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I've been taking my son for a ride every weekend and there are a couple of sections where we get off the road and into a nature strip, you could call it light gravel although as we get close to the end of this section the ground ripples which is partly cushioned with grass.

My son and I are always relieved that we have suspension seat post, of the coil spring inside the seat tube type, that do a decent job.

Since the vibrations are dampened by the spring coil, I wonder if a suspension seat post of this style can potentially extend the life of a bicycle frame that often get off road and into gravel ground and sometimes submitted to going up and down the curbs.

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I wonder if a suspension seat post of this style can potentially extend the life of a bicycle frame that often get off road and into gravel ground and sometimes submitted to going up and down the curbs.

Depends on what you compare to.

If you compare to a skilled rider who is not tired, the skilled rider would anyway use leg muscles to dampen all shocks, if riding on uneven ground. Compared to such a skilled rider, there is no extra protection.

However, if you compare to a tired and/or careless rider, who is just sitting on the saddle even though the ground is uneven, then the suspension seat post is a massive benefit to the fatigue life of the frame.

So yes, a suspension seat post can increase the lifetime of a frame. The way frames fail is by fatigue (exception: carbon fiber doesn't fatigue), and by reducing the forces the frame sees increases the fatigue life, perhaps in the case of steel even from a finite number of kilometers to an infinite number of kilometers. Aluminum always has a finite fatigue life, but the increase in frame lifetime can even be 100x - 1000000x (or even more!) 1 if you use a suspension seat post.

Carbon fiber frames fail by encountering forces they were not designed to withstand. In this case, I suspect a suspension seat post could be of benefit too, since there's always a risk that if riding too fast on uneven ground on a crazy-light carbon fiber frame, that the forces the frame would need to encounter exceed its design limits.

(1): Yes, this figure is correct. See the aluminum fatigue life curve here: https://en.wikipedia.org/wiki/Fatigue_limit

Note that decrease from 45 ksi to 25 ksi increases number of cycles 100-fold, and that decrease from 45 ksi to 15 ksi increases number of cycles 100000-fold. It is perfectly reasonable to assume that a suspension seatpost would reduce the loads the frame has to withstand to a value even much smaller than one third of the loads in a non-suspension-seatpost-frame. Of course, this applies only to fatigue life. For example if you increase the fatigue life of your frame million-fold, but at the same time don't ride more carefully nearby cars, it is possible you only experience 10x the frame life since a collision with a car kills the frame. Also, on aluminum frames you will eventually have worn away bottom bracket threads, which is the end of the frame, and the bottom bracket threads don't care that your frame has million times the fatigue life due to a suspension seatpost, they will wear out at their natural rate anyway.

So the figure 100x - 1000000x means for fatigue life. There may be other reasons a frame fails.

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  • @Rеnаud a skilled rider, also when riding in the saddle, will still watch the ground before them, anticipate any larger bumps and prepare for them by pretensioning the legs so they take most of the impact instead of the back (or in fact momentarily get out of the saddle). And even when not actively doing this, a skilled rider will have much less weight on the saddle than an unskilled rider, because the legs are always pushing with substantial force against the pedals.
    – leftaroundabout
    Commented Sep 3 at 7:57
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    Good answer BTW, but why post bogus figures like "100x - 1000000x" lifetime increase? That's obviously unrealistic.
    – leftaroundabout
    Commented Sep 3 at 8:00
  • When going through rough terrain, it's recommended to shift to a higher gear and pedal through it as it will increase pressure on your pedals and offload your saddle.
    – MindDBike
    Commented Sep 3 at 14:12
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    @leftaroundabout The figures are not bogus. I explained them now, they quite clearly match the fatigue life curve Wikipedia has for aluminum.
    – juhist
    Commented Sep 3 at 15:48
  • "In this case, I suspect a suspension seat post could be of benefit too, since there's always a risk that if riding too fast on uneven ground on a crazy-light carbon fiber frame, that the forces the frame would need to encounter exceed its design limits." This is pure nonsense, even road frames will not fail unless you go for large stunt jumps. But then you do not sit in the saddle.
    – Vladimir F Героям слава
    Commented Sep 5 at 6:48

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