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Glens's Guide to Building a Beetleweight

This is not meant to be comprehensive guide on how to do everything this is just meant to be an overview of what you need to get started and where to go to find more information.


Combat robots are dangerous that is one of the reasons why putting them in an arena and watching them destroy each other is fun. This means that they are also capable of destroying you. Despite this building robot can be done in a safe manner if you think about what you are doing and take a few precautions. All the weapons you can build basically rely on the transfer of large amounts of energy into the other robot whether by spinning, flipping, launching, ect. So simply don't test these sorts of weapons in situations where you are not adequately protected. Do it in an arena or equivalent area with safety measures in place to ensure you don't get hurt!

Secondly a driving robot (even without a weapon) running into your ankles could easily break one, or cause sever damage, so make sure your robot can be properly controlled and have some form of barrier it can't drive over between it and yourself before letting it loose for testing. When testing your robot on the bench you should always have your wheels elevated.

There is many other things which I haven't covered here but if you have any doubts before doing something don't do it.
I take no responsibility for any injury sustained by building anything shown in this guide. If you don't have the skills, tools or patience to do it properly and safely STOP NOW

We now return you to your regular program.


Most events now use the standard RFL rules set. Jump to the Rules Section and have a read before you go much further.

Just quickly before you ask. "Why can't I build an EMP or use radio jammer or use a gun on the robot?" Have a think about what would happen...
With an EMP you would put the two robots in the arena one would set off EMP... other robot would sit still. Not going to be the most interesting thing to watch. There is a reason why there is restrictions on the weapons. Whether it is for safety, to keep things interesting or just prevent undue damage to the arena. The rules have been refined over 10 years so they are pretty well thought out. So just ask yourself why before complaining about the restrictions.

What will you need?

To start your path to competing in the beetleweight division your going to need some crucial parts no matter what style of robot you decide to make, this guide will serve as a brief run through of what you will require, mainly;

Radio controller
Drive train
Speed Controller

Radio Controller

You will need some form of radio controller in order to control the robot! You can also make an autonomous robot as these are within the rules, but that is beyond the scope of this article. Most controllers are standard RC hobby type units. There have been many custom solutions in the past involving RF computer gear (zigbee etc) and even Infra red doorbell controllers in one case, but with the current price and reliability of Radio control gear, as low as $35-40, you will find that this is the most popular solution and the one we will focus on.

If you don't have one already

Jump on HobbyKing and grab a 2.4ghz any model you like should pretty much work these days. A Hobbyking T6A V2 6channel radio is <$50 including shipping and is a good place to start. Just don't forget to grab the PC programming cable.

For more info on radio choices then jump to the Radio Control section

If you already own a radio

The important question now is "Is it a digital radio?"
Any 2.4ghz radio will be digital and some of the older radios have digital signal encoding which make them much more robust to interference. These should all work fine in a robot.

If you have an older style radio, such as a 36-40mhz aircraft radio or a 27mhz ground radio, either FM or AM, then it *MAY* be possible to use. In featherweight competitions their use has largely been discontinued, even banned in some events, as the interference that is a problem with these types of radios can be a serious problem with the larger robots. However on the much smaller scale of a beetleweight size robot, they are still permissible.

Personally i would recommend you stay away from them and just go for the cheap 2.4ghz radios mentioned above. It is a small investment you will use for years upon years for all your robots and the frustration saved from battling with interference issues, which often occur sporadically at events (trust me i know all too well!), is well worth it, not to mention having a giant 30cm long wire antenna trailing from your small robot is just asking for someone to attack it! The 2.4ghz radios have extremely short antennas that can be buried inside even a metal robot so that is one worry you wont have.

Just a few years ago a digital radio would have set you back $300+ however now you can get one for <$50 which you just put in your robot and it works. No making antennas, no sitting there scratching your head as to why the robot isn't working. So my advice is save yourself the pain and just spend the few dollars on a proper radio. It is well worth it.

Drive Train

There are many options out there for the drive train. The small size of these robots opens up a world of possibility when it comes to selecting drive motors and gearboxes.

At the simplest level, you can simply buy a cheap toy radio control car and liberate the drive units from them, Giving you a pair of gearboxes, motors and wheels all ready to use. If you get lucky you may even have a controller and radio all ready to go! You must make sure that the car has "tank steering" however, If it has standard car steering then you will likely only get one drive unit.

These toy drives are often quite powerful and fast and are a good way to build a robot quickly. However the main downfall is they are often hard to mount with wierd mounting points if they even have any at all given some are cast into the plastic frame of the toy and are rarely very durable, requiring replacement every event or so. So you have to use your own discretion there to find a suitable car to hack.

Stitchface is a prime example of a robot using this drive train. It even won the event!

Yes your standard hobby servo can be hacked to work as a drive motor!

By following the *servo hacking guide* you can make the servo turn 360 degrees instead of a set angle. The pro's of using servos are the low cost, the inclusion of a speed controller in the servo (you dont have to buy any so they are very cheap) and the integrated gearbox and mounting tabs.

However they are very slow and fragile and difficult to mount a wheel too because of the short splined shaft meaning you have to use the servo horn or buy an expensive ready made wheel for servos. Because of this they are rarely used.

The most popular solution at the moment are the range of gearmotors from ebay. They are reasonably cheap at $25 for a pair posted, and offer a great combination of speed and power while being small, lightweight and easy to mount giving you a range of possibilities in your design instead of being limited by the large size and shape of the rc toy drive setups.

The downside to this is that you need to make or purchase your own hubs to mount your wheels to the output shaft, or find a wheel that will fit straight on. Many builders will make custom hubs for your choice of wheel for a small fee if you ask on the forum.

