Gear Ratio Computers- I'm a Dummy.

#1
On my current project, I am using a standard clutch and a jack shaft. I am running at 5.4:1 and I want to build in more bottom end. Both primary and secondary drives are #35 chain. The clutch is a 12t Max Torque.

Space and clearance issues prevent me changing any sprockets on the jack shaft that would serve my purpose, leaving me only the clutch.

So I figure a ten tooth clutch will do it. Only I cant get a ten tooth clutch with a ten tooth sprocket in #35 chain pitch.

So I ordered a ten tooth clutch in a #41 pitch, and the matching #41 pitch jack shaft sprocket to replace the 18 tooth #35.

Today I got the replacement #41 17 tooth sprocket. It's huge. That was my first clue. An 18t #35 sprocket has a diameter of 2.1 inches, while an 18t #41 sprocket has a diameter of nearly 2.9 inches. That is huge. (You cant get a #41 sprocket in 18t, but that is beside the point)

Further looking into this, that 10t #41 sprocket is significantly larger in diameter than a 12t #35 sprocket, so the clutch I just ordered with only ten teeth is going to be larger in diameter than my #35 with 12t.

All of these years doing this, all of the hundreds of times looking at calculators and spread sheets, it never dawned on me that I wasn't looking at "teeth" at all, but diameters of wheels that might as well have been pulleys, meshed gears, or sail boat winches.

I challenge anyone to show me one of those "on line" calculators that make allowances for other chain sizes, or even one that says it's for a specific chain pitch.

And I am MORE than happy for someone to tell me I am going at this wrong, and too many mind altering substances while listening to Stairway to Heaven destroyed one too many brain cells. :laugh:
 

chrisr

Active Member
#2
I may be missing something - so I apologize in advance, but when it comes to chain size, chain pitch that doesn't matter when figuring gear ratios; its all about the amount teeth on the driver and driven - e.g: 10t front/ 60 tooth rear is still a 6.0:1 ratio no matter the chain type. Just like a 10t on the pinion and 41t ring gear on a rear end, no matter the size of application (truck, car, van) its still a 4.10:1.

https://www.summitracing.com/parts/fms-m-4209-88410/overview/

https://www.bmikarts.com/Go-Kart-Gear-Ratio-Calculator-Jackshaft-Setup
 
#3
On my current project, I am using a standard clutch and a jack shaft. I am running at 5.4:1 and I want to build in more bottom end. Both primary and secondary drives are #35 chain. The clutch is a 12t Max Torque.

Space and clearance issues prevent me changing any sprockets on the jack shaft that would serve my purpose, leaving me only the clutch.

So I figure a ten tooth clutch will do it. Only I cant get a ten tooth clutch with a ten tooth sprocket in #35 chain pitch.

So I ordered a ten tooth clutch in a #41 pitch, and the matching #41 pitch jack shaft sprocket to replace the 18 tooth #35.

Today I got the replacement #41 17 tooth sprocket. It's huge. That was my first clue. An 18t #35 sprocket has a diameter of 2.1 inches, while an 18t #41 sprocket has a diameter of nearly 2.9 inches. That is huge. (You cant get a #41 sprocket in 18t, but that is beside the point)

Further looking into this, that 10t #41 sprocket is significantly larger in diameter than a 12t #35 sprocket, so the clutch I just ordered with only ten teeth is going to be larger in diameter than my #35 with 12t.

All of these years doing this, all of the hundreds of times looking at calculators and spread sheets, it never dawned on me that I wasn't looking at "teeth" at all, but diameters of wheels that might as well have been pulleys, meshed gears, or sail boat winches.

I challenge anyone to show me one of those "on line" calculators that make allowances for other chain sizes, or even one that says it's for a specific chain pitch.

And I am MORE than happy for someone to tell me I am going at this wrong, and too many mind altering substances while listening to Stairway to Heaven destroyed one too many brain cells. :laugh:
It doesn't matter what chain pitch you're using. An 18 tooth #35 and #41 have different pitch diameters yes, but so will your #35 or #41 driven of say, 45 teeth. These two differences cancel each other out so the end ratio is the same for an 18/45 in #35 chain or #41 chain.

Or you can think of it like this: An 18 tooth #35 sprocket has a circumference of 6.75" (18 teeth x 3/8" pitch). An 18 tooth #41 sprocket has a circumference of 9" (18 teeth x 1/2" pitch). The 45 tooth driven sprockets have circumferences of 16.875" for #35 and 22.5" for #41. To get the ratio, divide the driven circumference (or teeth or pitch diameter, doesn't matter) by the driver circumference (or teeth or pitch diameter):

For #35: 16.875/6.75 = 2.5:1 ratio

For #41: 22.5/9 = 2.5:1 ratio

This is why ratio calculators don't ask for pitch.

And hey, Stairway to Heaven is a killer song!:punk: Say what you want about it but that buildup to Robert's high notes toward the end gets me goin'.
 
