The First Build.

[AlphaRunner85]

After picking up a second hand go kart my friends and i have been addicted to the kart. So when we had the luck to trip on a free gx390 in great condition we said "Its time to build."
We also got lucky with a welder as things at work are fallin apart. So now we have motor, means, and we need a plan.
I have drawn up some early plans and i might beg just a little for any advice. So here are pics, sorry if they are crappy.

http://i401.photobucket.com/albums/p...r/PICT0035.jpg
rear suspension

http://i401.photobucket.com/albums/p...r/PICT0036.jpg
Pulley clutch

http://i401.photobucket.com/albums/p...r/PICT0037.jpg
drive train minus the tensioner pulley

http://i401.photobucket.com/albums/p...r/PICT0038.jpg
rear drive assembly

http://i401.photobucket.com/albums/p...r/PICT0039.jpg
Automaticsih pulley clutch setup thanks to bkrider113 and modelengineer for the plans basically.

http://i401.photobucket.com/albums/p...r/PICT0040.jpg
bottom frame

http://i401.photobucket.com/albums/p...r/PICT0042.jpg
side frame

http://i401.photobucket.com/albums/p...102109_001.jpg

Im sorry about so many pics but suggestions? What would be ideal for the main(lower) frame? I was thinkin square steel but i dont know the differences in the different types.

[modelengineer]

On the pulley clutch drawing, if the kart is driving to the right of the page then the tensioner is on the wrong side of the belt. It needs to be on the slack side under acceleration.

Square steel is a good idea. I'd be using 30mm or 1 1/4" square. Might even go for 1 1/2". Around 2mm wall thickness is a good size for 1 1/2" tube.

[AlphaRunner85]

so the tensioner pulley needs to be on the underside of the clutch assembly?
and we are all pretty big guys (over 225lbs) so i think we will get 1-1/2".

[modelengineer]

Yeah. Just imagine when the engine is trying to drive the pulleys, one side of the belt will go slack and the other will have lots of tension on it. You want to tension the slack side.

[AlphaRunner85]

Thanks for the input! We are planing on doing a full suspension also, and after looking at several kart suspension threads with various ideas i liked dual a-arm
front suspension. i will try to draw up something by tonight.
And for the rear we were thinkin a hinged lower rail with 2
shocks higher in the frame and a pair of leaf springs below the lower rail. Once again ill try to draw it up becauseThanks again for any input.

[AlphaRunner85]

I did some calculations and it looks like i messed up. For drive system



on the engine i was thinkin a 3" pulley to a 4"driven pulley on jackshaft with a 36t sprocket to a 60t final on axle. and my numbers say 2.2:1 ratio. that seems really high, maybe too much for the rig? we are planing on at LEAST 18" tires for off road, i was hoping for another set a bit smaller for on road use.

[r_chez_08]

have you found the speed calculator yet? put in 3600rpm for the stock gx390. i would try a 5:1 or 6:1 ratio. mabie a 2" pulley to 4" pulley, then 20t, and 60t.

[AlphaRunner85]

Originally Posted by r_chez_08

have you found the speed calculator yet? put in 3600rpm for the stock gx390. i would try a 5:1 or 6:1 ratio. mabie a 2" pulley to 4" pulley, then 20t, and 60t.

i have and with you ratio im lookin at like 59mph-ish, we have a few parts on order for the motor, new valve springs, air filter adaptor, and exhaust and of course we will be removing the governer compleatly so i figured the calculator at 4500rmp.

[r_chez_08]

no its not, 1:5 is 49 @4500 rpm, and 1:6 is 40mph with 18" tyres.
edit: oops, is it 1:5, or 5:1 ect im not sure. also, what kinda speed did you want?

[modelengineer]

I'd be going for a smaller driving sprocket on the jackshaft, rather than messing too much with the pulley diameters. You don't want to make the pulleys too small or they won't engage very well and the belt will get hot. The size you have is good.

[AlphaRunner85]

Originally Posted by r_chez_08

no its not, 1:5 is 49 @4500 rpm, and 1:6 is 40mph with 18" tyres.
edit: oops, is it 1:5, or 5:1 ect im not sure. also, what kinda speed did you want?

Im sorry i didnt put in that we found a good deal on some 22" tires

I'd be going for a smaller driving sprocket on the jackshaft, rather than messing too much with the pulley diameters. You don't want to make the pulleys too small or they won't engage very well and the belt will get hot. The size you have is good.

thats what i was thinkin i was worried about slip with a tiny drive pulley. Wouldit be impossible to build a 2speed trans with the belts? i was thinkin like a bicycle gear shift setup
but alot beefier with 2 pulleys on the motor and 2 on the jackshaft and a guide made of 3 pulleys in a u shape on a hinge to shift gears.

[AlphaRunner85]

Did some more figures and with a 3" drive a 4" driven on the jackshaft to a 18t sprocket and a 60t on the axle. that gives me a final drive of 4.33_:1 which will give us a better bottom end while still giving us a top speed of 69mph with the 22" tires. The 3" drive pulley should be big enough not to slip too much but still nets a good ratio. Our welder got delayed a few days but should be here friday. So thursday we should be able to get at least the 1-1/2" square steel for main frame and we need to get an angle grinder for cuts. a chop saw would be ideal
but a bit too expensive.

[AlphaRunner85]

Hopefully the frame will look like

this!

[modelengineer]

I don't think a GX-390 will pull 70mph, IMO you should gear it down even more. I'd aim for around 50-55mph top speed. That way you should get a decent take off.

[dpaxson]

put some triangulation in that frame

[r_chez_08]

Originally Posted by modelengineer

I don't think a GX-390 will pull 70mph, IMO you should gear it down even more. I'd aim for around 50-55mph top speed. That way you should get a decent take off.

[AlphaRunner85]

OK so here is the revision and a couple of our potential suspension setup







I appreciate all input because i want to get the design as good as possible before
we start cutting and welding anything. Im no engineer but it looks pretty stable to me
it shouldnt be able to twist too much, but i have no idea how well it will withstand a hard landing

and this is a draw out of a front suspension assembly

[modelengineer]

Don't bother with the leaf spring, it's not needed and will add a lot of weight. Other than that, the rear suspension looks really good. I'd advise making the rear shock mount moveable so you can adjust the spring rate.



It's a bit hard to see, but the shock mounts in between the flat bar in the middle. As you can see there are about 10 different holes to put the shock in.

Also, I'd add one more red line to that drawing from the top corners of the green x-brace to the bottom of the vertical red supports, just to complete the triangulation.

[AlphaRunner85]

Ok will do on the final support. do you think that the black lines being the lower frame will be sufficient or needs more? and i was looking into some adjustable shocks but that would be less expensive im sure and easier to adjust. would it be possible to run the adjustment rails up the main frame rather than the swing arm.
how heavy is the steel in your swing arm and i was thinking a dual hinge on either side rail rather than one single hinge point... ???? better worse indifferent?

[modelengineer]

No problem with the adjustment rails on the main frame. You can still use adjustable shocks, it's just that the rails provide more adjustment. The shock could be used for fine tuning, but I haven't found the need on mine.

Bottom frame looks good to me. The steel in the swingarm is mostly 30x30x2.5, but the base rails are 50x20x2.5. It's a bit overkill, the plans call for 20x20x1.6 support instead of the 30x30 but there have been reports of them cracking. I didn't want to take the chance.

On the dual hinge I can't see a problem as long as it is braced a LOT. The advantage of the single attachment is that it goes some way to stop twisting of the swingarm, which is what causes the welds to crack.
If it's braced properly then I don't see why the double attachment wouldn't work.

Good luck mate, getting close to buying some metal!