Seat Tube height – What difference does it really make

I just saw this pic comparing old Spectral frame to new Spectral 29. They’ve always been known to have stupidly high seat tubes.

But these days with kinked seat tubes where you CANT insert post all the way in anyway, what difference does having a shorter tube make?

From the overlay picture it looks as though in the new frame I wouldn’t be able to get my (267mm long from collar to actuator) OneUp 180mm (shimmed to 170) dropper in as low down as I would have in the older frame, despite the newer one having a ‘shorter’seat tube.

Am I missing something here? If anyone can help me understand would be appreciated. Am I simply ignorant to the fact that all these bike designers are aware of this and in fact when they have shorter seat tubes they consequentially also allow the dropper post actuator end to insert further down closer to the Bottom Bracket and that fact just isn’t clear from the picture? So universally across the board a slammed saddle will always be lower in a shorter seat tube?

It was issues like this that made the Trek Remedy a no go for me, I thought I wouldn’t be able to get the height I wanted out of my saddle at extension without losing as much drop as I wanted to not snag on my thighs every once in a while. This would make choosing a next bike just that bit more difficult if I also had to worry about max seat post insertion depths etc as that number is VERY rare to come across on bike geo charts.

I hope its OK that I linked the photo from FLOW Mountain Bikes review of the Spectral 29?

Basically… Tall seat masts are ****ing stupid and in 2021 if you’re making a bike that way you may be a very good designer but you’ve missed the basics and you’re doing it wrong. It’s not like this is new, it’s been a bad idea for a decade as it stops people from sizing up and it has NO advantages. Well maybe it lets you make a weaker front triangle but… just, don’t do that, sorted.

Kinked seat tubes, sometimes that’s just where the pivots want to be so it’s a bit more excusable but still, if you’re designing a long travel bike and it won’t allow a long travel seatpost, **** off, you’re doing it wrong.

Basically, if you design a bike that can’t have at least as much dropper post travel as it has suspension travel, you should lose your job. I ride an ancient Trek Remedy 29, I went up a size and it still comfortably takes a 200mm dropper post. It’s not boost but it also takes a 3 inch 650b+ tyre. That’s because boring as they are, Trek employ good people that actually think about stuff like this and as a result good bikes come forth. Meanwhile in 2021 people are still routinely getting this stuff wrong and it’s not just doing it wrong, it’s doing it wrong when it was sorted in a previous decade.

ARGHbkajef;oi!onE1!

Bike companies have to take into consideration riders at the upper end of sizing as well. The XL Spectral has a 490mm seat tube. That’s actually one of the shorter seat tubes I’ve had and would comfortably fit a 200mm dropper at my saddle height. I suppose it could be a little bit shorter but there are riders taller than me with higher saddles that might then find a problem getting minimum insertion. Marin, for example, have gone too short for some. Ideally, offer more sizes, but very few brands do that. Do I want to ride something so low it looks like a trials bike? Could look almost as silly as the days of riding a barn gate. Do I need a 200mm dropper that’s more likely to go wrong? Very rarely.

Kinked seat tubes are annoying. Makes it harder to tell what the angle really is at a particular saddle height. I imagine it’s to do with pivot placement on the Canyon. Can’t think it’s for tyre clearance under compression as the chainstays aren’t exactly short and the seat tube angle is fairly steep. Orbea partially solved the pivot thing by adding another tube for it on the Occam.

In the old days conventional wisdom said the minimum seat post insertion was measured at the crossing point of the seat tube and top tube because that was where everything supported everything else. So a tall seat tube above the top tube was for looks only and did nothing useful.

Or am I wrong?

Bike companies have to take into consideration riders at the upper end of sizing as well. The XL Spectral has a 490mm seat tube. That’s actually one of the shorter seat tubes I’ve had and would comfortably fit a 200mm dropper at my saddle height. I suppose it could be a little bit shorter but there are riders taller than me with higher saddles that might then find a problem getting minimum insertion.

Conversely, even at 6′ 2″ I could only just fit a 150 mm dropper into that size frame because of my (relatively) tiny legs. So for me shot seat tubes are pretty much a prerequisite when looking for a bike if I want to have a reach/effective top tube length that actually fits my upper body. My last couple of bike purchases (it doesn’t happen too often) have involved a lot of time spent with a spreadsheet to see how much reach I can get while still being able to fit in a 150 mm post.

That Canyon would be no good for me.

I am 5Ft 10, (I.E. bang average), but I have a long torso and short legs.

My Large SC Hightower is the right size for me – as its the right length, however I can only run a 125mm dropper, and thats with flat pedals, and 165mm cranks.

An off the peg large Hightower comes with a 175mm travel dropper!

If the seat mast were any talle relative to its size, it owuld have been a no-go.

Edit – seemingly the same issue as @bigginge above then

Personally it makes bugger all difference for those of us with +35″ inside legs :-)

But the engineering of seat tubes to take longer internals is definitely a benefit, for example I can put a 200mm dropper in my On One Scandal HT.

The seatpost may have a deeper insertion than it first appears.
If you draw a straight line down from the back of the seat tube into the bottom of the lower seat tube it goes down deeper than the other frame.
My Kenevo also does this and was a concern before I bought it.
Looks like theres not enough room for dropper insertion because of the linkage pivot placement.
Cleverly they’ve made the seat tube wide enought so that the bearings and mounting boltsdon’t get in the way and the seatpost goes past them.

