Designing a gravity racer part 1

By Ben Everard. Posted

HackSpace mag editor, Ben Everard is planning a gravity racer build. For the uninitiated, gravity racers are vehicles designed to go down hill as fast as possible. Many races include chicanes, jumps and other obstacles to make the race entertaining for spectators. If you’ve not seen one before, take a few minutes to check out this year’s London race:

soapbox2

Fancy dress isn’t always mandatory, but it’s part of the theatre of the occasion.

The maker community is famously good at sharing details of their builds. It’s common to see makers post schematics, source code, STL files and more to help others understand (and build upon) their project, but we’ve found relatively little about gravity racer designs. This post is the first of a few where we pick through the available information and try to come up with our design.

Watch any of the online races and it’s clear that wheels are what make or brake a soapbox racer. This collection of the top five crashes from the Redbull London 2017 racer show that a three of the top five crashes were caused by wheel, or wheel-mount, failure: https://www.youtube.com/watch?v=p85s4zxG_yk (the remaining two were: one seat came off the cart and one was going too fast to make a turn). So, wheels are important, but what wheels should you use?

Here’s what the Scottish Cartie Association has to say about it (which is the most authoratitive source on building gravity racers we’ve found (from http://scottishcarties.org.uk/resources/construction/wheels):

“The de-facto standard wheels for soapbox racers and gravity sidecars in the UK are 20 inch BMX wheels. This is because they are strong enough to withstand the side loads experienced when cornering and readily available. There are also a good selection of tyres for them.
...
Other wheels used include go-kart wheels, pit bike, scooter and mini moto wheels. These are very strong, and give good grip when cornering, but have high drag on long fast straights.”

That’s pretty clear then – if you want to perform well, BMX wheels are the ones to go for. That gives us an easy source of cheap second-hand wheels. But what are BMX wheels? There’s a pretty wide range of bikes that come under the name BMX from glorified kids bikes to serious bits of kit capable of handling some pretty extreme jumps. In terms of wheel strength, bike wheels are often judged by their spoke count. More spokes equals a stronger wheel. 36 or more spokes per wheel seems to indicate a ‘proper’ BMX wheel.

The choice of wheels naturally leads us on to the question of how to mount them. Bike wheels are obviously designed to be mounted on both sides of the wheel, but most four wheeled vehicles (like cars) only mount on one side of the wheel. You can mount a bike wheel on one side only (known as stub mounting), but should you? Again, the Scottish Cartie Association says this:

“BMX wheels with 14mm spindles can be single side or "stub axle" mounted. If the spindles are smaller than this the wheel will need to be fitted in forks or swing arms”

This is hard to verify. While there’s lots of footage of soapbox crashes online, and lots of these involve the wheels falling off, it’s near-impossible to tell from a video if a 14mm or 10mm axel is used (that's 1/2 inch or 3/8th for any Americans reading). There’s also lots of footage of carts with stub-mounted BMX wheels surviving, and again, it’s hard to verify what axels these have. Supporting the rear wheels on both sides is a fairly simple proposition, but on the front it’s slightly more complex due to the need to turn the wheels. It doesn’t seem like this should be too complex though.

Perhaps the most surprising thing for us is that while wheel-mount failures are one of the most common failure points of wheels on soapboxes, relatively few have forks or some other method of supporting the wheel on both sides. Are we overlooking the design complexity of this?

So, we’re left with three choices:

1) Use go-cart (or similar) wheels which are designed to be stub mounted. They’re stronger, but also slower

2) Stub-mount 14mm BMX wheels and hope that they hold

3) Add front forks to the design

We would love to tell you what the right option is, but unfortunately, until we’ve actually built the cart and put it to the test, we can’t say for sure. We’re planning on going with option 3 though. What do you think? Let us know in the comments below, and if you’ve got any experience in this area, please get in tough with us on social media or at hackspace@raspberrypi.org.


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