# Cheap DIY electric bicycle?



## LHoT (Aug 28, 2008)

Hello all!

I am looking to find a cheap(ish) way to give some power to my bicycle. Cheap because I am only 16, and working for only $0.25 over minimum wage. Speed isn't terribly important to me, well, compared to what I've seen on this site (I saw 75mph and went, "...wait... WHAT?!")

My bycicle is a K2 Crush (Ha ha ha, Sports Authority). Front suspension + Rear shocks. 28" wheels. Weighs aprx 20 pounds.

Here is what I would want ideally.

Speed: 40MPH, will settle for as low as 30 to keep it cost effective.
Distance: 150 miles or more.
Weight: Total weight (Batteries, Motor, Bike) under 40 pounds.

Can it be done? If so, how much is an estimated cost (I would like for something near, or below 1.5k). I do realise that I haven't looked into this as much as I should have, but I am not completely decided yet.

Thanks in advance,
Derek


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## JohninCR (May 6, 2008)

LHoT said:


> Hello all!
> 
> I am looking to find a cheap(ish) way to give some power to my bicycle. Cheap because I am only 16, and working for only $0.25 over minimum wage. Speed isn't terribly important to me, well, compared to what I've seen on this site (I saw 75mph and went, "...wait... WHAT?!")
> 
> ...


150mi with 40lb motor and battery package simply isn't possible. 50mi at 20mph...maybe. Once you get above 20mph the wind resistance gets geometrically harder to overcome.

Whatever you decide, be extremely careful using an aluminum bike. With a hub motor, torque arms to distribute the force along the frame instead of just in the dropouts is mandatory. 

35-40mph in that price range is definitely achievable, just not your distance goal in terms of price or weight. 
btw 75mph is E-motorcycles, not E-bicycles. You might have the brakes for 40mph, but not brakes or frame for much more.

Good luck with it,

John

EDIT: Sorry, I didn't realize you included the bike in the 40lbs. 20lbs of motor and batteries, you'd need to machine the gearing solution to be able to use a 2-3lb RC motor, and the best lithium battery packs money can buy to get the 50 miles at 20mph I mentioned.


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## LHoT (Aug 28, 2008)

Thanks for the speedy response.

Well, I guess I can deal with a heavier bike. I can maintain 30MPH on it for about 10 minutes flat ground, no wind. (I have monstrous legs, I run a 5 minute mile, and play a game called ITG at level 16 (Look up on youtube "ITG Rynker Vertex Beta" (He's my friend, not me, but that's about what I am capable of )) Anyways, the main thing I'm looking for is a nice long range. I suppose 50 miles would be alright, now that I think of it. Still looking for 40MPH though (With assisted pedaling... I would assume that matters).

Sorry for how sloppy my thoughts are at the moment. It's late, and I should be resting for tomorrows ride to school. (10 miles away, long story short, I moved and still go to the other school district)

So, summed up....

35-40 MPH with assist pedaling
50-70 mile range
40 pounds for the batteries and motor together. 60 including the bike.

Doable? If so, how much. (sorry, cost is a prime concern again)


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## JohninCR (May 6, 2008)

LHoT said:


> Thanks for the speedy response.
> 
> Well, I guess I can deal with a heavier bike. I can maintain 30MPH on it for about 10 minutes flat ground, no wind. (I have monstrous legs, I run a 5 minute mile, and play a game called ITG at level 16 (Look up on youtube "ITG Rynker Vertex Beta" (He's my friend, not me, but that's about what I am capable of )) Anyways, the main thing I'm looking for is a nice long range. I suppose 50 miles would be alright, now that I think of it. Still looking for 40MPH though (With assisted pedaling... I would assume that matters).
> 
> ...


Since you're a powerful rider, I'm sure it's possible. The problem is that E-bike stuff isn't designed for those speeds. Yes, you can over-volt them, but I'd question durability for those kind of sustained rides. Push them hard and they're less efficient, meaning more heat.

I'm at about 35mph now, and hope soon to be in the 40-45mph range soon with reasonable durability, but my trips are just local errands of a few miles. I don't pedal though, except hills and the first 10-20ft of take offs., but I'm an old fat guy. I've got some samples of motors for small motorcycles in route that could handle the speed and range, but they're 20-25lbs and will need significant batteries for range.

You'll need a motor capable of multiple kilowatts continuous, and at high efficiency, but the problem is that the existing motors capable of this are a bit more powerful than you need, so they are in the 25-30lb range. This includes Mars (the cheapest at $450), Lynch and Perm (most efficient, but expensive, and still heavy. Then you need batteries, which are going to be the most expensive part of your build, because you need high power Lithium. Go look at RC lithium battery pack prices to see what I mean.

