Gravity Racer, take III

Yes, I am committing to another build. It is an addiction...

Let me start by saying that I was extremely happy with how the Mk V worked out this past year and I am not sure that anything else I would build will be able to out run it. It will be difficult to put aside a car that is running within a sneeze of the track record which was set on a MUCH more favorable (HOTTER) day. Despite this, I think I need to try. This is not the radical departure I was contemplating last year, but a fairly conventional trike layout, with one front wheel and two rear wheels. Why you ask? Read on.

At SEMA this fall, I chatted with someone who runs at Bonneville with a sidecar motorcycle and we had a conversation I just couldn't get out of my head. He asked me why I did a couple of things a certain way and then told me what he did and why and what the results were from their wind tunnel testing. Sometime during that ~10 minute discussion, I realized I was going to be building a new car for this year.

I dug up the go-kart front end and acquired an old junked bike to put together a prototype chassis, adding in the front wheel of a pocket bike. This minimizes the height of the running gear underneath the bodywork. I think there's a bit more rolling resistance with smaller, wider, and stickier the kart tires, but I am hoping the smaller size turns into an aero advantage once at speed (>20mph or so). In switching to a single steered front wheel, it also simplifies/reduces the components out in the airstream as well.

My main concern is stability through the Learning Curve. I was pulling almost 1g laterally through there last year and I really don't want to have it flop over on me. The track and overall width will be similar, although narrower, but I am also thinking I will be able to keep it even lower overall than the Mk V as well. Dynamically, I don't think the corner cares if I have one wheel in the lead or in the trail position, as long as the height of the CG is low enough to keep some weight on the inside wheel. I would be curious if anyone has any evidence to the contrary. There was a lot of discussion when the Delta wing came out. I am not afraid to give the concept my own shot.

So, without further adoo, I introduce the prototype for the Scuderia Kouba Mk VII:

It doesn't look like much right now but it will come around. The handlebars are on there just as placeholders, it will not be head first. I have the steering planned out regarding how to get it steered while laying down with my head at the stern. At this point the proof of concept indicates that it can be done, so I will be giving it a shot.

I have a new frame started which should be much more square and flat, and it will carry a wider track rear axle- it will be the go-kart front end with fixed tie rods, so that I have independently rotating rear wheels. I still need to figure out a few dimensions for it but it's definitely on its way. Watch this space for further updates.
People are constantly (continuously?) re-inventing the three-wheeled vehicle. It has a fundamental problem- the end with one wheel has zero roll stiffness and, accordingly, the thing's handling is atrocious, Morgans included.
Roll protection will absolutely be incorporated. It may not be SCCA spec but I don't want to scratch my skid lid.

I admit, I am skeptical regarding its cornering ability. The single thing making me go forward with the project is the rest of the fleet I have built is 3-wheeled and corners just fine. I am well aware of the Robin film and its roll-over talents. Looking at its layout, it's quite obvious as to why it turns turtle at the slightest hint of a corner.

Looking at the Morgan, the motor is down between the wheels and the cockpit is both narrower and lower than the Robin, making it respond differently to directional changes. Looking at the Mog, would it be any more or less stable if its driving direction was turned around? Physics would still apply the same force to its CG, introducing the same moment about its roll center, right? I've seen the Hamster flog one around pretty liberally (I think he has a thing for them) and it seemed to be under control.

Also, the Mk V is 22" tall at its tallest point. It was that tall because they are 20" tires and there's ~1.5" of ground clearance. The Mk VII has 10" tall tires, so it will only be as tall as I need to see over my own feet. The rear end I will be using is a bit wider than the one shown in the pic above- maybe not quite twice as wide, but close. I think the geometry can be made to work.

So yeah, there's no lateral weight transfer on the 1 end but there is massive roll stiffness on the 2 end. Based on the mechanics working out very well on the other three entries which have been down the hill successfully (albeit in reverse), I am willing to give it a try.

I am also willing to admit that I may be wrong, but I'll never know if I don't try.

Doug S.

The protoplasm may be 70, but the spirit is 32!
Lifetime Supporter
Glad to hear you're going to do this again, Chris! Last year's race was a great "write-up". I'll be anxious to see what comes of your creativity and driving this coming season.

