Severe necro-posting I know, please excuse me. It's just this is exactly what I'm grappling with currently.
I'm not a particularly good trials rider, but I can make a rough imitation of most of the key trials maneuvers - Double-blips, zaps, splatters etc.
I think the excerpt below has a fair bit of relevance to the discussion:
sn0wchyld said:
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.
This is in fact what many (most?) trials maneuvers require - rev up, then snap the throttle closed as you drop the clutch. It actually creates more "leap" than holding the throttle open.
Holding the throttle open as/after you drop the clutch is also a recipe for disaster, as you loop out at speed or half way up a vertical climb!
There's a lot going on, in that fraction of a second between popping the clutch and the bike leaving the ground.
One aspect is the reaction of the suspension - current designs all squat under power to some extent, so if you don't cut the power just as the bike starts to leap you will get very little lift from the stored energy in the rear spring. The bike will feel glued to the ground compared to when the throttle is cut. The only way I can imagine to achieve this timing is with a flywheel. A FW gives a very short impulse of power, then as the wheel spins up to speed the chain tension drops dramatically, allowing the suspension to extend. Hard to coordinate that level of timing with a twist grip throttle, but automatic with a flywheel!
Another aspect was alluded to in this thread, that of cutting power instantly. Again, very hard if not impossible with a twist grip (advanced trials riders use one finger on the clutch, right up close to the lever pivot in order to get fastest response). And the key in many instances is to cut power - not brake, not roll off power, just go into freewheel mode really quickly.
Associated with the clutch is the flywheel - trials bikes have big, heavy flywheels - far, far heavier than similar capacity enduro or MX bikes. The flywheel provides a couple of key characteristics:
- it smooths out power delivery, both the pulses of an ICE, and the vagaries of the rider's throttle hand. This maximises traction.
- It stores energy, not just for popping the clutch, but for rolling the bike up steep or slippery obstacles. If you rely on the flywheel to carry the bike up, the risk of wheelspin is far reduced - if the wheel does slip it will not accelerate, only continue gradually slowing down, potentially regaining traction. In contrast any amount of throttle on when the wheel slips and and the spin will increase and the bike will completely lose traction.
A misconception that I see and hear frequently is that, "I've got so much torque I power wheelie at will, so how can I possibly use any more that a flywheel might provide?"
The error in this thinking is not considering the time factor. Any decent ICE trials bike can flip you onto your a***e with just a twist of the throttle, and yet even fairly ordinary riders like myself regularly spin the engine up and dump the clutch. The key is that the big impulse only lasts for a moment, enough to shoot the bike forward and stand it up vertical, but no longer. Good trials riders are often in the air, bike vertical or beyond, and with the bars against their chest - when the (unpowered) back wheel hits something, it stops and the bike rotates back to a more sensible attitude. This rotation around the back axle is what presses the back tyre onto the obstacle with enough force that the bike can climb vertical walls, mostly relying on the flywheel to carry them up.
Having a handful of throttle on when the wheel hits the face risks a loop-out or the rear wheel bouncing or spinning.
With a big, heavy flywheel it becomes imperative that you have some way to cut the power instantly (eg at the top of a high wheelie, or vertical climb), rolling off throttle is not going to cut power to the rear wheel fast enough.
Simulating a mechanical clutch and flywheel electronically might be achievable, but it's not just a matter of turning power on and off. For instance how do you simulate the instant power on/off when wanted, and also simulate the effect of flywheel inertia carrying the bike up a vertical face without any wheel acceleration if it loses traction for a moment?
Not impossible perhaps, but a very specific setup indeed.
Finally, going back to the "rev, cut the throttle and simultaneously dump the clutch" move, it's really, really hard to be jumping up and forward as hard as you can (required to get the bike airborne and travelling from a standstill) and at the same time be twisting the throttle down. Yet with a clutch it's beautifully synchronised that as you squat to prepare for the jump you twist the throttle open, then when you jump up, forward and dump the clutch, your hand is naturally rolling the throttle off.
It is perhaps worth noting that the Electric Motion trials bike linked to earlier in this thread has this year integrated a mechanical clutch into it's drive train. In it's previous "electronic clutch" form it simply wasn't up to the abilities of the other serious electric trials bikes on the Trial GP circuit, all clutched.
Watching the old model up against the clutched electric bikes it was obvious that it just couldn't develop the same "pop" as the clutched electrics, or the ICE bikes either for that matter. Every review of the new version comes away glowing with how much better the bike is with a clutch.
As I say, I'm no champion trials rider, but this is the best I've been able to come up with based on what I can do, from watching a lot of video and thinking long and hard about it.
I believe a trials bike without a clutch simply isn't capable of some of the fundamental contemporary trials moves. Perhaps in years to come both the electronics and the riders will progress to the point that clutches are not needed, but it's a fair way off yet I think. It's much more about subtlety, timing and control than just power. As with 99% of trials riding.
Which perhaps explains why I'm currently grappling with grafting a Kawasaki wet clutch onto my LMX motor.
