Another thread here got me thinking. (I know scary huh?) What is it about the way semi-truck tractors are built that they can tow so much more weight in comparison to themselves than a pickup? Does the second rear axle provide huge amounts more stability or is there more to it?

Another thread here got me thinking. (I know scary huh?) What is it about the way semi-truck tractors are built that they can tow so much more weight in comparison to themselves than a pickup? Does the second rear axle provide huge amounts more stability or is there more to it?

The "Load" is acually carried on the Tandem axles of the tractor and trailer, 34K# each or more, I spec'ed 40K# tandems on my tractors.

The steer axle, although it will carry ~ 12K# Points the vehicle.

Hope this helps.

Idahofox

A lot of it has to do with the tires and suspension utilized. Remember that most heavy duty pickups are usually limited by the load rating of the tires that are used.

Oh I see, so that's why the wheels are so far back on the trailers. This puts more load on the tractor and because there's so much weight on the tractors tires it has more control over all the weight in the trailer?

I think this is making sense to me.

Loading up the drive wheels is more a matter of drive traction, ride quality, and bridge laws than control. You will see many trucks with the trailer axles moved forward for city deliveries.

Tires and axles for weight our Lowboy has 5 axles and 26 tires for IIRC 135K. You move the 5th wheel and trailer axles for weight distribution and to affect turning radius you can make a 53' trailer turn like a 48' trailer by moving the trailer axles.

Woah I didn't know you could do that. Learn something new everyday. This forum rocks! :D

and also 2000 ft lbs of torque helps for the pulling aspect

And an 18 speed

The majority of the rigs are running more like 1000 ftlbs at the flywheel. Having enough gearing makes up for it most of the time. 10 spd seems to be the most popular. City trucks do well with 9 spds.

All the load capacity is based on the axles/tires though. Typical drive axle can carry 20,000 plus and pull 40,000 or more. Load rating each tire starts around 5,000 lbs. Oh, can't forget about the suspension. Next time you get a chance, take a look. Notice the huge torque link on each drive axle. Sometimes it looks like half a spring leaf. Other times it is a rod with rubber bushings on each end.

Controlling the load has little to do with the weight of the powerhead, or even horsepower. It is all suspension with a driveline to handle the torque.

Eaton has a fairly good overview of suspension selection on there web site. They cover torque, load, and duty cycle factors to pick axles, suspension, trannys, and ratios.

The axles on the HD pickups are rated at over 10,000 lbs, but you are hard pressed to find rubber that lets you use the full potential of the axle.

And, Tractors have a big honking frame. That's also a plus for weight capacity.

I second everyone else, tires, axles, springs, drivetrain, and frame.

A typical 18 wheeler when fully loaded will have 12k on the front axle, 34k on the drivers (rear axles of the tractor) and 34k on the trailer axles. Most of them have a 5th wheel that will slide to adjust the weight on the steering axle. Most trailer axles will slide to adjust weight on the drivers, the ideal setup when loaded to a typical 75,000 - 80,000 lbs is 11 - 12k on the steer axle, 32 - 34k on the drivers, and the trailer carrying the balance. Most states will only allow 35,000lbs on a set of tandem duals as maximum weight. Since you can never seem to load the trailer "just perfect", you have the ability to move the weight around as needed. Being able to "suck the axles up" to make tight corners is definitely a bennefit, obvioulsy you can't drive that way for very long (at least not near scales) because a considerable amount of the weight will be transfered from your tractor to your trailer tires.

Not to second guess you Fingers, but actually most over the road trucks are running closer to 1,500 lb/ft of torque on average. 1,000tq is a pretty small motor, not too common in OTR trucks.

Pwdr & Fingers,

When "double nickel" speeds were nation wide, 1K tk was about average for "fleet" trucks, so we must concede that Fingers was right at one time. When the "electronic" engines with integrated computer controls could get much better fuel averages at higher tk and hp, the 55 mph national speed limit became moot, so some States took the bold lead and raised it, despited the Fed's threats to witdhold highway funds...this all settled out of court, but that is another story. Since then, tk and hp's began to climb as emissions and mpg's got better, and 500 hp and 2k tk is rapidly becoming a common spec. for fleet trucks, so Pwdr is also right.

RF

Pwdr & Fingers,

When "double nickel" speeds were nation wide, 1K tk was about average for "fleet" trucks, so we must concede that Fingers was right at one time. When the "electronic" engines with integrated computer controls could get much better fuel averages at higher tk and hp, the 55 mph national speed limit became moot, so some States took the bold lead and raised it, despited the Fed's threats to witdhold highway funds...this all settled out of court, but that is another story. Since then, tk and hp's began to climb as emissions and mpg's got better, and 500 hp and 2k tk is rapidly becoming a common spec. for fleet trucks, so Pwdr is also right.

RF

I stand corrected. City trucks are still running with about 300-350 HP torques in the 1000 range. Just ran over the powertrain spec guide for a KW. 400 min HP recommended for OTR with 1600 torque for 80,000 lb

Goes to show how often I get a new truck I guess.

I'm guessing that the thread poster is wondering why I say twice the pickup weight is a limit I use when towing. And it does have to do with the difference between a OTR 18 wheeler and a normal pickup trailer. Normally a pickup trailer has the axles near the center of the trailer load, this allows more freedom of movement of the trailer compared to a OTR trailer. In an emergency situation or an equipment failure, effectively 1/2 the trailer weight can be pushing sideways on the hitch if it tries to rotate.

The brakes are a separate but important issue. Should the trailer brakes fail (OTR trucks have airbrakes which activate if they become disconnected) you still need to stop the load with your pickup brakes. The reality is that many electric brake setups are prone to failure due to the primitive design of most the electrical systems that operate them.

You will notice that the most the pickup mfr ratings are close to the 2:1 formula anyways.