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ThecarT ransmissionBible - howtransmissionsandgearboxworkincludingmanuals, automatics,clutch,CVT,crash gearboxes,differentials,limited-slipdifferentials, 2wd, 4wd, awdandmuchmore.

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The Transmission BibleI am a pro-car, pro-motorbike petrolhead into basic maintenance. This site is self-published to spread my

knowledge. By reading these pages, you agree to indemnify, defend and hold harmless the author, any

sponsors and/or site providers against any and all claims, damages, costs or other expenses that arise

directly or indirectly from you fiddling with your vehicle as a result of what you read here. If you hurt your

vehicle or yourself, don't blame me. 

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Transmission, or gearbox?

Latest blog entry

10/15/2012 07:00 AM

Remember that time you had trouble getting petrol? Sorry about that.  

If you're a reader in the UK, you'll likely have no problem remembering the fuel protests back in 2000. Did you know I was theinstigator of what ballooned into a national crisis? A crisis that had fuel drivers blockading their own refineries? That had airports

shut down because they had no fuel for the aircraft? That caused Blair to finally admit he was propping up the National Health

Service with money fleeced from motorists? The crisis that, for the first and only time in the recent memory of UK fuel pricing,

caused the government to drop the tax rate? Yeah you can blame me for that.Late spring 2000, a colleague of mine and I were moaning one morning about the price of petrol at the time and we decided that

apathy had ruled long enough and that it was time to do something about it. We set up a website called Boycott The Pumps and

filled it with interesting facts and figures. At the time I was running the Speedtrap Bible - a site I later sold - so I had rather a large

following of readers. One simple email to all those readers was all it took. We decided to organise a pump boycott - a mechanism

that has notoriously failed to go anywhere every time it was tried before, because of apathy. "But if I don't buy petrol today, I'll just

 buy it tomorrow, so what's the point?". The point isn't that you're buying petrol on another day, it's that you're sending your elected

officials a very clear message. And holy crap this time it actually worked. It went nationwide when the Daily Mail picked up the

original email from me, and publicised it on the AP Wire and the front page of their paper on the same day. The Sun, Mirror, Star 

and Telegraph all then picked it up and it just ballooned from there. I must have given 100 radio and newspaper interviews and I

was on TV for what seemed like 3 weeks straight from local news stories to the BBC nationally. What really caught the attention of 

the public though was when the farmers and truckers started to get vocal - rolling roadblocks on the motorways, blockaded

refineries and such. At that point it had a much snappier name - wish we'd thought of this originally, but we didn't - it became Dump

The Pumps. I had to distance myself from it at that point because the press were invading every aspect of my life, and my family's

life, and the life of a lot of my friends. But by that point it didn't matter - the damage had been done. By September 2000 there werequeues at petrol stations all across the country. WW2 jerry cans were the hot item to buy and military surplus stores ran out of them.

Shortages were everywhere, the refineries were blockaded, trucks were being turned away in France, aircraft weren't landing. Blair even went on TV and told everyone not to panic, and that was brilliant because then people reallystarted to panic. That was when

the army reserve were called out to try to maintain some order and use their fuel trucks to supply forecourts across the country.

So what was the point of it all? On the day of the original protest, it worked - petrol retailers all across the country reported much,much less take than normal. I proved that one person can make a difference. Yes the price of petrol has continued to go up ever 

since, and the drop in fuel duty at the time was only 1p. But put that into perspective : no government before or since has

ever lowered fuel duty. It's not the amount that counts. It's that I forced them to do it using a grass-roots campaign. Even today's UK 

government is extremely wary of what happened in 2000, to the point where I've seen it referred to several times since in a "we

don't want that to happen again" sort of way.

