Trial riders need their clutch?

[sn0wchyld]

im struggling to think of a clutch dump behavior that couldn't be matched by a more dynamic throttle response (And possibly just some extra phase current) - think about whats going on - a clutch dump is using the flywheel momentum coupled with the relatively fast action of dumping a clutch to provide a very fast, very high spike in power output (and a resultant high spike in torque/thrust).

Perhaps whats needed (for EV trials) is not so much a clutch, but a switch (possibly attached to a normal clutch leaver) that enables a higher throttle ramp. Theres no doubt a controller can dump large phase currents into a motor to produce large spikes in torque (think controllers like the old infinion units that dumped full battery current into the motor for a breif moment on startup) so its not a question of capability as much as a question of response times. It takes much longer to twist a throttle from 0-100% than it is to release a clutch (given dumping a clutch effectively achieves the same thing, ie going from 0-100% throttle in a heartbeat).

A simple solution may be to just have a clutch like leaver that reduces the max throttle from 100% to 0% as its applied, enabling the rider to apply the throttle to 100% without any power actually being applied, and then release the 'clutch', thereby going from 0-100% throttle much quicker than they could by twisting a throttle alone. Couple this with an appropriately programmed and beefy controller and a motor that wont saturate too early and you have your clutch dump effect, no actual clutch required.

tldr: seems to be more of a programming/setup issue than an actual lack of a clutch.

 

[macribs]

1+ snowchyld. And well explained.

 

[Rube]

Consider that a clutch enables increasing rpm which riders believe increases gyroscopic action of the engine and resists tilting. A common practice on larger road bikes or sports bikes with tight geometry. I'm skeptical of the benefit for slow speed lower weight applications.

 

[jonnydrive]

I recall one incident where I needed the clutch to dump the ICE power instantly, and the throttle was not fast enough. An e motorcycle would have had the same problem.

For situations like this with e power it might be important to connect a clutch lever to an ebrake that would apply reverse power to the motor to cancel the differential momentum of the motor nearly instantly but not actually brake to a stop. Essentially emulate a clutch's behavior or something similar and even better. It requires taking just the right amount of energy out of the system, but this can be done, and very quickly.

Combine this with strong variable throttle controlled deceleration (which might be enough by itself). The goal is to make the electric machine more precisely controllable than the gas trial bike. It would require some special engineering but perhaps it has already been done by the trials bike manufacturers, or they will soon get there.

Interesting ideas for these applications.

Hi,
I do exactly what Alan B said on on my bike. I replaced a clutch lever with a regenerative analog brake. If you need to stop the engine you can pull the level and the engine reduce its spin and the bike inertia. You can easy control by software how much "aggressive" the analog brake can be.

It is also very useful in deep brakes because it avoid the real wheel to block and eventually sliding.

Stefano

 

[MrDude_1]

as noted above throttle response can theoretically be almost instant.

The problem with these EV theories is that no one is mass producing this theoretical solution at the moment.


Just like how in theory EVs can be acceleration monsters, but in reality they're all about the same as a 4cyl economy car.*





*tesla and other expensive performance EVs excluded...

 

[fechter]

Most electric motors are limited by the saturation of the iron in how much torque you can get, even for a short blip. Spinning up a flywheel and dumping a clutch can give a much higher torque peak. Seems like it wouldn't be all that hard to put a clutch on an electric trials bike. It might need a jackshaft or a specialized clutch design, but certainly seems doable. You might want some additional flywheel weight too. You wouldn't have to worry about stalling the engine.

 

[bionicon]

maybe it will be good to start another tread with all the controllers that will unleash full power at zero rpm

 

[macribs]

Feel free to do, if so pls cross link so those who follow this thread becomes aware.

I think it is well within the topic to list those controllers here as well. If you got one or more controllers that comes to your mind feel free to add a post here about them.

 

[Slozukimc]

Used to own a Beta trials bike. I can’t see an electric drive without a clutch accelerating like a trials bike when you wind it up and dump the clutch. They will literally leap off the ground if you know how to load the suspension and work the clutch.

Mike

 

[danielrlee]

Used to own a Beta trials bike. I can’t see an electric drive without a clutch accelerating like a trials bike when you wind it up and dump the clutch. They will literally leap off the ground if you know how to load the suspension and work the clutch.

Mike

What, leap off the ground like this?:

[youtube]0JtW3rhfCE8[/youtube]

 

[Alan B]

There's my video again (I was behind the camera shooting the vid)... That was a fun day, and Luke's bike was amazing. He's way beyond that now.

