carbon v titanium seat posts

[threebikesmcginty]

Bravo that man, I doff my hat in envy

That looks like a classic mumbo-jimbo calculation!

Wonder what happened to him, disappeared up his own slide-rule probably

 

[MacB]

That looks like a classic mumbo-jimbo calculation!

Wonder what happened to him, disappeared up his own slide-rule probably

He lives on, in my heart

 

[RecordAceFromNew]

Bravo that man, I doff my hat in envy

Nah. I was merely getting a bit concerned why my backside was not reaping the alleged benefits of such amazing seatposts!

 

[RecordAceFromNew]

That looks like a classic mumbo-jimbo calculation!

Wonder what happened to him, disappeared up his own slide-rule probably

My sincerest apology if you are suddenly feeling less BOING from your carbon seatpost(s) on any of your 3 bikes!

 

[threebikesmcginty]

My sincerest apology if you are suddenly feeling less BOING from your carbon seatpost(s) on any of your 3 bikes!

Carbon, moi!?

 

[mickle]

Anyone who is suffering from "road buzz" should check out the quality of their tyres, the pressure they run at and their saddle. That is where harshness can be reduced (along with a more relaxed frame of course). Changing the six inches of rigid tubing connecting the saddle to the frame will only make a difference in your head and your wallet.

Really? How?

 

[Smokin Joe]

Really? How?

Relaxed angles, longer wheelbase, both soak up the bumps better.

 

[Rob3rt]

RecordAce, one thing which may be pretty important in this carbon vs alloy seatpost debate is the fact that some full carbon seatposts, especially the higher costing ones are built as one piece with any setback being achieved through curving the post etc and the clamping mechanism is also carbon and part of the the one piece post, whereas alloy ones tend to be a pipe, with a clamping mechanism (which also achieves the setback) attached at the top. There are a few exceptios like the Thompson lay back seatpost etc though.

 

[RecordAceFromNew]

RecordAce, one thing which may be pretty important in this carbon vs alloy seatpost debate is the fact that some full carbon seatposts, especially the higher costing ones are built as one piece with any setback being achieved through curving the post etc and the clamping mechanism is also carbon and part of the the one piece post, whereas alloy ones tend to be a pipe, with a clamping mechanism (which also achieves the setback) attached at the top. There are a few exceptios like the Thompson lay back seatpost etc though.

In terms of importance I suspect design differences at/near the clamp are driven by integrity or weight considerations, since the closer to the clamp the less effect bending has on deflection. In any case as you mentioned such variations can be implemented in different materials.

Coincidentally, the alloy post I mentioned earlier is a KAlloy with setback in one piece that looks like this. At 220mm long and again 27.2mm diameter, it weighs merely 10% (~20g) more than my 250mm Campag Record carbon post, at a cost of around 1/10th...

 

[mickle]

Relaxed angles, longer wheelbase, both soak up the bumps better.

Really? How?

 

[snailracer]

Relaxed angles, longer wheelbase, both soak up the bumps better.

Really? How?

When the back wheel goes over a bump, the whole bike rotates about a pivot point located at the front wheel axle. Imagine a straight line of length L drawn between that pivot point and the rear tyre's contact patch. Imagine another straight line, of length S, drawn between the pivot point and the saddle. When the back wheel hits a bump which causes it to move, say, 10mm, the saddle will moved by 10mm * S/L.

The aspect between saddle and front wheel changes very little because reach is fixed for any particular rider, and head tube angle and rake vary little between bikes. Therefore, S is much the same for any particular rider, regardless of whether they are riding a long- or short-wheelbase bike.

The length L, however, can be appreciably longer for a long-wheelbase bike, and manifests itself as a longer rear triangle.

Bottom line, a long wheelbase bike has a smaller S/L ratio, which means a bump causes less movement at the saddle.

The saddle movement is at right angles to the line S. The seat post sticks out at roughly 45 degrees to the arc of saddle movement, so is subject to both compressive and bending forces (which is why telescopic suspension seatposts don't work very well - the bending force creates significant stiction). A shallow seat tube angle results in the seat post being further misaligned with the saddle arc, and so a higher proportion of the force goes into bending, rather than compression. Per unit of force, bending results in much more seat post distortion than compression, so, bottom line, a shallow seat tube angle results in the seatpost bending more, thus absorbing more shock.

The extreme theoretical limit would be reached if the seat tube angle was so shallow, it ends up at right angles to the direction of saddle movment i.e. coincident with the line S. At that angle, the seatpost would ONLY be subject to bending forces and ZERO compressive forces, which maximizes bending and shock absorption.

