Weight Management

Every combat robot is a balancing act between weapon, armor, drive and electronics, all within a strict weight limit. Being even slightly overweight can cost you a match or keep you out of an event, while every gram you save can go into something useful. This page covers planning a weight budget, saving weight, and making weight at events. For the weight classes themselves, see Robot Classes.

The rules

• Weight classes are set by each event; the SPARC names and limits are listed in the SPARC Tournament Procedures. Robots that walk, shuffle or use unusual drives can weigh more (see Multibots, Walkers and Shufflers).
• Re-weighs: under SPARC rules, officials can re-weigh a robot at any time. A robot less than 5% over must get under weight before its next match; more than 5% over, and it forfeits its previous match too. Repeat offenders can be disqualified.[1]
• NHRL uses its own rules: being up to 2.5 oz over earns a penalty point and a chance to fix it, while anything more is an immediate loss by TKO.[2]
• Weigh your robot the way it will fight: with its battery, in its heaviest configuration (with the heaviest weapon option or armor set), and with any removable link in place. Check whether your event weighs the weapon lock.

Plan a weight budget

Before you build, write down the weight of every part and add it up. A spreadsheet works well, or your CAD software can calculate it from the materials you assign.

• Start with the parts you're buying (motors, ESCs, battery, receiver, wheels) – their weights are known.
• Decide how much weight to give the weapon, armor and frame. There's no fixed rule: a spinner might put a large share of its weight into the weapon, while a wedge or control robot might put more into drive and armor.
• Leave a margin for wires, screws, glue, heat-shrink and the things you'll inevitably add later. Small robots in particular gain weight from hardware and wiring faster than you'd expect.
• Aim to finish a little under the limit, so you have room for repairs and changes at the event.

Saving weight

• Choose materials by weight, not thickness – compare at equal weight (see Armor and Materials).
• Size the battery to what you need. Calculators like the Tentacle torque and amp-hour calculator help you avoid carrying more battery than you'll use (see Batteries).
• Remove material where it isn't needed: pocket or hollow out plates and printed parts, and use less infill or fewer walls in 3D printed parts where they aren't loaded (see 3D Printing).
• Shorten wires and use appropriately sized ones (see Wiring and Soldering).
• Use lighter fasteners where loads are low: shorter screws, fewer screws, or titanium or aluminum hardware (see Fasteners and Hardware).
• Combine parts: a frame part that also mounts the motors, or a weapon that also self-rights, saves weight twice.
• Don't save weight in the places that take hits – a lighter robot that breaks isn't a better robot.

Making weight at events

• Bring a scale that can weigh your whole robot, and check it against the event's scale if you can.
• Weigh again after every repair or change – replacement parts often weigh more than the originals (see Pit Kit and Event Checklist).
• Have a plan for quick weight savings if you're over: a lighter wedge or armor panel, a smaller battery, or holes you can drill.
• Some events allow extras, such as a small camera, above the weight limit – check the event's rules (SPARC's optional camera rule allows it if officials approve).

See also

• Robot Classes
• Drive Systems – how heavy drive motors usually are
• Spinner Design – how weapon weight affects energy