Gravity Racer, part IV

Chris Kouba

Supporter
The Portland Adult Soapbox Derby returns to Mt Tabor on the third Sat in August this year (8/20), and I think I am signing up for the madness once again. Rumor has it the organizers are trying to get the current record holders to come run again this year for a true King of the Mountain battle. Time to find my A game.

In usual fashion, there have been some rule changes based on our participation. I'm not sure whether to be flattered or insulted... One of which is any given "team" can only enter two cars, which seals the record of the Scuderia being the first and only team to ever sweep the podium. Another new rule? Despite a very explicit rule about having legible numbers on your entry, you can't paint your cars in the same livery. Great.

The best new rule though is that our brakes, helmet, and all other safety gear is no longer included in the budget cap. This means I stand a chance of actually stopping in the braking zone. This is extremely motivating for me, so once again, I think a new build is in order.

To that end, the build has started:


So, "the build" doesn't look like much at this point, but I believe this hub will permit me to have a 17mm axle spinning a 16" tire/wheel combo wearing an actual disk brake. The wheels will be shorter and hopefully cause less aero drag while still being able to roll smoothly over any pavement roughness.

I am still debating layout and set up, but will likely be some version of the 2F/1R trike style. Goal is minimal cross section and drag with the ultimate goal of breaking the course record. We were at 71.9s last year and the record is 79.1s. Getting that last 0.8s will be quite a challenge.

This thread will likely develop slowly due to the other projects at the homestead. It is going to happen though, and I am looking forward to seeing where it goes.

If you're new to my involvement in this event, this is how the last ones went together and ran down the hill:
2017
2018
2019
2020 - cancelled
2021 - cancelled
 
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Bill Kearley

Supporter
I started and ran and the soap box races in Kitimat BC for a few years. For the second year I bought 20 sets of wheels from Akron, the real thing, then gave them to any one that wanted to enter.
 

Randy V

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I may have a nice set of wheel chair wheels in Texas, but it will be a couple months before I get home...
 

Chris Kouba

Supporter
I may have a nice set of wheel chair wheels in Texas...
Hard pass, but thanks. The first build started with a set of wheelchair wheels- the solid rubber tires- and they were horrible. MASSIVE amounts of flex and the NVH was enormous. Making the switch to pneumatics was a no-brainer.

These will be proper wheels and tires with proper braking capabilities (for the first time!). Not that they are proper soapbox wheels, but they'll do well for this event.
 
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Randy V

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Hard pass, but thanks. The first build started with a set of wheelchair wheels- the solid rubber tires- and they were horrible. MASSIVE amounts of flex and the NVH was enormous. Making the switch to pneumatics was a no-brainer.

These will be proper wheels and tires with proper braking capabilities (for the first time!). Not that they are proper soapbox wheels, but they'll do well for this event.
These are apparently pneumatic.. I surely understand your wanting to get away from hard rubber and weak hubs... I will PM pics of wheel/hub...
 

Chris Kouba

Supporter
OK, massive left turn. I was getting skunked on spoke calculations for the new wheels and getting frustrated. So I did a bunch of thinking, dug out the go kart hardware which I picked up cheap in 2019, and whipped up a quick chassis, achieving some pretty encouraging results.

The wheels spin remarkably freely and while it doesn't roll as limitlessly as the 20" BMX tires do, I think it's good enough to get us into the speed ranges where less aero drag (much smaller frontal area) will win out over the increased mechanical drag (rolling resistance). These initial trials are certainly enough for me to proceed with the experiment.

The current chassis:


So for entering one car, I will be building two. I have a problem...
 

Markus

SPRF40
Lifetime Supporter
Hello Chris,
Every year I'm awaiting your new and more refined version....;)
I like that you are willing to leave old and worn out pathes to explore new once.
Maybe I can add one idea: do you think it would make sense add dimples, like in a golf ball, as they
create a tiny layer of air around the ball that significantly cuts down drag?

That logic should also work in the smaller tires!?
Rubber wear should not impact as you only make one down hill pass at one time?

Not sure how to put those dimples on, but with your abilities that should not be a problem.

Regards,
Markus
 

Randy V

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Rolling resistance...
Just out of curiosity - Have you ever heard of or thought about using Cambered Tires on the front?
Those tires have a substantial amount of meat on them, they could be shaved on a lathe with a belt sander. (pro-tip - Open window and have a big fan going)...
 

Neil

Supporter
Unless the course is a straight line, the 3-wheel topology may be disadvantageous. The end with one wheel has zero roll resistance, making its handling spooky. Of course, the "missing" wheel doesn't create drag or rolling resistance.
 

Chris Kouba

Supporter
Thanks guys.

I have thought about surface texturing but haven't put effort into it. The easier option to pursue there is application of vortex generators, and I haven't done much with that either. That may change this year, but let me get chassis with bodies on them first and I'll see what bandwidth I have left.

Instead of cambered tires, giving them some extra pressure has ballooned them up a bit so they are only rolling on a bulged out center strip. This is probably sufficient to reduce the resistance while still affording me the desired grip through the Learning Curve. I will see how it rolls and adjust from there.

