Riding Mower Racer

[Carbon Fibre]

Hi! I garbage picked a Craftsman 10 horse mower a while back and plan on making a steet legal full-bodied "go cart" out of the bitch.
I started by taking the drive pullies and switching them so the big blade drive pulley now powers the 4-speed transaxle (reverse,too!). This invoved hours with a sawzall, cutting fluid, a bastard file and a big hammer and punch. I kept the motor and transaxle , this section of the platform, and front sispension and steering gear and put the rest back where iI found them. I'm planning on making the framerails out of laminated layers of lewan (1/4" plywood) so that they can bridge the front and rear sections I kept. The problem was the rear motor and transaxle section is 26" wide, and the front suspension mounts are 7" wide, and I want the seat between the framerails. So, the lewan is thin enoufh that I can curve it, and I'll glue 3 layers of it together With fiberglass resin. My feet will actually be outboard of the framerails, with a clutch, gas, and brake pedal. I'm also making the body out of door skin, which is 1/8", and covering both in a light layer of fiberglass cloth. My windows will be thin lexan (light and cheap), and the seat will be canvass mounted to a frame with bungee cord
Enough for now. I'll post pics later for your amusement.

Oh, anybody got any tips on how to tweak my motor? I'ts geared to do 60+ but I could use more power. Wait, who couldn't use more power?

[mike@af]

Good Lord...I'd be scared shitless to ride a go kart with a wood frame. Fiberglass, wood, feet outside the rails, 60mph? Do you have some sort of a death wish?

If I were you, well if I didnt want to die, I would build the frame out of 1.25" .010 wall DOM tubing. Make it low to the ground, and wide. Small wide tires for good handling as well. Make the rear track wider than the front.

As far as engine performance, Im not sure what kind of engine is in that lawn mower. The old B&S Raptor series motors have cammer kits and stroker kits available. You can also bore them, and do port and polish. Little things go a long way with those motors.

[Carbon Fibre]

The whole idea is cheap. Plus I HATE steel. The wood is easy to work with and will definitelybe strong enough when laminated. The front suspension pivots in the center, so torsional siffness is pointless. I'm going to make a beefy footwell, bit the front wheels will stop anything I crash into from reaching my feet. I do plan on incorporating a "rollbar" into the chassis and the seat frane with thicker plywood.
As for the motor, a kart motor is way beyond budget. I was thinking of milling down the jug to raise compression, but the valve lifter rods? would then be too long. I could cut them in the middle and weld them, however accurately, or shim up the rocker arms. A good port and polish job, a better or modified magneto and coil, and bigger carb would help immensely. I have seen air pumps like a supercharger for small engines in the back of popular science years ago, if I chain drove it that would only require me to get better spark-and retard it-, and a bigger carb.
Oh and my ass will be about 4" from the tarmac, with a thin floorpan, so the whole thing should stand about32-36"from the ground

[mike@af]

I'm going to split this up, and address all your points with experience of building and designing, and racing go karts/shifter karts.

Originally Posted by Carbon Fibre

The whole idea is cheap. Plus I HATE steel. The wood is easy to work with and will definitelybe strong enough when laminated.

The wood will still twist when carving around turns, or split. I wouldnt want to do anything more than soap box derby with a wood frame. Steel is your friend. Laminated wood is still weak when it comes to torsional stiffness. With a beefy motor, you need a beefy solid frame with super tight clearences. You wont get that with wood. Unless you plan on carving the entire thing out of a red wood tree.

Originally Posted by Carbon Fibre

The front suspension pivots in the center, so torsional siffness is pointless.

Refer to above.

Originally Posted by Carbon Fibre

I do plan on incorporating a "rollbar" into the chassis and the seat frane with thicker plywood.

Plywood roll bar...the thought of that alone is scary.

Originally Posted by Carbon Fibre

As for the motor, a kart motor is way beyond budget.

Depends what kind of kart motor you're looking for. The basic raptor series motor can be found on a lot of B&S tools at garage sales for a minimal cost.

Originally Posted by Carbon Fibre

I was thinking of milling down the jug to raise compression, but the valve lifter rods? would then be too long.

If you want to increase compression on a motor like that you need to build up the piston, not deck the head.

Originally Posted by Carbon Fibre

I could cut them in the middle and weld them,

Machine them down on a lathe. You wont be able to get them accurate enough and strong enough. Once you weld it you will have killed the hardening in that area, and you will have valves bendings or snapping. Even if they are heat treated properly.

Originally Posted by Carbon Fibre

however accurately, or shim up the rocker arms.

Shim the springs for faster, harder, and tighter closing of the valves. If you notice, you lose compression since the valves on most small engines do not close in the seat fully.

