Why should I not change gears in a vehicle whilst applying power to said gears?

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I ride my bike a lot and if you change gears while pedaling you’re met with a lot of resistance and the bike creates a very audible and tactile *crunch* when the gears shift which is regarded as you making a mistake and a sign of a novice rider. And, while I’ve never driven a manual transmission car, I do know that you should also disengage the accelerator when depressing the clutch to switch gears and the transition is smoother if you get really good at this just like on a bike.

I understand that changing gears while accelerating is “bad” but I do not understand what damage is being done and why it is being done. It’s also odd to me that I can’t switch gears while applying power to a vehicle, but I can switch gears in a stand mixer without needing to shut it off.

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Okay, when you are pedaling the bike, you are pulling the chain tight and dragging it over the gears, which pulls the gears and spins the wheel.

When you switch gears, you are lifting the chain up or down and to the left or right to align it with the next gear, to give yourself an increase or decrease in speed.

So, when you pedal and shift at the same time, you are applying two different and opposing forces onto the chain, one that is trying to stay engaged with the gears and one that is trying to disengage, which causes the chain to drag improperly, and you risk damaging the chain or the gearset.

In a car you won’t damage anything because the clutch decouples the drivetrain. This also means the engine will suddenly have almost no load on it, so if you press the clutch, but don’t disengage the throttle the revs will quickly rise to the max.

Won’t damage anything during shifting, but if you then let off the clutch with the revs at full, you’ll get a sudden jerk of acceleration and some unnecessary wear on the clutch.

You can shift gears while still pressing the accelerator. Nothing stops you from doing so. However it is not recommended as it might damage the clutch.

The clutch is basically a spinning disk that connects the transmission to the engine via friction. If you shift up, the goal is to lower the RPM of the engine while still going the same speed. This means that while you shift, the RPM of the engine needs to lower in order to match the new RPM of the transmission. By pressing the accelerator while shifting up, you increase RPM of the engine when the goal is to lower it. The clutch needs to match the RPM of both before fully engaging (otherwise the car will stutter or shut off). A larger difference in RPM results in more friction and damage than necessary. Also, since the goal is to lower RPM, there is no point in adding gas while shifting up.

While shifting down, you could in some cases add gas (while going faster than you should at the desired gear). The goal here is to increase the RPM of the transmission and if your engine is not at the same (higher) RPM, the car will stutter too. Adding gas increases RPM of the engine better matching the RPM of the transmission.

Small side note: if you keep pressing the accelerator while pressing the clutch, the load on the engine will be removed (as the transmission is disengaged), and this will suddenly increase RPM as the engine has no load for the same amount of gas. This could make the engine redline and run hot unnecessarily.

You won’t suddenly kill your bike transmission but you might find that your are going very fast through them like cogs and sprocket with dead teeth after a few hundred miles when it should hold up thousands.