Gearing question?

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shane73

New Member
#1
#1
I know this isn’t a minibike but it should help anybody with minibike gearing.

I have read tons of gearing threads and played with lots of calculators but all just tell me what numbers I will get. If I end up with the same number in the end – the best combination I used to get it – that’s what I can’t find.

I am putting together a kart and I am having trouble with the gearing. I am looking for more torque / power than speed. 14 – 20 MPH would be fine with me. I understand the ratio thing somewhat. A 10 tooth clutch and an 80 tooth sprocket with no jackshaft will give me 8:1. Now if I take and add a jackshaft and go 18 tooth in and 14 out and change the sprocket to a 62 tooth, I end up with 7.91:1. Again if I go 20 in the jackshaft and 10 out and change the sprocket to a 40 tooth, I get 8:1. The last one sounds the best to me because it would give the most ground clearance.

I just don’t know enough about how this stuff works to make the right decision. All I know is right now I have 18” tires and more than likally the clutch I get will be 10 – 12 teeth.

I can make the same ratio several different ways. Is it better to use a smaller sprocket and a bigger jackshaft sprocket or larger wheel sprocket and smaller jackshaft if it comes out to be the same ratio?

I also don’t know what pitch chain to go with. The 35 looks to be more common and you can use larger sprockets without it being to large in diameter. A small tooth 35 pitch wheel sprocket would give the most ground clearance but the 41 pitch looks to be a lot heavy duty-ier. Is one better than the other – smaller – more common – easier to maintain – etc.?

I want to buy this stuff soon but I don’t want to make a mistake and wish I had done it different later.
 
steven Durham

steven Durham

#2
#2
Lots of things that can be done on a Kart that is more difficult on a mini bike. You can mix your chain use or stick to one size Adding the jack shaft is a plus in that you can change
your selection in multiple places. Clutches often come with different sprocket sizes and if you use a plate sprocket and hub on the jack shaft then you changing your primary gearing and leaving your secondary gearing alone. I have a 5/8 inch bore hub that uses different plate sprockets to archive the different gear ratio's Bonanza did that on some of their mini bikes.
Steve :scooter:

 
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shane73

New Member
#3
#3
steven Durham said:
Lots of things that can be done on a Kart that is more difficult on a mini bike. You can mix your chain use or stick to one size Adding the jack shaft is a plus in that you can change
your selection in multiple places. Clutches often come with different sprocket sizes and if you use a plate sprocket and hub on the jack shaft then you changing your primary gearing and leaving your secondary gearing alone. I have a 5/8 inch bore hub that uses different plate sprockets to archive the different gear ratio's Bonanza did that on some of their mini bikes.
Steve :scooter:
Click to expand...
Yep, thats the reason I had decided to do it this way. If I can just find the answer to my few questions, I am ready to start building. I guess I can go ahead and order my axle, pillow bearings, jackshaft plate, and stuff like that though. I was just trying to order it all at the same time and save some on shipping.
 
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shane73

New Member
#5
#5
Thanks - I already had that site on my list though. I have found several places to get the sprockets and hubs at pretty good prices - BMI has some of the best prices on the split 35 sprockets, hubs, and the jackshaft sprockets. I got a list here of places with the best prices I have found on the 41. It seems 41 varys from place to place. Soon as I figure out the best way to go, im ready to order. I figure someone will have the answers here soon. I see a few good deals on ebay every now and then too.
 
Oldsalt

Oldsalt

Well-Known Member
#6
#6
You say you want a top end of 14 to 20 MPH. So use 17 as the top speed at 3600 RPM [max for a briggs type engine unless you wish to live dangerously].

The formula to find the proper final gear ratio is:

Gear Ratio = [RPM X Tire Diameter] divided by [MPH X 336]. Plug in 3600 as the RPM, measure the outside diameter of your rear wheel and plug that into the formula, and use 17 for the MPH. The number that falls off the end will be your final ratio.

