Tandem Slide Calculator
Type in the three weights off your scale ticket. Get how many holes to slide, which direction, and what your axles will weigh after the move — before you pay for a reweigh.
Enter your weights and hit WEIGH IT
How this calculator works
The calculator reads the three weights off your CAT ticket and checks them against the federal Interstate limits — 12,000 lb steer (by typical tire rating), 34,000 lb drives, 34,000 lb trailer tandems, 80,000 lb gross. Gross gets checked first: over 80,000, no slide on earth fixes it. If gross is legal but one group is heavy, the tool figures how far over, divides by what one hole on your rail moves, and rounds up — you can't slide half a hole, and rounding down leaves you overweight.
≈ 250 lb/hole on 4″ centers · ≈ 400 lb/hole on 6″ centers
forward → weight moves off drives onto tandems · back → off tandems onto drives
Direction is automatic: heavy drives means the tandems go forward, toward the cab, so the bogie sits closer to the freight and carries more of it. Heavy tandems means slide back and hand weight to the drives. The ticket then prints your estimated after-slide weights and re-checks the group that's receiving the load — the classic mistake is clearing your drives by burying your tandems one hole past legal. If no hole makes both groups legal, it tells you that too: that's a rework-the-load problem, not a slide problem.
Two worked examples
Example 1 — drives heavy on a 4″ rail
You scale at 11,240 steer, 35,120 drives, 32,480 tandems. Gross is 78,840 — legal — but the drives are 1,120 lb over the 34,000 limit. On 4″ centers a hole moves about 250 lb: 1,120 ÷ 250 = 4.48, which rounds up to 5 holes forward. The estimate lands at 33,870 on the drives and 33,730 on the tandems. Both legal, but with only 130 lb of daylight on the drives — reweigh to confirm, and don't top the tanks before you do.
Example 2 — tandems heavy on a 6″ rail
A reefer loaded tail-heavy scales 11,900 steer, 31,880 drives, 34,760 tandems — 78,540 gross. The tandems are 760 lb over. On a 6″ rail one hole is worth about 400 lb: 760 ÷ 400 = 1.9, so 2 holes back. That lands the tandems at 33,960 and the drives at 32,680. Legal by 40 lb on the tandems — which is thinner than freight settling on a hard brake. If the rail allows it, take the third hole: 33,560 and 33,080 puts honest margin on both groups and costs you nothing.
From the road
Scale before you leave the shipper's town. The first CAT weigh costs about what lunch does, the reweigh is pocket change; the overweight citation at the chicken coop starts in the hundreds and climbs with every pound. A shipper's "it always scales fine" is not a scale ticket.
The slide itself will fight you some days. Pins seize with rust and road grime — set the trailer brakes, pull the release, rock the tractor against the locked bogie until they pop; winter can take a few tries and some percussion. Then get out and count the holes yourself — an unlocked bogie or a miscounted slide shows up at the worst time.
Two things quietly change your numbers: fuel and state law. Diesel runs about 7 lb a gallon and lands mostly on the steer and drives — 120 gallons is 800-some pounds your ticket didn't show. And California's 40-foot kingpin-to-rear-axle limit caps how far back the tandems can live: drivers scale legal in Arizona and pick up a bridge-law ticket in Barstow. The state-by-state picture is in our guide to sliding tandems and state bridge laws.
FAQ
Which way do I slide if my drives are heavy?
Forward, toward the cab. Moving the tandems closer to the freight shifts weight off the drives and onto the trailer tandems. If the tandems are the heavy group instead, slide them back, toward the rear — the weight walks back to the drives.
How much weight is one hole worth?
Rule of thumb: about 250 lb per hole on 4″ centers and about 400 lb on 6″ centers. Trailers vary, so check the rail spacing on yours. When it's close, slide one extra hole and confirm on the reweigh.
What are the legal axle limits?
Federal Interstate limits: 12,000 lb steer (set by typical tire rating), 34,000 lb drive tandem, 34,000 lb trailer tandem, 80,000 lb gross. Some states allow more on non-Interstate roads, and bridge law adds axle-spacing rules on top.
I'm over gross. Can sliding fix it?
No. Sliding only moves weight between axle groups — the gross never changes. Over 80,000 lb means removing freight, burning off fuel weight, or getting an overweight permit.
How do I physically slide the tandems?
Set the trailer brakes so the bogie stays planted, pull the release to retract the locking pins, release the tractor brakes, then ease the truck: backing up slides the tandems forward, pulling forward slides them back. Let the pins snap in, tug against them, and count your holes.
Does fueling up change my axle weights?
Yes — about 7 lb a gallon, landing mostly on the steer and drives. Scale half-empty and top off before the coop, and a legal ticket can turn overweight. Weigh with the fuel you plan to run, or leave margin.