An album of the various newspaper headlines is here if you're curious.  Dump the pumps. The local paper - the Bracknell news - has

the story of me giving up the initial protest.Chris - www.carbibles.com 

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That question depends on which side of the Atlantic you're on. To the

Europeans, it's a gearbox. To the Americans, it's a transmission. Althoughto be truthful, the transmission is the entire assembly that sits behind the

flywheel and clutch - the gearbox is really a subset of the transmission if you want to split hairs.Either way, this page aims to deal with the whole idea of getting the

power from your engine to the ground in order to move your car (or bike)forwards.

Recommended link: Butts of Bawtry

Manual gearboxes - what, why and how?

From the Fuel & Engine Bible you know that the pistons drive the maincrank in your engine so that it spins. Idling, it spins around 900rpm. Atspeed it can be anything up to 7,500rpm. You can't simply connect a set

of wheels to the end of the crank because the speed is too high and too

variable, and you'd need to stall the engine every time you wanted tostand still. Instead you need to reduce the revolutions of the crank downto a usable value. This is known as gearing down - the mechanical process

of using interlocking gears to reduce the number of revolutions of something that is spinning.

Check out AmericanMuscle for all your aftermarket Mustang parts. 

A quick primer on how gears work

In this case I'm talking about gears meaning 'toothed wheel' as oppose to

gears as in 'my car has 5 gears'. A gear (or cog, or sprocket) in its mostbasic form is a flat circular object that has teeth cut into the edge of it.

The most basic type of gear is called a spur gear, and it has straight-cutteeth, where the angle of the teeth is parallel to the axis of the gear.

Wider gears and those that are cut for smoother meshing are often cutwith the teeth at an angle, and these are called helical gears. Because of 

the angle of cut, helical gear teeth have a much more gradualengagement with each other, and as such they operate a lot more

smoothly and quietly than spur gears. Gearboxes for cars and motorbikes

almost always use helical gears because of this. A side effect of helicalgears is that if the teeth are cut at the correct angle - 45 degrees - a pairof gears can be meshed together perpendicular to each other. This is auseful method of changing the direction of movement or thrust in a

mechanical system. Another method would be to use bevel gears.

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The number of teeth cut into the edge of a gear determines its scalarrelative to other gears in a mechanical system. For example, if you mesh

together a 20-tooth gear and a 10-tooth gear, then drive the 20-toothgear for one rotation, it will cause the 10-tooth gear to turn twice. Gear

ratios are calculated by divinding the number of teeth on the output gearby the number of teeth on the input gear. So the gear ratio here isoutput/input, 10/20 = 1/2 = 1:2. Gear ratios are often simplified to

represent the number of times the output gear has to turn once. In thisexample, 1:2 is 0.5:1 - "point five to one". Meaning the input gear has tospin half a revolution to drive the output gear once. This is known asgearing up.

Gearing down is exactly the same only the input gear is now the one with

the least number of teeth. In this case, driving the 10-tooth gear as theinput gear gives us output/input of 20/10 = 2/1 = 2:1 - "two to one".

Meaning the input gear has to spin twice to drive the output gear once.

By meshing many gears together of different sizes, you can create a

mechanical system to gear up or gear down the number of rotations very

quickly. As a final example, imagine an input gear with 10 teeth, asecondary gear with 20 teeth and a final gear with 30 teeth. From the

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input gear to the secondary gear, the ratio is 20/10 = 2:1. From thesecond gear to the final gear, the ratio is 30/20 = 1.5:1. The total gearratio for this system is (2 * 1.5):1, or 3:1. ie. to turn the output gear

once, the input gear has to turn three times.This also neatly shows how you can do the calculation and miss the middlegear ratios - ultimately you need the ratio of input to output. In this

example, the final output is 30 and the original input is 10. 30/10 = 3/1 =3:1.

Collections of helical gears in a gearbox are what give the gearing down

of the speed of the engine crank to the final speed of the output shaft

from the gearbox. The table below shows some example gear ratios for a5-speed manual gearbox (in this case a Subaru Impreza).

