Sunday, December 2, 2012

(181) Driving According to the Chump #8: Transmissions

In addition to the onboard videos I post to Vimeo, I also have a Youtube channel. On Youtube I have a lot of racing stuff, mostly the same stuff as on Vimeo, but I also use it for other things I am interested in as well, which is why I usually post the blog related videos to Vimeo instead of Youtube.

On my Youtube channel, I have video from the Exotic Autocross I did last month, as well as a couple of my more exciting and interesting race onboards. Some of these videos have a good amount of views, some of them have very little. But what is surprising is that in spite of the allure of a Ferrari or a Lamborghini, and the excitement of a race car crash, my most popular video continues to be this:


My feet.

Why?

I couldn't tell you. But I can make a couple of guesses, and the reality is probably a combination of all of these factors and more.

For one, good racing footwork videos are sort of hard to come by. Broadcasters do not show it much, and amateur racers are usually using the camera for recording the track. Many cars also do not have much room in the footwell, so getting cameras down there can be tough sometimes.

For two, drivers (like me) probably don't think videos about their feet are all that interesting. I certainly didn't expect that video to get very many views at all, much less over 10,000 (it's at 14,500 as I write). My second-most-watched video is also of my feet, but on my simulator pedals. That one's at 8,000 views. Not exactly front-page material, but compared to my other videos that is a lot.

And for three, I think people just want to learn about transmissions and how to use them. The major point of watching racing footwork is to see heel-toe, a downshifting technique.

So let's talk about using transmissions. Then I will talk about heel-toe and how to learn it (and it's permutations, like double-clutch shifting). I start with the basic mechanics because it is important to know how those work for various types of transmissions, so that you can apply the footwork accordingly. For conciseness, I will omit old technology that isn't really used any more, like sliding mesh transmissions, and I won't go into too much technical detail because that is readily available elsewhere if you're mechanically inclined. For now, anyway, this blog is about driving, and even doing the basics is going to make for a really long post. I will omit left-foot braking, because that is it's own post that I will do at some point.

I'll start with something modern, but basic (mechanically).


Constant Mesh

Constant mesh transmissions go by a lot of names. "Dog box" is one, another is "crash box", among others.

A constant mesh transmission is named so because if one gear is rotating, all the gears are rotating because they are all meshed with each other. The gears are separated from the drive wheels by bearings, and from the engine by the clutch. Moving the gear stick moves a selector fork which moves a smaller gear called a dog. Sometimes it's also called a "dog clutch". It's kinda both a clutch and a gear. The dog is what engages the gear to the wheels, so that the gear is not spinning freely on the bearing. In a 6-speed transmission, you will have 3 dogs for the forward gears; a dog for 1st and 2nd, another for 3rd and 4th, and a third for 5th and 6th. The dog just moves back and forth between the gears and meshes with the face (side) of the selected gear with it's teeth. Once a gear is engaged, the other gears continue to spin freely on their bearings until the driver shifts and a new gear is locked in.

So, since we have two gears trying to mesh teeth, they have to be spinning at a similar RPM in order to do so. If either one is going too fast relative to the other, such as, say, upshifting from 1st gear to 4th gear, the gears will not be able to engage and they will grind, adding some nice metal chunks to the transmission oil. Oil loves metal chunks.

Not really.

However, the transmission should be set up so that changing - briskly - from one gear to the next in sequence (for instance, 1st to 2nd) should be a very doable affair. The adjacent gears should be well within the "engagement window" for the dog, as long as you don't sit there for a hundred years with the clutch pushed in. If you do, the dog will lose too much RPM and fall out of the window.

If the dog and the gear are perfectly synchronized, the gear will engage without a sound. To say that is hard to do is the grandest of understatements. Fortunately, they do not have to be perfectly synchronized - they only need to be within a certain RPM window. If they are within the window, but not synchronized, the gear will make a "clack" noise as it is engaged. If you've ever sat next to a bike at a stop light while he selects first gear, you'll know the sound. It's a nice, satisfying "thwack".

But I don't ride bikes, so that's as much as I can say about that.

The method of working a constant mesh transmission in a car (usually a racing car these days) depends on the type of gear selector the car has. The car could have any number of selection methods. It could be a classic H-pattern selector like in most street manuals, or it could be a fore/aft lever, or it could be a paddle system; and these systems can have further functionality via electronics and/or hydraulics.

A dog box transmission, without a housing. The yellow bits are the selector forks.

