H: Hubs, Wheels and Tyres Trevor Southwell re brakes
- Thread starter fogrider
- Start date
The need to grind, or turn, the linings to drum size is important as any kind of distortion in shoes, pivots, cam boss etc plus drum wear is accounted for. What I now see is that the "dips" on the shoes are placing the linings away from the design ideal of 90 deg ,being biased much nearer the pivot than the cam. As the self servo effect at the leading shoe lining is more nearer the cam, I rather think that is where the Vin brakes are losing some effect. I shall have some re-lining done to test this, more to correct what I see as a built in error, rather than another mod. Does'nt require any major engineering either - same re-lining cost too.
Sadly I won't be able to report on the result until late spring. I doubt it will be a monster change to the good, but every bit helps.
Has anyone already tried moving both linings towards the cam ? And how far ?
Sadly I won't be able to report on the result until late spring. I doubt it will be a monster change to the good, but every bit helps.
Has anyone already tried moving both linings towards the cam ? And how far ?
I've always had what I consider good brakes on my Rapide......this on normal road riding and often 2 up with my wife on the rear on rallies around Tasmania and so on........but years ago I took my bike around lakeside race track and after one good haul on the brakes into at that time was a very aggressive corner.....not a full stop, but a 90 degree "S" bend that required very heavy application of the brakes.......after that my brakes were pretty dead, and less effective than before.......These brakes do have limitations........The brakes now work well and I would not touch them (these do have the new stronger shoes with good hard linings).....But on a future higher spec Vincent build, I will be using a much larger 4 l/s brake that is more capable of stopping with more effect and longevity.....It will work well, and look good without the "Modern look" of discs.
This is what I did regarding disc brakes. I wrote it up and posted it on the Coventry Section website. Good enough to produce a looong black skid mark to avoid hitting the car that pulled out in front of me.
https://coventryvoc.uk/machine_mods/disc_brake%20conversion/Disc%20Brake%20Conversion.html
I am not certain this is true. I have no "dips" on my shoes and I have the linings on the center of the shoe. When calculating the brake factor of a drum brake, it is based on the mu of the brake lining and the angles of where the linings start and stop on the brake shoes in relation to the centerline of the axle hole (drawn through the center of the cam and pivots). The mu of my linings is just below 0.50 (AM4 is 0.44 mu). With the stock Vincent brake geometry, they work very well. I have also used in the equations the mu numbers from various linings that produce very high brake factors. This led me to believe that the original geometry was good, but the mu numbers of the original shoe linings were way too low.
These shoes have a hardened and ground steel H48 that is cast into the shoe.
I do support your experimentation because the more data that is available the better off we all are. Best of luck.
David
It could be that I’m missing something ( like grey matter), but how is that method different from sticking shims between the cam and shoes?
That became painfully obvious with the yet to be ridden Lighting replica. With Neal’s shoes and heat treated alloy brake plates, next in line was that mount. Without the existence of the front mudguard stay, when the front brake is applied, you can see the leading edge of the mudguard move downward because the FF32 is bending. How does the FF32 bend without moving the fork blades (right where that spindle is located)? Do they move enough to make a difference….. I have no idea…in its current condition there is no convenient way to hang an indicator on there. When assembling the girdraulics, they should drop under their own weight. My gut feeling is that with the FF32 deformed the same amount, that would be less likely. Would it make a difference to the girdraulics performance under heavy braking. Probably not, but that might depend on the state of the bushings.
Anyway…. either a thicker FF32 or add a brace.
In the '90s, I made a SS plate that was reasonably thick, but I was ignoring all the other weak links. More recently when discussing the brakes on the racer, I discussed the stock cable system vs. the twin cables with David Tompkins. David had taken my advice on the cables and used the Motion Pro Longitudinally Wound cables. He decided that two cables were better than dealing with the balance beam and bridge plate (with the outrigger) and went to the twin cable set-up. We had discussed the bridge plate blocking the airflow to the head and I had urged opening it up with two holes for some cool air flow. So, the first iteration was the Comet Racer:
I had good airflow to the head and the thick aluminum did not seem to bend. Scott was using David's front brake with the new shoes I made for them and he went to a different SLS brake. He used the LW cables with a mock bridge plate with two holes for airflow. There is nothing to bend in the system:
David's bike is on the right and Scott's is on the left. David's bridge plate provides lots of airflow and its only duty is to hold the front mudguard. Scott's is the more traditional Vincent set-up. David's braking improved greatly after going to this system. The design seems very effective, but most owners would prefer the original design over function.
Mike Hawthorne split the difference by bracing the balance beam and then bolting it in place so it would not rotate. He then ran two cables to the balance beam. I don't know if this setup bends. It appears to be fairly sturdy.
I think Dave's setup has proven to produce the best results with the least amount of weight. It is also easy to adjust.
David
Re Davids' photo of a brake assembly what a brilliant set up. Stiffened ally backplate, superior shoes, the works ! I did'nt know such a brake existed. They are clearly a leap forward in eliminating many of the old flex and sponge issues.
Re the centre of the linings, I can't find my reference as to the 90deg thing, but David quotes 90 from the centre of a line between pivot and cam. I am referring to 90 from a line between hub centre and pivot, that is a much different lining centre and accounts for the difference in lining location.
Where the hell is the reference I have for that ?
I'm trying to keep my B as original looking as I can, ( to honour the previous long term owner, RIP J.B. ) which leaves me with standard 7" drums and an unsupported balance beam - hence my need to wring every last ounce out of the original brakes internals. Some small external things might end up visible, but that's inevitable.
I have decided to leave the pivot pins alone, except for the stiffener, but I must say the Bert Weitz mod is a rather good idea. I'll hold that one in reserve.
Re the centre of the linings, I can't find my reference as to the 90deg thing, but David quotes 90 from the centre of a line between pivot and cam. I am referring to 90 from a line between hub centre and pivot, that is a much different lining centre and accounts for the difference in lining location.
Where the hell is the reference I have for that ?
I'm trying to keep my B as original looking as I can, ( to honour the previous long term owner, RIP J.B. ) which leaves me with standard 7" drums and an unsupported balance beam - hence my need to wring every last ounce out of the original brakes internals. Some small external things might end up visible, but that's inevitable.
I have decided to leave the pivot pins alone, except for the stiffener, but I must say the Bert Weitz mod is a rather good idea. I'll hold that one in reserve.