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When I recently cracked my frame near the headtube, every bike shop that I went to, asked me explicitly "Did you put a longer travel fork?".

Putting aside that apparently fork travel and fork crown height are two entirely different measurements, and that the fork height is actually critical, how does a higher fork contribute to higher stress in the frame?

Also:

  • What is the effect of lower fork than the original?
  • Does bike A, being sold with higher fork than bike B, mean that A's frame is stronger? Or is it just different geometry?
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  • 3
    Added a new tag "reliability", which seems to come up a lot lately: reusing chain quick links, squelching brakes etc.
    – Vorac
    Commented Oct 1, 2013 at 8:29
  • Might be a duplicate of safety, though I do not think so. Please vote one of this comment or the upper one. If this one gets more upvotes, I will remove the new tag.
    – Vorac
    Commented Oct 1, 2013 at 8:33
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    Longer forks mean a longer lever, transmitting greater forces to the headset.
    – andy256
    Commented Oct 1, 2013 at 9:34
  • As Andy says, a longer (when fully extended) fork will apply more leverage to the headset. But the fork would have to be 30-50% longer, headset to dropout, for this effect to be significant -- 2-3 inches would be negligible. (I suspect the fork just looked longer, and they were asking out of curiosity, or simply wondering if the fork had been changed.)
    – Daniel R Hicks
    Commented Oct 1, 2013 at 11:27
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    My guess is its a reasonable indication of how the bike is used - no one increases fork length to ride to the corner store....
    – mattnz
    Commented Oct 1, 2013 at 20:19

1 Answer 1

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how does a higher fork contribute to higher stress in the frame?

By creating a longer lever, and stretching the end of that lever to a greater angle, it transmits more force to the bottom of the head tube (the part of the frame where the steer tube passes through). This can cause damage to the head tube itself or where it joins both the down tube and top tube. How does this happen? That longer lever is essentially trying to pry the head tube off of that joint.

What is the effect of lower fork than the original?

A fork with less travel (or similar travel but a shorter overall length) will steepen the head angle which will make your steering a bit quicker and more responsive. Now, this does reduce the force applied to the lower part of the head tube, but due to the change in angle, increases the force applied to the upper part.

Does bike A, being sold with higher fork than bike B, mean that A's frame is stronger? Or is it just different geometry?

Simply: no. Frames are like any other product where the visual inspection does not guarantee that the assumption is true. For instance, most tapered head tubes with fork travel X are probably stronger than a straight head tube with the same fork travel. The purpose of the bike will also determine a relative frame strength. A dirt jump bike with a 100mm fork will handle way more stress than a XC frame with 100mm fork. The trade off is usually flex, weight, or another metric.

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