Let me give you some numbers to keep in mind while you search and plan. With pedal assist guys with good e-bikes are using 12-15 watt hours per mile, but that's at 20-25mph. At 40mph, with heavy pedaling, you're probably looking at 30-50wh/mi (an efficient motorcycle uses about 150wh/mi). 100wh of the best batteries weigh 2+ pounds, and cost over $100 for raw cells you have to make into packs.

It sounds like you won't be happy without the performance, otherwise I'd suggest starting with something lesser. Battery prices are expected to come down over the next year as production volumes increase, and significant increases in capacity per unit weight are years away from becoming available. 

The marked improvement I see coming is in motors to suit your specific needs. Companies simply haven't been designing for that, but changes are starting to happen. Start saving now for batteries, then once 2kw continuous with 5-10kw peak brushless motors that weigh only 8-12 pounds become available, get one and buy whatever batteries you can afford. Build your bike and add to your battery pack over time to get to the range you want.

Little considerations for you are things like extra large chainrings to be able to pedal assist at high speed without a cadence that will wipe you out after a mile. That kind of specialty stuff isn't cheap either.

John


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## LHoT (Aug 28, 2008)

Cost may be non issue soon! I was talking to my teacher today about if it would be possible to get credit for this. She had said, "Absolutely." Then told me that it may be possible to get a "sponsorship" to build this! If this could happen, my budget may go up DRASTICALLY. If this turns out to be the case. I would be extremely pleased. Now, I've been thinking about how to do everything. so I have some questions now.

The BionX kit... it is essentially what my mental picture was. So I have some questions about it's motor. How does it work? How efficient is it? Etc.

Solar Charging. Possible? If I can get this "sponsorship" it is one thing I would LOVE to add. But ways of doing it I am not too sure of. I would think panels in between the triangle of the frame, on both sides, and possibly is the wheels, although I'm sure adding it on the wheels would be very very difficult.


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## skullbearer (Jul 9, 2008)

I disagree with John, if people are getting 30-50Wh per mile at that speed then you need about...

40mile = 1 hour

50W/hour = 1 mile

so if I'm doing this right, 50x40Wh per hour @40mph, so 2000Wh/hr or 2000W (2kW) of Power to maintain the speed.


If he can maintain 30mph on his bike for some period of time, short or otherwise, that's what? By your numbers I'm estimating about 20-25W @30mph, maybe more given the lower quality of his bike (I can do near enough as doesn't matter 30mph on my $250 mountain bike with $20 100psi road tires from Performance Bike and keep it up for almost 40miles and I'm not in good shape).


So let me to that calc again... 25x30= 750Wh or 750W to maintain that speed.

We see here the drastic difference in need between the two speeds, but this means 2kW is more than he needs. I bet he could do near enough to 40 as his bike could survive for a few years with a 1kW motor. I've done 45mph downhill on my own bike and its Chrome-olly which is more flexible. Aluminum is much much rougher at those speeds, and my experience working with cheap aluminum bikes at a bike shop tells me you might have some serious issues at 40mph with your bike. Even if you can do it I'd very gradually work into that speed, watching your rims and forks, and every stressed part especially welds... I've heard some horror stories about those cheap aluminums snapping at welds or bending their main frame at lesser speeds on only mild bumps.


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## JohninCR (May 6, 2008)

Skullbearer,

We're not as far off as you think, and yes a motor rated at 750W may be capable of doing 30mph, however, that doesn't scale up to 1000W for 40mph, because due to the effects of wind resistance the increase in power required isn't linear. Think about a car...a 100hp motor may get you to a max of 100mph fairly easily, but no way putting a 200hp engine will get you anywhere close to 200mph. The difference is due to wind resistance, which on bikes becomes the biggest hurdle on flat terrain for speeds over about 25mph.

For durability with long rides I suggest not pushing a motor to it's limits, and a higher powered motor than the minimum required will typically dissipate heat better, and heat is the real enemy since inefficiencies all along the line turn into heat. Don't forget about acceleration and hills. Someone who wants a 40mph bike doesn't want to go 15mph up hills or take 200yds to get up to speed.

A 2kw motor is about right, though with a lot of pedalling 1200-1500W may do the job as long as he's not hauling a load with him (camping gear, ice chest, a load of school books, or whatever).

John




skullbearer said:


> I disagree with John, if people are getting 30-50Wh per mile at that speed then you need about...
> 
> 40mile = 1 hour
> 
> ...


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