this post was submitted on 11 Jun 2024
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This seems counterintuitive. I would love to hear why.
Most of a tractor-trailer's stopping power is split between the trailer brakes and the tractor's drive tandems. If there is not enough weight on those axles, the tires can't grip the pavement properly. If I apply too much power to the brakes the wheels can start bouncing or just lock up and start skidding if the ABS system is acting up.
Most tractor-trailers you see on the road in the US are designed to weigh 60,000 to 80,000 lbs (~ 27,000 - 36,000 kg). For comparison, a Honda Civic weighs roughly 3,000 lbs (1360 kg). Every system on the truck is designed around moving that amount of mass safely. With an empty dry van trailer your looking at closer to 30,000 lbs (~ 13,000 kg). Makes a difference in performance. Ride is rougher, takes longer to stop.
Thank you. That's fascinating.
In the age of computer-controlled ABS and brake assistance systems, that just sounds like poor programming.
There's no reason why the computer shouldn't be able to take current weight into account and deliver more braking power to the tractor when the trailer is empty.
Some probably do, tech has advanced quite a bit since I started driving in 2008, but the newer tech tends not to be installed widely when it first comes out due to how unreliable tech becomes under the working conditions that are normal in the trucking industry. Fleet owners want their equipment on the road making money, not in the shop costing money, so they tend to wait till a tech proves itself to be reliable. Plus upgrades costs money, so they tend not to happen till a unit is replaced with a newer model, which can take a while.
Most large companies in the US have an experimental fleet where they try out new tech first, before they roll it out to the rest of their fleets. They are looking for cost effectiveness, reliability and driver response. The smaller owner operators, which make up the bulk of the trucking industry, tend to follow (slowly) after them. And as old as the trucks are, the trailers are often even older. While most trailers in my company's fleet are less than 3 years old right now, the oldest trailer (now mostly used for hauling pallets back to Chep) was built in 1992 according to it's data plate. If it's ABS system is newer then 2008, when it was last active in the fleet I'm a monkey's uncle, and I'd pay long odds it's still the original system from 92.
Great insight, thanks!
Is ABS a thing with air brakes?
Not only a thing but required.
Sounds like you’re talking about icy or wet roads. I’ve never had a trailer do that on dry pavement and I can definitely stop faster emptying than full.
I really wish that were entirely the case. The distances I quoted came from safety trainings I’ve had to take over the years. Given my personal experiences during that time, I think they were from before ABS was mandated. And I had a lot of ABS failures when I was OTR and few close calls as a result of those failures. That’s one of the reasons I chose to switch to running a yard truck 5 years ago. Far less stress.
When ABS failed on dry pavement and I needed to stop in a hurry, the affected tandem would tend to lock up and bounce along the ground. Nerve racking and scary when there’s traffic in front of you, but not near as bad as on wet or icy roads. The sheer terror of feeling one of my axles start sliding under me.
If I had one word of advice for drivers new to the industry, it would be to drive as if none of the safety systems on the truck and trailer exist because in my experience they will fail exactly when you need them.
But when they do work they are f-ing magical.
I would imagine it has to do with traction and ability to apply braking forces without skidding the wheels.
Even in a pickup truck, it's easy to skid the rear wheels (antilock brakes aside) with the bed empty because the brakes can easily overcome the traction of the tires. This is why pickups have height sensing proportioning valves.
That makes sense.