Short answer: the spring above your roll-up door carries almost the entire weight of the curtain, and the cables transfer that help down to the bottom rail. When either one goes, the door gets brutally heavy, hangs crooked or crashes down. This is the one repair on a box truck that we tell people flatly not to attempt themselves.
How the counterbalance actually works
Above the rear header there is a shaft with a spring wound inside or around it. Cables run from drums at each end of that shaft down to the bottom of the curtain. When the door is down, the spring is wound tight and holding a lot of stored energy. As you lift, the spring unwinds and gives that energy back, which is why a door weighing well over a hundred pounds can be raised with one hand.
Everything that feels wrong about a failed door traces back to that balance. Lose the spring and you are lifting the raw weight of the curtain. Lose one cable and one side of the curtain gets the help while the other doesn't, so the door racks in the tracks.
Signs the spring has failed
- The door suddenly weighs far more than it did last week.
- It won't stay up — it creeps or slams down when released.
- A loud bang from the back of the truck, then a dead door.
- Visible gap or separation in the spring, or a shaft that spins loose.
- The door stops part-way and needs two people to move.
Signs a cable has failed
- The door hangs crooked — one side lower than the other.
- Slack cable coiled at one side, or cable jumped off its drum.
- Frayed strands — "meat hooks" of broken wire you can feel with a rag.
- The door binds in the same place every time, and always to one side.
- Rust and kinks where the cable has been rubbing on something it shouldn't.
Fraying almost always starts at the drum or where the cable turns. A frayed cable is a countdown, not a maybe. Once you see broken strands, that cable will part — usually at the worst possible moment, which for a box truck means mid-load at a dock.
Why springs and cables wear out
Springs are rated in cycles, and every open and close is a cycle. A truck doing twenty stops a day burns cycles at a rate a truck doing three stops never will, so a busy delivery unit will need a spring long before its neighbour in the yard. On top of raw use:
- Grit and dust in the drums and tracks accelerate cable wear.
- A misaligned track makes the curtain drag, loading the spring harder every cycle.
- A bent slat does the same thing.
- Repeatedly slamming the door shortens everything's life.
- Corrosion at the cable ends, especially where water sits.
That's why we look at the whole door rather than just replacing the failed part. If the track is pinched and we only fit a new spring, the new spring wears out the same way. The related symptoms are covered in roll-up door won't close.
What a proper repair looks like
Done right, the job goes roughly like this: unload the tension safely, inspect the shaft, drums, cables, rollers and track together, replace what has failed and anything else that's obviously on its way out, re-reeve the cables evenly, then wind the spring to the correct tension for that curtain's weight and cycle the door repeatedly to confirm it balances and holds anywhere in its travel.
A door that has been set correctly stays where you leave it, moves with light effort, sits square in the tracks and closes without needing to be forced onto the seal. If it needs a shove at the last foot, something is still wrong.
On site or in the shop?
Springs, cables, rollers and track alignment are all on-site work for us. We bring the tools and common parts to the truck, because a door failure usually means the truck can't be moved safely anyway — this is exactly what mobile box truck repair is for. What comes to the shop is a rear frame or header damaged badly enough that the door opening is out of square, since that needs the truck off the road. The general rule is set out in mobile vs shop box truck repair.
Before you call
- Note whether the door is stuck up, down or part-way, and whether it's loaded.
- Look for slack or broken cable at both sides and say which side.
- Say whether you heard a bang, and roughly when.
- Give the truck's location precisely — address, dock, cross streets.
- Year, make and box length if you have them.
We take calls in English and Spanish, 24 hours a day. Call (480) 251-1521. If you run multiple units, door hardware is the single best thing to check on a schedule — see cutting fleet downtime and our fleet repair and full service pages.