trayer350

I've seen the reference to "effective seat tube." Does that mean what it is effectively to the rider of the road bike, in other words a 53 cm seat tube is really 54, or does it mean something else?

AnthonyG

You will see the words "effective" and "actual" used in conjunction with seat tube length and top tube length. The effective seat tube length is from the centre of the bottom bracket to the centre of the top tube. Modern frame designs means there may be some seat tube sticking out past the top tube so the actual seat tube length is longer.

With top tubes if you measure along the top tube from the centre of the headset tube to the centre of the seat tube you get actual top tube length. Now with compact frames (sloping top tube) to get the effective top tube length you need to measure parralel to the floor efectively negating the angle on the top tube.

Generaly the effective length is the important one.

Regards, Anthony

55/Rad

This term is most often used to describe the top or seat tubes for compact frames, where the sloping angle of the top tube literally shortens the length of the tube but doesn't change the fit geometry.

55/Rad

Trogon

Seat tube is seat tube - it's generally measured from the center of the BB to the center of the top tube, or the top of the top tube or the top of the collar. Some funny guys measure from the top of the bottom bracket to one of those locations.


In the case of compact, short seat tubes are really effectively "longer" because they slope and the difference (in comparison to a traditional frame) is made up by seat post. I.e. a 50 seat tube on a sloping frame might be comparable to a 54 seat tube on a trad frame. They may be trying to point that out such that someone in need of a 57 in a compact doesn't end up buying a 60.

biker7

You see many references today which are the result of a throw back to how frames have been measured for many years. With the onset of compact or sloped top tubed bikes...both seat tube and top bars now are typically rated in actual versus virtual which is the same as effective. What that means is...with a sloped top tube or compact geometry frame bike...actual seat tube length will be less than effective or virtual seat tube length for equivalent steerer tube length relative to bike size which historically has been from bottom bracket to top of top tube on conventional or horizontal top tube bikes. The way to measure the difference between actual and effective or virtual seat tube length for a compact/sloped top tube bike (there is no difference on a conventional horizontal top tube frame)...is take a carpenter's level and set is right on the top bar intersecting where the top tube connects to the steerer tube. The level will diverge from the top tube as it progresses rearward due to the slope of the top tube. Where it intersects above the seat tube or seat post defines the effective or virtual seat tube length "which gives the bike its size rating." Actual seat tube length is always less than this value for a compact frame bike.
HTH,
George

AnthonyG

Actualy I'm going to have to disagree with some of your post and some of my previous post.

I've just measured the seattube on my Giant 42 cm frame. 42 cm is from the centre of the bottom bracket to the very top of the seat tube. If I measure it from the centre of the bottom bracket to the centre of the top tube it would be 37 cm. If I measure it from the centre of the bottom bracket to where the top tube would be if it wasn't a compact frame it would be 48-50 cm. I didn't take this measurement that accurately. Giant called it a 42 cm frame.

Now the problem is that not all manafacturers use the same standards. Buyer beware.

Regards, Anthony

biker7

Interesting. Post the frame geometry for your particular model Giant Anthony if you would. Most manufactures of compact geometry bikes size their bikes based upon effective seat tube lengths versus actual to not mess with the sizing convention that has been used for years which would further confuse buyers. My 61 c-t-t Bianchi which is a mildly sloped top tube bike has an actual seat tube length of 58 cm for example.
George

sydney

What you originally wrote is alot of nosense, and illustrates once again that generalizing does not fly. Bianchi does it one way others do it their way. With compact sizing you need to look at specific geometry charts and pay attention to what manufacturers are saying about measurements.Compact TT is usually listed as both actual and virtual.It's virtual that counts.Even with a conventional frame seattube can be measured c-c, c-t and c to top of an extended seattube.

Steelrider

I'll agree with this 100%. You would need to look at the geometry specs to know what the method of measurement is and understand how they measured it on your current bike to have a shot at knowing what the impact will be on you.

For instance, on a Kestrel Talon the seat tube extends almost 2 inches above the horizontal top tube and even though it is not a compact bike, they list an "effective top tube/seat tube intersection" because of the fact that the top tube is not round - so they measure to an effective top/seat tube intersection as if the top tube was round!? Yeah I was confused too when I first started looking at 'em, but read up on it, compared it to my current bike, then rode some.

I think Anthony's first post was generally correct, but in an area where cm's make a big difference, you have to know exactly how it's measured for what you're riding or looking to buy.

biker7

As usual, you have no idea what you are talking about. Almost "all" compact frames are listed as "virtual sizing" versus actual seat tube length...as I stated "for frame sizing" particularly as more and more frames evolve to sloped top tube geometry. Almost all frame geometry charts list both actual and virtual but it is the "virtual seat tube length" that determines "the frame size". Why?...because it removes the variability in top tube slope from bike model to model to maintain some semblance albeit a strained sizing convention.
George

AnthonyG

Well all the compact framed bikes I've seen in Australia are sized Centre-BB to top-ST or centre-TT. No one sizes compact framed bikes centre-BB to where the theoretical flat TT would be for marketing reasons. There trying to convice short people that they can comfortably ride thier bikes so their not going to scare them off by labeling them with a larger than nessecary theoretical size.

For the record my 42 cm compact Giant has 71? head angle, 75? seat angle and an effective TT of 520mm although I've carefully measured the effective TT at 510mm.

Regards, Anthony

biker7

Again...the actual seat tube length is pretty meaningless without knowing the rest of the geometry of the bike when you are talking a compact frame. To remove the ambiguity of virtual versus actual seat tube length, Giant choses to use a generic reference for frame size versus size their frames by virtual or actual seat tube length. So what you say Anthony is partially true in that Giant does not provide a virtual seat tube length...but instead they choose even a more generic and some would maintain a less clear convention which is XS, S, M, L, XL. These sizes do correlate of course to actual seat post length but only meaningful if you factor in slope of the top tube which determines the overall frame height based upon steerer tube length and angle. I believe Trek lists their slope tube bikes with both...a generic size reference and virtual seat tube length by comparison....virtual seat tube length being a better barometer of fit because it includes the effective height of the frame and harkens back to how conventional horizontal top tube bikes are judged for size
Cheers,
George
Here is a frame geometry chart for the TCR from Giant Australia's website which illustrates our discussion:

sydney

Don't you get it. He is pointng out the the fact that your generalization doesn't fly. Everyone doesn't do it like Bianchi. And again, even a manufacturers virtual seattube measurement won't be the same from make to make,due to differences in how different ones do it.

biker7

Dam...was hoping you got hit by a car. Yeah numb nuts...I get it. I admit guilt though. I violated my cardinal rule. I responded to a thread where you participated. Its hard to argue with the weird kid (you) that rode the short bus to school.
Have at it boys...I'm out.
George

AnthonyG

Just for the record my Giant frame has 42 cm labelled on it, not XS. They go 42, 46.5, 50 and up. They use XS, S, M, M-L, L, XL on the web site but on the frame themselves they use cm measurements. Centre BB to top ST.

Regards, Anthony