If the clutch has [for instance] 12 teeth multiply the answer by 12 and you have the number of teeth required for the rear sprocket. Then your top speed at 3600 RPM will be right at 17 MPH.

The 336 is a 'constant' in the formula to account for 'time' [because we are dealing with miles per HOUR].
 
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shane73

New Member
#7
#7
Thanks Oldsalt - yea that’s about what MPH I have been using. It comes out to be a little over 10:1 so I have plenty of torque. Here is a good calculator that makes it a lot easier on the math part of it.... Jackshaft Ratio Calculator

I spent another 12 plus hours researching my questions last night and it seems there are a lot of people wondering the same questions. I did run across a very good thread over on 4cycle.com and it does make a difference as far as racing. They said they couldn’t explain it either and said logic would make you think it wouldn’t but - several people - same day - same weather conditions, same motor, same chassis, same everything - all they did was change the sprockets - went from 12/72 to a 10/60 or a 9/54 and they said it handled different in the turns, pulled different out of the turns, different on the straight-aways – leaving everything else the same. With racing, every little thing makes a noticeable difference - even if nano-seconds. I don’t know if physics says it shouldn't - but many said it does -for a fact.

I feel better now knowing my concern and questions has some merit to it now anyway. I am not looking or don’t really care about making my kart a half second faster out of the corners but just by choosing to buy one over another and making less stress on a shaft, or less stress on the clutch, I care about. Just my way of thinking – I would think if you divide it up – it would equal out the torque on all the shafts, axle, and bearings. Im sure there are a magic combination that has to do with weight, tire to surface coverage, foot pounds of torque, and all the technical stuff that’s way over my head.

I guess – unless some engineers or physicist on here can answer my questions – I am just going with what I think may be best. What I have in my head….. I figured a 10 tooth on the motor. I think the smaller on the motor would put less pressure on the gears than a larger gear and less pressure on the crankshaft bushings. Then I think a 40ish tooth on the axle sprocket. Im thinking the smaller the more ground clearance I get and less chance of damaging the teeth or chain. I also think too big of one would puts more stress on the axle. A bigger one would make the chain coming off easier. A smaller one would wear the teeth faster – all the power distributed over say 45 teeth verses say 20 – puts the motor torque in contact with more metal. Plus I think the smaller on the axle – the larger (more metal) I get to use on the jackshaft gears. I figure to divide the metal out over the gears.

Now I have 56” circumference of tire – 10 tooth engine driver – 45 tooth axle – a engine that will WOT at about 3600 RPM. I know I want about a 10:1 ratio and a top speed of 14 – 20 MPH. I can use several different combinations of jackshaft gears to get a combination that’s close to the two. I think I need to divide it out so as one gear doesn’t have to take more torque over the least metal than it has to.

I can also go back and change the drive gear and sprocket gear and use a different combination of jackshaft gears and get the same exact final output I want but my way of thinking makes me use the 10 tooth starting and 45 tooth final gears.

I also want the gearing where as the clutch spins over “engagement speed” most of the time. This may mean putting the clutch on the jackshaft instead of motor – I may have to look into that. This means I need to go with a ¾” jackshaft and bearings.

I think the chain size needs to be a 40 pitch because its larger, fits deeper in the teeth, and it has about 1,700 more pounds of tensile strength. It is also the next most common used one other besides the 35 pitch.

I think the sprockets need to be 40 pitch because of the chain I need to use and it has larger, deeper teeth. That means stronger and less chance of chain slipping over the 35 pitch.

Now, my brain may be over looking something minor or major. I am sure one combination is better than another for one reason than another. I know when a multi-million dollar kart company gets ready to put out a new line of high end karts with jackshafts and stuff, they don’t just say “we want the kart to go this fast, what gears you got laying around over there George”? Im sure there are a technical reason and explanation behind each combination of gear sizing on each design.

