What's Better - Heavy or Light?

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I'm building a soapbox cartie and I don't know whether to make it as heavy as possible or as light as possible. What should I do?

Physics says that - all other things being equal - a heavy soapbox will accelerate quicker and have a greater terminal velocity than a light one, so if you add weight to your cartie it will go faster.

Cool - so that means I should make it as heavy as I can?

Not necessarily. If it's a straight drag race where you need little or no braking until you get to the finish line, then yes the heavier you can make it the faster you will go. However, if you need to do any braking for corners then it depends on how good your brakes are. A lighter cartie will be able to brake later and so have a lower time on certain hills. There is an optimum weight for any given hill which depends on how good your brakes are.

Oh OK - so how can I tell what my weight should be?

You can use SCA CartieSim to model the speeds of any given cartie on any given hill and determine what is the best weight to be.

Hang on - you said heavier is faster on straight hills. Doesn't that just mean the heaviest cartie will always win?

Not necessarily - there is a lot more to it than that. If you put more weight in your cartie (securely bolted in place, of course), it will accelerate quicker. However, that doesn't mean that your heavy cartie will always beat a lighter one. Aerodynamic drag, rolling resistance and moment of inertia will all affect the speed. Of those three, aerodynamics has by far the largest effect, and a sleek light cartie will usually have a higher top speed than a heavy and unaerodynamic one

What's the heaviest I can make my cartie?

That depends on the event, but typically somewhere between 80 and 100Kg without the driver. Check with the event organisers.

Doesn't weighing them without the drivers give an advantage to the heavy drivers? Why don't you weigh the carties with the drivers in them? Wouldn't that be fairer?

In theory it might be fairer, although on short twisty tracks the extra weight of a salad dodging driver might actually work against you unless you have really good brakes. However, we don't run races in theory. We run them in real life. Many teams want to run different drivers in different heats, and most event organisers have neither the time nor the patience to wait for all the teams to swap out their drivers, change the ballast so they are back at their optimum all up weight and then re-weigh them before each heat. So we weigh the cartie once and don't allow ballast to be changed. In any case - as explained above - light carties often do very well.

Is there a limit on the amount of ballast I can use?

Sometimes. It depends on the race. However, you should not be thinking in terms of adding huge amounts of ballast to get up to the maximum weight. If you want to make your cartie heavier, add structural mass to make it stronger rather than just heaving dead weight into it.

What about centre of gravity?

That one is easy - make it as close to the ground and as near to the middle of the cartie as you can. If you have a high centre of gravity you risk rolling over on corners. If you have your centre of gravity too far forward or too far back, you will be liable to spin on corners.

OK. So I'm still confused. What should I build?

  • Aim for around 70-80Kg without ballast.
  • Make it strong, low and wide to keep it safe and stable. 
  • Make sure there is enough diagonal bracing to keep the chassis stiff and stop it from deforming if you crash.
  • Use solid ballast weights securely bolted in place to fine tune the centre of gravity and/or optimise for a particular course, but don't overdo it. 
  • If your cartie is a long way below the weight you want to be, add structural weight to make it stronger rather than just bolting on more ballast.

And if you want to go really fast, stop obsessing on weight and concentrate on making it as aerodynamic as possible. And make sure your steering and brakes are up to it.



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