Im gonna build a "gravity bike" and see how fast I can roll down a hill.
Ive thought about all sorts of possible configurations for things with the ultimate aim of going as fast as possible on a road while being propelled by nothing but gravity.
Essentially that brings two different forces into play with gravity pulling you down hill, and friction resisting your motion... The two forces will balance out at some point and that's your maximum speed... Im aiming for something above 100Kmh.
In order to minimize friction I want to be as stream-lined as possible and with the smallest possible frontal area. To maximize the forces from gravity I want to be as heavy as possible.
... and (letting survivability have its say) I also want sufficient control to be able to avoid obstacles, stay on the road, and stop in a hurry if I have to.
And Ive decided that Ill take the project in steps too so that I can prove out different aspects of my design as I proceed and don't go spending large amounts of money on things that wont work.
Initially I was very attracted to the idea of starting with a recumbent trike that I could modify a bit and build a stream-lined fairing for... But a decent trike is a very expensive piece of hardware (minimum $2000) which was a bit much... So, then I considered building something from scratch but that would cost me a lot too and Ide end up with less functionality than the commercial trike route.
So then I decided Id consider two wheels rather than three, though the idea of getting a flat tire while traveling at these speeds on a bicycle does scare me quite a bit more on a bike than on a trike!
So I looked into recumbent bicycles but they still cost about $1000 minimum.
And so I finally settled on something much simpler as a start point... A BMX bike.
Im going to modify a kids BMX bike as a base platform.
It may sound a bit strange but it has a bunch of the features that I want: Its got a very low centre of gravity with the small 20" wheels. A kids bike is usually well made but uses thick walled heavy gauge steel tubes... which is fine for my application as it adds weight and strength. I can get one with disc brakes too and that makes it way cheaper than having my own set of custom 20" wheels built. And finally of course its much cheaper than the fancier options.
So, I looked at quite a few second-hand bikes on Craigslist and they are often very cheap or free but I couldn't find any disc-brake bikes around (new teck for BMXs). But I did find a relatively cheap new one and I went and bought that for a couple of hundred dollars.
The BMX bike in original configuration.
So, the mods so far include:
Remove seat, peddles crank-set and chain... I will not be peddling this anywhere :)
Invert the bike frame... It gives the head-set a steeper rake that makes the bike more stable at speed.
Invert the handle-bars... Lets my hands be much lower and further forward.
Add foot rests to the rear axle stubs and put spoke covers on the rear wheel.
And the bike currently looks like this:
And while that looks really odd for a bike, its not really a regular bike any more. All I have to add now is some foam padding to the top tube for my body to lay on, a strong rear wheel fender so my back-side does not rub on the rear wheel, and a couple of knee/shin rests that will attach to the seat-post. Then it will be ready for some test runs.
It is ridden something like this (and yes, absolutely, I will be wearing full motorbike riding protective equipment!:
Photo copyright J. Maus
If my road tests are successful then I expect Ill be building a full body streamlined fairing for the bike... I may continue to use the bike frame as is or I may decide to do some cutting and welding to lengthen out the wheel-base (more stable ) and maybe also try a fully prone riding position (probably good for straight runs but not so good for twisty roads).
Oh and just in casy you were wondering, a bike like this is in fact completely legal on the public roads (assuming it does not exceed the speed limits) because it has standard bicycle brakes and can stop and turn the same as a regular bike (unlike street luge or skate-boards etc).
Anyway, that's my gravity bike project that is "underway"... Ill post again when there is something to say, though that may not be till next spring!