I haven’t tried and counted yet but, I bet I could make a 10:1 (+/-) gear ratio with an output of 19MPH (+/-) at 3600RPM with a 56 tire circumference with about 1000 different gear combinations. If clearance, availability, and price is not an issue, then that only leaves my original questions.

Its not a great big deal and im not trying to be picky but, I hate (and im sure a lot of people hate) to buy all the parts for a project, put hours of labor into it, get it finished and then find out later it could have been better for the same amount of (or less) time and money. Or also later someone say – you could have done it this way and it would have been a lot better.
 
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steven Durham

steven Durham

#8
#8
Shane73: First off I hope you have a strong heart and are in good health. But your going to worry this project to death and yourself included.
First off if you go to a go kart track every racer there will have lots of gearing combinations ready to go for quick changes made for track conditions and even weather. And when you buy a kart it is just the chassis no tires, no engine, no clutch WHY it is because there is umpteen products to let's use the term I heard so often they want the ( HOT SET UP ) and it goes on and on .
I had customers who though they had every T crossed and ever i dotted and often it was a disastrous day because they would not listen.
I once had a customer who build a kart his way with the best of everything took 2 years to build and it was his vision. took that Kart to Pismo Beach Sand Dunes and got to ride for less than 10 minuets before he totaled his frame.
Not wishing to stomp on your parade but take a look at what karts like the Manco and others that use just a clutch. And you will see that most have torque converters to handle the gearing questions you have. quit worring about so much I would stick with #41 chain not #40 as most avalible gearing is for #41 chain
Steve :scooter:

 
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shane73

New Member
#9
#9
I had everything else figured out in no time – it was just these sprockets. I have the option to choose almost every size for each place and they all are about the same price. So it was just the simple part of choosing the size for each place that was stopping me. I defiantly understand the part about listening. I try to read and listen to every bit of advice given - To me or someone else. Especially on something I don’t know much about and the other person has experience at. Its just these dang sprockets – no one knows the answers for me to listen to on this one, lol. Around here where I live, all the places have ready to go karts. I haven’t really ever paid attention to the set up on them but I bet they do have torque converters on them.

I would love to put one on this kart but when I drug it out of the back yard, I set a $300 limit on fixing it up. I needed a seat cover, seat belts, clutch, tires, rims, paint, chain, and a motor. It was going to take about $120 to replace the 5” tires, tubes, and bearings. I knew I could do a totally new live axle for about the same price. I already had several 18” tires and rims, and a few smaller things. $100 for a new 6.5HP motor - $75 for clutch, chain, and jackshaft - $75 for axle, sprockets, brake, and hubs. That leaves me about $50 for odd stuff. Looks like I still will end up going over my limit though.

I am actually working on 2 karts at the same time. The other one is a Manco Dingo with 99.9% of its original parts, decals, and stuff. It has a CAT99 driver on the motor and what looks like the 40 series for the driven. It’s what ever came factory though. Im the second owner, my friend never rode it much. I just say 40 series because it has the wear buttons like the 40’s do and it came with a red spring. It already had its factory gearing go it saved me all this worry.

You got me wondering on the 40 / 41 chain though. All the gears / sprockets I have been looking buying said for 40 / 41 chain.

I was looking at this clutch because its priced good and you can change the engagement springs out in a minute or so and they are only about $3 something a spring. It also takes heat well.

Hilliard Extreme Duty 3/4" Clutch 10T #40/41 (404610)

Hilliard Extreme Duty 3/4" Clutch 10T #40/41 (404610), www.bmikarts.com : BMI Karts & Supplies : Go Karts Parts : Racing Go Kart Parts : GoKarts : Versailles, Ohio : USA : Indoor Track : Tips

The manufactures website says to use either chain too.

