transmission operation
There are a bunch of youtube videos and
other articles that help. After reading on Prime that Honda recommends against skipping shifts because it increases synchro wear, I decided that I wanted to understand how a transmission works finally. As I understand it, here are the basics (for the present discussion, we are focused on (E)(ii) below):
Mechanics:
- The input shaft comes from the clutch and engages with the intermediate shaft (aka countershaft, layshaft, cluster gear).
- The countershaft contains gears for each of the ratios that the transmission will use; they all spin together with the input shaft (main drive gear).
- The gears on the countershaft are always engaged with gears on the mainshaft (output shaft).
- While the mainshaft gears are always spinning with the countershaft, they freewheel on the output shaft.
- Each mainshaft gear has a dog clutch: one piece that rotates with the output shaft because it is splined to the output shaft, and one piece that rotates with the countershaft. Because each dog clutch attaches to the output shaft through a spline, each may slide on the output shaft. Between the two halves of each dog clutch is a conical piece that is spring loaded--the synchro.
- The output shaft is always connected to the drive wheels, so it reflects the speed the car is moving.
Operation:
A) When the transmission is in neutral with the clutch out, the input shaft is rotating and therefore the countershaft and output-shaft gears are rotating but are not connected to the output shaft, which is not rotating.
B) Clutch in: the input shaft slows because it is no longer driven by the clutch; eventually it is stopped and you may shift into first gear; shifting into first causes the first-gear output-shaft gear to slide along the output shaft; the first-gear synchro engages its mating portions between the output-shaft gear and the countershaft gear (when the car is not moving and the clutch is out, this is easy because both of the mating gears are motionless).
C) Clutch out: engaging the clutch causes the input shaft to turn; as it is connected to the output shaft through the countershaft gears and first-gear dog clutch, the wheels start to turn.
D) Shifting to second:
(i) clutch in disconnects the input shaft from the engine; but the wheels are driving the output-shaft gear and countershaft gear;
(ii) when you move the shifter into neutral, the output-shaft gear is no longer engaged with the output shaft, so the output-shaft and countershaft gears begin to slow because of frictional losses;
(iii) now you move the shifter to second; this pushes the second-gear synchro halves together (one is on the moving portion that is attached to the output shaft with a spline and the other is fixed to the countershaft gear); as the two halves touch, friction between them causes the speed of the countershaft gear to match the speed of the output-shaft gear; when the speeds are the same, the teeth of the dog clutch may engage and the shfiter moves into second;
(iv) releasing the clutch connects the engine to the input shaft; the input shaft is now moving slower than before because of the different gear ratio for second compared to first; your engine has slowed during the shift so that is good.
***
E) Shifting from third to second:
(i) When you disengage the clutch and move the shifter to neutral, you want to connect the output shaft to a higher-speed output-shaft gear--given the speed of the input shaft when you were in third, the second-gear output-shaft gear was moving faster than the third-gear output-shaft gear;
(ii) as you push the shifter into second, friction in the dog-clutch synchro causes the countershaft assembly to speed up, matching the speed of the second-gear output-shaft gear; once the speeds match, the shifter may slide into second as the teeth of the dog clutch engage.
(iii) letting the clutch out at this point causes the speed of the engine to match the speed of the input shaft; if you have not blipped the throttle, then the engine speed is at or lower than it was before the shift, and the transmission needs to accelerate the engine to match the input shaft; if you have appropriately blipped the throttle, the speed of the engine matches the (now higher than before) speed of the input shaft and clutch disengagement is smooth.
With all that in mind, we can consider the effect of blipping the throttle on engaging second gear during a third-to-second downshift. Since the clutch is in at this point, the effect should be nil. All the blip does is accelerate the crankshaft and half of the clutch assembly; without a connection to the transmission, the clutch can have no effect on the transmission.
Double clutching (or double declutching, as my father might say) is a wholly different story. In this procedure, as you move the transmission to neutral on your way from third to second, you let out the clutch as you blip the throttle. This has the effect of speeding up the input shaft and countershaft--including the second-gear output-shaft gear that you want to engage with. Now when you slide the shifter towards second (with the clutch once more depressed), the second-gear output-shaft gear is going much closer to the speed of the output shaft and the synchro has to do less work to match the speeds so that the dog clutch may engage. Thus less synchro wear.
---
Notwithstanding the above, on which I am more than open to correction, I understand your point that as a matter of experience, blipping the throttle seems to help engage the shifter during downshifts, not just smooth the clutch engagement. I can suggest two theories as to why that may be true. First, perhaps the clutch does not fully disengage so that blipping the throttle does impart some torque to the input shaft and thus does affect the speed of the countershaft to some degree. Second, blipping the throttle takes additional time and shifts focus, thus perhaps just changes one's perception of the whole event. Other than that, I'm at a bit of a loss otherwise.
I fear this is too far afield the original post, but I find the discussion entirely interesting. I have always wondered about Honda's don't-skip-gears advice, considering that when you are upshifting, if the engine slows appropriately (by waiting the correct time before engaging a new gear), skipping gears should not cause any additional synchro wear. On downshifts, it seems appropriate unless one double clutches, in which case again no additional wear should result.
My dad has a 1931 Model A, which does not have synchros in its gearbox. Driving it is somewhat harrowing compared to a modern car, as each shift is an event. If you don't get it the first time, it can be extremely difficult to recover.
-jason
