How ABS Makes Bad Tires Worse

Swedish Brakes

Anti-lock brakes (ABS) have been common long enough that most drivers consider it an essential safety feature and have experienced its effects.  On dry pavement, the brake pedal hums as the vehicle comes to a quick and controlled stop.  On wet pavement, it vibrates a bit more but again brings the vehicle to a quick and controlled stop.  Yet in winter conditions, it can often seem like it’s working against you.  There are at least two situations where ABS can adversely affect winter braking performance.

The Snow Wedge

On hard surfaces such as dry and wet pavement and cold ice, the tires rely on friction for grip.  Static friction is greater than dynamic friction, so the tires have more grip when they’re not sliding excessively.  ABS provides consistently near-optimal braking by keeping the tires operating in that static friction range, and allows for steering control regardless of driver input to the brake pedal.

On soft surfaces such as dirt and snow, the tires must dig into the surface for grip because it is too loose to provide adequate friction.  Maximum braking force will occur when the wheels are locked and a wedge of the soft material builds up in front of the locked tires.  ABS can prevent this wedge from forming.

With summer tires and most all-season tires, the wedge can be the primary means available to provide reasonable braking performance.  If the tread depth is too shallow, and the lugs too large and closely-spaced to dig into the snow adequately within the tire’s contact patch, it relies heavily on that wedge.

Winter tires, on the other hand, have the deep, widely spaced tread blocks necessary to dig into the snow.  Not only does this provide outright grip within the contact patch, it also digs in strongly enough to allow for the intermittent slight locking of the ABS to build a bit of snow in front of the tires.

So winter tires solve much of this problem.  It doesn’t even matter which ones.  Any winter tires meeting the RMA mountain/snowflake symbol will perform well in deep or loose snow.  But they still stop a bit quicker in those conditions with locked brakes.

Partial Brake Locking

On hard surfaces, many ABS systems can be a hindrance if the surface is particularly slippery and not uniform.  The shortest stops occur at the threshold of tire lock, just before ABS activates.  Modern ABS works well enough on most hard, high-grip surfaces that the difference between ABS braking and threshold braking is negligible.  You would have to be very skilled to beat ABS with any consistency in that situation.

On a mixed-condition road surface, the grip level of the four wheels can vary dramatically.  For example, one side may be on slippery ice while the other is on hard packed snow.  In this situation, even light brake application causes the icy side to lock.  The brake pedal begins to pulse, but the driver is still using nowhere near the maximum capability of the snow side so he continues to apply firmer pressure.  This causes the brake pedal to pulse even harder such that the driver’s ability to threshold brake the other two wheels becomes impossible.  The vehicle begins clumsily cycling between releasing the brakes and jerking them on, demonstrating to the driver at every jerk that there is more friction available than the brakes are using, if only the system could cycle more delicately and independently.

Without ABS in that situation, the icy side locks and the driver makes a minor steering adjustment and continues to threshold brake the other side to a controlled stop.  The sliding icy side tires are irrelevant as they are unable to contribute significant braking force regardless of braking technique.

This problem may not apply to all anti-lock brake systems, especially on newer vehicles.  Ideally, the system would have enough fine control of individual wheels to provide smooth and calm threshold braking at every contact patch.  But not all systems are ideal.  On complex surfaces, ABS functions much better on some vehicles than others.

Regardless of ABS performance, this problem can be virtually eliminated with a set of good winter tires that provide predictable and adequate grip on all possible surfaces.

Disabling ABS

If you don’t have winter tires, then a set of winter tires should be the first step to dramatically improving your ABS system.  If you already have winter tires, it’s still possible that a new set of winter tires will be a substantial improvement.

Disabling ABS is not as risky as it sounds.  Unlike stability control systems, there is little, if any, indication that ABS actually improves safety.  The European Commission has concluded that “while injury crashes decrease (-5%), fatal crashes increase (+6%). A recent study, however, indicates that anti-lock brakes may not contribute to crash prevention at all.”  NHTSA studies found that ABS reduced crashes by 6 to 8% overall, but did not reduce fatalities, and fatality rates actually increased in adverse conditions (rain, snow, ice).  A vehicle still stops quite well with locked brakes, as long as you want to travel in a straight line and don’t mind some vehicle rotation while doing so.

However, I still don’t advise it to anyone who doesn’t already know that they want to do it.  Disabling ABS is a last resort.  It’s usually only desirable in lower speed braking situations during city driving and on heavy snow days, but typically can’t be conveniently reactivated so it ends up remaining disabled during highway and freeway drives as well.  If nothing else, ABS can prevent flat-spotting or stud damage from frequent locking on mixed surfaces or even one major lock-up at higher speeds.

Keep in mind that, on most newer vehicles, disabling ABS will also disable the stability control system.  Stability control has been proven to substantially reduce collisions and fatalities.  So don’t disable that unless you truly hate it too.

Driving Without ABS

The typical sort of person who wants to drive without ABS learned to drive without it – or any other electronic controls – and spent many years afterward without it, in the process locking up a wheel or two or four on many thousands of separate occasions.  They already know that stopping and turning should be done separately.  That, on slippery surfaces, any braking reduces the ability to turn, and any turning reduces the ability to brake.  And that, on higher traction surfaces, turning while braking induces instability.  they know that turning while braking during street driving is a bad habit at any time.  When enough wheels lock during a braking event that the car begins to rotate, they let off the brake, correct the steering, and re-apply threshold braking in a fraction of a second using only muscle memory.

If you don’t have experience without ABS and you still make the choice to disable it, you also need to learn these things.  Not just the idea of these things; they need to become something that happens without any thought.  As much as possible, where it is safe to do so, practice threshold braking, locking the brakes, making steering corrections to the inevitable rotations, and only steering when off the brakes.  Lock the brakes, let off and make an evasive maneuver, settle the car, then reapply the brakes.

Developing your driving skills will improve the driving experience, but the only way to drive safely is to respect the potential consequences of misjudging your limits.  Leave plenty of room for error, especially around other vehicles.  You aren’t always going to get it right, regardless of your equipment.

 

 

2 Comments

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2 Responses to How ABS Makes Bad Tires Worse

  1. Adrian

    Good article. If you’re inclined, you guys should consider an email sign-up so new content will be emailed directly to interested readers.

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