Today while driving I had to make a quick shift to a lower gear that was not as smooth as it should've been. I've been taught that while shifting to a lower gear you have to be more careful when releasing the clutch than while shifting a gear up. Why is this? I understand the basic mechanism of transmission and clutch. When shifting up, the car can "bump" slightly if the rotational speeds of the engine and wheels are far apart. But why does this effect seem more noticeable when shifting down?
That's because when you upshift, you select a lower ratio, so your clutch speed drops. Since you took your foot of the gas, the engine rpm also dropped, and now the clutch and engine speed are close to synced, and hence little force is felt when you engage the clutch. The best way to upshift is to relieve the throttle a little(not fully), so the rpm will drop a bit as soon as you declutch. Meanwhile, shift up and clutch. After practice, no force will be felt. Every gearshift will in/-decrease rpm by about 20-25%. So aim with the throttle at a rpm 20-25% higher or lower when resp. down- or upshifting.
When you downshift, you select a higher ratio, so the clutch speed increases. You should apply a little throttle so the engine spins up and matches the increased clutch speed. Again, you'll feel no force when engaging the clutch.
If you don't do this, the clutch will have to make up for the difference in speed from the engine and the clutch. That costs time and gives you the annoying feedback.
The best way to downshift, is to keep your foot the same on the throttle as you were while cruising. As soon as you disengage the clutch, the engine rpm will increase. Now quickly downshift and re-engage the clutch. While you briefly disengaged the clutch, the rpm rose just enough to match the clutch in the lower gearing, if you did it right. But you have to shift and clutch quickly. It will require some practice, but you'll have supersmooth shifts and impress your passengers.. :p On top of this, you'll save on some wear on the clutch.
One factor is that engine braking is stronger in lower gears - releasing the clutch suddenly into a lower gear has the effect of applying a sudden braking force to the car, causing a jolt that you can feel as the engine spins up to match the speed of the wheels. When moving into a higher gear, the engine braking is reduced compared to your previous gear.
There will be a jolt due to the sudden engine braking effect whenever the engine is spinning slower than ideal, and has to be 'dragged up to speed'. This is much more likely to happen when downshifting than upshifting.