H-pattern

You might find an H-pattern gear stick in an older Formula Ford or a GT car built before the turn of the century (that's Y2K, not 1900). Up until about the early 90s, F1 cars also had H-pattern dog boxes.

Firstly, in order to get moving, you need to almost simultaneously push the clutch in and select first gear. The reason being, the gear needs a little RPM relative to the dog in order to mesh while the car is sitting still. If both the gear and the dog have stopped rotating, neither are going to mesh because the teeth are almost certainly out of synch. Thus, if you sit there for more than 3 or 4 seconds with the clutch pushed in, and then try to select a gear, you probably won't be able to do it because the gears have stopped moving. To fix this, move the stick back to neutral and release the clutch pedal to get the gears moving again with the engine. Now push the clutch and promptly select first gear. Thwack. You're ready to go.

Shifting on the move is pretty much normal. Just push the clutch and move the stick to the next gear. As long as you don't dawdle the gear will go right in. It will probably make another thwack if you have a quiet engine. Same goes for up and down shifts. The fun begins when you want to stop using the clutch.

This is a more natural progression than it sounds. A lot of dog box drivers who think they are using the clutch actually are not. They may dab at it a little bit, but for a fast shift with a dog box the clutch doesn't have time to go all the way to disengagement and back before the shift is done. If it does, the driver is slow at shifting the dog box. Since the gears only need to be in a general RPM window to engage, the clutch is really only there for getting moving and for allowing the driver to pull the car out of gear if needed (like with a stuck throttle that literally holds the gear in place unless the torque is disconnected from the transmission - hopefully via a clutch and not an engine explosion).

So all you really need to do to make a shift in a dog box equipped car is to stop the engine torque, and to do  that all you have to do is lift off the throttle. Once you've done that you are just one quick, decisive movement away from a lightening fast shift to the next gear. This needs to be done, however, before the compression of the engine sends negative torque back through the transmission as it drags the car's speed down, making it hard to pull the car out of gear again. You solve this by putting pressure on the gear stick prior to lifting. Then, when you snap off the throttle, the gear will come out automatically and you can engage the next one, promptly. That's called "pre-load".

Now that we know how to use an H-pattern selector, the explanations for "sequential" shifters are going to be much quicker.


Sequential stick

You will find sequential sticks in current-day Formula Fords and F2000s, as well as in most modern GT racing cars like Porsche Cup cars and Le Mans GT cars from the late 90s to late 00s. A sequential stick has a stick in the same place as an H-pattern, but the stick is only able to move forward and back. You usually pull the stick backward for an upshift, and push it forward for a downshift. This is done for efficiency of movement - if you are accelerating, the G force is pushing you back, so it is easier to pull the stick backwards. When you are braking hard, the G force is pushing you forward, so it is easier to push the stick forward to downshift. Some of these sticks can be fairly stiff to move, so it helps a lot in long races.

When you move the stick, you are only moving a ratcheting drum with grooves cut in it. The selector forks are set in the grooves so when the drum rotates, it moves the appropriate selector fork (and thus, the dog) to the next lower or higher gear. The other internals of the gearbox are the same as before.

The ratcheting selector drum in a sequential dog box. The grooves are clearly visible.

Since most of the internals are the same, we use the same methods as the H-pattern for shifting. Clutch in and select first gear quickly, and apply pre-load to the stick before shifting. If you don't do it right you may find yourself stuck between gears, in what's called "false neutral". The gearbox is in neutral, but if you push forward or back on the stick to up or down shift, you will find a higher-numbered gear that is not next to "true neutral" (those being 1st and reverse). To fix a false neutral, you'll need to use the clutch and upshift or downshift to the gear you were originally trying to select.

If you are in true neutral, to get to reverse, you usually need to pull a knob or a lever to be able to select reverse. This is to keep drivers from going down too many gears, going past neutral, and selecting reverse in slow hairpins. While you are moving, first gear is usually as far down as you can go, at which point the stick will simply not be allowed to move forward.

As for pros and cons, it is much easier to do no-clutch shifts in a sequential, because the stick only has to go one direction and you don't need to find the gate for the next gear. Of course, you can't skip gears, so there is one downside.

Some stick sequentials have a hydraulic system instead of direct linkage. This makes it easier to change gear, especially in powerful cars.


Paddles

With a paddle system, you will generally have two paddles behind the steering wheel - the right one upshifts, and the left one downshifts. The paddles send signals to a hydraulic system that moves the same sort of ratcheting drum as before.

