Conflict: Cheerful About (a few) Seat Post Size Choices

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Evolve & Erase

Seat post sizing differs from every other discussion of bicycle standards in one essential way. Where options for headset dimensions, hub sizes, freehub drivers, brake mounts, and rotor sizes have expanded over time – leading to a steady drone of derision from the collection ridership – there are significantly fewer seat post sizes in use now than at any point in the history of the mountain bike.

We owe this fact to the rise of the dropper post as standard equipment on any performance mountain bike. Even the budget-friendliest hardtail and full suspension bikes that do not have a dropper post as standard equipment come ready to run a dropper post from one of the four standard sizes – 27.2mm, 30.9mm, 31.6mm, or, yes, 34.9mm.

When I first started working in a bike shop, and it’s still true for classic bike collectors, your rig could have a seat post outer diameter – a frame’s seat tube inner diameter – anywhere from 25.0mm to 32.4mm in 0.2mm increments. Okay, it’s a slight exaggeration, but there were over a dozen sizes to account for and if your frame fit a 25.2mm post and I only had a 25.0mm and 25.4mm in stock we were SOL. Add in 22.2mm BMX rigs and then 34.9 when Scott Bicycles introduced the size with the introduction of the “world’s first carbon freeride bike” the Ransom. I’ll come back to that.

I’ve owned mountain bikes that used 27.0mm and 30.0mm seat posts. I’ve witnessed a few older 30.0mm aluminum bikes carefully reamed to 30.9mm and drilled for a stealth cable exit, to accommodate a dropper post longer than what’s available in 27.2mm. To say that, at least regarding seat posts, we live in a simpler time is an understatement.

The elephant in the room is, of course, the 34.9mm seat post-size – not seat clamp size – and the feeling amongst those who care that it should erase all other sizes from consideration. I admit that I don’t understand the end goal, are we trying for heavier frames or heavier dropper posts? Both? Are there that many riders who want a post longer than 240mm (available in both 30.9mm & 31.6mm post sizes) never mind that it’s 160mm-to-200mm travel droppers that most folks seem to have happily chosen for themselves?

I suppose 34.9mm is the obvious choice because it’s the newest option, but that thinking hasn’t worked out for press-fit bottom brackets, or ‘Super Boost’ 157 hub spacing, has it?

Three Size Justification

I recognize that for many brands seat post size is part of their brand identity. Rocky Mountain uses 30.9mm, Trek uses 31.6mm, etc. Some companies, for example Specialized and Kona, have embraced a world with multiple sizes, to introduce 34.9mm for certain models. But let’s ignore the hard-ingrained seat post-jingoism for a hot second and justify the existence of cycling’s three most common sizes.

Here I’ll briefly note that while I’m fairly agnostic about seat post sizes – I can always shave some weight and increase options by shimming a 34.9mm seat tube – the number of different clamp sizes (seat tube outer diameters) in use today drives me insane. The Pembree DBN (Double Barrel Nut), which I think is the cleanest little luxury clamp on the market, is available in ten sizes. Ten sizes across eight collars even.

For those keeping score, Wolf Tooth ‘only’ makes seven sizes and Hope is trailing them with six. Chromag makes four. I noted the latter to confirm that their 36.0mm clamp is the same size as the 36.4mm that other brands make – fitting many Trek, Yeti, and Marin frames amongst others. I know this because I like to run a quick-release seat clamp, yes, even with a dropper post, theirs is my favourite, and I’ve run their 36mm on examples from all those brands.

Asides put aside; I’ll start with justifying the continued existence of the 27.2mm seat post size. One word, comfort . For folks running a fixed post in their road or gravel bike, the Thomson post in a smaller diameter, like 27.2mm, provides a notable improvement in ride quality. If your gravel or road bike, or any bike really, has a round fixed post of any size, shimming in a 27.2mm Masterpiece is a ride different you must try to believe.

Steel, carbon, or aluminum, if I was lord over bikes for a day, all of these non-mountain bike rigs would come with a 27.2mm internal seat post diameter. And they would all come with routing to run a dropper post as it’s the best way to serve the most riders.

I wouldn’t stake my reputation on it, but I’m also not the only rider who has noticed – placebo or for real – that the smaller shafts on 27.2mm dropper posts make them more comfortable for long miles in the saddle. Plus, the 60-125mm lengths that companies are making groadie (gravel-roadie) posts in work great in the 27.2mm dimension. The best I’ve used have been from BikeYoke, Crankbrothers, and PNW, but several brands are making them now.

