I can check my dynamo works by spinning the wheel on the workstand or just riding, but I'd like to do some more involved testing. I'd like to develop some custom electronics to use the extra power available at high speeds (for charging), to drive an additional rear light that's always on even if the front light isn't, etc. The first step is to measure the output. I can make a start with a battery oscilloscope in my top tube bag, but that won't get me very far.

Road speed is something like 300 rpm, and I can't achieve that by just spinning the wheel by hand, let alone work my oscilloscope at the same time, so I'd like a hands-free solution. Ideally this would work on the bike, as I'd like to try this with the existing lights as well.

I have tried running the chuck of a cordless drill against the front tyre, handheld. The speed ratio should be about right given the diameters of the chuck and wheel, but there's too much slippage from the hard curved plastic. Something cheap and easy to fit/remove would be ideal.

  • 1
    Take inspiration from the dynamo guru: peterwhitecycles.com/headlights.php
    – Paul H
    Oct 18, 2019 at 15:17
  • @PaulH I'm now thinking along those lines, but mounted to the fork
    – Chris H
    Oct 18, 2019 at 15:19
  • Since it is for testing and development, you don't actually need to have the wheel on a bike. Remove the wheel and couple the axle to the dril or another suitable motor. A short length of rubber hose and hose clamps may be used as a flexible coupling to avoid stressing the axle and drill's bearings.
    – Jahaziel
    Oct 18, 2019 at 22:38
  • @Jahaziel I don't technically need it on the bike, but I will soon want to test with the lights that are currently fitted to it, to be sure my circuit doesn't cause problems with them. Also spinning the axle with a drill will spin the connector, and whatever testing I do needs wires attached
    – Chris H
    Oct 19, 2019 at 7:03
  • @ChrisH, you are right, I hadn't fully thought may suggestion. An alternative for the drill is to attach a rod (as if it where a drill bit) and wrap around some material like strips of inner tube to enlarge the diameter, with a final layer of anti slip tape. That should be easier to make as close to round and concentric as possible. Another thing that may work for your project is the use of bike rollers (I Assume the dinamo is on the front hub). You'd need an assistant to pedal the bike though.
    – Jahaziel
    Oct 19, 2019 at 14:43

1 Answer 1


Sounds like you have the same requirements as the Rubbee, which is a small wheel pressed against the tread of the tyre.

Have you got an old inner tube (perhaps a 2" MTB sized one) where you can cut a slice off, and mount that over your drill's chuck?

If the diameter of the chuck isn't quite large enough, a wooden disk with a larger radius could be mounted in the chuck with a bolt on the disk's center.

Don't forget that some drills have gearboxes with multiple stages. 2 speed is common.

A last resort might be to disregard any battery powered drill and check the effective RPMs from a mains powered drill.

I've successfully mounted a spoke magnet in my pillar drill and used it to test a bike computer sensor. (It topped out reading 150 km/h)

  • 1
    Some good ideas there. I though about a wooden disc but decided centering would be tricky without a lathe. My mains drill (normally in a press) has only a little more speed (my cordless is good) but a lot more torque at full speed; it's a proper keyed chuck so would need a disc of some sort - but it has a mounting collar. Perhaps multiple layers of tube or tube over some firm foam would increase the diameter and allow some deformation for better grip
    – Chris H
    Oct 16, 2019 at 10:23
  • Now you mention the Rubbee, there's a chance I could get my hands on a 24V e-bike motor
    – Chris H
    Oct 16, 2019 at 10:24
  • 2
    Hardware stores sell trolley wheels in many sizes. You could also use a disused balancer wheel for a child's bike.Use a bolt as an axle to hold it with the chuck.
    – Carel
    Oct 16, 2019 at 11:01
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    The Rubbee is a conversion kit for electric propulsion of regular bikes that drives the rear tyre with a roller on its tread?
    – gschenk
    Oct 16, 2019 at 11:17
  • 1
    @Carel my garage doubles as a junkyard; I might be able to find a wheel of some sort, then need to mount the drill. I'm inclining towards using the mains drill as I can use a foot switch to turn it on/off and it's got a mounting collar
    – Chris H
    Oct 17, 2019 at 10:23

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