Folding Electric Bike

Introduction

After redoing the first serial hybrid bike with a full suspension and thoroughly enjoying the weight savings of using lithium, I thought about building something lighter. Not sure if I will ever build such a bike as described, but I did some investigation…

Goals

The goals of this build would to refine the build process and produce something more portable. Less welding, more scouring websites for the most appropriate components.

Requirements

  • Reduce the weight of the bike to 45lbs
  • Retain the improved ride of a suspension bike
  • Reduce the build time
    • I should be able to assemble the bike in one full evening
  • Refine the aesthetics of the bike

Initial Thoughts

I’ve created a parts spreadsheet, and the numbers don’t look good. Building a bike from parts is more expensive than just going out and buying the whole bike (at least for low end components). I may be better off finding a suitable donor model and just improving it. I’m investigating folding bikes more and more because of the small wheel size and steering. I could replace the seat and add a boom and be on my way. Unfortunately, folding bike’s (with a suspension) aren’t cheap, and I’ll be throwing out almost the whole drive system, which seems like an awful waste.

April 2nd Update

Folding bikes appear to fall into several categories. Here they are in ascending price level:

  • Heavy, no suspension: CitizenBike Tokyo, $160, 29lbs
  • Heavy, full suspension: CitizenBike Miami $180, 35lbs
  • Light, no suspension: CitizenBike Gotham, $260, 30lbs; Downtube NS9 $350, 24lbs
  • Light, front suspension: Downtube IX $300, 26.5lbs
  • Light, full suspension: Downtube FS $390, 26.5lbs
  • Light, expensive components, different types of suspension: Dahon >$900

The number of options is staggering and I’ve had to think about overall goals for this bike. Is 2.5lbs less weight and a front suspension worth the $140 jump from the Tokyo to the Downtube IX? Since a new Zoom front fork can be had for $40– no. The Downtube is a higher quality bike, but the following components need to be thrown out anyway:

  • Brake levers (don’t have motor cutout switches, although I suppose I have gone without them so far…)
  • Gear cluster (replaced by sprocket and adapter)– actually the Downtube uses a cassette, so the whole wheel may have to go
  • Derailleur
  • Bike chain
  • Seat

What about for a full suspension. Does 8.5 less pounds justify the $210 for the Downtube FS?

Battery Weight & Cost

Voltage Capacity Chemistry Weight (lbs) Cost Max Bike Weight (total: 45lbs, 5lb motor)
24V 10Ah LiFePO4 6 $550 34
48V 7Ah Sealed Pb 22
$80
18
24V 12Ah Sealed Pb 18 $80 22
24V 20Ah Sealed Pb 28 $120 12
24V 26Ah Sealed Pb 36 $160 4

The CitizenBike Miami must use lithium. 12Ah lead batteries can be used with the Downtube bikes if the rules are bent a little, or if there is some weight reduction in the drive system. I believe the drive system is actually heavier than 5lbs but I don’t have a scale to verify this.

My next step then is to research the lithium batteries further.

Battery Suppliers

LiFePO4 batteries don’t like to be discharged at more than 2C continuously (or so I have read). Using my ammeter, I measured that at 24v, 17 amps provided fairly aenemic acceleration. 20 amps would be about minimum to justify carrying the electric bits around. 30-40 amps was plenty. So, at a minimum, I need a 24V and 10Ah battery pack, but preferably 15Ah.

I tried scouring Google, VisforVoltage, and Endless-Sphere for battery info. ZEVA Australia recommends PHET batteries and has some pretty cool tests to back them up. 3.2V and 1.1Ah and $2.80 each. 8 cells in series and 14 in parallel is 112 cells or $313. Ouch. Bill Dube of KillaCycle recommended the A123 cells he is using, but the only way to get them cheaply is to disassemble DeWalt battery packs. They’re 3.2V and 2.3Ah and anywhere from $10-18/cell. You need 56 cells, but the dewalt packs have 10 batteries in them, so 60 minimum. The lowest price I have seen for the DeWalts on ebay is $80/pack, but $100 is typical. $500 minimum. Pricey! LifeBatt is advertising in Gmail, but they are super expensive. Yesa is also well regarded, but apparently they don’t perform too well. The latest craze is to ebay Chinese “duct tape” batteries. Quite a few people seem to have good reviews [and #2, a huge thread] of the duct tape batteries from PingBattery on eBay. The stock is constantly changing, but the buy it now price for 24V and 10Ah is $200. That’s short of what I need but the best price I have seen so far. Watching the auctions for a while now, most people opt for the bidding war. 48V and 20Ah goes for about $600, and 24V and 10Ah goes for $300. Doesn’t make much sense, but that’s how eBay bidding works…

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