I know that 30.9mm and 31.6mm post sizes have long been subject to discontented ‘can the industry just pick a winner’ chatter but I can happily justify the existence of both, absent specific brand’s reasons of history or nostalgia. Back to the explosion in seat post clamp sizes, I could happily bring us back to three with a snap of my fingers.

Using a 34.9mm clamp (outer seat tube diameter) works perfectly with a 31.6mm post for the thinner tubes of steel and titanium mountain bike frames. This keeps the weight down with zero additional challenges to manufacturing or design.

It sounds like a tiny difference, but where larger clamps started showing up is companies that insisted on 31.6mm posts for aluminum and carbon frames trying to get their warranty rates under control but using larger outer diameters. I’ve seen so many broken aluminum and carbon frames at the seat tube top tube junction, including examples of aluminum frames where the seat tube visually wasn’t reamed in the center.

Ransom 34.9mm

The 34.9mm seat post inner diameter is just like Super Boost 157 (SB-157) hub spacing. It’s a standard that was created, if not popularized, for a specific reason, or reasons, which have long since expired but have clung to life through new justifications.

In the case of SB-157, the folks at Pivot introduced it to combine three seemingly intractable design features in a full-suspension bike with the release of their 2015 Switchblade.

1. The shortest chain stays in its category at 428mm because riders at the time were obsessed with short stays.
2. Clearance for a 27x3.25” rubber because no one was sure where Plus tire sizing would stop.
3. Fitment of a front derailleur.

I bring up SB-157 because like the dual-births of 34.9mm none of those features matter anymore. When the largest tire most riders want to run is a Maxxis 2.6” or 2.5” that’s just the 2.4” with bigger side knobs, any rideable chain stay length is achievable with Boost 148mm spacing. And the current trend is towards longer chain stays anyway.

When Scott introduced 34.9mm seat posts with their 2006 Ransom they were solving a design goal. How to keep from ripping apart carbon fiber frames with interrupted seat tubes. It was a solid riding bike with a dead aesthetic as the industry was thankfully abandoning the limited seat height adjustment, shock-killing potential, and unsupported design of interrupted seat masts.

The 34.9mm seat post diameter caught its second wind around 2016, a decade after the Ransom was released, and again the rise was down to durability. Riders were clamoring for posts with more than 150mm travel and some dropper post manufacturers were convinced that making reliable longer travel dropper posts required a larger seat tube size.

I know of at least two brands that were planning to switch their mountain bikes across to 34.9mm seat tube diameters but didn’t when dropper manufacturers started making 170mm, then 200mm, and now longer dropper posts in 30.9mm and 31.6mm sizes that are lighter weight than 34.9mm posts with no obvious decrease in durability.

I Remember, Joel

The 34.9mm seat post standard could have become the only size for mountain bikes. Perhaps a duopoly with 27.2mm ruling gravel, road, and commuting while 34.9mm reigned over any application that required longer than a 125mm dropper. It did have an honest chance at success, but two forces conspired against it.

First, dropper post manufacturers produced longer travel posts in legacy sizes. At lighter weights than 34.9mm options to boot with the only selling point of 34.9mm droppers being that thicker shafts are stiffer – like anyone is complaining about their dropper post not being stiff enough. Maybe there’s a 34.9mm-friendly future where folks want dropper posts with more than 240mm of travel, where the rear tire somehow still clears the saddle at the bottom out, but I doubt it. If anything, I think we’ll see folks running relatively shorter travel droppers combined with higher bars.

Second, the industry chose to call the standard thirty-four-point-nine-mill. Credit to Joel Smith, then of X-Fusion, now the big wheel at Reserve Wheels for calling this out in 2015. If the standard had been called thirty-five-mill, whether calipers read 34.9mm or 35.0mm doesn’t matter, the adoption would have been much more prevalent, and it would likely be the de facto mountain bike standard today.

Just look at handlebars where 35mm clamps stormed the spec. charts before anyone could pause long enough to wonder if it was necessary from a structural perspective or beneficial in terms of ride quality.

I admit that, like SB-157 hub spacing, the 34.9mm seat post size has stubbornly held its ground and even added a proponent or two. Will the chorus grow from there? I wouldn’t bet on it. But if you do find yourself with a 34.9mm frame don’t stress about it. Specialized sells an excellent 34.9mm-to-30.9mm shim you can use with their bikes or any others to enhance your dropper post options and drop some grams at the same time.

Check our Cy's argument for 34.9 mm seatposts here.