ABS in the dirt/mud in trucks

Out of interest, what are the current generation of ABS brakes like in say a Volvo FM or FMX like on dirt roads or mud?

Car ABS is scary in these situations, can be the same as no brakes (luckily you have a hand brake or can go into low range to turn it off). Are trucks as bad? Never drive a truck with ABS...

Very interested in this too, had no idea ABS was working on big brakes.
Are they full air or air/hydraulic?
Rodders will get a laugh from this but back a bit my mate and I tried to invent an ABS system for a sugar cane brake van we were building.
We fitted Hall effect wheel speed sensors to each axle and ran it all from a PLC.
Brakes were conventional rail brake blocks but with individual truck boosters on each axle set and having each booster's dump valve controlled from the PLC.
Well the idea was OK but in practice we couldn't charge/vent the booster air with any sort of accuracy and the whole show skidded wonderfully, flat spotting wheels.
Client (rightly so) ditched the lot and continued without the ABS.
On dewy rails and with a long string of cane wagons, the remote controlled braking system was an ideal application for ABS.
In hindsight we probably should have gone air/hydraulic using components from existing automotive applications.

Jeffo, from July this year every new variation submitted for a compliance plate for a new trailer has to have ABS brakes and automatic slack adjusters, and from January next year, every new trailer has to have ABS brakes and automatic slack adjusters.

Funny thing is, we bought 2 new axles a couple of months ago, then bought another 2 last week, the 2 we bought last week have the ABS sensors and auto slacks, and they are the same price.

This sounds like conventional "S" cam drum brakes and with a 30/30 booster or such?
Is there a dump valve mounted on each booster?

After just assembling an ABS system on Stuart's new trailer

Thanks for the explanation Zuffen, it sounds almost as primitive as our attempt back in the 90's but of course with much faster processor speed.
We found air was too "bouncy" to give the minute reductions in braking required to come even close to the efficiency we see in cars using hydraulic ABS systems.
I imagine each booster is supplied via varying duty cycle pulsations but really thought the dump valving would be mounted right at the booster, not from a central unit.
My 90 GTR Skyline uses a "G" sensor, wheel speed sensors plus other inputs to tell the AWD assist computer how much torque to send to the front and it's reaction speed is lightning fast, hydraulic actuation of course.

The ABS in an air brake system works by reducing the air pressure to the particular booster until the wheel begins turning at the right speed again. The system doesn't pulsate like a hydraulic setup on a car.

Swishy might not like this one, but it goes to show how effective ABS can be, although it doesn't really answer Wedgetail's question...



I would imagine that the truck would have a switch, possibly available as an option, to turn the ABS off for off road use. Have you asked the dealer?

OK so maybe they're not as bad as car ABS by the sound of it. Still can only see it as being worse in the mud/dirt. Volvo dealer said no you certainly can't turn them off. Not that he was aware anyway. Maybe if you have low range like in an AWD truck it turns it off? Big points to the KW with it being only an option anyway till 2015.

Can you remove a fuse like in a car maybe? Legalities aside...

Legalities aside, it is as simple as someone with a laptop with the Volvo service program, linking up to the truck's computer and turning the ABS off. You will find that trying to disable it by removing a fuse or trying to bypass the system will result in the truck going into limp home mode or something like that. But if you could get it turned off then you won't have it for bitumen use and that would be bad....