The 1996 Bronco with manual hubs and transfer case (part time), the ABS 
cannot be turned off. The vehicle has limited slip differentials both 
front and rear that are the spring loaded multiple friction disc 
variety. With everything locked up, stops on slippery, the ABS does 
kicking in occasionally, but not near as frequently when 4wd not locked 
up. The limited slip does pretty much keeps things turning at the same 
rate. With slightly different depth tire treads side/side on the front, 
with the hubs locked, it will pull to one side. When rotating tires for 
the winter, need to pay attention to that. On tight corners on dry, 
locked up, slow speed, sometimes can feel tires slipping, kind of like a 
hop, probably part due to the non-constant velocity cross/yoke U-joints. 
On very slippery tight corner, locked up, might go straight, where the 
forward component overpowers the side component with steering hard to 
one side. On other hand very stable straight ahead on sheet of ice, can 
cruise on ice at 65 mph with no concern for staying on straight road, as 
long as don't have to stop the 5300+ pounds fast.
 
McReynolds, Alan A wrote:
> Indeed it is true on gravel, sand, and other loose material.  In my experience loose gravel stopping distances are more like 2x not +20%.  That experience is mostly on dry desert gravel and sand.
>
> An off-road vehicle should have a manual or automatic means of disabling the ABS system.  For example, locking any of the differentials on my truck automatically turns off the ABS.
>
> ...Alan
>
> -----Original Message-----
> From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On Behalf Of Apptech
> Sent: Friday, January 25, 2008 11:08 PM
> To: PIC List
> Subject: [EE]:: ABS increases stopping distance by typically 20%+ on gravel surfaces
>
> A competent authority has assessed that ABS brakes INCREASE stopping
> distances on gravel (metal) surfaces by typically 20+ percent.
>
>         http://en.wikipedia.org/wiki/Anti-lock_braking_system
> A 2003 Australian study[2] by Monash University Accident Research Centre
> found that ABS:
>
>   a.. Reduced the risk of multiple vehicle crashes by 18 percent.
>   b.. Reduced the risk of run-off-road crashes by 35 percent.
> On high-traction surfaces such as bitumen, or concrete many (though not all)
> ABS-equipped cars are able to attain braking distances better (i.e. shorter)
> than those that would be easily possible without the benefit of ABS. Even an
> alert, skilled driver without ABS would find it difficult, even through the
> use of techniques like threshold braking, to match or improve on the
> performance of a typical driver with an ABS-equipped vehicle, in realworld
> conditions. ABS reduces chances of crashing, and/or the severity of impact.
> The recommended technique for non-expert drivers in an ABS-equipped car, in
> a typical full-braking emergency, is to press the brake pedal as firmly as
> possible and, where appropriate, to steer around obstructions. In such
> situations, ABS will significantly reduce the chances of a skid and
> subsequent loss of control.
>
> In gravel and deep snow, ABS tends to increase braking distances.
>
> On these surfaces, locked wheels dig in and stop the vehicle more quickly.
>
> ABS prevents this from occurring.
>
> Some ABS calibrations reduce this problem by slowing the cycling time, thus
> letting the wheels repeatedly briefly lock and unlock. The primary benefit
> of ABS on such surfaces is to increase the ability of the driver to maintain
> control of the car rather than go into a skid - though loss of control
> remains more likely on soft surfaces like gravel or slippery surfaces like
> snow or ice. On a very slippery surface such as sheet ice or gravel it is
> possible to lock multiple wheels at once, and this can defeat ABS (which
> relies on detecting individual wheels skidding). Availability of ABS
> relieves most drivers from learning threshold braking.
>
> But part of the answer is that on heavy snow, locked wheels can be useful
> because they gather up a "wedge" of snow which helps to slow the vehicle.
> ABS allows this wedge to clear every time the wheels are unlocked. The same
> can apply on sand in some conditions.
>
> A June 1999 NHTSA study found that ABS increased stopping distances on loose
> gravel by an average of 22 percent [1].
>
>
>
> _________
>
>
>
> Very interesting paper on ABS brake accident stats.
>
> No great relationship to gravel roads.
>
>
>
>
>
>
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