I have 76 BMW R90/6 sidecar rig. It has the pitifully inadequate single ATE caliper and 260mm disc. An inmate of the ADVRider airhead forum (Matt Fish) has come up with a design to modify the lower leg caliper mount to accept a radial Brembo caliper, and disc adapter to mount Ducati 320mm discs. A left side lower leg is easily modified for use as a right side lower, allowing for dual 320mm discs with radial calipers. A braking system orders of magnitude better than the stock ATE brake. I have purchased the parts from him for a single 320mm disc and caliper. I also have a K100 front end that I am looking to do something with. My K100 front end already had larger discs and dual non radial 6 pot ISR calipers. The calipers and adapters are long gone, so I asked Matt (R90X Designs) if he had adapters to use radial calipers and larger discs on the K100 forks. Here is his reply:

I’m very close to that. The K100 uses similar components in braking to the later airheads. The fork spacing is different, so shims would probably be required. I already have adapters to fit 320 discs to OEM calipers, going to radials would not be to difficult, although the small front wheel could require removing the calipers to mount.
It works with the mono shock airheads using a K1100 style axial 4 pot. https://www.instagram.com/p/CPGkrKPBJal/?utm_medium=copy_link
Basically the forks are just turned around K100 lowers. A spacer under the hub adapter is probably all that would be needed to make this work.
I’d have to make some calculations based on measurements to nail down the offset.
The main reason I haven’t made a radial caliper set up for the mono shock airheads is lack of interest. Owners have a perception that the brakes are good enough on these bikes. I’ve only sold a hand full of adapters. The radial set up would be expensive to make at first. Do you think there is much interest in the K bike community to upgrade the calipers?
I think big discs and calipers would be really nice on these bikes. If I had one, that’s the first mod I would make.
Thanks
Matt
 
Matthew Cooper
R90X Designs
www.r90xdesigns.com
2835 Franklin Street
Bellingham, WA 98225

Is there any interest in big brakes and 320mm discs for K bikes?

If you have those ATE caliper(s) then you have my sympathies but, IMO, airheads are just fine with twin Brembo 2-piston calipers and 285mm discs - as on dual-disc airhead monos (and early K100s). Note that master cylinder piston size and pad choice have a significant effect too.

A mono front-end will go on the earlier airheads and, with a simple spacer/adapter, you can retain the earlier spoked or snowflake wheels.

If you want more on a K then it's not that difficult to fit the 4-piston Brembo calipers from the 16V. Or you could go all out and put a K100 16V or K1100 front-end with 320mm discs on. K100/K1100 parts are cheap and readily available from breakers. The customisers put GSXR front-ends on so I don't see many takers for your option.

Radial calipers sound cool but, unless you're riding #46 are unlikely to make any discernable difference. And I would worry about your mate's use of reversed K100 fork legs 'cos that is likely to mess up the steering geometry.

stanthomas wrote:if you have those ATE caliper(s) then you have my sympathies but, IMO, airheads are just fine with twin Brembo 2-piston calipers and 285mm discs - as on dual-disc airhead monos (and early K100s). Note that master cylinder piston size and pad choice have a significant effect too.

If you want more on a K then it's not that difficult to fit the 4-piston Brembo calipers from the 16V. Or you could go all out and put a K100 16V or K1100 front-end with 320mm discs on. K100/K1100 parts are cheap and readily available from breakers. The customisers put GSXR front-ends on so I don't see many takers for your option.

Radial calipers sound cool but, unless you're riding #46 are unlikely to make any discernable difference. And I would worry about your mate's use of reversed K100 fork legs 'cos that is likely to mess up the steering geometry.

I don't think he was saying that he reversed the fork legs. The airhead mono front ends have their calipers in front of the fork leg and the K100 has it's calipers behind the fork leg. The fork lowers appear to be the same. Reversing them won't have any affect on steering geometry because the axle is centered on the axis of the lower leg.

arbalest wrote:Reversing them won't have any affect on steering geometry because the axle is centered on the axis of the lower leg.

Oh no it isn't (welcome to panto season ).

I can post a picture of an R80 mono fork if you insist...

https://i.servimg.com/u/f63/20/27/38/97/k100_f10.jpg

https://i.servimg.com/u/f63/20/27/38/97/r100rs10.jpg

I would like to point your attention to the location of the axle on these lower legs.

Here is a picture of the fork leg of a 1986 R80 monolever (same forks on all mono airheads). Note the spindle offset 8mm in the direction of the brake caliper, i.e. forward. Reversing would increase the trail.

On the original point : R90X uses a radial caliper on the ATE fork leg(s) 'cos the radial mounting position 'works out' with the aluminum block he fits into the fork legs in place of the original caliper. Since the same problem is not present on post '81 airheads and Ks, using a radial caliper is unnecessary and quite possibly worse than an axial mount caliper.