Oscar Forand's Kart
I just completed my newest race kart that incorporated a lot a ideas I learned from your site. Pictures are attached, but here are some of the details:
Maximum frame width - 26". Wheel base - 44". Maximum width (rear wheels) - 51". Height to top of roll cage - 50".
It became obvious real quick during construction that the roll cage was going to make the frame to stiff. Jpg 2 looks down where the upper frame and roll cage are joined to the lower frame through a 3/4" bolt pivot point. The frame was still to stiff to flex much, so the lower frame crossmember in front of the seat was also removed providing plenty of lower frame flex. To keep the lower frame flexible, the small rack and pinion steering box is attached to a dropped section of the upper frame, and the foot pan in front of the steering box is is equipped with rubber bushings underneath to allow the upper and lower frame sections to move independantly.
The front spindles are Azusa kids kart types that have been flipped 180 and the original steering arms cut off. New steering arms were fabricated with Ackerman angle built in. Castor and camber are zero. The 1-1/4" rear cromemoly hollow steel axle is fitted with a hydraulic disk brake system and it works great!
You can see the dual chain drive through the jack shaft. By changing the input or output or both gears on the jack shaft the the engine RPM can be set for maximum @ the end of the longest straight. I moved to jack shaft to a location behind the axle to lengthen the drive chains to give the links more time to cool and cut down on chain kinking. This seems to be working good.
The engine is mounted on a 1/2" aluminum plate that has 4 slotted 3/8" bolt holes as does the jackshaft mounting plate. This allows the jackshaft and engine to be quickly moved to tension the chains.
The engine has a lot of work done to it! Originally a 6.5 HP Honda 4 stroke, it now makes 16 HP @ 6800 RPM! It has 12.5:1 compression and must run on 114 octane off road gas. It has a cam with 230 degrees duration, uses a Mikuni 24mm motorcycle carb, and a centrifugal clutch the engages @ 3000 rpm. The aluminum flywheel used in this motor makes acceleration of this kart scary! I still learning how to drive it, cause right now it's easy to over drive the track we have with this kart.
The solid rear axle definitely requires a different driving style than the one wheel drive karts! The steering in the older karts was about a half a turn lock to lock, this one is a full turn. Also seeing how it drives, if I was going to build another one, I think I would keep the wheel base @ 48" to reduce how quick it reacts to steering input.
Further follow-up from the kart builder:
The kart rims are aluminum single piece Douglas with a 3 -5/16" American pattern bolts. I purchased them in 2007. The rears are 6" dia X 8-1/2" wide and are fitted with Burris dirt track racing tires of 11 X 8 - 6 size. The fronts rims are 5" dia X 6-1/2" wide and run 11 X 5.5 - 5 tires. The entire set, plus mounting and shipping, cost me about $400.
I fabricate my own jackshaft mounts from 2" X 3" X 3/16" steel angle, and use a 3/4" shaft to mount the gears on because most of the hardened tooth, off-the-shelf gears I need come with that bore size. I will say this, I have a small machine shop in my garage I do side jobs with now that I'm retired (I'll be 66 this November) and having your own milling machine, lathe, hydraulic press, mig welder, etc: makes a big difference in what you can fabricate yourself!
The stock Honda's are governed @ 3700 to 3800 rpm. The governor and "low oil alert shut down system" are the first things removed when modifying these engines.
The frame is functioning fine, the front end works and flexes almost as if it had a simple spring and shock suspension system.
The only other problem I have had in track testing so far is the gravity feed fuel systems inability to keep up with the engine while racing.
A Mikuni high volume pulse type fuel pump is going to be added to the system.
Still have a lot of track testing to do to with varying tire pressures to optimize performance, but this thing definitely gets the adrenaline flowing!
Thanks for all the good info on your website and the info you gave me directly!