With a road tractor and lowboy hooked up, what is the correct braking ajustments? Should truck brakes come on at same time as trailer or slightly before or after trailer? Lets assume an average weight load of 40,000lbs on the lowboy.
In theory, the adjustment of the slacks (if that is what you are talking about) should have little effect on the order of brake application. They should all be adjusted the same, that is, 1-2" for a type 30 chamber by the book. Some of the relay valves have different "crack pressures" to try to manipulate timing somewhat. I am trying to remember, years ago in an air brake class they talked about a company that ran triples and had 3 different crack pressure valves on their trailers so there were A, B, and C trailers to try and get the rearmost trailer to come on first and keep the combination in line. As you can imagine this was a nightmare to keep things in the right order and Bendix made a valve that had a "zero" crack pressure to try to address this problem and get them all to come on at the same time. With the truck, the way they come from the factory they do air brake balance tests and measure application speed very carefully, and try to get the rear coming on first because the brakes are bigger, traction usually best, and also they can help snub the front tires into the ground for better traction before they start to apply. I'm not a lowboy guy so I'll leave the specifics to somebody else.
Are you looking for what is the proper stroke of the maxi cans, or if people have changed the valves to make their truck apply brakes in an order? Here the law says less than 1" travel if manually checked, and less than 2" if applied
And don't forget less than 90 degrees.
I have seen trucks with a factory setup that goes over 90 degrees. I don't know if it's right or wrong but is there any official literature that points to this? I guess the way to change it is by adjusting the clevis position on the push rod, but a little over 90° seems better than a lot under. The loss of leverage is the same either way.
Its law here in Nova Scotia. I think the idea is that as the shoe wears the leverage gets more advantageous vs going the other way. But don't quote me on that.
Proper adjustment is that when loaded the trailer brakes pull harder than the drive and steer, the drive pulls harder than the steer. When empty the steer axle pulls harder than the drive and trailer. In the old days you had to adjust this manually with a brake proportioning valve in the cab, then they came up with auto valves and now they are going towards computerised ABS systems. When I adjust brakes, I like to lift the axles off the ground and adjust till the brakes grab and then back off a 1/4 turn or the wheel turns freely. It's a good idea to get the feel of your trailer brakes by applying them at a certain speed with a certain load to determine if if they are effective, particularly a good idea before going down hills. Also heat is a good indicator of what your brakes are doing, after a good braking pull over and feel the rims for heat (don't put hand on brake drum). Loaded the trailer brakes should be hotter than the drive and the drive should be hotter than the steer. Also every axle group should be even. One of those point and measure thermo heat measuring thingies would be handy, which can also be used to check other parts of the truck for temps. http://www.amazon.com/Actron-CP7875-PocketTherm-Infrared-Thermometer/dp/B000FJR0SG you can buy the ghost hunting kit for 72 bucks
For the most part what I do the same as others . I'll snug the brakes tight then back them off a 1/4 turn on the manuals . Auto's won't back off well so its measurement . Ideally the trailer brakes should be a tad ahead but rarely ever works that way . If you have the same trailer everyday you can manipulate adjustment . The outfit I work for you can wind up shagging 6 different trailers a day . Pretty tough to adjust brakes for your truck when someone else may be doing to same thing . The tractor mount boom truck I drive weighs 33,000 but the next truck may weigh 18,000 . My truck does not even a hand valve to manually operate the trailer brakes . For the most part I snug up for a loaded trailer , Then when I know I have a problem trailer. I just back off and let traffic have a mile of room ahead of me . That gives me time to put slight pressure on the brakes and let the air kinda catch up to truck and trailer .
well that is a very good ?? the brake ajustment depends on the brake if the set up has (cheap) brakes on it you better ujust them snug if you have good brakes like the haladex GG"s then you back them off 1/4 of a turn. i myself hardly use brakes i have very good Jakes!
We just put all new drums and shoes on the tractor and lowboy. I adjusted them snug and backed off a 1/4. Put a load on and went appox. 20mi had several stopping and turning events and when I arrived at shop the trailer brakes were hot, even had faint smoke odors. Tractor brakes were less hot, still could not touch with finger and steering brakes were less hot, yet. It seems as though I needed all the braking that was there but feel I have the trailer to tight. Not really sure what to do next on adjustment??
It might be a little early yet to 2cd guess yourself , Give it at least one load before you back them off because the shoes need to seat in ..... I have yet to see a set of shoes and a set of drums that have the same arch . Keep a wrench with you , you will need it
While I agree with all above, I back off about a 1/4 turn then tap the drums with my wrench. If i hear a dull thud I back off just a touch more until I get more of a clear ring. If the s-cam bushings are worn a bit it will allow the lower shoe to rub. 3/8's of a turn usually is where I end up. I also check to see that they are all equally as warm (more or less) after traveling. Jacking an axle up an listening for shoe vs. drum contact is good, but not too practical and time consuming if you adjust brakes every week.
We actually put all new s-cam bushings in also, so I will make another haul with it and see if the trailer brakes seem too hot again, maybe they are not seated to the drums yet as Tiny says. If they still seem too hot after the next haul, I will back them off a little. Thanks
JB, I was taught when I started out, whenever I haul a load in the rain, to look at all the drums, because the ones that aren't steaming, aren't working. The job of brakes is to take the kinetic energy of the moving truck and convert it into heat, after all. You said you had several stopping and turning events in 20 miles. If you live way ut in the sticks like I do, drive a mile or two and coast to a stop, then feel them. Good Luck, Mitch
One thing to consider, is that the smaller the drum (less width and especially less diameter) the hotter it is going to get to do its share of stopping the same load. Depending on how small they are the lowboy brakes may just get hotter, i.e. you have less stops before they start to smoke than the tractor brakes, less mass of iron to do the same thing. This is all completely variable based on the combination and design of each component.
This is not criticism, more perplexed. Even with a load, in just 20 miles, seems to me that getting brakes that hot would take some extreme braking. :beatsme
Update: made another 30mile haul with the 953C on, upon arrival the lowboy brakes were not smoking at all but hot to the touch. I think now the new brakes and drums were just getting seated properly to each other. as Tiny had mentioned.
That sounds better. :drinkup
Not really, you can get hot brakes in 500m and yeah new brakes will usually stink and smoke. Don't you people bed your brakes in?
Hendrik, What is the procedure to "bed your brakes in" I am unsure of what that means. Thanks
Brake pads need to be heated up and seated on the drums/rotors. http://www.ebcbrakes.com/troubleshooting.html http://www.tirerack.com/brakes/tech/techpage.jsp?techid=85
Changing the length of the slack adjuster(center of the s-cam to the center of the pushrod clevis) is one way truck/trailer brakes proportioning is set up. The truck will typically have slacks adjs. with a different length than the ones used on the trailer.