How I'm Building a Homemade Dirt Bike with Big Tires

I've always wanted to built my own dirt bike / off road motorbike. I'm never going to do motocross - I don't have an interest in it, and I'm not going to jump. That what dirt bikes are made for - jumping. I wanted something that was kind of a cross between a dirt bike and a mini-bike - the quality of a dirt bike but with low center of gravity, so it it will balance almost on it's own. I want to be able to ride dirt roads, have my feet touch the ground easily, and if it falls over, I won't have a hard time picking it up. No company makes a bike like this, so I decided to build my own.

Why I'm Building This
Most dirt bikes have high strung, single cylinder engines that are specialty engines - the frames are designed around the engine. So it's all one unit, and you could never take an engine out of something else and use it in that frame with extensive modifications.
I wanted to build something that was generic and would work with any engine. That's kind of how mini bikes are - you can get the engines very easily and cheaply. Most use Harbor Freight Predator engines, or you can swap these in. All Mini bikes are in the 200cc range. I have never seen a mini bike with a 400cc and above engine from the factory.

So what I wanted to do was build a quality dirt bike frame that is designed for 400cc single cylinder industrial type engines. It's got to have full suspension and lots of travel - I want it to have basically unlimited suspension travel. I want the frame to be able to touch the ground and not bottom out. When you're riding it, it should feel very plush and soak up bumps like they aren't even there. This is not hard to do, or it shouldn't be hard.

Dirt bikes are made for motocross racing, and mini bikes are made for children. Where is the bike for the average guy that just wants to ride across some desert or down a dirt road? A bike like I want doesn't exist.

Wheels and Tires
I decided to do with ATV wheels and tires - 25" tall and 12" rims. It's a standard size that you can get anywhere. The big advantage here is that these big, low pressure tires with soak up bumps and make the ride even softer. It won't be as responsive as thin dirt bike type tires, but that's not what I'm looking for. I want something that can roll across just about anything - rocks, sand, mud, you name it. Dirt bike tires, because they are thin and small, have no flotation. They will just sink into soft terrain. Big wide ATV tires will just float over everything. Here are the tires I'm using.

Frame Design
You have to build the frame for the type of engine you're using and figure out how the transmission is going to work. The industrial engines I wanted to use don't have a gearbox built in like a dirt bike engine, so you have to use a CVT. Most CVTs are made for go karts and mini bikes in the 200cc range. They are not good quality. I think if there was more demand for CVTs for bigger 400cc engines, there would be more options. Just not many people are using these bigger engines - I don't know why. Why not use a slightly bigger engine when the weight difference is negligible? The 400cc engines have so much more torque and power.

I needed to build a frame for these "big block" industrial type engines. They won't fit into a cheap mini bike frame just because of the way they are designed. They are bigger engines, but not that much bigger. The frame only needs to be slightly bigger.

DOM Tubing - I use 1 1/4" by .095 wall. This strikes a good balance between strength and weight. It's a little heavy, but it gives you a lot of strength. It's a small frame so I can use less tubing, which reduces weight. I don't need to add a lot of smaller, supporting tubes because my main tubing is big and strong.

Here's a photo of my current design:

CVT & Engine Power Output
I'm using a 460cc Wildcat Engine from EC Carb, and when fully built it could produce 30-40 horsepower. Most go kart/mini bike torque converters can't handle that much power. So I had to get creative. I decided to use a secondary pulley off a Polaris RZR 900. I'm not sure if this will work yet, but I don't see any reason why it won't. The belt is huge, and it's way more than what I need for this application. Hopefully, it shifts and works correctly with obviously less power than a 900cc SXS would have. It's going to sit on the outside of the frame and connect to a jack-shaft that runs through to the other side of the frame. The drive sprocket will run off the opposite side. The tire can be as wide as I want.

The biggest issue is fitting the secondary pulley - it's big, and there's really not a good place to put it except on the outside of the frame like I'm doing. It's a dirt bike so there's not much space.