Decreasing rolling resistance is vital in Zipp’s quest to Making You Faster. Welcome to Part II of a four-part podcast and blog series on the four barriers of speed identified (and attacked) by Zipp’s TSE approach to wheel innovation. In this second feature, we talk with Zipp advanced development engineer Ruan Trouw about the second barrier of speed: Rolling Resistance.
Zipp's Total System Efficiency (TSE) defines Rolling Resistance as Power loss created by the friction of the tires on the ground. It depends on tire performance but also tire patch and tire pressure. The other barriers to speed are wind resistance, gravity, and vibration loss. Below is an edited transcript of our conversation with Ruan about rolling resistance:
How did we broaden beyond looking mainly at aerodynamics for speed gains to consider factors such as rolling resistance?
Rolling resistance has always been there, and we’ve always looked at it. But not necessarily from the standpoint of the rim. It was mainly in terms of the tire. We didn’t look at how we could alter the rim. It wasn’t until the complete redesign of the 303 Firecrest that we started looking at how can the rim shape, tire-bed shape, all those things alter the four components of TSE (wind resistance, rolling resistance, gravity, vibration loss) for total system efficiency.
What did Zipp learn from creating the 303 Firecrest Tubeless Disc-brake wheelset, its first with a hookless rim?
Moving to the straight wall (hookless) rim unlocked something in rolling resistance that we didn’t necessarily know was there. The fact that the tire has a wider stance on a straight-walled rim compared to a crochet (hooked) rim helps with the tire’s shape, which ultimately affects the tire’s rolling resistance.
What is a simple explanation of rolling resistance?
Many things impact rolling resistance. How sticky the tire is will play into it, as well as the static friction of the tire to the ground. The factor that makes up a considerable portion of rolling resistance is the elastic hysteresis of the tire. In simple terms, it’s the tire deflection as you roll over it to create a certain contact patch—that deflection requires the tire to compress molecules. It requires plies within the tire to rub up against each other. All that extra movement within the tire creates heat, which is energy lost out of the system. We try to minimize the amount of movement within the tire to reduce those extra movements that happen, which essentially leach heat out of the system.
By widening the internal width of the rim, for the same tire size at the same pressure, you will end up with a shorter contact patch, which requires less sag in the tire and, therefore, a more efficient system.
–Ruan Trouw, Zipp advanced development engineer
What role does the wheel play with the tire’s contact patch on the ground?
The contact patch plays a big part in the amount of hysteresis in the tire. For the same rider weight and tire pressure, that area (contact patch) that supports that rider will stay the same. What we do is change the shape of that contact patch or area. By changing the shape, we can reduce the amount of hysteresis in the tire. Imagine a skinny tire on a skinny rim at 80 psi; when a rider gets on the bike, that tire compresses and creates a long, skinny contact patch on the ground. The longer that contact patch is, the more the tire must deform to make that length, which means your tire is compressing quite a bit. So, if you have a bigger and wider tire on a wider rim, to create the same amount of area in the contact patch, the tire does not have to squish as much, which gives you a shorter contact patch. The amount of squish in the tire—that sag or compression—reduces the hysteresis. You don’t have as much compression, so you don’t have as much extra movement within the plies of the tire, which leads to less loss overall.
So that ride rim (internal width) and lower tire pressure are essential?
Yes, a rider will need a certain amount of pressure to keep them from bottoming out, and the pressure itself dictates how much area is required to support the rider. The pressure needed to support the rider might remain the same, but if you change the contact patch, you can still increase your rolling efficiency and lower your resistance.
How does Zipp seek to optimize its approach to overcoming the four barriers to speed identified with TSE: wind resistance, rolling resistance, gravity (weight), and vibration loss?
We look at the whole system. We’re not in the business of perfecting one of these. We want to get the wheel to be as fast as we can make it. You must look at the tradeoffs between improvements in each one of the areas. For example, if you have higher tire pressure to reduce your rolling resistance, you will increase your vibration loss. You need to compare all four of them and make sure the net of the four is the best system.
How should riders evaluate the effect of rolling resistance on their rides?
The biggest thing to look at would be road surface texture. If you are riding on something smooth, you should dial your pressure in the match that surface, minimize the amount of vibration losses and maximize the wins in rolling resistance. (See Zipp Tire Pressure Guide) If you’re going to travel, it’s good to see what the roads look like and adjust your tire pressure accordingly.