The traction patch of a pair of road tires is frighteningly small for the speeds we regularly subject them to—about the size of a pair of postage stamps. Most of us like to forget this little factoid when talking about what we wrap our multi-thousand-dollar wheels in and focus solely on the go-fast numbers like rolling resistance, drag figures, and transition to the brake track. But what if someone hadn’t forgotten to make a set of extraordinarily “grippy” tires and included all the latest talking points, too? Bontrager is that company. The R4 Aero is that tire.
The “Aero” in the tire’s designation refers to the “wings” on the side of the tire that come in contact with the brake track’s lip, smoothing the transition between the tire and the rim and reducing the vortices and turbulent airflow that it generates. Using CFD, Bontrager has figured out that air turbulence is much more than a minor problem for wheel and tire combinations. This realization is clearly evident with the R4 Aero, which when mated to the Aeolus 3 rim shape, is about 15mm deeper than the Aeolus on its own. Therefore the R4 Aero becomes incredibly competitive from a drag perspective. That means our set of Aeolus 3 D3 Clinchers with the R4 Aero tires are nipping at the heels of a set of 404 FCs with a set of Continental GP4000S. That isn’t just fast, that’s scary fast.
Where the R4 Aero excels when compared head-to-head with other tires, however, is in Crr – or rolling resistance. For really detailed, technical work, such as a tricky mountain descent in the rain, a higher rolling resistance is desirable as it denotes a “stickier” rubber (more grip than a lower rolling resistance tire). Think about it in this fashion: for efficiency, lower rolling resistance; for corner carving, higher rolling resistance. Since the name of the game in triathlon is efficiency, lower is better on the bike split. And this is where the R4 Aero delivers. In a category that measures in 0.0025 for the R4 Aero, the next closest competitor would be the GP4000S at around 0.0034. It is undeniable that the GP4000S is an absolutely phenomenal tire, and tunnel data shows that it is incredibly aero on some wheelsets, too. But while it might be a little faster than the rubber from Bontrager, it simply cannot hold a candle to the efficiency on the contact patch.
As for the feel of the tire on-bike, our time with the R4 Aero thus far has been overwhelmingly positive. It rolls exceptionally well, has excellent puncture protection at 220 tpi, and has a supple feel to it when simply cruising. Under hard acceleration you can feel the tire dig a bit before it really seats itself on the asphalt and launches you forward. But but the aero wings on the tire—especially mated to the Aeolus D3s we’ve been testing—really do seem to give the wheel and tire combination an aero effect that bumps it to the next level. While we haven’t had a chance to test the tire in the wet, we have no complaints about the tire in dry conditions whatsoever.
So far we are quite enamored with the new rubber from Bontrager. It is certainly pretty, grippy, and fast when you want it to be, and that’s all we can ask for in a race tire. The puncture protection is what catapults it up to the “superstar” class of the tire world. And in the next thousand miles or so, we’ll be looking carefully at what tires we reach for when mounting up the review wheels.