Fork Deflection Test
by Damon Rinard

What is deflection?

Deflection is flex. All forks do it. A fork with lots of deflection is flexible, and can feel squirrelly or soft for heavier riders, but may be perfectly matched for lighter riders or riders who want a little more comfort. A fork with very little deflection is stiff. A stiffer fork can be jarring over bumps, but often is more precise in handling.

Why is deflection such a big deal?

It's not, really. There are other factors that go into a decision of which aftermarket fork to buy, if any. And many people of all sizes enjoy forks that have a wide range of deflections, so there are exceptions to any rules I might try to come up with based on rider weight, etc. But you may be considering buying a new fork, and knowing just how it stacks up against what you have already ridden, helps you predict how the ride of your bike might change.

If you are looking for more precise handling, buy a fork that is stiffer. If you are looking for comfort, buy one that is more flexible (or one that has other design features that provide comfort in other ways). If you are looking for light weight, you must decide how flexible is too flexible for your taste, or you may be interested in which forks have the best stiffness-to-weight ratios. And aerodynamics are almost always more important than other factors if solo speed is your goal.

How my test was conducted

I performed a non-destructive quasi-static deflection test. In other words, I anchored the steerer tube horizontally 5 cm from the fork's crown seat and hung a 47.5 pound weight on the dropouts. In all cases a Dura-Ace hub was installed with the skewer tight. For the Lateral (side-to-side) deflection numbers, the hub axle was vertical and deflection was measured with a dial indicator in the direction of loading. For Longitudinal (in line with the direction of the bicycle's travel) deflection, the hub axle was horizontal and the fork blades curved toward the ground. Again, I measured deflection in the direction of loading.

Why not measure torsional

Torsional deflection is the result of a twisting load on a fork. An example of a torsional load is the effort you make when trying to remove a stuck handlebar stem: you twist the handlebars while holding the front wheel with your knees. I didn't measure torsional deflection because I believe it has almost nothing to do with how a fork experiences loads during real-life road riding. If riding did load the fork torsionally to any significant degree, we would have to wrestle the handlebars to keep the front wheel steering correctly. Even during cornering, a rider applies virtually no torque on the bars. In fact, many well-balanced riders can corner quite sharply with "no hands." You can't apply or resist significant torque without hands on the bars.

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

Now we get to the controversial part. For years, framebuilders' conventional wisdom held that a fork ought to be flexible longitudinally and stiff laterally. People thought that such a fork would have comfort (achieved with forward deflection) and precise handling (due to the lateral stiffness). However, I am presenting a new theory: longitudinal stiffness is more important for precise handling than lateral stiffness, and lateral deflection can measure quite high before a rider will experience handling troubles. There are two reasons I believe this is true.

One is that longitudinal loading must be higher than lateral loading. I say must because I havn't measured a fork during riding to be sure. (This is, after all, just a theory.) Because of gravity, the rider's weight 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.)

Also contributing to the longitudinal load (and very little to any lateral load) are the impacts absorbed when the rider hits bumps. The magnitude and direction of bump forces are related to the speed and direction of the bike, and whether the brakes are applied. Now, we presume that a rider who is in control of the bike always goes forward (never sideways), so the direction of any bump force is fore and aft, not lateral. That takes care of the direction of the bump force. It must be true that the magnitude of the bump force increases with speed. In other words, the faster you go, the harder you hit bumps. No surprises here.

These large fore and aft loads indicate the relative importance of longitudinal stiffness because as the loads increase, the deflection increases, and that changes the fork rake at any given moment. If the fork rake changes, the steering geometry of the bike changes, and the rider has to adjust to the bike's constantly changing feel and response. The only time a rider does load the fork laterally is while tossing the handlebars side to side. This happens out of the saddle of course, during sprinting or climbing, for example.

The other reason I believe longitudinal stiffness is more important than lateral stiffness is that people like the handling of forks that are stiffer in the forward direction, such as steel forks, or the Kestrel EMS fork. Okay, my bias is showing: the Kestrel fork tests well in the longitudinal direction, and I used to work for Kestrel. But seriously, how many people do you know who wish their bike didn't have a Kestrel fork, or who don't like the way it rides? The numbers show that of all the forks I tested, so far only steel forks have matched the forward stiffness of the Kestrel fork.

Will you notice a difference?

Some riders seem not to notice anything about their bikes. Other riders notice everything. Some riders even notice differences that don't exist! Truthfully, the differences between forks are pretty small. Nice light wheels make a much more noticeable difference. Padded handlebar tape or slightly more or less air pressure make about as noticeable a difference as a new fork, in my experience. But many riders rave about the improved sprinting a stiff fork gives them, or the new found confidence on fast downhills. Others love the comfort they can finally enjoy on longer rides after installing a more flexible fork. The bottom line is you might 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. Perhaps the best way is to use wider tires, since tires are far more compliant than any rigid fork can be.

 

 

How to read the table

Brand, Model are the brand and model of the fork tested.

Lateral is the lateral deflection in millimeters.

Longitudinal is the longitudinal deflection in millimeters.

Weight is the weight of the fork in grams, normalized to a standard 175 mm steerer tube length.

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

Notes:

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.

See also The Rinard Frame Deflection Test

Edited and converted to HTML by Sheldon Brown

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Copyright 1996 Damon Rinard

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