Gearbox explanation

[Andy_Pagin]

I'm planning to do the DAS this spring, meanwhile I'd like to get my head around how a bike gearbox actually works, bit tricky as I'm on a twist & go. Anyone got a reasonably simple diagram/explanation I could study? I find I get on better using anything mechanical if I know how it actually works.
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Yamaha Vity -> YBR125 -> FZS600 Fazer -> FZ1-S Fazer

[Fizzer Thou]

It is a very complicated piece of engineering,better explained by

https://uk.foxstart.com/search.php?q=motorbike+gearbox&cx=009900900170867307223:6nxc54q8niu&cof=FORID:9&ie=UTF-8&rls=en:uk:p&src=ffsb#920
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Always have a 'Plan B'

[mad4it028]

works just the same as your mountin bike gears apart from the clutch which basicly alows the engine to keep spinning without turning the gear box when pulled in

thats the simple way of looking at it
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[Vincent]

Basicaly, you have two, ("Main" or "Drive"and "Lay"), shafts with gears of varying sizes on each. The gears can slide up and down the shafts and mesh with each other to provide the different ratios. Above the shafts are two smaller shafts with selector forks on. the "prongs" of these forks fit into grooves on "Dogs" which are like the gears in that they slide up and down the shafts, but they don't have any teeth, (gears). The dogs, when moved by the selector forks, move the gears into the correct position. The gears on the drive shaft spin with the shaft to produce the drive to the back wheel.

Best I can do.
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[stinkwheel]

hth
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Teflon-Mike]

I'm a mechanical engineer by training; I spent eons sat in dusty lecture rooms learning about gear forms tooth loadings and all that kind of thing..... but opening on up? STILL a pandora's box!

They are full of 'bits' intricate little mechanisms that move stuff around, and hold them in place! Thrust washers, bearings and splined shafts, THEN there are the actual cogs!

Principle is very simple....

Imagine two rubber wheels of the same size, pushed together so that when you turn one, friction between them turns the other, but in the opposite direction.

Wheel has circumference; which is mathmatiucally pi time diameter, but if the wheels are teh same size, circumferences are same distance so one turn of drive wheel causes one opposite rotation of
driven wheel, right?

OK... Big wheel / Small wheel.....

If you have a small wheel, distance round circumference is small; big wheel, circumference bigger..... BUT when they touch, surface speed is the same, or they would 'slip'....

So, for small wheel to travel same distance as big wheel, it has to turn faster....

This means; if you have small drive wheel, turning big driven wheel, you have to turn the little wheel lots of times, very fast to make bigger wheel turn less revs more slowly.....

This is 'Reduction' Gearing

If you drive the big wheel against little wheel, you can turn it quite slow, but little wheel will go quite fast.

This is 'Over-Drive' gearing

Got the idea?

Now! MOST car & motorbike gear-boxes provide reduction gearing.

Wheel; roughly what? 17" diameter with maybe a 180/50 section tyre on it. That's 430mm of wheel, and two 90mm sidewalls.... lets call it 600mm for round numbers..... times pi (3.142.......) we get 1900mm, lets call it 2m.....

Now lets work in Kph for easy maths. We want to go 120Kph....

How fast does the wheel have to turn?

Well, each wheel rev gives us 2m of road. So 100Km would take 60,ooo wheel revolutions.....

We want to do that in one hour, so, 60 minutes in an hour, we'd want to do 1000 revs per minute... that's RPM....

OK.... we have engine..... its delivering 'power' on a shaft, spinning at some number of revs per minute.... whats it say on the rev counter? Tick-over is something like 1,200 rpm, red line is at something like 12,ooo rpm.

Lets say our engine is a little one, and we need to be reving it up somewhere near max power to do 120Kph..... for easy reckoning, LETS say we need to rev it to 10,000rpm at the crank....

So, crank turns 10,000 revs, we want the wheel to turn 1000 revs... so we need exactly 10:1 reduction gearing.....

Or a gear pair where little wheel (on crank) is turning big wheel (on rear wheel axle) 10x slower...

So, if we have a gear on the crank with 20 teeth, we need a gear on the back wheel with 10x that number of teeth.... 200...

