Is the Legal Minimum Tire Tread Depth Enough?

TireNo doubt this morning was an intensely exciting one for a driver that I passed by. Chances are good that the tread depth of her car's tires were not top of mind before today. 

She had obviously done some panicked steering on the rain soaked highway judging from the marks in the median and the amount of grass and mud in the fast lane.

Others had already stopped to help and aside from being stuck in the median she and her vehicle appeared relatively unharmed.

Minimum Tread Depth

This turned my thoughts to the tread on my tires and the question of how much tread is needed to stop and steer properly on wet roads. We know that tires are considered to be worn out at 1.5 mm or 2/32nds of an inch unless they are winter tires, in which case the limit is 3 mm or 4/32nds of an inch of tread depth. Is that really enough?

Tread Wear Bars

This image shows the tread wear bars that appear across the grooves of a worn out tire. These bars indicate that the tread depth is 2mm deep or less.

Image of wear bars showing tread depth of a worn out tire

It appears that if you intend to drive on wet roads 4 mm may be the minimum tread depth needed to stop and steer effectively. Without at least this much tread the tire cannot move water away from the tread fast enough to maintain adequate traction. To drive with less tread is not illegal until you reach the legislated minimums, but it may not be safe.

Tire Gauge

One last thought and that is to buy a decent tire gauge to keep in your glove box. Use it every couple of weeks before you start out and make sure that the tires are inflated to the specifications on your vehicle's tire placard. Proper inflation helps tread do its part to prevent hydroplaning.

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Can I suggest as well that proper tire inflation improves life of the tires as well as increases the fuel economy of the vehicle.

An interesting topic, here.  Tires that are almost worn out, even beyond the legal minimum, can still provide good grip - but only in dry conditions.  Once the roads get wet though, obtaining grip is wholly obtained by being able to keep rubber in contact with the road, and this is dependent on three things:

  1. How well the tire can channel water, dissipating it via the treads.  A good wet weather passenger car tire can channel about two gallons per second; a Formula One racing rain tire can channel about five gallons per second.  Thing is, in a Formula One race, the moment the track conditions start to dry out, they'll swap to intermediate tires (otherwise the rain tires will rapidly overheat), and shortly thereafter to slicks, once there's a moderately dry racing line.  We don't have these options on our passenger vehicles, so must select the best compromise for all conditions.
  2. The amount of water on the road.  The greater the depth of the water, the tougher it is to channel it out of the treads.
  3. The speed of the vehicle; you're not going to hydroplane (that's when the tire can no longer maintain contact, and starts to float on the surface despite the weight of the vehicle) at 20 km/h.  But at 100 km/h it doesn't take a lot of water to interfere with grip.

Did you know that the hydroplaning phenomenon was first discovered and understood in the fifties/sixties with the advent of passenger jet aircraft?  Their landing speeds were so much higher than turboprop aircraft, that despite their huge weight, they couldn't effectively brake as they were floating down the runway if it was wet.  Big problem.  The research into tire tread design for these aircraft then filtered down to car tires.

Incidentally, if you don't have a tire tread depth gauge, your tires will almost certainly have visible wear bars to show when they have reached their legal minimum and must be replaced (though in some conditions - see factors above - they may already have become potentially dangerous long before).

As to tire pressure, this has to be one of the most neglected safety aspects.  Even if a tire doesn't leak (and most do, ever so gradually) what was correct pressure in the heat of summer will be significantly lower in the winter due to the change in air density.  And it's not just the ability to channel water properly that's of concern, here - an underinflated tire will not corner as well, it won't provide the same braking grip (or acceleration grip, for that matter), it will adversely affect your gas mileage, it will wear out faster and so need to be replaced sooner, and in extreme cases (particularly at freeway speeds) the constant deformation of the sidewall will result in a blowout.  That's right, blowouts aren't normally caused by hitting sharp debris, they're caused by too little pressure; when you see bits of tire carcase on the freeway, they're usually from one of the tires on a truck or trailer that has been running when deflated (it's not visually obvious with dual wheels when this has happened).

Incidentally, tire pressure can often be safely raised higher than the vehicle's tire placard; particularly if the tires are not the originals that came with the vehicle, and/or you're operating with a lot of weight on board (think family vacation with luggage etc).  What does it say on the sidewall?  May be radically higher than the placard, so you may want to judicially experiment.  Also, in a front wheel drive vehicle, I would recommend rotation about every 8,000km, no matter what the manufacturer says is necessary; they will absolutely last longer and wear better.

As to the exciting moment experienced by the woman who ended up in the median?  I wonder if she ever considered that the camber of the road on the freeway is to the left when you're in the fast lane, so more likely to send you that way if you're lacking grip?  I wonder how she reacted when she found that her steering control seemed to be compromised due to the lack of traction resulting from tire and road conditions?  Smoothly, gently, calmly?  I don't think so!  And yet, if you think about it, if you're driving in a straight line, even if your coefficient of friction is suddenly reduced, there is absolutely no reason to lose control.  Cars don't 'went out of control' like you're always reading in the newspapers; drivers lose control, because they don't know what they're doing, or what's happening.

If you're driving at speed in wet conditions, then make sure the cruise control is off, relax your shoulders and elbows, 'feel' what's happening through the wheel, leave the brake alone if at all possible, but don't try to accelerate hard either (imagine there's an orange under your gas pedal, with a cut in the skin, and you want to slowly extract the juice without damaging the orange any further), keep your eyes up, (shift to 5th or 6th if you're in a standard) and there is no reason whatsoever to lose control so long as you're gentle with the vehicle.

The hardness of the rubber is a significant factor as well.  High performance summer tires are often better in the wet than all season tires, which are often designed for long life over other characteristics.  Also, tires gets harder with age, and can become terrible in the rain even when there is lots of tread depth remaining.  (And the harder the rubber gets, the longer the tread will last, all while the tires have become unsafe.) The general advice today is to replace tires once they reach the 6 year mark, regardless of wear.