Extreme Duty Centrifugal Clutches - Go-Kart, Racing Kart, and Youth Snowmobiles

If you have the type children like mine who feather the gas – this looks like a pretty good clutch for not getting burnt up. It can be made to enguage anywhere from 1200 RPMs to 3000 RPMs

The sprocket says Sprocket For #40/41 Chain

#41 Sprocket 45T w/ 1" Bore (400227), www.bmikarts.com : BMI Karts & Supplies : Go Karts Parts : Racing Go Kart Parts : GoKarts : Versailles, Ohio : USA : Indoor Track : Tips

and looking at the chain sizes look to have the same pitch and roller diameter. The roller width is wider by 1/16” but that’s only 1/32” extra hanging off the side of the sprocket gear. The pin inside the roller is 1/32” bigger but that would only be noticed when the chain was taken apart.

Chain - Purchase by the Foot (4006**), www.bmikarts.com : BMI Karts & Supplies : Go Karts Parts : Racing Go Kart Parts : GoKarts : Versailles, Ohio : USA : Indoor Track : Tips

The sprocket above is just like the one I was getting but not from BMI. I was going to get all my sprockets / gears from SurplusCenter though because I can get all 3 for the price of that one.

All that said though – I do see some of the clutches only list #41 for chain size. They don’t mention #40. But….. Take that same clutch and Google it – it will be listed at another retailer as for #40 / #41 –
And coming from someone like you – you mess with these things way more than me – you have a reason for saying that. Is there a difference in the sprockets and clutches? From looking at the chain guide link I posted above – there shouldn’t be a difference in size other than width. #40 being the 1/32” wider on each side of the gear tooth - #40 should fit any #41 sprockets / gears and if the clutch face is not all over the sprocket – it should fit any #41 clutch sprocket.

I have no clue – until I found that chart – I didn’t even know there were more than 2 sizes. I always thought 40 / 41 were the same. One was just an older way of labeling chain or something. That chart may even be wrong or I am looking at it wrong.


so now you have gave me an aneurysm to go with my heart attack. I just thought I had all my gears / sprockets picked out.
 
Oldsalt

Oldsalt

Well-Known Member
#11
#11
Tikker said:
Do you mean the rear tire in this formula?
Click to expand...
Yes. The diameter [and hence the circumference that it will roll in one revolution] is a necessary part of any formula that deals with RPM, or final speed, or gear ratio.

In my humble opinion it is best to have the following formulas on a piece of paper next to the calculator in your tool box. No computer or bookmarked site is necessary.

To determine the TOP SPEED using any final gear ratio and RPM and tire diameter:

MPH = [RPM X tire diameter] divided by [final gear ratio X 336]

To find the final GEAR RATIO when the RPM and MPH and tire diameter is known:

Gear Ratio = [RPM X tire diameter] divided by [MPH X 336].

To determine the RPM when the MPH and gear ratio and tire diameter is known:

RPM = [MPH X gear ratio X 336] divided by tire diameter.

If the RPM, MPH and gear ratio are known and you wish to find the rear TIRE DIAMETER:

Tire Diameter = [MPH X Gear Ratio X 336] divided by RPM.

PM me if you have any questions about your application.
 
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Tikker

Tikker

New Member
#13
#13
OldSalt, It all makes sense to me. Actually reminds me of school days.
Great formula to have on hand since I am currently doing a "build" and want to add a jack shaft set up, of some type. Would like to do a torque-a-verter but cost is going to prohibit going that far.
The online calculators I have found, using this formula, are great for doing the different sprocket options and writing those down, as well.
Thanks.
 
Oldsalt

Oldsalt

Well-Known Member
#14
#14
I am glad to possibly be of help for your future projects. As far as I am concerned it is indispensable to determine the gearing and etc, based upon intended use, rev limit, and tire size, BEFORE building any minibike. Even with a TAV it is necessary to plan on the final chain drive ratio.
 
Tikker

Tikker

New Member
#15
#15
Didn't have an interest in the technical points of building projects until my more mature years.
This information has given me pause to actually consider how I'll be using it and possibly prevent revisions being required for bad results.
 
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