Originally Posted by Carbon Fibre

A good port and polish job,

Yes, that will give you a couple ponies.

Originally Posted by Carbon Fibre

a better or modified magneto and coil,

Dont bother messing with the ignition system on a small engine. It will cause you lots of problems try to modify that. All you can really do is change the plug and plug wire.

Originally Posted by Carbon Fibre

and bigger carb would help immensely.

No it wouldnt. The engine can only take soo much fuel at a time. The carb is proportional to the engine. High power small engines do not need a larger carb. If you insist on changing the carb, look at an ethanol carb.

Originally Posted by Carbon Fibre

I have seen air pumps like a supercharger for small engines in the back of popular science years ago,

I thought this was a cheap project. Super charger kits for small motors are $999.99.

Originally Posted by Carbon Fibre

if I chain drove it that would only require me to get better spark-and retard it-, and a bigger carb.

As opposed to a...turbo? Once again, dont mess with ignition as the timing is almost always dead on, and dont get a bigger carb.

Originally Posted by Carbon Fibre

Oh and my ass will be about 4" from the tarmac, with a thin floorpan,

Thats still pretty high.

Originally Posted by Carbon Fibre

so the whole thing should stand about32-36"from the ground

Your CG is too high. You better compensate by making the car much wider, or much slower.

[Carbon Fibre]

reply to GT Mike
Thanks for all the exellent info. I'm still buildinf with wood. Wooden boats sustain much greater loads than automobiles do, especially in sailboats where the pressure from the mast is concerned. Also I was involved in the construction of a raceboat with a 400 hp Buick V6 from a Grand National that the hull was made of 2 layers of doorskin laminated with 6oz. fiberglass cloth. It held up at 60+, and on the water offshore is like skipping off of broken up concrete.
The front axle is pivoted in the center to give it articulation. unless I want to change this, there will be no torsional load on it at all. Also, the framerails will be about 8" tall boxed in with the floorpan, footwells, and rollbar. Ever look at formula one body tubs? The plywood acts as a core, only addidn strenght along its vertical and horizontal axis. The fiberglass is what makes this a composite, keeping the benifits of the wood and adding its own properties of strenght, like in joint areas. I got the idea fir using wood from a naval architect in Tampa I knew very well that told me I should build a car with wood instead of fiberglass, and then proceeded to show me a Triumph based racecar with a wood! chassis! It had been racing, and performed well, with no stiffness orfailure issues.
Wood and fiberglass are both alot stronger than steel for the same weight. The engineered shapes are much different, are more organic to be efficient, but are superior. My kart will weigh less, therefore I have lower loads to worry about.
As for the center of gravity, the onl thing that really weighs anything is the motor and transaxle. My plan, now that I've given it sime more thought, is to put the transaxle behind the motor, with it flipped over and backwards. Then, the drive pulley would be on the bottom and still turn the wheels the right way. This would let me lower the motor with the belt still lining up, and I could tilt the assembly down so the pulley was close to the cround (with a cover), thus dropping my center of gravity about 14"!
Again, thanks alot for the motor help. Do the mower motors use a governor, or is it determined py valve float or something?

[Carbon Fibre]

My front tires are pretty skinny, which should induce understeer. Rear trak 32", front 29". I guess I could sit in a more reclined position and get a 30" height, but I'm 6'3", so it'd suck.

[Carbon Fibre]

hand lap the valves

[mike@af]

With that said, I'm assuming you will engineer the chassis well. I've seen people think they can use 2x4s as frame rails and build a rectangle for a chassis that falls apart after a little rock. I wish you luck with it, and hope to see some of your progress on it soon.

The motors use governors, centrifugal governors. They usually open up between 1,000 and 3,000RPM. Once removed, the motor can move up between 5,000RPM and 8,000RPM. Depending on the motor, 5,000RPM is about as high as I'd want to go if I planned on not rebuilding that motor.

On a side note, I did see an insane B&S motor that revved up to 15,000RPM!

[mike@af]

Originally Posted by Carbon Fibre

hand lap the valves

Yes, get some of that lapping cuttin grit (Is there a formal name for it?) to help seat the valves better.

[TheSilentChamber]

I use Clover A3 and A6 for lapping valves.

[MPWR]

Saw this, and just had to share....

[TheSilentChamber]

It even has adjustable front suspension.

[mike@af]

I'd be scared to ride that.

[Carbon Fibre]

Fuckin awesome chopper! Can I ride it? It's like the only 2x4 thing that'd work. Drum brakes n all!
Steel is fer queers!

[Carbon Fibre]

But not your project, Mike. I was speaking from an engineering standpoint.