Lets see what happen within a few months after the Brexit...
Eric in France
Eric in France
2mm might be good, take that from the info I gave you in MPH 499. Believe me I tried a whole lot of different ways of moving the Pivot pin, I think Berts method was the best as far as improvement was concerned.
A lot of the time the pins are bent in the brake plates, this bending is probably the biggest weak point as far as I can see. A small bridge plate across there would help, but the problem has come from poorly fitting shoes. Another cause would also be grease from the bearings getting on the linings, which then causes the rider to apply the brakes harder to the point that something bends. When the linings are bonded on, I feel sometimes the clamps that hold the lining to the alloy shoe are done up too tight, when the assembly is baked in their oven, the shoes can end up bent.......Meaning the original radius of the shoe becomes smaller, this shifts the cam face of the shoe closer to the axle. This becomes obvious when you machine the linings in the lathe and you can see the shape of the lining material. The brakes are a continual issue on these bikes just as the breather system is also in the same basket. They are marginal design faults that Vincent owners have had and will continue to do battle with. It is trying to come up with a decent "Fix" that tries to keep things looking original that is the real challenge.
David Dumfey sent me the article of the Bert W. Modification quite a while ago.
I was first tempted to do it but finally didn't for some reason.
The design actually turns your sls brake into a servo brake. While I am not against trying that due to the small size of brake lining and drum diameter I think I have to tell you that there is a common belief that this kind of brake is unusable and highly dangerous on a motorcycle.
With a good 2ls front brake or even more with a 4ls as my Grimeca you can feel the self servo effect while breaking hard. Usually the brake linings need a certain temperature to work best. With the Grimeca on a cold brake you need to squeeze the lever hard for breaking hard. Once the brake is up to temperature and having to break hard you feel that the self servo effect starts as without changing the pressure the braking is getting harder still. You actually have to ease the grip on the lever to counteract the increasing bite of the brake.
Now imagine this self servo effect actually presses the second brake shoe even harder into the drum than you will ever be able by squeezing the lever and it all happens really fast... This sounds like big trouble.
Secondly from an engineering point of view what you want is the brake shoes to return to a specific resting point when not using the brakes. With this mod the shoes are only centralized by the spring near the brake cam and two little rubber O-rings at the bottom which I am sure will loose their shape very soon.
Every servo brake I had seen while working in the trade had a positive stop for the leading edge of the second shoe who was moved against this stop by a substantial spring.
Please don't get me wrong. I am the last person that does not like modifications and as I mentioned at the beginning the mod may and probably will work well due to the small breaking area and drum diameter. But a servo brake always has something vicious in its behavior and I would rather bite the bullet and pay for a readily available known to work 2ls conversion.
Saying this I will probably build my own 2ls conversion just because I like doing things, but I will stay away from a servo brake for sure
Bernd
I was first tempted to do it but finally didn't for some reason.
The design actually turns your sls brake into a servo brake. While I am not against trying that due to the small size of brake lining and drum diameter I think I have to tell you that there is a common belief that this kind of brake is unusable and highly dangerous on a motorcycle.
With a good 2ls front brake or even more with a 4ls as my Grimeca you can feel the self servo effect while breaking hard. Usually the brake linings need a certain temperature to work best. With the Grimeca on a cold brake you need to squeeze the lever hard for breaking hard. Once the brake is up to temperature and having to break hard you feel that the self servo effect starts as without changing the pressure the braking is getting harder still. You actually have to ease the grip on the lever to counteract the increasing bite of the brake.
Now imagine this self servo effect actually presses the second brake shoe even harder into the drum than you will ever be able by squeezing the lever and it all happens really fast... This sounds like big trouble.
Secondly from an engineering point of view what you want is the brake shoes to return to a specific resting point when not using the brakes. With this mod the shoes are only centralized by the spring near the brake cam and two little rubber O-rings at the bottom which I am sure will loose their shape very soon.
Every servo brake I had seen while working in the trade had a positive stop for the leading edge of the second shoe who was moved against this stop by a substantial spring.
Please don't get me wrong. I am the last person that does not like modifications and as I mentioned at the beginning the mod may and probably will work well due to the small breaking area and drum diameter. But a servo brake always has something vicious in its behavior and I would rather bite the bullet and pay for a readily available known to work 2ls conversion.
Saying this I will probably build my own 2ls conversion just because I like doing things, but I will stay away from a servo brake for sure
Bernd
Bernd, all good points that you've raised and not trying to be smart but by it's very definition the 'leading shoe' is always self servo so twin-leading shoe is twin servo. If memory serves me well the modification being discussed is typically called 'duo-servo' and commonly found in many modern car rear drum brakes as well as many mid 60's autos. I suppose duo-servo differ from twin-leading in so much as the trailing edge of one shoe actuates the other shoe. Like you I too would be concerned that one end of both shoes are floating off the brake plate without any substantial support.
The design of the common drum rear brakes on contemporary cars actually allows the shoes to completely self center, as the shoes are actuated at one end by a slave cylinder with two opposed pistons which themselves can move left or right to center the shoes. At the opposite end of the shoes there is an adjuster that is generally like a small screw jack. This holds the two shoes apart, but the shoes are firmly held against this adjuster with a stiff spring. The two shoes also generally have linings of different lengths to assist in offsetting the servo action and to help with the ware characteristics of this set up. The end where the adjuster is can basically move at will and has no "anchor" at all. Of course this type of brake has a massive advantage of being hydraulically operated, most drum brakes use a line pressure at around 600 PSI which is far better than cables and levers. I think the manufacturers of Drum brakes for bikes have to be mindful of the design of "Servo".......self energizing brakes, so as the front wheel does not "Lock up" too easily, which would end up with results nearing a design that approaches the dangers of a front end that can readily go into a "Speed wobble"..............
.............
.............Hello 0_0,
You are right with the naming of the brake type. That was just how we called them in Germany 35 years ago. I just realize I am getting old writing like that....
Anyway I found a link to a simple picture that compares the different types of shoe actuation and breaking force per shoe.
It could make things easier to understand for everyone
https://www.google.es/search?q=duo+...ELVAKHThrD_0QsAQICw#mhpiv=9&spf=1497608842279
Cheers Bernd
You are right with the naming of the brake type. That was just how we called them in Germany 35 years ago. I just realize I am getting old writing like that....
Anyway I found a link to a simple picture that compares the different types of shoe actuation and breaking force per shoe.
It could make things easier to understand for everyone
https://www.google.es/search?q=duo+...ELVAKHThrD_0QsAQICw#mhpiv=9&spf=1497608842279
Cheers Bernd
Twin leaders are great when moving forward, not so good going backwards. That's why most cars have leading/trailing jobs, when reversing the trailing one becomes leading.