Words: Dan Roberts
Illustrations: Taj Mihelich
What is the seat tube angle?
Originally, the seat tube angle was one measurement along the axis of a bike’s physical seat tube to the horizontal, measured in degrees. On many hardtail, road and gravel bikes this is still the case, as we have one straight tube from top to bottom.
On a side note, as a hardtail goes through its fork travel, the seat tube angle gets steeper. On a suspension bike the seat tube angle gets slacker as you go through the bike’s travel. This is why hardtails can get away with seemingly slacker static angles in comparison to full suspension bikes.
Why do we have more than one on bikes?
As full suspension bikes became more popular, we had the problem of the wheel moving around and interfering with bits of the frame. Even with small diameter wheels and small amounts of travel the wheel or seat stay bridge could come into contact with the back of the seat tube.
The physical seat tube then needed to be moved away to create some clearance for the moving wheel and frame parts. But this meant the angle of the physical, or actual, seat tube and the angle to the center of the bottom bracket, our virtual seat tube angle, weren’t the same.
Some hardtails with large wheels and short chainstays also needed the seat tube to be moved out of the way to provide enough tire and mud clearance.
With suspension travel and bigger wheels, that same straight to the BB seat tube we used to have would cause some issues. Engineers and designers had to move the physical tube out of the way to provide some clearance. And this is where we started to generate more than one seat angle and some confusion.
How do we measure the different ones?
Our current two seat tube angles are measured as follows.
The actual seat tube angle is pretty straight forward, measuring the angle to the horizontal of the actual seat tube tube.
The virtual, or sometimes called effective, seat tube angle is a little more difficult to visualise and is where our confusion and problems lie. It’s an angle measured from horizontal to the line from the center of the bottom bracket up to the intersection of the actual seat tube angle and the top of the head tube.
What are the downsides to the current way of measuring?
The actual seat tube angle is a good measurement, but it only tells part of the story. The virtual seat tube angle, as it currently stands, is misleading and makes it difficult to compare bikes or even know if the one we are looking at on paper is going to fit us properly.
Most of us pedal our bike with the seat considerably higher than level with the top of the head tube. For example, I run a seat height of 820mm from the center of the BB, which is 144mm higher than the top of the Madonna V2 head tube.
The further we go above this imaginary line, the further back our seat goes and the slacker our virtual seat tube angle is in real life compared to the one on paper. Comparing bikes can also be tricky as two bikes on paper can have the same virtual seat tube angle but depending on their actual seat tube angle, and a few other factors, they can have drastically different seated positions.
The same is true of just one bike with multiple sizes. Often the virtual seat tube angle is the same for all sizes while the reality will be much different. Sometimes geometry tables only quote one seat tube angle without stating which one it is, leading to possibly more confusion.
What could we implement to make measuring seat tube angles better?
Our current measuring system is a bit dated. Our modern suspension bikes have evolved a lot in recent years with the increased front centers, longer reaches and increased wheel size and travel.
Some of those factors help the seat tube positioning conundrum. A longer chain stay gives more room between the wheel and seat tube, whereas the longer dropper posts, with their increased insertion, have made packaging the now shorter seat tubes into the frame trickier.
We need to add a couple more measurements into the system to help paint the full picture of where our seat will be and make it easier to understand how our bike will fit us.
Our actual seat tube angle is a good measurement, telling us the angle of the physical seat tube and the line along which our seat post or dropper will move.
Adding in the forward offset from the center of the BB to the actual seat tube angle gives us another side of the triangle and allows us to then calculate what our virtual seat tube angles will be for our actual seat heights. Although the trigonometry behind this is pretty involved, so there’s no easy formula to follow. Most of the time we can just draw it out in a CAD package and easily extract the angle.
There are a couple of ways of measuring this forward offset at the moment, with some using a horizontal measurement and others doing perpendicular to the seat tube.
The last, and probably most important step, is defining that seat height at which the virtual seat tube angle will be measured. The seat height is measured from the center of the BB along the virtual seat tube angle line to where it meets the actual seat tube angle line and is representative of where your seat will actually be when you’re pedalling.
Is it difficult to understand?
On paper it sounds like a pain to figure this out, but some brands are already giving this information in their geometry.
