Front Brake needs some hand muscle - ATE single disk

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jjwithers
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Front Brake needs some hand muscle - ATE single disk

Post by jjwithers » Tue May 02, 2017 4:01 pm

I got a bike back from the dead and put a really old set of brake pads on it, but with minimal wear on them.
It is a R90/6 front end with a single disk, and a solid rotor (R90s style without the holes in it). The rotor has a wee bit of grooving in it but nothing too bad.
I rebuilt the master cylinder (It is an early '74 style), I added a stainless brake line, and the brakes are not as responsive as I would like.
They take some serious squeezing of the handle to really stop the bike at high speed/emergency situations.
My thoughts are - ride it more and more and hope the pads mate to the disk better.
I'm considering different pads.
Or I am considering scuffing up the pads.
Suggestions or thoughts?

-Josh
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'77 R100S - http://www.joshuawithers.com/r100s
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Airbear
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Re: Front Brake needs some hand muscle - ATE single disk

Post by Airbear » Tue May 02, 2017 7:50 pm

Josh, I faced the same problem with my '74. The situation becomes particularly dangerous when loaded for camping or carrying a passenger - there simply is not enough leverage to stop the bike quickly, and it comes down to the master/slave ratio, as well as cable friction and stretch in that crazy hybrid system.

My solution was to fit an aftermarket, handlebar mounted Magura 13mm MC, from Amazon. Here's a thread I started back in 2012:

viewtopic.php?f=1&t=5461&hilit=magura+13

This will only work if you have the /5 style switch gear, which you should have on your '74, and you don't mind going away from stock. I am still delighted and impressed by how well my bike stops.
Charlie
and Brunhilde - 1974 R90/6
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Graduate, Wallace and Gromit School of Engineering and Design (Pending)

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jjwithers
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Re: Front Brake needs some hand muscle - ATE single disk

Post by jjwithers » Tue May 02, 2017 8:05 pm

Very cool! I am going to use that on a new build... for sure. I have been thinking about the /5 controls with a master cylinder for a while.

However, i have the mid to late 70's controls. They are the /6 and /7 style.
Perhaps there is a 38mm caliper that I should have used vs a 40mm?
I have the 38 sitting around... i think.
The master cylinders for the under tank, single disk /6 models, are 14mm for the early and late style master cylinders.
I have already sunk a bunch of money into these brakes. But if switching calipers works, it won't hurt my wallet because I have the calipers lying around (i think).
Converting to the 13mm BMW style handlebar mounted master cylinder might be an option... but those aren't cheap and I would only do it if it guaranteed the improvement.
www.beemersandbits.com
'77 R100S - http://www.joshuawithers.com/r100s
'73 /5 Toaster Cafe bike http://www.joshwithers.com/slash5
'67 Bridgestone 175 Hurricane Scrambler
'66 R60/2
'72 BMW 2002
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Airbear
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Re: Front Brake needs some hand muscle - ATE single disk

Post by Airbear » Wed May 03, 2017 12:51 am

It's all about the master to slave ratio, so the 40mm caliper currently fitted will provide better leverage than a 38 caliper, and the change to a 13mm master at the top end is really significant. BMW simply got it wrong in those early days. Since you have the later controls a used bar mounted MC from a later bike may be your best option. I gather it is possible to have the existing MC bored and sleeved to a desired diameter but you would still have the inefficiencies of the cable system to deal with.

I used every trick in the book to get my original brake to work for me, including parting with hard-earned dollars for that damned MC refurb kit (why can't we just buy a fekkin' cup seal on its own?) The brake still felt like squeezing a lump of wood and its ineffectiveness forced me to adjust my safety margin to well below preferred fun limits. On the plus side it did introduce me to the pleasures of 'gentlemanly riding'.
Charlie
and Brunhilde - 1974 R90/6
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Graduate, Wallace and Gromit School of Engineering and Design (Pending)

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jjwithers
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Re: Front Brake needs some hand muscle - ATE single disk

Post by jjwithers » Wed May 03, 2017 1:15 am

I'm worried I might need wider bars for the MC to fit with the S fairing.
But if I understand you correctly, do not go to a 38mm caliper and going from a 14 to a 13mm master cylinder will help. Regardless if it is aftermarket or BMW's master cylinder. right?
www.beemersandbits.com
'77 R100S - http://www.joshuawithers.com/r100s
'73 /5 Toaster Cafe bike http://www.joshwithers.com/slash5
'67 Bridgestone 175 Hurricane Scrambler
'66 R60/2
'72 BMW 2002
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Re: Front Brake needs some hand muscle - ATE single disk

Post by Rob » Wed May 03, 2017 12:24 pm

The OEM master cylinder on my '79 R65 was 13mm. I seem to recall hearing that the unit from the early R65 was the hot setup for this conversion.
I can't find the size of the caliper pistons, but both sides were live, and most R65s only came with a single disc.
Rob V

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Re: Front Brake needs some hand muscle - ATE single disk

Post by jagarra » Wed May 03, 2017 2:27 pm

My bike, before I converted, had a 40mm caliper and a 14mm M/C, still was very wooden feeling, didn't stop worth a crap. Going to a 13mm handlebar unit may be the best option.
I too have thought of having the original under tank unit sleeved smaller, but then a new problem appears, getting a seal and a piston to mount the seal on to make it work.
1974 R90/6 built 9/73
1994 BMW R1100RS
1964 T100SR Triumph
1986 Kawasaki Concours

Seth
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Re: Front Brake needs some hand muscle - ATE single disk

Post by Seth » Wed May 03, 2017 7:37 pm

I would suggest going to dual disks.
You'd need a right fork leg, another caliper and a splitter for your hydraulic line.

The 40mm calipers are better than the 38, as it improves the ratio. The smaller master gives you a softer lever feel, and some better leverage, but the second disk provides more brake pad surface area.

I'm running a single disk master (machined for the second port so it looks original) on a '75 R90S, a '77RS and a '77S. The braking with the single disk master is better than the larger dual disk unit (16mm), but it's still not like modern brakes.

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jjwithers
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Re: Front Brake needs some hand muscle - ATE single disk

Post by jjwithers » Wed May 03, 2017 9:10 pm

If you have a right leg, and a wheel with a dual disk, with tire mounted... then yea!
:)
I have another bike (77 R100S) with dual disks and a handlebar mounted master and it brakes very well.

I built this other bike's front end with spare parts, all in good shape. But it looks like I will need a 13mm handlebar mounted master cylinder unless a miracle happens to my current set up.
There are no 13mm under tank masters... right?
www.beemersandbits.com
'77 R100S - http://www.joshuawithers.com/r100s
'73 /5 Toaster Cafe bike http://www.joshwithers.com/slash5
'67 Bridgestone 175 Hurricane Scrambler
'66 R60/2
'72 BMW 2002
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Ken in Oklahoma
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Re: Front Brake needs some hand muscle - ATE single disk

Post by Ken in Oklahoma » Thu May 04, 2017 4:42 pm

jjwithers wrote:
Wed May 03, 2017 1:15 am
I'm worried I might need wider bars for the MC to fit with the S fairing.
But if I understand you correctly, do not go to a 38mm caliper and going from a 14 to a 13mm master cylinder will help. Regardless if it is aftermarket or BMW's master cylinder. right?
No, not quite right. The master cylinder area and the caliper area(s) are just part of the ratio equation. There is also the mechanical 'leverage' ratio to be considered. That is, how much movement of the hand lever results in how much excursion of the master cylinder piston.

And the under tank master cylinders add yet another ratio to the equation stew due to the leverage ratio at the back end of the master cylinder.

Ken
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