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Installing a Front Disc Brake on a Series ‘C’


highbury731

Well Known and Active Website User
VOC Member
I know what you mean Bill but as you have probably worked out I'm more more interested in function than how it looks. Do you remember the twin disc conversion that Paul Dunstall offered for sale on his Norton bikes? It was really tidy. I think the calipers were cast in with the alloy fork leg and it used two smallish disc. Quite neat but I not sure how good it was.
The Dunstall twin disc set-up had the calipers cast into the fork sliders. The set-up was reckoned to be ineffective (and expensive) IN PERIOD. It might be improved with a smaller diameter master cylinder, but you would be hard pressed to find one to find out - few were sold. If any survive, they are probably stashed under work-benches.
 

Chris Launders

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VOC Member
With the hoses at the top it is easier to bleed the calipers back to the master cylinder instead of out of the bleed nipples, air tends to rise anyway. Just push the pistons back one caliper at a time.
I think the twin Honda Superdream discs are about the neatest, and almost the correct bore and offset as well.
 

highbury731

Well Known and Active Website User
VOC Member
With the hoses at the top it is easier to bleed the calipers back to the master cylinder instead of out of the bleed nipples, air tends to rise anyway. Just push the pistons back one caliper at a time.
I think the twin Honda Superdream discs are about the neatest, and almost the correct bore and offset as well.
If you are going for Superdream brakes, use the larger 250N discs not the 400N ones. Twin smaller discs on the 400. There may be period discs of larger size but same offset, perhaps on the CB900F
 

andrew peters

Website User
VOC Member
I've been following this thread with interest and have researched this subject (and other ideas to improve my Rapide) almost to death!
I'm nearly finished putting disc om my bike and feel it's about time I threw my hat in the ring as I may be able to help a little, after all, without the help of you lot (Vincent riders generally) I would have even more to work out by mainly guesswork..
So like I said I'm not completely done yet but I can keep you all posted, I'll even take some pictures later (soon)
Discs.. yes Honda 250... 'correct' offset and 5 hole pattern, centre bore is about 20 thou small. easy. I drilled 5 more holes and made a thin sheet ally cover ring for the other 5 holes, I trimmed a flat off a set of 5 new bolts as you need slightly long mounting bolts.
I then made ally plates to support the stock speedo drive and hide the Honda disc centres. I had to build a boss to move the drive box outboard slightly but the gears line up now. Also on the 'cover' plates there is a boss to line up with the fork leg stop, this boss is centre bored to locate with the caliper bracket 'stop peg'
I used Kawasaki Zepher calipers (I would have like opposed piston calipers but I need spoke clearance and moving the discs out I lost alignment for the speedo drive) as the mounts on the caliper are not handed, only the floating two piston caliper, which has a bleed nipple at the top for easy bleeding. The caliper bracket is 6 mm ally pivoting on stainless counter-bored spacers on the new 4140 axle. 'stop pins' are also stainless locating on special 'top-hat' threaded plugs that locate into the brake disc cover plates (the cover plates don't really rotate as they are held but the axle tightening but I figured they would be better pinned as the speedo drive needs to stay in one place. (the left side disc cover is just a cover.. although in black with un-drilled disc is does look a little less modern) The front fender stay is bolted in the same place on the fork leg, the bolt going through the caliper bracket as it may stop the caliper dropping down, but its not in any tension either way. the other fender stay is off a welded lug on the caliper bracket, I didn't like the fender stay going around or bending over the caliper.
My ideas aren't totally original I admit, like I said I research a LOT before using the best ideas and making them work for me. It really is quite a bit of work but I believe it looks and should (I've not tried it yet on the road, other work I'm doing as well, including Normans Steering head mod and AVOs)
Yes, I know someone will ask for pics and I will post some.
While I'm here I will thank Everyone in the VOC and all Vincent owners for being so willing to share their experience and advice.. believe me, it's valuable and appreciated, and a lot more useful than the nonsense I've read on Harley and BMW forums, (yes, I have a couple Harleys and a BMW, old bikes not those new fangled things!)
 

davidd

Well Known and Active Website User
VOC Member
Andrew,

We are all very lucky if our mods are derivative. There has been so much good work that has been done by others. Different designs are just that...different. If something clicks with you, use it...if not, don't use it.