Gear RatioRPM of gearbox output shaft

when the engine is at 3000rpm

1st 3.166:1 947

2nd 1.882:1 1594

3rd 1.296:1 2314

4th 0.972:1 3086

5th 0.738:1 4065

Final drive - calculating speed from gearbox ratios . It's important tonote that in almost all vehicles there is also a final reduction gear. This is

also called a final drive or a rear- or front-axle gear reduction and it'sdone in the differential with a small pinion gear and a large ring gear (see

the section on differentials lower down the page). In the Subaru exampleabove, it is 4.444:1. This is the final reduction from the output shaft of 

the gearbox to the driveshafts coming out of the differential to thewheels. So using the example above, in 5th gear, at 3000rpm, thegearbox output shaft spins at 4065rpm. This goes through a 4.444:1

reduction in the differential to give a wheel driveshaft rotation of 914rpm.

For a Subaru, assume a wheel and tyre combo of 205/55R16 giving acircumference of 1.985m or 6.512ft (see The Wheel & Tyre Bible). Eachminute, the wheel spins 914 times meaning it moves the car (914 x

6.512ft) = 5951ft along the ground, or 1.127 miles. In an hour, that's

(60minutes x 1.127miles) = 67.62. In other words, knowing the gearboxratios and tyre sizes, you can figure out that at 3000rpm, this car will bedoing 67mph in 5th gear.

Making those gears work together to make a gearbox

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If you look at the image here you'll see a the internals of a genericgearbox. You can see the helical gears meshing with each other. The

lower shaft in this image is called the layshaft - it's the one connected tothe clutch - the one driven directly by the engine. The output shaft is theupper shaft in this image. To the uneducated eye, this looks like amechanical nightmare. Once you get done with this section, you'll be able

to look at this image and say with some authority, "Ah yes, that's a 5-speed gearbox". 

So how can you tell? Well look at the output shaft. You can see 5 helicalgears and 3 sets of selector forks. At the most basic level, that tells youthis is a 5-speed box (note that my example has no reverse gear) But how

does it work? It's actually a lot simpler than most people think althoughafter reading the following explanation you might be in need of a brain

massage.With the clutch engaged (see the section on clutches below), the layshaft

is always turning. All the helical gears on the layshaft are permanentlyattached to it so they all turn at the same rate. They mesh with a seriesof gears on the output shaft that are mounted on sliprings so they

actually spin around the output shaft without turning it. Look closely at

the selector forks; you'll see they are slipped around a series of collars

with teeth on the inside. Those are the dog gears and the teeth are thedog teeth. The dog gears are mounted to the output shaft on a splined

section which allows them to slide back and forth. When you move the

gear stick, a series of mechanical pushrod connections move the variousselector forks, sliding the dog gears back and forth.

In the image to the left, I've rendered a close-up of the area between

third and fourth gear. When the gearstick is moved to select fourth gear,

the selector fork slides backwards. This slides the dog gear backwards on

the splined shaft and the dog teeth engage with the teeth on the front of the helical fourth gear. This locks it to the dog gear which itself is lockedto the output shaft with the splines. When the clutch is let out and the

engine drives the layhshaft, all the gears turn as before but now thesecond helical gear is locked to the output shaft and voila - fourth gear.

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Grinding gears. In the above example, to engage fourth gear, the doggear is disengaged from the third helical gear and slides backwards to

engage with the fourth helical gear. This is why you need a clutch and it'salso the cause of the grinding noise from a gearbox when someone iscocking up their gearchange. The common misconception is that thisgrinding noise is the teeth of the gears grinding together. It isn't. Ratherit's the sound of the teeth on the dog gears skipping across the dog teeth

of the helical output gears and not managing to engage properly. Thistypically happens when the clutch is let out too soon and the gearbox isattempting to engage at the same time as it's trying to drive. Doesn'twork. In older cars, it's the reason you needed to do something called

double-clutching.Double-clutching, or double-de-clutching (I've heard it called both) was aprocess that needed to happen on older gearboxes to avoid grinding the

gears. First, you'd press the clutch to take the pressure off the dog teeth

and allow the gear selector forks and dog gears to slide into neutral, awayfrom the engaged helical gear. With the clutch pedal released, you'd 'blip'the engine to bring the revs up to the speed needed to engage the next

gear, clutch-in and move the gear stick to slide the selector forks and doggear to engage with the next helical gear.

