A version of this article was originally
published in Triathlete Magazine

I am entering my second year of triathlon and want to upgrade to a carbon aero fork on my bike. What should I look for when choosing?”

Ben, PA

Upgrading a basic fork to a higher quality fork is one of the most effective ways to improve the way your bike rides. The fork affects everything from the aerodynamics of your bike to the way it handles. I’ve outlined the main variables that should be considered for triathlon use below.

Aerodynamics: The fork is a leading edge on the bicycle and the less the fork disturbs the air and encourages laminar (undisturbed) air flow, the less drag there will be. For this reason, a true aero fork can save some notable time – in the area of four minutes for a rider traveling at 20mph over an Ironman distance. Without getting overly involved in drag and axial force, you can get a good idea as to how a fork will perform aerodynamically by looking at its aspect ratio – a representation of how wide a fork is to how deep it is. It has been shown that forks with aspect ratios of about 3:1, meaning that the fork’s blades are 3 times as deep as they are wide, perform best aerodynamically. Forks like the Alpha Q Aero, Oval Concepts Jetstream and the Reynolds Ouzo Aero forks are designed close to this 3:1 ratio.

Weight: The deeper profile (aspect ratio) of an aero fork’s airfoil shape requires more material than a standard road fork. For this reason, aero forks are a little heavier than their comparable non-aero road fork counterparts – usually in the range of 100 grams (under ¼ of a pound). Even with this difference, a high quality aero fork is still a fair amount lighter than the stock forks that come on most entry to mid-level bikes. Weight’s largest affect is on acceleration and swing weight, which are not primary variables in most triathlons. If your biggest concern is to go as fast as possible while in the aero position, focus on the fork’s aerodynamics over its weight.

Vibration Damping and Comfort: One of the best reasons to install a new fork on your bike is to improve the quality of the ride. All the vibration from the road that enters the front-end of the bike passes through the fork. The shape of the fork and the application of the material(s) used to construct it determine how well the fork absorbs vibration. Carbon fiber is a very workable material that offers virtually infinite shaping possibilities. This has allowed many carbon fiber forks to provide a more compliant and damp ride than what is reasonably achieved with other materials. However, just because a fork is made of carbon does not mean that it will automatically be more comfortable. The shape and construction of the fork’s internal structure are what really matters. Forks built with narrower diameter, thinner walled tubing and/or a curve in the blade will flex easier (thus absorbing more shock) than blades made with larger diameter, thicker walled tubing, and/or straighter blades. The Reynolds Ouzo Pro Aero uses curved carbon fork blades as well as a full carbon steerer column and crown area (the portion of the fork where the blades and the steerer column connect) to create one of the smoothest riding aero forks available.

Torsional Stiffness: Torsional stiffness is how well a structure resists twisting loads. If a fork twists or flexes too much side-to-side, handling and tracking will be compromised. On a full carbon fork (carbon blades, steerer and crown area) the higher the modulus (basically the density) of the carbon used in constructing the fork’s crown area, the better it will fight torsional and side-to-side loads. Higher modulus carbon, of course, costs more than lower modulus carbon. On forks that are built of mixed materials (usually aluminum and carbon) the quality of the aluminum used and the diameter and the thickness of the material at the crown will determine how stiff it is torsionally. The heavier and faster the rider, the more torsional stiffness matters.

Geometry: A fork’s influence on frame geometry is one of the most overlooked, but important items, to consider. Every frame is designed around a specific fork span and rake, which (along with wheel size) effects handling and stability by determining what is called “fork trail”. Fork trail is complicated and requires an article all its own to describe well, so I’ll focus on describing how changes in fork span and rake can directly influence handling.

Span is the distance from the top of the fork crown (where the lower cup of the headset is installed) to where the axle on the front wheel attaches. If you install a fork with a longer span, while keeping the rake constant, you will effectively slacken the frame’s head tube angle. This will tend to make the bike more stable and less reactive. The opposite will happen if you shorten the fork span while keeping rake constant.

Rake is the distance from the bike’s steering axis (an invisible line that follows the frame’s head tube angle down to the ground) measured at a perpendicular out to the wheel’s axle. Rake works counter-intuitively. If rake is the only variable altered, a shorter rake fork will generally make a bike more stable and less reactive, while a fork with a longer rake will make the bike less stable and more reactive.

The best recommendation I can make is, “Unless you are looking to change the stability and handling of your bike, don’t roam much from the span and rake numbers of the fork your frame was designed around.”

Price: Full carbon aero forks will run $350-$500. For those on a tighter budget, the aluminum steerer tube and crown equipped Reynolds Ouzo Comp Aero ($260) should prove one of the best riding “economy” aero fork options when it hits the market soon. Another inexpensive aero option is to find an original equipment fork that was pulled off a stock triathlon bike that was upgraded at the time of purchase. Also, if aerodynamics can take a back seat to ride quality and handling, consider one of the many quality carbon road forks starting around $200 from companies like Kestrel, Profile and Reynolds.

Best of luck in the pursuit of speed.

Ian

Originally published April 2003/Copyright © 2003