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I was told yesterday that BMW made the swinging ATE brake with two different size pistons. The story was that in the first production runs, there was a smaller piston and that sometime in 1975, they moved to a 40mm piston. My bike is from the first half of 1975, as determined from the different size pushrod seals.

This suddenly opened the door for yet another incremental brake system improvement.

Can anyone verify that pre-mid-1975 R90s had a smaller piston in the swinging ATE brake Caliper?

Thanks
Brad

Not sure if this answers your question, but there is a number "40" cast onto the larger calipers.

Tim Shepherd wrote:Not sure if this answers your question, but there is a number "40" cast onto the larger calipers.

As the story was told to me, they stamped the "40" in for only a short time and that after the transistion year or so, they dropped the mark. So if it says 40 its a 40 - but if it doesn't say 40, it may still be a 40. Anyway - I was hoping someone could confirm that there was actually something less than 40 made in early 1975.

And while I am asking: How much difference would I get by changing from the smaller to the larger size?

bbelk wrote:

Tim Shepherd wrote:Not sure if this answers your question, but there is a number "40" cast onto the larger calipers.

As the story was told to me, they stamped the "40" in for only a short time and that after the transistion year or so, they dropped the mark. So if it says 40 its a 40 - but if it doesn't say 40, it may still be a 40. Anyway - I was hoping someone could confirm that there was actually something less than 40 made in early 1975.

And while I am asking: How much difference would I get by changing from the smaller to the larger size?

Brad, I can't confirm the point at which the caliper size changed from 38mm to 40mm, but I believe it was late '75, to tie in with the '76 model year. Both my '76 R90S's have the 40mm calipers, and both are stamped "40". All of the 40mm calipers I've seen have got "40" stamped into the body (facing away from the wheel), including those off the later /7 bikes.

TBH, I'm not sure that you'd notice much difference with a caliper change alone.

I caretake a '75 R90S. I rebuilt a caliper earlier this year and it used a 38mm piston and seal.

The difference in stopping power between the 40 mm and 38 mm caliper would be the same as the difference in the area of a 40 mm compared to a 38 mm. In other words, if the 40 mm has 8% more area it will put 8% more force on the back of the pad with the same hand lever force at the handlebar.

Just calculated it...40 mm piston has 10.8% more area then a 38 mm piston.

George Ryals wrote:8% more force

Thats almost 10% !!

Cool!

RealOEM lists both 38mm and 40mm repair kits for all /6s up through 9/1975. Then in 10/76, only 40mm is listed.

Andy Schwietzer's book "Boxer from /5" that in the 3rd quarter of 1974 (I assume that is calendar year not model year) the calipers went from 38 to 40mm. At the same time, the disk was drilled. So, likely tying the size increase to the drilled disk is the way to go.

Kurt in S.A.

Kurt in S.A. wrote:RealOEM lists both 38mm and 40mm repair kits for all /6s up through 9/1975. Then in 10/76, only 40mm is listed.

Andy Schwietzer's book "Boxer from /5" that in the 3rd quarter of 1974 (I assume that is calendar year not model year) the calipers went from 38 to 40mm. At the same time, the disk was drilled. So, likely tying the size increase to the drilled disk is the way to go.

Kurt in S.A.

Well, my 12/74 (1975 model year) /6 has drilled disc and 38mm. Caliper unmarked when I got it as to size, even less marked now.

melville wrote:Well, my 12/74 (1975 model year) /6 has drilled disc and 38mm. Caliper unmarked when I got it as to size, even less marked now.

Is your unmarked caliper markedly less marked now?