The DIY Junkyard E-BMX Friction Drive

I have seen youtube videos on a regular basis that show an old cordless drill being used as a drive-motor for a small bicycle. I’ve frequently responded that a cordless circular saw is a much better candidate, and one day recently I decided…I needed to put my money where my mouth is. If you are interested in a beautiful and sophisticated high-performance and long-range ebike, then this is not the article for you…go away!

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Why a Saw, and can we use a Chain-Drive to a Wheel?

I noticed a while back that a lot of “weekender grade” cordless tools are made pretty cheaply. Not horrible, but…also not great. I have come to the conclusion that these manufacturers are making the batteries to last as long as they can affordably design them to last, but…once the average battery pack wears out, there is no incentive to design the actual tool to last any longer than the first battery pack.

I might use my cordless drill once a month, but a professional tradesman might use his for several hours each workday. In that case, those professional customers will pay extra to get a tool that can work almost constantly, and still last for many years. They will have several battery packs, so that…when one is low, there are several charging, and that provides a fresh one whenever they need one. A high-quality heavy duty tool like that is something a tradesman would buy new batteries for when the batteries get old.

I mention that because…when the weekender-grade tools get old, they have almost no resale value. The used Ryobi circular saw (with small 5-1/2 inch diameter blade) that I purchased from a Pawn shop only cost me $28 (there were a lot of DeWalts there too)…and it included a working charger and 5S / 18V battery.

Drills are designed to be pushed straight down inline with the motor shaft, so the bearings are configured to hold up in that application. However, if you use the side of the chuck as a friction-drive roller against the tire tread (with radial pressure)…it can be fun to see that it works, but….doing that for any length of time will fry the already worn bearings.

If we use a circular saw to power a friction drive, the stock bearings are being used in exactly the direction that they work best. Not to mention that a typical circular saw will likely be more powerful than an average cordless drill. Even if they are both 18V, the saw will have a bigger motor that draws more amps, providing more power.

If you’re wondering why I’m using a friction-drive configuration, there are several benefits to doing it this way, when using a circular saw. Cheap used cordless tools are not that powerful, and a friction drive is limited to how much power you can apply to it anyways, so I think its a good match. If you used a high-powered motor, the friction roller would just slip instead of driving the tires tread.

Since this isn’t going to be an impressive hot rod for an adult, it is natural to build this project up for a child…and that will affect some of the other decisions we make. This can be a fun parent / child weekend project. And besides…if it turns out to be a total failure, you will still end up with a cheap cordless circular saw, amirite?

We “could” use the circular saw to power a chain that connects to a sprocket, which is attached to an aluminum adapter disc that is mounted to a bikes hub (in the location where a brake disc would normally be mounted). The immediate problem is that the saw doesn’t have a freewheel, and when it isn’t powered, the wheel would be driving the saw.

But…if you incorporated a rear bikes hub (from a common and cheap 20-inch wheel BMX) as a jackshaft between the saw and the wheel…it could work. You could even use a 3-speed IGH, to make your saw-drive a 3-speed.

I would only attempt to chain-drive a bikes wheel-hub with a smaller wheel, like a common 20-inch, since a taller wheel would tend to bog it down. Of course then, you would have to incorporate a significant reduction in the drivetrain, because this saw runs at about 3200-RPMs. It’s “doable”, but complex. However…this project is all about doing this as cheap and as easy as possible, so…the friction drive set-up is what I went with.

I have a lot of experience with friction drives (it was my first article for electricbike.com, click here). There are two general styles of engagement. You can either have a roller that does not touch the tire when you are pedaling, or you can have a freewheeling roller that is always touching the tire. Having a “no-touch”  system means you would have to use some method to engage it, like moving a lever, perhaps operated a re-purposed brake handle. Trying that is also ‘doable’, but I never liked that, and it would get annoying after a while.

The famous Kepler RC-drive uses the start-up torque of the motor to make it swing into the moving tire. Once the roller is spinning, it only has to touch the tread for its spinning motion to draw it deeper onto the tread. You simply power-up the motor, and the unit swings onto the tire. In our project, a light spring, or the location of the pivoting hinge will position the center of gravity so that when the saw is un-powered, it hangs near the tire, but doesn’t actually touch it. I almost made one of those, but the last method I described earlier seemed faster and easier.

