I seem to remember being advised to change my tire pressure if I have to drive in the rain or on wet roads. Problem is, I don't remember whether MORE air, or LESS air than stock specs is better.

Or, does the TYPE of tire have to be considered first?

ON the one hand, I recall reading that adding an additional 2-4 PSI above stock reduces the chance of hydroplaning.

ON the other hand, I also recall reading that deflating a tire 2-4 PSI below stock would put more rubber on the road, or increase the surface area of the contact patch, and thereby increasing rolling resistance.

I can see how having more air would reduce hydroplaning, but I can also see how having less air would reduce the tendency of the tire to lose its grip.

So, which is it?

 

First and foremost, the type of tire is the most important when considering the hydroplaning resistance. Tread depth, design, and rubber compound are all factors.

I would say that the amount of air is negligable for the amount of wear it'll put on your tires if you forget about them. If you really deflate 2-3 psi, that'd really only increase the amound of rubber on the road >2%, or about 4-5 cm2 of tire on the road. Not worth it if you ask me

 

r u really going to jump our of the car if it starts raining to change the pressure ??

 

I understand tire and tread design are most important...and along those lines, there is the issue of keeping the tread grooves open that channel water away. I could logically see where an underinflated tire would inhibit these grooves from properly channeling the water.

I could ask one of the engineers at Goodyear who designed the Triple Tread if it makes a difference.

 

Yes, if I would if I knew that adding or removing air from my tires would make a significant difference, and I could ask someone to do it for me.

 

you would think less...

 

not exactly less.

Look at this for a moment. What about the condition of the tire? If the tread width is fully planted on the ground and the outside of a tire is more worn then the inside, then water will be trapped in the centre of the tread path.

There are many scenarios, but look at winter rally racing, in the snow and ice, they use the skinniest tires possible, in order to cut through snow. Not glide on it like the super wide tires of a bmw x3 or x5.

 

Yes, skinny tire has more PSI between the tire and the road surface. That's what you want in rain and snow.

 

If the road is just wet, then wider is better. I don't think it will make a big difference going wider, but in theory it is.

But usually wet roads come with some standing water. For standing water your tire tread needs to cut thru the water to get to the road, and narrow tires with the deepest tread work best for this. Snow is similar.

Dave

 

Here's some test data from Goodyear and NHTSA. Both tests are using vehicles that have 30 psi front/rear stamped on the placards; i.e., the recommeded amounts.

According to Goodyear's tests, stopping distance (on concrete) generally increases with lower tire pressure. The only exception is at 25 mph.

With fairly deep water on the road, (0.050 inches is equivalent to 1 inch of rain in an hour) lowering inflation to 17 psi and increasing speed to 45 mph increases the potential for hydroplaning and much longer stopping distances.


Braking Distance (in feet) provided by Goodyear
Wet Stopping Distance (0.050" water depth)

Surface Speed 17 psi 25 psi 29 psi 35 psi
Concrete 25 mph 47.4 48.2 48.2 48
Concrete 45 mph 182.6 167.2 167.4 163.6


On the NHTSA data, travelling at 60mph, stopping distances decreased with greater inflation, but only from 25psi and beyond.

Braking Distance (in feet) from NHTSA testing
Stopping Distance from 60 mph

Surface 15 psi 20 psi 25 psi 30 psi 35 psi
Wet Asphalt 158.5 158.6 162.6 161.2 158.0
Dry Asphalt 144.0 143.9 146.5 148.2 144.0

In general, these data suggest that the road surface and depth of water on the road have a large influence over stopping distance. Both the Goodyear and NHTSA tests results imply that tire inflation can have a significant impact on stopping distance.

Bottom line: inflating a tire above the minimum suggested inflation indicated on the tire placard improves (shortens) stopping distances.

So, when it's raining, putting in an extra 5psi above specs may be the difference between rear-ending the car in front of you and preventing a nasty accident.

That's worth getting your T-shirt wet, isn't it?