dyno a happy time?

[alex]

has anyone ever done it?
I want to dyno mine, where could I have it done.
Do they make a smaller dyno for powersports applications?

 

[bluegoatwoods]

I'm a dyno man, as a matter of fact. I've made a nice living, except during market soft times, for more than thirty years at it. The ones I deal with are far too large for happy times or any bicycle ready engines.

Smaller dynos can be found in some electric motor shops. Maybe the smallest eddy current dynos could be sensitive enough to give realistic readings. But the shop isn't going to have any kind of stand for your motor. You'll have to rig that up yourself.

But imagine your engine sitting in a stand. Remove the gear that drives the chain. You might need to rig up some sort of extension of the shaft that turns the gear. Now rig some sort of brake to that shaft. I don't think it'll need to be terribly strong, I doubt if these engines put out all that much torque. At this point you're gonna want some cooling. Running water over that brake will do. Mount a fairly sturdy handle of some sort on this brake assembly and pitch that handle toward the front of the engine. For simplicity's sake, make the handle one foot long. As you are applying the brake have this handle rest against a bathroom scale. As you apply the brake the handle will push down on the scale. The scale will read X pounds. Since it is mounted one foot from the center of rotation, this means that the engine is delivering X foot/lbs of torque.

Now if you can rig up a tachometer, you'll have a dynamometer. You'll be able to measure how much torque you are getting at how many rpms all the way through the engine's operating range. If you want an actual measure of horsepower, that's easy to calculate from torque and speed.

 

[graucho]

Alex, cool idea.
bluegoatwoods, great info.

 

[bluegoatwoods]

Calculating things just a bit, I'm beginning to think that even a bathroom scale might not be sensitive enough.

Just at a guess, I used 2 horsepower at 2500 rpm as a starting figure. That would mean that the engine is only putting out about 4 foot lbs of torque. Maybe you would have to use a postal scale.

I suppose the good news is that cooling wouldn't be a huge problem. You'd still want to run some water over that brake. But I suppose that it wouldn't be so much that you'd need to build splatter shields and such.

Does anyone know if my guesses were in the right ballpark. Does two horsepower at 2500 rpm sound right?

 

[bluegoatwoods]

Naah, my numbers can't be even close. If I imagine using my hands as the brake, then I just know I'd be trying to hold onto more than 4 ft/lbs.

Someone will come along who has a better notion of how much torque at how much speed we should expect.

 

[bluegoatwoods]

Wait just a minute............never mind that nonsense about taking readings off of the main output shaft. The rear wheel will work well enough. There'll be some loss of reading due, perhaps, to drive train friction and chain slop. But it's bound to be small.

Your brake can be a standard bicycle brake. Maybe a double to make sure of enough strength. But it won't be mounted to the bike. You'd mount it to a pivot standing behind the bike. Make the reach to the pivot exactly one foot. Then have the arm reach another foot behind and have that push on the scale.

The only problem left would be the tachometer. But if that can't be installed on the engine, then just measure wheel rpms. putting a tach there shouldn't be a big problem.

You'd hoist your bike up off the floor and pin the throttle down. Then you start applying your brake. When you start to get measurable readings on the scale, then you start writing down your results. i.e F rpms-G foot/lbs, then E rpms-H foot/lbs, then

D rpms-I foot/lbs then C rpms-J foot/lbs, and so on until your engine is about ready to stall.

You'd end up with a torque/power curve for your engine from wide open throttle, no load, right down to lugging conditions.

When I was first thinking about taking those readings off of the output shaft, I really didn't think that it was worth doing. It was more like thinking out loud.

But going to the rear wheel would mean no modification or disassembly of the bike.
The engineering shouldn't be all that hard.

This idea might be workable.

 

[alex]

I found this video earlier, the hard part with a happy time would be to get the wheel spinning.
http://www.youtube.com/watch?v=uChOPuEbfAI

 

[Creative Engineering]

Naah, my numbers can't be even close. If I imagine using my hands as the brake, then I just know I'd be trying to hold onto more than 4 ft/lbs.

Someone will come along who has a better notion of how much torque at how much speed we should expect.

This is something I have messed around with in the shop for other applications. I have been thinking about building a bench Dyno for small engines.

HP= [Torque X RPM] /5252

Torque = HP / [RPM/5252]

Actually you're not that far off...there was one thing you forgot to consider.

The Torque output at the jackshaft is multiplied 4.1 times. The ratio of the crank gear to clutch drive gear is 4.1:1. With the exception of some small frictional losses the HP at the jackshaft will be the same as at the crankshaft.

Using the advertised values for a 70cc, (3.5HP @ 6000), the Torque at the crankshaft would work out to about 3 Ft. lbs. At the jackshaft there will be about 12 Ft. lbs.

I know 12 Ft. lbs. doesn't sound like much but you have to consider the speed at which it is produced, (HP). If the engine were turning 6000 RPM, the jackshaft is turning 6000/4.1 or 1463 RPM. Imagine setting a, (click type), Torque wrench to 12 Ft. lbs. and operating it in a full circle at a rate of 24 clicks per second!!! 1463/60 = 24

Jim

 

[bluegoatwoods]

Creative Engineering, thank you.

3.5 HP at 6000 rpm is something that I can work with.

I carried on this thought in a new thread titled "Building a bike dyno shouldn't be hard", if you're interested. Unless I missed something, I laid out a design that should be cheap and useful to serious MB mechanics.

By the way, have you seen any figures for the torque that should be produced down at some low, lugging, rpm? Torque rise, in other words. That would be useful as well.

 

[Creative Engineering]

Creative Engineering, thank you.

3.5 HP at 6000 rpm is something that I can work with.

I carried on this thought in a new thread titled "Building a bike dyno shouldn't be hard", if you're interested. Unless I missed something, I laid out a design that should be cheap and useful to serious MB mechanics.

By the way, have you seen any figures for the torque that should be produced down at some low, lugging, rpm? Torque rise, in other words. That would be useful as well.

The final drive is about 4:1 so wheel Torque will be 45-50 Ft. lbs. if 3.5 HP is actually being produced by the engine.

Jim