Calculating top speed

[bswan0508]

Hi everyone

I am currently building my first kart and I am having trouble figuring out the top speed it's about complicated for me just wondering if anyone has any incite into the matter.

I have mounted the motor which is an old arctic cat 440 I have the primary clutch on the motor then the secondary clutch is on a jack shaft obviously these open and close at higher rpms. The primary is roughly 7 1/2" diameter fully closed. The secondary is a complete guess as I haven't taken springs out to see diameter when it's open if I had to guess it's somewhere between 3-4" in diameter fully open. Again a guess at this point. The engine operates at about 8000 rpms from what I'm reading again not 100% sure on that. The drive sprocket on the jackshaft is 16 tooth and the driven sprocket on the axle is 40 tooth. The tires are 18" in diameter. And the kart with two adults is right around 700lbs. The motor pushes about 60hp.

I hope somebody can help me out here. Also open to suggestions on sprocket sizes for better gear ratios as I can change out if necessary. I dont want it to be so powerful it just burns rubber which I'm thinking might be the case.

I've attached some pictures of the motor jack shaft and rear axle


*EDIT*
Ok correct me If im wrong but the top speed I calculated comes in at 80mph the way I got there is by finding the gear ratio of the primary and secondary clutches which is roughly 2.14:1 considering the secondary at 3.5" in diameter. Which puts the jackshaft rpm at 3738 I then plugged that along with sprocket sizes on the jack shaft and axle as well as tire size into a simple go kart speed calculator I found through Google search and it came up with a top speed of 80 mph. Obviously my information on the secondary isn't perfect as like I said I don't know the exact diameter of it but just curious if I went about finding this number the right way.

 

[itsid]

tough to tell.. that's an old salsbury clutch for all I know
since the mid 70ies it's what's known as Comet 102C.

the driven you have should be ~10 3/4" in OD right?

so that'd match the 102 100D combination in terms of dimensions rather nicely
(and since the larger comets are essentially derived from the salsbury clutches also give a good approximation of the ratios no matter if salsbury or comet 102C)

that means we talk 3.62:1 in low and 0.94:1 in high gear.
(if your driven is only 9 13/16" in OD it gets even worse... 3.28:1 in low 0.78:1 in high gear)

actually that engine should run at 7k rpms if not race modded for all I know..

doesn't matter
16 to 40 is a terrible idea with 18" wheels
(remember that the drive wheel on a sled is MUCH smaller than 18" in diamter!)

that makes it a 2.5:1 chain ratio

that's overall 9.05:1 low and 2.35:1 high

and if we assume that you do not ruin your terribly expensive belt the first try
you'd get a take off speed (engagement is @2800 rpms) of about 17mph
which IYAM is pretty terrible for a buggy that heavy..

Anyways.. top speed for 7k rpms (if at all possible to reach) would be 160mph
(which is absolutely impossible to reach... 110mph might be doable at best)

with 8k rpms it just gets worse... 180'ish mph theoretical top speed. (still 110'ish at best possible)


Sooo with 8k rpms high speed ratio overall should be in the 3.8:1
and with 7k rpms in the 3.3:1 region

means you want 56T on the axle for a 7k rpms and 65T on the axle for a 8k rpm engine

everything less is a waste of torque for no additional top speed at all.

'sid

 

[bswan0508]

Wow thank you so much for the info you obviously know your stuff the driven is 10 3/4" OD. I'll definitely be changing out the sprockets. I probably should have been asking these questions before I got this far. But was designing and building the kart as I went and I'll admit I don't know much of the technical stuff that you just enlightened me to.

 

[Kasea]

I'm im the same boat with my kart gonna watch this thread closely and soak up all the info I can .

 

[Budget GoKart]

Glad your building your own frame instead of butchering one your a great person! I hope that thing is a blast when its done!!!

 

[bswan0508]

Glad your building your own frame instead of butchering one your a great person! I hope that thing is a blast when its done!!!

Yeah. I had bought a different kart before and was gonna modify it. But after looking at it and seeing everything I would have to change and rebuild to get what I wanted It ended up being less time consuming and actually cheaper material wise to build it from scratch. I did however fix that one up rebuilt the motor that was on it and ended up flipping it for more then twice what I paid for with very little invested into it.