Steve’s Bike Tech
This month I thought I would write about wheels and tires. Aside from the frame, no part of a bicycle has more influence over the ride and handling characteristics of a bike than the wheels and tires.
Although they are very different structurally, wheels and tires share some important qualities. Choices about these qualities will have a significant effect on how your bike feels and responds, on how much energy you expend, on maintenance and replacement expense, and on convenience and safety. Consider the various properties that both can have: efficiency, weight; compliance, durability and expense.
EFFICIENCY. The efficiency of wheels and tires boils down to two factors: friction, and, in the case of wheels, power transfer. Friction come from two sources: the drag created between two solid surfaces (bearings and hubs, tires and the ground), and aerodynamic friction between a solid and the air.
Let’s examine wheels first. Most wheels sold have reasonably good bearings, so the differences in bearing friction are effectively very small. However, if you race, do very long rides or like to go downhill fast, you will get some benefit from better wheel bearings. A lot of wheels manufactured today have sealed bearings. These are long-lasting, require little or no service, and are relatively easy to replace. They also have more friction than loose ball bearings.
Shimano and Campagnolo wheels and hubs, at least in the better quality models, have loose balls made of metal (mostly) that rotate in a ring around the axle. They are coated with grease to reduce friction and wear. This type of hub has less friction, but is more susceptible to contaminants (dirt) and moisture. The grease is more likely to dry out or get washed out by riding in wet conditions. The bearings also cause wear to the cones and races that hold them in place, more so if not re-greased periodically. With regular service, this type of wheel bearing can last just as long as sealed bearings, but if not serviced regularly they can cause expensive problems. There is a new fad in high-end bikes: the use of ceramic bearings. I won’t go into the properties that make ceramic superior to metal for bearing use, but the benefits are measurable mainly at higher sustained speeds. They are available in sealed and loose bearings and are extremely expensive (from $200 - $400 for a wheelset.)
Tire friction, obviously, comes from the interaction between the tire surface and the ride surface. It is affected by three factors: the hardness of the tire compound, the width of the tire, and the air pressure inside the tire. All tires involve tradeoffs between rolling resistance, comfort of the ride, traction, maneuverability and durability. There are a great many different tires available currently, so it’s wise to check the manufacturers web site or get opinions from fellow riders about a given tires design goals. Current research shows that tires actually have lower rolling resistance at about 10% less than their maximum recommended inflation, plus there is better traction on turns, less tire wear, and higher compliance (comfort) than with higher pressures.
There is no practical effect of aerodynamic drag on tires, but it is a major factor in wheel performance. The number, distribution and shape of spokes, and the shape of the rim have a great influence on bike speed. However, this is only significant at speeds of over 20 MPH, so be careful of buying wheels on aerodynamics alone. Wheels with deep rims and bladed spokes can reduce drag in calm conditions or a direct headwind, but can actually have a detrimental effect if the wind comes from the side, even a little. And deep aero rims are often heavier and have a stiffer ride. If you do long-distance riding in mixed terrain, especially with a lot of climbing, you will probably get more performance benefit from lighter wheels.
Power transfer. The final factor in wheel performance is power transfer. How much of your pedaling power is transferred directly to forward motion is affected by two wheel characteristics: torsion stiffness (side to side flex) and how quickly the ratcheting mechanism inside the rear freehub engages when you start pedaling. Some wheels that are torsionally stiff can give a harsh ride, so it’s best to do some careful research before buying wheels to make sure the design goals match your riding goals.
WEIGHT. The weight of wheels and tires has a major effect on bike performance. Consider that any part of the wheel/tire that is closest to the outside of the circumference is magnified, because rotational weight increases as speed increases and the greatest effect of rotational weigh is towards the outside, as this travels furthest for every revolution of the wheel. The effect of lighter tires and tubes is significant, but the tradeoff is usually faster wear for tires and more susceptibility to flats. Wheels can be lightened in a number of ways, from reducing rim weight to using lighter spokes, fewer spokes, or placing the spoke nipples at the hub instead of the rim (again, think rotational weight.)
Durability. The weight tradeoff on wheels is reduced durability and reliability. Fewer spokes on a wheel greatly reduce rotational weight, but each spoke carries higher tension with an increased risk of breakage. And with higher spoke counts, if one breaks on a ride there will be less rim deflection than lower spoke count wheels, so a better chance of being able to finish your ride.
There is also a curve of diminishing returns for lower wheel weight, where the expense goes up exponentially for every bit of weight saved. The service department of one major wheel maker recently told me that if a consumer purchases a light-weight high-end wheelset, he or she should expect at most 2 years out of it! Of course, not all wheelmakers have that kind of attitude.
COMPLIANCE. Compliance, or the comfort of the wheel-tire interface with the rolling surface and the bike frame, is probably the most significant factor in how much road shock the bike transmits into your body. This translates into greater or lesser ride comfort, especially on longer rides. Higher compliance tires sometimes wear faster than those with a stiffer ride, but they may provide slightly greater traction on curves or wet roads. Look at the thread count in TPI (threads per inch) to help evaluate tire compliance – a higher thread count usually makes a more compliant (and expensive) tire. Folding tires generally have the smoothest ride.
For wheels, you will have to find a balance that suits you between compliance, efficiency, weight and durability. For most riders, a wheel with low profile rim, good quality hub, and 28 or 32 double butted spokes (thinner in the middle, thicker at the ends) should be a good blend. Also, pre-built wheels from Shimano, Campagnolo or Mavic are generally good choices if a bit more expensive.
In summary, as with most things related to bicycles there are more choices than ever before. Careful research is recommended before purchasing new tires or wheels. There is an inverse proportion between weight and expense, with durability and compliance sometimes suffering also as weight decreases. But it’s no fun to ride on wheels or tires that are too heavy.