Drive Axles Over on the Scale Ticket
At a Louisiana weigh station on I-10, your 5-axle tractor-semitrailer shows: steer axle 11,900 lb, drive tandem 34,650 lb, trailer tandem 32,900 lb, gross 79,450 lb. The cargo is 42,000 lb of bagged sand loaded by the shipper; the trailer tandems are at the middle of their slide range, and the fifth wheel is in its factory position. The steer axle tire rating is 12,000 lb; drive and trailer tires are rated 34,000 lb per tandem. The scale officer says the drive axles are over. You have 1,100 lb of room on the trailer tandem and only 100 lb on the steer axle. The shipper can move pallets, but that takes two hours. You need to decide whether to slide the trailer tandems, move the fifth wheel, or redistribute the load, and then verify the result.
1. Which axle is over and by how much? What is the best first adjustment, and why?
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The drive tandem is over by 650 lb (34,650 vs 34,000). The best first move is to slide the trailer tandems rearward. Moving the trailer axles rearward increases the share of the trailer load carried by the trailer tandems and reduces the load on the fifth wheel, which transfers weight off the drive axles. The trailer tandem has 1,100 lb of room, so a 650-lb transfer can be corrected without overloading it, provided the transfer is not larger than expected. Moving the fifth wheel forward would also reduce drive weight, but it adds weight to the steer axle, which has only 100 lb of room. Redistributing 42,000 lb of bagged sand is slower and should be the fallback. After sliding, reweigh all axles.
2. After sliding the tandems rearward, what new risks must you check?
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Check that the trailer tandem did not exceed 34,000 lb and that the steer axle did not change unexpectedly. Recheck gross weight, tire ratings, and the bridge formula. Moving the tandems rearward changes axle spacing and can change the bridge calculation, though the greater spacing between the drive and trailer tandems usually helps the bridge limit. Also check trailer clearance and turning behavior: a longer rear overhang can increase off-tracking and swing, so the driver must allow more room in turns. Confirm the slide pins are locked and the trailer brakes and air lines are not stretched or damaged. Do not leave the scale without a new ticket.
3. If the trailer tandems are already at the rear limit, what are your options?
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Do not force the slide. Options are to shift cargo within the trailer toward the rear, which reduces drive weight and adds trailer weight; reduce the load; or move the fifth wheel forward only if the steer axle has enough capacity. Here the steer axle has only 100 lb, so moving the fifth wheel is not a good first choice. Shifting cargo must keep the load secured and must not put too much weight on the trailer tandems or the rear axle. If none of these can be done safely, the vehicle should not leave with the drive axles over the limit. The driver should ask the shipper to adjust the load and get a new scale ticket.