should not have a pivot but an abutment which allow the shoe heel to slide on and wedge into the drum. I believe to get the best result in a cable operated brake contrary to a hydraulic brake this is where you gain most of the braking force.
In the literature I could find till now I couldn't find a real advice how to design this abutment for that reason, so it will be a typical trial and error scenario.
So there will be a great force pressing against this abutment. Ideally you'll also need at least 2 springs per shoe to return them into a positive stop position.
So while the breaking power will be improved over a 4ls brake, I did not have the guts to build a servo brake and be the test rider.
Maybe I am just a whimp.
 

davidd

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Hi there David, is that type of brake the same as Patrick used on the Isle of Mann racer that used a Sealy brake. It is one that we discussed some time ago.........

Greg,

Based on the description, I thought the Seeley shoe was a duo servo brake. It turns out that when I finally saw the inside of the shoe it was much more of a standard leading/trailing shoe. What Seeley did was add a floating cam as opposed to a floating pivot. It turns out that this is a fairly tricky thing to do. The cam pivot needs to be tight enough not to be pushed about, but loose enough to move as the leading shoe wears more than the trailing shoe. In the normal leading/trailing shoe you get a situation where the leading shoe wears faster than the trailing shoe and the leading shoe begins to be held from touching the drum with full force until the trailing shoe wears down a bit. This is called "equal work condition" but should be called "unequal work condition because you get this back and forth unequal wear as the shoes exchange decent wear rates.

I suspect many have noticed this. When you finish doing a brake job the brakes stop really well and your are patting yourself on the back. In a short while the leading shoe has worn a bit and the trailing shoe has not. The braking gets a little tepid. Now you are stuck in this cycle of one shoe wearing and then the other. Bill Hancock suggested using a soft lining on the trailing shoe. The lining would wear quickly and keep this uneven wearing cycle duration at a minimum. Seeley floated the cam in an attempt to do the same thing. It occurs to me that the floating cam could keep the leading shoe close to the drum, but it would not wear down the trailing shoe any quicker. So, you would have to swap shoes on the drum, front and rear to keep them equal, but I do not have any experience with the Seeley. I would note that Cam Donald liked the new Godet Flash, but I remember him saying the brakes were not good.

Duo servo brakes are used by the thousands, mostly in trucks as Bernd mentioned. I think that is due mostly to small cars having been converted to front and rear discs.

I still would not worry about spragging. I would start with a relatively low friction lining. Figure out where the front and trailing edge of the lining should be cut for the best braking and then increase the friction of the lining. As long as the brake is stable (not grabby) you're good. It is difficult to get too much servo action in Vincent brakes. You can't use the highest friction linings as they require too much heat to work. Very high friction linings require several thousand degrees of heat to wake up and stick. This is why many modern brakes are made from carbon. On a smaller scale, we would see this as the AM4 problem. The friction is great, but only if you are able to generate enough heat.

David
 

vibrac

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I am a big fan of UK safetek who usually supply me an AM4 substitute for racing. The next time I think I will try a different mix on the trailing shoes probably the mix I use on the rear as I have found there is little point in adding very hard lining there as engine braking uses up a lot of rubber adhesion on tight corners and sliding into corners is a bit millennium :p and way beyond my capability's nowadays
 

hadronuk

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All very useful info.
I like that the mod could be done without altering the external appearance of the brake.
But having read the comments above, I don't think I will attempt it.
With the Triumph, is easier to just fit a later TLS backplate instead.
 

greg brillus

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I think the good or bad description of drum brakes is relative to the rider as well..........Most of us are very familiar to old bikes and drum brakes, and the capabilities of them. A lot of "A" grade riders who mostly ride modern high powered bikes with excellent brakes all say that on the older machines the drum brakes are poor..........To me this seems quite normal, as how could any drum match disc brakes..........Now it is not a matter of the drums not being as effective at braking, it is how the drum cannot dissipate the heat fast enough for the brake to return to full operating capacity. This is where the top riders see the brakes as not good.........It must be said too that a lot of classic race bikes built to modern specs are far more powerful than the originals, whereas the brakes can only be improved so much by comparison. This is a "Very" limiting factor in how fast these bikes can be pushed around the race tracks.........Even with "Poor brakes" these top riders can still ride the old bikes amazingly fast..........That is a rider showing their skills at best.
 
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