Small PRS Car: The Search for Wheels

Good wheels are really expensive. I mean, they probably aren’t to those who are better versed in go-karts and other small vehicles, but I’m cheap. My novice showed when I was oh-so-ready to put my faith in the good ol’ Harbor Freight Special. I posted some questions on the PPPRS (not really sure what the two extra P’s are for) Facebook group and got some advice from team WaffleKart, seconded by the MIT brigade, to upgrade the Harbor Freights with better tires and bit of welding. I’ll keep trolling Craigslist, but as of right now I’m going to go with WaffleKart’s aptly described Franken-wheel method.

I’ve developed quite the bill of materials now, and instead of posting a new version every week, I’ll just put the Google Sheets link right here for those interested and talk about any changes or additions in my updates. Same goes for the constraints and criteria. Right now the BOM is just basic materials and components that make the car go. Also there’s some sensors for A+PRS.

Blog posts are super boring without pictures, so here’s a Scion xB that’s dressed up like a toaster. If my head, torso and arms didn’t have to stick out of the roof of my small car, I would definitely pursue something like this.

Someday these PRS posts will be about tangible progress instead of lots of brainstorming.

PRS Begins

This marks the start of my endeavor to trick out a children’s toy to go fast, drive by itself, and maybe even become self-aware. I have begun to spec out a BOM for motors, speed controllers, steering and drive hardware, and other nonsense that goes into building a tiny car. More information about the competition is at the end of the table.

I’ve had a lot of ideas about how to make this car be human and autonomously controlled without removing or adding any parts, and the goal is to just flip a switch to go from human to autonomous control. I can’t help but fantasize about the style of the whole thing. Should it look like a sports car from the future? Should it be a movie or TV reference? Should it look like a giant toaster? The possibilities are endless.

This is my annoyingly misformatted list of initial constraints and criteria. As you can see, most of them are just rules from the competition, but you have to start somewhere. I’m sure more will come to mind as I continue designing.

Constraints and Criteria
Req. Num. Criteria Rationale Treshold Target Stretch
R1 Car dimensions shall be 62″ x 36″ x 72″ while containing driver [1.4] Stated in the rules Dimensions are exceeded Power Wheels Dimensions N/A
R2 Wheels shall have pneumatic tires [1.5] Stated in the rules Non-pneumatic tires Air-filled tires N/A
R3 Wheel base (axel-to-axel length) shall be at least 18″ [1.5] Stated in the rules < 18″ Power Wheels Dimension N/A
R4 Wheel track (wheel-to-wheel width) shall be at least 12″ [1.5] Stated in the rules < 12″ Power Wheels Dimension N/A
R5 Motor(s) shall be driven with at most 48V [1.7] Stated in the rules 12V 24V 36V
R6 Batteries shall not be LiPO/Li-Ion chemistry [1.8] Stated in the rules No batteries LiFePO N/A
R7 There must exist a throttle-controlled motor controller to transfer power from the batteries to the motor(s) [1.9] Stated in the rules No controller ESC Microcontroller with ESC
R8 A unique number must be visible on the side of the car [1.11] Stated in the rules No number Any Available Number 77 or 0x4D
R9 A 6″ x 1.75″ label shall be present for the Moxie Board [1.12] Stated in the rules No moxie tag Generic text Nice Photoshop logo
R10 Front and rear bumpers shall cover 75% of the width of the car [1.13.1] Stated in the rules No bumpers 75% coverage 100% coverage
R11 Front and rear bumpers shall cover at least 1″ of vertical space 4-6″ from the ground [1.13.1] Stated in the rules No bumpers Aluminum tube bumper Aluminum curved bar
R12 Bumpers shall be covered in an impact absorbing material [1.13.1] Stated in the rules No coating Pool noodle Rubber coating
R13 Bumpers shall not protect wheels of the car from side contact [1.13.1] Stated in the rules Wheels not exposed Wheels exposed N/A
R14 The car shall extend beyond the driver in every direction when viewed from directly above [1.13.2] Stated in the rules Any open sides Enclosed cockpit N/A
R15 The car shall provide front and side protection for the driver’s feet [1.13.3] Stated in the rules No protection Front and side protection N/A
R16 The car shall have mechanical disk brakes capable of a full stop [1.13.4] Stated in the rules No brakes Mechanical disk brakes Mechanical disk and motor braking
R17 Batteries shall be secured so that they remain in position in the event of a tip or roll-over [1.13.4] Stated in the rules Batteries not secured Velcro straps Metal tie-downs
R18 The car shall impliment a kill switch that stops all current flow from batteries to the motors [1.13.6] Stated in the rules No kill switch Kill switch N/A
R19 The kill switch shall be accessible from outside the car [1.13.6] Stated in the rules No accessible kill switch Accessible kill switch Easily removable and accessible kill switch
R20 The kill switch shall be easily identifiable as such [1.13.6] Stated in the rules Unobvious kill switch Red kill switch N/A
R21 The car shall implement a fuse with a value such that the maximum possible power output is 1440W [1.13.7] Stated in the rules No fuse Correct fuse size N/A
R22 The car shall implement front and rear LED lights [1.13.1(2)] (Optional) Stated in the rules. For illuminated driving at night No lights LED head and tail lights Functioning head, tail, reverse, and indicator lights
R23 The car shall be able to drive in reverse [1.13.2(2)] (Optional) Stated in the rules. For backing out of crashes Only forward drive Forward and reverse drive N/A
R24 The car shall implement a horn [1.13.3(2)] (Optional) Stated in the rules. For fun No horn Generic horn Selectable horn
R25 The cost of the car (excluding safety systems and power wheels toy) shall not exceed $500 (batteries are counted for 50% fair market value) [1.14] Stated in the rules > $500 chassis $500 chassis < $400 chassis
R26 The car shall reach a top speed of 20mph Competitive speed < 20mph 20mph 25mph
R27 The car shall play music Fun No music Music loud enough for driver Music loud enough for spectators


Here’s where you can find out more about Power Racing Series(PRS) and also the autonomous portion: A+PRS, which, to the best of my knowledge, is run by SparkFun and only takes place in Boulder, Colorado.

Can’t wait to get this one underway, stay posted for more brainstorming and hopefully tangible progress.