E: Engine Speet Brake Installation Query

Gary Gittleson

Well Known and Active Forum User
VOC Member
That's the "other" VOC site. I still don't understand why there are two sites. I have asked. You first have to log into the other site and then go to the link posted here. The link shows somewhat blurry photos from the MPH article Tom Newman posted. I haven't had time yet to find the issue but it was sometime in the last six months or so.
Gary
 

bmetcalf

Well Known and Active Forum User
VOC Member
I couldn't find it on the Forum Resources, Maybe Graham can post it.

The VOC website has a lot of info and on-line renewal that club members should make available to themselves.
 

davidd

Well Known and Active Forum User
VOC Member
Gary,

Your figures seem incorrect to me, but I fully admit that I have had trouble with this approach before. If I summarize what I believe the argument has been it is that the twin cables split the force generated at the hand lever. Thus, 10 lbs. at the lever turns into 5 lbs. (per cable) at the end of each brake arm. The Vincent balance beam is different: 10 lbs. of force on the hand lever yields 10 lbs. of force on the inner cable (on the primary side) and 10 lbs. of force on the primary side of the balance beam from the sheath or outer cover. That force is in turn pulling up 10 lbs. on the timing side of the balance beam (and the attached brake arm), meaning the the 10 lbs. of pull is generating a total of 20 lbs. of pull at end of both brake arms...double the pull of the twin cable system (ignoring for the moment the different distances travelled.)

To simplify even further, I think the argument has been that the Vincent system has no stationary brake anchors where the hand lever force is lost to the frame. The Vincent has a moving brake anchor (the brake anchor is the primary side of the balance beam), that utilizes the force lost with the frame brake anchor by transferring it to the timing side brake arm. The balance beam acts as a moving brake anchor transferring the force lost by the frame brake anchors to the timing side brake arm.

I have not seen any simple explanation of the physics of the differences of these two systems and why they yield such different results. It is most likely my lack of schooling in this area, but I would be interested in know how the physics work.

I don't know in which MPH Tom's article appears, but I remember reading it. I posted the Higgins articles here.

David
 

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  • Brake Lever Ratios Higgins 2.1-630-14.pdf
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Nigel Spaxman

Well Known and Active Forum User
VOC Member
You won't get extra braking force from the balance beam. Any lever type device if it increases force will loose movement in the exact proportion. Going back decades in MPH there are lots of people arguing that this device somehow magically increases the force on the brake shoes, it doesn't.

If for example the tension in the single brake cable caused by squeezing the lever with a force of 100 lbs and movement in the cable of 1/2" Then you will have 100 lbs of force on each end of the balance beam. If you pretend there is no stretch in the brake cables and all the flex is in the brake then you will have 1/4" of movement of each brake lever and 100 lbs force on both levers.

If you have twin cables you will end up with only 50lbs tension in each cable but you will have the full 1/2" of movement at both brake levers. (this is how the Speet system works) With the extra movement of the cables you can use much longer levers on the brake cams, twice as long in theory, maybe a bit more really. With twin cables and half as much force on each cable there must be less stretch. The stretch does seem like the waste in the system.

If you want to retain the balance beam as John McDougal did with his Speet brakes, then you will need levers on the brakes about half as long as the ones that come with the Speet brakes (they are made for twin cable that move twice as much) John modified his Speet brakes in that way.

I have the Speet brakes with the twin cable set up. With this set up you have to make sure that both cables have even tension in them when the brakes are applied. The balance beam does that automatically. You can fine tune the brakes a bit. When I first used these brakes I had levers with 1 1/8 distance between the pivot and the cable. That made the lever to hard, so I welded up the lever and reduced the distance to about .800" which works really well. The proper distance, between pivot and cable that the Speet and Vincent brakes are designed for is .875". By shortening this a bit I gained a bit of leverage. If you keep your brakes well adjusted you can set them up for a bit more leverage, and have better brakes.
 

Gary Gittleson

Well Known and Active Forum User
VOC Member
Sorry, but I must disagree with some of this. When 10 lbs are applied to the primary side, that force will move two things at once, the brake cam lever on that side and the balance beam in the opposite direction which in turn applies a force to the other brake, thus distributing the 10 lbs into two halves. The brake cam lever will never see all of the 10 lbs. They are halved because the balance beam has its pivot in the center. Had it been off-centered, the splitting would have been proportional to the ratio between the lengths of the two sections, right and left.

Another way to look at is to imagine fastening a clamp to the cable on the primary side just below the balance beam and trying to squeeze the clamp and balance beam together. That's what the setup is really doing. That pulling together is obviously dividing the force evenly between the cable below and the beam above.

Still another way to think of it is this. Imagine two springs attached parallel to each other to a fixed object and a bar joining their other ends. Suppose we pull on the bar in the center with 10 lbs of force. Will each spring see 10 lbs? Whatever force they experience, they will apply in return. If so, wouldn't their forces add together and pull back with 20 lbs? If we can do that, we are well on our way to building a perpetual motion machine. If we pull with 10 but feel 20 then we are pulling with 20 so should feel 40..... It's obvious that the two springs will work together and return the same 10 lbs of pull: opposite and equal. Therefore, each spring will see and oppose 5 lbs assuming they are of the same strength.

In any case an important point is that if we are to use the balance beam to actuate brakes designed for direct pulls, we will introduce a doubling of the handlebar lever movement the system was designed for. That's why Nigel had to modify the lever. The standard lever has less leverage than what the Speet brake requires because in the standard setup, there's a mechanical advantage provided by the balance beam. And that's why if we use the balance beam with the Speet brakes, we need to shorten the levers at the brakes to reduce the leverage there to compensate for the extra leverage provided by the beam.

Another way to automatically evenly distribute the force is to use a whiffletree or whippletree https://en.wikipedia.org/wiki/Whippletree_(mechanism). I think I've seen such a setup on a bike somewhere, as well as disparaging comments regarding them. I don't remember where I saw that. Any thoughts about that?

Gary
 

Nigel Spaxman

Well Known and Active Forum User
VOC Member
Gary, the first part is wrong. When you apply the brake there will be tension in the cable 10 lbs if you have very weak hands. That tension pulls up on the left hand brake lever, but it can only do that by pulling down on the balance beam (which then pulls up equally on the other side). If the force in the cable is 10 lbs at the handle bar then the force is going to be 10 lbs on both cables. You are not building a perpetual motion machine because the travel is halved.

The reason I had to modify my lever is instead of using the original type that has the 7/8 pivot distance. That is the type the Speet brakes are designed to use as well. I used a Dourhety lever because I like those. They have the 1 1/8 pivot distance.
 

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
This is how I did it after seeing Erik's and Mr Speet's comment when he tried it at the French Rally and said it felt more effective than his own set up, but it was on a Comet and not a twin and the Comets always had better braking distances than twins as Ted Davis could confirm. My pulling power is just behind the axel so no need to make an OMEGA link to miss axel. That is a WM4 rim I have used with the Trail Rider tyre 90 x 90 tyre.
bananaman.

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