Steer tires blowing going downhill or on a curve?

Note how a typical 3030 air chamber has a diaphragm on the spring side that is the same size as the service brake?

Note how the typical 3030 air chamber spring brake starts start to apply real pressure at ~20-45 psi?

Ever drive a truck with a applied brake pressure gauge? If so you would notice that light braking is at around 40 psi, heavy braking is about 80 psi with panic stops above 80 psi.

Ever drive a city transit bus with a door interlock or any other heavy vehicle with a winch brake? Typically those add a BVA-85 to the foot valve, which applies 85psi and is much stronger than the spring brakes even if you exclude the non-spring brake axles. At 85 PSI which the above part name alludes to.

Pulling the yellow knob will cause you to slow quickly but less quickly than a panic stop.

When I use to sub for school bus our training required us to do this every single year.

Putting way too much faith in how strong those parking brakes are will get you in trouble both when parking and when you need to use them in an emergency. You will stop like you have about a 20-45 psi applied brake pressure with you steer axle brakes out...it is not some violent super fast stop.

 

Here is a diagram.


B: Emergency diaphragm
D: Service brake diaphragm

The most common air chamber size on drives and trailers is the 30/30, which the spring brake diaphragm is 30 square inches and the service brake diaphragm is 30 inches.

Basically the PSI you first start to feel your spring brakes come on is the max spring brake application pressure equivalent on the service brake side.

Being either afraid to pull that knob, or pulling it too late will end in disaster. If you are moving down hill let's say and you lose your brakes, if you accelerate to a speed that relatively light braking won't stop you, neither will your spring brakes.

Don't believe the tall tails about people being violently thrown by pulling the yellow knob. When you are light you may flat spot your tires, but when loaded it will be a fairly gradual stop.

 

@seagreg I’m not used to driving crap equipment like you because I consider 45 psi applied brake pressure a hard stop. I was happy you admitted that the stunt of pulling the parking brake at 30 mph might flat spot the tires. You do that stunt in my truck I guarantee you will ruin tires. Your bus comparison is apples and oranges because the unladen weight is a higher percentage of loaded weight.

 

Pulling the parking brake may skid the tires on a bobtail,

I did it all the time on my trucks and it never skidded the tires with a trailer , not even an empty one

y’all should try it in a parking lot some time .

 

I suggest reading 49 CFR § 571.121, and noticing that a brake application of 45 psi at a heavy weight being dangerous would make your "non-crap" equipment unlawful to use on public roads. Even bobtail, a brake application of 45 psi causing you to lose control would also be illegal to drive on the road.

You may not use more than 20 pounds day to day, but as others have said, bobtailed this may happen, but look at the tables on 49 CFR § 571.121 related to permitted stopping distances and the emergency brakes,

A truck that can't deal with 45 pounds of applied pressure is the unsafe truck.

 

With the note that if you pull it at about walking speed it may stop abruptly, but with a little speed it will stop in a controlled way.

Emergency brakes are designed to stop a vehicle in a controlled fashion, and in the case of a blowout of a steer tire, as they don't have spring brakes it has the advantage of not using a tire that may have very little sidewall left to do so.

 

I'm with you. Just because the pop out of the valve may be down to 45 psi, that doesn't mean the springs won't activate the brakes at much higher pressure. Most well maintained trucks I have driven, the brakes won't release until 70-90 psi, nearly double the 45 psi he is suggesting. Performance standards for service brakes, notwithstanding, spring brakes activated at 70-90 psi will lock up brakes if they are properly adjusted. Likewise, Old chambers, even those properly adjusted may not hold a trailer tire from rolling when trying to slide the tandems.

 

There is a difference between service applications of 45 psi, vs spring brake at 45 psi. And just because the button pops out at 45 psi, doesn't mean the springs brakes aren't starting to hold at a higher pressure. I have had a dryer leak from the Inlet side, before the check valve. Every application of the brakes decreased pressure in the system, but there was no leaks. I took it to the shop, and the brakes began to drag as pressure fell below the cutout, and it locked up about a mile away from the shop as pressure fell past 70 psi. The yellow button did not pop out.

 

Ok I don't have a dog in this fight but on more than one kw can over I've seen like 3 or 4 knobs for the brakes and specifically remember one being referred to as an emergency brake.

I know things change and am not saying one way or the other because I really only have a tiny bit of experience.

I know at 20-30mph it won't cause you to skid even with a light load. It will do a fairly abrupt stop tho.

Instructor pulled the knob when another student driver tried to run a stoplight

 

Note I said, drags, not pops.

And no, while the spring takes 70psi to fully compress, the pressure that the spring applies to your brakes is exactly related to the pressure that you feel that drag against in a 30/30 chamber.

The energy required to compress the spring enough to prevent dragging is exactly equal to the same pressure on the service side with the spring breaks released.

It is basic physics with both air chambers having the exact same area of 30 square inches.

This is also related to why winch brakes or working brakes only apply 85psi, so that when they are used with the parking brake on the effective pressure doesn't exceed 125 PSI, which is more than the design pressure.

The air pressure that the knob pops at is often below that 35-40 PSI, and you can feel the spring break dragging if you push in the knob before you build pressure although a check valve will typically hide that in a pressure drop so you will typically only notice it when building air after the truck has sat for a while.