California's Kingpin-to-Rear-Axle Limit

KPRA is not gross weight and not the bridge formula

A scale can do two different jobs in one stop. It can weigh the combination, and it can look at where the trailer axles sit behind the kingpin. Passing the first does not pass the second. Kingpin-to-rear-axle length is a state equipment limit. It is independent of the federal bridge formula and of the 80,000-pound gross cap that drivers treat as the interstate ceiling. A legal weight can still be an illegal trailer setting.

Weight limits by state owns the gross and axle chart. This page will not rebuild that chart, will not print a bridge-formula table, and will not tell you what a steer axle "usually" weighs. If the only problem is pounds, you are on the wrong guide. If the pounds were fine and the officer still talked about the slider, you are in the right one.

The practical mistake is treating a CAT ticket as a full legality letter. The ticket is a weight at a moment. It does not measure the kingpin. A driver who photographs the ticket and leaves has not checked the length the California rule is about.

The limit scale houses apply, and the section to quote

The feet on this page come from the California Department of Transportation vehicle-length page, opened while this guide was written. That page says it paraphrases the Vehicle Code for brevity and tells the reader to open section 35400 for the full legal wording. The legislature's own HTML did not load in this drafting session, so this article does not pretend to quote the statute's commas. If the statute and the paraphrase ever diverge, the statute wins. Open it before you argue a citation.

What Caltrans states, as the paraphrase of the exception in section 35400, is a semitrailer being towed by a motortruck or truck tractor, if the kingpin-to-rearmost-axle distance of the semitrailer does not exceed 40 feet for semitrailers having two or more axles, or 38 feet for semitrailers having one axle. The words are "rearmost axle." They are not "center of the rear axle group." A scale-house habit that measures to the middle of a spread tandem is not the wording on that page.

The same Caltrans page paraphrases the longer combination exception in section 35401.5. On the National Network and terminal-access routes, a semitrailer may be not more than 53 feet in length, with two or more rear axles and a maximum 40-foot KPRA, or with a single axle and a maximum 38-foot KPRA. Route access is part of that exception. A 53-foot trailer is not a statewide permission to ignore where you are. Caltrans also describes reasonable access for fuel, food, lodging, and repair within one road mile of identified exits when that access is safe, and a terminal-access process. This page is not a map of those routes. Use the department's current truck-access information before you leave the network because a yard said the street was fine.

Two more paraphrases matter and are easy to flatten into a single "40 feet everywhere" sentence. Caltrans says section 35401 lets a city or county restrict the kingpin-to-rearmost-axle distance to 38 feet, but not less, on highways under its jurisdiction, and that the department may recommend restricting KPRA on certain highways, but not to less than 38 feet. It also paraphrases section 35401.1: a combination with a KPRA of 38 to 40 feet may be operated on local highways only where the owner or operator deems it safe. "The state is 40" is not a complete answer on a city street. Read the section and the signs. Do not invent a local limit below 38 from a blog.

A 53-foot trailer can fail it

Trailer length and KPRA are not the same dimension. A 53-foot semitrailer has more box behind the kingpin than a 48-foot trailer. If the tandems are shoved all the way forward to make a dock, or to put more weight on the drives, the distance from the kingpin to the rearmost axle grows. That is how a trailer whose overall length is allowed on the STAA routes still fails the KPRA cap in the same paraphrase.

The fix, when the slider has room, is to move the tandems rearward so the rearmost axle comes closer to the kingpin's limit, then measure again. "Rearward" and "forward" get swapped in yard talk. Forward, toward the cab, lengthens KPRA on a typical slider. Rearward, toward the back of the trailer, shortens it. Confirm the direction on your trailer before you announce that you fixed it. Some sliders are already at the stop. If the stop still leaves you over the cap, the setting is not legal just because the pin will not move farther. The load has to be rearranged, the trailer has to be a different length, or the trip does not use that route. Do not cut the trailer, do not unpin on the shoulder, and do not decide the officer "probably won't have a tape."

