Maybe I should know thus but I don't 😳

However if a manufacturer gives the effective top tube length of 22.5inches how do you work out the actual (circa/as a rule of thumb)?

For a standard hardtail frame of medium size (17 or 18") I'd minus 0.75" from effective/virtual TT…obviously a rough guesstimate though.

I'd get a spirit level and measure the head tube to 'seat tube' seat post length.

you can't, unless you have a fair other measurements too

is there any useful information to be learnt from knowing the 'actual' top tube length?

My 16" 456 is 22.5 actual, my old medium Chameleon was 22.5 effective and I need to know if I'm making a terrible mistake buying a new frame..

is there any useful information to be learnt from knowing the 'actual' top tube length?

more than there is from any "virtual" measurement 😉

but the only measurement that would translate into telling us how a bike fits is actually the downtube length.

So the way I understand it, is, that the effective top tube length is the horizontal distance from the centerline of the head tube, to the center line of the seatpost. Thats the important measurment when comparing frames, and the actuall length dosent matter. Two bikes with the same actual length top tubes could have completely different effective lengths because of the angles, making one in reality much smaller than the other.

If your trying to compare your 456 frame to one your buying, your better off measuring the effective length of the 456 as reference.

But thats not all! unless you sit at the same hight as the top of your head tube, your seatpost is going to be raised quite a bit higher, and as the seat tubes arnt vertical, but angled back, the higher you go, the further your seat moves back. So you need to look at the seat tube angles too when comparing frames!

If you find a bike with the same effective top tube length as your 456, but with a slacker seat tube than the 456, and you run the seatpost quite high, your going to be more stretched out than your current bike.

I hope than made more sence than my free stroke screw adjustment explination? lol 😆

Slide saddle on rails or use a longer/shorter stem.
Are head and seat angles the same?

minus 3/4 to an 1" but all so take into account st angles 72/73 deg would make a bit of differants

the important dimensions when looking at whether an mtb frame will fit is the distance from the BB to head tube, with this measurement, top tube is irrelivent (assuming you know head and seat angles)
chainstay length, wheelbase and bottom bracket height are irrelivent to fit but very important for how it will ride.

Your better off not getting too fixated in single numbers like that. You could have two bikes with the same effective top tube measurement, but with different angles on the head and seat tube, it'll have a big difference on how it feels.
The only thing I can really add is that the On-One and the Cham will be very different beasts to ride.

The actual will probably be 0.9" less than the measurement for effective no? But does differ from frame to frame of course.

I need to know if I'm making a terrible mistake buying a new frame..

well it will be "all the bike i will ever need" for a month or two then something new will come out that you want.
perhaps you should think about the resale value of the frame in 3months time?

more than there is from any "virtual" measurement

Well, it's distances between two points that are of more use than what path the bits of metal take along the way there…

Other than that, what GW says is – as usual – sage advice. However, effective top tube length combined with seat angle tells you virtually the same thing as down tube length. So if you happen to be comparing two frames with the same seat angle then effective top tube is a good measurement to use (otherwise you can treat an extra degree of seat angle as being very roughly equivalent to about half an inch of extra ETT). And also assuming you're using the same fork, MTB forks varying wildly in length and sag…

(otherwise you can treat an extra degree of seat angle as being very roughly equivalent to about half an inch of extra ETT).

this is such a generalisation it's silly, 26" wheel mtb frames come in everything from 12" – 22" seat tubes and 20"-25" top tube lengths.

ETT is a crap measuring system IMO, always has been.

the important dimensions when looking at whether an mtb frame will fit is the distance from the BB to head tube,

maybe, but I think what you really need is to separate into vertical & lateral distances (2 more virtual measurements) rather than the length of a tube whose angle to the ground is unknown.

If you're posh you prob want bb height too ?

well not really.. frames are designed around a certain fork axle to crown height (so fixed), you're right, BB height varies but not by a huge amount +-1"? (don't know about anyone else but I know what BB height I want down to a few mm for every type of MTB I ride)

not my point; a long, low xc racing frame could have a similar BB-HT length as a taller, shorter frame (say DH bike) but they aren't the same fit, are they ?