The point here is that a clutch connected to stored mechanical energy can make a big pulse, but there are limits to the energy transfer that a clutch can handle and the available energy (most of which gets wasted in the slipping clutch). While we can imagine this or that is better, you really have to get to the numbers to see what is possible. In the above video the motor is brushed, and we know a good BLDC motor can exceed that. It had impressive launch capacity even with all that.

 

[sn0wchyld]

Most electric motors are limited by the saturation of the iron in how much torque you can get, even for a short blip. Spinning up a flywheel and dumping a clutch can give a much higher torque peak. Seems like it wouldn't be all that hard to put a clutch on an electric trials bike. It might need a jackshaft or a specialized clutch design, but certainly seems doable. You might want some additional flywheel weight too. You wouldn't have to worry about stalling the engine.

im not so sure a flywheel will actually yield that much extra torque - i suspect its more to do with getting the engine up to its power band, and putting that power through to the wheel (much like high phase amps, in both cases most/all the power is wasted as heat in order to generate large torque at 0 (wheel) rpm). back of the envelope calcs on a 1kg flywheel at 10krpm and 150mm dia yield about 2kw of power, assuming the clutch is dropped fully over 1 second - and this assumes that none of the energy is wasted as heat (also assumes my calcs are correct heheh). 2kw isn't much of a bonus to a 25kw motor... but spinning at ~10krpm where it puts out 25kw rather than 1000rpm where it might only put out 2kw is.

 

[DanGT86]

I was hoping somebody would do the calcs. This one can actually be answered with math but I wasn't sure how to figure it.

If the electric motors's power output is a product of the torque and RPM, AND the torque is proportional to current, then wouldn't you need infinite current at zero rpm to equal a clutch dump? If that is the case then the clutch would seem to win. However, there is clutch slippage in the system from both physics and rider control. So the clutch slippage slows down that impulse over a longer period of time. If that longer period of time is enough that the phase current and rpm of an electric motor could produce equivalent power than its a wash and the clutch is unnecessary.

My gut feeling is also that the clutch enables the gasoline engine to get to its full rpm power potential faster than it could ever ramp up under load. The rotating mass of the flywheel and motor probably have less to do with the power output than the powerband of the motor.

I think its telling that the majority of manufacturers of electric dirt bikes have chosen to not include a clutch. However, KTM, EM and Zero have higher budgets than us diy builders so maybe with the limited phase current most of us can afford there would be some utility to a clutch. I would't want to add a bunch of flywheel weight to something like an RC motor mid-drive but if it were a 20lb mid-mounted hub motor than the mass is already there so adding a clutch might not hurt anything. Wish I had the money to try one of each.

Maybe someone with a BRAMMO could do back to back acceleration tests dumping the clutch vs just going full throttle?

Sn0wchyld, could you post the math/formulas you used to figure that?

 

[fechter]

The amount of instantaneous "power" you can get from a clutch dump depends on the stored energy in the flywheel and how fast the clutch can engage. It may be limited by clutch slip. Yes, we can do the math on this (but I'm lazy), but a fast clutch dump will produce several times more torque than you could get with an unclutched electric motor. Maybe as much as 10x more. A motor will be limited by iron saturation and probably winding resistance.

 

[Slozukimc]

One other aspect of a clutch is better low speed control. If the motor is already spinning at speed the clutch can be used to accelerate and control the power much more precisely than a throttle on any sort of engine/motor. Not to mention the heat will no longer be as big of a factor in the motor and controller.

Mike

 

[sn0wchyld]

The amount of instantaneous "power" you can get from a clutch dump depends on the stored energy in the flywheel and how fast the clutch can engage. It may be limited by clutch slip. Yes, we can do the math on this (but I'm lazy), but a fast clutch dump will produce several times more torque than you could get with an unclutched electric motor. Maybe as much as 10x more. A motor will be limited by iron saturation and probably winding resistance.

i really hvae to disagree. perhaps on the sizxe of motors that could prctially fit on a trials bike it may be so (though that im doubtfull of too, we've all seen lukes deathbike for one example). if it produces 10x more, it needs to produce 10x more power, as torque and power are directly (and linearly) proportional, which seems unlikely. i think its simply the fact that the engine produces more power at 10krpm than at idle, and clutch dumping, while disipating much of this power as heat, allows far more torque to be produced at the wheel.

the easy way to test this in real life is to spin up your bike wheel to high rpm, and then hit the brake really hard. if your bike doesn't leap off whatever stand you put it on, then the energy released by this poor mans 'clutch dump' isn't a significant contributor to the behaviour of the bike. you could do the same experiment putting a disk brake onto a RC motor shaft, with a bit of extra weight as a flywheel. spin it up to ~10krpm , release the throttle and slam on the brake.if the test rig you attach this all too doesn't need to be bolted down to stop it leaping arround, then the stored kinetic energy's contribution to movement is minimal. not a perfect analogy but it gives some idea of the forces involved.

