Top drive - yes, I can see the point you're making. Even though the rear axle will essentially be a single gear, its arc (from the chain crank) will still diverge from the arc of the swing arm as suspension is brought into play. It therefore seems likely I will require some form of chain tensioner to compensate for the small difference this will cause. Thanks for mentioning this.

yeah because the pivots not concentric with the centre point of the front chainset - not many FS bikes are due to bearing issues mind.

 

Try shimano alfine sprung tensioner - or for the cheapo option use a rear mech locked out with a bit of cable - if you stick the head of the cable into the barrel adjuster and pull mech to desired position and tighten cable to hold it - no need for a shifter

Are you going for this kind of swingarm shape?

Are you going for this kind of swingarm shape?

 

Here's a technical drawing I found of the K2 (which has been very useful for planning out) and I'd like to shape the swing arm as closely as I can to its original shape. That's not gonna be entirely possible with rectangular tubes - and I suppose I could build the arm as a frame of round tubes (following the contours of the original) and weld a tapering 3mm flat plate onto each side.

 

I've seen pictures of the same K2 bike with an alu swingarm (obvious from the weld lines)... maybe Proflex brought this out to deal with cracking problems in the CF version?

 

Edited September 17, 20169 yr by Slopes

Their is nothing to stop you doing a straight swing arm is there?

That would simplify the design perhaps even lighten it?then an offset mount for the shock.

straight arm would fracture at the point where the spring is attached.

im still struggling with the aspect of this being cost effective ? or is there an emotional attachement to these proflex frames

A straight swing arm in the rectangular tube would be the easier option (and the preferred one in terms of my metalworking abilities). Although I'm not sure if a straight tube would clear the crank/BB.

straight arm would fracture at the point where the spring is attached.

 

trex - is there anything I could do to prevent this happening? Such as raising the height of the shock mount to its original position and welding the two arms together with a hefty central 'shoulder'?

im still struggling with the aspect of this being cost effective ? or is there an emotional attachement to these proflex frames

 

So far it's shaping up to be very cost effective. And - from a design point of view - I like the look of the bike... it's one of a few from that era that lends itself to conversion (CF swingarm notwithstanding).

Get your swing arm fabricated into box sections from flat plate then no need for the heavily loaded weld just in front of the shock mount. You could then carry the side plates on to form the dropouts.

Get your swing arm fabricated into box sections from flat plate then no need for the heavily loaded weld just in front of the shock mount. You could then carry the side plates on to form the dropouts.

 

chris_n... that sounds like a good solution, but I think it'd be beyond my abilities - would like to keep the welding I'll have to do to a minimum.

What equipment do you have to cut/machine/weld/design with?

Well, a friend has a TIG but to be sure, I might ask in the local cycle shop if they can weld alu. If I go with rectangular box tube, I can ask the supplier to cut pieces to the dimensions I supply - or cut them here on my mitre saw. As for the initial design, I'm pretty good with Fusion 360... so I'll start by making a precision drawing of the new swingarm, taking measurements from the old CF one (once the frame actually gets delivered!).

I have the frame now- looks in great condition... barely used, if at all I've decided to keep and use the carbon swingarm for the time being - as rebuilding the item in alu will be time-consuming and tricky - with its various angles. I'm not intending to use the bike off-road at the moment so I can gently test out the existing backend with the motor fitted.

 

The dropout gap is 135mm - as bending the arms is out of the question, I'll opt for a version of the Xiongda hub at that width. Presumably, if I'm limiting myself to a single gear at the back, I could fit in a disc brake too (?)... if not, I'm ok with a V brake on the back with a disc on the front forks.

 

The Xiongda website appears to be down currently so I do hope they haven't gone out of business (has anyone had any recent dealings with them?). I also note that Panda Bikes has stopped selling this product - which is a shame as I was very templed to buy the whole kit from them.

 

I have to decide upon 36v or 48v. I definitely want to keep within the 250w legal limit, but also want the best performance I can get - any help with a good overall set-up would be welcome.

Edited September 24, 20169 yr by Slopes

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).

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.

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.

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.

Edited September 25, 20169 yr by 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.

 

 

http://i68.tinypic.com/25kmng6.jpg

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

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.

Edited September 25, 20169 yr by Slopes

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

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 :-)

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.

 

 

http://i68.tinypic.com/25kmng6.jpg

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...