We will discus here just basics on most difficult but very sexy shifting techniques for "normal drivers". Of course all of explained is in constant use in race and performance driving. To have a complete picture of performance driving, take a look at links on the bottom of this article.
Rev-matching refers to matching the engine speed to the transmission input speed before re-engaging the clutch. This is particularly desirable when downshifting for a corner. In a hard corner, the suspension needs to be stable and the tires need to be loaded so that all of their traction is being used to generate cornering force. If the engine is at a different speed than the transmission input when the clutch is re-engaged, this will generate a force on the driveline that will upset the stability of the suspension (weight Transfer) and will cause the tires to exceed their traction capability if the corner is being taken near the limit. It is necessary to rev-match if a corner is to be taken near the limit. It isn't necessary for slower corners, but it will make them smoother.
Another benefit is that "heel-and-toeing" allows you to downshift at the last moment before entering the turn, after you have started braking and the car has slowed, so the engine speed when the lower gear is engaged will not be too high.
As the power band of most rally cars is high in the rev range, this technique can also be used to ensure that engine rpm does not drop below the power band of the car while under braking. If this happened there would be a delay between the drivers accelerating after the corner and when the car responds; this is especially true in turbocharged cars. This technique ensures that maximum power can be reached the instant the brake pedal is released and the accelerator fully depressed. But this can be achieved with left foot braking too.
Double clutching is a technique for rev-matching the transmission's intermediate shaft to the output gear that is to be selected. This is a function that is normally handled automatically by the transmission's synchro-mesh gears (or synchro's). It is useful to double clutch for extreme gear changes, for older transmissions with worn synchro's, or simply to save wear on the synchro's. It's also an entertaining thing to do. Most drivers, when downshifting, merely press in the clutch after letting off the throttle, jam the stick in a lower gear, and then let out the clutch. In modern vehicles, this is possible with synchromesh transmissions, but is usually accompanied by a sudden rearward weight transfer, if not with any less-than-harmonious engine/transmission noises. This sudden weight transfer can have an adverse effect on handling. Perhaps more importantly, however, is that the life-span of the synchro meshes may be shortened with this practice. The solution, then, is to double-clutch!
Before I get into the specifics of rev-matching and double clutching, it would probably be good to review the inner workings of the clutch and transmission.
Gasoline engine has a limited output bandwidth of approximately 1000 to 6000 RPM. It can't operate from 0 RPM (like an electric motor can), so it needs a device (the clutch) to disconnect it from the drive train so that it can idle while the car is at a standstill. The power is not constant from 1000 to 6000 RPM, so it is also necessary to have different gear ratios in the transmission to extend the car's useful operation speed range and to maximize available acceleration. The clutch also serves to disconnect power to the transmission when changing gears. The clutch is integrated with the engine's flywheel. There is a disk with friction material, similar to a brake pad that is connected to the transmission input shaft on a sliding spline. This disk is surrounded by the flywheel surface on one side, and the pressure plate surface on the other side. The pressure plate is spring loaded to squeeze the clutch disk against the flywheel, effectively making a solid connection. The pressure plate can be moved by pressing down on the clutch pedal. This releases the coupling pressure between the flywheel and the clutch disk so that they can rotate independently.
There are three states the clutch can be in:
CLUTCH DISENGAGED (pedal pressed down): The engine flywheel and the clutch disk can rotate independently. No power is being transferred.
CLUTCH SLIPPING (pedal brought up just to the point that the clutch starts to grab): The engine flywheel and the clutch disk are still rotating at different speeds, but power is being transferred. If the clutch pedal is continued to be brought up properly, the speed of the engine and the clutch disk will be brought together. This is the state that clutch wear occurs.
BTW, just for training, with your car parked and with engine off (or running in neutral gear), SLOWLY push your clutch pedal all the way down. Try to feel difference in force needed to push. Somewhere in middle of the way (usualy 1/3 of the way), you should feel something like small “step”, like small change in force needed. After this point force is again the same as before. Now, try slowly to release pedal all the way up. You have to feel same “step” on more or less same position. This position is the point where clutch starts to grab. It's good to remember this position because in fast gear change you don’t need to push clutch pedal all the way down to release clutch, but only a bit more of point you find in steps before explained. One more thing. Clutch slipping point is easier to feel in “sportier” cars, cars made for faster driving and of course in sport and high performance cars with classic clutch pedal. Family cars for slow cruising from A to B can have clutch pedal smoothed down with hydraulic actuators, and this slipping point can be unrecognizable.
CLUTCH ENGAGED (pedal up): The engine flywheel and the clutch disk are locked together. Full power is being transferred, and no clutch wear is occurring.
More on clutch you can read here!
The transmission has an input and an output. There is mostly five or six forward gear ratios (highest gear is most often a straight 1:1, not an overdrive) and one reverse gear ratio. The forward gears are all constant-mesh, that means that the gear teeth for all ratios are always engaged with each other at all times. Instead of sliding a gear out of engagement with another gear, the gear is disengaged by disconnecting it from the shaft that it is on. Only one gear ratio pair can be connected to the shaft at one time. The reverse gear is only sliding gear whose teeth actually slide out of engagement when it's not being used.
