gearing help

[zenking12]

Hi I have a radio flyer kart build. Been away from it for a while. My build post has been closed. Don't know why, maybe because I was gone so long.
I need help with picking the right sprockets. I have a 72 teeth axle sprocket and can use it but here is what I have.

10" tall tires, a 79cc 3HP Predator motor. Its going to be for grand-kids needs to go a little slower that a teenager or older would want. I will want to ride it at car shows but it does not need to be fast. I'm thinking if I go with the 72 axle sprocket and a 12 centrifugal clutch, it will give me a 6:1 . Do you guys think this will work ? I really don't know much about this stuff and am reading as much as I can. I think it would make the kart go 12 to 15 mph. Is that enough to get the kart to move from a stop ?

Thanks for any help.
Dan

 

[Poboy kartman]

Threads are closed due to inactivity, a quick PM to a moderator or a request here should get it reopened.

I would go a little higher on the sprocket or smaller on the clutch....

 

[mindymogul]

Well, right now you're running a good 6:1 ratio, and with a engine that small I wouldn't go much higher. IF and I really mean IF you wanted to push it you could go with a 60t which would give you 5:1. Only thing is it'll be a slow start and the weight of your grandkids ad a factor in as well. Right now you're reaching 18mph if they Feed Ah the Beans! If you went to a 60t you'd gain ALL of an astonishing 3mph.

 

[chancer]

I have 2 79cc predators on kids karts. Lightweight karts converted from "Razor Electric buggy" 12" rear tires. http://www.diygokarts.com/vb/showthread.php?t=29265
I started with 11 tooth clutch and 72 gear. about 6.5:1 Anyway the thing would barely take off from a stop, No HP, and was then too fast for my 8yr old if it finally got moving.
So I redid it and made a jackshaft ended up about 10 or 11:1.

Now has plenty of torque and just enough top speed to scare them.

 

[mindymogul]

^----Listen to him.

 

[zenking12]

Sprocket

So in an effort to save money if I kept the 72 tooth sprocket and went from a 12 to a 10 tooth centrifugal clutch what would that do for me. Would it get the cart moving a little bit quicker and have a little less top end ?? Thanks. Dan

 

[chancer]

Well it might help. You are going in the right direction. I mean a clutch is like 20 bucks on ebay so it is a cheap way to try.
My experience was with a 11clutch and 72 with 13" tires.
So with your 10" tires and 10/72 depending on weight it should get the kart moving. and the kart speed calculator says top speed at 15mph

 

[number9]

Keep in mind your ground clearance for the rear sprocket. What may be plenty on a garage floor could bottom out on normal ground and ruin your sprocket.

...

 

[Poboy kartman]

Well it might help. You are going in the right direction. I mean a clutch is like 20 bucks on ebay so it is a cheap way to try.
My experience was with a 11clutch and 72 with 13" tires.
So with your 10" tires and 10/72 depending on weight it should get the kart moving. and the kart speed calculator says top speed at 15mph

Just so you understand....that's really faster than the average NFL playa can run...(if you average the whole team)...

And when you're only a couple of inches off the ground...it seems really fast!!!!

 

[chancer]

15mph sounds slow, but yep in the yard (1.5 acres) The kids are extremely pleased with the speed.
And 10" tires and a 72 tooth only gives you .75in ground clearance.

So in the long run count on adding a jackshaft.

 

[zenking12]

number9 and everyone else is right less than an inch sprocket to ground. Would a 60 tooth sprocket give a lot more clearance? Also I cant find the chart that figures out the mph sprocket ex..Can someone help me find it.
thanks

 

[itsid]

blunt advertising ahead...

on your mobile device (yes.. sorry)
you can use my kartcalculator ( web address: app.kartcalc.net preview: www.kartcalc.net)
just play with the numbers until you're happy;
it'll warn you as soon as your rear sprocket gets too big or the ratio is simply not feasible.

'sid

 

[Ryafor]

blunt advertising ahead...

on your mobile device (yes.. sorry)
you can use my kartcalculator ( web address: app.kartcalc.net preview: www.kartcalc.net)
just play with the numbers until you're happy;
it'll warn you as soon as your rear sprocket gets too big or the ratio is simply not feasible.

'sid

Ugh I hate that thing. Lol it's like playing a game of operation with my son. Everytime I hit calculate its like I touched the wall and the guys nose lights up and bzzzzz. Everything Ive read says 6:1 is a good start. I plan on 12 tooth and 72 on 13" tires. I can understand the sprocket warning but not the gearing one. I already ordered my sprocket but not my clutch. I'm stumped. Trying not to throw out all the details cause I feel like I'm hijacking altho gearing help is gearing help so hopefully the op will understand. I've done searches etc. I'm just estimating my kart will be 100lbs and max 200lb rider. 7hp @ 3600 rpm. Sorry not trying to hijack but questions may bring answers to the op and future members doing searches etc that may prevent more unnecessary threads

 

[Poboy kartman]

Ugh I hate that thing. Lol it's like playing a game of operation with my son. Everytime I hit calculate its like I touched the wall and the guys nose lights up and bzzzzz. Everything Ive read says 6:1 is a good start. I plan on 12 tooth and 72 on 13" tires. I can understand the sprocket warning but not the gearing one. I already ordered my sprocket but not my clutch. I'm stumped. Trying not to throw out all the details cause I feel like I'm hijacking altho gearing help is gearing help so hopefully the op will understand. I've done searches etc. I'm just estimating my kart will be 100lbs and max 200lb rider. 7hp @ 3600 rpm. Sorry not trying to hijack but questions may bring answers to the op and future members doing searches etc that may prevent more unnecessary threads

Sooo.....what's the problem? ???! Sounds like you've got the right gearing....

