6-Wheel Rover – Rover Base – Not wild about the Wild Thumper Chassis

Let’s start from the ground up, shall we?

I took the easy route and bought a premade 6-wheel robot chassis. I would, however, urge you to think about building your own, or at least purchasing a different model. While my chassis is certainly going to suffice, it has a few design flaws that, had I known beforehand, would have had me reject it.

Nonetheless, it is what the robot is based upon, so let’s look it over.

Dagu Wild Thumper Chassis

The rover is based upon a prebuilt chassis from Dagu, called the Wild Thumper. I purchased mine at RobotShop.

I bought this chassis as it looked sturdy, and some videos I saw on YouTube certainly made it seem capable of running outdoors. And, for the most part, it is a pretty decent chassis. It has room for a couple of LiPo’s plus some electronics, and the chassis material is pretty sturdy.

But it also has a few design flaws, two of which are pretty serious in my opinion.  I’ll get to those in a bit, first let’s look at the specifications of this chassis.

Chassis Specifications

• Aluminum construction
• 6 x DC motors – 6V @ 5.5 Amps
• Independent suspension for each motor
• Two center motors also have linked suspension
• 6 x Wheels with balloon tires
• Two battery/electronics compartments

The chassis came completely assembled, except for the wheels. They fitted on with a single Allen screw each, and the key was provided in the kit.

There was a one-page “instruction manual” provided with the chassis.

My kit was “prewired” with the motors going to a terminal strip. However, the wiring online was incorrect, as they wired the center wheel to the wrong motor group! No big deal in my case, as I wasn’t planning on using their wiring anyway, but it doesn’t say a lot for quality control.

The real issues came, however, when I examined the build.

Design Flaw # 1 (Fixable)

I spotted the first issue almost immediately.

The six brushed DC motors each have two leads, the positive and negative power leads. These wires are, sadly, only 20 gauge, I was hoping they would be at least 18.

But that’s not the flaw. The flaw is in how the wires were routed.

The Dagu Wild Thumper chassis is constructed of aluminum channeling, which has several M3-sized holes drilled in it. I’m sure you’ve seen similar products, as this sort of chassis construction is used in many robot designs.

Well the motor wires, all twelve of them, were routed through the holes on the chassis. No special provision was made for protecting the wire insulation, which on my Wild Thumper was already showing signs of wear. Not surprisingly, the wires move whenever the motors flex, which they are designed to do.  The wires were literally scraping on the aluminum chassis.

Wear and tear on a brand-new unit!

This, fortunately, was a repairable flaw. So I did what the manufacturer should have done in the first place, I drilled out a few of the existing holes and put a rubber grommet in each one. Then I passed the motor wires through the grommets.  Now, the wires are protected during movement.

If you need to modify your chassis to do this, make sure that you position your “grommeted holes” in a place where running the wires won’t interfere with the suspension.

Design Flaw # 2 (Unfixable)

Unfortunately, there is a much bigger flaw in the design of the Wild Thumper.  And, unlike the wire routing issue, I can’t easily fix this.

After spotting the mis-wired motor and correcting the wiring design flaw, I was on alert for other issues with the chassis.  So I took a screwdriver and Allen key and went around the chassis, tightening screws and bolts as required.

It was while I was tightening the screws holding the motors on that I spotted the design flaw.

The motors on the Wild Thumper are shaped like an upside-down “T”, with the motor shaft coming out of the “stem” of the T. The bottom, i.e. the horizontal component, has a mounting screw on each side in the center.

But you can’t over-tighten this screw – because it also serves as an axel! If you tighten it, the suspension stops working. And if it gets too loose, then the suspension fails.

And nothing short of a complete motor and/or chassis redesign will fix it!

For a chassis that costs as much as this, I really feel this is a serious design flaw. It’s certainly one that keeps me from recommending the chassis to you, and I may eventually have to replace the base units.

I also wish that the motors had encoder outputs, but I’ll admit that I knew that before I purchased the chassis.

Motor Specifications

For those of you smart enough to take my advice and build your own chassis (or purchase a different one) I’m providing the specifications for the motors included with the Wild Thumper.  You can use these specs as a guide to finding suitable motors for your rover.

• Rated voltage: 6V DC (Min. 2V Max. 7.5V)
• Stall current: maximum 5.5A
• Maximum Locked current of 5.5A
• No load current per motor: 350mA
• Motor RPM: 10000 + / – 5%
• Output shaft speed: 295rpm + / – 5%
• Stall torque is 4Kg/cm
• Locked-rotor torque: 4Kg/cm

Moving Forward

As I’ve invested a good amount of both money and time into the design based upon the Wild Thumper I’m going to continue with it, at least for the time being.

Down the road, however, I’ll probably rebuild the base. I can keep the remaining components and just stack them onto the new base. One that also has a suspension, and hopefully motors with speed encoders.

But, until then, the outdoor robot is based upon the Wild Thumper 6-wheel drive chassis.