Why All Dropper Posts should be 34.9

Words Cam McRae
Date Dec 6, 2019
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2-Minute Expert is a new feature on NSMB that distills technology and other concepts into short, easily digestible chunks.

Have you ever wondered why the majority of modern mountain bikes accept 31.6 or 30.9 mm diameter seat posts? Those standards are held over from the days of aluminum tubing sizes and while it's stupid to still have both of those sizes, Stefan "Sacki" Sack's dream is to nuke both in favour of 34.9.

As the designer of the BikeYoke dropper post, Sacki told me his post is the first 34.9 that makes use of the extra diameter, and it turns out the advantages are significant. 34.9 posts that predate the BikeYoke Revive in that size used the same diameter upper tube as 30.9 or 31.6 posts, and simply beefed up the lower tube to decrease the inner diameter, and added unnecessary weight. Sacki instead scaled up the entire mechanism and, most notably, the inner tube. It went from an outside diameter of 25mm to a colossal(!) 28mm. That doesn't seem like much, but physics often baffles intuition, and that's the case here.

There are 3 main reasons to obliterate 30.9 and 31.6 diameter seat posts in favour of 34.9, each with compelling supplementary benefits.

1) Strength and Stiffness

With a dropper designed around a 34.9mm diameter, the upper tube can be larger. As mentioned earlier, the difference is small in terms of dimensions but large in terms of an increase in robustness, with the inner tube becoming over 40% stiffer. This becomes important since leverage increases as dropper travel extends. A heavy rider with his saddle pushed back can exert massive force and flex when pushing his dropper down. This flex can make the tiny gap between the upper and lower tubes (as small as 0.1mm for 30.9 or 31.6mm posts) close so that contact occurs, increasing wear and producing added friction during actuation.

Many dropper malfunctions are the result of the seat collar pinching the lower tube so the upper tube can no longer move freely. The 34.9 OD lower tube will clamp at lower force, resisting compression under clamping force better.

Finally, 34.9 will allow for bigger drops because of the stiffer and stronger upper tube.

2) Space for Internal Hydraulics

Sacki put this eloquently: "All 'standard' posts have to deal with small seals, high pressures, and low air volumes, which is one reason hydraulics are so prone to failure. More space means more stable hydraulic designs can be achieved." So 34.9 will allow for lower pressures, higher volumes and larger seals. All that means fewer failures, less maintenance, and improved longevity.

3) More space Between Moving Parts

Using BikeYoke's Revive as an example, the gap between the upper and lower tubes is only 0.1mm at the narrowest points for the 30.9 and 31.6 posts. This means excess flex could close that gap, increasing friction during use and wear over the long term. A larger gap in these smaller sizes would either reduce the inner diameter of the lower tube, making it prone to binding from seat clamp pressure, or a smaller diameter upper tube, which would be more flexible, causing more problems. With 34.9 Sacki was able to double this gap to 0.2mm leaving more space for any flex (which will be reduced as well as mentioned above) and allowing for smoother deployment, even for heavier riders with the longest droppers available.


In this capture from the 3D image, the gap we are talking about between the upper and lower tubes is between the bright green (upper) and yellow (lower). The 34.9 on the left allows for a 0.2mm gap while the 31.6 and 30.9 can only manage 0.1mm without encountering issues - and that presents its own issues.

What's the catch?

There seems to be a very small weight penalty to pay for all of this goodness. For BikeYoke's Revive dropper in 185mm length, the 30.9 weighs 560g, the 31.6 weighs 580g and the 34.9 in 185 drop is 630g. 50-70 grams seems like a reasonable tax for higher strength, increased reliability and smoother operation. The challenge now is getting bike companies on board. Obviously this should be relatively easy to do with carbon frames since there is no relationship between tubing size and seat post diameter, but it shouldn't be a problem with modern aluminum forming techniques either.

Having two seat post sizes is a pain in the ass for everyone; bike shops need more stock, consumers often can't use their old seat post with a new bike, and manufacturers have to make more models, increasing cost for complete bikes and after market parts.

Dear bike industry, can we make this happen, please?

For more on BikeYoke, head to their site.

Check out our teardown of BikeYoke's Revive Post to get a look at the internals, or our full review of the Revive here.

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+5 Cam McRae sacki Niels Skyler Andy Eunson
Heinous  - Dec. 5, 2019, 11:30 p.m.

> Having two seat post sizes is a pain in the ass for everyone; bike shops need more stock, consumers often can't use their old seat post with a new bike, and manufacturers have to make more models, increasing cost for complete bikes and after market parts.

It's arguable that this is more of a positive for customer focussed outfits like BikeYoke than some of the larger OEM players, where obsolescence and incompatibility drive sales and the tolerances and accelerated wear ensure cheaper design elements aren't an issue as the posts don't last long enough.

I'm running BikeYoke posts on all my bikes and the quality and design is a step up from the rest.


