Step 2: Frame reinforcement
The next step is to beef up the frame or make alterations that will work with your components.Its important to realize that these mowers will be racing on what tends to be really rough dirt track. They have no suspension, thus the frame takes a severe beating. Reinforcement is critical to avoid having the frame flex and ultimately crack from fatigue. The rear of the frame was cut about 6" from the rear. Throughout the build, I used 1x1 square tubing which is easy to weld and work with. This is what I used to create the square frames in which the mounting brackets were welded into to hold the rear axle bearings. I chose to use a 1/1/4" rear axle because that size is highly common and thus easier to get parts like wheels and sprockets for.
These square frames were welded into the frame, then the end I cut off was welded to the back. The minimum height requirements for my class is 4" from the frame to the ground. So its important to know what size wheels you plan to use and where to mount the axles in order to meet that requirement. The lower you can go, the better handling the mower will be. Mine site just at 4" off the ground.
Next, I welded two lengths of square tubing along the top of the axle brackets to the front tubular frame. I did this because the transmission will go underneath. A piece of diamond plate will cover it, and above will be the seat. This will give me easy access to servicing the chain and transmission and also protect me from flying debris or potential chain failures.
I am using a right angle gearbox for this build. Why? Because the other choice is to use a 3-5 speed gearbox used as standard equipment on mowers. These work fine, but it also means you'll have to change the grease in them and perhaps invest in hardened gears since the originals will strip out much easier. With a right angle gear box, or RAGB, there's only two moving parts. Plus, they are made for higher speed applications and therefor perfectly suited for this application. More simplicity means more reliability.
Additionally, I am using a centrifugal clutch. This is a higher quality, higher HP rated unit that is heavier duty than typical go-cart clutches. The springs can be adjusted for higher or lower engagement.