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I am commuting in rainy Seattle, and the rain has started. Right now I have Continental Gatorskin tires on my commuter, but yesterday I noticed I was braking to stop the wheels and skidding quite a bit on wet roads. I know I can gradually adjust braking on front and back to come out of the skid, but I was wondering if i should replace my tires to get a little more grip. My LBS seems to think thin slicks are fine however. Is there a study or data on tire types on wet pavement?

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Are you skidding the front or only the back? –  ChrisW Oct 12 '11 at 2:02
    
I have various tires on my various bikes, and I can say that the bike with Schwalbe Kojaks gets great traction in the rain. (The dual pull caliper brakes may also have something to do with that.) –  Neil Fein Oct 12 '11 at 5:04
    
Looking online, Kojaks seem very slick with no tread also. And they get great traction. –  Ash Machine Oct 12 '11 at 5:22
    
I am from Seattle too and have the same problem now. Skidding only in the back though. –  anton2g Oct 12 '11 at 15:31
    
I am skidding on the back (and I carry a pannier there) but go light on front brakes, else I'd flip. –  Ash Machine Oct 12 '11 at 16:31

4 Answers 4

up vote 18 down vote accepted

If you are riding on the road, slicks are fine in the wet. If you need to go over any mud, etc. then you'll need something else.

From Sheldon Brown:

Bicycle tires for on-road use have no need of any sort of tread features; in fact, the best road tires are perfectly smooth, with no tread at all!

Unfortunately, most people assume that a smooth tire will be slippery, so this type of tire is difficult to sell to unsophisticated cyclists. Most tire makers cater to this by putting a very fine pattern on their tires, mainly for cosmetic and marketing reasons. If you examine a section of asphalt or concrete, you'll see that the texture of the road itself is much "knobbier" than the tread features of a good-quality road tire. Since the tire is flexible, even a slick tire deforms as it comes into contact with the pavement, acquiring the shape of the pavement texture, only while in contact with the road.

People ask, "But don't slick tires get slippery on wet roads, or worse yet, wet metal features such as expansion joints, paint stripes, or railroad tracks?" The answer is, yes, they do. So do tires with tread. All tires are slippery in these conditions. Tread features make no improvement in this.

Also worthy of note is:

Car and truck tires need tread, because these vehicles are prone to a very dangerous condition called "hydroplaning." This happens when driving fast in very wet conditions, which can lead to the tire riding up onto a cushion of liquid water. When this happens, there is a sudden and total lack of traction.

Bicycles can NOT hydroplane because:

  • A bicycle tire has a curved road contact. Since a bicycle leans in corners, it needs a tire with a rounded contact area, which tends to push the water away to either side.
  • A bicycle tire is narrower, so not as much water is in contact with the leading edge at once.
  • The high pressure of bicycle tires is more efficient at squeezing the water out from under.
  • At high speeds, hydroplaning is just possible for car tires, but is absolutely impossible for bicycle tires.
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I'm tempted to post this on skeptics.stackexchange.com just to see if anyone can dig up some research to back this up :) –  Nathan Baulch Oct 13 '11 at 17:19
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Good idea. For those that want to follow that question, it's at skeptics.stackexchange.com/questions/6572 –  Wilka Oct 14 '11 at 10:25

First you need to analyze the pavement a bit -- smooth pavement or rough, is it generally pretty clean or is there a light coating of sand or dust, or perhaps a heavy coating?

Narrow, high-pressure bike tires do not "hydroplane" at non-supersonic speeds, so you don't need to worry about that. But there is still a thin layer of water between tire and road surface, sufficient to impact the coefficient of friction, and some sort of "tread" or "siping" helps significantly. Any dust or sand exacerbates the problem, and a smooth surface (especially worn-smooth asphalt) can get treacherous. I don't know of any hard numbers (though I would suspect they're out there somewhere), but Wikipedia has this to say (about auto tires):

Treadless racing "slicks" on dry roads give maximum traction. These have no sipes, no grooves, and no tread blocks. They also have very poor traction however on even slightly wet surfaces.

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In this case, the concrete road is very wet, rough texture (so cars brake better), free of debris and mud, probably a little greasy. –  Ash Machine Oct 12 '11 at 16:34
    
That's a good condition, so long as things are not terribly greasy. You do need, when riding in wet, to observe the road more closely and steer clear of oily spots, etc. –  Daniel R Hicks Oct 12 '11 at 18:18

The conti's really are just fine.

There's just less friction on wet roads and so your rear wheel is going to skid at a lower deceleration. A wider tire will be a little more helpful because your contact patch is larger.

I know that some material compositions are better than others. In particular, Michelin carbons and the avocet carbon 15's are claimed to have more grip in wet conditions.

But then you have to balance that with puncture resistance. In addition to reduced traction, you also have a greater probability of getting a flat in the rain. Which one is worse? It's up to you. The michelin's and avocet's are nowhere near as durable as the conti-gatorskins (there's a reason they're highly popular as high mileage urban tires). But if you're doing office-park crits in the rain you're better off with the michelin/avocet's.

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The size of the contact patch doesn't vary with width: it varies with tire pressure, rider weight, and braking (which moves the weight forward onto the front wheel). Theoretically at least, tire width affects the shape of the contact patch but not its area. –  ChrisW Oct 12 '11 at 14:13
    
There is a slight effect on the area of contact due to the stiffness of the tire, but probably not much. More significant, probably, is the unevenness in contact pressure due to tire stiffness -- a stiffer tire will have a less even contact pressure (which is probably good on slick surfaces). –  Daniel R Hicks Oct 12 '11 at 15:30
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@ChrisW, Well yes, if you're riding a wider tire, you're usually also riding a lower pressure. Generally speaking, wider tires are better for adverse road conditions. If the OP is having problems with wet urban roads, he'll be a little better off with wider tires. –  Angelo Oct 12 '11 at 17:21

Just to add on to what Wilka said about hydroplaning, people involved with the physics of planes landing are particularly interested in hydroplaning. Sheldon Brown of course talks about this.

Even with automobiles, actual hydroplaning is very rare. It is a much more real problem for aircraft landing on wet runways. The aviation industry has studied this problem very carefully, and has come up with a general guideline as to when hydroplaning is a risk. The formula used in the aviation industry is: Speed (in knots) = 9 X the square root of the tire pressure (in psi.)

I've found a chart here which on page 9 includes this plot:

Hydroplaning

You can see that at the pressures we operate at ones risk of hydroplaning is negligible.

Regarding your concerns about flipping, Sheldon Brown has a very informative page, "Won't I Go Over The Bars?", which details why you should not be concerned with flipping over the handlebars if you are using your front brake appropriately, and shows that when traction is not an issue using just your front brake is the fastest way you can stop. He also speaks about braking in the conditions your asking about (traction impaired).

As long as I am not braking on a manhole cover I have never had issues stopping in the rain on 23C slick tires using just my front brake.

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