I know what BB height I want

that's why I said it may be important

this is such a generalisation it's silly, 26" wheel mtb frames come in everything from 12" – 22" seat tubes and 20"-25" top tube lengths

Yes, seat tube length makes a difference, but generally you're interested in comparing two frames of a similar size (and usage – eg I'd want a much smaller frame for techy DH than for mile-munching XC) – in which case ETT is adequate as long as you remember to factor in the seat angle.

I've never seen a down tube length published, so unless you can take a tape measure to a frame (and even then it's going to be hard to measure accurately) it's the best you'll get.

surely seat-tube length makes NO difference to fit as long as it's not longer than you can accommodate or so short that you can't find a long enough seatpost

assuming same fork (& same headtube length) the whole point of ETT is that it obviates seat tube length and gives a fixed horizontal length that allows you to relate saddle position to top of the steerer (assuming you set saddle to same height). I agree ETT isn't great, though

not my point; a long, low xc racing frame could have a similar BB-HT length as a taller, shorter frame (say DH bike) but they aren't the same fit, are they ?

er.. if that distance is the same then yes they are!

they will however still feel very different since the DH bike will have a considerably slacker H/A and shorter stem

oh, yeah, and DH bikes are actually longer than XC bikes

Bez – nah, unfortunately they are never published but it's what I go with when buying a frame, sometimes I measure it myself, other times I have asked a shop or another rider with that frame to measure for me.

do you own a DH bike & xc race bike then GW ? I'd be interested in their linear distance between BB & crown race. My guess is they're not the same (let's say "down to a few mm" in your case)

actually, better still, why not measure BB-grips instead ? That should even out effects of differing stems, bar widths etc

surely seat-tube length makes NO difference to fit

Ok, I wasn't quite precise. It's not seat tube length per se, but if you're using "ETT plus seat angle" as your ballpark fit figure then the effect in terms of ETT of one frame having a slightly different seat angle to the other is roughly proportional to the distance from the BB to the height at which you're measuring ETT. Which is, roughly (or at least proportional to) seat tube length, for a conservative frame design at least. In practice for modern MTBs I guess that will cease to work below a certain frame size because forks are so long these days, but for road frames and the sort of large and outdated steel pipes I ride it's quite adequate 🙂

better still, why not measure BB-grips instead ?

Because frames don't have grips…?

well that's kind of my point when I wrote it – you can precisely measure the down tube all you like, and then bugger it all up with different stems & bars

I'm imagining that GW sees DT length as a surrogate for hand position ?

DH bikes are actually longer than XC bikes

well, wheelbase definitely. ETT as well ? (maybe I guess, since they're intended for shorter stems, see comments above)
If ETT is longer, and if they're also typically taller I can't see how we can believe the DT is the same length ??

(If I had a DH bike & a xc racer I'd be out with the tapes but I also don't think I'm sensitive enough to have bikes that really "fit" me in the 1st place)

do you own a DH bike & xc race bike then GW ?

I have owned lots of both, but I now ride XC on a dirtjump frame as I have no interest in riding uphill efficiently (a proper XC race bike would be too detrimental to fun on the descents/flat for me)

I'd be interested in their linear distance between BB & crown race. My guess is they're not the same (let's say "down to a few mm" in your case)

sorry, you've missed my point. 2 frames for very different styles of riding are not necessarily meant to fit you the same way.
but FWIW Yes, my DH bike and hardtail and mini DH bikes downtubes' are all within a few mm of each other.
if I was exclusively dirt jumping I'd want a slightly shorter downtube on te hardtail, if I was exclusively racing XC I'd want longer.
if I was racing DH competitively rather than just riding DH for fun I'd want about an inch longer downtube on my DH bike.
the mini DH bike I'd be happy with as it is in that department.

do you get what I'm saying now?

I do get what you're saying now, but I don't think it's:

"the only measurement that would translate into telling us how a bike fits is actually the downtube length"

(I think we agree, mostly)