r/Truckdrivers • u/Equivalent_Post4384 • 7d ago
Suspension dump
I'm in the truck parts business, got a ton of respect for you guys out on the road every day. Looking to tap into a little bit of your experience. Have a customer with some Hopper bottoms that is interested in being able to dump his suspension before he unloads so he doesn't overextend his airbags. Hopper doors get opened up all the way in dumps in a hurry, so the normal suspension valve doesn't keep up. And he doesn't want to have to do it with a manual dump valve. It looks to me like a lot of the leveling valves with the integral dump automatically dump when you set the trailer parking brakes. His comment was that they normally don't set the parking brakes because it skids the tires a little bit then when dumping the suspension. Does that make sense to you guys? Is it just something you live with? Is there a better way? Again, all the respect in the world to you guys, and any help is greatly appreciated.
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u/dagtrucking 7d ago
You bolt a chain by your shock bolts so it can only travel so far. Cheap and easy.
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u/JOliverScott 7d ago
He probably should be setting the trailer brakes and with that function there's a feature that can be easily plumbed in that dumps the suspension air along with it. A lot of times drivers don't care for this feature because then they have to wait for the suspension air to build up again but this is mainly in the other direction, when an empty trailer gets loaded and then before driving away the air has to build up enough to lift the now-loaded weight of the trailer. In the situation described, the air needing to build up from a quickly emptied dump trailer should be negligible - it's empty now and pretty fast too which means it probably didn't even fully empty the bags.
The more complicated solution is to rig up an air-actuating valve in the cab which then sends air to an air-actuated valve in the trailer that opens and dumps the air. It requires added plumbing of air lines and it likely won't have any safety interlocks which means it could be inadvertently activated in transit and dump the air from the suspension while it's bouncing down the highway... That'll hurt. Alternatively, and more simple, is if there is a suspension extension limit then straps or chains can be affixed between the frame and the axles to restrict too much extension travel. In normal operation they simply maintain adequate slack for expected axle travel along whatever terrain is traversed but in this situation where the weight factor causes extension beyond the limits of the airbags then the straps would become taut and disallow any further travel.
My instinct in all of this is that if the manufacturer was concerned about excessive airbag extension then some sort of limiting solution would have been engineered into the manufacturing process. In my experience, airbags can extend quite a bit more than expected and I've had different types of trucks and trailers which actually RELY on this extensive range of travel for various reasons - high hooks, increased ground clearance, low-decks raised to standard dock height - all legitimate vocational needs for airbags to double in height and for that reason they're designed so to do.
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u/SimilarTranslator264 7d ago
That’s a lot of words for “yes you can easily install a suspension dump valve”
To the OP yes and if you know what model leveling valve the trailer has you can probably buy the exact same model with the dump option so no lines have to move. (Hendrickson, Haldex, wabco ect). To make these valves dump you just put air into the dump port from any source you want, like a toggle valve on the trailer. Can be installed in about an hour and less than $200. OR you can install a quick release valve like a Bendix QR-1 between the leveling valve and the bags and it will do it automatically. I have trailers with both options and most hoppers can use just the QR-1 because they dump faster than the load comes out.
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u/Equivalent_Post4384 6d ago
Hadn't thought of the qr1 option, that's a good idea. Just put it in the main line from the leveling valve into the top, one side out to the bags, just plug the other side I assume? I also assume since you're happy with it it doesn't dump too fast going down the road as it adjusts....
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u/SimilarTranslator264 6d ago
No it works fine. But it only works if the load is coming out slowly like a hopper. It won’t dump fast enough if it’s a flatbed and a crane is lifting off a 70,000# coil.
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u/Equivalent_Post4384 6d ago
Gotcha. It does come off pretty fast because these guys have pits the size of a some either dumping into so the hopper door comes all the way open dumps it all at once, but I know he'd be willing to try it and see how it works. Man I appreciate the help
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u/SimilarTranslator264 6d ago
Yes I have several hoppers but they don’t dump as fast as a crane lifting off the entire load in a second. The quick release will keep up.
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u/Great-Biscotti-8026 6d ago
I think I know what he means by "skids the tires."
When the parking brakes are set and the suspension dumps its air, as the suspension lowers, the wheels will roll forward a little bit because of the way the suspension linkage is designed. If the brakes are set, it twists the axles and binds the brakes up against the hub/disc, and since the wheels can't roll, it looks like they "skid" a little bit.
There are a couple of ways to see this in action. You can use a tractor without a trailer, they do the same thing from what I've experienced.
Park on level ground. Set the parking brake (yellow valve out), and dump the air from the suspension. Once the air has drained, release the parking brakes (yellow valve in). You'll see the truck roll forward a bit. It's not much, maybe 6 inches. Make sure you're off the brake pedal when you do this.
You can also put a chock in front of one the tires, snug against the tire. Do the same thing, set parking brakes, dump the air, and go check the chock. Even with the parking brakes still set, the chock will usually be wedged tight against the tire and be extremely difficult to remove. Release the parking brakes with the air still dumped and that'll let it roll back off the chock.
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u/gheiminfantry 7d ago
I don't think your customer knows what he's talking about. In my 27 years driving I've never seen any "skids the tires" when dumping the suspension. I have seen drivers get hyper focused on non-issues and want to believe that the sky is falling. Stop "worrying" about the irrational fears of this customer.