Gravity Racer

Chris Kouba

Chapter One: Mental Commitment, Design Definition, & Initial Build

Portland has an adult soapbox derby each summer ( and an acquaintance of mine built an entry and ran it last year, having a BLAST. I found out about it and decided I had to do it, so I started my build this fall...

My design concept:
  • Fit in my truck bed- so a footprint of 4'x8' or less
  • Completely removable front axle assembly
  • Lowest possible rolling resistance
  • Minimal cross-sectional area
  • Good brakes
  • Simple
  • Strong
  • Heavy

Pondering the build for a while, it seemed like the fewer the bearings, the lower the friction losses might be (although this will increase the loading per tire). I chose a reverse trike layout, hiding the single rear wheel behind my fat head instead of putting two rear wheels out in the air stream. That pretty much laid out the design for me- narrow, low, 2F/1R trike. In my head, I saw the front half of a Swift DB1, except lower and narrower, with a single 26" wheel behind the cockpit.

What wheels should I use? I wanted something which offered minimal rolling resistance, had reasonably robust bearings, was secured by an appropriately-sized fastener, and was available cheaply. For the front end, I found them in the shape of a used wheelchair (acquired off Craigslist for $20). For the rear end, I enlisted the front wheel from one of my mountain bikes, complete with 8" rotor and hydraulic brake.

Beside that, all I needed to source was steel. This was easily done as there's a steel yard maybe 12 minutes from the house.

I read somewhere that one of the early British GP teams (Cooper?) designed their entries by drawing on the shop floor with chalk, so I figured this was good enough for me. I moved the GT out of the way, ran the broom around the floor a bit, and got on with the task at hand.

With the wheels set up just inside 48" wide and the rear wheel placed roughly along the centerline, I laid out the space which my body would occupy in the chassis. Then I narrowed it a bit. I made sure the fronts of the front wheels were slightly behind the front of the chassis (roughly 8' from the aft of the rear wheel) to ensure the whole thing would fit in the truck bed. It looked a bit like this:

I then started chopping steel. First thing to do was build the front axle. I wanted to be able to store the car with minimal footprint, so I wanted an axle I could simply remove and then stand the car up in the corner or something... to get it out of the way. This axle will be held in with U bolts secured to the frame. The upright would be a simple machined billet alu piece with the wheel bolted to it with a 1/2" bolt, and it'd pivot in a C channel section of steel. It would look like this:

Front end solved, I turned to the rear end. I was using a 20mm thru axle wheel, so I just machined up two pairs of pillow blocks with a 20mm hole, and put another set of holes in them so they can be bolted to the frame. Kinda like this:

I then built a very simple fork, which became the first member of the actual chassis:

Presumably you can see where this is going... Once the fork was made, it didn't take much to connect it to the front axle- just a bit of box tube:

And by the end of the first day, it looked like this:

Yeah, still needs steering and brake mounted, floor boards, seating, bodywork, nosecone, etc... but it was off to a great start if for nothing more than proof of concepts.

Sounds like fun. I built a soap box derby car when I was 13. The local Elks Lodge built a track special for it. Still have the "Official" wheels axles & steering wheel & the plaque all participants got (Somewhere LOL). Think low center of gravity & frontal area, weight forwards.

Terry Oxandale

Skinny Man
If it's anything like cycling, overcoming aerodynamic drag is going to need 80% of your design prowess. Assuming max weight has its limit as well. Time Trial bikes have some pretty unique and low-drag wheels, but they get expensive (carbon disc being the ultimate).
I like it... question, was there a reason you decided to steer the front axle where it has two wheels and not just the single rear?

I hope you keep posting the build log.. very cool
This is worth a look:


Guy Martin is a top bloke. Well known for his TT exploits.
One of my favorite characters of motor sport. How can you not be drawn in by this mans infectious personality. A REAL DARE-DEVIL!!

Thanks for posting.

Chris Kouba

Google "Top Gear Reliant Robin test" for the reason we went 2F/1R. I can't seem to post the link to it without getting an error. One wheel in front is more unstable than two.

Next update shortly...


