Curious if there is some advantage to having short(er) inner / outer bridge lengths?

Hello Again -

As a quick reintroduction, I am a Research Engineer at the University of Texas - Center for Transportation Research. I am trying to learn more about truck efficiencies in connection with a project I am working on to evaluate potential new Oversize / Overweight corridors serving coastal ports in Texas. As some of you may know the Port of Brownsville can sell OS/OW permits for $30 each that permits a truck with up to 125,000 lb. GVW to operate on State Highway 4 / State Highway 48 from the Port of Brownsville south to the Texas / Mexico border.


I am studying the specific permits that are being sold which document the axle weights and spacing. I can calculate the inner bridge length (distance from the center of the first drive axle to the center of the last axle on the trailer) and the outer bridge length (distance from the center of the steer axle to the center of the last axle on the trailer) based on this information.


These permits allow many different truck configurations and axle group weights, but the most common are:

5-axle Class 9 truck weighing about 90,000 lb GVW

6-axle Class 10 truck weighing between 105,000 - 115,000 lb GVW.

What I am curious about is that I'm seeing relatively short inner and outer bridge lengths on a number of Class 9 and Class 10 trucks.

Texas legal limits are minimum 51' outer bridge and 36' inner bridge. I am seeing Class 10 trucks with 40' outer bridge and 28.3' inner bridge lengths ...... I can't figure out what advantage a trucking company would gain by operating with short bridge lengths (these trucks are legal because of the permit, so no question there).

Do shorter inner or outer bridge lengths make it easier to maneuver the truck in tight spaces?

By the way, nearly 99% of the trucks originate in Mexico. I've seen some photos of these trucks and some are cab over and others conventional tractors. I have heard that cab over's are easier to maneuver in urban areas, but I almost never see them along IH 35 in Austin. I have also heard that the cab over design limited the size of engine that could be fitted and that truck driver's prefer the conventional rigs for that reason and also for the additional crash protection.

In Texas, logging trucks, milk transporters and a few other specialty trucks are also allowed by state statutes to run with shorter inner and/or outer bridge lengths.

Your comments about why short inner or outer bridge lengths would be preferred by trucking companies would be appreciated.

Thanks very much,

Mike

 

Hello Groo

Thank you very much for the insight. I had not thought about the obvious fact that a shorter truck will be lighter.

I know that though trucks manufactured to operate in Mexico often look just like trucks that operate in the US, their curb weight is about 15,000 to 20,000 lbs. heavier due to thicker chassis rails, heavy duty axles and suspensions necessary to operate on rough Mexican roads. Thus, as you pointed out, making the truck shorter will lighten the curb weight of the truck and increase cargo weight.

Texas laws allow logging trucks and milk tankers to operate with shorter bridge lengths -though again, for the milk tanker I can't understand how they could benefit since shorter inner bridge would mean shorter tank = less tank capacity. However, I have stopped at one dairy trucking operation in north Texas and talked with one of their operators and looked at their trucks - the trailers did not look to me like the trailers were shorter than typical 18 wheelers in Texas (53' trailer is most common, though Texas was grandfathered to allow up to 59' trailers which some grocery haulers and cube-out operations take advantage of. The dairy truck operator indicated the tank capacities for most of their trailers was about 6,500 gallons.

Thanks again, I appreciate your comments.

Mike

 

Groo -

Thank you very much - again Obviously, I haven't been thinking the entire problem through, else I wouldn't keep missing the obvious - I had only thought about tank length - not the diameter.

I was once on an ATA Truck Maintenance Council (now called by a different name) Task Force that was studying tire impact on pavement damage. At that time, wide base single tires were being seriously considered by US trucking operations due to the reduced rolling resistance and resulting increase in fuel economy (about 3 - 4%). I understand that about 70% of EU trucks run with wide base tires.

I listened to some of the TMC fleet operators including a major national petroleum hauler talk about the advantages of wide based tires in terms of the tank and trailer design. A wide base tire and wheel is narrower than a dual tire assembly - thus there is additional room provided between the back of the wide base tire and the tanker chassis rails. The fleet operator indicated that switching to wide base tires not only reduced their curb weight (a set of wide base tires and aluminum wheels were about 1,800 to 2,000 lbs lighter than a set of dual tires and wheels as I recall); but it also allowed this company to consider whether to set the trailer chassis rails further apart and lower the tank - lower tank = lower center of gravity = reduced roll over potential = greater safety. Or they could lower the tank and increase load due to the weight savings of the wide base tires and wheels at about the same level of safety as before. In this case, the lower cg of the tank + additional liquid weight balanced out to give about the same rollover potential.

The main downside to the wide base tires, as I recall, was their weight. They were much heavier than a single tire of a dual tire set and therefore a trucker could not possibly change the tire if there was a flat - and since there was only one tire in a specific wheel position, there was no way to 'limp in' to the next town on the remaining good dual tire if one went flat.

Your comments brought back this memory - and it makes perfect sense.

I do see some petroleum haulers running with wide base tires in Texas - and some grocery store chains also run wide base tires for fuel economy and added cargo. Previously one big concern with switching to wide base tires was that the state legislature would change the laws and reduce the pounds per inch of tire width rules. This was back when a wide base tire was running with pressures over 130 psi which was a concern for asphalt pavement rutting. Also, some truckers in the western US were singling out their dual tires as if this was the same as running a wide base tire. This caused majore damage to some roadways and resulted in those state legislatures in reducing the pounds per inch rules. However, Texas changed our laws in a different way and DPS uses the tire manufacturer's load rating on the tire to check load compliance. The Motor Carrier Division still uses pounds per inch of tire width when permitting loads oversize / overweight loads (650 in/tire width) - but this is only for permitting purposes.

In any case, thanks very much again. I'm really learning some interesting insights from The Truckers Report forum.

Mike

Have a nice Sunday -

Mike