Sprocket advise before I pull the trigger

[bnt5]

I am thinking about changing out my 60 tooth drive sprocket with a 78 tooth. I currently have a 12t driven and a 60t making my ratio 5:1 which according to a pro I spoke with sucks for torque. Before I tear everything apart and order the new sprocket, am I heading in the right direction with a 6.5:1 ratio?

Also would changing the drive sprocket on the clutch froma 12t to say a 10t be the same as changing out the larger sprocket on the axle?

The kart has 20" tires on the back and very little torque for trail riding. Just wondering.

 

[DCProductions]

When you say 12t driven, are you referring to a torque converter or a centripetal clutch? Centripetal clutches do not work very well on trails. They are best where there is no starting and stopping. With 20" tires on your kart )even with a torque converter) I would suggest gearing to 6.5:1. With a centripetal clutch, I would suggest an even higher ratio.

 

[Krustalien]

Making the drive sprocket smaller and making the driven sprocket larger both have the desired effect, more torque. More torque, but bare in mind lower top speed.

 

[racerc2000]

a 10T would give you a 6:1

@3600rpm your original setup is for 45mph on the top end
dropping to a 10T states 37mph

a 78T is 34mph.

a 10t with 78t is a 7.8:1 and 28mph.

this is all at 3600rpm the normal governed rpm

now you may not want to do this but a smaller tire would help more then gears as 20 is huge.

dropping from a 20 to 18 would drop your speed from the 45 to 38mph with your current gears. and be physically less rotating mass.

a 16" tire would give you the same gearing result as a 78T @34mph with stock gearing. but lining 2 carts up one with 12/60 16" and 12/78 20" even though the top speed is identical I feel the 16would be faster on all counts.

im guessing a torque converter is out of the question?

 

[bnt5]

The buggy has a comet 40 torque converter on it, with a 9 hp robin subaru that I just took the governor off of. The sprocket on the driven torque converter (jackshaft gear) is 12T and the main axle drive sprocket is 60T. I measured the available room for the axle sprocket and I am maxed out with my 60T, so I was wondering if I can just change out the TC sprocket (jackshaft gear) to a 10T instead to get the same desired effect? After looking at the setup on the kart I noticed that there is a backup gear/sprocket in the middle of everything so not sure if this changes things. I added some pics to show how everything runs, between the final drive 60T sprocket and the 12T jackshaft gear (torque converter sprocket) there is 2 other sprockets involved....arrgghhh. I guess if I change the 12T to a 10T and everything else remains the same I will still raise my gear ratio and get more torque....

 

[racerc2000]

yes having 2 chains and 4 sprockets does make gearing difficult to determine.

if possible you may want to try what you were thinking.

run the 12T direct to the 60t and see the diff.

if possible could you give the sprocket specs?

12t to ?? with ?? to 60t it will help to figure out a real ratio and what mods could be done.

dropping to a 10T may not give the desired result when geared in a 2:1 or 3:1 jackshaft setup. it may be better to mess with the sprockets on the jack shaft.


you may also be able to flip the jack shaft when we have sprocket sizes a proper ratio can be given for all your options

 

[bnt5]

12T to 25T then 18T to 60T
2.08:1 3.33:1

 

[racerc2000]

your current set of sprockets adds up to 6.9:1

changing the base sprocket from 12T to 10T would give you 8.33


flipping your jack shaft sprockets would net 3.6:1 so thats a no go.

since it would prob be harder to find a sprocket under 10T

you can get a 8.33:1 if you swap sprockets to

12T 30T 18T 60T

or

12T 25T 16T 60T for an 7.81:1 ratio. with the torque converter you would prob keep a decent speed to. this is assuming those jackshaft sprockets are just that 2 sprockets separately mounted on the shaft.


if those sprockets are a single unit. due to the 60T being at its space limit. your only choices would be to find a smaller then 10T or make a new jack shaft and pickup 2 new sprockets for it.

but the lowest cost would be to bypass the jack shaft and use a 10T or smaller directly to the 60T

10T 6:1
9T 6.66:1
8T 7.5:1

using the 12T would drop it to 5:1 so not good.

 

[Krustalien]

I don't know if I am understanding this correctly. Going by those ratios, you don't get 5.41:1.

Forgetting the torque converter, I believe you get a ratio of 6.9:1 with a 12 tooth driver > 25 tooth jackshaft input > 18 tooth jackshaft output > 60 tooth axle.

You multiply the ratios 2.08:1 and 3.33:1, not add them.

Looking at the images, I'd suggest making the jackshaft input sprocket larger, to say 30, which gives you an overall of about 8.3:1.

But, a torque converter, 9hp and 6.9:1 I would have thought would be fine already. Obviously it's not, but still...

Just wanted to point out that the math makes your current ratio 6.9:1.

 

[racerc2000]

you are correct. it was late and my brain wasnt working for some reason

http://www.compgoparts.com/TechnicalResources/JackshaftRatioCalculator.asp makes calculating easy

6.9:1 would be fine for 16" or 15" tires. but 20" are another story.

6.9:1 with a 20" tire should net 40mph @4800rpm(assumed conservative no governor)
6.9:1 with a 15" tire would be 30mph @4800rpm
to match this basic go kart performance
9.38:1 with 20" would net 30mph @4800rpm

ps... I think I fixed everything