Scale sizes for brass tubing

I’m currently collecting parts and materials for a small-scale functional prototype. I’ll be using 6″ diameter wheels up front and from that I’ve been scaling down dimensions from my design graphics to match that proportion. Presently, my small-scale prototype will be 22% the size of the real deal. That’s not quite quarter scale, and should be plenty big enough for me to build most components in about the same way they’d be built in full size. If I also scale my materials as much as possible, it’ll give at least some idea of structural rigidity (although not material rigidity). More importantly it’ll hopefully reveal the majority of mechanical complications. I’m going to use brass tubing because it’s inexpensive, readily available, and I can solder it together similarly to how I’d weld the real thing out of steel. I can also easily bend curves and other complex shapes without too much effort

So at 22% scale, my equivalent brass tubing sizes are as follows:

1″ steel tubing (the majority of the structure) = 7/32″ brass tubing [rounding to 1/4″]

1-3/4″ steel tubing (the heavy bits of the roll cate) = 3/8″ brass tubing

1/4″ steel plate = 1/16″ brass plate

These materials should be lightweight enough to be easily workable. Once it’s all soldered together, it ought to be pretty sturdy. I’ll be able to find small bearings, bushings, and other hardware at the local R/C car shop. Now that I have both a working band saw and a drill press, I should be able to fabricate pretty much anything at that scale. I wonder if I could even mill parts on my dremel press from solid brass if I take small bites. We’ll see.

4 thoughts on “Scale sizes for brass tubing”

  1. That’s really a great idea. Have you found a place to get a 1:4 scale human to use for the driver and judge the fit?

  2. Well more importantly, I’d need a 22% example of ME! I’m less concerned about nailing down fit in this model as I am in confirming all of my assumptions about counter-steer leaning, stability, suspension, and so forth. I imagine the actual driver compartment will be taken up by servos, wiring, and batteries.

  3. Oh, wow. I didn’t know this was going to be an actual functioning RC. I thought you were just talking about a scale model. That will be awesome. So it sounds like the 22% scale came from the size of the tires, you must already have tires from somewhere that you want to use then?

    On the counter-steering topic though, I wouldn’t take too much of the info you get from an RC model as very good info on how the full size trike will steer. I’m no RC expert, but I think “good” working counter-steered RC motorcycles haven’t been widely available until just recently (like, in the last 5-10 yrs), and many of them used all kinds of tricks and “crutches” (drag bars, etc.) to make them stable because real counter-steering uses lots of feedback to the driver, which an RC driver obviously doesn’t get. I think the latest RC motorcycles though get pretty good performance by hooking up the steering servo in reverse (ie. Tx steer left = Front wheel steers right) and then attaching the servo to the steering head with a spring (or worse, rubber fuel tube) so that the feedback from the front tire bends the the linkage (or compresses the spring) to get the final steer angle during a steady state turn.

    Even so, there’s still discussion online about “blipping the throttle” to get a motorcycle to stand up out of a turn, so there are apparently still “tricks” being used to get them to work reasonably well.

    All that aside though, I still think that a working RC StreetLiner would be AWESOME, and that you could probably sell it for a pretty penny when you move on to your next build stage. That is, if you don’t want to just keep it yourself.

  4. 22% did indeed come from a set of stroller wheels that I got at our awesome local surplus junk store. They’ve got some good mass to them and have solid rubber tires on them. They’re not quite the profile I’d prefer, but they’re likely close enough for my purposes. If it comes to it, I can always chuck them up in my lathe and round the tires a tad. Based on the wheels, the SSP (small scale prototype) will be not quite 18″ wide and the better part of 24″ long. That’s a good thing as it should behave less like a toy and more like the real thing.

    Building just a scale model, as fun as that would be, wouldn’t be nearly as useful as something that will actually move and steer. I had an R/C motorcycle when I was a kid that did have little outriggers on it, and it did indeed steer “backwards” which I found puzzling at the time but understand completely now. Feedback systems aside, I think that a lot of R/C motorcycles have to have outriggers or other “tricks” to keep them stable for no other reason than they don’t have proportional radios in them. They might have a two-step throttle and the steering is basically all left or all right and you end up pulsing that input to get the vehicle to do what you want. I’m going to use a proper, fully proportional radio setup so that I can make very precise throttle and steering inputs. I’m not going to try to rig any sort of feedback system into it to make up for loss of “feel”, I’m just resigning myself to a learning curve. If I can’t make that work, I may incorporate a leaning counterweight system of some sort to simulate the driver’s shifting weight. Probably just a small chunk of lead on a slider that I can move with a servo — make it a left stick input. I also plan to do some sort of tilt lock, although I’m not quite sure how to do that mechanically at this scale just yet. I haven’t run across any 22% working disk brakes. Then again, I haven’t looked yet either.

    If I’ve built it well and everything is reasonably tight and true, the natural state of the vehicle should be upright when it’s in motion. At least, that’s the theory. Then with enough space, I should be able to learn the steering inputs it requires to make the vehicle turn as I like and only then will I have a good sense that counter-steering alone is enough to initiate and terminate the lean. I’m very confident it will, but this R/C prototype will be the lynchpin of this entire project. If I can’t get it to behave the way I like, my first recourse will be to adopt an MP3-style front suspension. If that doesn’t work to my satisfaction, then I’ll probably scrap the project all together — maybe change gears and just build a custom scooter of some sort or a 4-wheel cycle car.

    If the SSP behaves to my satisfaction, I’ll completely button it up solid and then foam/fiberglass a body and wheel pants for it. It’ll be good experience making that plug and molding the body. It’s been quite a while since I worked with fiberglass. Who knows, a fully functional SSP could even invite some sponsorship for the full build. As with all of this project so far, half the fun will be documenting it and getting people’s feedback and input.

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