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Old 05-10-2021, 01:32 PM   #1
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Can you lock up your airbrakes?

Still getting a feel for a class A RV...

Clearly, takes longer distance to stop than a car...

Brake pedal feels much different than on a car with hydraulic brakes...

Slowing down at a regular pace is no issue... Or even at a moderate/fast stop.
Brakes work fine, no squeals, bus stops.

Tried some "panic stops." Going 40 mph, quickly applied the brakes. Stopped faster, brakes did "squeal" some, but never got the sense they were going to lock up. And I was pressing pretty hard...

On a car, the brake pedal takes little pressure to push through the full range, until the wheels lock up. On the air brakes, seems like the response is proportional to how hard you press the pedal - ie, takes a lot of force at the end...

My truck driving buddy drove it, says it feels normal...

Normal? Can you guys easily lock up your air brakes?

Pads by the way look fine, PO had freighliner adjsut the brakes a year ago...
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Old 05-10-2021, 02:08 PM   #2
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Ive never tried to lock mine up but I believe they would if i push hard enough. Ive had a couple panic stops before with no lockup and it feels like it could throw you ( and everything else not bolted down) to the windshield if you weren’t belted in. Agree the response is proportional to pressure applied.
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Old 05-10-2021, 02:09 PM   #3
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First, sounds like you might benefit from reading the air brake section of a CDL driver manual. You can get one from your local DMV's website.

Assuming that you have automatic slack adjusters on your coach, they should not need to be adjusted. If the Freightliner shop had to adjust them, either they are not functioning properly or you did not make enough firm pedal applications to cause them to self adjust.

One of the things I was taught when going through training to drive semi as to do the daily pre-trip brake inspection, including the pump-down test. This portion of the brake check (where you firmly press the brake over and over again with the engine off to test the low-air warnings and automatic parking brake application) help to ensure that the auto slack adjusters do their thing.

My air brakes will lock the wheels if I want them to, but it takes putting the pedal all the way down and doing so suddenly. Not something I want to do though as you have no control over a locked up wheel, especially a front wheel. The effort on my pedal stays pretty constant over the full range of travel though, so I can't explain why yours requires more effort towards the end of the travel. Perhaps you have an obstruction in the pedal travel from carpet or a floor mat?
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Old 05-10-2021, 02:30 PM   #4
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First find out of you have anti lock brakes.

If you do, then no you can't lock them up, but you will hear the antilock spitting like a paint ball gun.

If no antilock, you'll need to do a few hard stops to burnish the brake shoes, before they will.
Gentle driving and engine/exhaust brakes don't let the brakes work hard.
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Old 05-10-2021, 03:09 PM   #5
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Husband has only cleaned everything off the counters one time and the tires didn’t lock up. Did barely get stopped in time although he’d put it on the shoulder of the road just in case.
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Old 05-10-2021, 04:01 PM   #6
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Rolling friction is higher then sliding friction on tires.

You don't want them locking up and skidding.
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Old 05-10-2021, 04:11 PM   #7
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As you have noted, Air brakes do work and feel different than conventional hydraulic brakes on a car. It takes a little different technique to lock the wheels although I don't recommend it. If you have your heart set on it here's what you need to do. Get the coach up to speed, 35 or 40 should do. Take your foot off the throttle and SLAM the brake pedal down. The quintessential "Panic Stop" will lock the brakes if you do not have anti-lock brakes. Attempting to ease down on the brake pedal normally creates too much heat and will stop the coach but will not allow the wheels to lock up in most cases. On a wet road, all bets are off.

This is not something I recommend you do! Locking the wheels is a BAD THING and does not slow the Coach, it actually speeds the coach up, (Skidding tire has less friction than a rolling tire) and you can and will lose control. Not to mention all the crap that will go flying inside the coach.

One last thing: Brakes NEVER get you out of trouble. Throttle and steering get you out of trouble. That is why you see so many skid marks at accident scenes. Much better to drive around it, than to slid into it.
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Old 05-10-2021, 05:20 PM   #8
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First it is possible to lock up anti-lock brakes .

I've done it on a car , wouldn't even think about it in my coach.

Anti-lock ,operation depends on the system controller seeing differences in each wheel speed sensor , if three wheels stop turning and one doesn't then you'll get anti-lock action to release brakes on the locked up wheels .
Lock up all four (6>18 ) at the same time ... well, we've all seen the marks on the highway.
Appling the brakes then increasing the pedal pressure to the max , will give an anti-lock system time to function , any wheel that looses traction, and locks up will be released and reapplied to give the best braking power .

When the wheels are locked up and skidding , you're sliding out of control , floating on liquid rubber , flat spotting very expensive tires .
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Old 05-10-2021, 05:26 PM   #9
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Thanks for the info... no antilock brakes...

Seems all is normal...

Just like immersing myself in any new hobby to learn as much as possible upfront...

Have no desire to slam on the brakes to infuse lock up...

Already read all the air brakes procedures, took all the online air brake dmv tests just for giggles...
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Old 05-10-2021, 05:36 PM   #10
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ABS keeps mine from locking up, even though I have all wheel disk brakes.
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Old 05-10-2021, 07:42 PM   #11
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Abs they shouldn't lock up
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Old 05-10-2021, 08:01 PM   #12
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Why don't you tell us what you have?

Anti-lock brakes have been around for some time!
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Old 05-10-2021, 08:09 PM   #13
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Everything you want to know about air brakes: https://www.fmcsa.dot.gov/carrier-sa...y-technologies


Air brake test: https://www.myrv.us/Pgs/RV/Air_brake.htm

Air drum brakes require significantly longer stopping distances vs air disc brakes, as explained in the FMCSA document.
Yet another reason not to drive faster than your guardian angel can fly.
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Old 05-10-2021, 08:28 PM   #14
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I am not sure if asking about locking up the brakes is the right question. However you are doing the right thing by doing some panic stops and getting a sense of you stopping capability before you need to do it for real. You did not list your rig but stopping a 30,000 lb rig is a different ball game than a car, no matter how good the brakes are. And you might as well get used to and be prepared for, the folks who do not want to ride behind a big rig and pull in front of you cutting off your stopping distance. It happens to often. I would also recommend you do your brake testing on a corner (when the wheels are not yet completely straight) and also in the wet. Last point, vehicle dynamics for any type of vehicle dictate that they are most efficient when you ask your tires to do one thing at a time. The shortest braking distance is straight line braking. If you have to take evasive action and swerve you will have to come off the brake some or you will not be able to steer, if you do not have ABS. All of those types of contingencies should be practiced and at least given some thought about your plan of attack should you find yourself in a panic.
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