Frame compliance - and target rider weight for frame size

In the Nerd Alert with Specialized’s Chris D’Aluisio, there was a discussion of the challenge that with newer frames gaining compliance through more flexible seatposts, when there is less seatpost exposed, there is less of that (helpful) compliance.
I see this as an issue for any rider that is at the smaller end of the range of riders for a specific frame size. For an example, with my size 52 cross bike (that fits me quite well) with a fairly horizontal top tube, there is only 30 cm of exposed post. While for someone else (with longer legs that also fits well of that 52) might have 100 mm of exposed post (and thus a smoother ride).
I wonder if anyone has measured the change in flex with various amounts of exposed seatpost - to get a sense of how important this aspect of compliance is?

And on a related issue, since bikes are sized on leg/torso/arm length - and not on rider weight, I wonder if any manufacturers have published their design rider weights per size? Again just to be specific, I am around 58 kg and fit on a size 52 bike, but I could imagine there are riders that could fit on a size 52 bike that weigh as little as 50kg and maybe as much as 75kg - and I would expect the frame/fork would behave quite differently if it had a design rider weight of 60kg versus 75kg. Thoughts? Data?

thanks for any feedback.


I don’t think it’s significant in mass production frames, because bikes are built to withstand a 120kg system weight (or so, I forgot the exact number) for safety standards, even the smaller sizes. If you weigh 60kg then everything is pretty much “overbuilt” for you.

From observation, if I’m carrying a 10kg backpack with stuff (which isn’t as bad as it sounds, but I’d still get a rack for anything really long distance) the bike doesn’t seem more comfortable or smoother to ride because the seatpost flexes more.

In theory, rider proportions (and hence, exposed seatpost) matter, but in practice if the frame is of the right size, it probably amounts to the effect of one or two psi more or less in the tires as far as compliance goes.


I had quite a long conversation with David Kirk (who makes the best looking steel frames out there IMO) about this. It’s not simply that many production frames are overbuilt for most riders - given that 110-120kg is the usual safety maximum - it’s that it gets worse on smaller frames. A smaller frame is (almost always) stiffer than a larger equivalent, and the riders are often lighter.

It’s one of the reasons that many riders of custom steel or ti frames swear they’re more comfortable. It’s got nothing to do with material, but more that the metal frame, being individualised for the 60/70/80k rider who bought it, isn’t as massively overbuilt as the production carbon options typically available to them.


While that’s all true, I’m not sure how much of a problem it is in practice.

Some people do buy custom, but I know lots of people who’re fairly light and ride mass production frames without any complaints or any desire to go custom.

I mean, what is an optimal stiffness for someone? I don’t think there’s a formulaic way to figure it out by looking at the rider size, weight and power and giving us how many N/m is the ideal stiffness of tubing for the rider, or how much a frame or fork need to flex for a given rider.

Vertical flex in a carbon frame is tuneable to a point, though few frame builders offer frames built with a custom layup. Like, almost nobody. It absolutely makes a difference, but it’ll co$t.

Owners of custom steel and ti frames will always tout their choices - it’s practically a religious matter. Steel and Ti do ride differently from Carbon. I build Carbon frames for a living, but most of my bikes are steel. It’s great stuff. Are my steel bikes more comfortable? Hard to say. I can definitely tell you that if I want to ride my fastest bike, there will be a carbon frame under me.

Seatposts are a great way to get some extra comfort out of any bike, if you don’t mind some trial and error testing different posts. In bike fits, I’ve seen some posts flex enough that I actually moved the saddle forward to offset the effect. There are companies that build seat posts for maximum flex and they’re absolutely worth trying.

Really, though, if you want to make any bike more comfortable, lower tire pressure is a good way to go. Very few people are riding their road tires as soft as they safely could. And if you’re worried that you might get pinch-flats, go up a tire size or two, if it fits.

thanks so much for all of the helpful comments and ideas!

I agree that wider tires and lower pressures are a good way to tune the ride comfort of an existing bike (depending on the route and intent of the ride).
One reason I was asking is that I am considering a new bike - and like the geometry of the new Crux but expected that since it is 5 years newer than my current Cross race bike (Fuji Altamira CX 1.3), which is also my gravel bike (with wider tires), I assumed it would be stiffer and that Nerd Alert podcast had me thinking about whether the frame should be more compliant and not force my tires to take on more of a role as tunable suspension (in addition to the aspects of traction and rolling resistance) .