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Greetings. I'd like to improve the braking on my 1975 R75/6. It has a single disk with the small single piston caliper mounted at the bottom rear of the left fork slider. I think there are other (later?) BMW sliders that will interchange with mine that have caliper mounting points up higher.

Right? If so, what model/year sliders do I need to look for?

BTW, I already have a a floating iron disk and will convert to handlebar master right away.

Thanx.

The handlebar master should be an improvement. In 1974 BMW introduced a twin disc ATE setup on the R90S. Used parts usually run in the $350 - $450 range. Swapping to any later model sliders will necessitate swapping the wheel and disc to a later model cast wheel as well.

With a floating cast iron disc and the correct pads for cast iron, I think the single disc will be fine if you get the correct size master cylinder. I would go with a 12mm myself.

Abstruse1 wrote:Greetings. I'd like to improve the braking on my 1975 R75/6. It has a single disk with the small single piston caliper mounted at the bottom rear of the left fork slider. I think there are other (later?) BMW sliders that will interchange with mine that have caliper mounting points up higher.

Right? If so, what model/year sliders do I need to look for?

I'm not sure I'm understanding your question right, but I understand that you're talking about the 'swinging' ATE calipers as used on your '75 /6 bike. All the lower fork legs that fit the swinging ATE caliper have the same mounting position on all of the ATE fork legs, so there is nothing to be gained there in terms of mechanical advantage. If I remember correctly the two variations of the calipers themselves (other than finish) is that some have 38mm pistons and the others 40mm pistons. So, even if you have the 38mm caliper, a 40 mm caliper isn't going to yield any significant improvement.

The single ATE caliper equipped airheads are notorious for their poor braking. Typical single caliper fittings include the R75/6, R90/6 R75/7, R80/7, and R100/7. The 'higher end, R100S, and R100RS bikes were fitted with two swinging ATE calipers and their braking performance is more highly regarded.

As might be expected, given the interchangeability of fork sliders has enabled the conversion of the single caliper ATE bikes to twin calipers. I have retrofitted my '77 R100/7 with twin discs and have the parts to retrofit my '76 R75/6 to twin discs. The result is much improved braking--but braking which in the scheme of things isn't all that great. And the major fly in the ointment is that everybody knows about the desirability of the twin discs and the supply of the relatively scarce R/H ATE mounting fork sliders is well known.

The above shortage has lead a lot of people to reequipping their /6 and /7 bikes with the much more efficient handlebar mounted master cylinders. The handlebar mounted master cylinders have the great benefit of getting rid of that damned leaky dirty under tank master cylinder. Plus the braking is better given the nature of the under tank master cylinder vs the handlebar mounted master cylinder. The problem (you guessed it) is that the new master cylinders and associated pluming makes the change over still very expensive.

Ken

If he goes for less plumes, he may be able to save some money.

Has anyone seen comparative stopping distance data on the single-disc vs. dual-disc systems? I rode both an R90/6 and two R90Ss back in the day, and I don't remember any difference in day-to-day feel or operation. My later dual-disc R80 and current (hah!) R100RT probably stopped with less input effort, but I don't know that they stop shorter than the old R90/6. One of the yesteryear road tests that I googled cited a 60-0 stopping distance for a single-disc R90/6 at 137ft., which doesn't seem obviously shabby, and none of the old road tests that I've briefly scanned say anything bad about how the single-disc bikes stop. I haven't seen a comparative figure for the R90S, but it's probably out there.

While I certainly felt more virile and worthy with an impressive fork-full of "dual-disc" braking hardware up front, I might've achieved the same self-satisfaction with an extra splash of Aqua Velva or Old Spice . . .

I'm installing an 11mm handlebar cylinder tomorrow. Stay tuned.

Abstruse1 wrote:I'm installing an 11mm handlebar cylinder tomorrow. Stay tuned.

I went from 15mm to 13mm on my 81 R100RS. The lever throw increased and feedback was much better. I'm wondering if the lever will hit the grip with an 11mm version?

Interim update: I fitted a handlebar MC from a RS100 or something like that. I put on a new EBC iron rotor (not stainless) and new EBC pads. The braking isn't any better. It probably stops tolerably, but takes a LOT of hand power.

With this setup, I think I'd need about a 12mm MC (currently have 14mm).

But I have a pair of R100 or somesuch forks on the way from the 'Bay, so I'll have the easy opportunity to fit a better caliper, maybe a 4-pot, in which case the 14mm may be better.

Stay tuned.