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Fork Deflection Test
by Damon Rinard
with additional comments by John Allen

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What is deflection?


Why is deflection such a big deal?



[Comment from John Allen: If a fork is weaker longitudinally than the bicycle's frame, it can spare the frame from damage in a curb impact or hard landing.. A stiffer fork tends to be stronger, though not necessarily.]

How my test was conducted


Why not measure torsional deflection?


[Comment by John Allen: torsional loading does occur in the case of "speed wobble" or shimmy of the front wheel. I suspect that the torsional flex of a suspension fork contributed to a very troublesome speed wobble I once experienced. However, Rinard did not test any suspension forks. With an unsuspended ("rigid") fork, most torsional flexibility is in the bicycle's frame.]

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What's more important:
lateral stiffness or longitudinal stiffness?


One is that longitudinal loading must be higher than lateral loading. I say must because I haven't measured a fork during riding to be sure. (This is, after all, just a theory.) Because of gravity, the rider's weigh loads the bike almost purely downward, especially in turns. Turning causes virtually no lateral load because a rider leans the bike so that loads are nearly perfectly in plane with the wheels. If he didn't, the bike would fall over. The rider's weight then exerts a larger forward load on the fork than it does a lateral load (on the order of, maybe, three times greater, as a guess.)




Will you notice a difference?


Is there more to comfort besides deflection?

Yes and no. There are two sources of discomfort in the road: gross bumps (like potholes or reflector dots you hit at speed) are one. The other is the smaller texture of the pavement itself. This texture produces a high frequency vibration as your tire rolls over the road.

The gross bumps can be made more bearable with a fork that flexes a little more. For extremely bumpy roads (like Paris-Roubaix), there are even suspension forks. But the second source of discomfort, the constant vibration, is harder to address. About the only way a normal road fork can lessen them is by the natural damping properties inherent in the material used to make the fork. Perhaps the best way is to use wider tires, since tires are far more compliant than any rigid fork can be.



[Comments by John Allen:

Mechanical resonance in the bicycle frame and fork depend on mass, not only on flexibility and damping. I think that the most significant effect of damping is to reduce the height of a resonance peak, which can magnify road vibration, increasing discomfort.

Damon Rinard provided no explanation for the graph below, which was in one of the two versions of this article on our site -- or maybe Sheldon deleted the explanation. The graph is a plot of forward (longitudinal) flexibility of forks against lateral flexibility. For most forks, they are roughly proportional. (Note that flexibility increases toward the right and top of the graph). The Kestrel fork and the Trek Classic Carbon fork both have an intermediate level of lateral flexibility, but the Kestrel fork has unusually low forward flexibility, the Trek Classic Carbon fork, unusually high. A traditional steel track-bicycle fork with round blades would have an unusually high ratio of forward to lateral flexibility, but apparently Rinard did not measure one.

The scale of the graph is in inches and that of the table in the next section of this article, in millimeters, but both are for the same measurements.]


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How to read the chart


Lateral is the lateral deflection in millimeters.

Longitudinal is the longitudinal deflection in millimeters.


Brand, Model Lateral Longitudinal Total Weight
Schwinn Paramount (1) 3.81 3.30 7.11 n/a
Merckx sloping crown 3.81 3.30 7.11 777
Colnago, straight blades 3.81 3.56 7.36 730
Kestrel EMS Composite (2) 5.33 3.56 8.89 526
Tange Silhouette, CrMo 5.33 3.81 9.14 590
Kinesis Easton aluminum 5.59 4.06 9.65 507
Dimension Carbon (3) 5.33 4.32 9.65 482
Time Carbon (2) 4.83 4.32 9.14 525
Look Carbon (4) 6.60 4.32 10.92 447
Fuji Titanium 4.75 4.37 9.12 n/a
SR Prism (2) 5.08 4.57 9.65 546
Trek OCLV 5.08 4.83 9.91 508

All steerer tubes were 1 inch in diameter.

  1. Waterford-made, using Columbus tubes with Henry James crown.
  2. Average of four samples.
  3. Now called Look LDS.
  4. With carbon crown and steel steerer tube. Much like the HSC1.
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