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How to make a racing lawn mower (Updated!)

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Step 2: Frame reinforcement

Picture of Frame reinforcement
frame_with_tranny_bracket.jpg
RAGB.jpg
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.
 
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What kind of clutch did you get and where did you get it?
McClellan412 years ago
ya hi i was wonderin what size sprocket i should use on the input to the ragb..i have 17t on my clutch and the ragb is a 1:1 ratio???

thanks
paullove3 years ago
the centrifugal clutch
where do I get 1 that is strong enough?
billybob63 years ago
whats the eaisiest way to reinforce the frame because i cant weld
bessner503 years ago
im makin one for goin muddin with my buddies and i need to get power how would you do that?
brodewald104 years ago
Where did you get your centrifugal clutch?
mowerracer (author)  brodewald104 years ago
I got mine off of Ebay. They cost around $140. make sure you specify for a 1" shaft.
maer354 years ago
where did you get the right angle gearbox
fast-dude5 years ago
how long is the 1x1" square tubing on top of the axle hangers that go most of the way down the frame?
mowerracer (author)  fast-dude5 years ago
I don't exactly recall. Most of the pieces were cut to fit as I went. The frame is not super-accurate. Lots of eyeballing.
fast-dude5 years ago
im thinking of building my frame by hand but i kinda need to know a rough estimate of the lengh and width of the one in the pics? can anyone help me with that?
mowerracer (author)  fast-dude5 years ago
The length is around 44 inches and around 17" wide for the middle, 38" from wheel to wheel on the front and back axles.. It is TINY, but I'm kind of a small guy. If you build your frame and plan to race, you need to download the specs from either the USLMRA, or ARMA which are the two largest racing mower organizations. They have all of the max width, height, wheelbase, and so on. Build to those specs and you'll be good because at these events, they WILL check your dimensions, so you'll want to make sure that they are correct.
beenie5 years ago
Where did you get your RAGB from? and how much did it set you back?
mowerracer (author)  beenie5 years ago
I got mine from Ebay. I have two of these, one as a spare. The first one was $30. The second was $50. They seem to be getting a bit harder to find. The cheapest are older RAGBs for riding mower decks. They typically have 2 bolts, but that will work fine.
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