Building a 250 Superkart.
by Pierre Martins
Late in the ‘06 South African 250 Superkart Championship a fellow competitor spun in front of me. I had nowhere to go and T-boned him pretty hard. My kart was a write-off and I had no option but to build another kart from scratch to complete the penultimate and final rounds of that year’s championship.
This is the story of the Stealth 250 Superkart I built for that reason. Sadly this kart didn’t last long. I crashed it again in similar fashion during the final round. Needless to say, 2006 wasn’t a good year for me in 250 karts.
Anyway, here are the pics and story. Click on the thumbnails to enlarge, and use your browser’s ‘back’ button to return…
Bare chassis, seamless mild-steel.
The grey colour I opted for this kart was normal 2-k auto paint.
It’s not a good idea to powder-coat a kart chassis. Powder-coating compromises chassis-flex and that will have an adverse effect on handling. Nowadays most Superkart chassis are Chrome-Molly, painted a thin coat of black paint.
The tolerances on a Superkart are minute and you have to be exact to the last fraction of a millimetre with regard to setup. Being 1mm out on your setup on a kart is like being 10mm out on a full-size car. Kingpin inclination, caster, camber, toe and wheel-track are all crucial issues.
Brakes on this kart were Kelgate double-ventilated full-floating discs with 4-pod callipers in front and a single 6-pod on the rear solid axle, with adjustable bias and braided lines.
Steering on this kart was a rack-and-pinion Kelgate item, imported from the UK. At the time some Stealth karts in South Africa still ran lawnmower types of steering mechanisms. Very high-tech, and that wasn’t too long ago. He-he-he…
Suspension? - Nothing, nada, zilch, but you can set caster and camber angles, wheel track and a few bits and bobs. The use of corner weight scales to set up a Superkart is required if you want well-balanced handling. Everything is rose-jointed. Back axle is solid, as in a pipe between the rear wheels.
And what about final drive? Diff? What diff? We’re talking chains and sprockets man! This is a modified shopping trolley for Pete's sake! Superkarts are chain driven, with sprockets ratios that are change-able from track to track, just like motorbike racing.
As is the case with most 250 Superkarts in South Africa, this kart ran a Rotax 257 two-stroke twin cylinder water-cooled engine good for about 85bhp on average, with a fully-sequential bike-type 6-speed gearbox.
Cable operated dry clutch is used only for pull-off. After that you just bang it up and down the box with a little lift-off on the throttle as you change, exactly as you would do on a bike. On this kart I installed a lever-type hand-operated clutch to the gear lever.
Top international karts make 95 to 110bhp and more, depending on who's telling the story. Overseas they also use Honda CR250, Yamaha YZ250 and Gas-Gas motors. In fact, most two-stroke engines good enough for 250GP Bikes are good to use 250 Superkarts.
Battleship Grey Under-Tray.
When building kart I’ve always felt better when that thin under-tray finally went on. I mean, it’s the only thing separating you from the track surface when you’re going 270km/h with your ass literally one inch off the ground.
For this kart I used a light-weight fibreglass flat under-tray with integrated rear diffuser and painted the thing battleship grey to match the chassis. I think it’s good colour if you wanna be able to spot possible failures like chassis cracks and other potential stuff-ups at a glance…
Fuel tank was a 15l aluminium home-made jobby with baffle plates to prevent fuel splashing inside the tank resulting in fuel-surge.
Oh, and by the way, the under-tray has to be bolted on loosely with rubber washers between the chassis and under-tray. Normal tap washers work well for this application. If you bolt it on too tight you hamper chassis flex and handling.
Under the Skin.
There is nothing on a 250 Superkart that is not there purely for speed. We’re talking bare essentials. No roll-cage and no belts in something that’s capable of top speeds in the region of 270km/h and that will turn lap times comparable to big ticket sports cars around medium-sized tracks.
It’s definitely not for the faint-hearted. No wonder David Coulthard said it was the most frightening thing he ever had to do when he tested world champion Martin Hines’ 250 Zip Eagle Superkart in 1994 when he was still a test driver for McLaren.
Ready to race.
As mentioned above, there is no safety harness. A snug seat is all that keeps you in the kart.
The big rear wing integrated the radiator and believe me, it wasn’t for show. At the speeds they’re capable of, 250 Superkarts are very susceptible to down-force. The attack angle of the wing aerofoil on this kart was adjustable to suit track conditions.
If you set the angle of attack too steep on the nose cone of a 250 kart you’ll have excessive down-force in front and that will make the front bite, making the kart will over-steer in fast sweepers. If the angle of the nose cone was too shallow you wouldn’t have sufficient down-force and the nose would push in fast corners, making the kart under-steer.
Believe me, you would not be able to drive one of these things flat-out without down-force.
And that was that.
The kart turned out pretty quick straight out of the gates, but I didn’t have much luck with it. On its first outing at Kyalami during the penultimate round of the 2006 season I touched another competitor as we entered Sunset Bend. His nose cone sliced my left front tyre and that was that.
At the final round another guy spun in front of me and I hit him hard enough to bend the rear axle and a few other things, necessitating another rebuild in the off-season.
Be that as it may, the kart was featured in a local magazine called Speed & Sound at the time. The article was written by a car-guy with no clue about 250 Superkarts, but you can view scans of the article by clicking on the thumbnails below.
Anyway, here are some pics.
Click on the thumbnails to enlarge,
and use your browser’s ‘back’ button to return…