Would a heavier soapbox car go faster than a light one? Is there any ways to make it go as fast as possible?


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jerm13868 years ago
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.
mnort381 year ago

There is one point that wasn't brought up, and that is the centre of gravity. Keeping your profile low within the cart. Remember when you pedaled down a road and the lower you got the faster you went. That was partially aerodynamics but the other part of the formulae was, centre of gravity.

CameronSS8 years ago
Yes and no. In theory, they should go exactly the same speed. Gravitational potential energy is mgh (mass times gravity times height), which is converted into kinetic energy, which is 1/2mv2. Since both have mass in them, they cancel each other out. This is the same concept as dropping a wooden ball and a steel ball of equal size-they both fall at the same rate.

In a soapbox derby, the most important way to increase speed is to reduce friction. This means having an aerodynamic body and wheels that spin freely and are perfectly aligned. Wheels that are crooked will all be trying to turn the car against each other, and energy will be wasted.
Where would extra weight be added to the back or the front?
Or would it be better to balance the weight evenly so the weight is the same on both front and rear?
nice clear answer, CameronSS.
tinker2345 years ago
yes a heviar one would go faster down the hill but keep in mind newtoons law
ANDY! (author) 7 years ago
WHy is it say WINNER?