Handlebar M/C Conversion Kit for R75/6

[Ken in Oklahoma]

I think with the handlebar m/c there is no issue of free play. There's nothing to adjust.

Ah! It would appear that I'm not very familiar with handlebar mounted master cylinders. Some of the newer bikes have the means to adjust the lever position, but that's not exactly free play.


Ken

[Ken in Oklahoma]

I did the motobins conversion last year. Since I have a Hannigan fairing with fairing-mounted mirrors, I asked if they could delete the included mirror (and credit me the difference) They replied that they'd send the RS/RT kit instead. I suspect that the difference in the kits is a mirror. They supply a machined fitting that essentially replaces the front of the undertank MC. On my kit the fitting had two tapped holes to connect to the two brake lines that a twin disc bike would have. They told me to plug the right-side outlet with a bleeder screw. I'm VERY happy with the conversion, but I was using a very old rubber brake line, and I replaced the brake line at the same time, so I really can't make a valid comparison. They have answered questions that I have e-mailed in the past.

Good inputs old R90 guy. Do you recall the master cylinder bore size on that RS/RT kit? I think the bore size in marked somewhere on the bottom of the master cylinder body.


Ken

[chasbmw]

I am using the 13mm mastercylinder kit from moto bins on my 38mm twin calipers on my R90/6. It works very well, lots of power and feel WO having to have gorilla hands. This M/C is twinpull so should work well with your existing throttle cables.

You may think that you don't need the electrical wires to brake level warning light, but on my bike one of them needs to go to earth or your indicator and neutral lights won't work!

[old R90 guy]

I'll have a look at the MC underside tomorrow for you, Ken.
Charles, I did nothing with the wires that formerly attached to the MC cap for the fluid level, other than wrap them in electrical tape, and tuck them out of the way. I've had NO issues with the indicator or neutral lights. (My bike was built in Jan. of '76, if that has any bearing on your issues.)

[chasbmw]

Old R90, the date of manufacture might well be relevant, mine is a 75 with the earlier wiring layout, no eart wires to indicators etc etc.
Charles

[chasbmw]

The comments under vanzens chart seem to point to a ratio of 27:1 for twin piston brake? Our bikes are single piston for each disc.

Looking at the chart my ratio looks to be 17:1. Made up of 2x 38mm pistons with a 13mm M/C.
Is this right or am I missing something?

The brakes work really well, much much better than I remember them from when I last had ATE brakes. I am in the cevennes mountains in France at the moment and my trip has involved so many mountain roads that the brakes have been well used. My hands are not large and this ratio puts the lever where I want it to give maximum control, without there being any chance that the lever would pull all the way to the bar.

Lying in my tent waiting for the sun to come above the mountains.

[Ken in Oklahoma]

The comments under vanzens chart seem to point to a ratio of 27:1 for twin piston brake? Our bikes are single piston for each disc.

Looking at the chart my ratio looks to be 17:1. Made up of 2x 38mm pistons with a 13mm M/C.
Is this right or am I missing something?

That is an excellent looking chart and is apparently quite useful to Vintage Brake and vanzen. I've saved it for future reference. I do, however, have one concern about it. Unless I missed it somewhere the chart is somewhat general in that it is not based on any particular master cylinder. I"m not concerned about the caliper "universality" because it is a direct line pressure vs caliper piston "push" sort of thing. With the master cylinder, however, there is an unknown factor in the picture. Specifically it is the geometry of the front brake lever. That is, how much travel of the hand lever results in how much excursion of the master cylinder piston? I can imagine that this varies from one brand of master cylinder to another. Or it may be that they are all pretty much the same because it works out that way in practice. That is, for a given movement of the hand you will want a certain travel of the piston.

Probably in practice the chart works well in virtually all applications, but for me there is a lingering concern. That is why I'm asking for anecdotal information in this thread. I was a bit sobered a couple years ago when Brad Belk reported that he was dissatisfied with the performance of his R90/6 single disk handlebar mounted master cylinder conversion. I wondered then if a different sized master cylinder piston might have worked better for him.

As for myself, with my impending dual disk handlebar mounted master cylinder conversion, I'm focused on the 14mm piston and eyeballing the 15mm piston. What I'm concerned about is picking too small of a size. With too small of a piston I'm concerned that the brakes will feel great--except when I'm in an actual panic stop. Then, with all that weight thrown forward onto the front wheel, the increased traction on the tire might result in the hand lever touching the grips before wheel lockup.


Ken

[DanielMc]

Ken - I fitted a 15mm handlebar M/C to a '79 R100S a few years ago along with braided hose and found things only slightly better than they were before - lots (I mean lots) of lever effort required and little feel. I ended up swapping out the front forks and fitting later, Brembo equipped, items and finally had a brake that was adequate.

I'm now planning on fitting a handlebar M/C to my '76 bike, and if I do I'll definitely go with the 14mm M/C. I don't believe excessive lever travel will be a problem but I'd certainly want more leverage than I got when I fitted the 15mm M/C.

[chasbmw]

Ken

I feel pretty sure that the13 gives me enough lever travel to lock the brakes if I wanted too, W/O running out of lever.......is there anyone you could ask who races these things? As in racing with repeated braking and high temps would show up any issues?

Would it be worth asking on the airheads list? Tom cutter does a bit of racing and has experience of the conversion.

I did have similar concerns B4 I bought my kit from bins, but the Internet consensus was smaller on the MC so I specced the 13mm from bins, the argument being that these were the first Bike discs that BMW used, and that they cocked up the ratios as well as putting the cable into the set up. My norton from the same era also had wooden brakes, but the norton ingle disc was stronger.

My view is that my setup is within striking distance of the brembo set up on the 1070 bike.

[Ken in Oklahoma]

DanielMC and Chasbmw, thank you both for those real life inputs. The 15mm master cylinder is now out of contention.


Ken