Friday, June 19, 2009


While browsing Goodwill last week, I came upon a ridiculously cheap "New Bright" Hummer H3 rc truck, the kind you find in Wal-Mart. There was no controller, so it was marked at $2, $6 if it had the controller. It had a 9.6V battery inside, and seemed to be in order. I figured maybe I could have some fun with it, use the body for future vehicles, since I've been looking into a crawler, what with their immense rise in popularity in recent years, and my thirst for the scale looks (of some) and the solid axles. My Rustler and 18R both have independent suspension, which is good, but I wanted something new.

Upon closer inspection, there were some cool things I noticed. My experiences with cheap RC cars as a kid led me to think that they used irreversible assembly techniques, so I couldn't tamper with anything as I usually loved doing. This little truck was held together with mostly philips screws, and some parts that snapped together. The entire battery/electronics compartment had a nifty locking mechanism and came clear out (after unplugging the steering motor and drive motor) with two screws.

However, the overall impression was still typical. There was no front suspension, at all. The front steering knuckles were mounted on stationary arms. Interestingly enough, they had shocks! same as the rear, except they just didn't do anything but look pretty.

The rear end had about a micron of suspension travel (laugh here). See if you can tell which picture is compressed and which is relaxed!

There was about an Angstrom of available articulation.

I thought about some chassis/suspension modification. Installing my own electronics would be very easy. However, any modifications would be difficult and I would have to custom-fabricate parts with materials that would have been costly.

It occurred to me then, what I could do. I had all I needed for a hybrid RC K'nex/New Bright monster truck!!!!! I had been wanting to do this for a year (here's the post to prove it! That was a working rolling chassis), but with all genuine K'nex parts (wheels, gears, etc etc). However, as I had been tossing around the thought for a year, every part of it just seemed very hard to do properly, and wouldn't perform well or be strong at all. The gearbox would have very many gears for a good reduction ratio, or I would have to use the weak supplied K'nex 130-motor-driven gearbox. The suspension system would be very weak and prone to breakage, as well as the steering system. The large K'nex tires are very narrow, and would have to be stacked, but I didn't even have any. The only working universal joint design I could find was HUGE and couldn't be trusted without tape.

On the other hand, the New Bright rear axle was a complete solid-axle drive unit, with its own motor inside. I could easily detach it and use it separately without any complicated mechanical connection.

I quickly set to work on a prototype.

The front axle is missing because at this point all I had was a chassis and a rear axle, which was a given.

Even though the axle link system I used involved only two links, which were hard-fastened to the axle, the knex rods were flexible enough to allow some articulation. The rods conveniently stuck into holes right under the shocks (which were very stiff, there would have been still more articulation had the springs been softer. However, the goal of this project was to be as cheap as possible).
There was more overall suspension travel:

When it came to the front end, I tried making an "axle" out of knex. It turns out that the difference in size of the knuckle pins and knex rods were too great, there was a lot of slop and the toe-in varied by as much as ~40°. So, I ended up cutting the "axle" from the chassis itself.

A knex rod zip-tied to it aided mounting. This project has taught me the value of zip-ties.
At this point, I had a rolling chassis+transmission.

Silly truck, that isn't your body!
That round part in the front turned out to be the only thing preventing optimum servo mounting, so away it went. I used the stock steering link to link the two knuckles together, but can you guess what I used to link the servo to the knuckles?

Custom blue plastic tubing?

That's right, it's a ball-point pen. Fit the bill perfectly, and was free since I had one laying around. (I just remembered, the cap from this pen went towards a headphone recable I did in 2007).
I installed all the electronics, and it was, well, slow.
Here are some more shots.

I've since made it waterproof (all I had to do was bag the receiver and esc), installed the motor from my car (which was way too fast, kept spinning inside the pinion gear), and rewound the original motor. It was loads of fun with the 18R motor, but I wanted to drive my 18R again (also the 18R motor melted the plastic gearbox housing a little bit and it upset the gear mesh :/)

I've been drifting the 18R, all I had to do was tape the tires. It's loads of fun and makes me want to keep it again. I'll post a crappy vid soon.

As for the rewound motor, it went from 30 gauge (or maybe even thinner) and NINETY-FIVE turns to this cool green-enamel-coated 26 gauge I got from radioshack and twenty-nine turns. I think it's safe to say I have a nice hand-wound modified motor lol. Maybe not nice since it's now my number-one source of problems, but it's a good in-between the performance of its stock form and the 18R motor.

As for "crawling" ability, it sucks. It's not 4wd and the gear on the axle is so big! It gets in the way very often. Also the tires aren't the best for the task (hard compound with chevron+spikes). I wonder if I cut off the spikes will it be better? Boil the tires? hmmmm

Still it's fun to try.

All that water isn't good for the axle, but some WD-40 fixed the problem and prevented new ones

Shot of the motor armature before and after winding. Sorry for crappy image quality, still using the phone.
That green wire looks so cool, the picture doesn't do it justice.

FINALLY, that's it for now. Actually there's more to post but I feel I need to start a new one. Tomorrow.

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