The Overweight Axle That Wasn't on the Ticket
You pick up a loaded 5-axle tractor-semitrailer at a Calgary yard. The shipper's scale ticket reads gross 39,500 kg, with drive axles at 17,200 kg and trailer tandems at 16,800 kg. You are legal on gross weight for the National Safety Code limits used in Alberta, so dispatch tells you to roll. Two hours later at a scale on Highway 2, the officer weighs you and finds the steer axle at 6,400 kg, drive axles at 17,300 kg, and trailer tandem at 16,900 kg. The officer also notes your steer tires are rated 3,150 kg each. You have no permits. The officer says the gross is fine but you still have a problem.
1. Identify which weight limit you have exceeded and explain why the shipper's gross-weight ticket did not catch it.
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The problem is the steer axle. Two steer tires at 3,150 kg each give a tire capacity of 6,300 kg, but the scale shows 6,400 kg - a 100 kg overage on tire capacity, which is a violation regardless of axle or gross limits.
Why the ticket missed it:
- The shipper reported only gross, drive, and trailer tandem weights. The steer axle was never on the ticket.
- Gross weight being legal does not mean every axle and tire group is legal. Each has its own limit: steer axle, drive axle group, trailer group, tire capacity, and registered/configured limits.
- A 5-axle load can be under gross and still overload one axle if the load is positioned too far forward or the fifth wheel is set wrong.
Lesson: always confirm per-axle and per-tire numbers, not just gross.
2. You must shed weight before moving. List the practical options and rank them by what you would try first.
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Options, roughly in order of practicality:
- Slide the fifth wheel forward (toward the tractor steer axle is wrong - you want to move load off the steer). Sliding the fifth wheel rearward transfers weight from the steer axle to the drive axles. Confirm drive axles have headroom: they are at 17,300 kg, so check the drive group limit before shifting more onto them.
- Slide the trailer tandems to rebalance trailer weight; this affects the trailer group and the kingpin load, which in turn affects the steer.
- Redistribute or remove freight - move pallets aft or offload if the load is front-heavy.
- Adjust air pressure / confirm tire ratings - only if a tire is underinflated or mis-rated; do not rely on this to fix a genuine overload.
Ranking: try axle repositioning first (free, fast, reversible), then load redistribution, then offload. Re-weigh on a certified scale and confirm every axle and tire group before departing.
3. The officer asks whether the tire rating or the axle rating governs the steer. How do you answer, and what is the general rule?
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The lower of the applicable limits governs. The steer axle must satisfy:
- the axle/steer weight limit for the vehicle configuration,
- the tire capacity (sum of the steer tire ratings), and
- any registered weight limits.
Here the tire capacity (6,300 kg) is lower than whatever the steer axle is rated for, so the tires set the ceiling. You are over by 100 kg on tires even if the axle itself would allow more.
General rule to state on the exam: a vehicle is legal only when it is within gross, axle group, individual axle, tire, and registered limits simultaneously - the binding constraint is the smallest one.