Blog Post Number 59 Written: 01-18-2023 Uploaded: 05-26-2023
I realise now, my design was always to grandiose, not just the 6×6 I started with or the 3×3 I went down to after that proved to be far too heavy. But they all had fancy long travel independent suspension and high amp draw motors that by extension required large battery banks to have run times of roughly 8.6 minutes (if running at max power the whole time.) And all of that just added too much weight.
I am going to draw up a much more traditional 4 wheel, box frame and use 1 inch square tube, supported by ¾ inch instead of 1.25 supported by one inch. And two motors for just the rear wheels instead of a motor for every wheel, and forgo the long fancy scissoring long travel IFS I devised, because the length of those A – Arms was adding seven or eight feet worth of tube and each. 8 feet of tube weights 6.5 pounds, plus you add a coil over shock and now yours at 10 pounds for half the front suspension. That would be half my weight limit just for front A arms and shocks. No motors, gears, chains, batteries, breaks, or even wheels and tires. I have a much simpler design, that will be much lighter, and sure it will limit me to an inch of travel not 6-8 inches, but to be honest, I would probably never be able to find or afford shocks of the length, and travel distance I needed, that were also light enough duty for a 300 pound loaded weight vehicle. Instead of 3000 pounds.
So in this case, the moral of the story is, lower my expectations, lower my standards, in order to lower weight and cost. Because I am still stuck to the 40/50 pound unloaded weight, 48x24x24 inch form factor, and sub $1000 cost.
Instead of investing in 6x 3000 Watt 72 volt 50 amp dc motors and all the batteries that it wuold take to power them (like $250 per motor) I am now looking at 2 24v 350 watt 20 amp motors. And if those two aren’t enough, perhaps I can go to bigger motors in the future.
That is my biggest limiting factor. I am not used to working at these smaller scales with smaller horsepower and torque numbers. I am not sure how much power is enough. And then I have to factor in gear ratios ets. I.E. that 350 watt motor comes with an 11 tooth sprocket. If I put a standard 60 tooth sprocket on my drive shaft, and then have 12 inch tires that 5.45 gear ratio at the motors max 3k RPM on 12 inch tires means ill only get about 6.5 mph. Right? Idk I didn’t double check the math, but I’m not sure how much power, with what gear ratio combination will be the appropriate combination of speed and torque.
If I converted the numbers right these $35 350 Watt motors should make .39 HP and .41 Ft Lbs each. 0.8 horse power and 0.8 foot pounds doesn’t sound like near enough power, but at the same time, I have seen adults drive barbie jeeps power wheels around, and those have less than half that. So idfk. I am torn between picking a number and building the thing, and if its not enough, iterating on the design, or figuring all the snot out I need to on paper first and building one thing that I know will be adequate that will save me the time and expense, (and clutter of outmoded parts) associated with iterating on the design. If the two 24V motors aren’t enough and I go to a pair of 48V 500 watt motors that’s going to double my weight and cost in batteries alone.
I’m not sure, I’m tempted to ask some of the engineers I work with about gear ratios and such, perhaps building a tiny multi speed transmission. I do have access to 3d printers and cnc machines so I could, but that’s more time, complexity, cost and weight and me potentially looking like a fool, or immature to my superiors at work. For now, I struggle with these problems, alone, in the dark. Or at least until someone reads this…
P.S. I have my own CNC machine now. though it is small, it’s enough for aluminum.
Chace Randolph 