I get all the 3 wheeler concerns for stability, but I can assure you that these cars are remarkably stable despite the long decreasing radius corner at the bottom of the course. The cars drift wonderfully through the corner and are very stable (as long as you don't hit any of the potholes- no suspension). I think they are the fastest way down the mountain, and the results seem to back that up- except for the year when I won with a 4 wheeler!
 

Ian Anderson

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How about inclined king pins?

Back in my karting days we did this so down the straights we ran on the inside edge and as you upturned the angles mean the tyre surface contachpts the ground.

Ian
 

Chris Kouba

Supporter
Based on the beam I already had built, I have what I would call "traditional" geometry, and seeing as it's already stuck to the chassis, I think I am sticking with it for now.

Neil- The 3 wheel configuration is plenty stable in this application:


Any instability is the result of running over bumps with no suspension at 40+ mph. We enter the big turn at just under 50.

Had a productive day yesterday. I completed the steering gear and put a floor into the gokart chassis. It rolls pretty well, although still needs some alignment work. It will be interesting to see how it compares with the other cars when we get them all out to test. It still doesn't roll as effortlessly as the bike wheels, but I think it's worth pursuing.

Testing under cover of darkness:




Progress with the bike wheel build is also being made. I did something last week which I've never done before- built bike wheels from components. I wanted a smaller wheel and no one makes disk brake wheels in any appropriate sizes for this, so I found compatible hubs and rims, figured out the spoke size with the help of my teammate, ordered custom spokes, and then spent a whole bunch of time on youtube university figuring out the magic. In the end, it was pretty magical.

Magic x 3 plus a spare:


Still waiting on tires (ordered) but have brakes in hand and will probably start working on some spindles this week. Based on the way the wheels spin on the stub axles, I have high hopes for this configuration too. It will be very exciting to see how they compare.

Chris
 

Ron Earp

Admin
That's a really neat project. Making wheels is probably something not many people do any longer but seems like it'd be basic technology.
 

Chris Kouba

Supporter
For what I am now calling the 10" chassis (go kart wheels), I fabbed up a master cylinder mount and brake pedal and started to bleed the brakes, but it didn't go well. The seals don't seem to be doing much sealing.... There are no P/N's on the components, so I took some pics and will be working with the shop where I got them to see if replacement seals are available. I have a horrible suspicion that they aren't, so my fingers are definitely crossed.

While investigating the calipers, I noticed something I hadn't previously. It's a floating caliper setup, with the caliper sprung to push the far side pad off the rotor, and the piston side pad sprung to retract the piston once pedal pressure is removed. It's pretty ingenious, I just hope I can get it to work!

I also spent some quality time truing the 16" spoked wheels this week and they are pretty decent at this point. Not perfect, but good enough to start with. I believe they will settle and creep a bit as well, so I accept that I will probably need to true them a handful of times- especially after getting a few miles on them. I have a set of import ATV calipers to run the front wheels. They should get the job done well enough for the stopping, but they don't have the fancy spring retract features. It will be interesting to see what difference that might make.

I assume the spoke built wheels is a niche thing- clearly people are doing it, but certainly not mainstream. I've been biking for years and am pretty mechanical, but only now have tried, albeit only because what I want is simply not available in the normal world.
 
^ wheel building in cycling started to disappear when Mavic intro'd the Ksyrium wheel set around 1990. Prior to that, all wheels were built up from components, so all bike shops had a somewhat competent wheel builder, and many serious cyclists built wheels. Now, most stuff is prepackaged. I'm sure there are a ton of used hubs sitting around in the world, not being used.
 

Chris Kouba

Supporter
Thanks Jac Mac. That red line is what I was shooting for but...

I've got that lower heim about as close to the wheel/tire as I trust it to be, given than I expect there will be some flex in the system- specifically from the wheel. The severe kingpin inclination points it just about right at the contact patch, and I have tilted the pivot axis backward to induce some caster trail in addition to some mechanical trail built into the uprights. With the brake disk as far away from the wheel center line as it is, my options to optimize the geometry are a bit constrained.

I've used all the concepts which have worked out with the prior racers, but gone through bigger hurdles to try to have everything come together on this one.
 

Chris Kouba

Supporter
It's been a productive few days. I have most of the 16" chassis sitting on the floor:



Yes, it is suspended.

I wasn't brave enough to try independent fronts so it has a live axle and single-sided swing arm in the back. Rotors are on all 3 wheels so stopping power should be leagues ahead of the prior versions. I have ordered some heims for the steering links which should be delivered this week and I will sort out getting some springs to hold the chassis off the ground as well. I think I have a concept lined out for the steering mechanics and for mounting the calipers too. If that's the case, I could possibly have a functional chassis in the next week or so.

In other news, I started to fill the master cylinder and bleed the brakes on the 10" chassis, but both the M/C and the calipers were leaking like sieves. The truly bad news is the components are old enough that there don't appear to be spares available for them. I am presently trying to figure out what to do about this. I would like to see what the chassis can do- you know, since I built it and all... but I don't want to spend an arm and a leg trying to find the parts. I will sit on this for a while and see what happens as I get the 16" chassis closer to functional.

Once the chassis is sorted, it'll be time to build the body. I am going fully enclosed this year, with canopy (I think). I am still developing the shape in my head, but shooting for the best guess at the lowest drag body I can fit over myself in the chassis.
 
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