CT70 Sprocket Ratio Chart

Dezdan

Dezdan

Well-Known Member
Can anyone confirm the rear sprocket on 74+ bikes? Most sources say 35T, but looking online at parts diagrams, 38T?
 
6

69ST

Well-Known Member
This is why I recommend adding final drive ratio (primary x top gear) as well as tire circumference. Sprocket ratio is a key part of that equation (and the only part over which you have any control) and thus, the sprocket ratio chart is a nice convenience. But, any of the aforementioned parameters can have a big effect. If you're off by more than about 3-5% you won't be as happy with the results as you could/should be.

Anyone wanting to make life easier, over the long haul, would do well to do the extra counting/research, measuring & math, at the start, and approach gearing on a revs per mile/mph per 1000rpm basis. Anything else is flying semi-blind, at best. Stockers are geared for ~4.77mph/1000rpm, regardless of model (K0, H, K1-later). Just raising that to 5.00mph/1000rpm, roughly a 5% increase, is usually enough to make the bike fall on its face..with few exceptions. FYI, available tire sizes, alone, can have circumferences that range from about 53" up 62", nearly 20%.
 
Dezdan

Dezdan

Well-Known Member
kirrbby said:
38T is only the K0's. All else were 35T.
...3 speeds that is.
Click to expand...
Thanks! That's what I thought, but the Honda PN for the 38T was stating it belonged to 74-94.

racerx said:
This is why I recommend adding final drive ratio (primary x top gear) as well as tire circumference. Sprocket ratio is a key part of that equation (and the only part over which you have any control) and thus, the sprocket ratio chart is a nice convenience. But, any of the aforementioned parameters can have a big effect. If you're off by more than about 3-5% you won't be as happy with the results as you could/should be.

Anyone wanting to make life easier, over the long haul, would do well to do the extra counting/research, measuring & math, at the start, and approach gearing on a revs per mile/mph per 1000rpm basis. Anything else is flying semi-blind, at best. Stockers are geared for ~4.77mph/1000rpm, regardless of model (K0, H, K1-later). Just raising that to 5.00mph/1000rpm, roughly a 5% increase, is usually enough to make the bike fall on its face..with few exceptions. FYI, available tire sizes, alone, can have circumferences that range from about 53" up 62", nearly 20%.
Click to expand...
Ya just made my head hurt! I could make a PDF that has variables inputted by the user to calculate data. I'm not sure at this point (will need to re-read your post a few times), on what exactly I need to do calculation wise. If you have any interest in furthering this, let me know!
 
6

69ST

Well-Known Member
It can be a tad on the migraine-inducing side, initially. But, it's nothing more than basic math. True, it does take some time to verify the primary & trans ratios, measure the rear tire and do the math. But, it's less time, effort and mess (not to mention back strain) than changing the sprockets a couple of times...or more. And, once you've worked out a table, you can easily figure out incremental changes with precision. With a new/unknown engine & tire combo, one can make a decent educated guess from the get-go; it's less hassle/headaches than testing out every sprocket combo between 17/31 and 15/35(!).

I've straightened out bikes that were no faster in 4th gear than in 3rd, the owner swearing that "something was wrong with the engine" since the recommended sprocket combo was being used...only to find out that it was necessary to drop one tooth at the C/S and gain one at the wheel, to compensate for the oversized tire...that also "couldn't possibly make that much difference".

The basic formula is: (engine rpm x wheel radius x .006)/final drive ratio = mph. Using 1000rpm as a constant: (wheel radius x 6)/final drive ratio = mph @ 1000rpm - a much more useful figure, in a number of ways.

With stock CT70 engines:
  • H-primary 18/67 = 3.722
  • K0 primary 18/67 = 3.722
  • K1-later primary 17/69 = 4.05

  • H models 4th gear 24/23 = .958
  • All others 3rd gear 21/25 = 1.19

  • H sprocket combo 14/45 = 3.21
  • K0 15/38 = 2.533
  • K1-later 15/35 = 2.333

Final drive ratios:
  • H-11.42
  • K0- 11.22
  • K1-later- 11.24

That's a difference of 1.6%, with the H-model having the shortest gearing...and more than enough rpm, at the top end, to overcome it.

Nominal spec tire size is 56.57" circumference/8.99" radius. Thus...
  • (8.99 x 6)/11.42 = 4.72mph/1000rpm
  • (8.99 x 6)/11.22 = 4.80mph/1000rpm
  • (8.99 x 6)/11.24 = 4.79mph/1000rpm

Now...let's do the math using a tire with a 61" tire circumference:
(9.70 x 6)/11.24 = 5.17mph/1000rpm. That's an 8% difference, dosn't seem like much but, it's more than enough to upset the proverbial apple cart. Actual tire sizes tend to be smaller and the actual difference is often closer to ~12%.

Okay, I've provided the primary, tranny and target final drive gearing for a stock, 72cc lump. All that remains to be done is taking the measuring tape to the rear tire and plugging that number into the formula. Just get within 1/2" of actual and you'll be less than 1% off, a very easy way of obtaining a precise result.
 
