Calculate how many holes to slide your trailer tandems, and which way, from your scale weights, using about 250 or 400 lb per hole or your own value.
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Sliding Tandem Axle Weight Formula
The calculator works in three modes. To find a slide when an axle group is above its reference limit, it rounds the required weight transfer up to a whole hole and checks how many holes the other group can accept:
Minimum holes = ceil(Overweight / PPH) Maximum holes = floor(Other group spare capacity / PPH)
To predict what a slide does, it multiplies the holes by the per-hole value to get the total weight shifted, then applies that shift to both groups. A forward slide (toward the cab) moves weight onto the trailer tandems, and a backward slide moves it onto the drives:
Shift = Holes * PPH New Drive = Drive - Shift (forward slide) New Trailer = Trailer + Shift (forward slide)
To estimate the observed per-hole value, use the same axle group’s weights before and after a known whole-hole slide with the load and configuration unchanged. Divide the measured weight change by the number of holes moved:
PPH = | Before - After | / Holes
Variables:
- Overweight is how many pounds an axle group sits above its limit
- PPH is the assumed or measured pounds moved per hole; the 250 lb and 400 lb presets are rough estimates, not values determined by rail spacing alone
- Holes is the number of slider holes the tandems move
- Shift is the total weight moved between the drive group and the trailer group
- Drive and Trailer are the current drive tandem and trailer tandem scale weights
- Before and After are one group’s weight on either side of a calibration slide
Sliding does not remove vehicle or load weight, so it does not reduce gross weight. This model transfers an equal estimated amount between the drive and trailer groups and assumes steer weight is unchanged. Moving trailer tandems back reduces their load and increases tractor load; moving them forward has the opposite effect. Reweigh all axle groups. The whole-hole mode checks both tandem limits: it distinguishes a combined tandem total above their combined limits from a whole-hole increment that cannot fit both groups even when their total fits.
Federal Limits and How Each Hole Shifts Weight
Normal federal Interstate references are 34,000 lb per tandem and 80,000 lb gross, subject to applicable exceptions. The Bridge Formula may impose a lower weight for an axle group or vehicle even when those references are met. Check your route, axle spacing, equipment ratings and kingpin-to-tandem restrictions; meeting the two tandem references alone does not establish legal loading.
| Axle group | Reference limit |
|---|---|
| Steer axle | 20,000 lb federal single-axle maximum; manufacturer axle rating may be lower. 12,000 lb is a common loading target, not the federal maximum. |
| Drive tandems | 34,000 lb |
| Trailer tandems | 34,000 lb |
| Gross vehicle weight | 80,000 lb |
The next table is the part most tandem guides leave out: what a single slide does to both groups at the same time. Every hole that takes weight off one group puts the same weight on the other. This example starts with the drives at 35,500 lb (1,500 over) and the trailer tandems at 32,500 lb, using 250 lb per hole. Watch the trailer column rise as the drive column falls.
| Holes slid forward | Drive tandems | Trailer tandems |
|---|---|---|
| 0 | 35,500 (over) | 32,500 |
| 1 | 35,250 | 32,750 |
| 2 | 35,000 | 33,000 |
| 3 | 34,750 | 33,250 |
| 4 | 34,500 | 33,500 |
| 5 | 34,250 | 33,750 |
| 6 | 34,000 (at reference limit) | 34,000 (at reference limit) |
At six holes both groups sit at the 34,000 lb reference. One more would put the trailer above it. With an assumed 400 lb per hole, the same starting weights have no whole-hole solution: three holes give 34,300 lb on the drives and 33,700 lb on the trailer; four give 33,900 lb and 34,100 lb. Their combined 68,000 lb fits the two references, but the whole-hole increments do not.
Example Problems
Example 1: fixing an overweight drive axle.
Your scale ticket shows the drives at 35,500 lb and the trailer tandems at 32,500 lb, using an estimated 250 lb per hole. The drives are 1,500 lb above the 34,000 lb reference, so 1,500 / 250 = 6 holes. A six-hole forward slide predicts 34,000 lb on each group. Follow your trailer and company sliding procedure, visually confirm all locking pins are engaged, and reweigh; these estimates alone do not prove legal loading.
Example 2: calibrating your own per-hole value.
Before a slide your drives read 35,600 lb. You slide forward 4 holes and reweigh at 34,000 lb. The measured change is 1,600 lb over 4 holes, so PPH = 1,600 / 4 = 400 lb per hole. This is the observed average for that slide with that load. Use it as a custom estimate for the unchanged load and configuration, and reweigh after further adjustments; it does not identify rail spacing.
Frequently Asked Questions
Which way do I slide to fix overweight drive axles?
Slide the tandems forward, toward the cab. That puts more of the load over the trailer tandems and takes it off the drives. To move the tandems forward you hold the trailer brakes and back the tractor into them. If the trailer tandems are the heavy group instead, slide backward by pulling the tractor forward.
How much weight does each hole move?
It depends on the trailer, load distribution, slider position and distance moved. The 250 lb and 400 lb presets are rough starting estimates; rail spacing alone does not establish weight transfer. For an observed average, weigh the same axle group on the same scale before and after a known whole-hole slide with the load and configuration unchanged, then divide the absolute change by the holes moved. Reweigh after each adjustment rather than treating one calibration as permanent.
Does sliding the tandems lower my gross weight?
No. Sliding does not reduce gross weight. If gross exceeds the applicable limit, changing cargo position does not reduce it; reduce the total weight or arrange an otherwise lawful load and configuration. If the tandem groups exceed their combined reference limits, a tandem slide cannot fix that combined total. Cargo redistribution may help axle distribution, but all axle, gross, bridge, equipment and route requirements still need checking.