+4 DMVancouver Carlos Matutes Niels Peter Scowcroft
fartymarty  - Dec. 6, 2019, 4:18 a.m.

The issues with droppers is there are 3 tubes in the same area - seat tube, dropper outer, dropper inner which all take up space and add weight.  Why not do away with one of the tubes and drop some weight at the same time like


+1 Pete Roggeman
sacki  - Dec. 6, 2019, 5:53 a.m.

This comment has been removed.

+11 Cam McRae Niels Andrew Major AJ Barlas Cr4w kmag76 IslandLife ManInSteel Metacomet Shrockie Tjaard Breeuwer
sacki  - Dec. 6, 2019, 5:53 a.m.

Hi, this is Sacki of BikeYoke speaking.

The idea behind this is nice.
However, please do not forget, that the bushings in your seatube will need to be extremly well calibrated and perfectly in-line with each other. If this is not the case, your post will get stuck or have play.
In the majority, we are still talking about aluminum frames (and they are having a comeback on high end bikes now, too). Those AL-frames are welded and suffer a certain amount of distortion after welding (heat). Seattubes naturally are not 100% straight. 
Frame makers will (and they do) hate you for mekingg them produce a frame with that tight tolerances. Frame makers are not keen on doing this. At all.
Then, you are also restricted to certrain frame designs, as the seattube needs to be straight and can not have a bend. Implementing dropper into the frame will simply not work with most of the current designs on the market. Designs like the new Santas, EVIL, the new Spesh Enduro (which IMHO are amongst the top-performing suspensions) are simply not compatible. And so are more than 90% of other frames.

So again: The idea is good, but at the moment an integrated dropper requires too much compromise on top-performing rear-suspension. Droppers need to go up and down reliably. That is what the customer cares about. Who would sacrifice kinematics performance for an integrated dropper that does not make a reall difference for the ride experience. 

P.S: You may want to check in some forums or with some owners or dealers and ask them about reliability, then you may know, why I personally do not think, that it is a good idea.


+2 Cam McRae Tjaard Breeuwer
Nouseforaname  - Dec. 6, 2019, 10:02 a.m.

Makes sense - if SRAM can decide that .55mm is enough to justify a new crank/BB interface I can see how an extra 3.4mm+ would give some advantages.


lukey  - Dec. 6, 2019, 11:24 a.m.

This comment has been removed.

+5 Metacomet ManInSteel Cam McRae Poz JVP
Beau Miller  - Dec. 6, 2019, 12:42 p.m.

2 minute expert, love it! More pls


+6 Heinous Niels Skyler sacki Cam McRae Tjaard Breeuwer
Metacomet  - Dec. 6, 2019, 12:44 p.m.

Yes.   Please industry.   Make everything 34.9.   For the love of God make everything 34.9.  Our existing 30.9 and 31.6 posts will work with a shim for backwards compatibility until we inevitably replace them as things wear out and/or better options come to market.  This is a standard the people want.   This is a universal standard the people NEED.


+4 Andrew Major Niels ManInSteel Velocipedestrian Tremeer023 Saša Stojanovic
Heinous  - Dec. 6, 2019, 12:55 p.m.

Why not make it 35 even, like modern bars?  One thing I do like about old road stuff is the consistency of dimensions. It’s just nice aesthetically


+3 Byron Cam McRae trumpstinyhands
Metacomet  - Dec. 6, 2019, 1:10 p.m.

SRAM it and call it 34.99?  lol ;-)


dodobob  - Dec. 6, 2019, 3:44 p.m.

34.9 is one of the tube diameters that can easily be ordered by manufacturers as a standard size if i remember correctly. don't know if it was outside (=for the dropper) or inside (=for the seattube).

can somebody elaborate? (kollege sacki?)


Sam James  - Jan. 4, 2020, 9:16 p.m.

Specialized use this size pretty commonly, amongst other brands. It would be nice if we could agree on this size. Personally I think we've done pretty well to get down to 3 common sizes in the last few years! (30.9, 31.6, 34.9)


+1 Tjaard Breeuwer
blaklabl  - Dec. 12, 2019, 9:01 a.m.

I totally agree with this article.  My frame has a 34.9mm seat tube, but was spec'd with a 30.9 post & shim.  Now, the post was a POS out of the gate, but I haven't really ever had a dropper I liked very well that was relatively trouble free.  I debated a OneUp and using the shim, or the Reverb 34.9 until I realized they used the same small upper tube as the smaller diameter posts.  Also not having great experience with a couple Reverbs prior, I gritted my teeth and paid up for the BikeYoke Revive MAX 34.9 and it has been stellar in almost 9 months of use.  Smooth, easy to compress, pops up solidly, honestly if it weren't for having to push a lever to activate it I would forget it was there.  One of the best "recent" purchases I have made for my ride and totally worth the money in my opinion.


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