gearing for my gokart

[xxmiaja]

I have a predator 420cc engine gokart with 18x9.5 tires in the back and 145/70-6 on the front it has a 9 tooth jackshaft sprocket with 54 tooth axle sprocket this axle is positive traction.im 260 lbs.is this the best gear ratio for me the cart will hit 35 mph it takes off pretty fast and will climb any hill..can I get more torque and speed from this kart or should I leave it like this also I'm running a 40 series torque converter

 

[itsid]

more torque, yes.. with a 60T axle sprocket for example.

more top speed yes, but although it's a big engine I wouldn't gear any lower,
so more speed will require more RPM from the engine..
with all that's tied to it..
governor removal, high airflow intake & exhaust..
and at one point all the nasty expensive billet stuff (con rod & flywheel)
maybe a different carb and so on and so forth.

So, if you want to stay within a reasonable pricerange, I'd say remove the governor
get a high flow filter and exhaust, then rejet your carb accordingly...
that shouldn't cost too much even for an engine of that size.

and keep your gear ratio.. you'll not add torque but top speed that way.
(just see where you end up that way)

If you want more torque, as mentioned increase the gear ratio;
you can play around with the newly found numbers.. to find a sweet spot right in the middle,
but I think with above mods and a 60T axle sprocket you'll hit 40mph and get ~11% more torque

'sid

 

[KartFab]

you may have clearance issues with the jackshaft sprocket and the axle sprocket if you go for a bigger sprocket on either.

if you want more speed, you can get 22x11-8 tires and hit 39-40 mph with the same sprockets you have on there now. Just will cost you like $120

 

[xxmiaja]

I can not understand how a 22 inch wheel will make the gokart faster this wheel is bigger and heavier.i am not saying that you are wrong I'm just wondering how and if i know how then i will put them on the cart

 

[KartFab]

Or get a bigger Jackshaft sprocket (10 tooth). That's probably cheaper.

One rotation of 18" wheel is shorter than one rotation of a 22" wheel. The engine goes the same rotations per minute regardless of tire size.

 

[itsid]

....

if you want more speed, you can get 22x11-8 tires and hit 39-40 mph with the same sprockets you have on there now. Just will cost you like $120

...
with the difference that he'll need almost 50% more torque to get the kart moving.
(thus reducing acceleration by a huge amount)
and even worse will not be able to climb much of a hill anymore..

'sid

 

[xxmiaja]

Ok so am I needing 50 percent more torque to move the 22 inch tires or if I move to a 10 tooth jackshaft sprocket

 

[KartFab]

22" tires here, 389cc engine, 54t axle, 9t jackshaft.

http://youtu.be/B18ybsN8Sf0

Cheesy video I know. Not sure about 260 lbs, but it could climb that same hill with me on it and it was a pretty hefty kart

 

[itsid]

Ok so am I needing 50 percent more torque to move the 22 inch tires or if I move to a 10 tooth jackshaft sprocket

the tires.
Some numbers:

sprockets	max accel.	speed@3600rpm	speed@5000rpm
18" wheels
 9 : 60		3.17 m/s²	28.90 mph	40.14 mph
 9 : 54 	2.85 m/s²	32.13 mph	44.62 mph
10 : 54		2.56 m/s²	35.70 mph	49.58 mph*

22" wheels
 9 : 60		2.59 m/s²	35.33 mph	49.06 mph*
 9 : 54		2.33 m/s²	39.27 mph	54.54 mph*
10 : 54		2.10 m/s²	43.63 mph	60.60 mph*

*speed is unlikely to reach

22" tires here, 389cc engine, 54t axle, 9t jackshaft. ...

I don't say it won't work.. I've seen your kart before

But since xxmiaja asked for more torque,
and increasing the tire diameter is doing the exact opposite;
I don't think that's the way to go here.

So all that's left is increasing the rpm for more speed,
and reducing the gear ratio for more torque.

'sid

 

[xxmiaja]

Yes I think you are right so I think I will keep what I have because my cart went up a hill just as steep as that if not steeper with no problems at all and I made a note to stop at the bottom of the hill then start.also in the video when I take off in the dirt or grass my kart gets sideways every time spinning both wheels...thanks for all you guys reply and time.and sid the chart was a big help.it is amazing that thing will get 50 mph at 5,000 rpm I wonder can i reach that rpm buy simply pulling the butterfly for a second

 

[KartFab]

Good choice. I have the same tire set up but with a smaller honda GX390. It tops out at 33 mph, but has just a sick amount of torque. I really like that part, but I'm switching to 22" tires so i can go 40, and have better ground clearance.... I like to keep the engines i have in 'stock' form (except for one in the works).

governor removal/aftermarket throttle set up/intake exhaust should get you the torque AND speed that you want (as itsid could only explain in detail! amazing). I'm afraid that once I start tinkering with all that performance stuff, that I will just end up buying all the expensive parts for more rpms.

so here is another angle on it (if you want to play around with gear ratios).... just buy a 10 tooth sprocket for 10 bucks, and swap it for the 9tooth. If you like it, keep it, if you don't, switch back to the 9tooth. Minimal risk/investment, and you can always go back, you have got nothing to lose! ....sounds like a sales pitch, but its the truth!

 

[xxmiaja]

I really like the looks of the 22 inch though.as for buying the high performance parts, the money is no issue buy I keep mine stock because I like my kart to be reliable and start every time with no engine issues.if I go adding all that to my engine I will wear it out faster...just my opinion

 

[itsid]

...it is amazing that thing will get 50 mph at 5,000 rpm I wonder can i reach that rpm buy simply pulling the butterfly for a second

Again 50mph is unlikely
(that's just a number taken from theoretical wheel rpm and circumference)
with speeds like that you'll have to deal with a reasonable amount of drag and rolling resistance..
But an engine like that should come close I think

...
so here is another angle on it (if you want to play around with gear ratios).... just buy a 10 tooth sprocket for 10 bucks, and swap it for the 9tooth. If you like it, keep it, if you don't, switch back to the 9tooth. Minimal risk/investment, and you can always go back, you have got nothing to lose! ....sounds like a sales pitch, but its the truth!

Actually not a bad idea at all..
10 bucks and maybe two more links for the chain shouldn't be too much for a nice test.
if it's good it's good if it's not.. no harm done

.. I keep mine stock because I like my kart to be reliable and start every time with no engine issues.if I go adding all that to my engine I will wear it out faster...just my opinion

Fair enough!
I don't think starting will be an issue, but of course you'll wear out the engine faster if you run it at higher rpms.
And if you keep the engine stock.. you can only have either more torque or more speed; not both.

or to keep it as simple as possible, if you have a sprocket hub
you can always get some split sprockets and change your gear ratio according to your current needs.
a 49T for more speed (roughly what a 10T JS sprocket would do)
the 54T you have and a 58-60T for steeper hillclimbs.

You'd need to dial in the chainlengths for every sprocket,
but if you keep the shortest chain (9:49) and with a second master link you only add the missing links per set up;
you can in fact even carry the sprockets and chains around in a small pouch.
And within three minutes you changed your gear ratio at the scene
No engine mods needed and you have everything handy.. more torque and more speed

'sid