Technical questions about manual transmissions

[gregxxxnt]

Hi, I'm passionate about cars and manual driving techniques. However, I can't figure out a few things about manual transmissions, so I've decided to ask you some questions:
1) What component slows down faster - the intermediate shaft or the engine RPM? (I talk about the situation when the clutch pedal is pushed and shift lever moved to neutral).
2) Regarding the upshift with the "eggshell pressure", there is a moment when the stick lever slips very lightly into the next gear. Is this the effect of relieving the synchro's at all? Or maybe they actively slows down the intermediate shaft, but with minimal wear?
3) I don't understand what's the point of double clutching on upshifts in an unsynchronized transmission. When the clutch is pushed down and the shift lever is moved to neutral the intermediate shaft is free to slow down. Why don't just lightly press the shift lever in the direction of the next gear, allowing it to slip into gear when the intermediate shaft slowed enough? In other words, what's the point of using the engine to slow down the intermediate shaft, if it should slow down by itself?
4) Which upshifting shifting technique is the best for reducing wear and tear in gearbox and synchro's? 1) Shifting with the constant, light pressure on a shift lever in the direction of the next gear, until the lever slips effortlessly. 2) Shifting with a complete pause on neutral position, than continuing the move to the next gear.
Thanks everyone

[potownrob]

Hi, I'm passionate about cars and manual driving techniques. However, I can't figure out a few things about manual transmissions, so I've decided to ask you some questions:

ok, i'm gonna give you the benefit of the doubt that you're not a spambot, despite your screenname, and answer your good questions.

1) What component slows down faster - the intermediate shaft or the engine RPM? (I talk about the situation when the clutch pedal is pushed and shift lever moved to neutral).

this might depend on various factors, such as flywheel weight, which transmission oil you're using, and what your engine's idle speed is but, given your scenario of having the clutch in (disengaged), the engine may drop quicker, but it won't slow down past idle speed, while the transmission will keep slowing down with the clutch disengaged.

2) Regarding the upshift with the "eggshell pressure", there is a moment when the stick lever slips very lightly into the next gear. Is this the effect of relieving the synchro's at all? Or maybe they actively slows down the intermediate shaft, but with minimal wear?

i believe it is, based on it being able to happen without using the clutch at all, but what do i know??

3) I don't understand what's the point of double clutching on upshifts in an unsynchronized transmission. When the clutch is pushed down and the shift lever is moved to neutral the intermediate shaft is free to slow down. Why don't just lightly press the shift lever in the direction of the next gear, allowing it to slip into gear when the intermediate shaft slowed enough? In other words, what's the point of using the engine to slow down the intermediate shaft, if it should slow down by itself?

that would probably make for a long shift. i believe truckers get around this by shifting without the clutch, but you have to know exactly where to let off the gas and shift, and you DO pause in neutral on a clutchless shift (don't try this at home!!). my dad (who drove a semi for a short time and learned on older trucks) demonstrated this to me a few times, and i was able to shift between at least 2-3 gears without the clutch in most of my cars when i took the time to test it out.

4) Which upshifting shifting technique is the best for reducing wear and tear in gearbox and synchro's? 1) Shifting with the constant, light pressure on a shift lever in the direction of the next gear, until the lever slips effortlessly. 2) Shifting with a complete pause on neutral position, than continuing the move to the next gear.

while one method might be better on the synchros and another better for overall gearbox longevity, i think at least most synchronized boxes are designed to be shifted normally, meaning letting off the gas as you push in the clutch, pushing the clutch to the floor and then sliding the shifter into the next gear. while the clutch is pressed in, the load is removed from the gearbox, so you don't have to worry as much about how you slide, guide or shove the shifter into the next gear; as long as you're not forcing it out of gear and into the next gear, i doubt any harm is being done. also, i don't know of any benefit of pausing with the shifter in neutral during a shift while the clutch is in.

Thanks everyone

np

[gregxxxnt]

Thanks for your answers. The first question comes from my last discovery about shifting with the constant, light pressure in Opel Astra G Classic. If I release the clutch pedal immediately after the shift lever slips into the next gear, the revs are perfectly matched. I figured out that the intermediate shaft slowed down faster than engine and maybe it is specially calculated by manufacturer to support smooth shifting. But I don't know Maybe this is not a rule for every car.

Your theory about double clutching on upshifts is very convincing for me. Perhaps the big engines in trucks have much more inertia than intermediate shaft, so it's easier to engage the engine in the process of bringing the intermediate shaft to lower revs if the trucker doesn't attempt to perform clutchless shift.

i don't know of any benefit of pausing with the shifter in neutral during a shift while the clutch is in.

I've heard the short pause in neutral enables the intermediate shaft to naturally slow down a bit by itself, without bringing the syncho's at the beginning, thus leaving them only a minimum amount of work to do.

[theholycow]

1) What component slows down faster - the intermediate shaft or the engine RPM? (I talk about the situation when the clutch pedal is pushed and shift lever moved to neutral).

This will differ greatly from one vehicle to the next. I imagine the intermediate shaft deceleration will be vaguely similar across similar vehicles (at least measured on the same scale), whereas engine deceleration can vary in the extreme...flywheel weight, internal friction, piston weight, and all kinds of artificial interference introduced by computers -- especially rev hang.

2) Regarding the upshift with the "eggshell pressure", there is a moment when the stick lever slips very lightly into the next gear. Is this the effect of relieving the synchro's at all? Or maybe they actively slows down the intermediate shaft, but with minimal wear?