finally, think about it this way... a good size inrunner (like a LR big block) has a rotating mass of about 1kg, at about 80mm dia. a controller can spin this up to 5000rpm in less than a second. (similar acceleration and mass as my theoretical freewheel, though admittedly a lower rpm, it is well within a order of mag.) - i can keep the motor body still just by putting my foot on it and holding it against the ground. all that inertia and torque induced by that rotor and its sudden change in rpm is easily managed and held at bay by my foot - if however the shaft was attached to something solid i'd have no chance of holding it stationary in that manner. ergo, the rotating mass has a negligable impact on the torque produced. same goes for an engine - the inertia in the rotating bodies could likely be held at bay by the mass of your body and the bike itself. its the power being produced by the engine and its transfer via the clutch to the wheel that makes all the magic happen.

another test i thought of - rev an engine up to 10krpm, release the throttle and before the rpms drop too much, dump the clutch (with no throttle). i bet it doesn't leap away anything like it does with the throttle still wide open. same rotating mass, same (or close enough to) rpm... same clutch dump. if my calcs are correct, the bike will jerk forwards far less aggressively, as the engine is no longer producing power, so its only the energy of the rotating mass that will contribute to any movement.

 

[speedmd]

Its not just a hard launch that is important, it is the measure of energy that you are trying to dose out and no more. Getting the right amount of motion is half the equation. Being able to stop it another big part. Watch the master and it may become clearer what is needed. I don't see it happening with out some sort of very instant motor brake function also if no clutch is involved.
https://youtu.be/iHLCdKjXFqY
https://youtu.be/YUHGRX6AH78

 

[Alan B]

Perhaps the clutch is just more precise a control than an ICE throttle. The fuel swirls around the carburetor and takes a while to get consumed. Both onset and cancellation of power is delayed compared to a clutch OR A FAST MOTOR CONTROLLER.

I notice on the Leaf EV that power control is much more precise than on ICE vehicles.

 

[Slozukimc]

Its not just a hard launch that is important, it is the measure of energy that you are trying to dose out and no more. Getting the right amount of motion is half the equation. Being able to stop it another big part. Watch the master and it may become clearer what is needed. I don't see it happening with out some sort of very instant motor brake function also if no clutch is involved.
https://youtu.be/iHLCdKjXFqY
https://youtu.be/YUHGRX6AH78

Toni Bou is absolutely amazing! His father literally raised him to be THE best trials rider in the world. Good job Dad!


Mike

 

[liveforphysics]

A clutch isn't whats wanted, its an aggressive and adequate torque pulse that gets the job done, and the clutch just happens to be the crutch an ICE bike uses due to native torque maps generally approaching zero below idle RPM.

EVs can be built to natively produce any torque map desired.

 

[DanGT86]

So then it falls back to the point I think Arlo was making about the existence of ev controllers with adequate current delivery. Are there reasonably priced controllers and motors capable of rivaling a clutch dump?

As far as modulation I think an electric motor's response and control is orders of magnitude finer than an ICE. The clutch is a band-aid for the poor response time of an ice.

 

[Slozukimc]

So then it falls back to the point I think Arlo was making about the existence of ev controllers with adequate current delivery. Are there reasonably priced controllers and motors capable of rivaling a clutch dump?

As far as modulation I think an electric motor's response and control is orders of magnitude finer than an ICE. The clutch is a band-aid for the poor response time of an ice.

But can that electric motor stay cool for hours of less than walking speed riding and the occasional wide open?

 

[DanGT86]

I dont think heat would be a huge problem. Lots of time for cooling between bursts of high power in trials. If it were an issue then the electric bike could be water cooled or fan cooled.

Delivering a precise amount of torque as needed has to be better than the ICE method of blasting out extra and using a clutch to grab only a portion of it. As long as the electric motor can deliver the instant impulse needed.

 

[Slozukimc]

I dont think heat would be a huge problem. Lots of time for cooling between bursts of high power in trials. If it were an issue then the electric bike could be water cooled or fan cooled.

Delivering a precise amount of torque as needed has to be better than the ICE method of blasting out extra and using a clutch to grab only a portion of it. As long as the electric motor can deliver the instant impulse needed.

It seems to me that the heat build up is more of a problem with the super slow speeds rather than the powerful bursts. We all know electric motors like to run fast to stay cool right?

Mike

 

[speedmd]

The potential of a large diameter motor with a light weight rotor and gobs of near instantaneous current to both launch and help stop it would put a ICE setup to shame in this app. IMO. Will be interesting seeing the developments.