Shorten Hub Motor Axle with Sawzall

[dudgggg]

Hi,

I need to shorten the axle on my front hub motor, it protrudes about 1,5 centimeters out of the dropout on the fork. It's a small hub motor (36V/250W, likely bafang 8Fun?) im trying to mount on a very (!) old linkage fork. I need to remove the protruding part of the axle.

Reason behind this is the narrow dropout on this 50+ year old fork and the fact that this ancient bike has only one front rod brake that doesn't really work well. I decided to go for a disk brake adapter that mounts right outside of the dropout and then attaches with a clamp to the fork to hold the brake. This is why I can't have the motor axle protrude there, the disk brake won't have its lower mounting point.

So in short, can I just saw that off? On the other side the cable comes out and I'm not messing with that. But as far as I can tell one side is just a threaded axle coming out, can i cut that off or are there any components that the motor needs to operate? Is there any reason not to saw off the protruding axle part?

I attached a photo of the part I want to remove.



I think most people wouldn't advise doing something like this in the first place, but I'm looking forward to hearing your thoughts. Thanks in advance.

 

[donn]

Reason behind this is the narrow dropout on this 50+ year old fork and the fact that this ancient bike has only one front rod brake that doesn't really work well. I decided to go for a disk brake adapter that mounts right outside of the dropout and then attaches with a clamp to the fork to hold the brake. This is why I can't have the motor axle protrude there, the disk brake won't have its lower mounting point.

I wonder if the inadvisable parts of this may include the disk brake plan. Not that I know anything about such matters, never had disk brakes, I just have the impression that they can put a great big hairy load on a fork if it wasn't designed with that in mind. How easy will it be to replace this fork, if it fails? "Linkage fork" - is this a recumbent? "Rod brake" sounds like more than 50 years ago.

 

[j bjork]

I am not sure how hard it is, but you can always try. Maybe you will need an angle grinder or something.
There are nothing sensitive in the axle. I would like too keep it to mount torque arms, but on such a small motor it might not be needed.

 

[dudgggg]

How easy will it be to replace this fork, if it fails? "Linkage fork" - is this a recumbent? "Rod brake" sounds like more than 50 years ago.

Pretty much impossible. It's not a recumbent bike, it's a very old bicycle that belonged to a family member and was recovered amongst all sorts of junk that I can date to be at least 50 years old, no idea of its older than that. I said linkage fork because it has separate bars with the suspension in front, the actual spring sits above the wheel where a V-brake would go. That's why I went straight to the disk brake plan, I don't want to mess with this bizarre assembly and somehow integrate a V-brake there. The fork basically looks like this:


But on mine there is only one big spring in the middle and the front bars are much shorter because they sit at a weird angle. I have never even seen this before and I have no idea how I would go about replacing this. Sorry if I messed up the terminology and it's not a linkage fork, I just googled it and found some websites referring to that as linkage.

There are nothing sensitive in the axle. I would like too keep it to mount torque arms, but on such a small motor it might not be needed.

OK good, so if that's just a "dead" piece of steel, it can definitely go. Torque arms, I hadn't thought of that yet. I will add one on the opposite site with the cable, the axle still comes out enough to add a thin plate there. Even if I don't need it on my small motor, it looks similar to the adapter that the brake will be mounted on and gives it a symmetrical appearance.

 

[motomech]

Get the correct thread die (probably a 1.25 M/M) and thread it on the axle first (w/ lot's of grease). Cut the axle as sharp and clean as possible and if it gets ragged, flat grind it w/ a hand held grinder. Then you can work the die off to clean the threads. Turn it back on carefully (start it by hand) for a final clean. The nut will be fussy going on, but it will.

 

[Drunkskunk]

That kind of fork is called a Monarch style fork. Great forks. Welcome to the forum.

Hey, if you're using one of those hinged disk brake adapters.. don't. There are better alternatives, and they won't require you to cut your motor. The hinged design has no lateral stability, meaning the brake caliper can wiggle side to side, causing drag and a horrible noise. Nails on chalkboard type noise. It's also overly complicated so has more failure points. Also, you likely won't be able to lock the axle down tight enough to the rocker arm. For every action, there is an equal but opposite reaction, meaning the axle is trying to twist just as hard as the motor, in the opposite direction.

There are MUCH better alternatives that let you replace the link with one that also holds the brake, Like this thing:
(Also, you need torque arms, one on each side would be best)

https://www.monarkforks.com/fork-store/fork-parts/disc-brake-adapter-detail

Alternative source... https://venicemotorbikes.com/shop/?product=disc-brake-adapter-drop-outs-for-monark-forks

 

[john61ct]

That kind of fork is called a Monarch style fork. Great forks. https://www.monarkforks.com/

Strong enough for say Grin All-Axle, or ideally a GMAC?

Would like to get decent regen for drag brakibg, heavy tandem/cargo, big hills