One of the best mods you have done is to fix the Girdraulic with a new stem (or short springs). That was one of the problems that I ran into with disc brakes on a Vincent. Norman noticed the same thing when riding a Vincent with a disc brake.

Best of luck!

David
 

vibrac

Well Known and Active Website User
VOC Member
I will file disc brakes under 'i would like' unless my new comet stoppers on racing plates is found wanting. I would like discs on the twin but would never try discs on Brampton's.
 

andrew peters

Website User
VOC Member
Up-date on the Caliper mounting, actually a small warning (advice really) to anyone thinking of pursuing the 'dream' of better (a lot better than most drum brakes I think) brakes....
Aligning the mounts... not at all easy, at least if you want the brakes to work properly, I guess you could mount the calipers so sloppy that they literally float into alignment, good idea but you'd need a Lot of brake lever travel to take up the 'play' before the brake actually starts it's braking job...
A caliper mount needs to be aligned straight and square to the disc, no twist or binding or things don't work, and our point is to have more than just a slightly better brake. By the way, one disc can be enough, again if it works properly, a larger disc is better (swept area, heat dissipation etc) Oh, and discs in the wet? never a problem, a brake pad will dry and warm a disc instantly! An advantage for us fitting a single disc would be that the stock speedo drive could be used on the other side to the disc and a flat disc can be sourced, and easier to mount than finding a suitably dished disc. Single discs work fine, with negligible fork flex in use, ie slowing and stopping.. although fork twist can seem alarming on ,say, a Harley "Wide Glide" (especially with extended forks) if you hold the brake lever and 'bounce' a stationary machine... on the road, like I said, not noticeable at all (in normal operation)
I don't really see a problem with discs on a Brampton, probably the suspension and damping will get a bit out of shape but the real problem would more likely be just the stresses on the 70 year old forks, and they really weren't designed with those stresses involved... try running into the car with ABS in front of you and see the effects of old girders succumbing to a bit of forced retardation....
Anyway, back to the immediate issue, alignment of the calipers... I found the caliper can only be tightly held by the axle, when the axle nut is tightened the mount is rigidly held in a 'sort of' square plane to the axle, except only 'sort of' as when the forks were made by the factory the machining of the axle hole faces were not necessarily perfectly square (to the axle) or even parallel, even the two fork blades are not perfectly parallel to one another, with drum brakes this is not so critical. For correctly functioning disc brakes alignment is very important.
So the 'stop-pin' needs some 'float' and the fender mounting bolt which was going through the mount was holding the mount slightly skew, this had to be modified with a shouldered spacer with some lateral play, the fender mounting bolt now has only that job to do, it never was in tension in any direction before, as I said, but now it's not interfering with the caliper mounting.
However, I still haven't completed the job, and many may be put off this task... I'm sure the "Dutch" brake or even the 8" unit is adequate, but I have been set on this route for a while.
My thought has also always been that the suspension/steering needs modification if the full potential of better braking performance is to be realized, yes quite likely this the real issue with Bramptons. Again I thank Norman, and AVO (and Thornton etc of course) for their constant striving to improve the Vincent.
I can imagine the PCV thinking he should improve the brakes but knowing the limitation of the stock fork design he hoped we'd all just plan ahead when stopping is involved..
"bloody hell! this geezer goes on..." I hear you say...
 

vibrac

Well Known and Active Website User
VOC Member
I have said before how Ron Kemp's am4 brakes bent the bottom of the Brampton's that a!one keeps the idea of discs on Brampton's a no go
 

