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?
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$\begingroup$ If you learnt to drive with a gearbox that did not have synchro on first and second you would not have this problem... $\endgroup$– Solar MikeCommented Mar 7, 2018 at 21:48
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3 Answers
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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.
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$\begingroup$ I don't understand what you mean when you say "clutch speed drops". When shifting to a higher gear, the engine rmps drop because now the driven gear is smaller compared to the driving gear (engine has to spin less to produce the same rotational speed of the driven gear). So just to clarify, what do you mean by "clutch speed"? $\endgroup$– S. RotosCommented Mar 7, 2018 at 23:05
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$\begingroup$ @S.Rotos As soon as you disengage the clutch, engine and clutch are disconnected from each other. That means they can rotate separately. You can rev the engine all you want, the clutch speed will stay the same; your vehicle won't accelerate. You can see the clutch and wheels as one part for simplicity. Now as soon as you upshift, the clutch speed drops, because of the lower gearing ratio. When you engage the clutch again, the engine has to match the clutch speed, which is lower than before, so the engine rpm is also lower than before. $\endgroup$– BartCommented Mar 9, 2018 at 10:31
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$\begingroup$ It's rather hard to explain in words, a picture, or video in this case, replaces a thousand words: youtube.com/watch?v=devo3kdSPQY It clearly explains how the clutch system works, and maybe you'll get what i'm trying to say. $\endgroup$– BartCommented Mar 9, 2018 at 10:32
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$\begingroup$ Try to google "revmatching". @Bart Why talk about something like "clutch speed"? Just stick to engine rpm since everybody knows what that means. $\endgroup$– MadMarkyCommented Mar 13, 2018 at 10:47
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$\begingroup$ Alright, I understand now. The video made it much clearer. Thanks! $\endgroup$– S. RotosCommented Mar 13, 2018 at 20:29
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Say 3rd gear is 3:1
And 2nd gear is 2:1
If you are at 4000 rpm in 3rd and down shift you take the rpm to 6000.
Clearly spinning an engine to 6000 rpm will have more resistance than spinning an engine to 4000 rpm.
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$\begingroup$ Down vote may I ask the problem? $\endgroup$ Commented Jun 13, 2018 at 23:27
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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.
answered Mar 7, 2018 at 17:40
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$\begingroup$ Nope. Mostly to do with clutch engagement rather than engine braking. Engine braking is actually very limited. Have you ever taken a car up to 50mph in 2nd gear, then taken your foot off the accelerator? The retardation is surprisingly little. $\endgroup$– AndyTCommented Mar 8, 2018 at 9:48
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$\begingroup$ @AndyT:As Paparazzi says - there is more resistance when the engine is spinning at high RPM. In your example, the engine is already spinning at high RPM, so there is no sudden increase in braking as the speed is dragged up. Furthermore, the literal first three words in my answer were "one factor is", indicating that this was not the only effect in play. Unless what I've written is actively incorrect (I'm happy to be corrected, but have not yet been convinced!) then I'm leaving this answer up, despite the negative feedback, as I feel it adds information not mentioned by other contributors. $\endgroup$ Commented Mar 8, 2018 at 14:53
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$\begingroup$ "engine braking is stronger in lower gears" - correct. "releasing the clutch suddenly into a lower gear has the effect of applying a sudden braking force" - correct. Linking the two - incorrect. "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'." - incorrect. "There will be a jolt due to the sudden clutch braking effect whenever the engine is spinning slower than ideal, and has to be 'dragged up to speed'." - correct, and basically what the other two answers already say. $\endgroup$– AndyTCommented Mar 8, 2018 at 15:17