Knobby Tire Sizing

[Sleepy122CID]

Knobby Tire Sizing - Where to Buy 7" Wheels?

I'm working on a "resto-mod" of sorts on some rust bucket of a caged off-road style rigid kart I picked up off craigslist. I started working with what I had, but after realizing how serious the rust was I decided the powertrain will be better off (and more fun) with a more serious setup.

It will start as a 13hp stock Harbor Freight engine with Comet 40 series setup. I plan on getting a 60 tooth final drive sprocket (#40 chain, live axle - 9 3/4") to start and then tweaking the clutch setup and/or sprocket if necessary to find the balance I desire.

I really have no idea what size tires to start with. The kart will be used off-road, but the course will be fairly smooth. There will be no jumps, but there may be mild bumps/ruts. Throttle response from a stop or low speed is my main concern, but I'd like to get something as large as possible (without bogging that engine) for ground clearance and shock absorption.

Any suggestions from anyone with similar setups? I've done a little research, but this seems like the most expensive part of the setup that I may get wrong, so I'd like to get advice from more experienced karters if possible. Also, should I run the same size front and rear or would it be beneficial to run a shorter (or thinner) tire in the front. I'm guessing it would at least be a good idea to run a narrower tire in the front to ease steering and decrease oversteer...

 

[97gokrter]

if you get then for 10" you can get them 5$ a peice

 

[devino246]

where?

 

[Sleepy122CID]

I'm not really trying to go super frugal... I was kind of thinking like 16" F and 18" R with a 10 tooth clutch sprocket and 60 tooth axle sprocket... something like that - but I don't know if that would be too serious... This is going to be an adult kart.

 

[Doc Sprocket]

That's not bad. My setup's not much different- 11hp, comet 40, 17.5" (measured) static tire diameter, 4.8:1 final drive. Works pretty well. With 13hp, that'll give it a little more grunt, and if you find it still lacking, go a little deeper with your final ratio, say 5.5:1.
EDIT- when you say "clutch sprocket", I assume you mean "jackshaft", as attached to your comet 40 driven, right?

And yes, smaller, narrower tires in front should help.

 

[Sleepy122CID]

Yes I do mean the jackshaft sprocket. I don't know where to find a 40 chain sprocket bigger than a 60 tooth or I would start with it. Any ideas?

 

[robbie]

60 teeth is great. If you want to do 4.8:1 or 5:1 as suggested, get a 12 tooth driver to go with it.

If you already have the two sprockets you mentioned, with 10 and 60 teeth, try it out. If you don't like it, then you could change either sprocket later. It's probably easier (and cheaper) to get the little sprocket in different sizes than the big one.

 

[Sleepy122CID]

Ok, well if I can find a 7" wheel for the front I'll try 16F and 18R. I guess even if I can't do it with just gearing I could always start modding that engine a little bit to help.

Thanks for the help guys...

 

[Sleepy122CID]

Anyone know where to find 7" wheels? I spent last night looking for what seemed like hours! I can find 5", 6", 8" no problem, but no 7"!? The reason I ask is because the front tire I was planning on using (to go with the AT18x9.50-8 rear) is a AT16x8-7. That does mean a 7" wheel, correct?

 

[Doc Sprocket]

Yup. Sorry, I have no idea where to get one, I've never even seen a 7" wheel. Maybe it's one specific ATV manufacturer.