This is where the saw drives a freewheeling roller that always rests on the tread. If the wheel is spinning, the roller is spinning, but…since it has an integral freewheel, the saw does not spin. You can make one of these from purchased parts, or…I wanted to see if I could get the rear hub from a single-speed BMX to work as a roller, and…it can!

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Making the Adapter Disc

There are two specific parts that are needed to make this concept work. One is to buy the 16T fixed sprocket that has six mounting holes, so it can be bolted the a disc brake flange. They are not common, but can be found online for about $20.

Then, we have to make an aluminum adapter disc that will bolt that sprocket to the shaft of the circular saw. For the disc-adapter we will be drilling and cutting a scrap 3/8th’s inch thick aluminum plate. That’s 0.375-inch, or…roughly 10mm thick (6mm/7mm plate would also be fine).

 

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I had a scrap of 3/8th-inch thick aluminum plate, but 1/4-inch would probably work fine too. the center-punch and felt marker shown helped, but I ended up not using the sharpened rod, which I planned to use as a scribe. I ended up using my old pair of measuring calipers as the scribe to scratch a precision circle.

 

If you perform the steps in the proper order, this can be done with a very simple tool-set, and it can be done with a reasonable level of accuracy. First, make a dimple in the center of where the adapter will be (with a center-punch if you have one). You don’t need to blacken that area, but I did that (in the pic) so the dimple shows up better.

Then, I feel it is necessary to lay the sprocket over the aluminum plate with the dimple in the center, as close as you can eyeball it. Blacken the six mounting holes with a felt marker.

Then, use a drawing compass or a caliper to scribe (scratch) a circle. You place one tip of the caliper in the center dimple, and then with the caliper set to the precise radius measurement (half the diameter of the “Bolt Circle Diameter” / BCD), and you rotate it so it scratches a perfect circle that will pass through the precise centers of all six bolt holes. If the measurement of the center of one bolt hole to the center of the bolt hole opposite of it is “X”, then…the scribe radius is 1/2-X.

 

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Figuring out where to put the holes.

 

Clamp the sprocket back over the work-piece with the BCD scribe-line centered through the six holes, then drill out three of the holes, evenly spaced at 12:00 O’Clock, 4:00, and 8:00. I’m only recommending three of the holes, because its easy to make a mistake on the first try, and this way you can re-do the job on the other three hole locations, if you need to. Plus, even if you get it right on the first try, you really only need three of the holes for this job.

The proper way to do this is to punch a dimple with a “centering punch” placed in the sprocket-holes, then drill a tiny pilot-hole with an 1/8th-inch bit (mine is cobalt), and then drill the holes to the proper size after that. I didn’t do this the proper way, and it worked out “close enough”…

 

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Measuring the diameter of the driveshaft…

 

Once you have the three sprocket-mounting holes done to your satisfaction, measure the diameter of the saw shaft (which varies by make and model), and drill out the central dimple of the adapter disc.

 

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Mounting the sprocket to the plate helps you see if you have the holes in the right locations.

 

I could have used a proper tap to cut new threads into the adapter, but the steel screws managed to create usable threads into the aluminum. If I had ended up keeping this, I would properly drill-and-thread the remaining three holes, and then use “threadlocker” fluid on the threads to keep the screws from loosening from vibration over time.

 

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Here, I am 90% done cutting-out the round edge.

 

In the pic above, I am 90% done cutting the outer edge with a Black & Decker 6.0 HP jigsaw that uses “quick-change” blades. I bought the steel-cutting Bosch blades and they easily cut the thick aluminum plate. I had to uses a zig-zag cutting pattern because of the tightness of the circle I was cutting.

 

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Starting to take shape!

 

Here, the hard part is over. One of my goals was to enable a cordless circular saw to be used as a friction-drive motor for a child’s bicycle…while still allowing the saw to go back to being a cordless circular saw afterwards for me!

Just below the saw (in the pic above), you can see the BMX single-speed hub that I plan to use as a freewheeling roller, and the short chain that will connect them. I cut an old rusty bike chain to the proper length with a dremel and its thin abrasive disc, and connected the ends with a “master link” (the shiny silver link shown).

Circular saws have an adjustment to control how much of the saw-blade protrudes below the flat deck. That adjustment will also act as our chain-tensioner. You set the saw as low as it will go, and once you put the short chain on it, lift the motor to tighten the chain and lock it into position.

 

I tried to make every part as cheap and simple as possible.

