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Learning how to build Lotus Seven replicas...together!
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 Post subject: Gravity (Soapbox) Racer
PostPosted: September 26, 2016, 7:16 am 
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I had a friend compete in a soapbox derby event for grown ups this year. All I needed was to hear about it and I was in!

He had built a traditional 4-wheeled entry, complete with the actual official soapbox derby wheels, axles and steering kit. For mine, I knew I was going to have only 3 wheels. And they would be much larger. I had seen others use bike wheels, but there are corners on the course which are taken at speed (~40mph). I have seen entries have their wheels turn taco upon laterally loading. A road bike wheel with a 120psi tire on it would be best if the course was only straight, but the empirical data would suggest that 20" wheels are about as big as you can go without risking a collapsed wheel at the cornering speeds attained during the event.

In thinking about alternative options, wheelchair wheels came to my attention. Talking it over with my friend Bill, he also arrived at the same conclusion. Upon scouring the internet, Bill located a $20 wheelchair (He's retired and has plenty of time to fulfill the role of chief procurement officer). Even if the wheels and tires were square, it'd be worth it for the development. Bill was off like a rocket to pick it up before it got scooped. The next day it was at his house. The wheels were round and in decent enough shape... We were in business!

The rear wheel is a gamble at this point. I confiscated a front wheel off of one of my mountain bikes. It is larger than the recommended 20" size (it's 26"), but it's also a good mountain bike wheel, and I know the abuse that they are capable of absorbing- I've dealt plenty of it! I am going to give it a try and see how it goes. I'm not too proud to figure out something else if necessary, but I think it'll be just fine. It has a 20mm through axle and has been easy enough to adapt to the frame.

For brakes (required by the race regs- and by common sense!), the mtn bike wheel has a rotor mounted on it. I intend to fab a caliper mount on the rear fork and just use the brake straight from the bike. I presume I can make that work well enough, but have yet to actually cross that bridge.

So with a few basic build concepts defined, the design brief became easy. I had a very strong mental image of what I wanted the final product to look like:

  • As narrow as possible.
  • As low as possible.
  • Will fit in my long bed pick up (less than 8' in length, less than 4' side-to-side).
  • Detachable nose cone for longer overall length and possible crush structure if I miscalculate something...
  • Steel space frame with alloy body.
  • Ackerman steering
  • Decent brakes
  • Heavy.

Bill came over last Friday and brought the prized wheelchair. We swept off the garage floor, got out the tape measure, took the wheels off the chair, and started taping out the general shape on the floor. This is what it looked like:

Image

The tape shows the basic outline for the chassis with a centerline and widths along the way at key distances. The front wheels are ~24" in dia, so we planned the front axle to be ~13" behind the front bulkhead of the chassis. Doing the opposite in the rear, the 26" o/a dia tire put the rear axle about 15" from the end of the chassis and the rear "bulkhead"/roll hoop about 30" from the end of the chassis. All the space in between the two bulkheads will be for me! I wouldn't get to start that day as Bill had to leave early and I still had to pick up the steel- which I did later that afternoon.

Sat morning, my schedule was clear. I started with a pile of tubing. These were full size sticks of thick wall 1x1 and 1/2x1/2 which were just slightly bent so they were on the rems rack. SCORE!!! I was out the door for $62. That's a good looking pile:

Image


My first objective was making a functional front axle. I would keep it simple- tube connecting two uprights which carried the 1/2" bolt which held the wheel. I would build in a bit of mechanical trail and caster (if I am using those labels correctly?) and hopefully have it handle respectably. The uprights are simple affairs which I machined from extruded aluminum stock. They are carried inside square box tube with one side removed to open it up and butt welded to the end of another tube connecting it to the other side. A close up of the right side, support tube only tacked in place:

Image

I fabbed a tie rod to hold the front end together for the time being and moved on to the rest of the chassis. This is what it looked like:

Image


Next I needed to start on the rear fork- for which I needed the straightest pieces from my occasionally slightly kinked sticks. In seeking the straightest ones, I found two matching lengths which had identical, gentle curves. These were perfect for my main chassis rails, and that is what they became. I found a bit of straight and square tube and built a rear fork which had the proper spacing for the wheel/rotor/axle assembly. After getting a start on this, I figured out how to mount up the rear wheel- using machined alu pillow blocks. I slotted the forks so I could adjust the alignment of the rear wheel (and overall wheelbase as well) and then squared it up to the best of my ability and tacked it together. Once everything was in the right place, I burned it all together.

The fork:
Image

Pillow blocks:
Image

Fork assembled:
Image

From here, all I had to do was connect the front axle to the rear fork I and was in business. I dug out the gently curved pieces I had found earlier and placed them on the "build table" (garage floor). I broke down the fork and butted it up to the rear of the chassis tubes. I picked a dimension for the front bulkhead width and cut another piece to close the whole thing up, and then I tacked it all together. Part way through the process, it looked like this:

Image

Once the main chassis layer was closed in, I welded two pieces of plate steel to the main rails and attached the front axle with u-bolts. Once this cooled, I attached the axle and I had a roller!!!

Rolling:
Image

At this point I called it a day and went into the house for some quality time with the wifey. I took another run at it on Sun but had only a couple hours to give. I managed to get the lower chassis framed up and attached to the main rails.

Sunday's end result:
Image


What it looks like with 200# of ballast aboard (it's... cozy):
Image


Steering is next on the prio list, then the brakes. I also need to close in the floor and lower sides before I really think about trying it on a hill- and we're trying to make that happen this week. This is nowhere near the level of sophistication of anything else on this board but it's pretty gratifying to see your design concept come together through your own actions and "skills".

More updates as time and progress allows (and if anyone's interested).

Chris


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PostPosted: September 26, 2016, 8:10 am 
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Location: Austin Texas
Cool project. I think you will find tires pretty easy to find for the front because I think BMX bikes for little kids use the same diameter wheels (It's odd but the bigger kids ride ride on 20" wheels, with the younger kids on larger diameter wheels).

Have you calculated where the CG will be? With corners on the course will it be easy enough to get the CG far enough forward?

If you haven't already run across this, it looks useful http://americansolarchallenge.org/ASC/w ... 060811.pdf

Dean


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PostPosted: September 26, 2016, 8:57 am 
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Hadn't seen it but will take a look. The CG should be somewhere around mid-chassis but I will have the option to weight it up front (it looks like 66/33 F/R is recommended from the paper?).

Chris


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PostPosted: September 26, 2016, 9:19 am 
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Cool, that should be lots of fun. I've never built one myself, but when I was a kid, one of my cousins was in a soapbox derby. I still remember him showing me the car that they had built when we visited one summer, and I thought it was the coolest thing ever. If I ever find something local, I'd like to do one too.
Kristian

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PostPosted: September 26, 2016, 12:02 pm 
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A tip I heard somewhere: ditch the pneumatic tire and substitute a large O.D. industrial "O" ring. You can get them in varying thicknesses and hardness and you match the hardness to the track surface and go faster.


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PostPosted: September 26, 2016, 12:31 pm 
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What a wild project. I've seen some videos of gravity racers on little things that look similar to the small ice sleds used in the winter Olympics. They really get going (50-60 MPH?). Yours looks to be much more sophisticated a concept. The roll bar and rear brake looks like a good idea! I'm not sure how those "sleds" I mentioned seeing in the videos stop, except at the bottom of the hill.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: September 26, 2016, 12:36 pm 
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That sounds like fun! For some reason, I keep thinking tilting 3-wheeler.

Maybe it's par for the course, but I do have to say that a rear-brake-only at least sounds potentially frightening/dangerous on a gravity powered (can't turn it off) downhill racer.

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PostPosted: September 26, 2016, 7:04 pm 
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Always Moore!
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Location: Pittsburgh, PA
Back in college we had a soapbox derby competition each spring between the different engineering departments. One of my friends made a 3 wheel car; he rolled in on the maiden voyage. Hopefully your luck is better! (orange Harbor Freight ratchet straps are decent seatbelts)

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PostPosted: September 26, 2016, 7:33 pm 
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This looks like so much fun.
Cheers,
Stewart.


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PostPosted: September 27, 2016, 9:32 am 
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Not to be a Debbie Downer here but I would seriously reconsider using wheelchair wheels under any kind of severe side loading. At least a spoked bicycle wheel can be re-laced with a radial pattern (straight from the hub to the rim with no crosses) to maximize the lateral capacity. If nothing else, it would be illuminating to do a simple static load/deflection on one of those wheels to see what you're up against. Clamp it by the axle horizontally in a vice, tie a known weight to the rim and measure the deflection. Compare to a spoked wheel of the same diameter. A decent spoked wheel will take a fair amount of side load (see video of BMX flatland or downhill mountain bike riding). Wheelchair wheels are designed to be inexpensive and low maintenance and pretty much NEVER see side loads. Now, I may be way out of line here not being familiar with what is commonly done on these but my 25+ years as a bicycle industry engineer is making my Spidy-Sense tingle. :shock:

Very cool project though! I'm looking forward to following the progress. :cheers:

EDIT: typo. *not*

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Last edited by Acerguy on September 28, 2016, 8:02 am, edited 1 time in total.

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PostPosted: September 27, 2016, 10:26 am 
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I agree with Keith. Also consider wheels from a trials bicycle. Those experience extreme side loading.

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PostPosted: September 27, 2016, 5:35 pm 
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Oh, it's maybe a bit late as you've got your steering already done but AutoSpeed has had some interesting articles on HPV trikes that might be useful. http://www.autospeed.com/cms/A_111099/a ... larArticle

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PostPosted: September 28, 2016, 7:16 am 
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Keith,

Your point is well taken, and yes, they may be a bit floppy. I think the trade off will be that they won't be able to generate the lateral grip and the resultant deflection, so that may mitigate the situation. I understand that using them is a compromise.

In addition to those concerns, while not generating the lateral grip, I may be scrubbing excessive energy simply trying to corner. I am not above dumping the whole front axle and starting over. It is designed modularly, with the front beam being held to the chassis with U bolts for easy removal. I've tried to make enough things of my own design already that I know this won't be the finished product- this is definitely a prototype chassis which I expect to have to iterate through a few design revisions.

Thanks for all of the comments and suggestions so far. If you have specific recommendations for wheels which are easily converted to a single-sided support, I'd be all ears!

Chris


Last edited by ckouba on September 28, 2016, 8:42 am, edited 1 time in total.

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PostPosted: September 28, 2016, 8:40 am 
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Well, again I'm not familiar with what is commonly done on these but it looks like you have a great start already. With the front end mounted on U-bolts you could actually play with the caster angle a little by rotating it. It's a super cool project.

Of note, unlike car tires/wheels, even if a tire says it is nominally a 24" tire, there are actually several different rim diameters that it can fall under. To be sure, check the sidewall of the tire and there should be what's call the ETRTO (or ISO) number. It should read something like 40-507 where 40 is the mm width and 507 is the bead seat diameter. The 507 is the diameter of what most North American 24" BMX wheels will be I believe. Something to be aware of if you go shopping for replacements.

EDIT: Now that I looks closer, are those solid tires on the front? If so, that's what you'll have to replace them with. I can't see any valve stems....

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PostPosted: October 3, 2016, 7:32 am 
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Acerguy wrote:
EDIT: Now that I looks closer, are those solid tires on the front? If so, that's what you'll have to replace them with. I can't see any valve stems....


Good eye Keith. You are correct, they are solid. Thanks for your pointers on selecting wheels/tires which match- I was unaware of the differences that all the 24" combinations can create. Knowing my luck, I wouldn't have matched them up correctly.

So how did they work? I can now confirm that the wheelchair wheels worked as anticipated. They do flex liberally under lateral loading but they do not catastrophically fail. They did provide an impressive amount of grip- enough to visibly flex them in a hairpin on our test hill. On the downside, the ride transmits EVERY minuscule nuance of the road surface and vibrates the vehicle immeasurably. I think whatever energy which may be potentially lost to pneumatic tire deformation will easily be pulled back from the lack of vibrating every nut and bolt (there might be 6 or 8 of them?) and the bodywork off the car. I couldn't see straight if I leaned back against the headrest.

Other than that tiny detail, everything else worked remarkably well. All my welds held together. It went side to side and stopped on command. It stayed on the road. It's also pretty fast, topping out at 43.3MPH on my fastest run. It genuinely handled reasonably well. It still needs some weight added- my friend Bill ran the racer which competed this summer (named the Hawksbill Flyer by its builder). It weighs 240# (plus ~210# of Bill). Mine probably checks in around 75#, plus 190# of me. When we started a run side by side, he simply walked away- to a point. Eventually I was able to hold the gap, but was never able to close it. Finished bodywork would likely help that as well...

Some pics of the day-

Loaded up and ready to head to the test site:
Image

Dennis, Bill, and Beau loading the Hawksbill Flyer:
Image

Prepping for my maiden voyage:
Image

After run #2:
Image

Making some adjustments to the Flyer:
Image

And to prove it moves, I offer... on-board video:

https://youtu.be/JayKtrFAacE

You can get a good feel for the ride quality(?) of solid tires watching this video.


I've got a number of other "real world" things to tend to now, but it was an excellent start to testing. As for what needs to be done next on the car- remove the paneling, triangulate the chassis, redesign the brake caliper mount, construct the front and rear bumpers, build the nose cone, whip up some paneling for the body, but the BIG thing (and probably step one!)- pick up some pneumatic tires and requisite wheels and get them onto the front end.

Chris


Last edited by ckouba on December 19, 2016, 9:02 am, edited 1 time in total.

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