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


vibrac

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VOC Member
Perusing the race regulations for BHR,CRMC and Landsdown it would seem that original size brakes are mandatory so however good the 8" brake offerings are, they are a no go, however at least one organisation (CRMC)allow twin leading shoe modification now I did read here about someone(Erich Kruse?) who had done such a modification and I expect there are some other documents somwhere on how it can be done if some kind sole could point me in the right direction....
I am quite interested in an internal linkage but perhaps that is too much to hope for
 

Old Bill

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Vibrac, are you referring to Aluminium Brakes in general chat? Internal mech for tls but eight inch drums? Look good but l think the price would be interesting!
 

roy the mechanic

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According to C R M C eligibility, any drum brake of any type may be fitted, provided it was used in period. So, basically take your pick.
 

davidd

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I think Erich's system is best for the 7" shoes, if that is what you are limited to. I did email Erich years ago, but my German is not good enough to converse about technical items. He did make drums and plates. Some of his plates have been twin leading shoe plates.Eric Kruse Germany.JPG

Twin Leading 7 Inch Kruse.jpg
His plate looks much like the one Paul Packman used (MPH 553) for his twin leading shoe conversion. Erich has flipped his levers forward and used two instead of the three levers that Paul used.

I would tend towards using the exposed linkages to start with. Bernd's design of the hidden linkages is brilliant, but on a racer I would like to be able to adjust the linkage lever ratios to get the maximum amount of braking force with the existing hand controls. That is a challenge with the hidden linkages.

You really want to get away from the Vincent brake shoes and use a good shoe. I do not know what Erich is doing about shoes. I would email Erich.

David
 

hadronuk

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VOC Member
I freely confess to owning a 1958 Tiger 100 with a 8" SLS as standard.
In spite of having better rigidity than a Vincent brake it is still remarkably poor.
Triumph later greatly improved it by making the shoes fully floating, so I was searching for info on how to modify my early brake when I found this:
http://victorylibrary.com/brit/SLS-TLS-c.htm

The page includes this diagram of how to make a SLS brake have a high self-servo action:
190mm-TLS-mod.JPG

As I understand it, only 2 mods are required:

1) The fixed pivot point is completely removed from the backplate and a shim placed on one shoe so that braking forces in the leading shoe are fully transferred to the trailing shoe and both shoes are now fully floating.
2) One cam lobe is ground off so that the remaining lobe only acts on the leading shoe. The trailing edge of the trailing shoe now abuts a constant radius on the back of the cam.

A few thoughts.
I can see it might give powerful braking, but it sounds like careful adjustments to the linings will be required to avoid locking up due to excessive self servo action.
The cam now carries all of the braking forces.
Additional provision such as anchor springs may be required to keep the shoes from moving sideways.

Anybody tried this?
 

Bill Thomas

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Ron and me used 2 cams per side and fully floating, On alloy Slater Plates, Early 70s.
It was good, But not as good as I needed. Cheers Bill..
 

Attachments

davidd

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VOC Member
I contemplated making a dual or duo servo brake for the front, because it appears to be a leading/trailing shoe. However, upon rereading the rules they specifically required the bike to have a trailing shoe. I saw that as a stopper.

The duo servo shoe has a primary and an secondary shoe. Those shoes are both linked together (where the pivot would otherwise be). The leading shoe has a cam and the trailing shoe has a solid abutment to hit so it does not move. That abutment could be the constant radius cam or a lump welded to the backing plate.

The real trick is that when you move the cam both shoes are forced by one the single cam to move in the direction of the rotating wheel, which they will gladly do. As the leading shoe (primary shoe) grabs the drum, the drum it is jamming itself into the into the trailing shoe (secondary shoe.) Because the back end of the secondary shoe cannot move due to the constant radius cam or the abutment, all that force causes the secondary shoe to be pushed hard into the drum. Because the back end of the leading shoe is connected to the leading edge of the secondary shoe it is pushing hard on the leading edge of the secondary or trailing shoe. Exactly what you want!

brakes-drum-chapter10-45-728.jpg

Just for clarity, the blue colored abutment at the top would be attached to the backing plate or it would be a constant radius cam.

The cam, which is not shown, would be pushing the shoe where the small red "block" arrow shows. The cam is shown at the top of the drum here, but it could be rotated to the bottom.

At the bottom under the link is another "block" arrow showing the transfer of force from the primary shoe to the secondary shoe. The leading edge of the secondary shoe is the one that is being forced into the drum by the link.

The one sided constant radius cam from a Triumph is shown here:
Triumph Sliding TLS Cam.GIF
The cam lobe is on the bottom in the photo and the other lobe does not exist, but is a constant radius so the shoe that is resting on this side will not actuate even as the cam spindle turns. This way it works one shoe only.

I have read that these brakes are better than twin leading shoes in terms of braking. I have also read the the primary shoe does not generate as much force as the secondary shoe. This would lead me to believe that a twin leading shoe might have a bit of an edge. I don't think you will have to worry about spragging with this brake design any more than on twin or 4 leading shoe brakes, but it pays to be careful.

David
 
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greg brillus

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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......... All the 4 l/s brakes i have played with have either circular anchors and strong springs, or a pin with a matching diameter hole in the shoe and a circlip to hold in place. These include the Suzuki water bottle, replica Ceriani 230 mm, and replica Fontana 250 mm.
 

vibrac

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VOC Member
Dual servo all seems viable and would be leagle for racing but I don't like the phrase " excessive servo action" I certainly would want conclusive practical examples of its use
 

greg brillus

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I think the excess servo action is probably down to careful set up and adjustment as the brake linings wear. In most old cars from the 70's if the linings get worn the brakes get very snatchy, almost send you through the windscreen when you touch them. This on drum brakes of course, and with a vacuum booster to add the extra fluid pressure.
 
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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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VOC Member
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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VOC Member
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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VOC Member
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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