BIT inconvenient, that. If we had a 'direct drive' from crank to rear wheel, we'd put a sproket on the crank, maybe 50mm in diameter, and we'd have to have one on the back wheel 500mm in diameter.... back wheel is ONLY 430mm remember, it would be bigger than the ruddy rim!

But you get the idea, and that wouldn't be the only inconvenience!

Directly driven, we'd have no way to interupt drive between crank and wheel, so when we wanted to stop, we'd have to stall the engine, AND with only one gear, we'd have to vary speed entirely with the throttle... be like riding every where in top gear, ALL the time..... not very easy or practical.

BUT, if you look at antique motorcycle's they DID run direct drive; but with a belt (much like your scooter)

The belt was looped over a pully on the crank and another bigger one as big as the wheel rim on the back wheel, that gave necessary 'reduction' to match engine and road speeds.

But left 'loose' the belt could slip, so the engine didn't have to be stalled when you stopped, and was put under 'tension' by an idler pulley being pressed against it to make it grip and act like a crude clutch......

At THIS point I will detract and mention the coffee grinder Zenith, motorcycle.

https://motorbike-search-engine.co.uk/classic_bikes/zenith_gradua_1912.jpg

THIS is particularly relevent to you, as it pioneered a novel means of varying the gear ratio 'on the move'..... which is STILL in use, and not much different to the 'variator' on your scooter....

Right; pioneering era, pre WW1. Bikes had these hide belt drives with tensioner pully 'clutch'. And the gear ratio was a compromise, between speed on the flat, and pulling power up hills.

Back to riuding every where in one gear; make that gear 'tall', or like big wheel driving little wheel, a 'higher' ratio, then back wheel will turn faster and bike will go quicker.... BUT, come to a hill, putting more load on the engine, a lower ratio would give more force....

Better mention a bit of physics; power = force x speed.

Straight & level, force is the drag of mostly wind resistance, which increases with speed.

Top speed is achieved when the FORCE you get at the back wheel balences the FORCE of drag.

Come to a hill, and you have same 'drag', but you also have to do 'work' against gravity climbing; the force is basically mass x acceleration due to gravity, or at least a fraction of it depending how steep the hill is....

So with increased load on the engine from going up a hill; you either have to slow down, or make more force.... and possibly both.

Bit MORE physics for you; the ratio between gear pairs INCREASES force in direct proportion to the DECREASE in speed.

So, big wheel turning little wheel makes little wheel turn faster, but with less force on it. little wheel turning big wheel, makes big wheel turn slower, but magnifies the force.

That MEANS that the 10X reduction from crank to wheel is magnifying the force on the wheel 10x.

Back to our hill... be really useful if we could say increase the reduction from 10x to 20x, and although we'd reduce speed to just 60Kph, we'd DOUBLE the force available to haul us up this hill.....

BUT, now we would need a rear wheel sproket TWICE the size of the wheel!

Simple solution; 'stage' the gearing.

If we had a gear on the crank, 50mm in diameter, turning a lay-shaft with a gear on it 100mm in diameter, we'd get a 2:1 reduction, or lay shaft would turn half speed. We could THEN put a 50mm gear on the lay shaft and drive the rear wheel with a sproket only 250mm diameter, instead of 500, right!

OK, lets do that.... only NOW we cant get to over 60kmh.... great up hills though....

OK, lets LEAVE the 500mm sproket on as well, and like a push bike gear set, when we want to change gear..... we've got to the top of the hill and want to come down the other side... slip the chain off the 500mm sproket and put it on the 250.....

Its that simple......

And the clever bit of push bike gears is the Campangnioloa invented 'De-Rail-er' device that shifts the chain between the two sprokets on the move, AND takes up the slack in the chain to account for teh different distance around teh different sized sprokets.

BUT thats a good forty years ahead of ourselves...

You get the idea of the two drive ratio's being 'useful'... one for speed, one for climbing.

And in the early days, that's how they got more than one ratio.

They had two pulleys on the back wheel. Often one on either side of the wheel, and they stopped at the bottom of a hill and swapped the drive belt between them to 'change gear'.....

OK... back to the Zenith coffee-grinder.... so named becouse of the coffee grinder handle above the petrol tank.......

SOME-ONE had a wonderful idea.... rather than having two seperate pulleys, wouldn't it be convenient if they could have ONE pulley, that changed size......

