Your gravity racer is going to need wheels of some sort, and unless you’re in a position where you can manufacture something from scratch, it’s a case of re-purposing wheels that are designed and made to fit something else. Choices I have seen include:
- Bikes (BMX are popular)
- Pit Bikes
- Mini Moto
Wheels are typically the first place people start when thinking about designing a gravity racer (they certainly were for me). I initially started looking at sack cart and wheel barrow wheels on eBay, but it quickly becomes clear that if you’re wanting to go fast, wheels like that aren’t really rated for speed and don’t have good enough bearings etc for the job.
I also soon realised that because a gravity racer needs brakes, unless you’re an adept engineer who can make bespoke things you need wheels from something that already has brakes attached. Bikes are most people’s first thoughts and there are many gravity racers that use bike wheels with either standard rim brakes, or more advanced hub or disc brake variants.
As I said above, bike wheels are very popular, but not all kinds will work. When a bike is ridden round a corner, the bike leans. This means that the forces are always directed straight down through the wheel towards the contact patch – the wheel doesn’t have to deal with any lateral (sideways) forces. For that matter, neither does a bike frame hence why they are very slim – all the strength is on the vertical plane.
In a gravity racer, the wheels have to cope with lateral forces during cornering. How much force depends on a few things:
- Tyre grip
- Sharpness of turn
A fast car with a lot of mechanical grip going round a tight corner will exert a lot of sideways force on the wheel. Less grip will mean you’ll have to take the corner slower (so not to skid), so less sideways force, and the same will happen if you’re going slower or the corner is not as tight – less sideways force.
What does all this mean? Well, basically, you need to choose a bike wheel that is strong enough to cope with the lateral forces. Because of leverage, larger diameter wheels exert more force on the spokes/hub. Mountain bike wheels of around 26″ can collapse during cornering. Racing bike wheels are also too big, and are of too skinny and delicate a construction to be suitable. The defacto standard of bike wheel seems to be the 20″ BMX style wheel as they are strong, cheap, and as they are smaller they have less aerodynamic drag. There are many examples of gravity racers using 20″ BMX wheels.
Assuming a 20″ BMX rim and tyre, the next issue is what hub to use. The key here is to use a hub that has as thick an axle as possible. Even with a tough 20″ wheel, the 3/8in axles that are common on bikes like this are not strong enough. Essentially, all the leverage and force that goes into the wheel during cornering tries to bend and snap the axle where it joins the chassis to the wheel itself. Tougher freestyle BMX wheels feature a 14mm axle, which is where you want to be at the very minimum. If you are happy to lace a specialist hub onto a 20″ rim there are even more choices available. This is also a likely option where brakes are concerned, as most BMX bikes use rim brakes. It is possible to use these on a gravity racer, but they look ungainly as it means building a frame to mount them on.
I asked if there are commercially available (i.e. not too expensive) BMX wheels with 14mm axles and disc brake mounts HERE. In short, there is not.
MORE TO COME