Also, a longer seatpost will bend more for the same bending force, so a compact geometry should also result in a smoother ride, all other things being equal.

The frame does not distort much, because the seat stays are fairly well in line with the saddle movement and see only compression, which will not measurably shorten them.

 

[YahudaMoon]

This is my fave post at the moment

 

[mickle]

When the back wheel goes over a bump, the whole bike rotates about a pivot point located at the front wheel axle. Imagine a straight line of length L drawn between that pivot point and the rear tyre's contact patch. Imagine another straight line, of length S, drawn between the pivot point and the saddle. When the back wheel hits a bump which causes it to move, say, 10mm, the saddle will moved by 10mm * S/L.

The aspect between saddle and front wheel changes very little because reach is fixed for any particular rider, and head tube angle and rake vary little between bikes. Therefore, S is much the same for any particular rider, regardless of whether they are riding a long- or short-wheelbase bike.

The length L, however, can be appreciably longer for a long-wheelbase bike, and manifests itself as a longer rear triangle.

Bottom line, a long wheelbase bike has a smaller S/L ratio, which means a bump causes less movement at the saddle.

The saddle movement is at right angles to the line S. The seat post sticks out at roughly 45 degrees to the arc of saddle movement, so is subject to both compressive and bending forces (which is why telescopic suspension seatposts don't work very well - the bending force creates significant stiction). A shallow seat tube angle results in the seat post being further misaligned with the saddle arc, and so a higher proportion of the force goes into bending, rather than compression. Per unit of force, bending results in much more seat post distortion than compression, so, bottom line, a shallow seat tube angle results in the seatpost bending more, thus absorbing more shock.

The extreme theoretical limit would be reached if the seat tube angle was so shallow, it ends up at right angles to the direction of saddle movment i.e. coincident with the line S. At that angle, the seatpost would ONLY be subject to bending forces and ZERO compressive forces, which maximizes bending and shock absorption.

Also, a longer seatpost will bend more for the same bending force, so a compact geometry should also result in a smoother ride, all other things being equal.

The frame does not distort much, because the seat stays are fairly well in line with the saddle movement and see only compression, which will not measurably shorten them.

I don't agree with most of this. What do you imagine is the measurable difference in the seat post bending movement between a frame with road racing geometry and one shaped like a touring bike. And the difference in bump force transmission between the same two frames as a consequence of their different length chainstays?


I'd suggest that the difference is so slight as to be unmeasurable - certainly a whole bunch less than the difference in compliance between - say - two different saddles or between 28mm and 32mm tyres. Or between 90 and 100psi in your back tyre. Or wearing chamois equipped shorts and jeans.

And, forgive me for stating the bleedin obvious, but a longer chainstay combined with a slacker seat angle puts the saddle right back where it started relative to the back wheel contact point.

That 'slacker' frames provide more comfort is a myth.

 

[snailracer]

I don't agree with most of this. What do you imagine is the measurable difference in the seat post bending movement between a frame with road racing geometry and one shaped like a touring bike. And the difference in bump force transmission between the same two frames as a consequence of their different length chainstays?

Fractions of a mm.

...And the difference in bump force transmission between the same two frames as a consequence of their different length chainstays?

Depends on the size of the bump. The ratiometric scaling of saddle displacement is a percentage of the displacment of the wheel as it passes over a bump. A long wheelbase might result in 5-10% less of the bump being transmitted to the saddle, due to the difference in chainstay lengths alone.

I'd suggest that the difference is so slight as to be unmeasurable...

It's umeasurable, but calculable and modellable using FEA tools, therefore I agree with Smokin Joe's claims taken at face value.
Personally, I consider it undetectable but others are welcome to disagree and claim their backsides get sore after 91 miles instead of 90.
Is it worth having? Well, some folks would say that a lot of tiny differences add up to a noticeable difference...

...And, forgive me for stating the bleedin obvious, but a longer chainstay combined with a slacker seat angle puts the saddle right back where it started relative to the back wheel contact point...

For those bikes where that is true, the slacker seatpost still bends more. For those bikes where it isn't (i.e. which truly have a longer wheelbase), you get the ratiometric decrease in bump displacement thrown in as well.

Here is a bike with an unusually slack seatpost. Despite the typical racey short chainstays, it was noted for its comfy ride but got banned by the UCI:

...certainly a whole bunch less than the difference in compliance between - say - two different saddles or between 28mm and 32mm tyres. Or between 90 and 100psi in your back tyre. Or wearing chamois equipped shorts and jeans...

Agree. Which is why my commuter has a sprung saddle and 1.75" tyres at about 60psi.