Engine output shaft ratio?

al nunley · Joined: 13 Nov 2006 Posts: 3066

Do different engines have different output shaft ratios? Not the gear the chain fits on, the ratio to that shaft.
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Randy Mckee · Posted: Wed Jan 02, 2013 3:35 pm Post subject:

Yes, counter divided by main gear provides the ratio for each gear in the gearbox as well as the final primary output.

Almost all gearbox motors differ -- even among the same type for different years. For example, the CR125 uses different ratios, and sometimes number of gears, depending on the year.
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al nunley · Joined: 13 Nov 2006 Posts: 3066

So, as I thought, and thank you, but do most drivers know what that final ratio is to the sprocket shaft?
I’m trying to build a spreadsheet to figure out the final ratio to the axle. Something to add to my nine sheets collection. Simple to do, but I think handy.
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If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
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Randy Mckee · Posted: Wed Jan 02, 2013 4:19 pm Post subject:

I'm guessing most drivers and tuners don't think too much about the internal ratios. It really only comes into play when comparing two different motors so you can guesstimate gearing when switching from one motor to another. For example, when I switched from ICC to spec honda, I used my ICC data and ratios to make an educated guess at the honda gearing for the tracks we frequent.

The most important thing (for most) is to know the high rpm on the longest straight, shift points around various turns and perhaps approximate rpm in each turn. All of these of course point back to gearing on the kart and not much thought given to internal gearing in the motor.
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Matt Dixon · Posted: Wed Jan 02, 2013 5:23 pm Post subject:

Vernon Head · Posted: Wed Jan 02, 2013 6:11 pm Post subject:

My '99 shop manual says 3.15.

al nunley · Joined: 13 Nov 2006 Posts: 3066

Vernon Head · Posted: Wed Jan 02, 2013 10:47 pm Post subject:

It says the primary reduction is 3.150 (63/20).

Jim McMahon · Posted: Thu Jan 03, 2013 12:14 am Post subject:

Al there is no final reduction perse, only primary. So you have x ratio from motor:transmission input, then the selected gear ratio. The CR125 is 3.15 primary. So the motor turns 3.15 turns for every single rotation of the input shaft for the transmission, so your "final" ratio would be primary ratio:selected gear ratio.

The ICC\KZ motors do differ in ratios and primary inputs...
You can look at the sheets here for the details: https://www.dropbox.com/sh/zi0qyls5ayg84gw/X304plSkoY
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al nunley · Joined: 13 Nov 2006 Posts: 3066

Steen Carstensen · Posted: Thu Jan 03, 2013 8:44 am Post subject:

You need to clearify whether you mean gear or sprocket. If I understand your question right, you are asking what the ratio is between crank rpm and output shaft rpm. To answer the question you need to know the primary gear ratio and then the gear ratio.

Benn Herr · Joined: 18 Jul 2001 Posts: 1584

Al,

From the Swedetech site:

Moto 125cc Honda CR125 Gearbox Ratios (Primary: 20/63)
1st Gear: 14/33 4th Gear: 21/27
2nd Gear: 15/28 5th Gear: 23/26
3rd Gear: 19/29 6th Gear: 24/24

It is a little unusual for top gear to be one to one. Most other engines have an over drive in top gear (or top two gears).

For example:

ICC 125cc TM K9B Gearbox Ratios (Primary: 19/75)
1st Gear: 13/33 4th Gear: 22/27
2nd Gear: 16/29 5th Gear: 22/23
3rd Gear: 18/27 6th Gear: 27/25

ICC 125cc Pavesi Gearbox Ratios (Primary: 18/75)
1st Gear: 15/32 4th Gear: 23/24
2nd Gear: 19/29 5th Gear: 25/22
3rd Gear: 21/26 6th Gear: 27/21

Moto 80cc Honda CR80 Gearbox Ratios (Primary: 17/70)
1st Gear: 15/35 4th Gear: 23/27
2nd Gear: 18/31 5th Gear: 25/25
3rd Gear: 20/28 6th Gear: 26/23
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Chris Reinhardt · Posted: Thu Jan 03, 2013 11:29 am Post subject:

Benn, not at all look, at the primary ratios on all the motors and that tells the story....

Moto 125cc Honda CR125 Gearbox Ratios (Primary: 20/63) 3.15

ICC 125cc TM K9B Gearbox Ratios (Primary: 19/75) 3.95

ICC 125cc Pavesi Gearbox Ratios (Primary: 18/75) 4.17

Moto 80cc Honda CR80 Gearbox Ratios (Primary: 17/70) 4.12

Bigger primary reduction means they have to overdrive it in the gearbox...

CR
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al nunley · Joined: 13 Nov 2006 Posts: 3066

Thank you Benn, just what I was looking for.
_________________
If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting

al nunley · Joined: 13 Nov 2006 Posts: 3066

Now I see that I was asking the wrong question. The question should have been “What is the top gear, gear ratio”
So I have four of them, are there more engines being used?
I think this ratio chart could be helpful if you ask someone, with a different engine, what gear ratio they are running. The top gear, gear ratio, could be different from your engine, so the final sprockets used would be different, if you want to arrive at the same ratio to the axle.
_________________
If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting