OK. Been off doing other things so just catching up with the activity here.
@DingusMcGee in order fo your questions:
- Wheel inertia is totally irrelevant - when we're talking balancing a trials bike we are absolutely, totally stationary. If you're moving even a 0.5kmh it's riding, not balancing in the trials world. A spinning flywheel doesn't in any way turn a bike into a self-balancing device, but when you're talking about people with the ability to stand balanced on their bikes with one or no hands on the handlebars, folding out the kick start and firing up a 300cc single, then looking over their shoulder to see if the back wheel is exactly where they want it, then a small advantage is very noticeable. There are trials riders who can balance on their bike, put their head where a seat would normally be (trials bikes don't have those superfluous jiggers) and roll up into a headstand, still balancing the bike. Trials is a different world to most bike riding.
- I can't give you complete numbers because the crankshaft plates are significant contributors to rotational inertia in a trials bike and I've never measured or seen measurements for them. I'd guess you could approximate those plates at 100mm dia and 12mm thick, times two. A trials flywheel varies between makes and models, but you could call the MoI 11,000 kg*mm^2 and not be too far either side of several bikes. The flywheels are steel cups. Below are a few samples. Density is an approximation allowing for the magnets embedded in the flywheel. Figures below were verified by doing inclined plane rolling tests - results were within reasonable error.
OD | ID | Width | Volume | Density | Weight | Weight | MoI |
mm | mm | mm | Cm^3 | g/cm^3 | kg | pounds | kg*mm^2 |
| | | | | | | |
139.2 | 85.5 | 46 | 435.94 | 7.83 | 3.413 | 7.509 | 11387 |
136.5 | 85.5 | 46 | 409.04 | 7.83 | 3.203 | 7.046 | 10386 |
146.0 | 100 | 42 | 373.28 | 7.83 | 2.923 | 6.430 | 11441 |
- RPM - pick any number you like
As I said, sometimes we're using clutch and flywheel virtually at idle (900rpm) other times it's hold the throttle pinned until the engine reaches constant RPM and wont accelerate further. Call it 9,000 rpm as a reasonable figure.
- Forget about the clutch. Only the basket is rotating when the bike is stationary and it's a puny little piece of aluminium that's maybe a couple of hundred grams, I can't recall the reduction of the primary gears, but maybe 4.5:1? So clutch contribution to inertia is zip diddly. Forget it.
- Trials bikes are approx 67kg +/- a couple kg.
I've seen other people do various calcs on some of this stuff, can't recall where now. But I have seen figures that show a typical trials flywheel at 6000rpm can lift 300lb several feet.
Here is a pic of a
YZ250 motocross flywheel, and a
250 trials flywheel. Vastly more mass out at the periphery!
@DanGT86
It would clearly parse out whether its the engine torque or the flywheel mass providing the torque.
I've said it before a bunch of times, and no doubt I'll say it again. The throttle is very often completely closed when the clutch is released. The engine contributes stuff all to the take-off torque in most cases.
Very advanced riders will hold throttle open for the very first part of the launch on really big stuff, but just take a look at a few videos of riders launching onto big stuff then try to imagine how you're going to hold the throttle open when your hips are between the handlebar grips! Yes, on the big stuff before they go they release the throttle tube, cock their wrist over the top then grab hold again so they can keep it open longer, but this is really very advanced. Mere mortals usually drive their wrist down, hold the throttle open then extend up. Your hand unwinds the throttle as you go, and the clutch release has to be late in your extension to get the reaction you need from the bike.
Somewhere I've got videos where I deliberately rev the bike up, shut the throttle completely, pause, then Go.
Guaranteed no throttle and a decaying RPM. The bike still gets airborne and climbs out on the obstacle. All flywheel except for whatever torque the engine at fully closed throttle produces. Heck, I've done the same on my electric trials while testing bigger flywheels. In that case there is ZERO power out of the motor and the bike still gets airborne.
Bikerpete, I would definitely not put the flywheel on the opposite side of the motor as the output shaft. Sending those torque shock loads through a 1/2 shaft would definitely destroy it. With the flywheel on the output shaft the shaft is never experiencing any more load than the maximum output torque of the motor which it is designed for.
My thoughts exactly. But the problem is putting the flywheel on the same end as the clutch makes it really, really hard. I've done that on my QS138, but it's not pretty.
It actually amazes me that 2-stroke cranks can withstand it. Crankshafts are 5 pieces - two main shaft halves pressed into the crank plates, then a crank pin pressed into the two plates with the conrod running on it. It's all interference fits, no splines. You can press the crank pin apart, replace the conrod bearings and press it back together and it all still works. That's with the flywheel on the opposite end of the shaft to the primary gear. It obviously works just fine, but it surprises me every time I see it.
But a 1/2" shaft is looking wildly optimistic to me! That's one of the reasons I go back to AstroFlight, then discount them again, and again. Disappointing. It's pretty obvious those motors are primarily designed to run something like an aviation prop -skinny little parrallel shaft with not even a flat, let alone a keyway.
And it's not quite true all the shaft experiences is motor torque if the flywheel is on the output end - in a geared drive there's going to be significant bending moment on the shaft too.
Throttle inputs add energy to the flywheel while clutch inputs take it away (provided the flywheel is spinning fast enough to make torque)
Nup. Completely falls over when you've got the throttle closed and you dump the "clutch". Zero throttle energy minus some figure from the clutch isn't going to get you very excited, or far up a 6 foot step.
Refer back to discussion above about closed throttle. This isn't optional, it's simple biomechanics, even if you disregard the fact that the best trials riders in the world use some form of that technique which suggests it's got just a little bit of value and shouldn't be chucked away just because we're moving electrons rather than hydrocarbons.
And look back here and elsewhere - EM did almost exactly that with the 5.7 - you could wind on throttle, but clutch cancelled it. Then drop the clutch and whatever throttle setting you had went straight to the motor. Hopeless. OK for beginners and low intermediate level, but totally useless beyond that. For many reasons that I really can't be bothered going into because it would take far too long and open a discussion I don't want to get engaged in.
Maybe it could be improved upon as Dingus is talking about, but I have to admit to being rather skeptical.
I've little interest in getting rid of a physical flywheel and a decent friction clutch, they just work so well. I am interested in seeing if electronics could effectively make that physical flywheel response adjustable.
I was going to ask this elsewhere, and probably will.
Does anyone have any ideas how you could approximate power consumption with only throttle input and RPM?
I don't need it to be accurate, I'm just thinking some figure that varies somewhat proportionally to real motor output. The number itself doesn't matter as long as I could apply a factor (experimentally field derived) that made that number useful to modify the slope of the RPM decay rate according to throttle position.
I'm considering the situation when you release the clutch with a progressively closing throttle - the decay rate slope will get steeper as throttle closes and motor power drops.
It's kind of icing on the cake, not so important for initial experiments.
