Thursday, September 3, 2009

Foot Drive System

While I'm waiting for a pile of parts to arrive, I've been working on the boxes in the feet for the motors and wheels. I bought a couple of Razor E150 scooters a while back at Walmart ($100 a piece). The kids have been riding them around the neighborhood, but I had to tear one of them apart and get the motor, wheel, and batteries out of it.

My foot shells are a tight fit. I built them from 1/4" plastic and there's not a lot of room to spare in there. So the dimensions for the foot drive have to be just so. I spent a frustrating day or two mocking up different designs and layouts with 1/8" MDF until I had a layout that would get it all in there.

Like lots of builders, there's not enough room in there for that motor and two wheels, so I have to go with one wheel inside. That has caused pivoting problems for some guys who have the foot shells attached by the single axle point like in the plans. But I built a little saddle for mine a while back (see an earlier post) and put a couple of threaded bolts into the bottom of the leg to attach the shell by. So I think I won't have the pivot problem.

I don't like the wheel in the scooter. It's too tall and skinny, and it seems like the sprocket is too far from the wheel. The wheel is about 1.5" by 5.75". So I found some caster wheels at Harbor Freight that are 5" by 2". That will give me more traction with the wider wheel and it will let the R2 ride down lower to the ground:

Then I unbolted the sprocket from the scooter wheel, drilled four holes in it, centered it on the new wheel and screwed it onto the rim/hub.

I was pretty worried about this when I was looking for new wheels. I didn't know if I could find one that had a hub that would work to mount the gear to, but this seems to be fine. In fact, this was the easiest part of this job.

I got $40 worth of aluminum at Blue Collar Supply (Sacramento) for building the boxes. I wanted to use rectangular stock tubing like Mike Senna and Vic Franco have been doing on their drive system (which looks like some excellent garage engineering). But to fit my foot shells, I'd need 3" by 4" rectangular tubing, which is a pretty exotic size. I don't want the drive box to stick down too far. So what I settled on, after looking around at all the readily available aluminum, was using some 3" by 1" channel and some 1/8" aluminum plate.

Then I cut out the trapezoid side pieces from the plate. Turns out that I didn't have a really effective way to cut these. The bandsaw had a thin blade on it that was drifting on the cut a lot, and putting an abrasive cutting wheel on my old table saw was noisy, dangerous, and slow. So I put a new blade in my hacksaw and saw them out with arm power. That sucked. My wrist and arm are still feeling it. But here are the results:

I got the pieces lined up, clamped them, double checked all my measurements, and cut the various holes. Notice the slot for the wheel axle to allow to tighten the chain. One advantage to this design over the boxed in version that Senna/Franco are doing is that I can get inside the box to work on stuff. You can see here though that I had to grind down a scallop out of the side wall of the channel. That's to make clearance for the gear and chain on the wheel. Not hard to put that in, but cutting the all four trapezoids in the Sacramento heat was not fun. Here are some shots with them bolted together and with the wheel and motor in.

I think it's going to work well. Notice the additional cross piece down in the corner. That's a 5/16" (or so) bolt --3.5" long I think, with a piece of 7/16" aluminum tubing cut to 3" for a sleeve. I'm also going to put sleeves on the bolts that hold the motor in. I replaced the bolts that were on the scooter motor with ones that are 1.5" to reach across. With the sleeves, the motor should be secure enough.

I hooked up the battery and ran these a bit. Getting the alignment just perfect will be important to make them run smoothly and more quietly. The motor itself is nearly silent, but when you get that chain and the sprocket going in the system, it can really scream. That's worrying me for when it's in the R2. I don't want the drive system to sound like a lawn mower. I've been tempted to order some belt drive motors and belts instead just to get the noise down. Maybe if some of you out there have the belt drives instead of the chain drives you can let me know if they run quieter.

But for now I have the chain motors and I'm going to try to make them work. I'm going to get the alignment very good, get everything secure, and maybe dampen some parts with some rubber tape and see if I can reduce the noise. A good way would be to get the whole drive box closed in and insulated with some kind of sound dampening material, but that'll have to be a later project.

Next I have some L channel 1.25" by 1.25" that I am going to cut and put on top of these to make a fitting that will slide onto the slot in the foot shells. I figure that will fortify the whole thing and make the feet more stable. More pictures then.

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