The synchromesh - why you don't need to

double-clutch.

Synchros, synchro gears and synchromeshes - they're all basically the

same thing. A synchro is a device that allows the dog gear to come to aspeed matching the helical gear before the dog teeth attempt to engage.

In this way, you don't need to 'blip' the throttle and double-clutch to

change gears because the synchro does the job of matching the speeds of 

the various gearbox components for you. To the left is a colour-codedcutaway part of my example gearbox. The green cone-shaped area is the

syncho collar. It's attached to the red dog gear and slides with it. As itapproaches the helical gear, it makes friction contact with the conical

hole. The more contact it makes, the more the speed of the output shaft

and free-spinning helical gear are equalised before the teeth engage. If the car is moving, the output shaft is always turning (because ultimatelyit is connected to the wheels). The layshaft is usually connected to theengine, but it is free-spinning once the clutch has been operated. Because

the gears are meshed all the time, the synchro brings the layshaft to theright speed for the dog gear to mesh. This means that the layshaft is nowspinning at a different speed to the engine, but that's OK because theclutch gently equalises the speed of the engine and the layshaft, either

bringing the engine to the same speed as the layshaft or vice versadepending on engine torque and vehicle speed.

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So to sum up that very long-winded description, I've rendered up ananimation - when you see parts of a gearbox moving in an animation, it'llmake more sense to you. What we have here is a single gear being

engaged. The layshaft the blue shaft with the smaller helical gearattached to it. To start with, the larger helical gear is free -spinning on itsslip ring around the red output shaft - which is turning at a different

speed because it's connected to the wheels. As the gear stick is moved,the gold selector collar begins to slide the dog gear along the splines on

the output shaft. As the synchromesh begins to engage with the largehelical gear, the helical gear starts to spin up to speed to match theoutput shaft. Because it is meshed with the gear on the layshaft, it in turn

starts to bring the layshaft up to speed too. Once the speed of everything

is matched, the dog gear locks in place with the output helical gear andthe clutch can be engaged to connect the engine to the wheels again.

 

 

o 

o  00:00 / 00:00

 

o 

o 

o 

o 

o 

o 

 

o o o o 

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What about reverse?

Reverse gear is normally an extension of everything you've learned above

but with one extra gear involved. Typically, there will be three gears thatmesh together at one point in the gearbox instead of the customary two.There will be a gear each on the layshaft and output shaft, but there will

be a small gear in between them called the idler gear. The inclusion of this extra mini gear causes the last helical gear on the output shaft to

spin in the opposite direction to all the others. The principle of engagingreverse is the same as for any other gear - a dog gear is slid into place

with a selector fork. Because the reverse gear is spinning in the oppositedirection, when you let the clutch out, the gearbox output shaft spins the

other way - in reverse. Simple. The image on the left here shows thesame gearbox as above modified to have a reverse gear.

Crash gearboxes or dog boxes.

Having gone through all of that business about synchromeshes, it's worth

mentioning what goes on in racing gearboxes. These are also known ascrash boxes, or dog boxes, and use straight-cut gears instead of helical

gears. Straight-cut gears have less surface area where the gears contacteach other, which means less friction, which means less loss of power.

That's why people who make racing boxes like to use them.Normally, straight-cut gears are mostly submerged in oil rather thanrelying on it sloshing around like it does in a normal gearbox. So the

extra noise that is generated is reduced to a (pleasing?) whine by thesound-deadening effects of the oil.