Chris Kouba

Chapter Deux - Functional Chassis and First Test Day

I had nothing to sit on, nothing with which to stop it, and nothing with which to steer it, so it was time to remedy that. I whipped up a bellcrank steering system with a simple tiller handle to do the steering duties and welded some plate steel in for flooring. With the rotor mounted on the rear wheel already, I also mounted up the caliper in the correct location to be effective. I also paneled up the floor and sides with alu to prevent road debris from perforating my body. Then I loaded it up and met some friends, who brought last year's entry along for more development.

Mine in the truck on the way to the test hill:

Loading up last year's entry (called the Hawksbill Flyer by its builder, Dennis, on left in Seahawks hat):

Dennis had run the Flyer in the official event last year, but it was wander-ous in its steering and woefully inadequate in its braking, and that combined with horrible handling, he was game to retire the entry. Bill thought that just didn't seem right, so he told Dennis he'd run it down the hill this year with Dennis' blessing. The objective for the day was to get a baseline for each car and see where we wanted to go.

Mine was easy- I pulled it out of the truck and ran off down the hill uneventfully.

Fresh out of the Scuderia Kouba garage and about to push off on its maiden voyage:

After the first run, .7 miles & 39 mph:

The Flyer was another story. Bill was all over the road and just about needed new shorts when he ground to a halt, eventually. The Flyer needed some adjustments...

Adjustments being made:

Over the course of the day, we managed 3 runs each. Performance-wise, we were close on top speed (Flyer had 43, I had 39) but I was unable to match the Flyer in acceleration off the line- presumably due to its higher weight and smaller (read: lighter) wheels. Bill would pull away initially and maintain the gap, although he spent a lot of effort just trying to stay on the road itself. When we decided to call it a day after the third run together (Flyer's brake broke), I did get to run all the way down the hill for a fourth run- 2.2 miles(!)- which is where I topped at 43.

On my runs, my car handled great and went exactly where I wanted it to go or stop, but it was not all smooth sailing. The wheelchair wheels flexed ENORMOUSLY when cornering (which I was expecting) but I didn't think they'd generate enough lateral grip where it'd be concerning. I was wrong. More importantly though, they were INCREDIBLY harsh in their ride on the pavement, and when I leaned my head against the head rest, the world went BLURRY.

All in all, it was a very successful day, but I had some revision work to get going. I needed a pneumatic front end to match the rear, and that would require completely rebuilding the front axle and steering.

Back in the shop after the testing:

Upon completion of the testing, Bill had reached the conclusion that my car was looking way more fun than his dance with disaster known as driving the Flyer. He said thought it'd be cool if we (read: I) build a second one for him. Bill is very active in another hobby of mine (slot car racing) and has done an enormous amount to support it locally. And he's just a good guy. So I said "Sure!" And the seed for the Scurderia Kouba Mark II was planted...
Having seen pics of some 'hills' you know of I am a bit concerned where the 'need for speed' of this thread might lead too!

Chris Kouba

Chapter Three: A Redesign for the Mk I and the Birth of the Mk II

After experiencing the astronomical NVH levels using the wheelchair wheels, I went back to the internet to ask it a question- what should I use to replace those pesky flexy wheels and solid tires. The answer it pointed me to was BMX wheels, but not just any BMX wheels. It appeared that the best option is a 20" wheel with a 14mm axle and as many spokes as possible. After additional exhaustive research, I was able to find 20" wheels with 48 spokes using 14mm axles.

So I bought four of them.

Why 4? Two for the Mk I and two for the Mk II. Bill was officially in for a build of his own. He picked up some tubing and the following week we got started- in the identical fashion as I did for myself: we set him up on the floor with another few pieces of tape and went about marking out a frame for him.

Framing up Bill on the floor:

I even put him to work chopping his own steel:

By the end of the day, we had something remotely resembling a new chassis, and Bill took the liberty of making zoom-zoom noises before heading home for the night.


At the other end of the shop, I was busy revising my front end. I wanted fully-functional steering geometry which required a completely new axle. I removed the old one thanks to its U-bolt installation and started building up what I thought would be a winning plan.