They may have improved the headline figure “SHORTER SEAT TUBE!! YAY!!!” but without following through to seat tube insertion depth. Users might start reporting that they can not get a satisfactory dropper in. Perhaps the seat tube is an open section there and the seat post can go lower than it looks. Certainly with the shaping, the bottom of the seat post looks like it could end up half way along that lower, rearward sweep down to the BB.

Well, high seat tubes and top tubes didn’t stop us BITD, but they’re certainly limiting when viewed through 2021 mtb eyes.

I’m 182cm and these days I wouldn’t buy anything with a seat tube taller than 460mm.

Because dropper posts.

A lot of otherwise very nice frames don’t have enough seatpost insertion to take advantage of long (170mm+) droppers. It’s often due to the pivots not just bends/kinks. It’s so annoying!

“ Personally it makes bugger all difference for those of us with +35″ inside legs 🙂”

I’m not tall but I’m about a 33” leg and that’s enough to cause trouble when paired with 185mm droppers, flat pedals and riding fairly heels down (if I was on clips and more on my toes my pedalling height would be a fair bit higher).

This just depends on how long your legs are. if you are short legged short is good if you are tall (me 193cm) a short seat tube can be an issue if the stack is also proportionately v low.

“…a short seat tube can be an issue if the stack is also proportionately v low.”

But there is no reason for seat tube length and stack height to be connected.

Stack height should relate to reach and what the bike is for (enduro bike should be higher stack than an XC bike).

Seat tube length should be long enough for the longest-legged riders (who’d fit that reach) to get pedalling height with a long dropper at a safe insertion depth but short enough for the shortest-legged riders (who’d fit that reach) to fit a long dropper.

if you’re making a bike that way you may be a very good designer but you’ve missed the basics and you’re doing it wrong.

Exactly how I feel about new frames that can’t fit a water bottle cage inside the front triangle.

“…But there is no reason for seat tube length and stack height to be connected…”

Exactly, but it seems to vary a lot for a bike of x size in the same category.

Kinked seat tubes, sometimes that’s just where the pivots want to be so it’s a bit more excusable but still, if you’re designing a long travel bike and it won’t allow a long travel seatpost, **** off, you’re doing it wrong.

Basically, if you design a bike that can’t have at least as much dropper post travel as it has suspension travel, you should lose your job.

agreed on both these points.

but what seems to get missed by many is that a dropper post needs the same (acutally a bit more) length in the outer/non dropping length to hide the retracted tube in.

Suspension design/pivots/ whatever limit where the bottom of post/actuator can be. the length between that point and the pedalling saddle height for the intended height of user is roughly spilt in half. Top half being the dropper stanchion/drop length and the bottom part being the dropper internals concealed within the seat tube. dropping the seat collar heigh beyond this point is useless and could lead to min insertions issues for people towards the top of the height range for that size.

Most people on here seem to get this, and I would hope designers do too – although they may not be running it to the wire with their designs.

Am I simply ignorant to the fact that all these bike designers are aware of this and in fact when they have shorter seat tubes they consequentially also allow the dropper post actuator end to insert further down closer to the Bottom Bracket and that fact just isn’t clear from the picture? So universally across the board a slammed saddle will always be lower in a shorter seat tube?

Go read the PB/vital comments or most FB groups and there will be any number of commenters saying “I wantz a 300mm drpper post so I can be slammedddd!!!” with absolutley zero consideration for where that 300mm of stanchion is going to go when the saddle is dropped.

It would be good to see a range of inseam lengths possible on each frame size for a given (or specced) post length… But as already mentioned, there can be a good amount of variation between dropper brands (actuator length and seat clamp height); saddle; pedal; shoe that could give 25-50mm variation between the max and minimum values.

Thanks for the input guys

@kahurangi @singlespeedstu – thats what Im hoping, that they do actually have deeper insertion points than actually appears.

but what seems to get missed by many is that a dropper post needs the same (acutally a bit more) length in the outer/non dropping length to hide the retracted tube in.

That above quote is exactly what Im getting at. Despite my itty bitty 29inch inseam I find the 180mm(170mm shimmed) OneUp dropper sits at the perfect height for long uphill slogs, and dropped it is just right, out of the way of catching my thighs (fat thighs/not enough thigh gap or any other similiar excuse lol). When I had the reverb in there it wouldnt drop enough to not snag on my thighs every now and then.

But obviously a 180mm dropper is going to be longer downstairs so it needs to go deeper into the seat tube otherwise when dropped its still going to be too high to not catch on my thighs. Hence the question does shorter seat tube = lower insertion point universally?

Surely if you are not sure one bike is right for you then you just buy another one that fits what you need?

Everybody has different body shapes and different ‘perfect’ bikes based on what they ride and their riding style so a range in styles and types is a good thing. There is no ‘right’ setup for everyone.

Just buy a different one that fits you and the riding you do.

Surely if you are not sure one bike is right for you then you just buy another one that fits what you need?

Exactly why I started this thread. If manufacturers dont quote max insertion how can we be sure that the bike we like with right geo, specs, price, colour, wheel size, blah, blah is right?

“ Exactly why I started this thread. If manufacturers dont quote max insertion how can we be sure that the bike we like with right geo, specs, price, colour, wheel size, blah, blah is right?”

Hopefully they’ll start realising they’re losing sales to companies that have added the max dropper insertion spec to their geometry charts, and have to follow suit.

Cotic and Banshee have this info on their charts, so as is often the case, the small manufacturers are leading the way.

Exactly why I started this thread. If manufacturers dont quote max insertion how can we be sure that the bike we like with right geo, specs, price, colour, wheel size, blah, blah is right?

if its that squeaky bum tight whether i’d fit on a bike i’d be looking elsewhere unless i could go and see it in person.