 

[Ryafor]

So in an effort to save money if I kept the 72 tooth sprocket and went from a 12 to a 10 tooth centrifugal clutch what would that do for me. Would it get the cart moving a little bit quicker and have a little less top end ?? Thanks. Dan

What size chain anyhow? Not sure but if its #35 I dont think there is a 10 tooth clutch. Well maybe. Idk the bore size on your pto.

---------- Post added at 04:11 PM ---------- Previous post was at 04:08 PM ----------

Sooo.....what's the problem? ???! Sounds like you've got the right gearing....

Lol I already stated the problem. I was good until I found that calculator. Now whenever I put in my info I get buzzzed about my gearing. Just has me worried.

 

[Poboy kartman]

Well....just put the info in a post...don't know why it's giving you fits on Kartcalc....but...we're here to help!

 

[Ryafor]

With 7hp
300 lbs
3600 rpm
13 inch rear wheels
just a 12 tooth clutch
72 tooth sprocket
this is what I get

My screenshot was an invalid file but it says "axel sprocket too big, may burn clutch, gear ratio too high. Suggests 7.2:1

 

[itsid]

the axle sprocket may get too big is because of #40 chain sprocket having ~11.5" diameter.. not leaving an inch of clearance..
(keep in mind that wheels compress under load.. so 13" is just the nominal diameter which might not be your effective diameter at all )
so there's the warning to let you know to double check

gear ratio too high (burning clutch) is because of the lack of torque in the lower rpms, with that wheels and weight.
the clutch might get too hot too fast... (especially if it's a cheap clutch)
taking that risk is up to you

Here's the Kartcalc output:
KartInfo by kartcalc.net:

[FONT="Verdana"]
[B][U]KART[/U][/B]
[INDENT][I]RW diameter[/I]: [B] 13 in | 33.02 cm[/B]
[I]Total weight[/I]: [B] 300 lbs | 136.08 kg[/B]
[/INDENT]
[B][U]ENGINE[/U][/B]
[INDENT][I]Original HP[/I]: [B] 7[/B]
[I]Asummed HP[/I]: [B] ~7[/B]
[I]Torque[/I]: [B] 10.21 ft lbf | 13.85 Nm[/B]
[I]RPM[/I]: [B] 3600[/B]
[/INDENT]
[B][U]TRANSMISSION[/U][/B]
[INDENT][I]Final ratio[/I]: [B] 6:1[/B]
[/INDENT]
[B][U]PERFORMANCE[/U][/B]
[INDENT][I]Max Wheel Torque[/I]: [B] 61.29 ft lbf | 83.1 Nm[/B]
[I]Acceleration[/I]: [B] 12.14 ft/s² | 3.7 m/s²[/B]
[I]Top Speed[/I]: [B] 23.2 mph | 37.34 km/h[/B]
[/INDENT]
[B][U]PROBLEMS[/U][/B]
[INDENT][B]Axle sprocket might get too big![/B]
[I]Sprocket diameters[/I]: [B] (#35) 8.64" & (#40) 11.52"[/B]
[B]You might burn your clutch/belt![/B]
[B]Gear ratio too high![/B]
[I]Suggested final ratio[/I]: [B] at least 7.2:1[/B]
[/INDENT]
[/FONT]

as a comparision, here's the output of the official Hilliard extreme duty clutch ratio calculator:

(read the notice? )

'sid

 

[Ryafor]

Thanks sid. Makes perfect sense now. I never took into account compression of the tires. The plan is #35 chain. Clutch will most likely be a cheap max torque or whatever they have at princess auto. I'm on a cell right now and can't read the info on the hilliard but I'll check it out later on my laptop. You think I'd be ok with any bigger tires without changing the axel sprocket or clutch count? I'm mostly mobile but will check out the kartcalc.net aswell.

Ahh I see max torque has an 10 and 11 tooth clutch on bmi for #35.

 

[itsid]

with #35 chain the sprocket diameter isn't an issue.
bigger tyres will make it WORSE though regarding the lack of torque.

A 10T clutch is available, but for #35 chain that's rare,
so check that info again, you might end up with a #40 clutch instead by accident.

10->72 is just what you'd want (aka 7.2:1 ratio) according to the calculator,
you can gear slightly higher if you wish (sacrificing clutch lifespan);
but in the end.. the taller the tyres, the lower the gearing must be.
going for bigger tyres, without adjusting the ratio will again put additional stress on the clutch...

I'd say no, do not get bigger wheels without adjusting the ratio down accordingly.

'sid