Cheers!...and good luck!

Thanks Doug!

Worked on the new car a bit this morning, getting the rear axle ready to attach to the new frame. Have it all laid out and aligned on the shop floor, just need to commit to a method of attachment. The rear axle is quite a bit wider than the live axle one from the prototype:

Frame members are just tacked together and the front fork is aligned and tacked in with two little scraps to hold it while I lock it in. Rear axle is just placed over the aft end of the frame. I need to figure out where the top rails of the chassis will be positioned and then start tacking them in as well, and then determine the wheelbase and weld in the rear axle.

From there I will do the steering system build out and then finish the brakes. After that it'll be time to make a body, so yeah, it's like I'm almost done already.
Rear steer is unattractive to me, as the primary method of directing the front end into a corner is to direct the rear end out. That feels like a recipe for disaster. I was privy to discussions about rear steer in a Formula SAE car where they tried to incorporate it but they found it a bit unstable in cornering. That's enough for me to walk away for a vehicle that will be used once a year and needs to be built for less than $500.

I've been thinking a bit about leaning but haven't quite gotten a manufacturable solution.

In the 80's, GM (I think) came out with what they called the Lean Machine. I was thinking about that as I started this prototype but have no quick and dirty way to make it happen. Maybe it's something to think about for next year, as long as the 1F2R concept plays out.
Rear steer is not a good choice. I asked Andy Green how he ever became comfortable with the rear steering of Thrust SSC and he replied "I never did really".
Time for an update...

I finally got a chassis functional enough to roll down a hill (no brakes mind you, but steering):

To see what it steers like, you can check out this quick video.

With sufficient lock and a tight enough turning circle, I was pretty comfortable it'd be worth developing into this year's entry.

Until I tried going down a hill.

We have a very modest hill in front of the house, maybe 20' of vertical and a very mellow gradient. It goes down to a saddle and then rises about another 8' to a slightly rising turn around a 90° corner. It is one of our standard testing yardsticks, start at the top of that and see how far it will go. Decent setups will roll up at least to the corner, with the overachieving Mk V from last year making it half way across our lot after that! That was not to be the case with the Mk VII. There was so much rolling resistance and bearing friction, it failed to actually make it across the saddle at the bottom of the hill. And that is with NO brakes. It didn't seem like it'd ever get to a speed where the anticipated aero advantage of smaller wheels would overcome the extra rolling resistance.

After being colossally disappointed with this result, I reflected on what to do next. I have a fairly square and usable chassis and can think of a few tweaks I would make to last year's entry if I were to build one of that configuration again. I didn't feel like tossing the chassis as it was a good start, and I can re-purpose the go-kart hardware for another project. The pocket bike front wheel will need to find a home somewhere else.

So being as objective/challenge oriented as I am, I decided to try to be the guy who shows up and wins 3 years in a row with 3 different (and new) builds for each year.

My first year was just a challenge to enter and win. The second year was to try and win again with a 4 wheel car. This year, I haven't really acknowledged it out loud yet, but I think my objective is the course record. To get that, I'll need to knock 1.5s off my time from last year's fastest pass (72.6s vs 71.1s). That seems like a fair challenge.

There were two extra bike wheels around which were spares from other builds, so I ordered a third one and some extra tires and missing hardware, and started planning. And then hacking.

With a tear in my eye, I hacked off the bike fork. I was particularly proud of how I got the steering linkage to function. I was really hoping it'd roll well, or at least well enough to make me think it was worth pursuing. I think the diameters of the wheels coupled with the width of the rears just ate up too much energy. I knew it'd be a tough one when it wouldn't spontaneously roll across the shop floor but didn't imagine it wouldn't make the bottom of the hill.

With some clamps and duct tape, I was able to start piecing together the front end. Much more attention was paid to minimizing the scrub radius and to ease Jac Mac's pain, I de-cambered the wheels this year. You can see it taking shape in this shot:

After this, I started reconstructing the steering components, basing the whole thing on a slightly modified version of the set up used on the Mk V last year:

After a little time with the hot glue gun, I got to this stage:

I briefly thought about running it in this configuration, but there was really no point. It just didn't want to roll. And if I'm going to make the effort, I'm going to do it right. So it was off with the rear axle. Cutting to the chase, here's what it looks like today:

The $64,000 dollar question is- was it worth it? The answer: unequivocally!

