There's a lot of physics that you could go into and I am an engineer, but rather than go down that tricky theory road, I'll tell you what I know from experience. It depends on the hill you're trying to race. In my racing experience I have driven a light kart and a heavier one. All other factors such as design, bearings, etc being equal, the light karts are faster off the blocks (especially on shallower inclines) and good on hills that remain steep. A lighter kart, however, carries less kinetic energy than an exact-design heavier kart, so it is more susceptible to wind resistance (since bearing friction should scale with weight, bearing losses do not affect one weight 'more' than another). If you are on a hill that is steep then has a flat section, the light karts will lose speed quicker than the heavy karts. In our most recent race, the heaviest kart had the highest top speed but placed fifth out of six! It took a long time for it to get moving. As for handling: a heavier kart will be harder to maneuver and will not recover from braking as quickly as a lighter kart Since wind resistance at starting-line speeds is practically negligible, the thing that will matter the most toward kart speed in the beginning of the race are the bearings/rolling resistance. Get the best bearings you can in the smallest size that is safe for your application. Cheap bearings have tendency to fail at higher speeds and with heavier karts. As for rolling resistance, large diameter tires with a small, smooth contact patch (think bicycle/motorcycle racing tires) will yield the best results. Smooth tires may not be safe in all applications, such as racing on dirt, snow, or wet smooth pavement. A heavier kart WILL compress the tires more, making for a slightly larger contact patch with the ground, increasing the rolling resistance. This is probably why lighter karts start quicker. At higher speeds, though (karts may get into the 50-60mph range on longer, steeper courses) your primary loss will be wind resistance, so make a kart that allows you to lay down and make your cross-sectional area as small as possible. Putting a body on the kart will decrease wind resistance too.