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#4
It doesn't matter what chain pitch you're using. An 18 tooth #35 and #41 have different pitch diameters yes, but so will your #35 or #41 driven of say, 45 teeth. These two differences cancel each other out so the end ratio is the same for an 18/45 in #35 chain or #41 chain.

Or you can think of it like this: An 18 tooth #35 sprocket has a circumference of 6.75" (18 teeth x 3/8" pitch). An 18 tooth #41 sprocket has a circumference of 9" (18 teeth x 1/2" pitch). The 45 tooth driven sprockets have circumferences of 16.875" for #35 and 22.5" for #41. To get the ratio, divide the driven circumference (or teeth or pitch diameter, doesn't matter) by the driver circumference (or teeth or pitch diameter):

For #35: 16.875/6.75 = 2.5:1 ratio

For #41: 22.5/9 = 2.5:1 ratio

This is why ratio calculators don't ask for pitch.

And hey, Stairway to Heaven is a killer song!:punk: Say what you want about it but that buildup to Robert's high notes toward the end gets me goin'.
Thanks for responding to my confusing gloobity goop Carbide! Yes, I grasp that it is all "relative" when you are using them all on the same chain. The ratio is the ratio. It's all just bigger.

I am not comparing one #41 primary drive with another. I am comparing the #35 primary drive I "had," with a #41 primary drive, I bought, because I was wrapped around having a ten tooth clutch driver. I did not account for the fact that a ten tooth #41 sprocket is larger than a 12t #41 sprocket.

I am limited in space. I can fit a 2.1" diameter sprocket on the driven end. That is maximum. I already have that in my #35 tooth driven. But that 10t #41 clutch driver sprocket is actually 1.61". The 12t #35 I had was 1.44", so I went the wrong way on my desired ratio. The only way to compensate that is to increase the diameter of my driven. But I can't, because the aperture (cut out) in the engine plate is limiting it. Any further down that road, and I start to weaken components.

When I was using my ratio speed for jack shaft spreadsheet, I input the sprocket teeth for the driven without realizing that the same 18 teeth on the #41 was going to be a third again as large, and in no way fit.

In reality, a 13t #41 driven will fit, and with that #41 ten tooth clutch, will put my gear ratio at 4.7:1 which is unacceptable. I either live with the 5.4:1, or take it apart and redesign the jack shaft mount, swing arm mount, pretty much the entire bike. LOL! If I were to hog out the driven aperture a bit more, I could fit a 20t in their and get 6:1 on the nose. That might be what I do, and add in some bracing around it.

That's $60 down the train for a brain fart.

Stairway was sort of passe by the time they used it for our HS prom theme song. I was always more of a Physical Graffiti guy, and little did I know that I wouldn't need any help at all, with artificially dimming the memories.
 
#5
I may be missing something - so I apologize in advance, but when it comes to chain size, chain pitch that doesn't matter when figuring gear ratios; its all about the amount teeth on the driver and driven - e.g: 10t front/ 60 tooth rear is still a 6.0:1 ratio no matter the chain type. Just like a 10t on the pinion and 41t ring gear on a rear end, no matter the size of application (truck, car, van) its still a 4.10:1.
Thanks for responding Chris, and you are right. But mere ratio wasn't the sum of my problem, it was one of allowable space. Or quite simply, to achieve the same ratio with a larger chain, one needs larger sprockets. Said like that, it sounds to simple. But when you're engineering a drive system and working with suspension and space limitations with limited brain cells, it gets more complicated. LOL!
 
#6
If the crankshaft was 5/8” OD on the output side, you could be running an 11 tooth clutch. Would that be enough? I will guess that you would end up at 5.85:1
Just a SWAG while I am in the shop counting sprocket teeth.
 
#7
If the crankshaft was 5/8” OD on the output side, you could be running an 11 tooth clutch. Would that be enough? I will guess that you would end up at 5.85:1
Just a SWAG while I am in the shop counting sprocket teeth.
It would be "better" but probably not where I want it. I have only tested the bike with no seat, and no pegs, and me sitting on it with my legs in the air AND my ass cheeks tightened, while accelerating. As tested, and with that engine, it is theoretically a 50 MPH bike. I don't need to be doing that on six inch "industrial" tires. So I'd rather have it down low where I could jam the throttle and feel real acceleration up through a blistering 22.8 MPH! :laugh:

This is my hot rod HS40. I wouldn't use my trials and tribulations and tintabulations as any sort of reference for another bike. If I didn't have a Honda swing arm in the way, I would have easily resolved this via both JS sprockets.

Here is a graphic display of the physical size differences in sprockets resulting from chain pitch choices.

 

chrisr

Active Member
#8
Pictures are worth a thousand words. The 35 chain yields a smaller form factor and depending on HP, may be a good option for you if overall sprocket size is a bigger concern in your design/needs.
 
