Understanding manual transmissions

[the_new_mr]

Hi everyone!

This is my first post here but I can see myself posting here a lot because I've always loved cars and have always wanted to know more about the technical side of things and it seems that there's a lot of people here that know what they're talking about.

My question is about manual transmissions and how they work.

I read this article on how manual transmissons work on howstuffworks.com and I think I understood most of it. There are just a couple of things that I'm not 100% sure about.



Referring to the image above, I get the fact that the input shaft (green), layshaft (red) and the gears (blue) [only two gears are shown here] all turn at the same speed. I get that the output shaft (yellow) that goes to the differential and the collar are attached and are therefore also moving at the same speed. I also get that it's the engaging of the collar's dog teeth with the gears that engages the gear.

Actually, I think I fully understand how the transmission works. What I don't get is double-clutching.

In old cars with no synchromesh, you had to sync the speed of the collar and next gear by letting out the clutch between gears. What I don't get is how letting the clutch out helps. I mean, when you the push the clutch in, the output shaft (and therefore the collar) are going to be moving at around the same speed but the input shaft, layshaft and gears will all slow down to a very slow speed once you let the clutch out again, right? So, how does that help? When you clutch in again, change to the higher gear and clutch out a final time, the collar is still going to flying around while the gears are now even slower than they were before.

Actually, after re-reading the article again, I think I got it. It says in the article that:
Quote:
In double-clutching, you first push the clutch pedal in once to disengage the engine from the transmission. This takes the pressure off the dog teeth so you can move the collar into neutral. Then you release the clutch pedal and rev the engine to the "right speed." The right speed is the rpm value at which the engine should be running in the next gear.

So, does that mean that you have to hold the engine at the correct engine speed with the throttle according to the speed of the car itself whilst changing gear?

And finally, in some racing cars, I know that some drivers double-clutch on the way down the gears (like in the old days). I get heel-and-toe and why you do it (syncing of engine speed with rear-wheels to prevent wheel lock and therefore maintain control). But do they double-clutch because they don't have a synchromesh or, even if they do have a syncromesh, to save wear and tear on the synchromesh itself?

Thanks in advance guys and sorry for the long post

[MagicRat]

Hi,
Welcome to AF

Sorry, I accidentally lost you pic. My fault and my apologies

First of all, you do not have to 'hold' the revs when shifting. You rev the engine a bit, then let the revs fall. When the revs fall to exactly the desired point, when you know the speed of both the input and output shafts are the same, you shift.

So how do you know? Through experience, based on road speed, the gear desired, engine pitch (sound) vibration, the 'feel' of the car.

Second, some cars, (not just race cars), especially old or high mileage ones simply downshift more smoothly and more quickly when double clutching. My '79 Mustang and '84 Supra do just this, especially when downshifting into first gear.

[The Dude]

When they talk about double clutching like that, they are talking about transmissions with no syncronizers, like a semi truck. You push the clutch in the first time to do what you said break the torque off the clutching teeth or dog and the collar is called a sliding clutch. Ok so we release the clutch and no you have to match engine speed to road speed or "output shaft speed" cuz we have no syncronizer to match speeds for us. Either revving the engine or letting rpms drop you find the "match" and pull it into gear again. You actually dont have to double clutch on a semi truck, just have to match speeds and sometimes double clutching helps if you have missed a gear and cant find the right one

[the_new_mr]

Originally Posted by MagicRat

Hi,
Welcome to AF

Sorry, I accidentally lost you pic. My fault and my apologies

Thanks and no problem

Thanks for the replies guys.

Thanks for confirming my thoughts about matching the road speed with the engine speed. I guess it must take a lot of experience to shift gear with no synchromesh. I suppose that after a while, you get to know by instinct as it becomes second nature from doing it so much. Kind of like the way you might have to blip the throttle more on a heel and toe downshift depending on the gear you're downshifting to and the current engine speed when performing the downshift.

I guess that's why you could nearly always hear the gearbox grinding in those old films when people were changing gear A lot harder than looks

[the_new_mr]

Originally Posted by MagicRat

Second, some cars, (not just race cars), especially old or high mileage ones simply downshift more smoothly and more quickly when double clutching. My '79 Mustang and '84 Supra do just this, especially when downshifting into first gear.

Was thinking a bit more about this recently. Is this because they are so old that the synchromesh has worn a little bit? And does the faster downshift come down to the fact that the synchromesh doesn't have to do as much work to sync the collar to the gear because the speed is already nicely matched?

Finally, if someone blips the throttle on a heel-and-toe downshift to match the revs, what's the point of letting out the clutch half-way during the downshift? Is it so that we are speeding up the layshaft instead of slowing it down? (as letting the clutch out again in neutral will slow down the green, red and blue parts of the transmission whereas not letting the clutch out half-way will mean that they have to slow down to the new engine speed)

Check this guy out doing using the double-clutching technique. It's sweet
http://www.youtube.com/watch?v=0j-3xIZK-Bk