E: Engine Speet Brake Installation Query

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Marcus asked Erik:
I would very much like some more of your knowledge on the modified Speet brake you have set up on your Comet after Vincent tried them and commented that they were better than his twin cable set up, but I like the thought of using the balance beam increasing the mechanical advantage and getting rid of those adjusters on the brake plate as it makes wheel removal much longer and a pain in the ass to readjust again .
 
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Gary Gittleson

Well Known and Active Forum User
VOC Member
I have Speet brakes on my Rapide and don't have that much of a problem removing the wheel. I don't disconnect the cables.
On the other hand, I have a friend who wanted to use the balance beam for aesthetic reasons. He made his own levers for the brakes to make that possible. It took some trials and errors before he was happy with the leverage. That part is tricky. I'll try to get some pictures and post them. He has made some extra levers, thinking someone else might want a set. I'll talk to him about that too and let you know. I plan to take a ride on his bike and if I like them, I might put them on mine as well.
 

davidd

Well Known and Active Forum User
VOC Member
I don't think anyone has ever figured out the mechanical advantage of the Vincent front brake. I have never seen it published in MPH and when I asked here I received no replies, just speculation. I suspect the speculation is correct, but the problem you run into is that it is difficult to mix and match because the formulae are not obvious. Thus, it is difficult to calculate the results beforehand.

I would suggest sticking with either system, but not interchanging parts unless you are willing to put the time into the experiment. If there is a mechanical advantage with the balance beam it is at the expense of losing lever travel. This may be fine, but it could also require a lot of fabrication (as Gary mentions) and trial and error. Also, if the brake shoes are stock it is likely that they are too flimsy to take advantage of any additional mechanical advantage.

You can use the balance beam as the adjusters if you make the beam stationary and run a cable to each end. Mike Hawthorne did this with his Flash racer.
2017 04.JPG


David
 

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Look forward to seeing the photos and dimensions of the linkages would be handy too.
Thank you for putting this thread into an appropriate slot
Bananaman.
 
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vibrac

Well Known and Active Forum User
VOC Member
I have never been convinced of the efficacy of the beam to increase pull. I believe Scott who were the only other advocates of the idea dropped it as well. The equilising of pull ( which is necessary) can be done at the lever this is how we did it on the Girdralic racer. looking at the Brampton bike we may have to leave it as standard for the moment

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Gary Gittleson

Well Known and Active Forum User
VOC Member
Well, here's my take.

Imagine that instead of a brake at the end of each cable, we have a spring with say 40 lbs/inch Hooke's constant. In that case, if we pull one cable (without the balance beam) 1/4", we experience a 10 lb force. If we use a Vincent Speet setup and pull both brakes together the same 1/4", we will have two springs applying 10 lbs resistance each for a total of 20 lbs.

It's easy to see that the balance beam will halve that force. The 1/4" movement at the "primary" end of the balance beam will be split in two, pulling that spring 1/8" and the other the same 1/8" via the 1/8" movement of the beam, thus causing a 5 lb resistance at each spring for a total of 10 lbs. So to get the same 20 lbs, we will have to move the cable 1/2".

That's a 2:1 mechanical advantage. But the levers of the actual Speet brakes were designed so as to apply proper braking with reasonable handle movement in a 1:1 setup. Using the balance beam with no other modifications would have us pulling the handle back to the bar before we have full braking. So one has to figure out how to hook up cables from the balance beam to the Speet brakes, which in standard form require a near-horizontal cable approach and to reduce the mechanical advantage. I just spoke to my friend (Tom Newman) and he reminded me that there's an article of his work on a fairly recent MPH. I'm going to try to find the issue. There's more to be said about this. I'm either going to get him signed up on the forum or act as liaison. More info to come.

Gary
 
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