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Discussion Starter #1
So I'm looking over my gearing spreadsheet trying to decide if I should drop one tooth in the back. And I'm looking at the formulas....haven't done a complete study of it...but a few quick numbers through the calculator and it appeared to me that if a person wanted a close ratio gearbox...and real short cut could be reducing the primary drive ratio.
Remember...the spread of gear ratios in the box as it relates to engine rpm is multiplied by the primary gear ratio...in my case 3.7:1.....so...if a person could reduce the primary drive ratio to say 3.2:1 then the "spread" between each gear in the box would be that much less....effectively producing a close ratio box with a taller first gear. Final drive sprockets could be changed to adjust for the same top end speed.
Since all the box ratios are multiplied by the primary ratio...it seems to me that it would work. For instance 4th and 5th gear would still be the same number ratio different...but their relationship to engine rpm would a smaller step do to reducing the multiplier of the primary ratio.
Anyone else put some thought into this?
JohnnyB
 

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Discussion Starter #4
Rosko...cool, yeah that's what I had in mind. Think I'll write a piece of software that will graph the spread of the gears with various primary ratios so I can get a feel for it.
Problem will be solving all the problems of changing primary gear sizes, clutch basket gear etc.
Thanks,
JohnnyB
 

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just be careful that you don't end up with a huge countershaft sprocket and tiny rear sprocket to get your normal gearing back. You can only go up/down so much with the countershaft sprocket before you run into space issues.

But heck, maybe other honda crank/clutch gears will fit. It'd be interesting to see.

You can mix/match the Yamaha parts from the TZ250 and TZ350 to change the primary ratio.
 

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Discussion Starter #6
I "think" if I went with a smaller primary ratio it would mean a smaller counter sprocket. But then I'd have to make sure it wasn't so small it caused clearance problems with the swingarm. I think if it's possible on a 175 it will only be a change of like 3.7:1 to maybe something like 3.4:1 or there abouts. Could only add so many teeth to the primary drive...and only subtract so many from the clutch basket until major changes would have to be made.
Hell...I'd have to write the software to see if it was even worth the effort. Or write a good spread sheet that would graph the mph/rpm spread and compare them.

Right now the spread for five gears at redline is about 30mph to 100mph I'd like it about 40-45mph to 100mph. Or...right now about a 2000-2300 rpm spread between each gear.....which means if at 9,000 rpm I'm not pulling hard enough I'd drop a gear and go to 11,000+ which gives me a about two seconds to change gears again. If I could drop that to about a 1500 rpm spread I'd go from 9,000 to 10,500 which would leave me some room to run in that gear before having to up shift again.

I wonder what would be cheaper and easier....buying/searching/researching dozens of engines to find a set of primary gears and clutch that would work...or just send out my gears and have them copied in the right pitches. The 175 uses a "double" primary drive setup...two drive gears and a double ring gear offset by half a tooth, instead of a simple "straight cut" gears.
With a close ratio box and a 12,000+ rpm redline the little bike would start acting like a "real" race bike.
JohnnyB

PS...just thought one upside and one down. Oil pump would pump faster....clutch basket would turn faster.



Edited by - jbranson on Jul 07 2007 1:03:44 PM
 

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Discussion Starter #9
If you know the final ratio of the shaft drive and plug in sprocket numbers until they match...then yeah that software should work for a shaft drive.
JohnnyB
 

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Discussion Starter #10
Well...only halfway through writting the software and during testing I can see that my theory was wrong.
Any change to the primary ratio to lessen the rpm difference between gears is cancelled out by the changes needed to the final drive ratio to bring top speed back up to normal.

Damn gearboxes.

JohnnyB
 

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i could have told you that. but i didn't look until now and you didn't ask. writing software to figure it out? how about algebra. remember that? guess you wasted some time on something that didn't make you any faster. how about making your own gears. that will take a lot of time.
parks
 

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Discussion Starter #12
I've wasted a lot of time on things that don't make me faster...in fact most things I've done don't make me faster...they just make me smarter the next time I try something. Similar to the way Turk makes his Bulto so fast....you know?
JohnnyB

PS. Yes I do remember...the software is full of formulas that I wrote. Would take a pretty nice calculator to plug in the 24 variables, program the calculations and plot the results...so I used the best calculator I have...my computer.


Edited by - jbranson on Jul 11 2007 10:48:38 PM
 

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Discussion Starter #13
Speaking of primary ratios...I wonder what makes a designer decide on one ratio or the other....just a matter of fitting the gears (crank/clutch) etc? As primary ratio goes down you spin the gearbox faster and increase inertia effects...but decrease the torque load on the gears...so you could make them lighter. As primary ratio increases you turn the clutch and gearbox slower but increase the torque load requiring a larger clutch and gears.
I guess it would depend on engine redline rpm to some degree...I mean if the engine turns 16,000 rpm...you probably don't want to turn the gearbox nearly that fast, so you'd go with a high ratio. And with something like a Harley with a low redline and lots of torque you could go with a lower ratio.
JohnnyB
 

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what you said...
slow clutches probabley work better. lighter gears are better. clutches on the crank aren't so good (ala yami ta) aren't most primary ratios by in large pretty similar? too big a difference between any two gears/sprockets/cogs seems to loose efficiency. the smaller diameter has to sustain higher tooth loads. i know in bycycles that smaller chain rings (front) wear way faster than big rings when used roughly the same amount of time. the same holds true for rear cogs but most people aren't strong enough to use their small cogs (down hill/ fast riding) too much. in trannies, the first gear combinaton is often thicker than subsequent gears due to large the difference in diameter between drive and driven gears. top gear combos are often 1:1 (or nearly) and lead an easy life. close ratio trannies often have tall 1st gears and therefore hold up better than their stump puller wide ratio brothers. notice how much noise your lathe makes when when turning big parts (slowly) as opposed to small parts (quickly)? reverse in cars makes the most raquet in part for the same reason...it's the shortest ratio in a gear box. (yes i know there is an extra gear involved to flip dirrection) a good friend of mine owns a gear making business and could probabley make a close ratio gearbox for you honda twin guys...if enough of you do it, the price might be reasonable enough for that unbelievable speed advantage you will get. i don't know the design of those gear boxes, but you might be able to throw away first, use second as a first gear and add a new top gear. usually the 1st/2nd jump is the largest and the 4th/5th is the smallest. wholah, a close ratio gear box by doing only one pair of gears...i'm done spewing,,,got to go to work.
parks
 

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Discussion Starter #15
I'd guess primary ratios vary from about 2:1 (Harley) to (4:1) high revving 600cc.

Mary's 50 has a quasi close raito gearbox....First was moved closer to Second and 4th was moved closer to Third ( 4 Speed). Fortunately they were off the shelf speed parts for that machine.

The 175's have a pretty typical gearbox in that, yes, the first to second gear jump is stupid. 1st to 2nd being about a 35% jump. 4th to 5th is perfect...about 14%,
The only gears that would really need changing are first and Second...moving both closer to third. Ideally...the first three gears would be changed. A tall first gear won't hurt much becuase of the 3.7:1 primary makes for a large robust clutch.

I've actually looked into getting the attachments for my mill to do some gear hobbing....the two problems I ran into were machining the dog section and the internal splines. And..I think first gear would require a new input shaft since that gear ( I think ) is made as part of the input shaft ( for sure on Mary's 50 ). Also...some of the gears are "Paired" which I imagine would be tough to machine.

The 175 could use some help...but luckily the gearbox is better suited for racing than a lot of OEM boxes I've seen. Except for 1st to 2nd it's pretty good.
JohnnyB
 
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