That trailer brake handle, how does it work?

I have a similar setup. I normally use it for the 5th wheel tug. If and when I blow a steer, I use it in hopes of saving the front wheel. If I pull it partially down, she will slow the rig without locking the trailer wheels. Thing is, you do have to know your truck.

 

If each tire has a 10" wide x 3" long contact patch with the ground, that is 30 square inches per tire.

If you have 34k on your 8 drive tires, that is 34,000 pounds on 240 square inches of surface area...or 141.66 pounds per square inch.

If you then drop a 3rd axle, even assuming 2000 pounds transferred from the steers in the process, that gives you 36,000 pounds over 12 tires which gives you 360 square inches of surface area...which is now only 100 pounds per square inch. Keep in mind that not only have you less psi on those tires, but the 2000 pounds that were transferred off of the steers in the process reduced that from 12,000 pounds on 60 square inches (200 psi) to 10,000 pounds on 60 square inches (166.66 psi).

Yes, you have more tires...but each tire is carrying less weight, so the available traction each tire has is reduced. It doesn't matter if you're talking legal loads or permitted loads, it works the same way. Heavier loads require the extra axles to spread the weight out more to lessen the impact on the road. It also allows those heavier weights to be carried within the manufacturer's specified limits of the tires, axles, suspension, and braking systems. If you aren't exceeding those limits, and there is no legal reason for the extra axles, you aren't gaining anything by using more axles to accomplish the same job.

 

You only THINK it works, because that's what you WANT to believe. Your belief, however, has no factual basis.

 

Broken down, you do have a very valid point now.

 

Knowing how to "play the whole piano" is important. Know what your truck is capable of. I use the trolley brake any time I want maximum braking WITHOUT locking up the steer tires. If the steer tires lock up as you are turning on a steep, slick downgrade then they stop turning. You go straight. That's a bad thing in a turn.

So when I face a steep, slick, downhill turn I (Gasp!) engage the Jake's on medium or low to give braking power to the drive axles and use the trolley brake to give braking power to the trailer axle. The steer tires turn instead of locking up and I can get around that curve just fine.

Decades ago I lived in Seattle. I lived on Queen Anne Hill in a house that featured a steep driveway with a turn to get to the street. I had a RWD car. When it snowed the only way to get down that driveway was to alternate between braking and coasting. Not quite quick pumping action. When brakes were applied the car went straight. Release the brakes and it turns. In fresh snow you leave tire tracks. Somewhere I have a photo showing the tracks down that driveway, not digital.

 

All these replies and no one has said what the Johnson Brake was designed to do. Just a few short years ago, trailers didn’t have parking brakes. You had to hook your airlines up, apply the Johnson bar to get underneath the trailer and do a tug test. You really don’t want to use it for braking as it isn’t properly proportioned to deal with modern ABS systems. And unsafe. I have seen many axle sets removed from a trailer by the operator using the Johnny bar as a brake.

 

But alas, your scenario assumes a whole lot:

1. It assumes that all tires have the same traction
2. It assumes the road conditions under each tire are identical
3. It assumes all tires are given the same amount of brake when the pedal is applied.

You are only considering weight vs. friction of the tire across the ground and not taking other variables into consideration. More tires in contact with the ground gives you the same amount of braking action as fewer tires but can give you better braking under less than ideal conditions because you have more contact patches with the ground meaning you have more chances to get the rolling resistance you need to stop your vehicle.

If what you said would be true, than the fastest vehicle to stop on the planet would be a unicyclist and we all know that's not true. So its FAR more complex than whether or not you add a couple of extra tires on the ground.

 

Can’t wait for the response.

 

Re-read my post. You missed the point completely, and your entire post is utterly ridiculous.
#1 Tires are a constant variable. Yes, some tires have a more efficient tread pattern to offer more traction than another less efficient tread pattern. Are you changing tires when you drop that 3rd axle? No. The tires that WERE on the ground are STILL on the ground, only now they are carrying LESS weight and therefore have LESS available traction. The drop axle is most likely equipped with ribbed tires, which doesn't help the argument you are trying to make...but even if they DO keep lug tires on the drop, they are most likely the same as what is on the drives anyway, which also negates your point.
#2 The road condition doesn't have to be identical under each tire, only similar to the condition it would have seen in the opposite scenario. This, too, is an irrelevant point because when a tire carries less weight, it has less traction available to it, and is therefore more likely to break traction. It doesn't matter if you're on ice or clean asphalt, the tire with less traction will break traction first.
#3 Brake adjustments aren't altered just because an extra axle was dropped. If your front left drive had a 1.75" stroke before, it'll have a 1.75" stroke after.

And in less than ideal conditions, weight is your friend. A tire rolling along with 144.66 psi of downforce will squeeze more water, snow, mud, or whatever else is between the rubber and the road into the tread grooves to be channelled out of the way than will that same tire exerting only 100 psi of downforce. That means those extra wheels you have beneath you actually INCREASE your likelihood of hydroplaning. Don't believe me? Jump in a 2wd SRW pickup truck on a snowy day...or even just a wet field...and drive around a bit. Then climb into a similar truck, only difference is this one is a dually. Where that SRW truck went with ease, the DRW struggles...and where the SRW struggled, the DRW gets stuck. Why? The available downforce is spread out over too great of an area to get the tires enough traction to move you.

And as for your unicycle remark, that's just as absurd as the rest of your arguments...even leaving out those "other variables" you're so fond of.

 

I'm simply amazed at how many people didn't pay attention in science or math class. This entire discussion ought not be necessary, because it is all quite rudimentary.