Xiongda Motor?

[anotherkiwi]

If the motor has 250 W stamped or cast on it it is 250 W it doesn't matter how many Watts you actually draw. Your legal limit is that the motor must stop assisting at 25 km/h. 48 V is the best performance option it is also the maximum legal limit for voltage (nominative you will be over 54 V hot off the charger).

 

[Slimjim]

I would just be careful about putting torque from the motor, or disc brake into the swing arm as it won't have been designed to handle torque.

 

[Slopes]

I would just be careful about putting torque from the motor, or disc brake into the swing arm as it won't have been designed to handle torque.

Yes I will be very careful - and I may well sort out an alu arm once I've completed this conversion to a working bike. I understand the issue here and I think a possible solution might be to run a tight 1mm steel cable from the back wheel axle down to the axle for the swingarm... this would transfer much of the motor's 'pull' away from the arm itself and back to the frame of the bike - whilst not inhibiting any movement of the rear end suspension.

 

[Slimjim]

If the motor has 250 W stamped or cast on it it is 250 W it doesn't matter how many Watts you actually draw. Your legal limit is that the motor must stop assisting at 25 km/h. 48 V is the best performance option it is also the maximum legal limit for voltage (nominative you will be over 54 V hot off the charger).

Yes I will be very careful - and I may well sort out an alu arm once I've completed this conversion to a working bike. I understand the issue here and I think a possible solution might be to run a tight 1mm steel cable from the back wheel axle down to the axle for the swingarm... this would transfer much of the motor's 'pull' away from the arm itself and back to the frame of the bike - whilst not inhibiting any movement of the rear end suspension.

Cables are a very light weight method to resist loads, but again I would just consider that fact that the hub will try to apply torque to the dropouts (I assume the hub has the 'double flatted' axles that key into the dropouts, possibly in conjunction with steel keying devices). The dropouts will try to resist/react against this torque by applying a bending moment onto their joint into the carbon swingarm. That joint should have been designed to withstand heavy loads from the rear wheel impacts etc. But not necessarily the action of the dropout being twisted by the hub axle, around the centre line of the hub. Your cable sounds like it would only try to prevent the swingarm 'lengthening/flattening'? Which the swingarm is already designed to resist.
The other issue with wires and cables is that they need to be pretensioned a certain degree to take up the stretch. Which is potentially loading your swingarm in the opposite direction - putting it under compression as the wire tries to pull the hub towards the pivot.

Could you run the cable instead from the pivot, to the end of a torque arm that constrains the rotation of the hub?

I may well be over complicating all this, and I could well be underestimating the strength of your swingarm.

It's just worth bearing in mind that the benefit, and drawback of using composites is that the particular area of the component can be designed to handle specific loads, in specific directions, with specific laying off the carbon layers. And loads in unusual directions can prove an unplanned problem. By I'm just guessing.

I'm impressed with your ingenious plans by the way, and that you haven't defaulted to just using a heavy and inelegant swingarm construction.

I'd also try to find some novel way to handle the loads just as you have discussed, but then I'd probably never actually get round to finishing the design, and the thing would never get built or tested.

Paralysis-by-analysis as they say.

Good luck, post some photos as you progress.

 

[Slopes]

Thanks for your considered response Slimjim. You raise some valuable points. To eliminate any 'counter tension' a taught cable might cause to the swingarm, maybe a 2mm steel rod (think coat-hanger wire) would work better instead. This could be attached in either of the ways shown in my image. The second shows an additional tension arm fixed to the dropout - although I think it's not required and the first design would work best.

 

[Deleted member 4366]

Don't forget that that member will be in compression, so you need rigidity, like aluminium tube. Also, the chain is in the way.

 

[Slopes]

Don't forget that that member will be in compression, so you need rigidity, like aluminium tube. Also, the chain is in the way.

Yup, I haven't got any of the crank/chain mechanisms attached yet, so I'm not sure if a rod would get in the way of this. I could just fit it to one arm (the non-chain arm), but that might not be effective.

EDIT: The function of the rod would only be to take away (or reduce) downward, vertical torque pressure from the length of the CF arm. It would have no effect on any non-vertical torque - which I think will be no greater than it would have traditionally been on the non-mototised bike. So, although the rod needs to be stiff, lengthwise, I don't think it would have to be rigid in the sense of a being a structurally fixed part of the member.

 

[Deleted member 4366]

have you already ordered the motor? If not, a mid-drive would be a much more sensible option.

 

[Slopes]

have you already ordered the motor? If not, a mid-drive would be a much more sensible option.

