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I just made an electrically assisted bike conversion......

Posted: Mon Jan 24, 2011 11:35 pm
by bobc
Basically just using up RC gear I had lying around from other projects.
I'll post a couple of pictures on the off-chance that someone might find some useful ideas among the insanity....
I've only run it with a small battery pack, but it actually behaved perfectly, straight out of the box - ran for a couple of miles at 12.5mph & was able to climb a 1 in 7 hill without pedal assist.
Things that might be useful: 1] how to attach a small sprocket to the shaft of a brushless outrunner:
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the cap screws pull a tapered thing into a tapered hole in the sprocket to grip the shaft taperlock style.
Taper reamers are about £3.50 from axminster tools (rather than ~£200 from dormer.......). 3.55degree angle on toolpost.
yeah I've got a lathe....... that is 6mm chain.
2] how to make any size or fitting of chain sprocket. Draw it in autocad (or something like) and send the dxf file to your local laser cutters. make in mild steel, stainless or ally, usually about 3mm thick - check chain specs. I got these back in 2 days for about a tenner each. Some minor fettling is advised - chamfer the sides on a grinder & clean the notches where the chain roller sit by pressing each one against the side of the right size twist drill (only takes a couple of minutes for a 90 tooth sprocket like this)
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that one is an 8mm chain sprocket mounted on a southpaw freewheel on a turned down disc brake mounting.
Here's the whole bike (yes it's a skip rescue case.... ;^)
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Hope there's something useful there.
The motor is actually a monster but running on a 40A ESC so not ridiculously powerful.....

Re: I just made an electrically assisted bike conversion......

Posted: Tue Jan 25, 2011 5:57 am
by retepsnikrep
Nice Bob. laser cutting of metal is bloody handy nowadays. If only i could use autocad :roll:

Re: I just made an electrically assisted bike conversion......

Posted: Tue Jan 25, 2011 9:25 am
by ex925
No, sorry, if you're trying to outdo the "Junkyard Dog" for scrap-recycling, I'm afraid you lose........
FAR TOO WELL DONE :D :D :D
Brilliant

All the best, from Ed

Re: I just made an electrically assisted bike conversion......

Posted: Tue Jan 25, 2011 10:12 am
by Jeremy
I like the neat "taperlock" type idea, far better than using grub screws.

I hadn't realised that getting one-offs laser cut was such a reasonable price, either. Cheaper than buying a ready made sprocket that would have been too small anyway.

For those without AutoCad I think even the free CAD packages will output 2D DXF files, which is probably all the laser cutting place needs.

Jeremy

Re: I just made an electrically assisted bike conversion......

Posted: Tue Jan 25, 2011 1:13 pm
by bobc
No I didn't like the grubscrews much either - part of the problem with them is that the shaft is about as hard as the grubscrew.
I get a good deal with the local laser cutters - they've made all sorts of bit for me now, 1st name terms & I always give them cash, but equally I know they're not giving me charity!!! It makes it simple, they're happy with my dxf's & the machine seems to be accurate to better than 0.1mm. I find it cheaper to buy finished parts with all the holes in the right place etc. from them than it is to buy the material alone!
If you do this MAKE SURE YOUR DXF is 1:1 SCALE - I use solidworks, saving the drawing is dxf form then editing the dxf to get rid of the frame and any index marks. If you do this, select properties of the drawing and change the scale to 1:1 before you import the part.
Thanks for the generous comments ;^) the mounting method is quite cool, using the layshaft bearing bolts to clamp the frame tube (the big plate is threaded so the bearings are held while you assemble it).

Re: I just made an electrically assisted bike conversion......

Posted: Tue Jan 25, 2011 2:04 pm
by bobc
as an aside - some back of envelope calulations of the torque capability of the taperlock fixing:
motor 3500W @ 8000rpm - 8000rpm is 840rad/s so motor torque is 4.1Nm

M3 screws - say root diameter 2.5mm; stainless steel say 200N/mm^2 stress for 0.2% - that's 982N per screw - there are 4 so that's 3927N clamping force.

3.55degree taper - that's 1 in 16 so normal force between shaft & taperlock = 16 * 3927 * (1-CoF) = 44kN
Coeff friction dry steel-brass = 0.3 so we have 13.2kN tangental force at 5mm radius = 66Nm
That seems like plenty of spare ;^)

Lots of dodgy estimates and simplifications to have a go at there lads.... fill yer boots! ;^)

Re: I just made an electrically assisted bike conversion....

Posted: Thu Jan 27, 2011 8:27 pm
by geekygrilli
Very smart - well done. I didn't realise Laser cutting was so reasonable. when my Dad used to get it done for work, I remember it being very expensive. .i.e. he made me cut and file stainless steel by hand cos it was cheaper for him and it built character!

I was thinking of using karting sprockets and chains (219). Large Sprockets are approx £7, and mounting hubs aren't too expensive, you can buy them with many different bores from 25mm to 50mm.
Smaller pinion type sprockets vary from £6 - £15.

Link here, if anyone is interested... http://cgi.ebay.co.uk/GO-KART-SPROCKET- ... 518c65a196

What cells are you running it on?

Re: I just made an electrically assisted bike conversion....

Posted: Thu Jan 27, 2011 11:35 pm
by bobc
Sounds as though the karting gear is pretty good value too. Last weekend it was run on 2 off 20C 2.2Ah 3S lipoly cells (i.e. about 22V), I think Matt's buying some 5Ah cells to give him (with luck) enough range - he has 2 big hills to climb so we'll see, I suspect he'll have to double up again.

Re: I just made an electrically assisted bike conversion....

Posted: Fri Jan 28, 2011 10:14 am
by Jeremy
A reasonable rule-of-thumb for a medium power ebike is to use a figure of between 20 and 30 Wh per mile, erring towards the higher figure for someone who's heavy, a route with lots of hills or someone who always rides at 20 to 25mph +.

The small capacity packs will give you a range of between 1.6 and 2.5 miles, adding some 5Ah packs in parallel will increase the range to between 5.4 and 8.1 miles. Pedalling a bit will significantly increase these figures, if he can pedal reasonable well at cruise speed then you may get the power per mile down to maybe 12 to 15Wh per mile or so.

I run 4 off 5S, 5Ah LiPo packs on my folding bike, connected to give 10S, 2P, which gives me around 37V at 10Ah, so about 370Wh, and a range of between 15 and 18 miles, depending on how I choose to ride. If I pedal a lot and slow down a bit (I normally cruise along at about 20mph where pedalling isn't practical because of the bikes gearing) then I can get the range up to over 20 miles.

Jeremy