A Friction-Drive (FD) is an electric bike drive system that spins a roller that’s pressed against the bicycles tire. FDs have been around for over 100 years. Small gasoline engines (the size of a chainsaw) have been used to drive a roller on a bicycle tire almost as soon as small gasoline engines were invented. A company that still makes these kits is staton-inc.com
A concern that some builders have about an FD is that…in wet weather, the roller might slip some. This is somewhat true in my experience, however…there are some unique advantages to an electric-FD that may make them worth consideration over a common hub-motor. Several creative FD builds have been mounted to the seat-post, and if you’re using a lever-actuated post-clamp, you can easily and quickly remove both the seat and drive from the bike with no tools. This can be useful to a college student who must park his bike outside, or an apartment-dweller who lives on the upper floor and wishes to leave the bike at ground level.
There is a limit to how much power we can be put through a friction drive, because acceleration is dependent on the roller keeping a solid contact with a small patch of the tire. Increasing the peak power of your drive by a little too much may result in the roller simply spinning-out. I have personally put 1000W through my system while maintaining constant roller traction, and several builds have been configured for a top-speed of 30-MPH. Both of these goals can be met with a drive that is surprisingly small and light, if you are willing to use some components that have been designed for RC model planes. (see our Astro motor story)
A builder named Kepler noticed that the style of motor frequently used for RC models is called an “outrunner”, and it spins its outer-shell. As a result, a drive could be made where the motor is the roller, rather than attaching a separate roller to the motor-shaft. He is from Australia, and he has begun marketing these drives to the public which he calls the Eboost Power Assist. (visit him here)
One clear feature of this shell-drive design is that the starting-torque of the motor is what causes it to swing out and grab the tire. Once they are touching, the spinning shell pulls itself deeper into the tread as needed. This also means that when the motor is unpowered, it does not touch the tire at all, and because of this, there is absolutely no drag when simply pedaling, so it freewheels exactly like a regular bike. This system is the smallest and lightest E-drive I can imagine for a bike, and is very quick and easy to install. This system installs and uninstalls so easily that you can decide to go with or without your drive system when you take your bike out. It takes less than 2 minutes to install the system once you are use to it. The Eboost can be used on 90% of bikes, but not bikes with rear suspension.
When using a shell-drive, the large diameter roller (2-1/2 inches for a 63mm diameter RC motor) will draw fairly high peak amps when powering-up from a dead stop. The constant-draw amps are very low due to the high reduction and high RPMs of these small but powerful RC motors. For a Kepler-style RC motor-shell drive, I recommend using an RC-controller that is rated for a minimum continuous output of 100-amps. If you want a physically small battery, you may need to use a high C-rate LiPo battery.
I have used some of the affordable generic Electronic-Speed-Controllers (ESCs) for RC-motors. Some of them suffered from a “loss of sync” failure. The ESC I recommend is from Castle Creations, and I have never had any problems with them. I would use the Phoenix ICE-100 (18V-34V, 100-Amps). Also, in order to reduce “voltage ripple” (which could damage the ESC), you should add two low-ESR capacitors to the red/black power inputs to the ESC. Like this $2 example: http://search.digikey.com/us/en/products/EEU-FM1H102/P12393-ND/613754
Why would someone want an FD instead of a hub-motor? They are not for everybody…
If you want to go faster than 30-MPH…I would only recommend a DD-hub and a voltage that is more than 48V. Be aware that even if you have a short commute, you may still need at least a 15-Ah size of battery to have an adequate C-rate when using the popular LiFePO4 battery chemistry.
If you have medium-difficulty hills, and spend most of your time at less than 27-MPH, most people seem to be very happy with 36V / 48V and a geared hub. Since the internal gears allow the motor to spin five times faster than the wheel, peak amps on start-up are lower than a DD-hub, and a modest 10-aH battery should be adequate as the minimum.
But…If you live where it’s pretty dry, you like the idea of an ultra-light drive, and you think you can be happy with a 1000W system that tops out at around 27-MPH, a Friction-Drive might be an option for you. And in spite of how small an RC-motor appears, the hill-climbing power is very good…
Parts list for a total of roughly $800
$350 (Aus) Eboost drive system, seatpost mount with space for one LiPo battery pack
= $800 plus taxes and shipping
For more information about the Kepler Drive system, read the endless-sphere post here.
Coming soon, information on the similar “Commuter Booster” friction drive from Adrian (currently in beta testing). Also, my personal prototype RC-drive where the roller and motor are separate (designed by EVTodd), and an argument for the possible benefits.
10 reason why you might choose a friction drive system over a hub motor:
- Friction drives look different, and appeal to the ebike geek in a lot of us because they were used historically on some of the first ebikes.
- Friction drives are the easiest install imaginable, so much so that your bike will be able to double as a regular bike since the drive system can be taken off of it in a minutes time.
- Friction drives offer much more power to weight than a hub motor.
- Friction drives mean no extra weight in the wheels, a basis for good ride feel.
- Friction drives mean weight is centralized, in the middle of the bike, making a well balanced ebike.
- Friciton drives are an ideal solution for a road bike or a fixie bike where there is not much space in the hub for a hub motor.
- Friction drives allow you to use a super cool rc motor to drive your bike.
- Friction drives are rare. Hub motors are everywhere, the basis for 95% of modern ebikes.
- Some people like the motor noise a friction drive makes….more of a sensation of riding an electric bike.
- Depending how configured, a friction drive can be more efficient than a hub motor.
- Hub motors are made in mass quantities and are therefore more cost efficient. More motor for the money.
- Hub motors are readily avialable.
- Hub motors are so cheap that if you blow one out you just replace it no sleep lost.
- Hub motors are comparably silent.
- Most hub motors can be ridden in the rain or wet conditons.
- Most Hub motors are super reliable and proven so.
- Hub motors make you feel like part of a huge crowd of like wised thinkers.
- Some hub motors can be over-volted to give you incredible performance, both top speed and hill climbing.
- Hub motors can be used on rear suspension bike, a friction drive cannot.
- Hub motors properly installed are much more stealthy than a friction drive unit
Written by Ron/Spinningmagnets, May 2012