Anyone Using Rear Disc Brake Adaptors?

Discussion in 'General Questions' started by 5-7HEAVEN, Feb 6, 2012.

  1. 5-7HEAVEN

    5-7HEAVEN Member

    I need disc brakes for a motorized bike I'll use as a daily commuter. The bike I've chosen for a project has v-brakes. Since the frame is aluminum, I'm leery about welding an adaptor onto the frame. I don't weld, so the job would have to be farmed out. I've mentioned it to bike shop employees. They act like I'm asking for an organ transplant.

    How about this?
    Last edited: Feb 6, 2012

  2. SimpleSimon

    SimpleSimon Active Member

    Your link returns:

    "This listing () has been removed, or this item is not available."

    Not sure what you were trying to show.
  3. 5-7HEAVEN

    5-7HEAVEN Member

  4. MikeJ

    MikeJ Member

    I did this conversion to my bike. Yes, it works just fine. Stopping power is superior. It is not cheap. But if I were to build another bike, I'd do it again.

    There is no welding. But there are unique tools involved, some careful shopping for parts, some disassembly and reassembly. Give yourself about 60 minutes of workbench time.

    The adapter arm and caliper sold for around $150 about a year ago. Be assured, you get only what you see in the photos. Do not assume you will receive a disc rotor or cable; you must purchase them separately. Be sure you get only the $40 160mm rotor from the bike store, the larger 200mm (?) won't work.

    Be sure to get the make and model of the hub/wheel you want to attach the caliper to. My wheel happened to have a Shimano 756 hub. I had to purchase a new wheel (about $100, gear cassette, good tube and tire extra) because my old V-brake wheel did NOT have the six-hole disc mounting surface (an absolute necessity!) If you don't have this six-hole mounting surface, ie, your bike came with V-style brakes, the wheel very probably does not meet this criteria. Go to a bike shop or the neighbor hot-rod and get a visual on the difference.

    If you decide to purchase a disc-ready wheel, match the compatibility list to what you can find on the axle housing. A Shimano axle clearly prints its model between the spoke flanges. This is important because Brake Therapy sends you an adapter (a bearing cone nut) that must be inserted into the left side of the wheel. Adapters differ because hub manufacturers differ between themselves.

    You have to have some thin bicycle-specific wrenches, 14 through 17 mm or something like that, available only from a bike repair store. Craftsman wrenches will NOT work, they are far too wide. And of all things, the cassette gear side of a cone nut you must hold is 2 mm different in size from the disc side cone nut. You need at least two hands. You must unscrew the disc side cone nut (wheel laying flat on a clean towel so you don't lose any of the 20-odd ball bearings packed in grease), remove the original disc side nut, (don't drop the axle or a ball bearing), and insert their provided cone nut adapter. Their adapter needs a little finessing to get it to fit just right, because it seats below the lip of the housing; have a pocket screwdriver available. Hard to envision right now, I know, but it is not difficult if you move slowly. (Ball bearing grease holds the balls into place, so don't get uptight about this maneuver. I did it right the first time; so can you.)

    Once their cone nut adapter is in place, you add another jam nut they provide (need a different sized bike wrench for this) and tweak the tightness of the cone nuts against the ball bearings. Not too tight, not too much slop. After you tweak fit half a dozen times like I did and compare wheel freedom against factory-built wheels, you feel comfortable.

    Bolting on the $40 disc is a no-brainer by this time. Just get the directional indicators going in the right direction. Wipe the disc of any fingerprint oil or grease film before use! Use a suitable solvent; I used generous amounts of rubbing alcohol.

    The Bike Therapy upper arm fits around the old V-brake boss snugly. The arm length is variable; I opened my about 3/4 inch; personal preference. Fit the axle carefully into the wheel dropouts, and the brake pad assembly slides right over the disk and the axle shaft. A pin locks the assembly into place.

    You will need a longer brake cable and housing, so back to the bike shop you will go with semi-accurate measurements. You can tell the guys in the work area what you did and they will still look at you like, "You did what? That's not normal."

    Pros: If you change bike frames, this whole assembly can go with you. Cons: Removing the rear wheel has just doubled in complication. There is a disc rotor you don't want to damage, removing the brake assembly must be done (kinda easy, really), and reassembly requires lining up four points almost simultaneously. It all just takes longer to accomplish. I have done it many times. (That's why my tires are about as puncture proof as I can make them.)

    And by the way, the brake rotor does NOT allow the common 2-stoke Chinese engine gearing; you can have one or the other, not both.

    Properly parts-matched and installed, the Bike Therapy brake is a good solid unit with better braking power than V-brakes. Costly to some people, but to me, there is no substitute for stopping power. (I later installed a new wheel, disc, and Avid-7 caliper up front as well. I can lock both wheels without trying too hard.)

    Some readers have commented that for the money I spent upgrading the brakes on my bike, I should have gone out and purchased a brand-new Giant medium-priced mountain bike. It comes factory assembled, tested, warranteed, and has the bike store staff for support until the engine goes on. They may be right....

    Have fun! ("Post Quick Reply". . . . Ha, that's funny!)

    Last edited: Feb 6, 2012
  5. 5-7HEAVEN

    5-7HEAVEN Member


    Thanks much for all the information. It is exactly what I wanted to know.

    So I guess this means no rag joints. No clamshell adaptors either, huh?

    Would you know if an internally-geared hub would work? Probably not, because of that special cone nut adaptor?

    I had intended to install a 6hp 169cc Robin/Subaru engine w/a left-sided single sprocket.

    Would that be possible?
  6. MikeJ

    MikeJ Member

    Any left-side sprocket is a no-go. My bike uses the cassette gears by means of a jackshaft.

    You might consider a belt-driven sheave. I have seen photos of those in use.

    Ask around how engine vibration affects aluminum frames. They are much more susceptible to vibration fatigue and cracking from vibration. I prefer a heavy steel frame made before 1995.
    Last edited: Feb 7, 2012
  7. Big Red

    Big Red Active Member


    I agree with Mike about aluminum frames. I've used a few and EVERY ONE of them broke. For a peddle bike, OK. For a motorized, NO.
    Shiney Side Up,
    Big Red.
  8. 5-7HEAVEN

    5-7HEAVEN Member

    I've had good luck with motorizing Raleigh male and female cruisers and Diamondback Response aluminum mountain bike frames.

    None have cracked; I've ridden them daily for 2-3 years.

    The DB has awesome welds.

    I'm leaning towards a shift kit, Torq-A-Verter and 6:1 gears w/engine governor. Max hp @ 3600rpm, max TQ @ 2400rpm.

    Still doing research.
    Last edited: Feb 7, 2012