Double Clutching

[ArkiStan]

Here is a question I've always had about blipping the throttle (matching revs) to downshift. I've never gotten around to asking anybody, so here goes:

When I match revs to downshift, here is how I've done it all my life:

1) Power is removed and clutch is disengaged (pedal down)
2) I blip the throttle while simultaneously shifting to lower gear.
3) Clutch is re-engaged (pedal up).

This has always worked perfectly fine and I only need to step on the clutch once. However, when I read online about "double clutching" when downshifting, it says that the process is as follows:

1) Power is removed and clutch is disengaged (pedal down)
2) Transmission is shifted from original gear to neutral
3) Clutch is re-engaged (pedal up). (Driver now has control of intermediate shaft speed by controlling engine speed)
4) Driver 'blips' throttle to match intermediate shaft speed to speed of new gear. (This takes practice to get the right match)
5) Clutch is disengaged (pedal down)
6) Transmission is shifted from neutral into new gear
7) Clutch is re-engaged (pedal up) and power is applied.

Is it really necessary to do this? The main difference here is that I blip the throttle while the clutch pedal is down, and the "double clutching" instructions say i need to blip the throttle while the clutch is up and gear in neutral, resulting in the need for the second (double) clutch. Have I been doing it wrong the whole time? But my method actually works and is much simpler/faster/smoother....


(Side note: The Automotive & Transportation forum used to be really fun when people actually talked about driving. Apex himself would chime in a lot. Ah the memories... How many people knew that before having to "settle down," and correct me if I'm wrong, Apex was once an avid street racer? I believe that's where the nickname comes from. :) )

[Markel]

As far as I know, double clutching is only useful when the transmission doesn't have syncromesh.

[ArkiStan]

As far as I know, double clutching is only useful when the transmission doesn't have syncromesh.

I thought other than tractors and trailor trucks, most modern passenger cars have syncromesh. I wonder why double clutching is so often discussed amongst regular drivers. Do racing vehicles not have syncromesh?

edit: i think the syncromesh works fine for a single-gear downshift, but if one needed to downshift two gears or (god forbid) more at a time, i think significant stress would be applied to the syncromesh, hence the need for rev-matching.

[ArkiStan]

I thought other than tractors and trailor trucks, most modern passenger cars have syncromesh. I wonder why double clutching is so often discussed amongst regular drivers. Do racing vehicles not have syncromesh?

From Wikipedia entry for Manual transmission (http://en.wikipedia.org/wiki/Manual_transmission)

Unsynchronized transmission
"When upshifting, the speed of the gear driven by the engine had to drop to match the speed of the next gear; as this happened naturally when the clutch was depressed or disengaged, it was just a matter of skill and experience to hear and feel when the gears managed to mesh. However, when downshifting, the gear driven by the engine had to be sped up to mesh with the output gear, requiring letting the clutch up (engagement) for the engine to speed up the gears. Double-clutching, that is, shifting once to neutral to speed up the gears and again to the lower gear, is sometimes needed. In fact, such transmissions are often easier to shift without using the clutch at all. When using this method, the driver has to time the shift with relative precision to avoid grinding the gears. The clutch, in these cases, is only used for starting from a standstill. This procedure is common in racing vehicles and most production motorcycles.

Even though automotive transmissions are now almost universally synchronised, heavy trucks and machinery as well as dedicated racing transmissions are usually not; such transmissions are colloquially referred to as "crashboxes." Non-synchronized designs are used for several reasons. The friction material, such as brass, in synchronizers is more prone to wear and breakage than gears, which are forged steel, and the simplicity of the mechanism improves reliability and reduces cost. In addition, the process of shifting a synchromesh transmission is slower than that of shifting a non-synchromesh transmission. For racing of production-based transmissions, sometimes half the teeth (or "dogs") on the synchros are removed to speed the shifting process, at the expense of greater wear."

So that answers my questions why "double clutching" is often discussed between race car drivers. I still don't understand why, in unsynchronized transmission you need to double clutch and in regular passenger cars i can match revs with a "single clutch." Am I not actually affecting the intermediate shaft by blipping with the clutch down?


.

[SteveB]

Markel is correct.
Blipping the throttle really isn't necessary unless you are at high RPM.
Based on my limited track experience, double clutching isn't necessary if you can master 'heel and toeing'. Very few, if any, drivers I know double clutch anymore if they are driving a car with synchros.
Check this out:

http://www.edmunds.com/ownership/howto/articles/45792/article.html

Read your post after I posted so I'll add, Many race car transmissions use straight cut gears which allow shifting without clutching as long as the driver matches RPM.