Dezdan

Dezdan

Well-Known Member
Thank you for introducing the math behind it! I was working through the formulas this morning deconstructing the excel sheet. I couldn't seem to figure out, where the '6' is coming from: (RPM/1000)*(6*((Tire Circumference/3.1416)/2)/((Primary Gearing Hi/Primary Gearing Low)*(High Gear Hi/High Gear Low)*(Sprocket Rear/Sprocket Front)))=MPH

I wasn't sure where you were getting the gearing numbers from, but that is making more sense now. I'm thinking about working up a sheet that shows MPH per gear. From elsewhere I've gathered the following:
CT70H
1st Gear Ratio: 2.692
2nd Gear Ratio: 1.824
3rd Gear Ratio: 1.300
4th Gear Ratio: 0.958

CT70K0
1st Gear Ratio: 3.364
2nd Gear Ratio: 1.722
3rd Gear Ratio: 1.190

I'll have to tinker with the formula a bit to figure it all out. Thanks!
~Dan
 
6

69ST

Well-Known Member
I discovered that formula decades ago, when I started building serious performance cars. It cuts through a lotta bullshit...like when someone talks about his 160mph pony car with a 4.56 ring & pinion. Primarily though, I used it to determine proper gearing, for any given application. Swapping-out ring & pinion sets is a little more complicated & costly than sprocket sets. :24: For anyone building a street car with a "Claybrook autograph" (85mph) speedometer and more than 200hp, knowing mph/1000rpm isn't just important, it can be a safety issue. It's what led me out of insane cars...and into somewhat less insane little bikes. 31.5mph/1000rpm x 5800rpm = "you're getting too old for this kind of :censored:stupidity"

The 6 is a simple bit of algebraic prestidigitation. Instead of leaving rpm as a variable and .006 as the constant, I made rpm a constant at 1000. 6 is cleaner than (1000 x .006). With math, lowest common denominator simplicity is best, imho.

Back in 2001, I worked-up a spreadsheet, showing speed for each gear, based on 30 different sprocket combos and two different 4th gear pairs, for the Honda Nice 110. Long story short, it was also based on faulty tire circumference input of 54". That led to my not trusting anyone but a tape measure :45:. Having long-since gained enough experience to know what works with that engine, and the numbers being close enough to use as a "poor man's tachometer" I've no plans for a redo. For a taller, or shorter, rear tire it's easy enough to figure out the percentage difference and work backward to figure out the proper sprocket combo. Really, it's no different for any other engine, including the stock, 49cc-based lumps. Once you have the baseline numbers, any changes in gearing (tranny/primary) or tire size are easily factored-in. Even with a bored/stroked tune, reasonable assumptions can be made...close enough to eliminate the need for blindly testing an armload of sprockets.

IMO...doing some math homework & research, up-front, ends up saving you time, not costing. It also demystifies the process of achieving optimal gearing.
 
Chrisqa

Chrisqa

New Member
racerx said:
It can be a tad on the migraine-inducing side, initially. But, it's nothing more than basic math. True, it does take some time to verify the primary & trans ratios, measure the rear tire and do the math. But, it's less time, effort and mess (not to mention back strain) than changing the sprockets a couple of times...or more. And, once you've worked out a table, you can easily figure out incremental changes with precision. With a new/unknown engine & tire combo, one can make a decent educated guess from the get-go; it's less hassle/headaches than testing out every sprocket combo between 17/31 and 15/35(!).

I've straightened out bikes that were no faster in 4th gear than in 3rd, the owner swearing that "something was wrong with the engine" since the recommended sprocket combo was being used...only to find out that it was necessary to drop one tooth at the C/S and gain one at the wheel, to compensate for the oversized tire...that also "couldn't possibly make that much difference".

The basic formula is: (engine rpm x wheel radius x .006)/final drive ratio = mph. Using 1000rpm as a constant: (wheel radius x 6)/final drive ratio = mph @ 1000rpm - a much more useful figure, in a number of ways.

With stock CT70 engines:
  • H-primary 18/67 = 3.722
  • K0 primary 18/67 = 3.722
  • K1-later primary 17/69 = 4.05

  • H models 4th gear 24/23 = .958
  • All others 3rd gear 21/25 = 1.19

  • H sprocket combo 14/45 = 3.21
  • K0 15/38 = 2.533
  • K1-later 15/35 = 2.333

Final drive ratios:
  • H-11.42
  • K0- 11.22
  • K1-later- 11.24

That's a difference of 1.6%, with the H-model having the shortest gearing...and more than enough rpm, at the top end, to overcome it.

Nominal spec tire size is 56.57" circumference/8.99" radius. Thus...
  • (8.99 x 6)/11.42 = 4.72mph/1000rpm
  • (8.99 x 6)/11.22 = 4.80mph/1000rpm
  • (8.99 x 6)/11.24 = 4.79mph/1000rpm

Now...let's do the math using a tire with a 61" tire circumference:
(9.70 x 6)/11.24 = 5.17mph/1000rpm. That's an 8% difference, dosn't seem like much but, it's more than enough to upset the proverbial apple cart. Actual tire sizes tend to be smaller and the actual difference is often closer to ~12%.

Okay, I've provided the primary, tranny and target final drive gearing for a stock, 72cc lump. All that remains to be done is taking the measuring tape to the rear tire and plugging that number into the formula. Just get within 1/2" of actual and you'll be less than 1% off, a very easy way of obtaining a precise result.
Click to expand...
Very good racerx. I'm glad you know how. I was struggling with this myself. I always thought the closer the sprocket ratio the higher the top speed so I ordered a 17 front and a 32 rear and got 34 mph from my 3 speed semi automatic hahaha boy was I wrong. I found out later the engine needs to be in it's maximum power band at top speed to get maximum mph. I changed the front from 17 to 15 and it went much faster. 39mph then again changed the front to 14 and it went 38mph so I went back to the 15 tooth. I'll try later after the engine lossens up from the overhaul
 
Last edited:
Top