Transition and Unno are two of the brands quoting the seat height at which they measure their virtual seat tube angle.
Unno currently only has one frame size available and so quote the angle for a seat height of 600mm, 650mm, 700mm and 750mm with the angle getting half a degree slacker as the seat height increases.
Transition, however, are giving out all the information. Actual seat tube angle, virtual seat tube angle, the seat height it’s measured at and also the offset from the BB. Transition use seat heights of 560mm, 600mm, 660mm, 720mm, 780mm and 820mmfor their XS, S, M, L, XL and XXL sizes respectively. Their actual seat tube angle remains the same for all sizes; as the seat height gets higher the virtual seat tube angle gets slacker.
As a rider, all you need to know is what seat height you use. And this is an easy measurement to acquire, measuring from the center of the BB to the top of the saddle.
If you want to make it more accurate then you can mask off your saddle and mark a line where the saddle is 80mm wide and use this as your reference point for getting your current setup and also replicating it on other bikes and with other saddles.
Once you know your seat height, and which size of frame you’ll be on, you can compare your seat height to that of the one stated for that size. If your seat height is higher than the one quoted then your virtual seat tube angle will be a little slacker. If your seat height is lower, then your virtual seat tube angle will be a little steeper.
If a brand is giving you all the information about offset, angles and seat heights then you can be sure that they are also designing their bikes carefully enough to have the seat in the center of the rails at the quoted measurements. This then enables you to fine tune your fit in either direction, not being backed into a corner with your adjustment. Moving your seat as little as 10mm can have the effect of over half a degree in virtual seat tube angle.
How does each measurement affect your saddle position?
Virtual seat tube angle – Simple. A slacker virtual seat tube angle puts your seat further back and a steeper one puts your saddle further forwards.
Actual seat tube angle – With all other dimensions held the same, a slacker actual seat tube angle will mean that for the same seat height the virtual seat tube angle will also be slacker, putting your seat further back.
Offset – With all other dimensions held the same, more offset will result in a steeper virtual seat tube angle putting your seat further forwards and less offset will result in a slacker virtual seat tube angle.
Saddle height – With all other dimensions held the same, if your saddle height is more than the bike’s design then your virtual seat tube angle will be slacker and your seat further back. Less than the bike’s design and it will be steeper and your seat further forwards.
What else is impacted by measuring seat tube angles differently?
Another measurement that is then rendered a little useless is the effective top tube measurement. This is the horizontal distance between the top of the head tube and the virtual seat tube angle.
Once again, our seats aren’t at that height when we pedal, so this measurement isn’t representative of the real world and can lead to further confusion when looking at a bike’s geometry on paper or comparing bikes.
If your seat is at the same height as your head tube, then you’re all good, and most downhill bikes aren’t that far off this. But the riding scenario for downhill bikes leaves no need to know or care about the effective top tube length – usually the seat position on a downhill bike is a function of clearances to the rest of the bike and the rider.
Top tubes are another topic for another day. But there also needs to be some thought into having a more meaningful measurement that better represents what we would feel on the bike. Some brands have already begun developing with the non-horizontal distance from the top center of the head tube to the stated seat height to give a more realistic measurement.
Food for thought…
Our current system of measuring seat position used to be fine before we really started to mess around with the seat tube position. But in today’s world it leaves a lot to be desired. Luckily a few brands have already started a movement to give more information to accurately show where your seat will be, and that matches up far closer to reality than what we are working with now. Fingers crossed that this catches on with more brands.
As a rider and consumer, the concept of seat heights, angles and offsets isn’t super hard to grasp. And armed with your own seat height information you can then get a better idea of how you would fit on a certain bike, or extrapolate from the information given from a brand to your unique case.
If nothing else, understand that seat height affects your virtual seat tube angle and knowing at the bare minimum these two measurements is a much better way to go for brands to communicate geometry and us to understand it.
Having an industry wide set of seat heights for the various size frames would make everyone’s lives a lot easier. But getting the industry to talk, let alone agree on anything is nigh on impossible.
Hopefully this edition of Enginerding has helped shed some light on the seat tube angle conundrum. Let us know what else you’d like us to go in depth with for more Enginerding articles.