With just a knuckle-dragger start (pushing the pavement by hand while seated in the car), I was able to make it down the hill, across the saddle, up to the corner and an encouraging distance along our yard. Yay! Twice. In the rain. Dragging my sweatshirt, and without bodywork/bellypan installed.

Even better, the steering geometry seems to work quite well. It doesn't have lot of self-centering, but it is very lightly weighted and very light on its feet. I still need to do more testing to see if I have front end drag minimized, but initially it's extremely encouraging. I really like it. It's got great potential. It's going to need some good brakes.

Randy V

Lifetime Supporter
I love this build....
Had you given any thought to a cable-pulley steering ala-SoapBox-Derby?
How much caster angle are you building in? Might have a bearing on steering centering..
I love this build....
Thanks Randy. They are a blast to build and run. They have solidified a team of people within my little world, and watching everyone come together to run them down the hill is incredibly rewarding. And so is winning!

Had you given any thought to a cable-pulley steering ala-SoapBox-Derby?
Based on our experience with this car, which is literally an oversize kiddie soapbox....'d need to place a gun to my head to see cable steering in entries I build. We chased steering system slack and vagueness all three years that car ran. No thank you!

How much caster angle are you building in? Might have a bearing on steering centering..
I'm sure it would, but the good Dr Jac Mac has planted the seed of zero camber in search of minimal energy loss and I am willing to give it a shot. Based on the performance of the Mk II, I know the front end doesn't need camber to make the Learning Curve at speed, so if there's a conservation of energy benefit to it, I am willing to try. I am confident enough I'll have the grip I need and the structure is stable.

Looking forward to more building and updates, because the real fun begins once the brakes are on it and it's capable of legit speed trials.

Stay tuned!

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Things are taking shape in the Scuderia's skunkworks. I have built up the frame and even committed to a canopy again:

I haven't finished all the running gear but have gotten to the point of starting the bodywork. It will be the same mould-less process I used for the Mk V:

Since I am budget-constrained, I will be solidifying the tape with paper mache or something, then fairing in the shape with joint compound again, and covering that with a layer of fiberglass. I've made some tweaks to the overall shape in hopes of reducing the drag. You can see much more attention has been paid to how I leave the air behind me than how I split it at the front. It's my personal rendition of a LH gravity car:


Randy V

Lifetime Supporter
Looking good!
I saw one of those Car Reality Shows that explained the process behind the construction of a big woofer surround. I found it fascinating that they use a fleece blanket stretched over the framework, then painted the fleece with epoxy resin. Once the resin cured, they finished it with a little bondo where needed. Nice and lightweight. Certainly not structural but maybe what you need?
We must have seen the same episode Randy. I am using a very similar process, except I will apply the body to the buck I create and then lift it off to remove the crap I stuffed underneath to shape it. It is the same process I used with the Mk V and worked out fantastically. Without a budget constraint, I would happily do it all in glass, but resin is expensive and I have every bit of budget already allocated.

After his rollover last year, I have been putting a new front end on Bill's car (the Mk II) as well. In just two work sessions, we have made MASSIVE progress- old front end removed, new components figured out and fabbed up. Car is back on its wheels and just needs a pair of tie rods and steering arms installed and it'll be back in business again. Some after shots of the Mk II:

Bodywork has begun!

Stretched cotton sheet over steel frame with stringers, paper mache adhesive applied to rigidify it. Once it's dried, some joint compound will be used to adjust the contours and then real fiberglass will be applied to the final shape. Once cured, the body will be removed, the internal buck dismantled, and the body reattached.

Quite pleased with the overall shape so far:

In addition, I'll be dropping of templates for the windscreen and side windows this morning to get them cut out. I have alu sheet for chassis sides as well... This thing is coming together, although the body will take me a bit of time.
First layer of joint compound on. It's a little thick in some places:

The windshield and side window pieces should be picked up today and I hope to get a second layer of compound on too. Fingers crossed I get it where it needs to be by 6/13 (2019 test day #1).

I am very pleased with how the shape is coming together. I will be working very hard to make sure it's symmetric and flowing. Looking forward to the final product!