Soapbox Racer Physics

The force F acting to move a soapbox cartie down a slope of angle θ is m g sin(θ), where m is the mass of the cartie and g is the acceleration due to gravity.

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What's Better - Heavy or Light?

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?

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Turn Your Phone into a Lap Logger for Under £60!

BT-Q818XT GPS Receiver Want to use your Android smartphone as a track logger but frustrated by the inaccuracy of your phone's internal GPS receiver? Simple - Install Trackmaster and the Trackmaster Bluetooth GPS Service on your phone then get one of these puppies and you have a state of the art race logger for under £60!

Team trackmaster and a BT-Q818XT with your laptop and SCA CartieSim, you can analyse the track before you have even turned a wheel!

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CartieSim Just Got Better

CartieSim, the SCA's cartie speed calculation application, just got better. Version has just been released and includes some significant improvements;

  • Added distance v. time chart
  • Much improved calculation of elevation data from online satellite data sets (SRTM and ASTER). Elevation is now interpolated from nearest 16 data points using bicubic interpolation
  • Modified formula and algorithm for determining speed to better account for the effect of wheel mass
  • Added modelling of drag plates (for Cairngorm)
  • Default new cartie data can be saved
  • Automatic insertion of extra course datum points on long straights to give better resolution of the elevation profile
  • Automatic detection and marking of corners 
The new version can be downloaded from here. Existing installations should update automatically when they are next run.
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SCA CartieSim - Soapbox Speed Estimation Software

Free Download

 Current version:

 Release Date: 29 Mar 2015

 System Requirements

SCA CartieSim Screen Shot

SCA CartieSim has been developed by the Scottish Cartie Association as a tool for cartie racers and race organisers. It allows cartie courses to be analysed to predict likely speeds and course times. In addition, it can highlight sections of the track where high speeds and tight cornering may cause difficulties and can be used to model the effect of chicanes on speeds.

Data can be loaded from Google Maps, GPS / SatNav data files and files created by Trackmaster, and the characteristics of individual carties entered to predict the effect of changes to weight, aerodynamics, etc.

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