 

I used some free pallet-wood for the part of the frame that the freewheel-hub mounts to. The actual drop-outs are another section of that scrap aluminum plate, cut to a rectangle shape and then I added a slot to fit the axle. I glued some 80-grit wet-dry sandpaper to the shell of the freewheel-hub to help- with traction.

 

Here you can see the cheap gate hinges, available at any hardware store.

 

The wooden triangulated section in the upper rear is firmly bolted to the handlebars with four U-bolts (see below). and the base-plate (plywood) that the saw is bolted to can pivot just above the roller.

 

These saws have a variable-speed trigger. I just need to attach the cable from a BMX brake handle to actuate a wooden pair of scissors to clamp onto it when you want to turn on the motor.

 

Here you can see why I made the baseplate rock forwards and back. Notice the angle of the base-plate.

 

In the pic on the left above, pedaling the bike when the motor is un-powered pushes the freewheeling roller forward, so it rolls along with almost no drag. On the picture on the right, the motor has been energized, and the spinning of the roller pulls itself backwards. This makes the roller swing down in an arc, and dig into the tire. If you look closely, you can see in the picture on the right that the pivot, roller, and wheel-axle are all in-line.

So…how do we calculate precisely where to mount the whole assembly so that the swinging mechanism operates properly? It’s really simple. You build the whole device, and then you clamp the whole thing together in the driving position (pic on the right) so the roller can’t swing forward. Then, you press it down onto the tire until it has the amount of pressure on the tire that you want, when the motor is powered up. Have a friend mark the wood where it crosses the handlebar in four places (in a square pattern). I used common 1/4-inch U-bolts (with a 1-1/8th inch interior width), to clamp onto the one-inch diameter handlebars.

Once the bike is pedaled with the power off, the rocking mechanism allows the roller to simply swing forward, until it is no longer applying any pressure to the tread.

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Doing the Math

One odd thing about friction drives is that…the tread of the tire will be moving at the same tangential speed as the tire tread. In other words, a 15-MPH friction-drive will provide 15-MPH whether it is attached to a 29’r wheel, or a 20-inch wheel. The two sprockets on the drive have a 1:1 ratio, so the roller will spin at the same RPMs as the saw-shaft. I measured its RPMs with a couple of laser tachometers, and they averaged a reading of 3200-RPM.

In order to find the hub diameter, I wrapped a thin strip of paper around the center of the hub that I’m using as a roller, and made a line across it. Once I put the paper strip back flat on the counter, I measured the distance between the two lines to find the circumference, which turned out to be 5-1/8th inch (5.125″). If that roller is spinning 3200-RPM’s, then its road-speed is 3200 X 5.125 = 16,400 inches per minute. Divide that by 12-inches per foot, and we have 1,366 feet per minute.

I could do the rest of the math, but I just went to a web-calculator and typed-in “convert 1,366 feet per minute to MPH”, and the roller (without a load on it) will be traveling at 15-MPH. Once we try to push the bike and the weight of a child, that might be a top-speed of 13-MPH (20 km/h). The motor has a very high reduction this way, but it is only 18V, and maybe 20A at the most, so…about 360W of power under the best of conditions.

I know you can find a much more powerful cordless saw if you search, like 24V, 36V, or even more, but…seriously, it’s a $28 used saw from a pawn shop. It includes the chain tensioner, the variable-speed throttle, the motor, controller, charger, and one battery. If your kid doesn’t like it? You have a cordless saw now!

Doesn’t the roller on a friction-drive slip in the rain? Yes.

Don’t the tires on a friction drive wear out faster? Yes.

Could you mount the drive-unit on the rear tire? Yes.

Doesn’t the small battery have a short range? Duh.

Isn’t this whole project slow, low-powered, and ugly? Yes, yes, and YES!

If you want a step-up to something better than this hillbilly set-up, I would recommend a Magic Pie in a 20-inch wheel. The controller is built into the hub-motor center, so 2/3rds of the wires are already tucked away. The only separate pieces are the throttle and the battery. It can be run with a 24V battery for smaller kids, or 36V if your child is older and can handle more power.

 

A 20-inch Magic Pie with the controller built into the center of the hubmotor.

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Written by Ron/spinningmagnets, June 2017

Grew up in Los Angeles California, US Navy submarine mechanic from 1977-81/SanDiego. Hydraulic mechanic in the 1980's/Los Angeles. Heavy equipment operator in the 1990's/traveled to various locations. Dump truck driver in the 2000's/SW Utah. Currently a water plant operator since 2010/NW Kansas


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