It is for the turning radius.

King pin to rear axle law is the proper name, it says how far apart your axles can be at maximum, it helps with turning to have the axles closer, but you also get more tailswing, when they are far apart, and this law only applies to trailers over 48 feet.

bridge law says how close your axle can be, for spreading the weight going over bridges!

People are always confusing the two!

CA does not have enough "low" bridges for this to be a concern. But yes, the issue you speak of makes a huge difference when you're trying to get a 53 foot trailer with the tandems all the way back under a bridge where the road is U-shaped immediately under it. But it's funny, CA, generally speaking, is more truck friendly than most any other state as far as navigating roads and turns with their relatively new wide[er] roads designed with 75+ foot long truck traffic in mind.

But, I think the true intent of the setting is to spread [gross] weight across a wider span ... but not "too wide". But seem very nit-pickey to me, regardless when you think about it.

http://www.fhwa.dot.gov/reports/tswstudy/TSWpaper.htm

... provides some pretty good information on King pin setting and "bridge laws"

Originally Posted by Pmracing

I had always wondered why they would want the tandems so far forward in CA. Wouldn't it be better to have the weight farther apart as the trailer rolled across the bridge.

Then it dawned on me that the law was not for going over the bridge, but under!

Imagine the parabola that is the road surface under a bridge. Picture your trailer stopped dead center under the bridge with your drives and tandems as close together as possible.

Now picture your drives and tandems moving apart. As that happens they will be climbing up the inclines, fore and aft, which obviously is going to raise the trailer. At a certain point that will, as we have all seen, transform the bridge supports into a can opener!

So am I correct that bridge law is for going under bridges, not over?

Mikeeee

Re-examine your statement, and you might come to a better answer.

Yes, spreading it out, helps distribute the weight for bridges better, and the opposite of that is...... closer together you can't load as much legally... or tear up the bridge/roads as much. They don't want trucks maxed out on weight. 73,000 for 5 axles with 40' kingpin setting.
The tail swing is an additional benefit in that the tighter the tandems, the less lead you need for a turn, meaning you can make sharper turns with less real estate with the tandems closer. There are some small streets once you get off the freeway.

Originally Posted by CAXPT

Re-examine your statement, and you might come to a better answer.

Yes, spreading it out, helps distribute the weight for bridges better, and the opposite of that is...... closer together you can't load as much legally... or tear up the bridge/roads as much. They don't want trucks maxed out on weight. 73,000 for 5 axles with 40' kingpin setting.
The tail swing is an additional benefit in that the tighter the tandems, the less lead you need for a turn, meaning you can make sharper turns with less real estate with the tandems closer. There are some small streets once you get off the freeway.

I thought tail swing was the back end, behind the tandems, swinging wide in the opposite direction of the turn. Similar to a dog wagging its tail.

Off tracking is the tandems tracking inside the turn compared to where the steer tires tracked.

Mikeeee

From recent experience I for once had no problem with them moved forward. Some tight turns in East LA.