While downshifting a motorcycle, why does the bike automatically slow down even though you need to blip the throttle and increase the rev before shifting?

Question

Seems counterintuitive

In: Engineering

Answer 1

I ride a Z400 (Her name’s Charlie and I love her):

If you are maintaining speed and try to clutch in and down shift, your road speed (tyre speed, final drive speed) will force the engine to speed up when the clutch is released. The engine will at the same time, force the final drive to slow, and cause the rear wheel to clatter. It’s uncomfortable and rough, and isn’t great for any of the parts of the bike.

So you blip the throttle to being the engine up to speed, matching it with your road speed in a lower gear.

You don’t hold the throttle, so when the clutch is released the bike begins to engine brake – a vacuum is created by the piston and valves, and that adds resistance to the engine’s motion. The lower the gear and higher the RPM, the stronger the resistance, the stronger the engine braking. This is why the bike slows faster in a lower gear than a higher gear, even when no throttle is applied.