In an effort not to hi-jack the disc brake installation thread https://www.vincentownersclub.co.uk...stalling-front-disc-brake-on-a-series-c.7357/, I thought I would answer here:
This is an early Seeley brake. I suspect Seeley himself found that brake difficult to deal with and had to move on to the less powerful four leading shoe brake that was more stable.
According to Newcomb and Spurr, two Ferodo engineers who wrote the book "Braking of Road Vehicles", it is the most powerful drum brake available due to its duo or dual servo design. I thought it was interesting because although disc brakes are the logical successor to the drum brake, the drum brakes can be designed to be quite powerful. The reality is that they are not.
Disc brakes seem to provide very stable braking while the drums do not. Linings tend to absorb moisture, they change shape between hot and cold and as a result they can be grabby or unpredictable. Discs are quite the opposite, except for water braking. Also, the price for good stopping with a drum is high fade to to heat. This is less likely with a disc.
Here is how it works so well:
The wheel is traveling toward the left. The shoe on the left is the leading shoe in the system or what is called the primary shoe in the duo-servo brake. When the leading shoe is forced into the drum by the cam, which is shown only by a stubby red arrow, that leading shoe grabs the drum and does not want to let go. At the bottom of the leading or primary shoe there is no pivot. Instead there is a link that pushes on the secondary shoe and is shown by the stubby red arrow at the bottom.
The trick is that bottom portion of the secondary shoe is actually the leading edge of what we would normally think of as the trailing shoe. But here, the primary shoe is jamming the leading edge of the secondary shoe into the drum, which gives the secondary shoe a lot of servo action. Thus, both shoes have high servo action even though there is only one lever. The secondary shoe cannot move because there is a stop or abutment on the backing plate.
These drums are still made by Molnar and Dick Hunt. I have never seen inside one, but I suspect the single cam has a flat that moves the primary shoe and a constant radius on the other side of the cam that acts as an anchor for the secondary shoe, possibly like the cam on the right:
These brakes could be said to have "knife edge" tuning. The clearances have to be carefully adjusted and if they become grabby, you are screwed. I am not surprised to hear that Surtees used one. If you are will to put up with the tuning the performance is top of the line. I would think that they are not a good selection for the street, but they do show that drums provide a high amount of braking performance.
I do think that Andrew's point about fade and heat is well taken. I believe it is futile to try and get the same performance from discs and drums. But, I am curious about the compromises that were made in the original Vincent design and whether we could be well-served by moving that line of compromises and upping the performance. I think it is worth a try. Street bikes are rarely used in a way that overheating the brakes comes into play. Finally, many new brake linings have been designed to produce more friction under high heat conditions (maximum fade conditions) than is available from properly warmed AM4 linings.
David
This is an early Seeley brake. I suspect Seeley himself found that brake difficult to deal with and had to move on to the less powerful four leading shoe brake that was more stable.
According to Newcomb and Spurr, two Ferodo engineers who wrote the book "Braking of Road Vehicles", it is the most powerful drum brake available due to its duo or dual servo design. I thought it was interesting because although disc brakes are the logical successor to the drum brake, the drum brakes can be designed to be quite powerful. The reality is that they are not.
Disc brakes seem to provide very stable braking while the drums do not. Linings tend to absorb moisture, they change shape between hot and cold and as a result they can be grabby or unpredictable. Discs are quite the opposite, except for water braking. Also, the price for good stopping with a drum is high fade to to heat. This is less likely with a disc.
Here is how it works so well:
The wheel is traveling toward the left. The shoe on the left is the leading shoe in the system or what is called the primary shoe in the duo-servo brake. When the leading shoe is forced into the drum by the cam, which is shown only by a stubby red arrow, that leading shoe grabs the drum and does not want to let go. At the bottom of the leading or primary shoe there is no pivot. Instead there is a link that pushes on the secondary shoe and is shown by the stubby red arrow at the bottom.
The trick is that bottom portion of the secondary shoe is actually the leading edge of what we would normally think of as the trailing shoe. But here, the primary shoe is jamming the leading edge of the secondary shoe into the drum, which gives the secondary shoe a lot of servo action. Thus, both shoes have high servo action even though there is only one lever. The secondary shoe cannot move because there is a stop or abutment on the backing plate.
These drums are still made by Molnar and Dick Hunt. I have never seen inside one, but I suspect the single cam has a flat that moves the primary shoe and a constant radius on the other side of the cam that acts as an anchor for the secondary shoe, possibly like the cam on the right:
These brakes could be said to have "knife edge" tuning. The clearances have to be carefully adjusted and if they become grabby, you are screwed. I am not surprised to hear that Surtees used one. If you are will to put up with the tuning the performance is top of the line. I would think that they are not a good selection for the street, but they do show that drums provide a high amount of braking performance.
I do think that Andrew's point about fade and heat is well taken. I believe it is futile to try and get the same performance from discs and drums. But, I am curious about the compromises that were made in the original Vincent design and whether we could be well-served by moving that line of compromises and upping the performance. I think it is worth a try. Street bikes are rarely used in a way that overheating the brakes comes into play. Finally, many new brake linings have been designed to produce more friction under high heat conditions (maximum fade conditions) than is available from properly warmed AM4 linings.
David
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