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Topic-icon R90/6 “Forkectomy”

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3 months 1 week ago #4787 by DSeero
R90/6 “Forkectomy” was created by DSeero
While exploring options to upgrade the front brake for my 1975 R90/6, I ran across a rider on Advrider who said he’d replaced his fork sliders with those from an early 80’s Airhead. Those forks have Brembo calipers.

Another rider suggested there might not be room for those calipers if used with spoked rims (which I have). This raises the question of whether the fork spacing is the same.

I’m not sure where to turn to find out how to go about this. I hate to fire the parts cannon, even though even buying an entire front end is relatively inexpensive.

FYI I’m familiar with the adapter to fit a Brembo caliper to an ATE slider. That remains an option, but I wanted to explore this first.

Crazy or Crafty?
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3 months 1 week ago - 3 months 1 week ago #4788 by Wobbly
Replied by Wobbly on topic R90/6 “Forkectomy”
Welcome !

DSeero wrote: Crazy or Crafty?

Neither. More like jumping from Mediterranean to Boardwalk, and missing all the wonderful orange, red and green properties in Monopoly.

► There are a lot of cheaper, easier intermediate steps you can take. You may have already taken these, but if so you didn't tell us....

• When was the last time you flushed all the brake fluid ?
• Have you replaced your brake pucks with new versions within the last 10 years ?
• Have to adjusted the eccentric screw lately ?
• Are you still running the OEM rubber-type, or the modern "stainless steel" type flexible hoses ?
• When was the last time you serviced your brake master cylinder ?
• Have you replaced your brake rotors with floating rotors from EBC ?

► The ATE calipers are single-piston, but very good for their day. With a selection of updates they can still hold their own. Of course, nothing like dual-piston Brembos. The first mod is the stainless steel hoses, because you are loosing about 25% of your breaking effectiveness with OEM hoses. Then pucks and rotors.

► One of the weak areas of the ATE system is the rearward cant making it very hard to completely rid the system of air. Another is the cable operation, which adds a spongy feel due to cable stretch. To cure these, the later model handlebar m/c is greatly preferred, but that also means changing out your RH electrical handlebar controls.

► Yes, the Brembos are decades ahead and therefore better. If it gets that far, then I'm all in favor of an complete front end swap with a dual caliper RS or RT front end as being cheaper, and an easier way to guarantee all parts fit. But you need to be careful. The early Brembo front ends still had the axle out in front of the forks. By 1988, the single-sided bikes (and the K-bikes) had moved the front axle in-line with the axis of the stanchions. That make the bike handle very "fast", requiring constant handlebar input. I see you look a little more "mature" and I doubt very seriously you want to leave the very "easy going" neutral handling of a 1975, just to be able to stop.

But even with a complete front end, you'll still want the "stainless steel" brake lines.

Talk to us.

Don't hide 'em, Ride 'em !!
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Last edit: 3 months 1 week ago by Wobbly.

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3 months 1 week ago - 3 months 1 week ago #4789 by DSeero
Replied by DSeero on topic R90/6 “Forkectomy”
Thanks for the reply.

Bike has a new master cylinder installed about 500 miles ago. Caliper adjusted recently. Not getting SS brake lines until I settle on a course of action. Also investigated sleeving the master cylinder from 9/16 to 1/2 (14 to 13mm in BMW speak) but held off on that as well until the question of one or two calipers, and what calipers, is settled.

My track bike is a GSX-R750 so I know what great brakes feel like. ATE seems like a total dead end. Don’t think anyone was ever happy with this setup. The most popular course of action - upgrading to R90S duals - isn’t an option any longer because those parts are scarce.

If early 80’s sliders fit, I’m there, just trying to find out what parts are needed. Looking at ‘83 sliders which presumably resolved the stiction problem.

- Do I need to change the entire front end, or just the sliders?
- Will the 80’s Brembos clear my wire wheel? A rider on Advrider who made the change was not sure.
Last edit: 3 months 1 week ago by DSeero.

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3 months 1 week ago #4790 by Wobbly
Replied by Wobbly on topic R90/6 “Forkectomy”

DSeero wrote: If early 80’s sliders fit, I’m there, just trying to find out what parts are needed.


Not being a BMW "parts swap guy", I cannot advise on this part of your question.

I do know complete Airhead and K-75 front ends are easy swaps. And that way, thrown in with the "deal", you'd be getting the updated hydraulic fork internals (possibly meaning better handling). And, I suspect that a pile of discrete parts is going to cost the same as the entire assembly, simply because there's less work for the breaker.

And too, in 1975, BMW was still into their "long stroke" fork phase of cushy, Cadillac-like, touring ride. I see your R90 has stubby bars, and I suspect you might appreciate more of the later model, taut, "sport touring" type ride and handling.

However, that's the most I can contribute.

All the best.

Don't hide 'em, Ride 'em !!
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3 months 1 week ago #4800 by DSeero
Replied by DSeero on topic R90/6 “Forkectomy”
A GIS search yielded a lot more threads on Advrider (under the general category of "My brakes suck") so I'm sorting through those.

In the meantime I bought a used master cylinder to have resleeved to 13mm, and a wire braided brake line, as the existing one could be quite old. Already have a new cable from handle to MC but haven't installed it yet.

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3 months 1 week ago #4801 by Wobbly
Replied by Wobbly on topic R90/6 “Forkectomy”

DSeero wrote: In the meantime I bought a used master cylinder to have re-sleeved to 13mm, and a wire braided brake line, as the existing one could be quite old.


Two points:
► Age IS a factor contributing to the ultimate failure of the hose, as the internal tubing has a finite number of flexures before internal collapse. However, if you can put your hand on the hose today, apply the brake really hard, and feel the hose expanding in your hand, then the hose is already worthless.

► All "stainless steel" hoses have the steel braid, but the better hoses have a vinyl cover over the braid because the braid will absolutely chew the living heck out of anything it touches, especially plastic or painted parts.

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