I have question about shifting.

[RTAF]

when we downshift the car, why clutch control is needed?

what will happen if I shift down without clutching?

Regards

[AxelRipper]

If you don't clutch, nothing bad will happen. We don't clutch upshifts or downshifts.

Why you would need to? I'm not sure. Probably causes less wear on transmission parts, but it is definitely not a necessity.

[NR-Cruelty]

Doesn't all of the Motorbicycle engines need clutch?
We use pneumatic shift which also pushes the clutch in our racecar, we found that if it doesn't push the clutch, shift cannot achieve once we lift the throttle too long.

[Owen Thomas]

Let me answer your question with another question:
What happens when you try to mesh two gears spinning at different speeds?
Contrary to what AxelRipper implied, I believe the answer to this question is the imminent annihilation of your gearbox. Now I'm not saying it's not possible to safely and reliably downshift sans-clutch, because it definitely is, but why would you bother risking it? The clutch is there for a reason.
Also, this question is HIGHLY dependant on what type of transmission you are using. I'm assuming it's a sequential motorbike engine, since you only mentioned downshifting, but come on dude, we can't help you if you don't give us any information.

[Mbirt]

Originally posted by Owen Thomas:
What happens when you try to mesh two gears spinning at different speeds? The dog clutches take over.

http://en.wikipedia.org/wiki/S..._manual_transmission (http://en.wikipedia.org/wiki/Sequential_manual_transmission)

[RTAF]

Thanks for reply.

I'm talking about CBR 600 engine. I heard for somewhere that we need clutch when downshifts.

[kcapitano]

Here is another question for you RTAF. You're driving along at 60km/h and 7000rpm. Now downshift without the clutch. What do you think will happen to your engine rpm? Your road speed? Your rear wheel speed?

Hint: research what a "slipper clutch" is and why some people use them.

[Paul Achard]

The clutch transmits torque from the crankshaft to the input shaft of your transmission. (How Stuff Works (http://auto.howstuffworks.com/sequential-gearbox1.htm)) Disengaging the clutch cuts this connection: the crankshaft and input shaft of transmission are now independently spinning. Now, when you shift, you modify the speed ratio between the input and output shafts of your transmission, the output shaft being directly connected to your wheels (forget about differentials for a second).
When you downshift, for example, your high-inertia wheels try to make your high-inertia engine turn faster while the engine tries to make the wheels turn slower. When you shift, you first disengage two sets of transmission dogs already rotating at the same speed, and then "smash" the shifting dogs onto the next set which are travelling slower. This impulse is what drives the engine faster/wheels slower, and it can cause a great amount of stress and wear on the dogs.
If the clutch is disengaged, however, all the dogs are driving are the very low-inertia clutch plates and input shaft, therefore a much smaller impulse is required to get everything going at the same speed. Since the clutch is made of friction plates, it "absorbs" the impact of re-engaging torque to the engine.

Since racing is high-maintenance and low-mileage in our case, the high impact of dry shifting is of lesser concern since we want to minimize shifting times. On a streetbike which you want to own for another 4**** miles, durability is more of a concern.

[Jon Burford]

Originally posted by RTAF:


what will happen if I shift down without clutching?

Regards

It depends how good you are, With our CBR upshifts are clutchless and downshift are the same, if you time it nicely and blip the throttle you should not have any trouble going without the clutch.

[RTAF]

my clutch was slipped when we drive for a while. I'm not sure about mileage but less than 200 km.

I think clutchless may cause this.

[MCoach]

Originally posted by RTAF:
my clutch was slipped when we drive for a while. I'm not sure about mileage but less than 200 km.

I think clutchless may cause this.


On our team, it has been found that it is best to only do clutchless downshifts while braking and very quickly. If you shift slowly, the dogs will chatter on each other (completely wrong technique in shifting with dogs). Under braking, without disengaging the clutch, the engine is decelerating to a much lower speed where the speeds start to get really close. Yes, the ratios stay the same, but the speeds are much closer in terms of rpm.


TL;DR
From our experience:

doing 3~4k downshifts during braking is MUCH easier on the clutch and trans than trying to downshift before you start braking significantly (8K+ downshifts).