Questions on rev matching and engagement points

[theholycow]

That's a good point, I forgot about that -- though with some cars, a really big downshift could be faster double-clutched.

[IMBoring25]

A few things to consider:

1) Saving synchros on downshifts is legitimate. Paranoid maybe, but legitimate.

2) Weak synchros will make a single-clutched downshift slow. Try to keep up with a DC in this situation and you'll grind, ultimately maybe losing the gear entirely if you do it often enough.

3) My dad actually mentioned the other day that, on a slick enough surface, it would be possible to skid the car by shifting the transmission, as the later half of the driveline imparts energy to the layshaft and input shaft through the synchros. Actually a real-world concern? You be the judge.

4) The synchro on first is typically weak. The aforementioned downshift into first, if at much more than walking speed, may never happen single-clutched.

[daleadbull]

1) Saving synchros on downshifts is legitimate. Paranoid maybe, but legitimate.

What makes it different than an upshift? From the syncho's point of view.

[theholycow]

Upshifts are usually smaller; you might skip 3 gears downshifting but much less likely on an upshift. Additionally, regular drag in the system does SOME of the work for an upshift anyway -- and does the opposite of what you want in a downshift.

Stepping outside of the synchro's concerns, it's often much easier to double-clutch a downshift (and even single-clutch rev-matching too) because you can raise engine speed quickly but there's no brake to slow the engine. (An engine configured for the job could drop its speed reasonably quickly without any real consequence elsewhere, but nobody cares about configuring an engine for that.)

[daleadbull]

Upshifts are usually smaller; you might skip 3 gears downshifting but much less likely on an upshift. Additionally, regular drag in the system does SOME of the work for an upshift anyway -- and does the opposite of what you want in a downshift.

That's a good point. How can you tell if you synchros are working extra hard on a big 6-3 downshift for example? As long as you take your time with it and the stick slips in easy, it means you synchro had enough time, am I right?

[IMBoring25]

Yes, if the shifter eventually goes in smoothly, the synchros have had time to do their job and keep the dog teeth from getting buggered.

However, the synchros are basically just friction surfaces. However long you hold the shifter against the gate, the synchro is working and wearing that entire time.

[theholycow]

That's a good point. How can you tell if you synchros are working extra hard on a big 6-3 downshift for example? As long as you take your time with it and the stick slips in easy, it means you synchro had enough time, am I right?

I can tell you that your synchros are working extra hard on a big 6-3 downshift. Consider it told!

If it takes a long time (given that you're using the same force) for the stick to slip in, it did a lot of work. If it had to make up 4000RPM, it did a lot of work (even if it didn't take long -- but then you have awesome synchros).

[daleadbull]

That's a good point. How can you tell if you synchros are working extra hard on a big 6-3 downshift for example? As long as you take your time with it and the stick slips in easy, it means you synchro had enough time, am I right?

I can tell you that your synchros are working extra hard on a big 6-3 downshift. Consider it told!

If it takes a long time (given that you're using the same force) for the stick to slip in, it did a lot of work. If it had to make up 4000RPM, it did a lot of work (even if it didn't take long -- but then you have awesome synchros).

What I meant is that, I take my time with the shift and don't force it in. I usually shift from 5-3 or 6-3 when I'm turning into my street or the parking lot at work, so its at pretty low rpms. I don't really rush the shift and I don't notice any additional resistance when shifting, so I'm guessing my synchros are fine.

Maybe I should start double clutching on those big downshifts. Hmm...

IMBoring, I don't hold the shifter against the gate. I usually just pause in neutral but the clutch is still down so its not a double clutch shift.

[ClutchDisc]

Since we are now talking about synchros, I had a question. When is really cold out, I drive a little ways and then stop at the end of the street. When I go to put it into first to take off again, it's really, really hard to get into first. This can't be the best for the synchros...what's the best thing to do so it's easy to get it into first?

[daleadbull]

Since we are now talking about synchros, I had a question. When is really cold out, I drive a little ways and then stop at the end of the street. When I go to put it into first to take off again, it's really, really hard to get into first. This can't be the best for the synchros...what's the best thing to do so it's easy to get it into first?

What I do is pull the stick slightly down towards 2nd then straight up to 1st and it slips in easier. I don't know where I read this but it helps in my car.

[ClutchDisc]

Since we are now talking about synchros, I had a question. When is really cold out, I drive a little ways and then stop at the end of the street. When I go to put it into first to take off again, it's really, really hard to get into first. This can't be the best for the synchros...what's the best thing to do so it's easy to get it into first?

What I do is pull the stick slightly down towards 2nd then straight up to 1st and it slips in easier. I don't know where I read this but it helps in my car.

I'll try that next time. Thanks

[IMBoring25]

Usually difficulty going into first occurs because everything is stopped and the gears don't line up (though a fast-spinning input shaft and a weak synchro is also a possibility until the clutch has been in for a few seconds). Once you're stopped, you can use another gear to realign the gears or briefly let the clutch out to spin the input shaft. For brief stops like stop signs, it's also possible to shift into first while you're still just barely moving.

[ClutchDisc]

Usually difficulty going into first occurs because everything is stopped and the gears don't line up (though a fast-spinning input shaft and a weak synchro is also a possibility until the clutch has been in for a few seconds). Once you're stopped, you can use another gear to realign the gears or briefly let the clutch out to spin the input shaft. For brief stops like stop signs, it's also possible to shift into first while you're still just barely moving.

It's only when it's cold it seems. I'll try another gear to line it up. I'm usually putting it in first while still rolling. Thanks!

[theholycow]

What I meant is that, I take my time with the shift and don't force it in. I usually shift from 5-3 or 6-3 when I'm turning into my street or the parking lot at work, so its at pretty low rpms. I don't really rush the shift and I don't notice any additional resistance when shifting, so I'm guessing my synchros are fine.

At low RPM the job isn't that huge, but it's still a lot bigger than a 6-5 shift.

Don't worry about the synchros, but if you want a good excuse to DC you've got one.

Since we are now talking about synchros, I had a question. When is really cold out, I drive a little ways and then stop at the end of the street. When I go to put it into first to take off again, it's really, really hard to get into first. This can't be the best for the synchros...what's the best thing to do so it's easy to get it into first?

Bounce off of 2nd like daleadbull, or do the no-gas low speed double-clutch 1st trick (ask your dad, if you don't remember the thread where I described it in detail to him).

[ClutchDisc]

Since we are now talking about synchros, I had a question. When is really cold out, I drive a little ways and then stop at the end of the street. When I go to put it into first to take off again, it's really, really hard to get into first. This can't be the best for the synchros...what's the best thing to do so it's easy to get it into first?

Bounce off of 2nd like daleadbull, or do the no-gas low speed double-clutch 1st trick (ask your dad, if you don't remember the thread where I described it in detail to him).

Ok I will try all of these and see what works best for me. Thanks all.