Shifting us stupid easy. Pull the right paddle to upshift. Pull the left one to downshift. You're done. The computer does the rest, including lifting the throttle and using the clutch, if necessary (most racing paddles don't use the clutch on upshifts, but street paddle systems usually use the clutch).

In racing cars, the clutch is usually manual as far as getting the car moving is concerned. Some cars retain a third pedal for this, others use a third paddle, or both a third and a fourth paddle behind the steering for the clutch. It goes back to automatic clutch for shifting.

The paddles on an F1 steering wheel. Top paddles shift, bottom paddles control the clutch.

But why would you have two clutch paddles? In racing cars that do a lot of standing starts, this works out to be very handy. In F1 cars, for instance, the driver will pull both paddles prior to the start. When the lights go on, the driver will let one of the paddles out to the slipping point of the clutch (let's say the left one), while keeping the other (right) one pressed down. When the start lights go out, the driver lets go completely of the right paddle, and continues holding the left paddle at the slipping point, gradually releasing it as he speeds up as normal. This way, stalls are reduced drastically, and the driver can get a more consistent start.


Synchromesh

By now you've probably figured out that normal street cars have very little of this behavior in their gearboxes.  Streets cars do use constant mesh gearboxes (remember, all the gears are meshed and spinning), but street cars have the addition of a new type of dog, called a synchronizer, or a cone clutch. That last name is pretty graphic as to how it works.

The dog still has to mesh with the face of the gear similarly to before, but this time there is a safety device between it and the gear. The cone clutch equalizes the RPM of the gear and the dog prior to meshing. This makes the gearboxes a bit quieter (no more "thwack" noise), and mush easier to use. But there is a downside. It adds quite a lot of resistance to the gearbox, and slows down shifts quite a bit.

You might think that simply removing or wearing out the cone clutches would turn your normal street transmission into a racing dog box. Unfortunately, it won't, because the dogs are designed with more teeth and are much harder to engage smoothly without the synchronizer. The engagement windows are much smaller. If you tried to shift it quickly like you might a dog box, you would just get an earful of grinding metal.

Synchromesh gearboxes generally have unsynchronized reverse gears, but, contrary to a somewhat popular belief, that is not what makes the "reverse gear whine" you hear while backing up. That's to do with helical versus straight cut gears.


Straight cut and helical gears

Straight cut (also called spur) gears are your traditional view of a gear. They have numerous straight teeth coming out of the gear. A helical gear has teeth cut diagonally along the circumference of the gear. Helical gears are used in street cars, and spur gears are used in racing cars.



Spur gear on top, helical gear on the bottom.


But why use these different gear designs at all? Well, helical gears are used in street cars because they are quiet. Spur gears whine when they rotate while meshed. Helical gears are stronger, since they have more area in contact with each other. But, for physical reasons I don't really understand, the transmission cases need to be stronger with helical gears. Since compactness and lightness is paramount in racing cars, that is why they utilize straight cut gears. Street cars also use straight cut gears for reverse because reverse is a lot less likely to be beaten on from bad shifting, so it doesn't need to be especially strong.


Downshifting

That's the basic mechanics and usage of gearboxes done and dusted, so now lets go a little more advanced.

Downshifting is probably the primary reason why my little foot-on-pedals video is my most popular. Downshifting is tricky to learn and takes a while to master. I had to practice on a simulator for weeks before I got decent at it, and months before I was doing it automatically.

Downshifting is tricky for the basic reason of needing to hit three pedals simultaneously with only two feet. But why do we have to hit all three pedals to begin with?

Think of a system of gears. Let's start with two gears, which are selectable, connected to an engine at one end and wheels at the other.

In 2nd gear, the ratio is 1:1 - meaning, for every revolution of the engine, the wheels also turn one revolution. In 1st gear, the ratio is 2:1, so for every two turns of the engine, the wheels turn once.

We start out moving forward in the lowest gear, and we reach 4,000 RPM after accelerating. This means that according to our ratio, the wheels are turning at 2,000 RPM. The engine starts to get noisy, so we decide to shift to 2nd gear.

Now, the wheel speed has not changed - we're still rolling at some MPH - but the ratio of engine RPM to wheel RPM has changed. Since the rotating parts of the engine are not connected to anything solid, the engine is the weakest link, so the engine loses RPM to match the wheels. Since the 2nd gear ratio is 1:1, the engine will spin down to 2,000 RPM, the same RPM as the wheels.

