Gravity Racer, take II

The lunacy is starting again.

Last year I (quite casually) chronicled the build of my entry for the Portland "Adult" Soapbox Derby in this thread, and cutting to the chase, WE WON!!

This year, instead of tuning up the winner, I have decided to actually just build another one from scratch again. The concept will be the same but the details will be different. The rear wheel dia will match the front, overall height will be lower, overall length will be longer, frontal area and cowl height will be reduced, better wheel skirts, and the final objective is to have a canopy/closed cockpit.

I assume there will be a pretty big target on my back this year and I was on the fence about building another one vs tweaking the original, until I found a really cheap deal on a full set of wheels and couldn't resist. So far I have been working on the packaging of the payload and the running gear and trying to get it lined up. I'm working with the same high-tech systems as last year: the shop floor and tape.

General layout:

In addition, I have the components for the front uprights fabbed up, just need the axle beam finalized before I can finish weld them together.

So not many cool pics, vids, or stories yet but I am looking forward to them!

Randy V

Lifetime Supporter
Looking forward to following this!
I have to wonder though - How long will it take your fellow competitors to catch on and find your threads?

Glenn M

GT40s Supporter
Good stuff! I remember reading all about last years entry...well done!
One thing though, IMHO, your seat, although very comfy may well be a tad heavy and un-aerodynamic? :laugh: :laugh:
Looking forward to following this!
I have to wonder though - How long will it take your fellow competitors to catch on and find your threads?

I think I'll fly under the radar on this forum. I have a facebook account on which I post maybe 3 times a year, so should be able to control myself there.

If they find me here, good on them!

Front end is almost built up now, need to devise steering pivots and plan out the steering and forward end of the frame's layout. Using some plywood scraps to do proof of concept. After that, the main chassis should come together fairly quickly.

Front end getting there:

Proof of concept chassis layout:

One thing though, IMHO, your seat, although very comfy may well be a tad heavy and un-aerodynamic? :laugh: :laugh:

Thanks for the heads up on the seat Glenn. I hadn't thought about that yet but see now that I will have to revise my plans!

Happy to post progress, glad people enjoy the saga.

Front end coming together even more...

Steering will be done by levers instead of wheel or tiller due to space constraints. They will be located approximately waist level when in the cockpit and as one is pushed forward, the opposite will retract. These levers will connect via pushrods to the bar below the front axle beam, with that bar being connected to the bellcrank for the front tie rods to do the actual steering. Simple, right?

Front end:

A close up, uprights broken down but bar and bellcrank visible, no tie rods yet:


Depending on where I end up routing the pushrods, the bar may be extended toward the bottom of the chassis. That will all depend on the size of the footbox and how much room I have along side my feet- or more accurately, how big of a hole I want to punch in the air at any given cross section.

There's no rocket science or anything ground-breaking with this project but I love the mental exercise coupled with the challenge to actually physically create the solutions and then see how it does on the hill. If only I could make a living at this!!

Pretty happy with how it turned out:

Google Photos

Pretty square, smooth movement, no slop in the pivot. Pretty good for an amateur.

If you're wondering why I complicated it in this manner, I am trying to think ahead and have the tie rods in the wake of the axle beam. I will end up building airfoils this time to streamline the beam and actually enclose the tie rod within it on the aft side.

Or at least that's the plan until I realize I can't...
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There was a challenge issued by the organizers. There hasn't been a 4-wheeled finalist for a really LONG time and the challenge is to build one to get there. It may not get me as fast as I need to win it all this year, but I am seriously thinking about taking up the challenge and seeing if I can pull another rabbit out of the hat.

I also started thinking incredibly non-conventionally, but I'll save that for 2019.
Per the pretty limited regulations, three wheels are required to be in contact with the ground at all times. The intent with their challenge was for a genuine 4 wheel entry to get through to the finals and while I may not end up being fastest on the hill for the day, I think I have a decent shot even with adding the 4th wheel.

If I stick with it, next year will have a very non-conventional design. This year seems too interesting to not try the 4 wheel challenge.
You need to be careful Jac... Great minds think alike- but I'm not sure you'd want to be lobbed in that bucket with me (although the bucket might need another label if I'm in it). That's one of the things I was thinking of doing.
Its a BIG bucket! One of those situations where the line between being clever or very silly can get blurred! Have a mate out here who used to race go karts & was keen to build a frame where he was prone lying face down & head first, had it built but parents stepped in and vetoed the deal. Seem to remember you climbing some lofty peaks in the past, how long before you take one of these creations to a higher & faster start point, Pikes Peak downhill for example?
I meant something like this Nick. Your twin wheel deal has similar problem to using very wide drag type rear tire on V8 bikes etc where leaning over in turn 'jacks' up riders effective seat height, uncomfortable sensation!


I finally took some time out and made some progress on the gravity car. I have committed to the four wheel challenge and will see how that all plays out. I am hoping that with an extra bit of weight and even smaller frontal area and better aero, we will overcome the extra drag and rotational inertia penalties.

The good news is I now have a roller tacked together. Next steps are chassis triangulation, tuning the ride height, and then fabbing up the steering arms, tie rods, and steering column. Once the mechanicals are sorted, the bodywork phase will begin.

I am quite happy with how it's playing out so far. I think we'll still have a decent shot at an overall victory again, but time will tell...

How things started on Sat:

How they ended up:
More progress being made. The cockpit is incredibly tight but I want good aero. I think I made the windscreen shape in a manner which doesn't generate a compound curve and will be able to bend flat sheet to form it. That's today's project- to at least generate a pattern for it.

Bodywork is the remnants of a roll of cheap duct tape, just to visualize the bodywork, ensure it flows reasonably, and check out sight lines from in the cockpit with the helmet on. Eventually it will be fiberglass and alu, like the original.

Super happy with how it's coming together!


Randy V

Lifetime Supporter
Coming along quite nicely!
Not that this will have any impact on your build but -
I’m curious how well bicycle wheels will work with cambering. With a bicycle, camber forces are minimal and only for a moment..