We have your scooter 'variator'......

Imagine a fan-belt pulley off a car. Its made of two plates, with a V-Shape in the middle, and the fan belt sits in the groove....

Now, chip out the sport welds, and mount those two half pulleys on bolts so that you can push them further apart.....

As you push the plates apart, the gap between them gets bigger and the belt would fall in closer to the middle to fill the gap, and run on a smaller diameter.....

SO... if you had TWO such pulleys..... you could get one to pull 'in' and one to push out, and if you were really clever, get the distance around the pulleys to stay the same, hence keeping the tension on the belt, BUT the drive pulley getting bigger as the driven pulley gets smaller..... the gear ratio between the two increasing....

THAT is your scooters 'variator' transmission. Two flexible pulleys and a rubber band, and a series of bob weights shifting the plates to change the ratio as speed increases..... which is ALSO how the automatic clutch works...... bit like a drum brake.... drum, two brake shoes, only the back plate is on the end of the crank, and as teh engine speed increases, so the centrifugal force starts to throw the brake shoes outwards against the drum, to transmit drive....

Clever, hugh?

OK, back to the Zenith. We're talking nearly a century ago; so it wasn't THAT clever; it didn't have the bob weights or anything; you changed gear by wionding teh coffee grinder handle, and that changed the distance between the plates and increased or decreased the gear ratio, the clever bit being the way the mechanism was arranged to keep tension on the belt at all times.

RIGHT.... so that is the advantage of gears, and the first 'variable' ratio transmission, precursor of the modern scooters variator, on the Zeneth.

Great on a low powered, low speed machine where the limits of a pure friction drive aren't an impediment.

As engines started to get better and turn higher rpm, and as the power they made increased, and the speeds they could achieve went up; proved rather inadequete; and all chain drive became the norm.

On some bikes with chain drives, like they had on belts, they had two seperate sets of sprokets and would stop at the bottom and top of hills to switch the chain between them.

Angus Scot, however had pioneered an inovative 'two speed' all chain transmission on his motorcycle in the very early days. Fixed drive to teh rear wheel, but he had TWO chain drives from the crank shaft to the reduction lay-shaft. The sprokets on teh crank were fixed, the sprokets on the lay-shaft not.

In 'neutral' engine turned both chains and the sprokets spun freely on the shaft.

BUT a heel and toe lever, would slide a 'dog-clutch' basically a hub, splined to the lay-shaft so it turned with the shaft; that had 'dogs' on it... lumps stuck out the side, basically, and when it slide to teh side, those lumps slotted into holes or engaged with matching lumps on free spinning sproket and so transfer drive from sproket through dog clutch, to shaft.....

Pretty simple; the chains and sprokets themselves never moved, just the engagement dog between them.

And THAT is pretty much how the modern motorcycle gearbox STILL works.....

Only difference is that the gears are direct meshing gears, not sprokets linked by chains, and there tends to be more of them, to give more ratios.

With the gears always meshing, and simply locked or unlocked from the shaft they are turning on, it is known as a 'constant mesh' gearbox.

And the clever bit is to arrange the 'selector mechanism' so that you can never have more than ONE gear pair locked to the shaft at one time, or the two cogs try and twist it at different speeds, and wither the box locks up, or the shaft breaks!

The most common means of engaging the gears, is with forked rods sliding the dogs between cogs, the forks slid forwards and backwards by slots in a selector drum, ratcheted around by the gear lever. Each gear lever 'click' rotating the drum, and moving the forks to a new position.

The car gearbox, is similar, but more commonly uses sliding gears. This introduces a problem that the gears have to slide 'into mesh', the peaks and troughs of the gear form fitting together.

If peak meats peak, then tries to push the shafts apart....

There are advantaces to the sliding mesh gearbox, and one of them is that the gears can be wider, and hence transmit more load, than the small 'lumps' on the side of a constant mesh gearbox; BUT adding a mechanism, 'syncro-mesh' to try and match gear speeds by way of friction cones that engage slightly before the gears, can reduce that or at least increase gearbox size, but does provide for a much smoother change.

Few bits of symantics for you; we talk of the nuymber of 'gears' in a gear-box.