But what is a dog box? Well - motorbikes have been using them since the

dawn of time. Beefing the system up for cars was the brainchild of aracing mechanic who wanted to provide teams with a quick method of 

altering gear ratios in the pits without having to play "chase the syncro

hub ball bearings" as they fell out on to the garage floor.

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Normal synchro gearboxes run at full engine speed as the clutch directlyconnects the input shaft to the engine crank. Dog boxes run at a half to athird the speed of the engine because there is a step-down gear before

the gearbox. The dog gears in a dog box also have less teeth on themthan those in a synchro box and the teeth are spaced further apart. Sorather than having an exact dog-tooth to dog-hole match, the dog teeth

can have as much as 60° "free space" between them. This means thatinstead of needing an exact 1-to-1 match to get them to engage, you

have up to 1/6th of a rotation to get the dog teeth pressed togetherbefore they touch each other and engage. The picture on the right showsthe difference between synchro dog gears and crash box dog gears.

So the combination of less, but larger dog teeth spaced further apart, and

a slower spinning gearbox, allegedly make for an easier-to-engage crashbox. In reality, it's still quite difficult to engage a crash box because youneed exactly the right rpm for each gear or you'll just end up grinding the

dog teeth together or having them bounce over each other. That results inmetal filings in your transmission fluid, which ultimately results in an

expensive and untimely gearbox rebuild.

But  it is more mechanically reliable - it's stronger and able to deal with a

lot more power and torque which is why it's used in racing.So in essence, a dog box relies entirely on the driver to get the

gearchange right. Well - sort of. Nowadays the gearboxes have ignition

interrupters connected to them. As you go to change gear, the ignitionsystem in the engine is cut for a fraction of a second as you come to thepoint where the dog teeth are about to engage. This momentarily removes

all the drive input from the gearbox making it a hell of a lot easier to

engage the gears. And when I say 'momentary' I mean milliseconds.Because of this, it is entirely possible to upshift and downshift withoutusing the clutch (except from a standstill). Pull the gear out of first, and

as you blip the throttle to get the engine to about the right speed, theignition is cut just as the gears engage.

Even the blip of the throttle isn't necessary now either - advanced dog

boxes can also attempt to modify the engine speed by adjusting thethrottle input to get the revs to the right range first.Of course even with all this cleverness, you still get nasty mechanicalwear from cocked up gear changes, but in racing that doesn't matter - thegearbox is stripped down and rebuilt after each race.

Before the gearbox - the clutch

So now you have a basic idea of how gearing works there's a second item

in your transmission that you need to understand - the clutch. The clutchis what enables you to change gears, and sit at traffic lights withoutstopping the engine. You need a clutch because your engine is running all

the time which means the crank is spinning all the time. You needsomeway to disconnect this constantly-spinning crank from the gearbox,both to allow you to stand still as well as to allow you to change gears.The clutch is composed of three basic elements; the flywheel, thepressure plate and the clutch plate(s). The flywheel is attached to the end

of the main crank and the clutch plates are attached to the gearboxlayshaft using a spline. You'll need to look at my diagrams to understandthe next bit because there are some other items involved in the basic

operation of a clutch. (I've rendered the clutch cover in cutaway in thefirst image so you can the inner components.) So here we go.

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In the diagram here, the clutch cover is bolted to the flywheel so it turnswith the flywheel. The diaphragm springs are connected to the inside of 

the clutch cover with a bolt/pivot arrangement that allows them to pivotabout the attachment bolt. The ends of the diaphragm springs are hookedunder the lip of the pressure plate. So as the engine turns, the flywheel,clutch cover, diaphragm springs and pressure plate are all spinning

together.

