Building my first kart:Verifying my ideas

[WoundedWarrior]

Hi everyone. Im new to this forum and have found many threads here that are very helpful in planning my kart. Based on what i have found i have settled on 10.5in rear wheels on a live axle. I got a 7hp powerfist engine from princess auto, and i am not planning on removing the governor or modding it at all so i dont void the warranty. Also got a 3/4in centrifugal clutch with 12teeth. My rear sprocket has 76teeth. |This gives me a 6.33:1 ratio.My guess is that the kart weighs between 250-350lbs with me riding it. Will this work? Any and all advice is appreciated.

I've debated adding larger wheels to the back for a higher top speed. Would this be beneficial to my kart or is it not worth it?

 

[BigWes]

wait a sec bro...that's a big sprocket on your axle. it's not going to be bigger than those 10.5 in wheels is it? I mean I think you are going in the right direction...you would probably be good with 13 inch tires with that ratio...

someone please chime in and correct me if I am wrong? I normally don't use centrifugal clutches but 13 inches is about tops I think and 15 is in the clutch burning zone.

 

[itsid]

it'll work especially with those small rear wheels.
10.5 is indeed a tad small, sounds like race kart fronts instead of vintage kart rears tbh size-wise .

Anyhow, Wes is right in assuming troubles with the sprocket.
the sprocket nominal OD is 9.2" (give or take) that's just a solid inch of difference counting in tyre squish.
Just half of that is sprocket to ground clearance...
so just half an inch a good centimeter for us metrics...
can cause some serious road bite quickly.

gear ratio is okay for the intended weight, in fact it'd be okay to lower the rear sprocket toothcount to 74 w/o issues.

And if you can drop the (or just narrow in on the lower half of your) weight span
(say 300lbs not 350 max) you can go with even less teeth ..

just 69 Teeth will still work and keep the clutch happy.
and apart from that allowing for some more ground clearance
(8.3" sprocket diameter.. almost an inch smaller)
it also allows for a slightly higher top speed

'sid

 

[BigWes]

it'll work especially with those small rear wheels.
10.5 is indeed a tad small, sounds like race kart fronts instead of vintage kart rears tbh size-wise .

Anyhow, Wes is right in assuming troubles with the sprocket.
the sprocket nominal OD is 9.2" (give or take) that's just a solid inch of difference counting in tyre squish.
Just half of that is sprocket to ground clearance...
so just half an inch a good centimeter for us metrics...
can cause some serious road bite quickly.

gear ratio is okay for the intended weight, in fact it'd be okay to lower the rear sprocket toothcount to 74 w/o issues.

And if you can drop the (or just narrow in on the lower half of your) weight span
(say 300lbs not 350 max) you can go with even less teeth ..

just 69 Teeth will still work and keep the clutch happy.
and apart from that allowing for some more ground clearance
(8.3" sprocket diameter.. almost an inch smaller)
it also allows for a slightly higher top speed

'sid

Thank you for chiming in sir. I am sure your words are far more supportive than mine.

I don't have sprocket measurements at hand but those initial tire results didn't seam right. I have a similar situation with a kart myself. I won't run it until it's right. Torque converters seem to give us the luxury of a little leeway to err with our gear ratios.

Thanks again...peace.

 

[WoundedWarrior]

Thanks guys! That is all very helpful! The rear sprocket is quite close to the same size as the wheels as itsid said. Ill gove it a try and if its sketchy maybe ill move to a 70 or 74t sprocket. Thanks again!

 

[mckutzy]

Here in Canada, getting a 70 or 74 is going to be difficult, as the Princess only sells 72 as the max, The next on down is 60...
That of course if thats your only place to get ones you have available...
A local bearing shop might be able to get what ever you like, but depending on the hub system you have, Like a weld on hub...

 

[WoundedWarrior]

Here in Canada, getting a 70 or 74 is going to be difficult, as the Princess only sells 72 as the max, The next on down is 60...
That of course if thats your only place to get ones you have available...
A local bearing shop might be able to get what ever you like, but depending on the hub system you have, Like a weld on hub...

Yeah im fine with a 72. From what ive heard it should work. I just hope my 76t works. The bolt holding the wheel on is so big that i would need to buy a new socket to take the 76t off, so im leaning towards slighty bigger tires.

 

[itsid]

i would need to buy a new socket to take the 76t off, so im leaning towards slighty bigger tires.

If you want to get an idea what change of wheelsize would be comparable to what gear ratio change, you can just calculate that.

Say you want the same effect as with a 72T sprocket

you divide your 76T sprocket by the 72T you want do go with
to get a ratio and apply that ratio to the wheelsize.

76/72 * 10.5 = 11.083

So an 11" Tire lowers the overall gear ratio as much as dropping from 76T to 72T on the axle while sticking with your 10.5" wheels.

It also works the other way around:

Say you find 12" tires and you want to know what sprocket size that'd compare to.

10.5/12 * 76 = 66.5

So increasing the wheel diameter from 10.5 to 12 inches would be comparable to dropping the axle sprocket's toothcount from 76 to 66/67.

And the best thing about it.. it also allows for some mixing up..

say you want to achive the same overall gear ratio as you would
with a 72T sprocket on an 11" wheel (the ideal 6:1 gear ratio)
still being unsure if toothcount or wheelsize should be changed.

you simply go both routes

for sprocket size:
72/11 * 10.5 = 68.73 (here's the 69T sprocket I mentioned earlier again)

for wheel size:
11/72 * 76 = 11.61 (11.5" ideally.. 12" is the closest size I'm aware of)

Now you know, a 69T sprocket gets you close and so would a 12" wheel or ideally a 11.5" OD tire (which afaik is non existing).

'sid

 

[WoundedWarrior]

Wow. I never knew that. That makes a lot of sense though. Thanks!