To work out a counterweight, multiply each weight by its distance from the pivot. Add those up on the load side, then pick a counterweight whose weight × distance on the other side comes to the same number. Put it as far back as you can, and it can be lighter. The surprising part is how much the far end of an arm matters: on a Technic-style barrier, the last nine holes of the boom need nearly three-quarters of the counterweight.
This guide walks through the method step by step on one real build, then shows where else it works. The Counterweight idea page has the one-line version.
Step 1: find the pivot and measure in holes
Every counterweight problem starts at the pivot, the point the arm turns about. On a Technic-style beam the easiest ruler is the holes themselves: count how many holes each part sits from the pivot.
The Parking Barrier Gate (level 2, about 20 minutes) is a good test piece. Its red 15-hole boom slides onto the pivot by its 5th hole, so it reaches 4 holes behind the pivot and 10 in front. A white 9-hole beam pinned onto the last two holes makes the boom reach 17 holes out. The counterweight, a big wheel, goes in the first hole of the short end, 4 holes behind the pivot.
Step 2: add up the turning effect of the load
Weight × distance is what engineers call a moment, or turning effect. A long beam's weight acts at its middle, so you can treat each beam as one weight at its centre.
Count each beam hole as 1 unit of weight, a "hole-weight":
| Part | Hole-weights | Middle, holes from pivot | Turning effect |
|---|---|---|---|
| Red 15-hole boom | 15 | 3 | 45 |
| White 9-hole extension | 9 | 13 | 117 |
| Whole boom | 24 | 162 |
The white beam is only 9 of the 24 hole-weights, but it gives 117 of the 162. That is the part people miss: weight far from the pivot counts many times over.
Step 3: divide by the counterweight's distance
Now pick where the counterweight goes and divide: 162 ÷ 4 holes = about 40 hole-weights. That is heavier than the whole boom, which is why the build uses a big wheel rather than a beam.
Move the counterweight closer and it must grow: at 2 holes behind it needs about 81. Take off the white extension and the 15-hole boom needs only 45 ÷ 4 ≈ 11. Try both in the lab, along with what happens when you get it wrong.
Interactive: Balance the barrier. The Parking Barrier Gate's boom with a counterweight you can size and move: predict how heavy it must be to balance the boom, what happens when you move it closer to the pivot or shorten the boom, and whether the barrier falls shut, stays or swings up when you let go.
Step 4: decide how balanced you want it
A perfectly balanced arm stays wherever you leave it, because tilting it shortens every lever arm by the same share. That makes it easy to crank, but it also means nothing pulls it shut.
So most builders leave the arm a little heavier than the counterweight. The barrier still lifts with a tiny turn, and it falls back closed when you let go. Too much counterweight does the opposite: the barrier swings itself open.
Step 5: test it on the real build
The barrier gate's own play notes suggest the test. Crank the boom up and down with the big wheel on, then take the wheel off and crank again. Then try the small wheel instead and see if it is enough.
- Crank with no counterweight and notice how hard it is.
- Add the big wheel in the end hole and crank again.
- Stop the boom halfway and let go. Does it fall, stay or rise?
- Swap in the small wheel. Too light, about right, or too heavy?
Because a wheel isn't measured in hole-weights, this is how you find the real answer: the rule tells you which way to change things, and testing tells you how much.
The same rule elsewhere
- The Balance See-Saw (level 1, about 12 minutes) asks whether one gear can beat two. Counting gear weight × holes, one gear twice as far out balances two, and the build has you test whether the real plank agrees.
- The Counterweight Trebuchet (level 3, about 40 minutes) hangs the battery box, with its batteries the heaviest thing in the kit, on the short arm. Here the counterweight is meant to win: it falls and whips the long arm up. The catapult vs trebuchet experiment tests how far it throws.
- The Nodding-Donkey Pump Jack (level 3, about 45 minutes) has a wheel on the back of its walking beam that stands for a counterweight. Its build notes say the balance has not been measured, so it is a good one to test with the method above.
- Outside the build table, a lift's counterweight is usually about the weight of the empty car plus 40 to 50% of its rated load, so the motor only lifts the difference. Tower cranes hang concrete blocks on their short back arm for the same reason.
Check yourself
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- What has to match for a counterweight to balance an arm?
- You move a counterweight half as far from the pivot. How heavy must it be now?
- Why does the far end of a long arm need so much counterweight?
- A balanced barrier is stopped halfway up. What does it do?
