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May 20, 2012
TECH NEWS BROUGHT TO YOU BY BIKEEXCHANGE
We all know that the fork is an important and integral part of the bike, but it’s difficult to define what makes various forks different, what’s marketing fluff, and how a fork can completely change the geometry and handling of a bike. I had a good long chat with Darren Baum about this topic where I bombarded him with my questions.
A fork’s purpose is obviously to hold and steer the front wheel. The biggest variable between forks is a characteristic called “rake”. Rake refers to the curvature or angle of the fork blades. More rake will equate to faster steering (i.e. less input required to make the bike turn). The product of head angle and fork rake gives you a measurement what is referred to as “trail“. Trail is a figure that will reflect how fast a bike actually steers. More trail equates to slower steering, less trail will make faster steering.
One extreme in terms of rake is a track bike, which has very little rake (normally under 40mm). Many people think that a track bike turns quick, but they actually turn very slowly. The other end of the scale is a touring bike which traditionally has a lot of rake (50mm plus) which makes the bike turn very fast.
I’ve talked about “rake” and “trail” before in Geometry of Bike Handling. This diagram will help you visualise rake and trail:
Another variable between forks is the crown to axel height. This is the measurement between where the headset sits onto the fork, and where the axel of the wheel is fastened. We’ll talk more about this in a moment.
What To Look For In A Fork?
A fork should be laterally stiff while able to dampen out vibrations from the road. Lateral stiffness is very easily built into a fork while torsional rigidity is not. Since the fork is fastened onto the wheel, the hub is a structural member of the fork and therefore easy to achieve lateral stiffness (much morethan what you feel when you bend a fork with your hands without the wheel attached). Torsional stresses on a bike aren’t huge, but having a torsionally flexible fork will make your steering seem sluggish on descents or fast corners. There will feel like there’s a lag between handlebar input and bike reaction.
The quality between various forks can widely vary. Most forks these days are monocoque (single piece mold.) The gold standard of bicycle forks is a brand called THM out of Germany. What makes THM forks so good is that there isn’t a wasted layer of carbon. Many carbon parts are built with excess carbon layers used as a buffer to make certain that there are no single points of failure or weaknesses. They do this instead of doing NDT (non-destructive testing) using ultrasound or x-ray which can get expensive and time consuming. THM eliminate excess material without compromising its lateral rigidity and safety by doing NTD on every single one of their forks. Their workmanship is superb.
Companies like ENVE make very good forks. They do have good techniques, good inspection processes, and extra layers of carbon to serve as a safety buffer. EVNE make a couple different types of forks. The only real difference is the weight. To save weight basically requires less material, therefore fewer carbon layers. Lighter forks will generally take more time and have better inspection processes.
One of the biggest difference between forks these days is that some look more aero than others. For example, the 3T Funda is a much more aero-shaped fork than an ENVE, Easton or Reynolds (not necessarily more aero though).
Replacing Your Fork?
An important variable you should look at when looking to replace a fork on your existing bike is the axel to crown height. If the axel to crown height is changed, so will the head angle and therefore the bike geometry. If you went out and bought a new fork with a lower axel-crown height with the same rake as your last fork, your head angle would steepen up (i.e. rake would increase) which would make the bike turn faster. You could counter-act that by using a fork in with more rake, but at the same time it would lower your bottom bracket.
If the bike was designed around a certain crown-axel height and you put a taller one on, it would tilt the whole frame backwards which slackens off the head-angle and seat-angle. This will make the bike handle/turn slower. These days it’s getting more difficult to swap out forks because they’re often integral to the geometry of the frame that it was designed with. These days there are many crown-heights between fork to fork.
To illustrate crown heights on various forks I measured the difference between my Focus and a Scott. There is a 10mm difference between them (the Scott being larger). This makes the Scott frame appear to look smaller while keeping a desired geometry. Scott is a good example of a bike with a massive headtube area and has made that axel-crown height even higher.
Fork legs have traditionally been curved so as to provide sufficient trail for stable handling of the bike. The first manufacturer to change their curved forks to a straight design was Colnago in ~1987. As the story goes, Ernesto Colnago and the Ferrari engineers discovered that a curved fork didn’t absorb road vibrations and shocks as well as a straight bladed fork. From this testing, Colnago’s PRECISA fork was born and many other brands followed, though the dampening characteristics of straight legs remains debatable. Today, the main reason that forks are often shaped in a straight line is because it’s easier to lay up the carbon and to manufacture.
The late Franco Ballerini on his way to winning Paris-Roubaix in 1998 with the Precisa straight fork. Not that the fork helped him win Paris-Roubaix twice...Photo: Cor Vos
Trek recently introduced its new Domane 6 Series with the IsoSpeed fork that claims, “generous ride-tuned sweep and shape increases compliance for greater comfort, and unique dropout placement optimises the wheelbase.” As you can see from the picture of the IsoSpeed fork, it looks a lot like a traditional curved fork, save for the placement of the dropouts.
Wheel and Fork Interaction
The thing to remember is that no matter what aerodynamic claims you hear, the fork and the wheel go hand-in-hand with overall aerodynamics. Because the fork that attaches to the front wheel is at the leading edge of the bike, the front wheel throws up turbulent air. Forks can certainly make a bike more aerodynamic, but only if the wheel interacts with the fork in a favorable manner. BikeTechReview.com did some interesting wind tunnel testing that revealed the effectiveness of different aero forks on a constant zipp 404 wheel.
We’ve seen manufacturers attempt to smooth this turbulence by putting slots in the fork. For example, Oval Concepts sold their patent to this technology to Ridley which was included in Cadel Evans’s time trial bike in 2008 (when he came second). We also saw Ridley introduce integrated breaks into their forks in an attempt to reduce turbulence caused by the brakes on the forks.
Darren Baum tells me that his all-time favorite riding fork of all time was “Woundup“. Scapin and Seven used these forks a lot in the past. It was a completely different fork design. The tube itself had an aluminium crown but the fork legs themselves were “wound up” with carbon. These were ugly as sin but rode beautifully. Darren tells me, aerodynamics aside, the Woundup was the greatest riding fork he’s ever ridden. Modern forks don’t come close to the ride characteristics of the Woundup.
I’d like to thank Darren Baum for sharing his technical expertise to help me write this article. Darren highly recommends reading anything by Tom Kellogg from Spectrum Cycles. He’s the fork guru in the industry.