I can tell you that we are recommending to our pro teams as well as pro-triathletes to run 21mm tires with both 808's and 404's, and NOT to run 18 or 19mm tires. We have seen no repeatable aero advantage in using 19 or even 18mm tires with these new rim shapes (though with the new 404, there was a slight tendency for increased drag with the 19mm tire, and feel that the increased ride harshness, reduced contact patch, increased rolling resistance and so on associated with narrower tires just can't really be justified on either of these rims.

Now, if you are running an H3, old Zipp 440, Campy Bora or Reynolds wheel, then we have definitely seen advantages in the wind tunnel to running 18 or 19mm tires over 21 or 22mm tires, and I would strongly recommend them for those applications from a pure speed perspective, but with the newer Zipp rim profiles, I can honestly say use a 21mm at a slightly lower pressure, and save yourself the added harshness of the high tire pressure as well as gaining back some improved handling and still maximizing aero benefits. These latest generation wheels have been fully designed around 21-22mm tires, so you can be confident that you are not sacrificing any performance with them.

The Corsa Evo CS will be perfect for that wheel, and I would recommend 100-125 psi (6.9-8.6 bar) depending on your weight. You will want to run 0.2-0.5bar more in the rear than the front to account for weight bias. I personally weigh 155lbs (70kg) and run my tires at 105 psi front (7.2 bar) and 110 psi (7.6 bar) in the rear. When I was racing and lighter I ran them at 100/105, but now that I'm heavier it is safer to run a few extra psi and I may go even higher on bad roads, which has worse rolling resistance, but better protects the wheels from damage in the event of hitting a pothole or something else which could damage a rim or tire. The better option is to run a wider tire at lower pressure on rough roads, but that is just not always feasible, so I would rather have higher rolling resistance and protect the wheels than lower rolling resistance and increased risk of damage to the wheel.

Higher pressure is definitely slower on anything other than perfect surfaces. Think of it in terms of a bunch of 1mm tall bumps in the road. If you have a lower tire pressure, the casing of the tire will deflect over each bump (we'll assume the casing deflects the entire 1mm) converting a small amount of energy into heat as the casing deflects, but the amount of energy necessary to compress the air is almost non-existent. Now at a higher pressure, we will assume that the tire deflects half as much. Now the bike and rider are lifted by 0.5mm and the casing deflects by .5mm, the energy necessary to deflect the casing by .5mm is less than it takes to deflect it by 1mm, but is nothing compared to the amount of energy necessary to lift the bike and rider by 0.5mm, so the end result is that the total energy requirement for the high tire pressure condition is much greater.

The other thing that happens is that on smoother roads, high tire pressures keep the casing from deforming over and into small cracks and crevices and over pebbles, which means that some of the deflection is transferred into the tire tread, which is not as elastic as the casing. Excessive tire wear comes about as the tire rubber begins to fail in shear as it is deformed by the road surface, and this generates heat as well as breaks down the cross-linking within the tread material.... overall, you are using more energy to go slower and you're wearing your tires out faster. The problem is that high tire pressures feel fast as your body perceives all the high frequency vibrations from the road surface as being faster than a smooth ride.

Lennard Zinn had a great analogy when he said that 100kph in a Jeep will scare the crap out of you but 200kph in an S class Mercedes feels effortless...the same is true of bike tire pressures, but it's just hard to convince ourselves of that. As athletes we tend to buy into the 'if some is good, more must be better' philosophy, but this is rarely true. Of course the tire manufacturers have given up on this and continue to try and make higher pressure tires as that's what the consumers demand, as I think that they've decided that it is easier to just give people what they think they want than to try to educate and argue with them :-)

Because the intended use of Zipp wheels is racing/high performance, the design is one that holds the tire very securely even under the most severe conditions. Tires can blow off of clincher rims for 3 reasons, either the rim is too small, the tire is too large, or the air pressure/force is too high for the tire, rim, or the interface of the two. Since our co-molding process requires the full tooling of the outer aluminum rim as well as the structural inner carbon hoop, the outer aluminum ring must be controlled to a diameter more than 2 times tighter than the ISO/ETRTO standard. To ensure secure tire fit we fit our tolerance into the upper 50% of the allowable size, so the rims can range from the nominal specified diameter to about 90% of the MAX specified diameter under the international standard, essentially the rims always tend to run larger rather than smaller. Likewise, many tire manufacturers try to run their tires in the smaller range of the tire size tolerance band due to the stretching of the tire and the bead with use, time, and heat. Clearly this combination can lead to a tight fit, but it is agreed by all that this is a very safe and secure fit.

On the pressure/force side of the equation, there are multiple critical aspects, but in particular the pressure you pump your tires to before the ride represents the minimum pressure the tire/rim will see during the ride. Tire pressure increases with brake heat in the rim, plus the tire becomes more elastic and will stretch with heat, so for example if you pump your tire to the Max 125psi and then descend the Alpe d’Huez, your rim may achieve 300 degrees F, the air pressure will rise due to the heat to roughly 150psi, and the tire will show increased elasticity…add in a high speed corner and the combined forces on the rim/tire interface can be significant. The Zipp rim has been designed to hold the tire when the system is hot and loaded, likewise the maximum pressure rating on the Zipp rim is set based on extensive lab testing of both burst pressure and fitment of dozens of tires, but also the hot bursting conditions of these tires. We feel that our fitment is one of the most secure on the market currently and offers additional security even when hot, as a result, on our rims most all tires will physically fail through the sidewall before blowing off of the rim, even when hot.

For tire installation, we recommend using a thin rim tape like Zipp, Rox, Ritchey or Michelin plastic. The added thickness of a tape like Velox makes the tire fit too tightly at the bead seat of the channel, making it very hard to stretch the tire over the bead on the opposite end. Also using the narrowest allowable inner tube helps by eliminating bulk under the tire during the install and drastically reduces the likelihood of pinching the tube during the installation.