A spread axle is the case where "center of the group" and "rearmost axle" disagree the most. The back axle is the one Caltrans' label points at. Measuring to the midpoint can make an over-length setting look legal on paper. Do not do that and then argue the paraphrase.

Sliding the tandems changes the axle weights

The slider is one bar that does two jobs. It changes KPRA, and it changes how many pounds sit on the trailer axles versus the drives. A move that cures the length can overload an axle group that was legal five minutes earlier. The reverse is also true: a slider position that made the axle weights legal at the last scale can be the position that fails KPRA at the California scale.

That is why the sequence is measure, slide if you must, measure again, then reweigh. Sliding tandems and weight distribution is the weight half of that sequence. How to use a CAT scale is the reweigh. Do not treat a slider hole number from last week's load as this week's answer. The freight is in a different place in the box. A nose-heavy load and a tail-heavy load do not want the same hole, and only one of those holes may also satisfy the rearmost-axle cap.

This page will not give you a pound figure for "each hole." That number depends on the load, the trailer, and the scale. Anyone who prints a universal pounds-per-hole is inventing it. Reweigh the combination you are actually driving.

Other western states are not the same as California

Nevada, Arizona, Oregon, Washington, and the rest of the map do not inherit California's paraphrase because they share a border or a lane. Some states measure to the center of the rear axle or the rear axle group. Some use a different footage. Some care about overall combination length and barely use KPRA. Copying "40 feet to the center" into a California argument is wrong on California's own page, and copying "40 feet to the rearmost axle" into another state is wrong until that state's code says it.

If you run a western tour, the pre-trip is a list of the states on the route, each with the section you opened, not a single number written on the visor. This guide will not build that list. The gross and axle chart is the other article. The KPRA chart for the other states is those states' codes.

How to measure before the scale

Measure from the kingpin to the rearmost axle. Not from the back of the cab. Not from the nose of the trailer. Not to the rear doors. Not to the mudflap. Not to the center of a tandem unless that tandem is a single axle, in which case the only axle is the rearmost axle.

On the ground, two people and a tape beat a guess from the catwalk. One end is the kingpin center. The other end is the center of the rearmost axle, the axle itself, not the tire's outer sidewall and not the bumper. If you are using a slider-hole chart the trailer maker printed, confirm that the chart's "40 foot" mark is defined to the rearmost axle and not to a rail sticker that measures something else. A hole chart from a different trailer model is not this trailer.

Write the measurement in the same note as the scale weights for that load. If you slide after the note, the note is stale. Do it again. A photo of the slider with the hole number visible is useful only if you know which hole corresponds to which distance on that trailer.

Local streets add the safety judgment Caltrans paraphrases for a 38-to-40-foot KPRA. "Deemed safe by the owner or operator" is not a slogan that defeats a posted restriction. If the city has restricted the highway to 38 feet, the paraphrase of section 35401 says that restriction is available to the city, down to 38 and not below. Do not out-guess the sign with the STAA sentence.

The combination length rules are a third topic this page will only point at. Caltrans paraphrases a basic 65-foot combination limit and separate doubles limits. KPRA compliance does not answer overall length. If the tractor and trailer together are the problem, you are outside this one measurement.

Check the measurement when the trailer is hooked the way it will be driven. A fifth wheel that is slid all the way forward or back changes where the kingpin sits relative to the cab, but the KPRA measurement starts at the kingpin, not the cab. Sliding the fifth wheel can still change axle weights and overall combination length. It is not a substitute for the trailer slider, and it is not a way to "add" kingpin length you do not have. Measure the trailer you hooked, on the fifth-wheel position you will use, then leave the settings alone until the next scale.

A reefer, a flatbed, and a dry van can share a 53-foot label and not share a slider range. The article you are reading is the measurement. The trailer in the yard is the specimen. Put the tape on that specimen before the scale puts a tape on it for you.

Frequently Asked Questions