Bicycle Helmet Safety Institute

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Chrono Bike Helmets for Time Trials

Summary: The Chrono (aero) style is a special aerodynamic helmet for time trialing. It is not suitable for street use. The tail can be a hazard in a crash. We have a list of individual models below the general explanation.

Chrono helmets are a very special type that is optimized for aerodynamics for use in time trials. The aero shape is advantageous at time trial speeds, primarily above 20 mph, so it is not of much use for ordinary street riding.

Early chrono models were shells only, and not certified for impact protection. Beginning in 2002, Louis Garneau introduced a chrono model certified to the US CPSC bicycle helmet standard. Two years later the European racing authorities required that chrono helmets used in time trials must meet the European EN 1078 bicycle helmet standard. That began a flurry of retrofitting as manufacturers tried to cram impact foam into their chrono shells. Depending on the amount of room available they were successful, but some had to redesign from scratch. Although they must sell very few of their chrono models, manufacturers believe that they lend prestige to the entire line.

The European CEN standard is less severe than the US CPSC bicycle helmet standard. Helmets built only to the CEN standard are less protective. One example of the difference is that CEN helmets are tested in 1.5 meter drops on the flat anvil, while a CPSC helmet has to perform at 2.0 meters. CEN helmets can be lighter and thinner, and usually are.

USA Cycling formerly accepted CEN helmets for races that it sanctions in the US, but reverted to a CPSC requirement starting January 1, 2010, as noted in the current USA Cycling Rule Book. In anticipation of that ruling and to sell in the US market, many manufacturers improved the protection of their chrono helmets.

The tail of a chrono helmet is not an asset for anything but aero shape. It is long and provides a great place to snag your head in a fall, twisting your head and neck. We don't recommend chrono helmets for street or trail use. Here is an email that demonstrates why:

"I was recently in a crash trying to avoid another rider during a triathlon. It was a typical fall with the bike sliding out in front of me and I landed sliding on my elbow, butt and head. This is generally not the most serious kind of crash but my comments are related to the helmet I was wearing, one of the time trial 'aero' helmets which are being used a lot more by triathletes. On my helmet the shell extends 5-6 inches past the head. The rear aero extensions were held intact by the plastic shell covering the helmet but the downward force from the back of my head broke the inner styrofoam shell into several pieces; the back retention clamp broke off; and the rear strap connection which did not go completely through the helmet (i.e. connected only to the styrofoam part of the helmet) also pulled out. With only the front ends of the chin straps attached to it the helmet came off my head."

Note that using a helmet like that in traffic, where the first hit is likely to be on a car, having the helmet come off could mean hitting the pavement with a bare head. In a time trial there is normally only one really hard impact, on the road. So the tail is a hazard. Here is the Snell Foundation's warning label, required on any helmet meeting their B-90TT or B-95TT standards for time trial helmets:

"WARNING: THIS HELMET IS NOT INTENDED FOR RECREATIONAL USE. This special use helmet has been designed to provide an aerodynamic benefit through an aerodynamic tail which in a fall or crash may reduce its ability to provide adequate protection. In a fall or crash the aerodynamic tail may cause the helmet to be pushed out of position thereby exposing the head to serious and/or fatal injury. Similarly, in a fall or crash a rider may be exposed to a strangulation and/or choking hazard from the retention system. USE ONLY ON A CLOSED DESIGNATED COURSE IN CONNECTION WITH SANCTIONED TIME TRIALING ACTIVITIES OR COMPETITIVE EVENTS."

That should be a caution to anybody considering the use of a long tail time trial helmet.

Riding in a chrono helmet is not simple if you want the full aero effect. Racers train in wind tunnels for best positioning because a degree or two of helmet tilt causes very large changes in wind resistance. If you have a long tail helmet, the tail must lie flat against your back for aerodynamics, so you can't tilt your head to look down at your computer or check your gears or relax your neck muscles without poking the tail up into the wind stream. Some riders use a humped over crouch, while others get down and flatten the back. If you have to stand on a hill you will lose the advantage unless you have practiced riding out of the saddle but bent over in a tuck. Those variables require a matching helmet shape for maximum efficiency.

The debate on long tail vs. short tail aero helmets continues. Casco opened it in 2009 with claims that their extremely round helmet performed better in actual riding, and there are many shorter helmets on the market now. If your head position is normally low, you may not need a long tail. Pros don't need them if a following car can give them a heads up before turns and they can keep the head down for most of a time trail. But racers with a higher head position and a need to see ahead may want to stick with a long tail model. Only a wind tunnel session can tell you for sure.

In 2013 a new category of aero helmet appeared: the aero road helmet. Not as slick as a chrono time trial helmet, but made more aerodynamic than a normal road helmet, often by covering the vents. Some have adjustable vents, or use vent covers. They are used by pros in some pack races, but abandoned for stages where ventilation becomes critical. They will not improve the average road rider's performance very much, but you might want one for the image, and some have the rounder, smoother profile that we recommend. We don't know if their performance for time trialing is comparable to other designs.

Can a dimpled surface that burbles air like a dimpled golf ball improve aerodynamics? Casco has used a section of six raised rubber dimples in the rear to lower air adhesion there. The Carrera Intruder has two panels of stippled material glued into indentations along the surface for a dimpled effect up the front, along the top and down part of the tail. The Lazer Tardiz has a dimpled rear section. Louis Garneau's Superleggera and Vorttice have a dimpled front section. So there is some indication that dimples can help, but no agreement on where they should be. And Garneau has a line of raised bumps behind their dimpled section across the crown that they say will accelerate the air and cause it to flow smoothly down the sides in the rear.

Vents are another question mark. Some helmets have none, or only in the rear. Some have small ones. And at the other end of the spectrum some have generous ones. The rider will generate heat in a maximum effort time trial. Is it worth giving up some aerodynamic advantage to cool the head? The answer may be different for different riders, since some give off more heat from the head than others. Lazer is the only one to claim that air taken in through the front vents exits at the rear and pushes the helmet forward. We would have to be convinced of that, since conventional wisdom has always been that maximum aerodynamics means no vents at all.

Any weights shown below are the manufacturers' claim. Accuracy is likely to vary, and there are size and accessory differences, so weighing the helmet yourself is the only way to be sure.

Chrono Helmet Models

Note: Some of these helmets are reserved for team use and are not available through retail channels. Some may even have been discontinued, since they do not appear in the company's annual catalog and it is difficult to track when a design is no longer produced.