#9
Pictures are worth a thousand words. The 35 chain yields a smaller form factor and depending on HP, may be a good option for you if overall sprocket size is a bigger concern in your design/needs.
Yes, and I WAS at #35 chain and sprockets, but I needed to go down in clutch tooth size to get a lower gear ratio. Thus my dumb decision to get a smaller tooth clutch, but in a #41- which was actually a larger diameter sprocket.

This is stuff my dumb-ass didn't compute when plugging in tooth sizes to a Jack Shaft ratio calculator. Zoomed right over my head.

My design and engineering constraints are obvious in my Full Suspension Cat Hot Rod build thread, of which I have been told isn't exactly rocket surgery, or brain science....good thing too, because the patient would have died on the table. LMAO! Trying to fit ten pounds of crap in a five pound bag.

On my next attempt, I shall take all of this into consideration.
 
#10
This should explain the results of my flawed thinking. It might not matter that we don't annotate the chain sizes on ratio calculators, but if you try and use them for something besides "how fast can I go," they end up being poor substitutes for engineering. Oh well, this is part of the reason I love this hobby. Thanks for the participation, even if you didn't say anything, and especially for those of you who didn't say "no shit Sherlock!" HAHA!

What I Had, and What Fits: 5.4:1


What I Wish Would Fit: 6:0:1


What I Bought Thinking It Would Fit With 10T 41 Clutch: 6.1:1
 

delray

Well-Known Member
#11
dave I see the easy fix. your going to have to bit the bullet and cut off the jackshaft plates and make them with more forward and up off-set.
not like the bike is painted and finish.

just do it....:grind:...…….:thumbsup:
 
#12
dave I see the easy fix. your going to have to bit the bullet and cut off the jackshaft plates and make them with more forward and up off-set.
not like the bike is painted and finish.

just do it....:grind:...…….:thumbsup:
Bwaa-Haa! I know. But there was a lot of work done after I set those plates. Swingarm, rear suspension, Cat/Azusa brake modification, swing arm, ride height, pretty much the entire bike hinges on where that swing arm pivot ended up.

Plates can not go forward, because the engine is there. As it is, I've given up half of the engine adjustment.

You know I have already been contemplating all of that. :scooter: I would essentially be starting over from scratch. I'll end my commentary here-in, and post what I decide back in the build thread. It's a pretty interesting challenge, and despite the fact that it "sucks," I find the fact that this snuck up on me fascinating.
 

delray

Well-Known Member
#13
well from the looks of it you could move the jackshaft another 1inch forward and maybe up little . keep your swing arm mounting area the same location. so nothing in the back get change. run your oil return line under the shaft and then up with some kind of tubing /steel/brass...etc.. even if you only could move it 1/2 inch or so that would still put you another couple teeth larger and that would make a big difference in the gearing.

just cut your plate in half so your not moving your swing arm and then weld a new plate in front of it with the correct off set you need and then weld the two back together when finish. or cut the whole thing off and make a new plates with the new adjustment and swing arm holes too and then weld it on.

your making it to hard on your self dave……….just do it...….:thumbsup:
final results are going to be a lot better when it comes to acceleration. that motor needs a lot of gearing make it come alive. no way of getting around it.
the other option is putting a stock hs-40 back on it......:laugh: I don't think so......:no:
 
#14
Thanks for responding to my confusing gloobity goop Carbide! Yes, I grasp that it is all "relative" when you are using them all on the same chain. The ratio is the ratio. It's all just bigger.

I am not comparing one #41 primary drive with another. I am comparing the #35 primary drive I "had," with a #41 primary drive, I bought, because I was wrapped around having a ten tooth clutch driver. I did not account for the fact that a ten tooth #41 sprocket is larger than a 12t #41 sprocket.

I am limited in space. I can fit a 2.1" diameter sprocket on the driven end. That is maximum. I already have that in my #35 tooth driven. But that 10t #41 clutch driver sprocket is actually 1.61". The 12t #35 I had was 1.44", so I went the wrong way on my desired ratio. The only way to compensate that is to increase the diameter of my driven. But I can't, because the aperture (cut out) in the engine plate is limiting it. Any further down that road, and I start to weaken components.

When I was using my ratio speed for jack shaft spreadsheet, I input the sprocket teeth for the driven without realizing that the same 18 teeth on the #41 was going to be a third again as large, and in no way fit.

In reality, a 13t #41 driven will fit, and with that #41 ten tooth clutch, will put my gear ratio at 4.7:1 which is unacceptable. I either live with the 5.4:1, or take it apart and redesign the jack shaft mount, swing arm mount, pretty much the entire bike. LOL! If I were to hog out the driven aperture a bit more, I could fit a 20t in their and get 6:1 on the nose. That might be what I do, and add in some bracing around it.

That's $60 down the train for a brain fart.

Stairway was sort of passe by the time they used it for our HS prom theme song. I was always more of a Physical Graffiti guy, and little did I know that I wouldn't need any help at all, with artificially dimming the memories.
Physical Graffiti was my favorite too. Great album!
 
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