Thinking of converting my commuting bike - questions and advice

[pajtaz]

Thanks for the replies, Nealh and vfr400. So in many responses, you say to run 36V motor at 48V (so 48V controller with 48V battery). But will this not decrease the lifetime of the motor if I am going to run it between 30 to 35 km/h?

And I guess I have to choose a controller that is capable of 20A to make sure I get the top speed but that can regulate current to lower values? I guess after some testing I will figure out the best assist levels in terms of current and the one that will get me 30 to 35 km/h should not be at 20A but lower (because I will be pedaling also). Only if I hit strong headwind should I be pushing towards the highest current setting.

Then how do I determine the motor winding? Manufacturers don't tell you in descriptions, I guess I have to hope to see a photo of the part number and the part number will tell me but how? From what you are saying, the reason why I would select 36V 201RPM motor vs 48V 201RPM is because the 36V 201RPM would be wound so that when higher voltage of 48V is applied, it will run faster whereas 48V motor will have lower speed at the same current.

All this points to:

• MXUS XF08C 36V 250W (because it is available to purchase but my concern would be with max speed and lifetime due to higher voltage combined with higher current)
• MXUS XF15C 36V 350W
• Q128C 36V 201RPM
• All of these would run with 48V 20A controller and 48V battery with 20 to 25A maximum discharge current.
• All of the above would need me to make sure motor runs as much as possible above 50% of maximum speed. I intend to use lower assist levels when starting or at crossings but should be looking to use higher assist levels everywhere else.

So to be clear, for my touring Kona Sutra, 700c wheels, 9-speed cassette, my weight 75 kg, riding flat occasionally against headwind, you would NOT recommend any 36V 328RPM or 48V 328RPM models to achieve 30 to 35 km/h? There are no 260 RPM models anywhere in stock or for my configuration.

 

[vfr400]

328 rpm will be perfect for what you want. You either use a 36v 260 rpm one at 48v or a 48v 328 rpm one. 260 rpm might be OK for 30 km/h, but not for 35km/h. You need a motor that spins to 1.33 times your modal riding speed to get good power at that speed because the power ramps down to zero as you approach maximum RPM.

 

[pajtaz]

328 rpm will be perfect for what you want. You either use a 36v 260 rpm one at 48v or a 48v 328 rpm one. 260 rpm might be OK for 30 km/h, but not for 35km/h. You need a motor that spins to 1.33 times your modal riding speed to get good power at that speed because the power ramps down to zero as you approach maximum RPM.

Thank you so much for this answer. This is what I really needed to know.

To all who contributed to this thread, thank you very much for taking the time out of your life to help me out. I will use your information to put together a configuration and hopefully purchase it by the end of the year.

 

[vfr400]

If you have enough mechanical nouse to install a kit, you should be able to figure out how to build your own wheel. Many on this forum have been in the same position as you and they completed their wheel build successfully. Sheldon brown gives a very clear instruction on how to do it. If you buy a rim and motor, we can tell you the spoke length and give additional advice.

Even if you can't succeed, at least you have everything to hand over to somebody who can, and you should at least be able to loosely assemble it, which will save them time and cost.

 

[danielrlee]

This hub motor lacing video tutorial from Grin is probably the best that I've come across:

 

[vfr400]

That's a good video. He uses exactly the same method as me. I like the tapered spoke idea to insert the nipple. I've been using a 13g spoke to insert 14g nipples. It just hooks in to the first thread in the nipple so you can then use it as a driver. That's not as good as the tapered spoke.

A couple of things he forgot to mention. When you have a dished wheel (off-set rim), the spoke tension will be different on each side. The difference goes up exponetially with the amount of off-set, so best to try and minimise the amount of offset with washers and spacers on the axle first to get the motor's spoke flanges as near to the centre of the frame/ forks as you can before dishing.

Secondly, a diamond shaped spoke key is better than a U-shaped one.
Diamond shape slot.

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U-shape slot

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[Gtechelec]

Would a carrera be easy to derestrict? (One of the ones that are somewhere around £1000 new?)

 

[vfr400]

No, not easy.
All electric bikes can be derestricted in one way or another. It's easy if you know how, but easy might mean a lot of work and cost. If you want an unrestricted bike, check what's involved first, which I guess is what you're doing.

 

[Kwozzymodo]

I have a rear hub motor, controller, crank mounted PAS and handlebar display on my set ups. Simple and straightforward to fit. The display can be used to set bike-specific parameters and limit speed. I use 36v batteries, either a 6.8ah or 10.4ah - which are good for 20-30 miles, depending on how energetic I am feeling!

I have removed the front derailleur from a couple of my conversions, and then adjusted the size of the single chainweel to meet my requirements (I like to pedal at around 80rpm). Anything from 40-50 teeth works for me.

If you want to do 30+kph, then a fast wind hub, say 328, is the way to go. I can do 40kph on mine.

Cost for the above is around €450, including the battery, charger, etc.