I'm adding a tag axle to a small hook lift truck I'm building. I've done some googling but haven't came up with some clear logic on how to plumb the brakes of the tag to the service brakes of the truck. Can anyone illuminate the issue for me/
One way to do it would be to plumb them in after the ABS modulator valves as shown here with tees. And then straight on to the tag brake chambers. This way if the ABS saw that wheel end locking up it would release the pressure to the tag axle brakes as well. This was commonly done with tandem axles.
Interesting, my assumption was I needed to connect the tag in front of the ABS. Connecting there would/could/should mean I don't need additional quick release valves, correct? T's with plastic lines back to and down the frame then back to hose for the drops down to the tags? Thanks for the idea!
You could connect before or after the ABS valves. If you connect before the ABS valve then the tag will stay locked up in the event of a sliding and ABS modulating rear wheel. But then you don't have to worry about left/right. One thing I would be concerned about is the ability of the tag chambers to release quick enough. So I would not make hoses to them too long or too small. Does it have a quick release valve for the service brakes or a relay valve? I can't see from the picture.
When you say tag axle is it a stationary or lift? I see the truck is a Freightliner. Truck Shop
I'll add a quick release valve, not worth the hassle of worrying about. I think it has a relay valve, but I can't swear to it. It's a lift axle, the truck is an International 4700.
Wow those crossmembers looked like FRTLNR. In any event plumb the service after the modulator. Most tandems are 2S-2M Truck Shop
Only thing is if it has ABS based traction control you might want to do something different. Traction control applies brakes to the wheel end that spins and you do not want brakes being applied to your tag while trying to get out of something.
You won't need a quick release valve on the tag it will exhaust through the modulator valve. And tags don't require sensors or ABS. It could have traction control but the tag won't effect it. Normally traction control is used with 6S-6M ABS. Just tee those service air hoses and run them to the brake chambers. Truck Shop
Intersting point, Birken. Truck Shop, why won't it matter if the traction control applies brakes to one side of the tag axle if the drive is spinning?
Because it is activating so fast on the spinning wheel power shifts to the opposite axle, the tag will hardly be affected. If your standing near you can hear it activating rapidly it never actually stops or locks any wheel from turning. If it did it would cause major issues. Truck Shop
If your out in the mud spinning around, you should have the tag raised up anyways (so it wouldn't matter if the brakes come on), to put all the weight on the drives.
Of course. I can think of times on pavement the tag could hang up the truck even lifted, although I’m not terribly worried about that.
Nice looking work though. Truck Shop
Thanks, that's high praise coming from you! Tomorrow should start to the final 20% of the job AFTER the truck gets moved out and the shop cleaned up! The tires/wheels for the tag axle are supposed to be here Wednesday so I can get on mounting the fenders and rear lights, the container is supposed to be ready Thursday or Friday. The tag axle's lift bag hits the bottom of the hoist, preventing the last ~2.5" of up travel. If the tag axle's tires make contact while lifted, nothing will keep it from raising the rest of the way and bottoming out on the frame.
I agree completely but when I give something to somebody else, or give advice, I try to cover every base.... Does this thing even have ABS traction control enabled anyway? Of course now that we have discussed it, you might find it desirable to add. One valve and a couple of wires and a switch was all it took on the one I did.
It has a light for it on the dash that is illuminated (green). I'm curious, how did you add traction control so simply?
Well it has been about 15 years ago so memory is not so great. Pretty sure it was a WABCO ABS module and yours looks like Bendix so not sure if any of this is applicable. But I think if the light flashes when you start, it has it enabled. I was reading the manuals and discovered that if it sees an ABS apply valve on the computer output, it is enabled simple as that. I also had to wire the light in the dash and a push button to disable so if you really want to spin the tires, you still can. (It will send a CAN command to cut throttle if both sides are spinning.) I had to scrounge up two Deutsch pins to populate the empty spots in the connector for the valve, plumb the valve (treadle in - tank air in - delivery to relay valve), and wire the dash. I remember doing something to verify it had control of the throttle (idle would drop about 100 RPM for a second). Then I took it to a steep driveway, dumped a pile of ice in front of one tire, drove up on it, stopped and then mashed the throttle (automatic). The tire spun about half a rev, then you heard some air noises and the truck climbed right off the pile of ice. You could do the ice test yourself if you think it already has it.
Great recollection! I could also lift a side and see it pulls, great idea. I've been lamenting the lack of a locker in the rear, although so far I've only driven the truck home, have never worked it.
Let me add. The way it works is by having an electric air valve ahead of the relay valve or quick release valve, whatever you have. When it wants to apply one wheel brakes it first uses the ABS modulator valve to lock off air to the wheels that are not slipping. Then it applies air with the additional electric valve to the relay valve and because of the lockoff it only flows to the wheel that is slipping. Then it modulates the air to make it do what it wants. IIRC there were a couple of options for the additional valve. One was a big chrome cylinder, that was the one I used mounted under the cab near the treadle because it was easy. Another is part of the relay valve. Less plumbing and mounting but more wire and possibly more cost. It's been a long time. I did not drive that truck much in bad conditions so I don't know how well it really works. But from my brief testing it was very impressive. But I like the idea of it over or in addition to an air diff lock. The air lock always pushes you in a straight line and if you are on a narrow road and throw it in, you can't get it out without turning the opposite direction or reversing. If I had to order a truck I would have both systems.
Biggest issue I can see in parasite plumbing the tag off the drive is the extra volume from and to those two chambers causing a ABS fault or not fully /applying/releasing soon enough fast enough. There is a reason each axle has its own QR plumbing.
Just to update this thread, I installed Ts behind each modulator valve and ran lines to each chamber. It’s working great! The traction control light is on (green), I’m not sure if that indicates a problem or not. It doesn’t seem like the traction control is working though; I need to get it scanned.