Most tire manufacturers agree that supple sidewalls and a thin tread make a tire fast, but the role of the tread pattern remains poorly understood. Most modern tires have either a completely smooth tread (slicks) or a coarse tread pattern similar to car tires. Many high-performance tires are smooth with just a few large sipes. None of these tread patterns are optimized.
Car tires have tread mostly to prevent hydroplaning. With their wide, square profile, a layer of water can form between tire and road surface. The tread pattern forms channels so the water can be pushed out of the tire/road interface.
Bicycle tires do not hydroplane. Their contact patch is too small and too round for that. This means that car-inspired tread patterns are not necessary on bicycle tires. Does this mean that no tread pattern at all – a slick tire – is best? Any tread pattern reduces the amount of rubber on the road surface… In the lab, it does work that way: Slick tires grip best on smooth steel drums.
Real roads are not as smooth as steel drums. An optimized tire tread interlocks with the irregularities of the road surface to provide more grip than the pure friction between asphalt and rubber. This is especially noticeable in wet conditions, when the coefficient of friction is reduced by half, yet you can corner with about 70-80% of the speed you use on dry roads. (Unless the road surface is greasy…)
The ideal tire tread has as many interlocking points with the road surface as possible. The “file tread” found on many classic racing tires does this. The ribs are angled so they don’t deflect under the loads of cornering or braking.
Why do race cars use slick tires, and not a file tread? The reason is simple: It would be abraded the first time the car accelerates. However, bicycle tires don’t wear significantly on their shoulders – the part that touches the ground when you corner hard – so we can use a tread pattern that is optimized for grip without worrying about wear.
Each Compass tires has three distinct tread patterns, each designed for a specific purpose.
- Center: Fine ribs serve as wear indicators. When the lines disappear, the tire is about half-worn. (The tread of our narrower tires is not wide enough for ribs, so small dots are used instead.)
- Shoulders: When the bike leans over as you corner, the tire rolls on it shoulders. A chevron or “fine file” tread pattern optimized grip.
- Edges: This part never touches the road (unless you crash). They serve only to protect the casing from punctures, so they don’t need any tread.
Supple casings make tires faster, but a supple casing is of little use when it’s covered by thick tread rubber. The fastest tire would have just a minimal layer of tread rubber, and many “event” tires are made that way. Unfortunately, that means that they don’t have much rubber to wear down until they are too thin to use. At Compass Bicycles, we call these tires “pre-worn”.
Compass tires have a slightly thicker tread in the center. A little more material there doubles or even triples the life of your tire, while adding minimal weight and resistance. (On our widest 650B x 42 mm tire, the added tread weighs less than 50 grams.) Once you have ridden the tires for a few thousand miles, they’ll be as light as the “event tires”.
On the shoulders and edges, the tread does not wear. So we made it much thinner to keep the tire supple and reduce its weight. The tread extends far enough down the sidewall that the casing is protect when seen from above. Extending the tread further adds little protection, but makes the tire less supple and thus less comfortable and slower.
Another important factor is the tread rubber. This is an area where incredible progress has been made in recent decades. In the past, you could either have good grip or good durability. I used to ride Michelin’s Hi-Lite tires, which gripped well, but rarely lasted even 1000 miles (1600 km).
Compass tires use Panaracer’s best tread rubber, which is amazing. Our tires are among the grippiest you can find, yet I just got an e-mail from a 230-pound rider who got 3786 miles (6093 km) out of a set of our 26 mm-wide Cayuse Pass tires. The wider tires spread the wear over more rubber, so they last significantly longer. (Don’t try to set wear records, but replace your tires once they get thin. The risk of flats, or worse, blowouts, is not worth getting an extra few hundred miles out of a worn tire.)
Tread color is another important consideration. Modern colored treads no longer are the “death traps” they used to be, but especially in wet conditions, the grip of tires with colored treads – including the Grand Bois Hetres we sell – is not quite as good as that of black treads. That is why we offer only black tread.
Click here for more information about Compass tires.
Update 9/25/14: Roadbikerider.com just published a review of the Compass tires. Click here to read it.