# building a bike dynamometer shouldn't be hard

Discussion in 'Performance Mods' started by bluegoatwoods, Jan 2, 2009.

1. ### bluegoatwoodsWell-Known Member

The other day a member named alex started a thread called "Dyno a happy time?" I gave him as much answer as I could right then, but with a few days to think about it I think I have a design in mind that shouldn't be hard at all. I doubt if I'll build one. I don't have the space and don't have enough need to justify it. But maybe you guys who do engine porting and many builds could make use of it.

Picture yourself hoisting a bike up off the ground. Now picture the equivalent of a see-saw standing right behind the bike. On the arm of the see-saw that points toward the bike is mounted a standard bicycle brake.
Place this brake over the rim of the back wheel. At the other end of the see-saw is a bathroom scale. Perhaps it sits on a platform right at the edge of this see-saw/torque arm.

The bike engine is running, clutch dis-engaged. So your drive wheel is spinning. Now apply the brake mounted on the torque arm. The bike's drive wheel will, naturally, pull up on this torque arm. At the other end of the equation the torque arm will push down on the scale. And this will give you a measure of the torque that your drive train is delivering to your drive wheel.

If you also have a bike speedometer attached somewhere, then you have a measure of the torque that is being generated at a particular speed. And from that you can calculate power. Though you will need to convert wheel mph to rpm. But that's not hard.

I'm assuming that the engines power is two horsepower at twenty mph. I have no reason for this particular number, except that I think I've heard somewhere that happy times are good for about two horsepower. If anyone has better numbers, that would be useful.

I'm calculating that 20 mph converts to about 244 rpm on a 26 in. wheel. If so, then the torque delivered to the wheel at this speed and power would be about 44 ft/lbs. A bathroom scale might be not quite sensitive enough to measure that with the accuracy that you really want. But that's no problem at all; you can use the principle of the lever to get your readings in the best range for accurate measurement. Imagine that you've built a torque arm that is 4 feet long. The fulcrum is mounted one foot from the scale and three feet from the brake. Now your torque reading will be
44 X 3 = 132 ft/lbs. This is right in the range of decent accuracy for even a cheap batroom scale. You can always calibrate your dyno by weighing a known weight, 5 gallons of water for instance, and adjusting the scale to the proper reading.

You'll need to cool the brake. But a little running water will do this and it will give another benefit. It'll lube those braking surfaces so that the loading action should be nice and smooth.

If you were to try to apply the load using a standard bike brake handle, you might find it difficult to load smoothly. But it shouldn't be too hard to rig up, perhaps, a crank arrangement.

This should work for happy time, rack mounts, or friction drives. It should even work for front hub electric motors. The bike would have to be hung a bit differently, but that's no problem.

Materials should be dirt cheap. It'll take a bit of shop space. Or it could be built outdoors.

I doubt if I'll build one. But the serious MB gearheads here might be able to make use of it. I hope it's helpfull.

2. ### crazymanNew Member

It should work, but not give any accurate numbers. Just before and after.

I recently had an idea similar to yours, for dynoing small engines. Take a car disc and caliper, some sort of hydraulic pump, and a pressure gauge. Mount the disc on the shaft, hang the caliper on a bracket, and hook up a pump and line to the caliper with a pressure gauge. Run it, and increase the pressure until it bogs it during acceleration. Mod it, and try it again. If it requires more pressure to bog it, you've made progress.

Last edited: Jan 2, 2009
3. ### bluegoatwoodsWell-Known Member

Yep, that would work. And if you could figure out how much psi holds onto how much ft/lbs, then you would be able to measure torque. Add a tachometer and you've got a dyno.

5. ### bluegoatwoodsWell-Known Member

Wow. The tachometer problem is solved much easier than I would have expected.

while I'm on here one thought does come to mind; if the brake is cooled with water, then that's likely to mean that this dyno won't work with friction drives. That's if there's so much water needed that it gets all over the tire surface. And it might need a fair amount of water.