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Wednesday, August 27, 2008

Friction Drive 2.0.2 Still Works, Except In Rain

I've been working on the treadmill motor idea off and on since the last post, but haven't made final designs yet, mostly because to my surprise, the Friction Drive using radiator fan motors and roller skate wheels has worked well enough for quite a lot longer than I expected, with little maintenance.

So far the only big problem was when I skidded on the lip of a shopping center driveway entrance as I was turning in (it is over 1.5" high--practically a curb for a bike tire, though probably not noticeable in a car or truck). I ended up too far sideways to recover and skidded out--I stepped off the bike with no problem but the bike hit the ground hard enough to knock a battery loose (had to re-zip-tie it on using the spares I always carry) and break one bolt and one rivet holding the motor plate on. A zip tie in a strategic location enabled me to keep going and get home after shopping, but it definitely wouldn't've held too long that way.

When I got home, it turned out a second bolt had been stripped a bit, so I took the whole plate off, redid the rivets holding the mounting brackets on the bike frame, and used the next size up for bolts. Took a bit of searching thru my junk boxes to find a set of matching nuts and bolts that also fit one of the wrenches I already carry with me (since I don't want to add another tool to it just to road-fix any motor issues). These bolt heads and nuts are a bit thicker than the originals, and thus on occasion if I am pedalling hard with little motor input (or during accelleration from stop when the motor is least effective) the left crank scrapes a tiny bit against one of the nuts, even though the nut's filed down as far as it safely can be. Can't really do anything to fix that, without using thinner-headed bolts I don't have (and would have to buy).

The rollers themselves have worn some, requiring occasional (2-3 weeks apart) retensioning of them against the tire, but no other issues have developed than those previously posted about. The tire itself doesn't show any excessive wear from it either.

The only major problem the system has is that when it's even a little wet, it really doesn't work. If I just *barely* throttle up once I'm already going, I can add a bit of motor speed to the bike, so I'm not completely pedalling, as long as it is not actually raining or I'm not riding thru puddles. During anything more than a light sprinkle, then even at low motor speeds the slickness is too much and the motor speed overcomes the friction, rather than transferring any torque at all to the tire, and the motor just spins at it's free-run speed at whatever throttle input level it's getting at the time.

Since this *is* Phoenix, Arizona, it generally doesn't affect my rides, since even when it's cloudy or rainy, it doesn't stay that way for long, and roads dry up enough to work with pretty quickly. During this season, it's a little tougher, and the other night my ride home from work was all pedal-power due to the nearly-torrential downpour the entire way.

It's other major disadvantage is the huge amount of power being wasted in the fan motors, since they're designed for low voltage (12V), high speed, and low torque (they drove small plastic fans to cool off a car engine in their previous life). Since I'm using them at 36V (actually about 28-30V since I can't get more than roughly 75-80% duty cycle out of the controller I've got on there, either because it's damaged or because it's designed to only do that), and at much lower speeds at those voltages than they'd free-run at, to drive a pretty heavy load, they get very very hot, perhaps 150F or more by the end of a less-than-2-mile run at their axle cores. Given that they're bolted to relatively large aluminum plate that gets significant airflow across it's entire surface during almost all of a ride, that's a lot of wasted power in heat. That's why I would like to go to the treadmill motor, as it was designed much more for this type of purpose.

Since I have only old SLA batteries to test with, I don't really know what the range would be if I had new batteries, but the most I get out of it when being conservative with it's usage is roughly 5 miles before having to stop for at least 30 minutes to let the system cool off and the batteries self-revive, which gives me another 2-3 miles after that before it *has* to be recharged for up to 14 hours to get that same range again. If I had new batteries that allowed higher C-charge rates, I could probably get a lot more range with a quicker turn-around at the end.

All in all, it's a definite success. For those reading and looking for a good method to drive a bike via motor, I can seriously recommend this particular one for it's simplicity of design and build, and low maintenance, as long as you're not in a wet area. If you are, you're gonna need different tires and rollers to generate higher friction when wet, which will probably wear your tire out faster. Other than that, and the motor waste heat/energy problem, it works pretty well, old batteries notwithstanding. :-)

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