The gearmotors are also supplied with a brass bushing on the output shaft instead of a bearing which can be prone to cracking, which leads to the shaft moving and gears stripping. However the chance of this can be reduced by belt driving your wheels or using foam wheels to absorb large impacts.

To control these your going to need an electronic speed controller. This will be covered later.


In This class, Lipoly batteries are almost exclusively used. Small Nicad and NIMH cells have been used in the past, but with the advent of extremely cheap lipoly batteries flooding the market and the fact they are superior to the nicad/nimh batteries in nearly all regard, It is strongly recommended to use them.

LiPo batteries

Lipoly batteries are the primary battery chemistry used for beetleweights. They are much smaller and higher performance then nicad/nimh cells of a comparable size as well as being quite a bit cheaper now thanks to our friends over at HobbyKing where you can find a huge range of robot sized lipos.

You should however be aware that Lipo's are also one of the most dangerous types, with reported cases of fires caused by physical damage to the batteries as well as incorrect or over-charging of the battery. Fortunately lots of progress has been made in the last few years which make these batteries suitable for use in robots and with due care, they will work just fine.

To select the right size of battery will depend on a few things. But first a few basic terms specific to Lipo's.

Batteries can only hold so much charge and this is the batteries capacity and it is rated in either MaH (milli amp hours) or aH (amp hours). A 1000mah battery will supply 1000 milli amps for one hour. Our fights last 3 minutes, thus this 1000mah (1ah, amp hour) battery while provide 20,000 milliamps (20 amps) for 3 minutes. You can work this out by

Minutes in one hour / 3 minutes (fight length) x batteries rated capacity in Amp Hours

This is probably the most important rating as you must have enough capacity to finish the fight, depending on how much current your robot draws. I've found that 500mah is about the dead minimum for a beetleweight spinner, which is the most current hungry kind of robot.

But given the above equation you could also say that in one second a 1ah battery could provide 600 amps of current for one second and this is where the next term comes in.

This is shown using the symbol C and it is a rather symbol rating.

A battery can only give out so much current, limited by its physical size and chemistry and this is shown by the C rating. A battery rated at 20C can only provide 20 times its Amp hour rating in amps. Using our 1ah battery as an example, if it were rated for 20c that would mean it could only provide a maximum of 20 amps of current at any time.

Maximum battery current = Battery capacity in aH x C rating

This is handy for weapons like hammers and spinners, where large motors will tend to draw large amounts of current. If your motor draws 40 amps and you try run it off our 1ah 20c example battery then its possible the battery can explode or at least be permanently damaged and your robot wont work to its full potential as its being capped by the limited current the battery can provide! So you must take this into account when designing your robot.

Designated by the letter S. A lipoly battery has roughly 3.7v per cell, this is just a function of the particular chemistry (in the same way lead acids have 2v per cell and nicads 1.2v). So to get more voltage you must link individual cells in series.

Generally you want to run components at their rated voltage. However for robots we can get away with running more voltage for a brief time. For example the 6v drive motors may be run at higher voltage for more power. So for this you will want a 3 cell battery (3x3.7 = 11.1v). This will also quadruple the current the motors draw so take that into account when selecting a speed controller.

The same applies to weapon motors.

So to select a battery you first want to select the right capacity, you can work it out mathematically if you want, but generally 500mah and up will get you by. Then you must work out the amount of cells you want to run. Most beetles use 3 cells but some even run 4 all the way up to 6 to get maximum power. And finally you want to Make sure the C rating will provide the current your robot will draw. If your only making a wedge style robot then you can use a much lower C (10-15) and therefore cheaper battery but if your making a spinner then you will want a high C (30-40 and up) battery.

For more information see the Batteries & Power section.

Speed Controller

Speed controllers are another item which you will require and are usually the most expensive part of your robot. I will give a brief run down of the controllers that have been used by fellow competitors and myself. There are plenty on the market though that you could use, but i cant say how well having never tried many!

A new development, these are based on modified Hobby brushless controllers, reprogrammed by Steve to work with brushed motors. To do this however you will need some programming gear, or someone on the may do the programming for you. These are still in early development however look very promising as they are by far the cheapest method.

Armour / Frame

You wouldn't have much of a fighting robot without some kind of armour! The use of a dedicated frame with armour attached or monocoque (using the armour as the frame) is completely up to you, but the vast majority use the monocoque style of construction because it allows you to have thicker armour without the added weight of a seperate frame at the expense of repairs being more difficult.

With the smaller dimensions of the beetleweight robots the use of more exotic materials is more widespread as you don't need as much obviously, so the cost is reasonable. Titanium and carbon fibre are common place in this class where as it's more a rarity in the larger ones. I will discuss the pro's and cons of the commonly used materials

One of the cheapest and readily available metals you can use, steel is also the heaviest listed which limits its use in beetleweight robots. Making one entirely from steel is almost out of the question because of this and thicknesses of usable weight are generally not stiff enough to survive.

However It has its place in high strength areas, like spinning weapon mounts and Armoured plows / wedges where the high strength and stiffness is needed but the small amounted needed keeps the weight down.

Steel is very cheap new at local supply stores and easy to salvage from other items. Hardened steel can be salvaged from old saw blades for example. The ability to weld it easily also makes construction simple.

Weapon System

Estimated Cost

So how much is this all going to cost?

Lucky for you the cost of new parts has decreased dramatically in the past handful of years because of the massive supply of hobby parts coming from Hong-Kong and China. For example the cost of the radio control unit itself has gone from roughly $300 to $40 and the prices look set to keep on dropping. You don't need the best gear in the world for a robot who's life span may be 30 minutes of operation, so these parts do us just fine most of the time.

So Robots are a lot more affordable then you may think! Here's an approximate breakdown, exact prices will obviously vary with the spec you need:

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