Custom dropouts thru axle

[Oldie]

Firstly I have small 36v 250w motor with a 17amp controller and 48v battery. The most I have seen it push is around 700-800 watts on the display.

I have a new frame I would like to convert using the setup above taken off my previous bike.

I have removable thru axles on my new frame.

Pictures attached.

What would be the best way to cut the dropouts to fit a 12mm axles hub motor?

My only thought and the easiest way according to me is to cut a bit of the axle on the non wire side. Slide in the other side and that should leave enough space to slot in the cut side. The frame is steel so I can flex it a few mm to make it work. Well that’s the idea.

My only concern is because the dropouts are round. Will the axles spin even I don’t use torque arms?

Can you think of a better way to cut the dropouts instead of the axle to get it to fit?

The drop outs attach to the frame using three screws.

 

[amberwolf]

Some thoughts below, discussion on various parts in more detail can be done if you have questions.

I have removable thru axles on my new frame.

How large are the thru-axle holes? (IIRC, the first ones were 15mm, and later shifted to 12mm)

If they are 12mm or larger, you do not need to modify them. Instead, you keep them as-is to keep the axle centered in the right spot of the frame, and install torque arms either inboard or outboard of those to secure the axle from rotating.

Torque Arms - Grin Products - Product Info

For every force there is an equal and opposite force, and for every torque there is an equal and opposite torque. Sadly with import hub motors this opposing torque is often born entirely by bicycle dropouts that are not up to the task, allowing the motor axle to spin out. Grin has pioneered the...

ebikes.ca

Since the thru-axle "dropouts" are removable, then as long as the frame does not have anything in the way of the axle going in and out of it, then you simply attach the torque arms and dropouts to the axle, then attach the entire wheel assembly with those to your frame.

If the frame does not have a "notch" for the axle to be lifted or dropped into, then you will have to modify the frame itself...but the removable dropouts do not need to be modified (if they are at least 12mm holes).

If they are less than 12mm and wont' slip over the axles, then you'd either have to modify them or replace them.

In that event, it's "simpler" to just skip using the existing removable dropouts, and make your own torque plates that bolt to the spots they did. If you don't have power tools (other than a drill), nothing more than patience and a set of files is needed to shape them, once you have cut the basic shape (hacksaw, etc) out of sufficiently thick steel. With the drill you can make a 8-9mm hole (not bigger) in the center of the spot the axle must pass thru, then file that out to the double-d shape of the axle, slowly and carefully till it is an exact fit to your axle (test fitting until it just barely goes onto the axle, and cannot wiggle rotationally; it's very important that the axle flats fit very tightly and flatly to the flatted parts of the hole). With this you won'tneed a torque arm; the plate *is* the torque arm and the dropout. Once it fits the axle, then file the outside to fit the frame, and

One source of such steel is the L bracket on some office chairs that connects the seat back to the seat--this is often made from >1/4" steel; I have successfully used this from a cheap goodwill find to make pinching / clamping droputs from for much higher power levels than you're using...and the chair is still a good roll-around seat, minus the back. As long as the L bracket is flat, and not curved or some other shape, it's easily usable for this.


You have to use some form of torque arm on at least one axle, the non-wire-side (because it is strongest with no cutouts for wiring), because you don't have anything else on your frame or dropouts to engage the axle flats and transfer the motor torque to the frame. Without this, the motor torque will, instead of spinning the wheel, will spin the stator and axle, twisting and ripping up the axle wiring.

I don't recommend modifying the axle itself, as it has little enough surface area on the axle flats to transmit it's torque as it is, and once modified it's difficult to undo.

 

[Oldie]

Many thanks for the detailed reply.

The thru axle dropouts are 12mm. The none thread side the motor axle slides in perfectly. The threaded side won’t slide in so will need drilling to remove the threads.

I was actually thinking about what you said about assembling it all the bolting the dropouts to the frame. I’m pretty sure I don’t have the space to do this. The bolts are on the inside. I was thinking of getting longer bolts with a nut and trying that.

Creating my own torque plates sounds good. I have a variety of tools. My only question would be how do I get round the derailleur hanger? I would need this for the gears.

With the low powered motor I have. What thickness steel would you recommend I use?

Thanks again for the help.

 

[Oldie]

Just an update, i am selling the frame and getting one with qr dropouts to make it easier.

 

[amberwolf]

You'll still want to get or make torque arms or plates; don't rely on the dropouts to handle the torque (none of them on pedal bikes were designed to do this, and most of the ones on ebike-specific frames weren't either).

It's not the wattage that is the issue, it is the teensy tiny amount of surface area on the axle flats that contact the dropout flats, vs the torque of the entire wheel pushing against the entire bike and load/rider, and that most dropouts don't *perfectly* fit the axle flats, so the axle can move just a bit inside them, and that can cause (at some point, sooner or later) enough deformation of the surface of the dropouts and/or axle to let the axle spin in the dropouts. At that point no torque is transferred, and it just very quickly pulls the wiring around the axle, usually damaging it (which itself can then cause other failures).