Each forward gear can be coupled to its shaft by a sliding locking coupler. This coupler connects splines on the shaft to splines on the gear. The coupler needs to be at the same speed as the gear splines to avoid grinding. (When people refer to "grinding the gears", it is actually the splines that are grinding, not the gear teeth). To synchronize the coupler with the gear splines, there is an intermediate device called a synchro-mesh.
The synchro-mesh is a lightweight ring with spline teeth on one side, and a conical friction surface on the other side. It is positioned between the sliding coupler and the gear splines. The gear also has a conical friction surface that mates with the surface of the synchro-mesh.
When a gear is to be engaged, the shift linkage selects a sliding coupler to connect to a gear. At this point, the coupler and the gear to be engaged are usually spinning at different speeds. As the coupler starts to slide, it first engages the spline teeth of the synchro-mesh ring. Because the synchro-mesh is so lightweight, it can virtually instantly change speed to match the sliding coupler that was just forced into engagement with it. It then becomes part of the coupler. As the coupler continues to slide towards the gear splines, the friction surface of the synchro-mesh ring is pressed into contact with the friction surface of the gear assembly. This friction causes the transmission's input shaft (which at this point is hopefully disconnected from the engine by the clutch) to be accelerated (or decelerated) so that the coupler and the gear are spinning at the same speed when their spline teeth finally engage. A synchro-mesh is limited in how much mass it can accelerate and how fast it can do it.
I think it's helpful to find some junk transmission parts to move around by hand to help visualize this process.
Let's see how double clutching work:
There are three separate spinning entities that need to be coordinated when shifting: The engine, the transmission input (I'm going to refer to this as the intermediate shaft) and the transmission output (which is directly related to the vehicle speed trough differential).
When the clutch is disengaged (pedal pushed down) and the transmission is in neutral (such as when shifting between two gears), the intermediate shaft is essentially free spinning. In normal shifting, we rely on the synchro's to control the speed of the intermediate shaft as it engages with the gears connecting it to the transmission output.
Decades ago, transmissions didn't have synchro-mesh. (Many large trucks still don't). On these transmissions, it was necessary for the driver to manually control the speed of the intermediate shaft so that it matches the speed of the gear to be engaged. This is done by the following process when shifting from one gear to the next:
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.
Steps 1 - 3 can be done casually or quickly. Steps 4 - 6 must be done quickly so that the intermediate shaft doesn't slow down again before it's engaged. If step seven is also done quickly, the engine will also be 'rev-matched' to the rest of the driveline so that engagement will be smoother.
Step 4 - if you need to brake in same time with downshifting, it's necessary to use heel and toe technique during all process, but especially in step 4.
It is worth noting that shifting to neutral may not be necessary. The driver can blip the throttle when the clutch pedal is depressed, then shift to the lower gear and release the clutch. While a modern synchronized transmission does not require double clutch shifting, a driver may choose to perform it in order to lessen wear on the synchronizer.
Let's see how rev - matching works:
When taking off from a stop and then going up through the gears, steps 1,2,6,7 above are the normal shifting method for each gear change. The synchro-mesh is more than enough to control the intermediate shaft speed. The engine will passively rev-match itself because it slows down naturally and this is appropriate when shifting up.
When down shifting, the engine needs to spin faster as it engages the new gear. This can be achieved by 'blipping' the throttle as the transmission passes through neutral. If the engine isn't manually sped up by the driver, it will be sped up by the driveline when the clutch is re-engaged. This can be OK for normal street driving, but if the car is cornering near the limit this can upset the suspension and the tire adhesion. Again, if you need to brake in same time with downshifting, it's necessary to use heel and toe technique during all process.
The synchromesh is usually adequate intermediate shaft control when down shifting one or two gears. Aggressive downshifts (like a 5 - 2 shift from high speed) can benefit by using double-clutch shifting to assist the synchro's. Shifting into first gear in a hard corner is nearly impossible without double-clutching and heel and toe technique (this is useful for those really tight hairpins, especially if they exit uphill).
I think that the biggest problem people have learning this technique is that they try to incorporate everything all at once. My suggestion is that you have to progressively learn things one at a time.
When shifting up from one gear to a higher gear both the engine and the intermediate shaft will naturally slow down. This will passively bring them near the speed of the higher gear. Because of this, active rev-matching or double-clutching are rarely required.
There are three indicators to determine how good a gearshift is: Was only light pressure required on the lever? Was the shift smooth? Was the shift fast?
To be complete, I should mention that there is one exception for forcing the shift lever. To get the very last little bit of acceleration out of a car, it can help to rush the shift sequence by forcing the shift lever into the next gear and re-engage the clutch with the engine already on-power. This can slightly reduce shift time and throw some extra inertia from the engine into the drive train. This technique definitely adds wear to the synchro's, the clutch and the rest of the drive train. If the car isn't being pushed to the point of lighting up the tires (which also accelerates wear), this technique isn't worthwhile.
