Seatpost

[Lee Legrand]

Hi All, hope everyone is well.

Is there a difference between an aluminum and titanium seatpost in terms of stiffness? As an engineer, I would think titanium seat post is stiffer because of its tensile strength, assuming the same moment of inertia but from reading online, it seems that the titanium gives more and not as stiff as aluminum which tells me sectional area is greater in aluminum than titanium.

Your thoughts?

Thanks

[Ken Freeman]

I think it's based on the modulus of elasticity ("modulus" for short).  The ability of a spring to deflect when pushed versus to resist deflecting is based on the geometry of th part and it's material springiness.  All steels have dang close to the same modulus.  Other metallic elements and metallic compounds or alloys will have different values for modulus.   I don't have the moduli of a range of elements or other structural materials ready to hand, much less in my head.   The parameter essentially represents how much a cube of steel (or of marshmallow) will deform if it is under pressure.  Stiffer materials deform less.

If you can maybe look up "engineer's bible" there should be a table of moduli, or just google something like "table of elasticity."

I'm an EE, not an ME or a materials scientist but I do a lot of systems analysis, so I've learned that following the right basic question 

will get you farther.

Ken Freeman, Ann Arbor, MI USA

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Ken Freeman
Ann Arbor, MI USA

[Lee Legrand]

The stiffness of a material is based on the material properties and sectional area.   Yes modulus of elasticity mattress but again, Titanium appears to be stronger than aluminium and E is higher for Titanium.  I am concerned that because it is stronger, is it built thinner to be equal or less in sectional area to have the same stiffness as an aluminum post or less.  Does you feel more bounce in rides with Ti than Al.

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[Shasta Mike]

An old thread over here on that subject.  Lots of differing opinions........  https://www.roadbikereview.com/threads/seat-post-material-does-it-matter.139276/

[Ken Freeman]

Stiffness of a material depends on elasticity, not geometry.  Stiffness of an object depends on material properties and on its geometrical matters.  K

Stiffness is not strength.

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[Lee Legrand]

You be wrong Mr. Freeman

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[satanas]

If both posts had the same cross section, the titanium post would be stiffer since Ti has a higher modulus. However, since titanium has a higher modulus, is stronger, and is also less likely to suffer from fatigue failures if kept below its yield point, a Ti post can be made with a much thinner wall than alu, and usually is. If enough post is exposed that means that a titanium post can flex more without any significant risk of failure, and my experience has been that a 27.2 American Classic titanium post flexes more and is noticeably more comfortable than an equivalent alu post - or most carbon posts, the Ergon/Canyon VCLS post being a likely exception. Since alu has a fatigue limit and titanium, steel (and carbon?) don't, the chances of designing and manufacturing a reliable structure that flexes significantly are better than with alu, if my understanding is correct.

However, my observations are based more on reading, test riding and practical use, not engineering theory; I'm not qualified to debate that, and am not going to do so.

Later,

Stephen

[Lee Legrand]

 I am referring to typical stiffness against flexure, not axial since axial stiffness would be minute or un noticeable compared to deflection or displacement due to flexure.  When referring to stiffness, cross section and material strength (tensile strength) are typical used for stiffness for flexure and E=Modulus of elasticity is typically referred to stiffness, axially. The two are related when analysis for members and some formula used E in reference to flexure but E is not primary but secondary.

If the post has give, it is mostly in the direction of flexure and less axial.  This is why I mentioned cross section because stiffness is related to cross section and tensile strength of a member.  Flexure is primary and most structural formulas use compression of that cross section and the unbraced length of the section as the strength that governs, not E. 

I think we got off topic, I just wanted to know what everyone experiences with titanium and aluminium posts.  Do they feel more flex in Al vs Ti.  Engineering, we can leave aside.

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[Ken Freeman]

I have been wrong many times and this may be one of them, but I don't think so.  

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[Lee Legrand]

https://www.engineersedge.com/beam_bending/beam_bending1.htm

Look at the equations.  The only place you find E, is in deflection equations.  When you are finding out strength, you need sectional properties.  When you are looking at stiffness, you need sectional properties.  Tensile strength is related to all that as well.  You can increase the stiffness of a material, by increasing the sectional properties resistance to bend. This works also axially as well. E can be constant in both situations.

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[satanas]

If we're talking about practical use it's IMHO/IME extremely unlikely that an alu post will flex more than a titanium one: most of the point in making Ti posts is comfort via flex, not weight saving; it's easy to make a significantly lighter post from carbon, and these can be designed to flex too, though this isn't necessarily the goal.

However, this is all generalizing, and unless there's a decent amount of post exposed (say at least 150-200mm+) the flex will be near zero unless some sort of suspension or flex enhancement has been incorporated. Alu posts do vary markedly in wall thickness, diameter, head design, etc, so unless one knows *exactly* what parts are being compared and how they are being used this is all basically guesswork...  :-(

Later,

Stephen (who's not especially fond of either equations or equines, but thinks equanimity is useful)

[Steve Weeks]

I have a titanium seatpost on my titanium bicycle (Airborne Carpe Diem, 2001). I don't notice that it's flexing, though the bike is quite comfortable.