Gear Ratio

Forest · Post by **Forest** » Fri Jun 24, 2011 11:55 pm

Hello All,
So lately I've been spending most of my time studying gear reduction ratio's and torque values.
Here's the basic equations I've been workings with:
Drive gear (A)/ Driven gear (B)= ratio reduction or increase

And the reverse equation: Driven gear (B)/ Drive gear (A)= torque per pound transfered to driven gear.

So going through the manual for my 61' Honda Dream 305 is states the following for mission gear gear ratio:

-First= 1 to 3.12
-Second= 1 to 1.74
-Third= 1 to 1.27
-Top= 1 to 1

Here's the numbers for transmission gears:
Main transmission shaft= 16T
Counter shaft= 21T
Low gear= 35T
Main shaft second gear= 23T
Counter shaft second gear= 28T
Main shaft third gear=27T
Counter shaft third gear= 24T
Top gear= 30T

Final drive sprocket= 16T
Final driven sprocket(at rear wheel)= 29T

I know that the final drive from 16T to 29T gives us a gear reduction of .55172 (A/B) and a torque ratio increase of 1 to 1.8125 (B/A).

So my question is how do you come to the conclusion of the resulting mission gear gear ratio's..
For example, how do you conclude that the top gear is in fact 1 to 1 ratio when the final drive sprocket (16T) is 13t smaller than the final driven gear (29T)?

Now all this may seem unnecessary but I can't stop thinking about it until I can completely wrap my mind around it.
Anyone who would love to clarify on this topic and even elaborate on the mechanics of the main shaft and counter shaft would be GREATLY appreciated! I hope my wording and descriptions are clear enough.

Thanks,
Forest
61' Dream

jensen · Post by **jensen** » Sat Jun 25, 2011 2:21 am

Hi,

1 to 1 means no reduction in a given gear set, the gears are equal.
it doesn't mean that 1 revolution of the crank is one revolution of the rear wheel.

when the driven gear is smaller then the driving gear we call it an overdrive.

I can go very deep into this, but that doesn't change the way your bike is geared.
you could make an exel sheet, to get an overview.

(I made a few, they are somewhere on the forum)

Jensen

teazer · Post by **teazer** » Sun Jun 26, 2011 10:41 am

Forest, there are three steps in the overall reduction from the crank to the rear wheel:

Primary reduction (crank sprocket:clutch sprocket)
Internal ratios ( inside the box)
Secondary ratio (countershaft sprocket: rear wheel sprocket)

To convert crank rpm to rear wheel revs it's
Primary ratio x internal ratio x secondary reduction

Just to complicate matters, the CA/CB series transmissions are indirect i.e they use the same shaft for input and output - unlike almost any modern transmission. Top gear set is 21:31 and to get that to 1:1 is because they essentially lock the input shaft and the output gear (on its own short shaft on the main shaft) together. Those 21:31 gears change the other ratios but not to gear.

It's a confusing concept, but that's how they work. if you have access to a gearbox you will be able to see how it all works.

Forest · Post by **Forest** » Mon Jun 27, 2011 11:51 am

Thanks for the help. I know its a confusing topic (for me at least) , but there is a lot of joy in fully understanding something that may have initially been a challenge. I'll still have to work on this one a bit.

And I'm sure your right, If I could get the gear box opened up and have a good look it would probably all make more sense.

Any suggested readings or further information is always appreciated.

Thanks again,
Forest

61' Dream

teazer · Post by **teazer** » Wed Jun 29, 2011 11:35 am

Forest, there was a thread here relatively recently where someone posted pictures of the trans in each gear.

IIRC the shop manual also shows the way that the trans works in each gear. It's not easy to wrap your brain around until you have the gears in hand and can actually see what engages.

redblk63 · Post by **redblk63** » Thu Jul 07, 2011 11:07 am

Forest,

As Teazer said, it is a bit complex. The Honda manual shows the flow of power in each gear. For transferring power in the indirect speeds (1st through 3rd), 4 gears are involved. If you can, find the excellent movie of this someone posted to this forum.

For example, with my 1963 CB77, the stock (before Xing) gear tooth counts are
main shaft countershaft
#teeth #teeth
1st 18.000 34.000
2nd 24.000 27.000
3rd 29.000 23.000
4th 31.000 21.000

The flow of power in first gear is from 1st main to 1st counter, then "4th" counter to "4th" main. This leads to an overall gear ratio within the transmission of

34/18 * 31/21 = 2.788

By the way, if you ever do have your transmission opened up, I suggest you look into Xing 2nd and 3rd gears, which gives a more even spacing. Many prefer moving 2nd closer to 1st, and increasing the gap between 3rd and 4th gear ratios.

Good for you for wanting to get your head around this! For me, the power flow diagrams in the manual were most useful.

Larry Young - Pasadena

redblk63 · Post by **redblk63** » Thu Jul 07, 2011 11:08 am

Forest,

As Teazer said, it is a bit complex. The Honda manual shows the flow of power in each gear. For transferring power in the indirect speeds (1st through 3rd), 4 gears are involved. If you can, find the excellent movie of this someone posted to this forum.

For example, with my 1963 CB77, the stock (before Xing) gear tooth counts are
main shaft countershaft
#teeth #teeth
1st 18.000 34.000
2nd 24.000 27.000
3rd 29.000 23.000
4th 31.000 21.000

The flow of power in first gear is from 1st main to 1st counter, then "4th" counter to "4th" main. This leads to an overall gear ratio within the transmission of

34/18 * 31/21 = 2.788

By the way, if you ever do have your transmission opened up, I suggest you look into Xing 2nd and 3rd gears, which gives a more even spacing. Many prefer moving 2nd closer to 1st, and increasing the gap between 3rd and 4th gear ratios.

Good for you for wanting to get your head around this! For me, the power flow diagrams in the manual were most useful.

Larry Young - Pasadena

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