Feature > Four Wheel Finesse
Anti-Lock Brakes and the Trail
by Jim Allen
This is the kind of situation that can get you in trouble with ABS. A long runout and the chance to build up a lot of speed. Plus, there are holes and dips that can unload opposite tires to engage the ABS. The key here is to keep below threshold speed. The scree (small chips of stone, generally kinda flat) is unpredictable. Sometimes, it will pile up in front of tires like gravel, but sometimes it gets under your tire like mini skis and you simply slide on a sea of flat stones.
When ABS (Antilock Brake System) first appeared in 1980 and later became standard in almost all motor vehicles, it was touted as the answer to many safety issues related to brakes and braking. No more tires-locked skids into the back of stopped semi’s and no more wild spins on wet roads when a driver taps the brakes.
I’m not gonna give you a full blown explanation of how ABS works, but a few words are in order. The gist of it is that the system uses wheel speed sensors to measure the rotational speed of the wheels and a computer controls hydraulic valves to make sure that under braking, all the tires rotate at the same speed. Sometimes all four wheels are controlled individually, sometimes just the rear two and sometimes the wheel speed of a solid rear axle is measured at the ring gear instead of at each of the wheels.
Everybody knows a skidding tire produces zero traction on pavement, so keeping the tires rolling at the threshold of traction reduces stopping distances on dry pavement. On wet or slippery pavement, unequal braking side to side can throw you into a skid, so antilock brakes equalize wheel speed side to side to keep the vehicle straight during the stop. There are debates going on about how much ABS has contributed to overall highway safety, but we’ll concede that it works well on the highway, giving average drivers an edge that only skilled drivers had before. But what about off the highway?
If the ABS system in your rig has a “dark side,” you’ll find it on the trail when it tries to outsmart you. This comes in many forms. The number one complaint comes while trying to stop on gravel, deep snow or mud. In any of these situations, locked front wheels actually are an asset because they will build up a pile of material in front that will act as a “dynamic wheel block” and actually stop you a little faster.
Weight transfer situations, such as a steep downhill, can cause problems, but backing down from a failed climb can be especially spooky in some rigs. Imagine you make it partway up, lose traction and stop. You put it in reverse and start to back down. Remember that your brake bias (the front brakes get more pressure than the rear because the front tires are more heavily loaded during stops) is mostly to the front, so that when you apply the brakes to slow the rearward descent, the front tires may try to lock up because most of the weight has transferred to the rear. The ABS prevents lockup and it can’t tell you are in reverse (actually some of the later smarter systems are a lot smarter). You end up mashing the brake pedal, which has gone hard as a rock and is pulsating madly, with all your might and the vehicle goes faster and faster. A similar situation can occur going downhill frontways, though it’s not as bad because your brake bias is working in the right direction at least.
Unless your rig has the technical sophistication of the ’04 VW Touareg, a lifted tire on a downhill might fritz your ABS system into sending you down a hill with the brake pedal chattering away. The Touareg has algorithms programmed into the ABS system that makes allowances for the terrain. If your rig isn’t at this level, the trick is to keep below the threshold speed that engages the ABS system.
(Courtesy VW)
The final problem is with lifted or unloaded tires. One scenario is when you are coming down off a rock with a tire in the air or one that’s very light. When you use the brakes, the least loaded tire, or tires, will lock up and the ABS will tell itself, “gotta equalize wheel speed” and you end up rolling off that rock a lot faster than you wanted to.
The above scenarios are more a problem with earlier generation ABS systems. The newer systems are a lot smarter. Some rigs will automatically cut off the ABS system when shifted into low range. Some manufacturers have programmed “off-highway” algorithms into the ABS control unit that will allow locked tires for more flexibility, again, most often triggered when the rig is shifted into low range. Some manufacturers have reprogrammed their systems to be less obnoxious overall. Prior to ’00, some rigs even had a manual cutoff switch for the ABS, though that item has now been legislated away. Overall, the level of sophistication seems directly linked to the vehicle price tag. While ABS is now mandated for all light duty rigs, prior to about 1995, only the more pricey rigs had it standard. The question now becomes, “OK, I got ABS. It’s been a pain in the butt. What can I do about it?”
Step one is to read your owners manual and find out exactly how the ABS works. Does it have any of the features mentioned above or is it an, “always on, always obnoxious” deal. The most important spec to note, especially with “obnoxious” systems, is threshold speed. There is a speed below which the ABS does not engage. I have not been able to get specs for every system out there, but it seems to be between three and six miles per hour on average. This knowledge gives you a driving tool to use in those tricky situations.
Let’s say you are at the top of a steep hill and you can see well enough that you don’t really need to stop before going over. If you go over at, say, three mph, it won’t take long for you to get up to 5 mph and when you lightly apply the brakes to slow a bit, there goes your ABS. That might or might not be a problem, depending on the surface conditions of the slope and so on. If you predict there might be an ABS related complication, the trick is to keep below the threshold speed. That starts by stopping at the top of the hill and creeping over, using the brakes to keep below that threshold speed (whatever it is). This also applies to failed climbs and lifted or unloaded tires. Sometimes the threshold speed is higher when the vehicle is first started, so shutting the engine off and restarting can reset the system to a higher threshold speed.
Finally, I will also tell you that some innovative folks have installed manual cutoff switches in their ABS systems. Don’t consider this a recommendation, and I won’t tell you how to do it, but if you have already considered it, I’d advise a lot of care and thought beforehand. If you’re gonna do it, do it right and consider all the consequences, technical and legal. I’ve heard of a lawsuit over a traffic accident that hinged upon a disabled ABS system.
To recap, the most important tool in dealing with your ABS system off-highway is the knowledge of how it works. If you know what it’s supposed to do, you can work within those parameters to not let it bite you in the butt.