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I see many mountain bike reviews and articles discuss the stiffness of frames, forks, wheels and other components. Here are some questions which cross my mind:

  1. What is the definition of stiffness?
  2. How do you measure stiffness?
  3. How does stiffness affect riding experience and performance?
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4 Answers

up vote 3 down vote accepted
  1. A Wikipedia definition is pretty much applicable. Stiffness is how much things resist bending.

  2. Stiffness is measured by measuring how much stuff bends when lateral force is applied. So, in the example of frame stiffness, you could put one end of the frame in a vise, horizontally, then hang a known weight from the other end and measure how much it sags.

  3. Because bending takes and dissipates energy, the stiffer stuff is, the less energy it robs from the rider. In general, you want everything in the system to be as stiff as possible, except the elements that are specifically designed to be flexible, like tires and shocks. Also, stiffer bikes (or any type of vehicle really) are more predictable in handling, because they have less degrees of freedom, are less prone to vibrations etc.

All that being said, the effect of stiffness is generally felt only by the performance oriented cyclists and is typically secondary to other considerations, like weight. For instance, for years MTB XC racing cyclists used the lightest forks possible like RockShox SID, even though they had the reputation of being "flexible like maccaroni" because weight trumped all else. Only in recent years, and especially in the road world where there is a fixed lower limit for bike weight in competition, manufacturers started to focus on stiffness as one of the main differentiating factors of their components.

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Stiffness is a frequent point of debate in cycling circles. For instance, touring bikes tend to still be made of steel rather than aluminum, because (among other reasons), for equivalent strength, steel is more flexible, and will absorb road shock better. A common complaint against early aluminum frames was that they were too stiff and rode too "harshly". (And note that a flexible frame does not "dissipate" energy to any significant degree -- steel has a very low damping factor, so the energy that goes into flexing the frame comes back out as "bounce" of a sort.)

OTOH, stiffness is valued in the "twisting" mode of the bike. A frame that is not stiff against twisting between front and rear wheels will be more likely to become unstable and vibrate at high speed.

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Allow my rather anecdotal answer, as for the third point in the question.

I used to ride a stock Trek VRX 400, did XC riding along with some jumps and drops. I was 18-22 years and I think my weight was around 120 lb. At that age the bike felt perfect, I loved that bike, I felt the bike very responsive, and each time I wanted to learn some new move, I felt no "resistance" from the bike.

Some time later, My VRX broke, so I acquired a Diamondback X2 (this is a 1999 model, I bought the frame in 2007, I think...). For some reason I also inherited another VRX (Trek VRX 200). So now I had two bikes. I used to ride both in the same trials and trying the same stunts. The X2 had very lower end components compared to the VRX yet I liked it much much better, I'll tell you why...

By that time I was 24, (I'm 28 at the time of writing) and my average weight had raised to 160 Lb. (Today it's 175). I had a little more strength by then, I used to try to measure it pressing a scale against a wall with my legs...

Now, the bike comparison: The VRX is noticeably lighter than the X2. The VRX is a lot more flexible. With just sitting on the bike, and applying a litle force to the right pedal I could see the hole bottom bracket part go to the left, It was evident, I mean, you could literally see the frame twisting. Both bikes had the same crankset sizes, but the VRX had a 11-32 cassette, the one in the X2 was 11-34. The two bikes had some components that where the same, as I used salvaged parts from my broken VRX to assemble the X2. Had the same shifters, same rear derailleur, pedals, and tires. However, the X2 had V-brakes, and lower end hubs and rims. The inherited VRX came with disk brakes, bontrager rims, a bontrager front hub and a DT Swiss rear hub. Both bikes where ridden with WTB Velociraptor front tires @40psi. The VRX had a Velociraptor Rear (specific) tire at 40psi. The X2 started with a generic tire and was later upgraded to a WTB Prowler XT. both @40 psi. The X2 was initially assembled with the Manitou XVert-t2 fork from the broken VRX400 and was later updated with a RockShox Tora 302, While the VRX200 was rode with a Manitou Xvert DC.

While riding, the X2 had far better acceleration and climbing. This was not because of the cassette, because I shifted to higher gears in the X2. For example, a hill I climbed using 2-2 (2nd front gear, 2nd rear) in the VRX, has easy for me in 2-4 or even 2-5 with the X2. With the X2 I was even faster than a friend who rides a Trek Y 26 superlite (Carbon), the same guy beated me up if I rode the VRX.

I the descents I quickly became more agile with the X2, mainly because of two aspects. The perceived stiffness of the X2 frame made me feel the bike had a faster response to my input, I also got a much better feel of the ground through the bike. For example, with the X2 I was able to tell exactly when a tire was about to skid.

I became so confident using the X2, that I started Downhill Racing with it, something I felt very reluctant to do with the VRX.

After about a year of riding both bikes, the VRX had turned into my backup bike, used only when the X2 was on the repair bench or once in a while to avoid rust...

To summarize, the Stiffness in my X2 made feel much more confidence, and provided me way better "communication" to and from the bike, much faster response and terrain feedback. I could tell exactly where the tires were passing. This developed into much more maneuverability, despite the X2 having a slightly longer wheelbase.

The VRX, although had a light feel, is simply inefficient, I was never able to ride the same slopes with the same times. It made me feel tired more quickly, and had a dorky sensation along rocky ascents.

In conclusion: the stiffer frame outperformed the other enough to compensate for being heavier, and as a whole bike, it also compensated the lower quality components.

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Stiffness is important when you have long tubes as in a tandem or elongated recumbent and your frame is made of a flexible material like steel. The frame can flex, especially with the pedalling of a strong or heavy rider which can lead to metal fatigue over time and can be disconcerting to the captain. For upright singles it's a non-issue.

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