But what happens if we decide we want more noise and we shift back down to 1st gear? The same thing happens, the engine gets dragged up to the appropriate RPM to match the wheel speed and the gear ratio, in this case 4,000 engine RPM for 2,000 wheel RPM. Except, it is a lot harder to spin up an engine than it is to spin it down, and we get a big jolt, the car bucks, and our passenger is very mad at us.

In racing, this unsmoothness is quite disastrous. It could cause you to spin, crash, or outright break something in the drive line. So what's the solution?

The solution is to pause before releasing the clutch, and spin the engine up using the method it was designed with - the throttle. Upon releasing the clutch, the car should not buck or jolt.


Learning rev matching

Using the throttle to spin the engine up - or down - to match the wheel speed for the gear you are in is called rev matching. The process goes like this.

You're in 3rd gear and you want to change down to 2nd, smoothly. Put the clutch in, select the lower gear as normal, use the throttle to raise the engine RPM, release the throttle, then release the clutch.

It sounds complex on paper, but there is only one new step to add - using the throttle. So let's practice that first.

The best way to do that is to sit with the car running, in neutral, with the hand brake on. We're just going to play with tapping the throttle. Tap the throttle, and try to hit 2,000 RPM. Keep tapping until you hit 2,000 RPM every time. As long as you get within about 200 RPM of your target, the shift would likely have been a smooth one. Different cars have different behaviors here - some rev up really fast, others are slower. You'll have to use different amounts of throttle to hit different RPMs in different cars.

Once you're comfortable "blipping" the throttle, try downshifting on the move. Accelerate up to 3rd gear, then hold speed. Clutch in, select 2nd gear, give the throttle a tap just like you were doing while stationary, and release the clutch smoothly. If you did it just right, you should not feel anything.

While you're practicing this, keep in mind that using more revs is better than using less. It is easier to be smooth - and easier on the car - if you over-rev than if you under-rev.


Heel-toe

It gets tricky when we want to do this rev-matching business while braking. If we're braking with the right foot and clutching with the left foot, what do we do about the throttle? The solution is heel-toe.

To practice heel-toe, as before, put the car in neutral while stationary. Put the ball of your right foot on the brake. Half of your foot will probably be hanging off the pedal. This is what we want. Now pick up your heel and, while maintaining pressure with the ball of your foot, place you heel over the throttle. Now press with your heel, again while maintaining pressure with the ball of your foot. Try to hit 2,000 RPM again and practice until you can do it every time, without moving the brake pedal.

The position. Assume it.

Now you should be able to transition to rolling practice just as you did before, only this time you will be braking while downshifting from 3rd to 2nd. Once your shifts are perfectly smooth, you've learned heel-toe.


Double-clutch

There is very little point to learning double-clutch with modern transmissions, but I will describe it.

Double clutch shifts are just like regular shifts, except there is one more step added - releasing the clutch while the stick is in neutral. A double-clutch shift goes like this:

Clutch in, move gear stick to neutral, clutch out, clutch in, move gear stick to next gear, clutch out.

The reason to do this is to basically do the synchronizer's job. It makes the dog synchronize with the gear, assuming the engine is at the right RPM. During a double-clutch downshift, the throttle is blipped while the clutch is out and the gear stick is in neutral.

There is very little reason to double-clutch with any constant mesh transmission, whether synchronized or a dog box. It is only useful in old "sliding mesh" transmssions, where double clutch was required to get the gears to spin so that they could engage.

Ten tips, and then this tome is complete.


Ten tips for using (modern) transmissions:

1. For a dog box, clutch in and select first gear quickly.

2. For learning heel-toe, it is best to first focus on rev-matching, then incorporate braking while downshifting once that is comfortable
.
3. When blipping, more revs is better than too little revs.

4. If downshifting and rev-matching for each gear in sequence is too much to handle, try skipping gears and making bigger blips to compensate. For instance, instead of 4th > 3rd> 2nd, just go 4th > 2nd, with a larger blip.

5. Downshifting is not for slowing down the car. It is for selecting the proper gear. The brakes slow down the car. Unless the car is badly set-up, then the engine can be used to help slow the car (for instance, the brakes are inadequate as in old Grand Prix cars).

6. When using a dog box, pre-loading the gear stick prior to shifting will make your life a lot easier.

7. If you get stuck in false neutral using a sequential stick, use the clutch to get back into the gear you were trying to shift to.

8. Double-clutch is pretty much useless in constant mesh transmissions.

9. If you read this entire thing, you are awesome.

10. The number one goal of heel-toe downshifting is to maintain constant pressure on the brake pedal, because slowing down is the most important thing a car does.