SHIFTING GEARS

SHIFTING GEARS

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  • On 17th May 2015

Ever wonder what happens when you are shifting gears? “The bike goes faster or slower. That’s what happens.” Well, true.  But that’s not what I meant.  What I meant is what actually happens inside the transmission box as you go from first gear to second, from second to third, form third to forth, and from forth to fifth?

I think crazy thoughts like this every once in a while.  Finding the answers is a lot of fun.

However, when rebuilding BLACKSQUARE’S engine, I was actually forced to think like that.  In fact, everybody will have to think along similar lines when assembling their engine’s crankcase.  Because the gear shift forks positioned in the upper crankcase’s shell will have to line up perfectly with their corresponding “grooves” in the transmission sprockets in the lower crankcase shell before you can bolt the two shells together.  Or else there will be trouble.

The HONDA Service Manual does not provide details on what position the gears and forks should be in prior to assembly.  Probably any; for as long as everything lines up.  But I couldn’t take that chance. So, I thought that I would assemble the crankcase with the gears and sprockets aligned in the “neutral” gear position.

The HONDA manual provides a good description of what all the parts are.  Here it is:

Transmission Parts HONDA manual

As you can see, it does say things like “Counter Shaft Second Gear” and “Main Shaft Second, Third Gear”, but I can’t read drawings well and don’t find the interpretation immediately intuitive.

So, to understand what is going on, I had to look inside the real thing.

Here is what the transmission assembly looks like sitting in the rear of the lower crankcase shell.  Lots and lots of parts.  And gears galore:

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HONDA CB550 TRANSMISSION

Here are the three forks (that do all the gear shifting) mounted in the upper crankcase shell:

GEAR SHIFTING FORKS

GEAR SHIFTING FORKS

These are the forks I referred to above when I said the forks must line up perfectly with their corresponding grooves in the sprockets.

When all the gear sprockets are perfectly lined up, as shown below, that’s “neutral gear”.  Again, note that the sprockets on the counter shaft (foreground) are directly behind and in line with the sprockets on the main shaft (background).

NEUTRAL GEAR ALIGNMENT

NEUTRAL GEAR ALIGNMENT

When shifting gears, only a few of the sprockets will change their position on their respective shafts.  I have identified those sprockets as A, B, C, and D in the picture below.  Why do only these sprockets move?  Because the gear shift forks are inserted between sprockets A and B, to the right of sprocket C and to the left of sprocket D.  They cannot shift any of other sprockets.  They can only shift the sprockets they are engaged to.

Neutral Gear with A B C D sprockets identified

Now.  Here is how it works.

When D is shifted to the right, that’s first gear as shown below.

FIRST GEAR ALIGNMENT

FIRST GEAR ALIGNMENT

Wait a minute! Just hold your horses there. Sprocket D is identified in the HONDA drawing above as Counter Shaft Fourth Gear, isn’t it?” Correct.  “How come shifting the fourth gear sprocket a little to the right puts the bike into first gear? Shouldn’t that put the bike into forth gear?” No.  It is a little confusing, I agree.  But, if you looked closely at the image above, you would see a couple of “teeth” to the left of the fork groove on sprocket D.  Similar “teeth” are present on the right side of sprocket D.  They engage into corresponding holes on the sprocket to the right of sprocket D, identified as Counter Shaft Low Gear (“low” means first), and this action makes that sprocket the one that drives the secondary shaft.  Which drives the drive sprocket.  Which drives the chain.  Which drives the rear wheel’s sprocket.  Which turns the rear wheel.  Which makes the bike go.

Okay.  How do you know that this is first gear?”  That’s the easy part.  You turn the main shaft by hand one full rotation and then count the number of rotations the counter shaft will do.  The HONDA Service Manual tells us that the gear ratio for first gear is 2.353.  This means that the secondary shaft will make one full rotation for approximately 2 and 3/8 rotations of the main shaft.  Which also explains why the bike cannot go very fast in first gear.  The main shaft spins fast when you twist the throttle, but the drive sprocket driving the rear wheel rotates relatively slowly.  (The opposite is true when the bike is in fifth gear.)

When sprocket C is shifted to the right, that’s second gear. (Note that when this happens, sprocket D returns to its “neutral position.)

SECOND GEAR ALIGNMENT

SECOND GEAR ALIGNMENT

Similar confusion exists here as well.  You shift sprocket C identified in the HONDA drawing as Counter Shaft Top Gear (“top” means fifth) to put the bike into second gear.  But when sprocket C moves, it engages into the sprocket to its right and that is the sprocket that becomes the sprocket driving the counter shaft.  It is identified in the HONDA drawing above as Counter Shaft Second Gear.

Following similar logic, when D is shifted to the left, that’s third gear.

THIRD GEAR ALIGNMENT

THIRD GEAR ALIGNMENT

When both A and B are shifted to the right, that’s fourth gear.

FOURTH GEAR ALIGNMENT

FOURTH GEAR ALIGNMENT

When both A and B are shifted to the left, that’s fifth gear.

FIFTH GEAR ALIGNMENT

FIFTH GEAR ALIGNMENT

Okay.  Let’s recap.  When you shift gears, the gear shift forks slide left and right and move sprockets A, B, C and D.  When these sprockets do so, they engage into their adjacent sprockets, which then transfer the motion of the main shaft to the counter  shaft.  Which, then, via the drive sprocket and chain, moves the bike.

Simple, isn’t it?  And yet so intricate and complex.  Considering how smoothly the HONDA CB550 shifts, I am in awe of this marvel of design and engineering.

But the questions never end, do they?  If the gear shift forks move the gear sprockets from one position to another and that’s how you change gear, what then makes the gear shift forks move?

More to come.

QUESTIONS?