Lawn Chief racing mower build.

Need to build something for an engineering project, I already have a killer off road mower that has done pretty much everything from the dunes, to cruise nights and drag races. I have a few other mowers yet to be built and it is time to build one of them into a racer. This mower will not be used in actual races, as there are none around where I live. It will mostly just be used to race around on, so there are no rules that I will need to follow. I’m planning on converting it to a live rear axle with a 700 transmission. I plan on lowering it a couple inches and lowering the front end also. I will be going to get a 17hp opposed twin this weekend for the engine on this. My other mower has a 17.5 tecumseh I built that has a ported and polished head with heavier springs, a custom cam, dirt bike carb, and makes a lot of high end power but not much low end. This is why I’m thinking of going with the opposed twin and keeping it stock, besides an exhaust, maybe a port and polish but I do not think it is necessary. The thing I am trying to figure out currently is what size rear axle I should use/how wide. There are many kits online that come with hubs bearings and a brake, just need to find which size and which one I should go with. Any help is appreciated

Sounds like an interesting project, i've been thinking about building a racing mower myself

Sounds fun! I have a tractor I'm planning on building into a racer just for myself too as just like you, there's no racing clubs around here either! Lol.

I personally would choose a axle width based on keeping the tires tucked just inside the fender pan for safety just in case you race some friends, I've seen things happen on the stock car track and the last thing anyone needs is a tractor Dukes of Hazzarding itself onto them!

I made my decision not to go with a kit, and just buying a keyed axle to cut to length so I can reuse the stock 3/4 wheels. I found a 700 series transmission, but it is a 3 speed and not a 5 speed. Will that effect top speed or just longer gears/ bigger gear changes? And for the axle, I think the best thing to do would be get a 1” keyed axle because parts are easy to find in that diameter and just go get the ends turned down to 3/4” to accept the wheels. The easiest thing to do would be get a 3/4” axle, but wouldn’t that bend under stress being so small?

3/4in axles would probably be fine as long as you had adequate support. Keep as little unsupported length between the axle bearings and the centerline of the tires.

In karting the thickness of the axle is often what determines the softness of the ride.
Although the go kart axles you see often are not really built the same
I guess 3/4 should do fine, many people run stock 3/4 shaftts

As far as the gearbox goes these aren't shift on the go so whatever you decide with your own pulleys and sprockets is more important.
I sound like a broken record but I am actually a huge proponent of using a cvt.

Especially if it's a racer and you keep the engine stock a cvt will get the most out of it.