It's not that the synchros are relieved in any way; it's the effect that you feel when the synchro has done its job.

3) I don't understand what's the point of double clutching on upshifts in an unsynchronized transmission. When the clutch is pushed down and the shift lever is moved to neutral the intermediate shaft is free to slow down. Why don't just lightly press the shift lever in the direction of the next gear, allowing it to slip into gear when the intermediate shaft slowed enough? In other words, what's the point of using the engine to slow down the intermediate shaft, if it should slow down by itself?

Where are you finding an unsynchronized transmission?

If you lightly press the shift lever in the direction of the next gear in an unsynchronized transmission, logically I am pretty sure it will grind until either it is synchronized or the dog teeth are all ground/broken off. I suspect you're better off jamming it in firmly than lightly pressing it, then the teeth will have to engage and skip the drama.

Now, if you had said "I don't understand what's the point of double clutching on upshifts in a synchronized transmission" I'd say this: I don't think there is a point unless you're insanely paranoid about synchro wear...but what are you saving them for if you'll never use them?

4) Which upshifting shifting technique is the best for reducing wear and tear in gearbox and synchro's? 1) Shifting with the constant, light pressure on a shift lever in the direction of the next gear, until the lever slips effortlessly. 2) Shifting with a complete pause on neutral position, than continuing the move to the next gear.

This is probably very heavily dependent on the vehicle; see my answer to #1. However, see also that last bit about insane paranoia...there's just no need to worry about this.

In fact, I can give you an extreme situation: I put a 1986 T5 manual transmission from a low-torque I4 engine behind the high-torque engine in my 1980 Buick. I had to piece together, hack, and modify to get a clutch system in place. I have never been confident that my clutch is completely disengaging. I fear that my synchronizers are getting abused severely. It's been 50,000 miles and it still shifts as well as any 1986 T5 has ever shifted.

With a properly engineered, designed, and manufactured OEM system you should never have to worry about driving the way the engineers expected you to drive, which is to shift with light to moderate pressure, not waiting around in neutral while the slushbox driver behind you rages and 3 cars that could have made it through the green light get stuck at a red light instead. If you are going to waste all that time waiting around in neutral, might as well double-clutch instead, it should be slightly faster (unless your engine RPM doesn't drop fast).

[gregxxxnt]

@theholycow, thanks for repying me, I guess you've misunderstood me Look:

I don't understand what's the point of double clutching on upshifts in an unsynchronized transmission.

I haven't asked about synchronized one. I don't use double clutching on upshifts in this kind of transmission, as it's pointless in my opinion.

It's not that the synchros are relieved in any way; it's the effect that you feel when the synchro has done its job.

Yes, but I think the light pressure on a stick lever means less work for synchro comparing to the shifting with more force. Does it work in this way?

see also that last bit about insane paranoia...there's just no need to worry about this.

Don't worry, I'm not paranoic at all I like being more and more conscious about cars and driving techniques.

[theholycow]

@theholycow, thanks for repying me, I guess you've misunderstood me Look:

I don't understand what's the point of double clutching on upshifts in an unsynchronized transmission.

I haven't asked about synchronized one. I don't use double clutching on upshifts in this kind of transmission, as it's pointless in my opinion.

Nope, didn't miss that:

3) I don't understand what's the point of double clutching on upshifts in an unsynchronized transmission. When the clutch is pushed down and the shift lever is moved to neutral the intermediate shaft is free to slow down. Why don't just lightly press the shift lever in the direction of the next gear, allowing it to slip into gear when the intermediate shaft slowed enough? In other words, what's the point of using the engine to slow down the intermediate shaft, if it should slow down by itself?

Where are you finding an unsynchronized transmission?

If you lightly press the shift lever in the direction of the next gear in an unsynchronized transmission, logically I am pretty sure it will grind until either it is synchronized or the dog teeth are all ground/broken off. I suspect you're better off jamming it in firmly than lightly pressing it, then the teeth will have to engage and skip the drama.

That was all about unsynchronized transmissions.

It's not that the synchros are relieved in any way; it's the effect that you feel when the synchro has done its job.

Yes, but I think the light pressure on a stick lever means less work for synchro comparing to the shifting with more force. Does it work in this way?

No. The amount of work that needs to be done is the same whether you do it fast or slow. Within a reasonable range of force there would be no effective difference. If you shove it hard then the force involved could be a problem.

[gregxxxnt]

No. The amount of work that needs to be done is the same whether you do it fast or slow. Within a reasonable range of force there would be no effective difference. If you shove it hard then the force involved could be a problem.

Does the synchromesh work in a "fixed" way? That means, does the cone clutch use the same amount of friction force to bring the gear to the same speed everytime, no matter if the driver uses the light or moderate force to shift? Or does the driver control the friction force of the cone clutch?

[theholycow]

Does the synchromesh work in a "fixed" way? That means, does the cone clutch use the same amount of friction force to bring the gear to the same speed everytime, no matter if the driver uses the light or moderate force to shift? Or does the driver control the friction force of the cone clutch?

Driver controls friction, then friction controls time...it's a friction clutch just like the clutch we operate with the left pedal. More friction for less time, or less friction for more time, the same energy is pumped into it either way. If you're so light that it never overcomes parasitic friction then you'll sit there forever wearing it slowly away. If you're shoving it so hard that it burns up then that's no bueno either. Everything else in between is splitting hairs.

[gregxxxnt]

Great explanation, thanks Now I feel that I understand it good