sprocket question

[ramairgtz]

Got my kart going last night and used my gps and the top speed was 28.9 mph. My front wheels are toed in alittle so it's probably safe to say it will do 30 when I fix them. Here's my set up gx390 comet 40 series, 10t jack shaft to a 60t on the axle and 18x9.50-8 tires. It will leave marks in the street when I take off so how much take off speed would I loose if I changed to a 11 or 12t on the jack shaft? I would be happy if it did closer to 40 but I don't want to loose too much take off speed any suggestions?

 

[ramairgtz]

according to the speed calculator a 12t should bring my top speed to 39mph where the 10t is 32mph but that wasn't figuring the torque converter

 

[ramairgtz]

Anyone know what gear ratio the 40 series is from idle to wot?

 

[Doc Sprocket]

I think it's around 2.43 on takeoff, and it's 1:1 high. I'm running 4.8:1 final with 17.5" tires, GX340 and Comet 40. I top out at 45 mph in a stretch, and it's a heavy kart.

EDIT- Just checked the manual, the above info is correct.

 

[r97]

i think realalistically you can expect 35 from 12t, go here and play around with yor gearing, http://gokartguru.com/engineering_tool_kit.php

 

[ramairgtz]

Toystory 4wd what gears are you running? Sounds like we have a similar set up

 

[ramairgtz]

i think realalistically you can expect 35 from 12t, go here and play around with yor gearing, http://gokartguru.com/engineering_tool_kit.php

Says with all the numbers put in I should be at around 32mph with mu current set up and almost 39 with a 12t. Didn't realize this til I searched but the specs say my kart weighs 443lbs

 

[r97]

remember i said realistically its safe to say if that calculator says your going 45 on a single seat kart you will prob be getting about 40, assuming you have a two seat kart with a cage you should look for the calculator to say about 47 to 50mph to get you to actually hit 40mph

 

[r97]

ithink that 14 to 17t sprocket will get you to 40+ maybe a bit more, but you wont be spinning the tires.

 

[Doc Sprocket]

Toystory 4wd what gears are you running? Sounds like we have a similar set up

Like I said, 4.8:1. How you achieve it doesn't matter. I've got 10T on the J/S, 48T on the axle. You could do it many ways and achieve the same ratio. For what it's worth, I could still go a little lower... But then, you've got a bigger engine.

 

[ramairgtz]

remember i said realistically its safe to say if that calculator says your going 45 on a single seat kart you will prob be getting about 40, assuming you have a two seat kart with a cage you should look for the calculator to say about 47 to 50mph to get you to actually hit 40mph

That calculator should be pretty accurate since it figures in the weight of the kart and of the driver. I have to re do my jack shaft set up anyways since those crappy bearings came apart so I'm going to weld the bearing holders to a 1/4 thick plate and bolt it on so I can move it if I need to play around with sprocket sizes. The kart has slotted holes all the way back so it will be very easy to do.

Toystory_4wd how is your take off speed?

 

[ramairgtz]

with a 12t and a 60T that should give me a around a 5:1 ratio which may me what I'm looking for

 

[Doc Sprocket]

Takeoff's not bad, the tires spin on dirt and gravel. It's enough to do donuts from a standstill, too. I don't have any pavement to run on, but if I did, I'd probably go a little deeper, 5:1, maybe a bit more.
I should probably also mention that my governor's been tweaked to top out at around 4200RPM, and I increased the tension in my driven to delay high range.

 

[r97]

That calculator should be pretty accurate since it figures in the weight of the kart and of the driver. I have to re do my jack shaft set up anyways since those crappy bearings came apart so I'm going to weld the bearing holders to a 1/4 thick plate and bolt it on so I can move it if I need to play around with sprocket sizes. The kart has slotted holes all the way back so it will be very easy to do.

Toystory_4wd how is your take off speed?

but it doesn't calculate wind resistance and friction ECT

realistic top speed upper 30's to about 40
http://gokartguru.com/engineering_tool_kit.php

low range

http://gokartguru.com/engineering_tool_kit.php

notice you wont spin your tires anymore

 

[freeman508]

ithink that 14 to 17t sprocket will get you to 40+ maybe a bit more, but you wont be spinning the tires.

no cause i have 13 40 and i can go 93 but ihave an dirt bike engine with 40 hp so lil difertent

 

[freeman508]

and i can burn out alot

 

[r97]

you have 13 40, and a 5 speed tranny and an internal reduction, prob about 6500 to 8000 rpm, 40 hp, and have you actually used a gps or speedo to find the real speed because past 60 your talking serious wind resistance.

p.s. remember that edit button?

 

[ramairgtz]

I found a big part of my problem. This spring seen in the pic below was bunching up and stopping the pedal from opening the throttle all the way. I'm talking about 1/4" of movement on the lever on the motor. So I backed the nut off and used a different spring on the side. I'm going to order a 11 and 12t from bmi karts tomorrow. I'm taking the kart out of town with me this weekend and I was trying to have everything worked out by then but I'll finish tweaking it when I get home.

This is off the subject but my driver unit keeps stating to engage when the motor is idling. Comet says it engages at 1600 rpms and my motor is supposed to idle at 1400 rpms. If I turn the idle on the motor down it wants to die so what should I do? If I watch the driver it will kinda pop over a little bit them go back and when the kart is on the ground the weight of it keeps it form moving so I guess it's not really a problem.

 

[r97]

when the kart is on the ground the weight of it keeps it form moving so I guess it's not really a problem.

problem solved, this is common and is called "creep" also your engine may not idle at exactly 1400 rpm and the tc may not engage at exactly 1600rpm

 

[ramairgtz]

Just making sure it wasn't a problem. I'll have my new bearings tomorrow and if I can get my seats mounted, front wheels aligned and the jack shaft redone I'll take it for a ride and see what it does. I'm putting money on 32mph