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Homebuilt ATV

NOTICE: This page will not be available after Jan 2004. Feel free to copy, archive, and redistribute it's content during the next two weeks.

 Drawings for Phase I

This is my first try at a ground up design for an offroad vehicle. It will be fully suspended with equal length A-arms in front and swing arm rear suspension. The front shocks are cheap ATV shocks ($13 ea.) with 2" of travel and 260lb/in spring rate. By connecting them about half way out on the bottom A-arm, I get about 5" of travel at the wheel. The steering hub will be a king pin design.

For power I'll probably use a 5hp Briggs and Straton engine. There are plenty of aftermarket parts to make these engines produce more horsepower and achieve higher rpms. Further down the road I'll try my hand at creating an electronic fuel injection system for this motor. The engine will drive the solid 1 inch live axle using chain and sprocket.

Braking will be accomplished with a single disk mounted on the live axle for now, in the future I hope the find a wrecked quad and used the disk brake front hubs to provide front brakes.

The frame is constructed of 1/16" wall 1" sqaure mild steel tubing. I'm using square tubing for ease of construction and to limit the number of new tools I need to buy.

My biggest challenge in designing the steering system on a suspended vehicle is eliminating the bump steer problem i.e. changes in toe angle as the suspension moves through it's travel. The trick is to make the steering linkages follow the same path as one of the suspension linkages. The image below shows the system I ended up with. The steering wheel is connected to a 'linear actuator' via chain and sprocket. The sprocket on the steering column can be changed to achieve different steering ratios. The sliding rod (linear actuator) acts much like a rack and pinion and connects to the tie rods and about the same point where the lower A-arm connects to the frame. The tie rod follows the same path as the lower A-arm such that there is no change in length i.e. no bump steer as the suspension moves through it's travel.

It's finally done!

Here are some cool shots of the completed kart. I'm very pleased with the final product. I've driven it mostly on gravel roads where it handles very predictably with slight catchable oversteer. Top speed is about 40mph as predicted. The clutch is a standard centrifugal clutch and slips a little on start up i.e. take off must be on level ground. The biggest impression when first driving the kart is how amazingly stable it feels, it seems like it would be impossible to roll it.

Specifications and Features

Phase III - bigger motor

After trying a number of different clutches to improve initial acceleration, I came to the conclusion that a centrifugal clutch just wasn't going to work. The next best alternative was to mount a belt driven torque converter but that would require a complete redesign of the rear end. Fortunately, about this time I came across a good deal on a parted out '87 CR125 dirt bike. The motor is water cooled and puts out about 30hp compared to 8hp from the Briggs motor!

With a bit of remorse I pulled out the die grinder and started pulling apart the rear end to accomodate the new motor. At this point, I have the motor and exhaust mounted, and the motor running.

This version of the kart also has a much better seat and a 5point harness .

Well it's been awhile but I finally finished phase III. The new motor is in, plumbed, wired, and ready to run. For the first test I took it to a gravel lot to try it out. I was completely unprepared for how much power and torque this motor had. Within 5 minutes I had bent several key supports such that the chain and exhaust kept falling off - back to the drawing boad. A few hours bending stuff back into place and welding more braces in and the kart was ready to go again. This time it run flawlessly. The kart is just unbelievably fast with this motor, and a lot more fun to operate too. The big problem now is to keep from spinning the tires :-) Quite a big change from the last motor where I needed a push start to get the centrifugal clutch engaged!


Where can I get the $13 shocks?
I purchased the shocks and a few other components from Northern Hydraulic.  You can request a catalog online.  Keep in mind that these low end shocks are really just springs, they contain no damping capability.  For low speed (<20mph) this makes for a very comfortable ride but if you want higher speed and higher performance, I would recommend using a motorcycle shock with adjustable damping and a choice of spring rates.

What are some other internet karting sites?
Rieken's Racing has a really complete online catalog and good service
Northern Hydraulic has a small selection of really cheap kart parts

How did you get more horsepower out of the 5hp engine?
First off, I didn't do this work myself, I had it done by a local 4-stroke kart racer.  He did this stuff as a side business and had lots of experience.  These are the basic things that he did to my engine.

On his engine dyno, my engine produced 7.5-8 hp at 5000rpm

Where can I get drawings for your kart?
The only drawings I have are here

Can you take more/better pictures of your kart, steering system, etc?
No, I sold this kart a while ago so I no longer have access to it to take more pictures

Can you provide better drawings, pictures, or descriptions of the steering system?
I get a lot of questions about the steering system. I used this system because I had all the parts in the garage so it was basically free to me. I guess there must be a lot of people in the same boat because I find if very surprising that people would want to use it. In other words, if I had my choice of components for the steering, I would MUCH rather have a real rack and pinion as it would be a lot sturdier than the chain drive system.
Having said that, here is my best description of the chain drive steering system. It's based on a similar principle as the rack and pinion except instead of a gear drive, the rack is actuated by the chain directly pulling on it. The rack consists of a straight rod that passes through the frame on bushings, the tie rods connect to each end of the rack. The chain is driven by a pulley on the steering shaft. The chain goes straight down to another set (2) pulleys that redirect the pull force from vertical to horizontal (parallel to the rack). Each end of the chain is then attached to the rack so that when the steering wheel is turned, that force is transmitted through the chain to pull the rack back and forth and eventually move the front wheels.
Hopefully this helps, if you still don't get it, examine the pictures at the top of this page carefully, they actually show pretty good detail of the steering.

What kind of welder did you use?
I used a Lincoln Weldpack 100 wire feed electric welder. I used flux core 0.032" wire with no shield gas. This system is very cheap ($350) and works quite well. The downside is that a lot of time is required to clean up the welds with a wire brush when you're done. If I could've afforded a MIG welder at the time, that would've been much preferred.

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Last update 10/24/98