Top tube length is a hopelessly imprecise way of judging "cockpit size" becasue its dependant on seat tube angle and length and head tube angle and length.
The horizontal distance from the bottom bracket axle to the centre of the bottom of the headtube would give an accurate way of comparing frame lengths?
MBR
i dont know mate bmx do it all from TT but mtb nearly all from seat tube as long as it feels good for you i reckon go with it. Example for me the chameleon just feels great couple tweaks i would make (read creaking b/b area) but the geo is near perfect
is this not why some manufacturers give you a "virtual" top tube length?
Measuring horizontally from the BB to HT will still not take into account the seat tube angle though.
Top tube length is a hopelessly imprecise way of judging "cockpit size" becasue its dependant on seat tube angle and length and head tube angle and length.The horizontal distance from the bottom bracket axle to the centre of the bottom of the headtube would give an accurate way of comparing frame lengths?
This idea really gets on my goat as it is blatantly wrong for xc/trail bikes which MBR test. The idea of sizing up bikes on their front-centre and wheelbase dimensions only is fine for DH bikes as those are the measurements that will give you some indication as to how the bike will handle, they don't take into account seat tube angle but that wont matter as you don't sit down on a DH bike. That's why Dirt measure the bikes that they test that way.
When sizing up a trail bike though you have to take into account how comfortable the bike will be seated as you'll be spending a good 50% of your ride in that position. The only measurement that gives you any indication of this is the virtual/horizontal top tube length. This figure is derived from the actual length of the top tube and the seat tube angle.
I think it is useful to know especially if you buy a roadbike.
Crucial for fitting.
TT length is about the only measurement that matters-assuming that the frame is properly designed so that every thing is in proportion.
>The horizontal distance from the bottom bracket axle to the centre of the bottom of the headtube
As mentioned, that takes no account of seattube angle - although the effective TT (ie horizontal distance) length is measured something like 10" down from the saddle, so there'll be some error according to seat tube angle, that error is going to be something like half or a third of the error you'd get if you used the BB measurement.
In any case, it's only a rough guide.
I suspect road bike geometry tends to be more consistent wrt head/seat angle.
Seat angle *is* important, it'll have an effect on standing/seated weight distribution together with wheelbase, and a slack seat angle will put your feet relatively further forward from your saddle.
Measuring horizontally from the BB to HT will still not take into account the seat tube angle though.
seat tube angle is irrelevant as well, thats why saddles and posts allow for forward and back adjustment.
True, but the point was about comparing frames geometries, before there's any faffing with seat fore/aft and stem length.
As I wrote, effective TT is *more* accurate than the BB version.
When sizing up a trail bike though you have to take into account how comfortable the bike will be seated as you'll be spending a good 50% of your ride in that position. The only measurement that gives you any indication of this is the virtual/horizontal top tube length. This figure is derived from the actual length of the top tube and the seat tube angle.
No. You can always move your saddle forwards and back - the position of that isn't determined by the seat tube angle. The only fixed points for fit are the headtube and the BB, hence why that measurement is the only one that makes sense. When I fit myself to a bike I start by getting the saddle in the right position relative to the BB - not only the height, but fore/aft relative to the BB. Therefore the seat angle is negated - and as the OP says the top tube length is irrelevant. Unless of course you like sitting a different distance behind the BB on different bikes.