Front brakes

Hervé Hamon is visiting and we are having a good discussion on Vincent front brakes. He says they are not working right and one of the reasons is that the brake arm on the backing plates is facing forwards and this reduces the lenght of the lever acting on the brake shoes inside the drum. Vincents are the only bikes with a forward facing brake arm and according to Hervé, this is a major factor in braking performance.

I on the other hand think that the Vincent's system with the balance beam above the wheel is just adding slop to the whole braking system which requires a longer travel from the brake lever at the handlebar and this translates to less force being applied. The only way to fix this I think would be to use some form of splitter right at the handlebar (à la Suzuki) or a junction box close to the handlebar lever (à la Yamaha TZ) and have proper rigid cable stops running to each backing plate, this way the lever travel can be kept to a minimum and the handlebar brake lever can have a good ratio providing more force.

Any takes on this problem?

Jean

Interesting stuff.
The balance beam mounting plate is well considered to felx, so, many add a stiffening plate.
I also know that Vincents do bennefit from extra heavy duty brake cables, and this is possibly linked to your comment about 'slop' in the system.
Interesting comments ref the forward facing levers... I can't quite work out in my head why it would make any difference which way the lever faces. The lever still turns a cam, which acts on the shoes. So I'd also be interested in others views on this.
However, we must remember that there are many out there who swear the stock Vincnet brakes are excellent when set up properly.
Here was my answer to 'the problem' though:

Fast Eddie said:

Interesting stuff.
The balance beam mounting plate is well considered to felx, so, many add a stiffening plate.
I also know that Vincents do bennefit from extra heavy duty brake cables, and this is possibly linked to your comment about 'slop' in the system.
Interesting comments ref the forward facing levers... I can't quite work out in my head why it would make any difference which way the lever faces. The lever still turns a cam, which acts on the shoes. So I'd also be interested in others views on this.
However, we must remember that there are many out there who swear the stock Vincnet brakes are excellent when set up properly.
Here was my answer to 'the problem' though:

Click to expand...

Hervé showed me examples of braced balance beams, they are not very good looking. The forward facing levers pulling up have a contact point inside the drum at the top, which is closer to the center of the hub which means the leading shoe is being moved with a shorter lever. I say that is insignificant but Hervé insists that the inch of difference is a major factor.

As proof to what I said, I asked him about other good and bad drum brakes, a perfect example is the Triumph's front drums, the twin leading shoe brake fitted to Bonnevilles before the "new" conical twin leading shoe brake fitted to the OIF models, the old one was great, the conical was a POS. The old brake was just like all good brakes, a series of levers transmitted the movement to both leading shoes with minimal movement, the new conical had two short levers and the cable pulls the two levers towards each other, twice as much movement as the previous brake. For sure the conical brake is cheaper than the old one, but the additional travel required a hand lever with more travel and thus less force.

Jean

PS I love your solution 8)

I have the Lightning back plates, Ferodo linings and Shadow drums on the bike I ride a lot. The brake is almost too powerful and will lock the front wheel at speed if pulled too hard. It acts more like a modern disc than a drum brake. I have pushed that bike pretty hard in the mountains, often two up and never run out of brake. Doing similar solo on the Commando when it had the stock Norton disc, the front brake faded to nothing. And nearly ran into the back of a Vincent that was being ridden the same and had no brake fade.
So I don't see anything wrong with the design, although there are some of them today out there that do not work well. This may be due to lining types or linings not contacting the drum properly.
My other Rapide has standard backing plates. It is a good brake but it requires a little harder pull than the one with Lightning back plates.

I see a lot of badly set up Vincent brakes on the show queens at Rallies. These are the bikes that come in on a trailer, sputter thru a couple of short rally rides, then get trailered home again. They level of finish is often way beyond original factory, with lots of "show Chrome" and "show polished alloy"
But one look at the position of the brake arms tells you this is not a well set up rider. The brake arms need to be perpendicular to the cable pull when the brake is fully on for max braking with a given line pull. Often the arms are off by 30 or 40 degrees or more on both the fronts and rears.
As far as the question of the arms being backward on a Vincent, I can't see how this makes any difference to anything. It is simply a lever with mechanical advantage which is based on the length of the lever from end to pivot . It could operate forward, backward or anywhere in between without making a difference to brake force, provided cable pull is constant and always at a right angle to the lever.

Here's my Oz Vincent in stopping mode:

https://m.youtube.com/?#/watch?v=amMPrwtuz2U

Glen


Glen

Good demonstration that Glen, don't think anyone could knock that braking performance from brakes of that age!

The direction of the arm makes a difference on a single leading shoe brake, because the cam lobe nearer to the shoe pivots imparts more movement to its shoe than the lobe farther from the pivots. This could be allowed for with an assymetrical cam, but makers didn't do that.

It seems to be accepted that the leading shoe should get the greater movement, even though that means less mechanical advantage. I use a Norton SLS brake (not on a Vincent) and with the arm facing to the rear, at the bottom of a backplate on the right hand side of the hub, it's "wrong."

Triton Thrasher said:

The direction of the arm makes a difference on a single leading shoe brake, because the cam lobe nearer to the shoe pivots imparts more movement to its shoe than the lobe farther from the pivots. This could be allowed for with an assymetrical cam, but makers didn't do that.

It seems to be accepted that the leading shoe should get the greater movement, even though that means less mechanical advantage. I use a Norton SLS brake (not on a Vincent) and with the arm facing to the rear, at the bottom of a backplate on the right hand side of the hub, it's "wrong."

Click to expand...

Your SLS front brake is correct, the leading shoe is the one towards the back of the bike and the most force is being applied to that shoe due to the greater leverage given by the contact point being furthest from the center of the hub.

The Vincent's front brakes are the opposite, the leading shoe which is towards the back of the bike is being activated by the cam with the shortest distance from the center of the hub, both sides of the two SLS front brakes are like that. Hervé think this is a major factor in the poor braking of the STOCK Vincent's front brakes.

Glen's example is different, he has different brake backing plates which may account for his good braking performance. Hervé will be performing braking tests on a rolling road where many factors will be measured. I think he will find that there is a lot of movement required to account for all the bellcranks and cables in the Vincent's brakes, lots of movement means less mechanical advantage from the brake levers so unless you have the grip of a gorilla, the brakes may not work as well as one with better mecanical advantage.

Jean

Jeandr said:

Your SLS front brake is correct, the leading shoe is the one towards the back of the bike and the most force is being applied to that shoe due to the greater leverage given by the contact point being furthest from the center of the hub.

Click to expand...

Yes, there is more leverage on the leading shoe, but less travel. The greater travel of the trailing shoe can stop the leading one getting its potentially greater pressure.

Correct me if I am wrong, I remember reading on the Vincent that with the beam operated brakes that on one side the cable outer is double wound because the inner cable is not pulling like a normal cable;
http://www.thevincent.com/vinbrake.htm

The long cable is very heavy with a fortified outer because it is supplying enough tension to operate two brakes. As far as the operation of the cable goes, it is exactly the same as any regular clutch, throttle,choke or brake cable on any other bike. This means that when the lever is pulled, the inner is in tension against the outer which is in compresssion. The secondary cable operates a bit differently. For that cable, because it is dead straight, the outer is only there for show, the inner does all of the work.

Glen