There are three basic scenarios involving downshifting that I can think of:
1) Downshifting (without braking) from an established original speed (such as when passing another car).
2) Downshifting while braking, and then re-accelerating in a straight line (no corner, such as when braking for a car that is turning up ahead).
3) Downshifting while braking to enter a corner.
(OK, I suppose there might also be occasion to downshift to enter a corner without braking, like when turning off a road with a low speed limit).
I listed these three scenarios in what I considered to be their order of difficulty.
Most people when they're learning to drive a manual transmission learn to deal with downshifting for a corner first. They do it without active rev-matching or double-clutching (which, of course, is fine). By approaching and going through the corner at a casual rate, there isn't a big speed difference in the engine, intermediate shaft, or the rest of the driveline. The synchro's can handle the engagement, and it's possible to smoothly engage the clutch while exiting the corner. With additional practice it's possible to use this type of technique to get through corners reasonably quickly.
What's necessary to get through a corner as quickly as possible?
I'm going to try to avoid getting too far into handling and suspension stuff here (a BIG, fun topic). I'm also not discussing how to choose a line or maximize braking. You can learn this if you follow the links. I've also avoided the topic of determining the best RPM and car speed to shift at. I'm touching these topics right now just barely enough to discuss shifting technique.
The tires basically have a fixed amount of traction on any given road surface. This traction can be used for braking, cornering, accelerating, or a combination of braking-cornering or accelerating-cornering. If the car is cornering near the limit, there is no traction available for braking or accelerating. It's that simple! Everything about traction limits is explained in my article about traction circle. Great stuff to read.
The suspension needs to stay stable and the steering needs to be a smooth as possible.
In simple terms, a good high performance cornering consists of braking in a straight line, smoothly transitioning into the corner with relaxing the brakes, holding the corner for its tightest section trough the apex, transitioning out of the corner onto the gas, and accelerating out of the corner's exit.
The two transitions are the most important parts here. The idea is to have the tires near their maximum traction while braking, and then to smoothly change the direction of the traction to sideways for the corner. It's the same thing when exiting the corner. The direction of the traction is changed so that car accelerates.
I want to re-emphasize that the goal is to hold the level of the tires' traction CONSTANT, while CHANGING only the direction of traction.
If there is a gap between releasing the brakes and steering into a corner, two things occur. The suspension becomes unsettled (so it can't corner as well), and time that the tires could have been working is lost. Again, it's the same thing when getting out of the corner to accelerate.
How does shifting affect all of this? If there is any jerking of the driveline when cornering near the limit, this will cause the limit to be exceeded because of the additional load imposed (traction circle, again), even though momentary, on the tires. The driver has to be concerned with interpreting the corner and the situation, and monitoring how the car is responding, all while trying to operate a steering wheel, shift lever and three pedals with two hands and two feet.
Most 'performance' corners require a reduction in speed. This implies that a downshift of one or more gears be made so that the car is in the correct gear to accelerate out of it. A fifth - second downshift is a big change.
When driving a fast corner, the downshift can't be done before braking because the car's initial speed would cause the engine to over-rev.
On a rear wheel drive car, power needs to be lightly applied to the wheels during the corner so that the suspension is neutral or oversteering slightly (this is touching on another subject). This means that the downshift needs to be done before entering the corner.
This leaves one place to downshift: towards the end of the braking phase of the corner.
The shift has to be done while braking, and the engine needs to be sped up so that it will rev-match the new gear so that the tires won't be jerked past their limit. All three pedals need to be operated at once. This is where heel and toe operation of the brake and the gas is required. Depending on how many gears down the shift is, the abilities of the transmission's synchro's, and the general pre-disposition of the driver, double-clutching may also be called for to get the shift lever into gear.
Recall the three downshifting scenarios I described above. Many people that I've observed trying to learn to rev-match and double-clutch (including myself) will try to learn it while cornering. This is the most difficult downshift. It's much easier to practice without braking or cornering. The braking can be added second, and then finally the cornering.
Brake timing doesn't have to be an issue at first if you practice at reduced speeds with moderate deceleration. The main thing to focus on is to get the downshift timed so that it is completed just before you start to turn the steering wheel. As the clutch is re-engaged, the gas should be held at the level where it will be during maximum cornering force.
The whole process of learning to shift well takes lots of practice and determination, but its well worth the effort. Everyone that I know who's learned to do this really ENJOYS shifting. I believe all of them have a really strong preference for manual transmissions. I know that I do.
Please remember to stay safe. Try to keep away from traffic when trying something new.
To have a complete picture of performance driving, take a look at Corners, Setup, Traction circle, Using tires, Left foot braking, braking, advanced braking, WRC braking technique, Slipstreaming, drifting, cornering, Heel and toe driving technique and steering technique articles