Can I use a rear motor hub with 7 cogs on a mountain bike frame with 27 speeds?

[Planet Indigo]

Will this work or fit?
The original non motorized wheel has 9 cogs and the appropriate shifter on the handlebar.
What are my options?
If I don't care about losing gears, can I just adjust the end on the derailleur?
If I wanted to retain 27 speeds, can I put the 9 cogs on the new motor hub? Can the 7 cogs there be easily removed and would have the same width as 9 cogs?
Thanks to anyone who could help on this.

 

[amberwolf]

If your original hub uses a thread-on freewheel cluster, and so does the hubmotor, then you can get the appropriate tool to match the splines on the clusters and move it over.

If your origiinal hub uses a freehub with slide-on cassette, and so does the hubmotor, then you can get the right lockring wrench / tool to match the cassettes' lockrings, and move it over.

If each hub has the opposite kind of cluster/cassette, you can't directly swap them, and would have to buy a 9-sprocket unit of the type your motor takes, that has the same sprocket sizes as on your present hub, to get the exact same pedal-drivetrain gearing.

Note that in any of those cases, you may not have the same chainline as before (angle of the chain from front to rear) for the same gearshift, and you will probably have to adjust the derailer high and low limit screws to match the position of the sprockets on the new hub (it will probably be slightly different than with the old hub).

Sheldon Brown's website has info on all these things if you need it.

 

[Planet Indigo]

Ok, thank you for your answer and those details!

 

[robby p]

First of all realize that once the bike is motorized you simply don't need as many gears.

On my first ebike I used a 9 speed freewheel, primarily because the shifter was 9 speed. Having that many gears actually becomes a little bit annoying because you still want the low gear for starting out and you still want the fast gear once you're at speed. On a powered bike you reach speed so much faster than a regular bike and it's hard for the hand on the shifter to even keep up.

You really only need 3 gears. 3 gear systems don't really exist though.

On my current bike I went with the SRAM GX DH 7 speed setup. Works great.

 

[Chalo]

You really only need 3 gears. 3 gear systems don't really exist though.

I just got one for a hub motor that doesn't have enough spacing for a 5 speed freewheel. But I'm going to use two widely spaced rings in front-- one for when the electrics are working, and one for when they aren't.

 

[999zip999]

I'll take that three-speed free will with a 11 tooth.

 

[Chalo]

I'll take that three-speed free will with a 11 tooth.

Sadly, because the body is the same format as a single freewheel, the minimum sprocket size is the same as a single freewheel (16t). For that reason, I am going to try a 60t chainring to see if that's a high enough ratio.

 

[robby p]

Thought I might get called out on that claim.

Gotta have an 11t on a ebike. I'm 58/11 on my set up. 45mph @ 100RPM

 

[Chalo]

Thought I might get called out on that claim.

Gotta have an 11t on a ebike. I'm 58/11 on my set up. 45mph @ 100RPM

Pedaling at 45 mph costs you more power as drag than you provide at the pedals. So only do it if you want less speed/shorter range.

 

[robby p]

Where's the power coming from? I can hit 50 on downhill but only if I peg the throttle, tuck and pedal my ass off. I'm on a MTB. Bike motor power limit is set to 2200W. I'm probably only good for 300HW.

There is exactly 1, ~0.7mi section of my 19mi. commute where this activity is necessary (or even possible). It's either that or hop up on the sidewalk. The overall impact on total range from this 1 spurt is pretty minimal. Most the time I'm cruising around 30mph. I get about 21Wh/mi.

Plus I like pedaling as long as I'm not spun out.

 

[Chalo]

Where's the power coming from? I can hit 50 on downhill but only if I peg the throttle, tuck and pedal my ass off. I'm on a MTB. Bike motor power limit is set to 2200W. I'm probably only good for 300HW.

You might need able to produce enough power in a short burst to overcome the extra drag, plus more. I know the last time I was on a stationary bike ergometer, I could hold 900W for at least a couple minutes. But I would not be able to do that for long enough to increase my average trip speed noticeably.

 

[4πr^2]

Interesting simulation. They seem to attribute additional drag area of almost 1 square foot just to pedaling? Maybe they are assuming different riding positions when pedaling vs not?

 

[robby p]

Well I'm a little guy. I have probably 25% the frontal area of Chalo who is half the size of my car. You know I used to live in Austin and I met you once or twice. I even saw your band play. You had some homemade amplifier in a bucket I think. We have a mutual friend, Scott Menzies. Tell him hi for me if you see him.

 

[Chalo]

Interesting simulation. They seem to attribute additional drag area of almost 1 square foot just to pedaling? Maybe they are assuming different riding positions when pedaling vs not?

That would be the additional swept area/turbulence of legs that are traversing up and down rather than staying put. You can't set that calculator for zero pedal rpm, so I don't think they're assuming a different riding position for that.

 

[99t4]

Interesting drag calculator. Where's it from?

 

[amberwolf]

I think that's

Bicycle Speed (Velocity) And Power Calculator

Bicycle Speed (Velocity) And Power Calculator, comparing different kinds of bicycles

kreuzotter.de