adlai

I’ve been looking at this product page here.

https://www.sturmey-archer.com/en/products/detail/s-rk3

it looks to me that the o.l.d. Is padded out with some spacers. Thus, if I removed those nuts it could fit on a 120mm. I might have to install a smaller disc brake, like 140mm instead of 160mm. Could this work?

Leisesturm

Need to know more. But sounds unlikely. Don't know any disc frames that aren't 135mm MTB. If Road, then 130mm. 120mm is a relic from the 5sp/6sp era. No discs back then. So, what's up? What are you working with?

adlai

https://bikesdirect.com/products/motobecane/uno.htm

3alarmer

.
...your link says it's a steel 4130 Chromoly frame. So you should be able to cold set the rear dropouts to fit your hub, if removing/adding spacers doesn't get you all the way there. I've never done it on a disc frame, though. So maybe I'm missing something.

Jay Turberville

Have you considered just going with a coaster or roller brake in back? Most of the stopping power is in the front disk anyway.

This Shimano Nexus has an O.L.D. spec of 122 mm. Of course it has a roller brake and isn't Sturmey Archer.

https://bike.shimano.com/en-US/produ...SG-3R75-B.html

The Sturmey Archer X-RD3 and XL-RD3 are actually a few millimeters narrower than 120mm. These have drum brakes.
https://www.sturmey-archer.com/en/pr.../detail/xl-rd3

Leisesturm

I think this is an excellent idea. A coaster brake is probably a bridge too far. No adult should even know how to operate one. But a sidepull? Even the drums wouldn't be bad. Way easier to put some washers on as outside spacers than remove spacers under the locknuts. O.p. should really think about it.

Jay Turberville

Ahhh - you forget the joy of getting that outside pedal in position so you could step down and slide the rear tire out.

tcs

The S-RK3 has three HMN405, 4.8mm locknuts on the RH side. Removing these and replacing them with a single HMN132, 3.2mm locknut will narrow the S-RK3 from 135mm OLD to 123.8mm OLD. Bet you can just 'spring' the rest.

I'm assuming the LH spacing can't be changed because of standard disc rotor spacing. Hmph. But maybe the Joytech 120mm OLD disc hub has unique rotor spacing - don't know nothin' about that.

A Sturmey-Archer X-RD3 three-speed hub with drum brake will slot right in your Motobecane Uno. Depending on the Uno's brake mount, it can be simple to secure the drum brake's lever arm to the disc mount...



...with a standard post/IS adapter.

adlai

I had a bike with a roller brake and it was kind of clunky and would fall out of place.

Also this bike im planning will have an ebike motor installed.

3alarmer

.
...I can foresee some possible issues with brake rotor and caliper alignment, given the construction details and mounting of the brake, if you use anything that does not exactly match your frame's spacing.
Personally, I would avoid doing so, by using a hub that fits the spacing exactly. But I wouldn't be buying such a bicycle, hoping to do an electric conversion, either. Good luck with your adventure.

Jay Turberville

A roller cam actuated brake or a roller brake?
https://sheldonbrown.com/rollerbrakes.html

I'd be more concerned about hub longevity than having a rear disc brake. That tcs has given some info that makes it seem likely that you could make the disc brake hub work. The big advantage of a disc brake for an e-bike would not be in total stopping power, but in the ability to have more heat dissipation for more frequent stops from higher speed. That's one of the often misunderstood advantages of regen braking on a hub motor e-bike. It saves wear and tear on the brakes.

squirtdad

this would be my concern....lot different situation than with a rim brake

squirtdad

Assuming front wheel hub drive?

this seems like adding a lot of stuff to a basic frame/bike and getting a bit of a mix vs getting a purpose built e-bike

Leisesturm

I was assuming mid-drive myself. That fork can't take a hub motor. Actually, that bike can't take an e-assist, to your point. The wheelbase is too short, the tire section is too small, and 160mm rotors ... meh.

adlai

Eh? It seems to me a mid drive would be fine.

The fork is cro mo steel so it seems it would be okay for a front hub. Probably thru axle is best for a front.

Leisesturm

Q/R is fine for a front hub if you use torque arms. Even if you go to thru axle you still need torque arms so why bother sourcing a new fork? IF I really wanted to e-assist that bike I would use 650B wheels so I could get some bigger rubber in there and I would use a backpack battery and mid-drive motor to preserve the handling. 500W is too much power for a front end regardless of how good the fork is. The front tires don't have enough grip to prevent wheelspin on any except the gentlest of take-offs.

Jay Turberville

That just isn't the bike I'd choose for conversion. If I wanted a lightweight ebike that would ride like a regular bicycle, I'd find one with 135mm dropouts and put a very small geared hub motor on the back. The backpack battery is a popular choice amongst cyclists that want their ebike to feel and handle as much like a regular bike as possible.

The Bafang GHA-3 would be an interesting choice except it has a 135mm OLD. It's combines an internal gear hub motor with a 3 speed internal gear transmission. Requires belt drive though. Seems likely to be an OEM product as well. But still a nifty idea for a city bike.

https://bafang-e.com/uploaded/news/2...0253574572.pdf

Leisesturm

I don't know ... as I read that Press Release, the GHA-3 does not have any kind of motor inside. It's a 'regular' hub albeit one with a 3-speed automatic tranny. I also read that it is for either chain or belt drive crank motor, or no motor at all. Being 135mm OLD it absolutely could be able to be sold locally to DIY types, but the efficiency is likely to be low enough that only builders of e-assist bikes interested in using it.

Leisesturm

Let's take a moment to ponder the late, great and gone before its time, Copenhagen Wheel. Now, try to imagine what might have been if those visionary R&D types had opened the envelope to accept a Backpack Power Source. I'm too old and too afflicted with Macular Degeneration, but <expletive> one of you Millennial types should be all over my idea. I won't even come looking for royalties. Run with it, I triple ripple dog dare you ...

Jay Turberville

Yeah - I was searching for a motor that I recalled that had an internal gear hub and thought I'd found it with that GHA-3. If I'd had been paying more attention the size would have clued me in to the mistake. The motor I had in mind was the BionX IGH-3. BionX is defunct and you can only get that motor used and it requires BionX controllers. So not worth the trouble unless you are a big BionX fan of some sort.

adlai

Update: it fits!

i Took the hub and removed two axle flats from the right side drive and I removed the fat axle flat from the disc side. I moved one of the thin axle flats from the right to the left. It fits rather perfectly.

I ordered an additional axle flat but it looks like that was unnecessary. So you can just buy the hub and alter it to fit 120mm disc.

the disc brake plus horizontal drops is weird though for flat tires. You have to remove the disc brake to get the wheel out of the flats.