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'75 R90/6 Forks, Wheels, and Final Drive

Discussion in 'Airheads' started by SAfrikanBMW, Jan 31, 2020.

  1. SAfrikanBMW

    SAfrikanBMW n00b

    Joined:
    Aug 14, 2012
    Oddometer:
    8
    Hi all

    I got a 1975 R90/6 that I am real excited to make a project bike. As is common with a lot of these forms, there seems to be a lot of interest and push for going to the two front disc brakes rather than the stock single disc. In doing some research, there are certainly ways of getting some nice inverted forks placed on the bike with a compatible triple clamp (see Cognito Moto), so Im thinking basically of doing a full front and rear wheel overhaul. Initially my thoughts are 19" in the back and maybe an 18 or 19" custom wheel in the rear. Naturally, switching to a modern inverted fork allows me to get the two disc brakes up front (with the new master cylinder). Has anyone done this with theirs? Any "gotchas" to be aware of?

    Also, on the rear wheel, how is the rear spline and final drive carried over to a new wheel set? I dont have much experience with shaft drive (Ive owned them, but never really dug into the intricacies of mounting standards). What spec's are there that when I order a new rear spoke wheel that I ensure I can continue to mount the original (or maybe new) final drive system? I assume there has got to be some interface that mates with the OEM hole pattern for the rear spline.

    Just as an FYI, on the Cognito Moto site, they appear to have a an hub adapter that will get rid of the rear drum brakes and convert them to disc. Keeping my eye on them
    #1
  2. Kiwiabbo

    Kiwiabbo Been here awhile

    Joined:
    Apr 16, 2014
    Oddometer:
    811
    Look up a few builders that have done some of this work. I think you maybe haven’t looked into it enough yet. There are a lot of changes that have to be made depending on what look you are after and what you want to sacrifice in doing it. Personally I would have looked to doing this to a later model mono but your bike to do with what you like.
    #2
  3. aptbldr

    aptbldr easy rider

    Joined:
    Apr 20, 2009
    Oddometer:
    1,320
    Location:
    Upstate South Carolina, USA
    Plenty of info on adapting - installing other front suspensions & big brakes to airheads will be found here.
    Same with rear swing-arm & wheels; maybe swap both to gain desired wheel & disk brake.
    Search, young man! :-)
    #3
  4. BicyclebBob

    BicyclebBob Been here awhile

    Joined:
    Jul 23, 2009
    Oddometer:
    508
    Location:
    Yuba City, California
    Both beefing up ATE brakes and full fork swap in here. https://advrider.com/f/threads/not-so-adventurous-r100.1334020/

    I can answer the rest of your questions about splines etc but encourage you to search around a bit. Sites like brook reams, 2 valve boxers videos, and this site. You’ll find most of you’re question and some of the questions you haven’t thought to ask already answered.
    #4
  5. SAfrikanBMW

    SAfrikanBMW n00b

    Joined:
    Aug 14, 2012
    Oddometer:
    8
    Thank you. This link here was incredibly helpful
    #5