dyno a happy time?

Discussion in 'Performance Mods' started by alex, Dec 31, 2008.

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  1. alex

    alex Guest

    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?

  2. bluegoatwoods

    bluegoatwoods Well-Known Member

    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.
  3. graucho

    graucho Active Member

    Alex, cool idea.
    bluegoatwoods, great info.
  4. bluegoatwoods

    bluegoatwoods Well-Known Member

    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?
  5. bluegoatwoods

    bluegoatwoods Well-Known Member

    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.
  6. bluegoatwoods

    bluegoatwoods Well-Known Member

    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.
  7. alex

    alex Guest

  8. 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

  9. bluegoatwoods

    bluegoatwoods Well-Known Member

    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.
  10. 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.

  11. bluegoatwoods

    bluegoatwoods Well-Known Member

    Yes, I calculated 56.6 ft/lbs.

    Now if we knew how much torque these engines should produce at, say, 4000 rpm, then we could establish a "spec" power or torque curve.

    The guys who port engines and design exhaust systems could compare and document their performance gains. And if a mod resulted in an "off" power curve, that could be useful as well.
  12. bluegoatwoods

    bluegoatwoods Well-Known Member

    Thinking about it a bit, I can see something else that I overlooked.
    Above somewhere I said something like, "If I pictured myself braking that output shaft by hand, then I know I'd be trying to hold onto more than 4 ft/lbs."

    But I wasn't taking into account the fact that that shaft has a radius much less than 1 foot. Probably something more like 1/2". 4 ft/lbs would translate into 4X12X2=
    96 half inch/pounds.

    Not important. I'd just feel silly leaving an error like that in print.
  13. stv1jzgte

    stv1jzgte Member

    I have dyno'd my 48cc slant head, FIGURES AT THE WHEEL! 1.6 kw or 2.2hp with 12 ft/lbs of torque.
  14. 48cc max 1.2kw(1.61hp)/6800rpms
    max speed estimate 35mph
    max engine torque 2.4nm/4500rpms(1.77lbs/ft)
    max no-load speed 8500rpms
    torque at rear wheel:
    50t 8.9lbs/ft
    44t 7.8lbs/ft
    36t 6.4lbs/ft
    34t 6.0lbs/ft
    32t 5.7lbs/ft

    66cc max 2.0kw(2.68hp)/5500rpms
    max speed estimate 35mph
    max engine torque 6.2nm/5500rpms(4.57lbs/ft)
    max no-load speed ????rpms
    torque at rear wheel:
    50t 22.9lbs/ft
    44t 20.1lbs/ft
    36t 16.5lbs/ft
    34t 15.5lbs/ft
    32t 14.6lbs/ft

    engine torque X overall reduction ratio = rear wheel torque
    Last edited: Jul 16, 2009
  15. stv1jzgte

    stv1jzgte Member

    Haha lmfao dude you followin me? these are guesstimates only people. He assumes you will get the same figures regardless of the tyre circumfrence
    Hows the stopwatch and gps going???
    Tell me dude when someone else dynos there bike and gets more than what you recon are you gunna dissmiss there results to?
    Last edited: Jul 17, 2009
  16. stv1jzgte

    stv1jzgte Member

    Sorry you said " Re: Dyno power
    my list is just too show the diffrence between engines and sprockets.
    the real numbers may be slighty higher or lower for diffrent engines and upgrades. please explain???????? calculators now ok!
    also your edditing all your posts to say different things dude whats up with that? You said 'a stopwatch and a gps thats a plan'! but its gone now???? haha your funny bro keep pedalin backwards!

    Before you edited it out you said that you got these figure on the dyno at your work? really what kind? oh and what ramp rate where you using? didnt know you used rolling road dyno's in the states, thought you were all using engine dynos?
    Really stop filling these threads with your mindless dribble.
  17. stv1jzgte

    stv1jzgte Member

    When i asked cabin if he was going to dismiss anyones figures if they dyno'd there bike, his responce?

    Dear stv1jzgte, cabinfever1977 has just replied to a thread you have subscribed to entitled - dyno a happy time? - in the Wild In The Streets forum of MotoredBikes.com:
    Here is the message that has just been posted:***************no , only yours.

    You wont find this post in here because he edited it out, see he only wants to stuff people around and is not interested in facts...... get a life bro!
  18. stv1jzgte

    stv1jzgte Member

    Far from bro!
    All i have done is post a result made on a dyno 12ft/lbs! you dissagree fine, then you post figures that you said you got on a dyno (what kind we will never know) but now i find its made on a friggn calculator? and you think im wrong hahahahahahahaha you should have left it on the other forum!
    Last edited: Jul 17, 2009
  19. stv1jzgte

    stv1jzgte Member

  20. hows your slant head motor working,lastime i heard your motor blew.
    thats what it said on your other post.