It wasn't. The concept was good but somewhere in the "measure twice, cut once" execution, some angles got goofed up and I ended up building in early Bugatti camber angles with no recourse to correct them. Back to the drawing board for a simpler-to-build front upright. I ended up with a single piece of square tube with two heim joints through welded in nuts for the main upright, and then adding on a stub to pass the axle through to secure it to the upright. Looks a bit like this:

I added on the arm for steering and gave it a shot. As the steering went to the extreme of either lock, the chassis lifted up- which means it's doing what I designed into it for self-centering forces. I was pretty proud- it worked like it should and has lots of adjustability built into the design as well.

Yeah, I made some seat cushions too:

As we chopped and burned more tubes for Bill's frame, I figured out how to integrate the original front end from the Mk I including adapting it to the new BMX wheels (24" to 20", plus the larger size axle). After another work session or two, we had both cars heavy on their wheels. Despite it being a dark winter's night and having no lights or reflectors on them, we dragged them out of the shop for a roll down the hill in front of the house.

Bill's maiden voyage:

Lined up side by side at a more responsible time to try our new rides:

And we had our first race:

The verdict- resounding success on all fronts. The incredibly harsh ride and vibration was gone and the steering geometry seemed to work and the car handled well. All the effort for the redesign was worth it.

Now we have to make them look like something...
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Chris Kouba

Now note the difference in noise using the pneumatic front end. This is the hill behind the house. It's fairy flat til the end where it dumps you out onto a main road- which is why I was yelling to Bill to brake. We rolled along in the mid-20's, and then quickly topped out at 33 on the little drop (we don't get rolling til about 50s into the video):

Google Photos
Who's car is the blue one? It looks like a conventional soap box derby car. I still have my official wheels, axles & steering wheel from the one I built about 1964. Now add a small block Chevy & you might have something LOL

Chris Kouba

The blue one is the official entry from last year, built in the same style and manner as the kids' soapbox derby cars. It was built by Dennis, who raced his own in the 50's, crewed by another friend who had raced on in the 60's, and pushed (yeah, we get to cheat at the start) by another who raced in the 70's.

The scary part of it is the car weighs in ~240 pounds, as does Dennis who drove it. I shudder thinking of the loads going into something designed for a 80(?) pound kid in a 50(?) pound car...

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Chris Kouba

Chapter 4: Moving on to body work

I wanted desperately to learn metalworking through this project, but after piling up some heaps of trial and error (with emphasis on "error"), I accepted the inevitability of going with glass and resin. Once this decision was made, it was on with the show.

Copying a trick I saw on one of the "Pimp My Ride" shows where someone made a trick speaker box with fiberglass and effectively some spandex, I found a stretchy sheet of fabric and stretched it over the chassis like the canvas coverings of old planes. Once stretched, I applied a coating of fiberglass resin and let it cure. This gave me a nice organic shape to work with for the top of the chassis.

Flannel sheet being put to good use:


End result:

This was a good start, but as anyone who's ever made something from scratch knows, you don't normally get everything done correctly the first time ever. So a version 2.0 was needed, this time just the chassis side- the nose cone will be a separate piece.

Chassis Bodywork 2.0:

I think it came out exceptionally well!

The metalworking wasn't a total loss. I was able to knock out some flatter panels with reasonable grace- I even wheeled out the top of the rear fender to a reasonably fender-ish shape. It all made it start to look like a vehicle.

Not great, but not bad either:

More to come shortly...

Chris Kouba

Chapter 5: Finishing the Bodywork and Putting the "Pretty" on!

Spilled some red paint on the white

Made the nose section

Paneled up the sides and installed a belly pan

Splurged on a quick release hub and used some old bike grips for a tiller

Just about "complete"

The official roll out in full Scuderia Kouba livery with my old man behind the tiller

In motion: Shared album - Chris Kouba - Google Photos

Just running it down that little hill in front of the house was a hoot. Had neighbor kids in it, neighbor "kid-dults" in it, the old man, me... Good times!

We've got a test day scheduled for Thursday, hoping for some good weather! Looking forward to seeing what the bodywork does for speed, and we've picked up some lead weight for ballasting as well. Going to have some FUN!

We're also officially entered in the Portland Adult Soapbox Derby for 2017. I will be car #2. It's Sat, Aug 19 if you're interested in checking it out. It's a bit ridiculous:

Free to all.

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