GEARS are actually the toothed wheels that transmit drive. And it takes TWO to create a GEAR RATIO.

So, if you have a 'six speed' gearbox, technically we have a six RATIO gearbox, and there ought to be twelve actual gears in it; one 'pair' for each ratio..

However; we tend to have some gears driving the gearbox.

There will normally be a gear on thr end of the crankshaft, and that will drive a gear on the clutch basket. Typically giving something like 3:1 reduction.

This is called the 'primary drive'

Clutch basket will tend to run 'free' on the gearbox 'input' shaft but friction plates in the clutch when engaged will transmit drive to the shaft.

The selector mechanism then shifts dogs between cogs to select what gear pairs are transmitting drive between the input shaft and an output shaft.

And on the end of the output shaft there will be a sproket, taking drive to the rear wheel via chain.

Typically there will be something like a 3:1 reduction ratio between the sprokets and this is known as the 'final drive'.

3:1 reduction on the primary drive, times 3:1 on the final drive, gives 3x3 or 9:1 reduction, which isn't FAR off the 10:1 reduction we wanted to begin with.

Rear wheel we want to turn forewards, so on a chain drive, with no reversal, we want the gear-box output to turn forewards, that means we need the input shaft to turn backwards, which in turn means that the crank shaft turning it will turn forewards, same direction as rear wheel.....

And with 9:1 reduction between crank and rear wheel, from primary & final drive ratio's we possibly DONT need any MORE reduction from the gear-box, so our TOP gear could be 1:1 two equal sized gears... but then we could have four or five gear pairs providing more reduction for acceleration and hills.

On a traditionalk car gearbox, there's no primary drive; the clutch is on the end of the input shaft, and driven directly off the end of the crank; final drive will still be there in the form of the differential crown wheel gear set, but with smaller wheels and more power from lower revs, we possibly only want perhaps 5:1 reduction in top gear, and possible to avoid an extra gear-pair simply locking shafts straight through the gearbox for a 1:1 drive for that.

Though on a bike, its more likely that on a five or six speed 'box'; 4th is 1:1, with first second & third, reduction ratios, 5th & 6th likely to be 'over drive' ratios, turning the gearobx output shaft faster than the input, though overall, the transmission, with reduction on primary drive and final drive WILL almost certainly always be giving gear reduction, reducing crank speed to rear wheel speed, and increasing rear wheel force over crank force in direct proportion.

Clear as mud now?

Good.... if you understand gearboxes you are FAR to clever by half!

Like I said; principles are pretty simple; in practice they can be rather horendouse, and the intricacies confounding, while the variations on how they work are enormouse.

BUT, Zeneth Coffee Grinder; grandaddy of your scooter's variator transmission.

'The' Scott & its two speed 'Dog-Box' grandaddy of the modern constant mesh motorcycle gearbox.

Cars gearboxes? Actually owe more to LATHES than anything else, curiousely enough.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[MickC]

So, pistony things go up and down, geary things spin round, dogs get involved somewhere (fed up of chasing cats and bikes) and wheeyl at back turns round!!

Teflon, thanks for bending my ead on a friday afternoon!!!
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Previous: CG125,XS250 (converted to 400),Z250A, GPZ550, Norton Commando, XS500, 78 Laverda Jota, GPZ1100,Harris bonneville, XV750 (mono shock),TR1

[whitedevil]

You probably wont find better than this
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GPZ500 sold ~ CBR600FS-2 sold ~ ZX6R sold ~ Street Triple R

[Pete.]

[Andy_Pagin]

Teflon-Mike,

Thanks, very detailed and believe it or not I actually understood all of it (A-level pure/applied maths & physics helped a bit, a Series IIa landie with overdrive helped a lot more) . The history lesson was interesting, I actually saw an ancient bike on the road in Tottenham a few months back with a leather drive-belt on the transmission and what looked like some kind of hand operated gear lever. I had a vague idea how scooter transmissions worked, now I can picture it in my head.

Whitedevil

Brilliant, perfectly illustrates how the gear change works.
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They're coming to take me away, ho-ho, hee-hee, ha-haaa, hey-hey,
the men in white coats are coming to take me away.
Yamaha Vity -> YBR125 -> FZS600 Fazer -> FZ1-S Fazer