gearing guidance

[kc8oye]

i'm trying to figure out where I want to start out with my gearing..

looking at starting with a predator 212 for now (Cheap)
will likely upgrade to a 420 eventually..

planning on 16x8 rear tires.

watched some videos on youtube of a couple guys getting 45mph out of a mod'd 212 in a lightweight cart and a substantial rider..

I'm certain I'm not to interested in going much faster then that...

not knowing much about these small engines.. I understand un-governed they can be pushed to 6000 rpm with some mods. (fly wheel namely)

so for lack of any where better to start with... 16" tires is 945 rpm @ 45mph..
6000 rpm / 945 rpm = 6.34

does 6:1 seem like a reasonable starting place for gearing?

I'm a fluffy dude and this cart isn't going to be particuarlly light weight..

 

[itsid]

I'm not as confident in 45mph with just a flywheel..
frankly I doubt it'll hit the 6k rpms without any more substantial mods at all.

5k rpms is close to a certainty with the governor removed,
5500 is what we assume (some hit a few hundred more, some not)
depends on the individual engine when the valve float kicks in
and it's setup (kart weight, gearing etc.) if it provides enough power to rev up further.

the higher your gearing the less likely it is the engine has the power to hit the targeted rpms.

So no matter what you do, the gearing must match the weight and wheels size, or even an engine which revs up to 7500 rpms without load cannot propell you past 3600-4000 while you're driving.

Back to the mods..
governor removed -> 5500
breath-easy (new intake and exhaust, new jet) to prevent the engine from choking up there.

You can add a timing key to shift the power up (you'll loose bottom end torque though)
which makes it even easier to hit the rpms.

More rpms needed?
then you will need to install a billet conrod, since that's even more likely to fail than the flywheel (it's also cheaper than a flywheel )
Anyways, consense around here: you need a conrod if you install the 18lbs valve springs.
those are able to prevent valve float before 7400-7500 rpms IIRC.

8° timing key to shift the powerband up a bit more
(or the ARC billet flywheel which comes with 8° advance built in)

You might want a bigger carb at this point.

And with that done (roughly a 220 dollar modification to a 100 dollar engine)
you should be able to get a solid 7k rpms.

IDK about the torque curve or if you need to throw even more money at it
(cam, lifters, bigger valves, higher compression, ratio rockers etc.)
to get the torque where you need it.

but 16" wheels are tall, and if you say you're not going to have a lightweight package either (kart plus driver upwards of 300lbs)

I doubt 6:1 is a good starting point....
frankly an unmodified engine requires more like 12:1 with some substantial weight and large wheels.

You might want to think about getting a TC!
it'll shift the ratio down a fair amount on takeoff and adjusts it as you go higher up again.

That way you can take off with say 13.5:1 and cruise with as little as 4.5:1
(i.e. predator, governor removed, breathe easy and nothing else installed chain ratio ~ 5:1)
I'd dial in on a safer assumption first (6:1.. maybe just 5.5:1)
and work my way up as I see the engine is capable.

theoretically that'd push you past 50mph
(it won't thanks to the power limit.. so you never overrev under load but upwards of 40mph is not too far fetched with a TC)

'sid

 

[Kartorbust]

Seeing as you are on a phone, use this calculator http://app.kartcalc.net/#home

Just change the settings to imperial from metric and you'll be golden. Just need the weight (driver and kart), tire size, torque converter or centrifugal clutch, tooth count on the t/c or clutch and tooth count on axle as well as whether or not you have a jackshaft. Depending on what your combined weight is, 6:1 Might be okay or too high and it'll recommend what gear ratio to choose from. It's a bit on the conservative side, so even though in reality your chosen ratio will work, it may give a recommendation to save the clutch or belt from burning out.

 

[Kansaskart]

I have a 212 with header, big jet and air filter kit on a light kart with 12inch tall tires and a tc to a 48tooth sprocket. I can hit about 38 to 40 and I weigh 250lbs.

Also the math is just a suggestion. Doesnt mean it would have enough kick to do what the numbers say.

 

[kc8oye]

yeah doing some reading here on the forums.... i'm sure I need to start around 12:1

i'm 360 lbs alone... the kart will problaby be close to 300 itself... the more i say it out loud... it sounds like I should forget about the 212 and look at the 420...

ultimately I want to put the 670 twin cyl on it... but not until I get the chassis ironed out

 

[Unhinged]

Typically one would start by asking oneslf: how fast do I want to go or maybe,how quickly do I want to go from naght to 45 or so. Gearing is typically a compromise between acceleration and top speed. It's your call!

 

[bob58o]

Typically one would start by asking oneslf: how fast do I want to go or maybe,how quickly do I want to go from naght to 45 or so. Gearing is typically a compromise between acceleration and top speed. It's your call!

Me thinks this one already started by telling us-selves that he wants to go around 45mph in the first post.

 

[kc8oye]

Top speed isnt my primary concern right now.. at least not until i can test the stability of my suspension

I started with a old school hot rod hairpin setup...discovered it wont articulate.... so i went with a friangulated 4 link setup... but that created some real heafaches with steering... i think now that ive had time research it.. might do a single a-arm in the front...with only 3 inches of shock travel, i dont think the camber curve will matter much

 

[jwc motorsport]

heres one for you guys. I have a tav2 with the blue/silver springs on the 300 gram weight. with the green spring on the driven. 18" tires, 585lb kart with driver, 17hp doug flemming motor 10 tooth driven sprocket going back to a final drive sprocket of 52 teeth. hhow fast should we be seeing if I put the driven spring in the front hole

 

[itsid]

NO Hijacking please!
but since this answer is quick.. here it is:
"no difference!!"

the setting on the driver only affects the "time" when the TC shifts, not how far.. so your overall speed nor maximum torque will change even a little bit, you only can shift into higher gear earlier (setting one) or later(setting 3) than usual(setting 2), nothing more.

'sid