Bill Thomas

Well Known and Active Website User
VOC Member
I guess it might how we ride, But I have had some sick making times with Cold discs as well as Wet discs.
Always thought they were a bit of a Con', Cheap to make etc.
Maybe good when things get too hot ie Fade, But the way some of them rust etc.
Cheers Bill.
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
I guess it might how we ride, But I have had some sick making times with Cold discs as well as Wet discs.
Always thought they were a bit of a Con', Cheap to make etc.
Maybe good when things get too hot ie Fade, But the way some of them rust etc.
Cheers Bill.
Dear Bill,
I've also come across discs in the past that are not great in the rain. :eek: Early disc brakes on bikes were often cast iron that indeed did go a bit rusty. I remember early MotoGuzzi had iron discs. Perhaps a bit of rust gave the pads something to grab on. Later stainless or non-rusting materials were used some of which were just as bad when wet. Most discs used now are drilled which makes them lighter and helps dissipate any water. The right pads to match the disc material makes a big difference to working in the wet and having minimal fade.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Dear Bill,
I've also come across discs in the past that are not great in the rain. :eek: Early disc brakes on bikes were often cast iron that indeed did go a bit rusty. I remember early MotoGuzzi had iron discs. Perhaps a bit of rust gave the pads something to grab on. Later stainless or non-rusting materials were used some of which were just as bad when wet. Most discs used now are drilled which makes them lighter and helps dissipate any water. The right pads to match the disc material makes a big difference to working in the wet and having minimal
The first disc I wanted, was a Yam' Stainless, We have to look smart !!, But was later told they didn't work very well, This was in the 70s, With a set of TeleForks, The supplier messed me about for ages,
So I bought second hand Commando forks complete, And fitted Alloy race yokes.
I seem to remember Honda did a small plastic guard, To stop water getting on to the discs.
Cheers Bill.
 
Last edited by a moderator:

davidd

Well Known and Active Website User
VOC Member
Andrew,

I never had alignment problems with the disc brake calipers. It may have been that I was lucky. I machined the two caliper mounts together and they attach to the forks through the axle hole and a bolt through the brake anchor, so there was no float in my caliper mount set-up.

As Tim mentions, the Bramptons tend to bend with a good drum brake. Kal Karrick bent the fork on his Series A twin racer in his first race outing with a Series A brake. The Brampton forks need the tiny brace that was done on the TT machines, at a minimum.

I stopped investing time in disc brake conversions when I started to race because I had to use drum brakes by rule. Drum brakes can be a lot more powerful than disc brakes, but as the drums become more powerful, they become less stable also, meaning that they can be grabby. Disc brakes rarely become grabby.

The reason drums can be more powerful than discs is because the drums with even a single leading shoe use the geometry and the friction of the lining to become self energizing. The disc does not have much self energizing, thus the need for hydraulics to increase the work the calipers do.

Vincent Speet's brake is designed to self energize a great deal, maybe something like 300% more than the stock drum. The 8" brake is maybe 100% more than the stock brake. Both brakes can perform much better or much worse by adjusting the coefficient of friction on the brake linings. Unfortunately, that is a more tedious process than just squeezing the lever of your disc brake harder! So, I think disc brakes are a good solution to better braking performance on a Vincent. For those who want to stick to drums I think Vincent Speet's brakes will do the job nicely.

David
 

vibrac

Well Known and Active Website User
VOC Member
Perhaps that prompts me to ask why hydraulic drum brakes were tried so little in those years before discs when they were already plentiful on cars, is that option re-visitable with modern parts? I would have thought that would have ticked a few boxes at a much lower cost than new brakes
 

davidd

Well Known and Active Website User
VOC Member
Perhaps that prompts me to ask why hydraulic drum brakes were tried so little in those years before discs when they were already plentiful on cars, is that option re-visitable with modern parts? I would have thought that would have ticked a few boxes at a much lower cost than new brakes
Tim,

I think it is an option to use hydraulic drum brakes. The reason you don't see many hydraulic drum brakes on motorcycles is the the manual drum brake is so powerful. The drum brake has to be detuned quite a bit to make it stable when aided by hydraulic pressure, like using low friction linings or twin trailing shoes, the latter being the most stable set-up. I would guess that most cars that use rear drums have very carefully matched friction coefficients for their linings.

However, if you think of using hydraulics on a stock Vincent drum I think it would do well, except that it would highlight the horrible pre-existing problems like the poorly designed brake shoes and the low friction linings provided as originals and replacements. The shoes would be bent into pretzels.

I think a better way is to design better shoes and experiment with high friction linings to see what optimal braking with the almost stock brakes are. This might provide some good results. If it does not, we can spend our money with Vincent Speet or the Club and get a better designed brake.

The alternative to both is the disc brake conversion. This is ideal for someone riding with modern bikes because it will provide similar braking performance. This is one of the reasons why I like large rotors. I don't think it is the best idea to make a weak disc brake simply because it is stronger than a poorly set up Vincent brake.

