Instability and Speed Wobble
Calfee Design has identified a cause of speed wobble (a.k.a. shimmy) and instability that can be prevented. Speed wobble is a dangerous condition that can cause the rider to lose control of the bicycle and crash. While a skilled rider can prevent and stop speed wobble, it is better if it doesn’t start. Some experts state that speed wobble is caused by the rider, which is technically true because the rider responds to the steering dynamic, initiating a resonance that causes the frame to act as a spring. While loose headsets and out of true wheels and frames can contribute to starting a speed wobble, we have found that fork asymmetry can also get it going.
Fork symmetry is defined as the symmetrical position of the fork dropouts in relation to the steering axis. Specifically, the equality of the distances from the dropout faces to the steering axis must be within a certain tolerance for the bike to ride in a stable and confident manner.
Traditionally, steel forks were cold set after welding or brazing to realign them after possible distortion caused by the heating and cooling of the metal. A diligent steel frame builder can align the fork blades to within a millimeter of symmetry. Certain well known builders align them to within 1/2 mm.
Carbon fiber forks cannot be cold set. They must be molded straight to begin with. We have found that a small percentage of carbon forks by various makers were molded with asymmetrical fork blades. Some are off by a little over a millimeter and others are off by two or more. Forks that are off by over 1.8 millimeters in symmetry have a good possibility of being prone to speed wobble. A symptom of a fork that is off by 1.8 mm or more is a noticeable difficulty when riding no hands at a slow speed (less than 10 mph). One has to lean to the side slightly to keep going straight. A bike with asymmetrical forks seems to corner better in one direction but not so well in the other. At speeds of 30 mph or more, the bike can develop speed wobble.
If your bike has the above-mentioned symptoms, the fork should be measured for symmetry. This is difficult to measure without proper tools. Calfee Design measures all forks for symmetry and is equipped to measure any fork. If any Calfee customer wishes to have their fork checked, please send it to us with a letter requesting a fork inspection. Non-Calfee customers may send their forks for inspection for a nominal fee. Replacements may be available for asymmetrical forks, depending on the individual fork maker’s policy.
Fork Inspection Setup:
- Fork blades must be square to the surface plate
- Rotate fork in V blocks to measure the other dropout
- Difference between the two measurements must be no more than 1 mm (Calfee tolerance)
Analysis of the Effect of Asymmetry:
The following illustration shows how a small amount of asymmetry translates to steering forces projected to the ground.
As a bicycle travels forward, the tire patch must follow directly behind the steering axis in order to travel in a straight line with no steering input. If the rider wants to go straight, It’s a simple matter of balancing on the bike so that the steering axis is lined up with the two tire patch centers. On a bike with a symmetrical fork, the rider balances symmetrically and needs almost no steering input. It is very easy to ride no hands on a symmetrical fork.
With an asymmetrical fork, the steering axis is not lined up with the center of the tire patch. The rider must lean to one side to move the front tire patch in line with the steering axis and rear tire. patch. A skilled rider can ride like this with little trouble, using very subtle weight shifting corrections. But they will find it easier to initiate a left turn on a bike with a fork whose steering axis is off to the left. Turning to the right is more difficult as it requires more leaning of the bike and steering input to bring the steering axis to the right of the front tire patch. Some riders will react more coarsely to the needed balance and steering corrections, possibly initiating a speed wobble. At best, the rider will spend more energy and concentration in making a high speed turn. In a race, it can mean the difference between winning, losing or crashing.
There are two interesting cases of repeatable speed wobble which occurred when the rider was descending on a curving road. At narrow speed range (from 35 to 40 mph), the rider can make a left turn easily, but at the right turn, he always experienced speed wobble starting up. Luckily, both riders had enough skill to maintain control and slow the bike down to diminish the oscillation. The forks on these two bikes were off the left by over 2 mm.
It is difficult to get test subjects for speed wobble experiments so we can only rely on the group of people who have had their speed wobble problems cured by replacing the crooked forks with symmetrical ones. But one test, conducted by a fork manufacturer, had three skilled riders try out three different forks. One fork was close to perfect symmetry, the second fork was just outside of 1 mm and the third fork was out by 2mm. The three riders didn’t know which fork they were testing when taken for a short ride. Each rider rode all three forks on the same loop. None identified any fork to be particularly bad, but they all identified the nearly perfect fork to be the best handling
What can be done?
The issue of fork symmetry is not yet fully appreciated by the fork producers. At this time Calfee is the only company requiring a tolerance on fork symmetry. It takes a little longer to make a carbon fork that is within tolerance. The fork must remain in the mold during the cool down phase of molding the fork. If the fork has a slight asymmetry after being released from the mold, it can be corrected when the dropouts are bonded. This is done by simply holding the steerer tube and crown race in a rigid clamp while the dropouts are being bonded. Obviously the bonding fixture must be accurate, but this is easy and commonplace with CNC machining of tooling. As the road cycling market becomes more demanding of higher quality products, perhaps the fork producers will invest in improving their processes and tooling.