I don't disagree with you, d8veh (and I certainly respect your opinion). It's just that I like setting myself design challenges and thinking through ways to make them succeed (elegantly, if possible). The good news is the Xiongda website is back online

 

[Slimjim]

Thanks for your considered response Slimjim. You raise some valuable points. To eliminate any 'counter tension' a taught cable might cause to the swingarm, maybe a 2mm steel rod (think coat-hanger wire) would work better instead. This could be attached in either of the ways shown in my image. The second shows an additional tension arm fixed to the dropout - although I think it's not required and the first design would work best.

A steel rod is a neat, light solution if you can make it fit without having to kink it around the chain etc.

However, I've just realised something- if you picture that the hub is trying to drive the wheel forwards, then it is in effect going to try to turn the bike in the opposite direction, I.e. lift your front wheel. Therefore a cable or thin rod won't resist that load as they will be under compression.

How sturdy are your actual dropouts, can you post a photo?

I'd be tempted by the second design, with torque arm. There are a few threads about the damage that can be done to dropouts from hub motor torque, however your installation would be no worse in that regard than any other hub motor retrofit.

If your dropouts look like those on your drawings of the alu arm, and have the replaceable derailleur hanger, then the drive side one might allow you to bolt on a sturdier version that acts as torque reaction plate by holding the hub axle on that side?

I still haven't managed to get my ex-Cytronix retrofit working, so no idea if I'm going to damage the dropouts on my Rockshox alu front lowers...

 

[Slopes]

Slimjim. The dropouts are exactly as shown in the photo I pasted up a page or so back (in fact that's my frame). The width of each dropout is 8.3mm.

From my understanding, the forward motion of the wheel will place a downward force on the swingarm - as shown in my diagram below. The area shaded in pink is where the item may be susceptible without reinforcement... ie, if the swingarm was made of (say) nylon, this is where it would bow downwards in response to the motor's driving force. I think the rod (in red) would take the torque strain away from the arm itself.

The vertical positioning of the back end of the rod may be determined by where it would avoid coming into contact with the chain.

 

[Deleted member 4366]

I think the rod (in red) would take the torque strain away from the arm itself.

The rod will go into compression, so will not support the torque arm. As I said earlier, if you want to use a compression member, it needs to be rigid, like tube or box section.

 

[Slopes]

The rod will go into compression, so will not support the torque arm. As I said earlier, if you want to use a compression member, it needs to be rigid, like tube or box section.

This is what I struggle to understand, d8veh. I can see that if torque was caused by the backward movement of the bike (if the motor was in reverse), the rod would be forced to compress (and thereby bend or bow). But it seems to me, with the forward movement of the bike the anchored rod would be forced to extend (which, of course, it would resist). This is what I try to illustrate with my diagram.

 

[chris_n]

The wheel is turning the way your pic shows therefore the hub is trying to turn the other way.

 

[Slopes]

The wheel is turning the way your pic shows therefore the hub is trying to turn the other way.

But aren't the hub and the wheel all of a single piece? The centre point of the torque is where the axle tries to spin in correspondence with the rest of the moving wheel (in a clockwise direction according to my diagram), but is prevented from doing so by it being fixed to the static dropout. This places a clockwise - downward - force onto the resisting dropout which spreads up into the arm it is attached to. This creates an 'extending' force on the rod (it tries to stretch out the rod).

 

[danielrlee]

The stator (stationary part of the motor attached to the axle) exerts an opposing force to the direction of the wheel rotation. Only the motor casing, which also contains the magnets moves with the wheel.

 

[D8ve]

Better to say the axel is pushing the frame the opposite way.
The arm is being forced counterclockwise.

Straight swing arms do work mine is well over 10 years old and still fine.

 

[Slimjim]

Agreed with above, if motor is trying to turn wheel clockwise in above pictures, it can only do so by trying to turn the swingarm anti clockwise. Equal and opposite reactions. I'm assuming also that both ends of your hub axle have double flats, which sit into the inverted-U shaped dropouts on both side. This is how the hub transfers the reaction torque into the dropouts, then into the swingarm. I.e. a tension rod or cable would have to be above the swingarm to be in tension, which would be more difficult to achieve a suitable shape of vertical torque arm.

 

[Slopes]

OK - I get it now. Thanks everybody... back to the drawingboard.

 

[danielrlee]

It's a lovely frame. Please consider taking the advice given to convert to a crank drive instead of hub motor. It'll make the bike handle more like it was intended to and won't risk ruining the swingarm.