DIY candidates

aswallie

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May 7, 2021
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I just came across this forum and I'm looking for a little guidance. We've had these bikes leaning in the corner of the basement for years and I'm wondering if they'd be a decent choice for upgrading to e-bikes. I have no reservations against drilling, cutting, or otherwise modifying the bikes in any way and the same goes for the actual conversion process. If it would be cheapest/the best option to build my own custom battery bank and hand assemble individual components that require custom mounting or soldering and wiring I'm okay with that too.

I don't really have a target performance in mind but I guess if I can get 35-45mph for 1 to 2 hours of battery alone that would be ideal but if that's too expensive I'm willing to compromise. We live in a very hilly area and a mile of 6% grade is not uncommon so a motor strong enough to carry a grown adult up that would also be ideal but if I need to put leg work in to assist I'm fine with that too.
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Spend more time researching and getting your expectations better aligned with reality. 20 mph is plenty fast for a regular bicycle. 30 mph is a risky and uncomfortable speed for a regular bicycle, and to go faster you should start with a motorbike or a sophisticated and very expensive fully suspended bike of race quality.

The green bike is junk that will take a lot of work and expense to make satisfactory. The red bike is garbage that will stay garbage no matter how much effort you waste on it.

My advice is to sell both of them, and use the proceeds to buy a simpler, sturdier bike that fits you, with more room and attachment points for add-ons.
 
RunForTheHills said:
Aside from what Chalo said, where would you mount a battery on those bikes? Under the downtube?

There's a limited amount of space there, or in a handlebar bag. You could mount a conventional rear rack on the red bike. The green bike can only have a seatpost rack (which doesn't support much weight) or a swingarm mounted rack (which would beat up the battery and the rear wheel).

None of these locations (not even all of them combined) would be suitable to carry the 2 to 4kWh of battery the OP would need to achieve his performance targets.
 
That's the kind of information I'm looking for, thank you for the reply. A bit of background, I'm converting an old school bus into an RV and I was looking for a second, more compact way to travel after we've parked the bus. It looks like plan A, an on road/off road motorcycle mounted on the rear bumper is back on the table.

If I lower my expectations to an average running speed of 15-20mph, with being capable of 35 if the need calls for it, what should I expect a system to run me if I wanted an hour to an hour and a half ride time assuming flat for up to 300lbs? If what I have is garbage I'd be willing to start from scratch and I've got plenty of fabricating tools and material laying around from the larger project.

Is there a general beginners guide? Or maybe a good YouTube series for starting from scratch?

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There is no need to fabricate a bike (unless you want to). Good donor bikes can be bought on Craigslist pretty cheap. Your new expectations are reasonable and there are many kits that can meet your needs. I bought the GMAC kit from Grin and it would do what you want, but is on the expensive side. It uses a rear geared hub motor. The Bafang BBSxx mid-motor kits are popular and affordable, but don't have a torque sensor. They only have cadence sensors. There are cheap kits available on eBay (around $200 without the battery). The Ebike General section is a good place to start, but there is also a build section where you can look at other people's builds.

To narrow it down, you are going to need to give more information about what kind of riding you will be doing: Mostly pavement, gravel and trail, steep hills, technical single track, commuting and grocery getting, etc.
 
aswallie said:
I don't really have a target performance in mind but I guess if I can get 35-45mph for 1 to 2 hours of battery alone that would be ideal but if that's too expensive I'm willing to compromise. We live in a very hilly area and a mile of 6% grade is not uncommon so a motor strong enough to carry a grown adult up that would also be ideal but if I need to put leg work in to assist I'm fine with that too.

Rather than an unqualified "No", it would be more instructive to provide a quantitative response illustrating the problem.

Let's look at lower and upper bounds:
300lbs at 35mi/h for 1 hour
300lbs at 35mi/h on an upright bicycle requires 50Wh/mi battery
35mi/h for 1 hour is 35mi
50Wh/mi * 35mi = 1750Wh battery

300lbs at 45mi/h for 2 hours
300lbs at 45mi/h on an upright bicycle requires 90Wh/mi battery
45mi/h for 2 hour is 90mi
90Wh/mi * 90mi = 8100Wh battery

RC LiPo packs can reach 225Wh/kg
1750Wh / 225Wh/kg = 8kg and $1500 of battery
8100Wh / 225Wh/kg = 36kg and $7000 of battery
 
Clear cut numbers, I like it. So is it the style and form factor that prices the batteries in that range? With the bus conversion I've been researching solar and lithium, 12v 100Ah battleborn runs about $1,000 but I can source parts from China direct and build the same capacity for around $400. I understand higher voltage is generally desired here and that increases capacity requirement but is that price you mentioned for pre fab kits? Do you think diy will bring it down?

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aswallie said:
Clear cut numbers, I like it. So is it the style and form factor that prices the batteries in that range?
35mi/h requires 2000W battery, and 45mi/h requires 4000W battery, so you'd need high energy and power density. RC LiPo is the cheapest way to get those numbers. The prices quoted are for commodity, but quality, RC LiPo.

aswallie said:
With the bus conversion I've been researching solar and lithium, 12v 100Ah battleborn runs about $1,000 but I can source parts from China direct and build the same capacity for around $400.
BattleBorn is overpriced, but China direct (Aliexpress) is risky -- usually garbage. Best to split the difference with a anaccredited US builder and wholesale dealer.

aswallie said:
I understand higher voltage is generally desired here and that increases capacity requirement but is that price you mentioned for pre fab kits? Do you think diy will bring it down?
I gave capacity in Wh; thus, high voltage does not increase capacity requirements. But yes, to maximize controller power density and minimize resistive losses, it is best to run as high of voltage as the controller is rated.
The prices mentioned are for naked LiPo packs, and would have to DIY'd into a complete pack.
 
aswallie said:
If I lower my expectations to an average running speed of 15-20mph, with being capable of 35 if the need calls for it, what should I expect a system to run me if I wanted an hour to an hour and a half ride time assuming flat for up to 300lbs?
300lbs at 20mi/h for 1.5 hours
300lbs at 20mi/h on an upright bicycle requires 20Wh/mi battery
20mi/h for 1.5 hours is 30mi
20Wh/mi * 30mi = 600Wh battery

600Wh / 225Wh/kg = 3kg and $700 of battery

Ultimately, anything is possible. It's just a question of cost and compromises.
 
Chalo said:
Spend more time researching and getting your expectations better aligned with reality. 20 mph is plenty fast for a regular bicycle. 30 mph is a risky and uncomfortable speed for a regular bicycle, and to go faster you should start with a motorbike or a sophisticated and very expensive fully suspended bike of race quality.

I broadly agree (especially for lay public), but among high-speed build and ride enthusiasts here 35mi/h on a quality hardtail build is ubiquitous, and I'd say low 40s on a quality full suspension build. But much depends on locale -- I wouldn't want to go over 30mi/h on any hardtail in my city though.
But past low 40s, experienced analytic design and unlimited budget is desirable.
 
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