[ArkiStan]

Markel is correct.
Blipping the throttle really isn't necessary unless you are at high RPM.
Based on my limited track experience, double clutching isn't necessary if you can master 'heel and toeing'. Very few, if any, drivers I know double clutch anymore if they are driving a car with synchros.
Check this out:

http://www.edmunds.com/ownership/howto/articles/45792/article.html

Read your post after I posted so I'll add, Many race car transmissions use straight cut gears which allow shifting without clutching as long as the driver matches RPM.

I agree that double-clutching is not necessary. As I said in my first post I match revs without double clutching. And heel-and-toe simply a way of matching revs while simultaneously braking.

It's a bit confusing to word my question accurately, but In the end, I guess my concern is....am i doing it wrong if I'm blipping the throttle with the clutch down? I don't think this is a question about the synchromesh, but rather the intermediate shaft. Do modern passenger cars have an intermediate shafts? If so, am I applying power to it when the clutch is down? If so, why were steps 3) and 4) even necessary in the double clutching instructions above?


(LOL. sorry this is a mess. I wonder if anybody understands my question. haha :hehehmm: )

[ArkiStan]

I have a feeling that the answer to my questions lies somewhere in this paragraph from the same Wikipedia Article (http://en.wikipedia.org/wiki/Manual_transmission). But I'm not familiar enough with the lingo to be sure.....

Synchronized transmission

"Modern gearboxes are constant mesh, i.e., all input and drive gears are always in mesh. Only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time, while the others are allowed to rotate freely. This greatly reduces the skill required to shift gears.

Most modern cars are fitted with a synchronized gear box, although it is entirely possible to construct a constant mesh gearbox without a synchromesh, as found in a motorcycle, for example. In a constant mesh gearbox, the transmission gears are always in mesh and rotating, but the gears are not rigidly connected to the shafts on which they rotate. Instead, the gears can freely rotate or be locked to the shaft on which they are carried. The locking mechanism for any individual gear consists of a collar (or "dog collar") on the shaft which is able to slide sideways so that teeth (or "dogs") on its inner surface bridge two circular rings with teeth on their outer circumference: one attached to the gear, one to the shaft (one collar typically serves for two gears; sliding in one direction selects one transmission speed, in the other direction selects the other). When the rings are bridged by the collar, that particular gear is rotationally locked to the shaft and determines the output speed of the transmission."

[mechmike0034]

I still don't understand why, in unsynchronized transmission you need to double clutch and in regular passenger cars i can match revs with a "single clutch." Am I not actually affecting the intermediate shaft by blipping with the clutch down?

Because "crashboxes" in trucks and older cars typically are of sliding-gear design, and not of the constant-mesh variety (http://www.edmunds.com/ownership/techcenter/articles/46029/article.html):


There are two basic types of manual transmissions. The sliding-gear type and the constant-mesh design. With the basic -- and now obsolete -- sliding-gear type, nothing is turning inside the transmission case except the main drive gear and cluster gear when the trans is in neutral. In order to mesh the gears and apply engine power to move the vehicle, the driver presses the clutch pedal and moves the shifter handle, which in turn moves the shift linkage and forks to slide a gear along the mainshaft, which is mounted directly above the cluster. Once the gears are meshed, the clutch pedal is released and the engine's power is sent to the drive wheels. There can be several gears on the mainshaft of different diameters and tooth counts, and the transmission shift linkage is designed so the driver has to unmesh one gear before being able to mesh another. With these older transmissions, gear clash is a problem because the gears are all rotating at different speeds.

All modern transmissions are of the constant-mesh type, which still uses a similar gear arrangement as the sliding-gear type. However, all the mainshaft gears are in constant mesh with the cluster gears. This is possible because the gears on the mainshaft are not splined to the shaft, but are free to rotate on it. With a constant-mesh gearbox, the main drive gear, cluster gear and all the mainshaft gears are always turning, even when the transmission is in neutral.

Double-clutching is a must with a sliding-gear transmission.

Bottom line:
Sliding gear = typically unsynchronized = old design = double clutch required
Constant mesh = typically synchronized = modern design = double clutch unnecessary

Rev matching is nice (and a bit of a lost art), and it may (or may not) save wear on the syncros (which are typically brass rings), but it really doesn't matter in a modern manual gearbox.

[ArkiStan]

I see that "constant mesh" is the answer to my question. Thanks MM34. That cleared things up. Maybe rev-matching doesn't actually aid in saving syncro wear, but I feel like when down shifting two gears at a time at relatively high speeds, withou rev-matching, there still is a noticeable jolt. And I feel I'm decent at blipping enough that it adds negligible delay to the downshift. Regardless if whether it does anything, i think it's one of the funnest parts of driving.

thanks for the info everybody. I've learned a lot. :thumb: