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:
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:
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:
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:
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:
Pillow blocks:
Fork assembled:
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:
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:
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:
What it looks like with 200# of ballast aboard (it's...
cozy):
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