I’m coming up to One Year of driving a stickshift.
I’m pretty comfortable with it by now, but there are still some things I don’t entirely understand. Double-clutching is one of those things that I’d like to actually do better.
The synchros in 1st and 2nd gear are a little worn, but it’s not too bad.
I have done the double clutch motion right before, but it’s not quite muscle memory.
Disclaimer: look I’m not an auto mechanic or engineer, I’ve almost certainly got some details wrong but I’m getting my head around it
What’s going on
From what I understand, there are 3 shafts spinning at their own speeds.
|===== Crankshaft =====| |===== Driveshaft =====| |===== Gearshaft =====|
They are connected to each other, in a way:
Crank --(0 to 100% clutch)--> Gear --(0 or 100% in-gear)--> Drive
Let’s say we have to shift from 5000 RPM in 1st to 3000 RPM in 2nd1.
We want to go from
|== Crank @ 5000 RPM ==| |== Axle @ 25 MPH ==| |== Gear @ 5000 RPM ==| (in 1st gear)
|== Crank @ 3000 RPM ==| |== Axle @ 25 MPH ==| |== Gear @ 3000 RPM ==| (in 2nd gear)
Now, if we just let off the gas, we know the crank/tachometer will drop pretty quickly.
But you can’t see a tach for the gearshaft, and it’s got no good reason to slow down. You think the bearings put that much drag on it? The oil? Hardly. The engine will be compression braking, so it should nosedive in RPM, especially at 5000 RPM 2.
So one way or another, the gearshaft has to have some energy taken off it. It’s a rotating mass, and we’re going to tap that energy one way or another.
Synchros are the traditional way. What a synchro is is a ring of metal that’s like the last ¼” of gear, with teeth and everything. One synchro per gear. What’s going on is the gearshaft gear has to align with the driveshaft gear, so we slide them next to each other, closer and closer.
=== - === === - === === - === === G S D -->
Now, you’d have to get pretty lucky to have them line up. The synchro is slightly cone-shaped and has ridges to “grab” onto the gearshaft. The ridges provide friction, the friction captures heat energy, which does slow down the gearshaft. Any remaining energy hits the driveshaft (probably not perceptible, it can’t be a ton of energy, especially compared to when we engage the engine).
The synchro provides friction, slowing down the gearshaft to match.
Eventually, the ridge is just worn down, and it’s just a loose gear that doesn’t really do anything.
It looks like about $100 to replace 1 synchro, probably $500 to replace all 5. That is not something I want to do. I mean, it’d probably be fun, and should last a long time, but I’d prefer doing it at a later, unspecified date.
Let me reiterate: the synchros are just a little worn, they’re not trashed. I can always get it in gear, though 1st gear sometimes is smoother the 2nd try.
Performing the double clutch
It’s simple: keep the engine engaged while it drops RPM. This whole notion of Keep The Clutch Down Until We’re Alllllll Done Shiftin’ does not seem rooted in reality3.
The simplest way to do it is to just pop it out of gear into neutral. Don’t do that! We can do a little better.
When we go into neutral, we only briefly disengage the engine. Clutch down & up.
What this looks like is tapping your toe. Down and up.
We let the engine drop or climb to the approprate RPM for the next gear, and then we clutch down, shift into gear, clutch out. That final clutch release will be guaranteed smooth if you dropped or climbed to the exact right RPM. If you do it right, there won’t be any jerk to feel at all4.
The 1st down-up is very quick, just to release the power as we slip out of 1st gear. We don’t want any power applied to Barely Engaged gear teeth, that’s not a good idea.
The 2nd down-up is your regular quick-down, slow-up clutching in.
Downshifts are no different, but the target RPM is way on the high side. If you’re not sure, spin the engine past 4000 RPM and slowly clutch in. What this looks like is slowing down, quickly clutching down-&-up into neutral, spin to 4000 RPM, quickly clutch down, shift in, slowly engage.
The engine will still settle down into the appropriate RPM, we just have to let it settle down from above the appropriate RPM.
I sometimes just need to See The Steps
@ 5000 RPM (in 1st) clutch down, just to get into neutral (in neutral) clutch up (ride the RPM down to 3000) (in neutral) @ 3000 RPM (in neutral) clutch down, smooth shift into 2nd (in 2nd) clutch up, slowly as usual (in 2nd)
and to downshift, which is always unusual; let me downshift from 3000 RPM in 4th to 4000 RPM in 3rd, that’s a bit more realistic5.
@ 3000 RPM (in 4th) clutch down, just to get into neutral (in neutral) clutch up (RPMs dwindling below 3000) (in neutral) Goose It! (RPMs climb to 4250, fall) (in neutral) @ 4000 RPM (in neutral) clutch down, smooth shift into 3rd (in 3rd) clutch up, slowly as usual (in 3rd)
this numbers almost exactly match an MR2 (or a Celica GT, any 1ZZ/5MT). And yes, you’re going just about 25 mph at 5000 RPM in 1st. ↩
engine braking is I’m pretty sure a mostly-fixed amount of force per-revolution (each cylinder only having so much volume), so we’re basically running an air compressor out the exhaust pipe (we literally are pumping air, quite a bit, for no further fuel cost). So engine braking is stronger at higher RPM. ↩
just ask my synchros ↩
of course, it’s a little fun to guess on the high side on an upshift, and we all should know it’s so easy to end up guessing the low side on a downshift (the whole car shits a brick like you double-kicked the brakes). ↩
you never downshift into 1st, are you crazy? You must be coming to a stop ↩