David
 

andrew peters

Website User
VOC Member
The first disc I wanted, was a Yam' Stainless, We have to look smart !!, But was later told they didn't work very well, This was in the 70s, With a set of TeleForks, The supplier messed me about for ages,
So I bought second hand Commando forks complete, And fitted Alloy race yokes.
I seem to remember Honda did a small plastic guard, To stop water getting on to the discs.
Cheers Bill.
73 up Hondas had the plastic "splash guard" behind the fork leg, the caliper was in front of the leg, not really sure what it did but I don't think it kept the disc dry...
 

andrew peters

Website User
VOC Member
Perhaps that prompts me to ask why hydraulic drum brakes were tried so little in those years before discs when they were already plentiful on cars, is that option re-visitable with modern parts? I would have thought that would have ticked a few boxes at a much lower cost than new brakes
60s and 70s Harleys had a hydraulic rear drum... a single leading shoe, it really was no better than the mechanical drum, they stuck with the big mechanical drum on the front... Harleys really aren't a good example to use when thinking of good brakes, they had large drums that were hopeless, in fact a performance mod in the US was to remove excess weight and who needs a heavy brake that doesn't do much anyway, especially as American racetracks require little braking, referring to Board Track to Flat Track.. (I have a '54 Panhead that I've put a disc on as I got so close to crashing so many times) Even when Harley fitted the Massive brake calipers on a 10" disc and 5/8 bore master cylinders they were still dire! So there is proof size isn't important, a caliper off a truck and it still doesn't stop!
I suggest hydraulic drums were never popular on motorcycles due to the size of (available) components, (the industry will usually source what is available from sub contractors, ie Brembo) the complexity of a hydraulic system and bottom line, cost!
 

highbury731

Well Known and Active Website User
VOC Member
Tim,
I think it is an option to use hydraulic drum brakes. The reason you don't see many hydraulic drum brakes on motorcycles is the the manual drum brake is so powerful.
David
A friend of mine converted his mid '50s Norton to hydraulic 2ls by using a brake plate off a car. The car, an early '60s Hillman, happened to be upside-down in his street being stripped. The plate went easily into the hub, a large nut happened to fit neatly as a spacer between brake plate and fork leg, and another fitted between plate and bearing area. He re-used the torque reaction lug from the Norton brake plate. A master cylinder was from a Yamaha he had scrapped a while before. The only expense was a brake hose to connect British brake plate fittings to metric master cylinder. It all worked a treat, excellent stopping.

I have heard of people using Mini 2ls brake cylinders and shoes to make their own hydraulic brake actuation. Owners have always reported being happy with results

Paul
 

davidd

Well Known and Active Website User
VOC Member
The most powerful drum brake of all time is the duo-servo brake.

Seeley G 250 Brake.GIF
It was not a popular brake, but it is superior to double twin leading shoe brakes in performance. It is considered a single leading shoe brake by racing organizations. It is not very popular because it is wildly unstable, but if set up properly it will probably out-perform a big twin disc. This brake design was also used extensively in cars and hydraulically operated.

David
 

passenger0_0

Well Known and Active Website User
VOC Member
The most powerful drum brake of all time is the duo-servo brake.

View attachment 21333
It was not a popular brake, but it is superior to double twin leading shoe brakes in performance. It is considered a single leading shoe brake by racing organizations. It is not very popular because it is wildly unstable, but if set up properly it will probably out-perform a big twin disc. This brake design was also used extensively in cars and hydraulically operated.

David
A Seeley front brake David?
 

andrew peters

Website User
VOC Member
The most powerful drum brake of all time is the duo-servo brake.

View attachment 21333
It was not a popular brake, but it is superior to double twin leading shoe brakes in performance. It is considered a single leading shoe brake by racing organizations. It is not very popular because it is wildly unstable, but if set up properly it will probably out-perform a big twin disc. This brake design was also used extensively in cars and hydraulically operated.

David
yes typical drum brake used on many cars and trucks from 1940s on.. complicated design, especially when 'miniaturized' for use on motorcycles, (read) expensive, less likely to be so viable for mass production compared to discs. Also heavy (reducing un-sprung weight more important on motorcycles than cars) and more maintenance than discs and more prone to heat fade. Cooling motorcycle brakes is not as easy as cars either, air scoops struggle to capture much cooling air as the front wheel is splitting airflow and deflecting air that that overheating little drum is craving. small drums get hot quicker than big drums. I'm really not convinced that any drum brake design even at its ultimate evolution can out-perform (in all practical situations) a disc brake (allowing the ultimate evolution of that design as well? ) imagine a wry smile at this comment please
 

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