Front wheel hub? Can I put in it in the rear?

MrJ

Joined
May 8, 2022
Messages
2
Good day. First post. Im running into a problem with a hub motor. Im a high school shop teacher and we decided to build the Silent speeder. So I ordered the plans, we cut up the frame, ordered the batteries, but I have run into a problem. All the Rear hub motors I see have 135mm dropouts. The rear dropout of the BMX we cut up is about 110mm. I understand those 135mm dropouts are for a gear sprocket assembly, thats why its wider. I cannot find a 110mm or so Rear Hub motor. So question....can I put a Front hub motor in the rear? and will it work ok? Plan B is to widen the rear stays to fit the 135mm distance. Maybe I would need to come up with some sort of spacer if I did have to use the 135mm hub. Not quite sure. Just wondering if anyone has had any experience with this issue. Really do not want to make a mistake when I order. Really want to complete this and let our school use it. Thanks in advance.
 
Joined
Feb 7, 2008
Messages
3,691
Location
Nottinghamshire England
Hi there

Put a scissor jack in the rear of the BMX and spread them to get 135mm
or
Put some M8 rodding in there and nuts on the inside and wind them out to 135mm - if you do this wrap some brake cable around it and tether near the BB to stop the rod winding out if your axle slots point forwards
or
just lay it flat on the floor put your feet over the part on the floor and pull on the upper piece till reaching 135mm

It is just that simple !

Paul
 
Joined
Oct 19, 2012
Messages
1,808
Location
Wakefield, UK
If you widen it you'll also need to realign the dropouts as splaying it to 135 will have them non parallel and that will not be a good thing with a hub motor. So long as it's a steel frame it'll be fine. If the frame is ally you run the risk of fatiguing it. Many front hubs won't have any means of attaching a sprocket for pedal drive but some will. If it's only got the regular 6 holes for a disc you could make a set of freewheel threads mount off that. Not as easy as splaying the rear. Many 135 motors will have a spacer fixed to the axle designed to clear a 7 speed freewheel. You'd then just need to remove it or cut it down so not needing to splay the frame or perhaps not as much. You'd then also have to cut off the excess axle you'd have. the axle may or may not be flatted that far back though so you may need to add those flats so it sits in the dropouts and can avoid twisting in them to supply drive.

I buy the 135 motor, remove or shorten the spacer and axle and then measure the dropout width you're left with. If the design doesn't need pedal drive then a front motor will do.
 

MrJ

Joined
May 8, 2022
Messages
2
If you widen it you'll also need to realign the dropouts as splaying it to 135 will have them non parallel and that will not be a good thing with a hub motor. So long as it's a steel frame it'll be fine. If the frame is ally you run the risk of fatiguing it. Many front hubs won't have any means of attaching a sprocket for pedal drive but some will. If it's only got the regular 6 holes for a disc you could make a set of freewheel threads mount off that. Not as easy as splaying the rear. Many 135 motors will have a spacer fixed to the axle designed to clear a 7 speed freewheel. You'd then just need to remove it or cut it down so not needing to splay the frame or perhaps not as much. You'd then also have to cut off the excess axle you'd have. the axle may or may not be flatted that far back though so you may need to add those flats so it sits in the dropouts and can avoid twisting in them to supply drive.

I buy the 135 motor, remove or shorten the spacer and axle and then measure the dropout width you're left with. If the design doesn't need pedal drive then a front motor will do.
Thanks for your input. That Silent Speedster that I wish to build .... it does not have a pedal drive. Pretty sure about that. So what you are saying is that a front drive hub would be OK to put in the rear if I do not use any gears/pedal drive? I pretty much have what I need for the build. Just need to decide on the front hub. Sounds like you have done some of this before. Do you have any favorite front hub suggestions? I have 4 12v batteries...as in the plans.
 
Joined
Oct 19, 2012
Messages
1,808
Location
Wakefield, UK
Check the size of the dropouts before buying anything. Some bmx bikes use a 14mm rear axle and most hub motors will fit 3/8 dropouts. For a no pedal machine a front will fit the rear subject to the above. If your frame is a 14mm axle you'll need to weld in something to reduce them.
 
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