Do suspension forks necessitate bigger/stronger tubing?

General Discussion
1

Does a fork increase or decrease the stresses a frame is exposed to?

And a related but (potentially) different question: Does a longer travel fork increase the stresses a frame is exposed to compared to a shorter travel fork?

It would be intuitive to think that suspension forks, and in particular long travel suspension forks, decrease the stresses a frame is exposed to when riding hard because impacts are absorbed by the suspension.

Now I was reading the very detailed/admirable documentation by Fairlight of their XC bike (https://fairlightcycles.com/wp-content/uploads/2022/05/Holt-Design-Notes.pdf). Here are two quotes from that pdf:

“In my opinion steel can excel when making a short travel hardtail. The word ‘short’ is an important one because as fork travel gets longer you have to use larger diameter tubes (usually with thicker walls) to deal with the increased forces.” (p. 2)
“The wall thickness @ 1.15/0.9/0.6/0.9 (with additional external gusset) means we have a strong tube to cope with loading forces from a 120mm fork.” (p. 8)

This would suggest that forks increase the stresses in a frame, particularly long travel forks.

It could be that Dom (Fairlight) just wants to say that suspension forks are an indication of rougher riding and therefore require stronger tubes. But that is not what it sounds like (see quotes above). :thinking:

2

Longer suspension forks have more atc/stack and thus a longer leverage while crashing into things, which leads to higher bending moments on the HT/DT junction.

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3

That makes sense. And that effect is larger than the suspension absorbing impacts might decrease stresses?

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4

Yep, the bending/buckling load cases on the HT outweigh the upwards shear loads by a few orders of magnitude.
The suspension is only acting in the upward direction and not in the fore/aft direction, and furthermore, the suspension is not taking the load off the frame. It only changes the way the fore is introduced and spread over time - the “ramp up” of the force is not as abrupt as it would be without suspension

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5

Makes sense. I hadn’t thought about that much because I’m not designing a suspension fork bike. But it is very useful when comparing tubing specifications between suspension fork frames and rigid fork frames.

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6

Shouldn’t then headsets with bigger bottom stack heights such as “External Cup” mean more stress for the DT and the TT? Those bigger bottom stack heights move the tubes further away from the impact which increases the lever just like fork length does.

The difference between a EC and a ZS headset bottom is roughly 8mm (in the case of Cane Creek it’s a 8mm difference) which isn’t totally insignificant. A 100mm fork has about 100mm more ATC than a typical rigid gravel fork.

7

Agreed.

Another factor: you will ride a 140mm bike much harder than a 120mm bike.

I did some hand calculations a while back, and the difference between the external cup and the integrated cup is marginal. Also, the external cup is not usually the limiting factor; it is the clearance between the fork crown and the downtube, which usually forces the DT higher on the headtube.

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