The clutch pedal is connected either mechanically or hydraulically to a

fork mechanism which loops around the throw-out bearing. When youpress on the clutch, the fork pushes on the throw-out bearing and it slidesalong the layshaft putting pressure on the innermost edges of the

diaphragm springs. These in turn pivot on their pivot points against theinside of the clutch cover, pulling the pressure plate away from the back

of the clutch plates. This release of pressure allows the clutch plates todisengage from the flywheel. The flywheel keeps spinning on the end of the engine crank but it no longer drives the gearbox because the clutch

plates aren't pressed up against it.

As you start to release the clutch pedal, pressure is released on thethrow-out bearing and the diaphragm springs begin to push the pressureplate back against the back of the clutch plates, in turn pushing themagainst the flywheel again. Springs inside the clutch plate absorb the

initial shock of the clutch touching the flywheel and as you take your footoff the clutch pedal completely, the clutch is firmly pressed against it. Thefriction material on the clutch plate is what grips the back of the flywheel

and causes the input shaft of the gearbox to spin at the same speed.

Burning your clutch  You might have heard people using the term 'burning your clutch'. This iswhen you hold the clutch pedal in a position such that the clutch plate is

not totally engaged against the back of the flywheel. At this point, theflywheel is spinning and brushing past the friction material which heats it

up in much the same was as brake pads heat up when pressed against aspinning brake rotor (see the Brake Bible). Do this for long enough and

you'll smell it because you're burning off the friction material. This canalso happen unintentionally if you rest your foot on the clutch pedal in the

course of normal driving. That slight pressure can be just enough torelease the diaphragm spring enough for the clutch to occasionally lose

grip and burn.A slipping clutch The other term you might have heard is a 'slipping clutch'. This is a clutch

that has a mechanical problem. Either the diaphragm spring has weakenedand can't apply enough pressure, or more likely the friction material iswearing down on the clutch plates. In either case, the clutch is notproperly engaging against the flywheel and under heavy load, like

accelerating in a high gear or up a hill, the clutch will disengage slightly

and spin at a different rate to the flywheel. You'll feel this as a loss of power, or you'll see it as the revs in the engine go up but you don'taccelerate. Do this for long enough and you'll end up with the above - aburned out clutch.

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something, a small donation to help pay down my car loan would be appreciated. Thank y

Motorcycle 'basket' clutches

It's worth spending a moment here to talk about basket clutches as foundon some Yamaha motorbikes. Even though the basic principle is the same(sandwiching friction-bearing clutch plates against a flywheel), the design

is totally different. If nothing else, a quick description of basket clutcheswill show you that there's more than one way to decouple the a spinningcrank from a gearbox.Basket clutches need to be compact to fit in a motorbike frame so they

can't have a lot of depth to them. They also need to be readily accessible

for mechanics to be able to service them with the minimum amount of fuss, something that's near impossible with regular car clutches. A basketclutch has a splined clutch boss bolted to the shaft coming from the

engine crank with strong springs. Metal pressure plates slide on to thisshaft, in alternating sequence with friction material clutch plates. The

clutch plates are splined around the outside edge, where they fit into slotsin an outer basket - the clutch housing. The clutch housing is bolted on tothe layshaft which runs back through the middle of the whole mechanismand into gearbox. Clever, but as usual, not much use without a picture, so

here you go.

In operation, a basket clutch is simplicity itself. A throw-out bearing

slides around the outside of the layshaft and when you pull the clutchlever, the throw-out bearing pushes against the clutch boss. The clutchboss compresses the clutch springs and removes pressure from the wholeassembly. The friction plates now spin freely in between the pressure

plates. When you let the clutch out, the springs push the clutch boss inagain and it re-asserts the pressure on the system, crushing the frictionand pressure plates together so they grip. And there you have a second

type of clutch.You should now feel proud that with all your newfound (and somewhat

geeky) understanding of clutches, you can go about your business safe inthe knowledge that you sort of understand how all this spinning, geared-and-splined witchcraft works.

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These pages were last updated on 19th July 2012.

Copyright © Chris Longhurst 1994 - 2012 unless otherwise noted.

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