A quick poll, if you don’t mind 😉

How do you define a frame size – if, for example, someone had a frame that was 60.5cm c-t (road frame), where would you measure from?

1. Centre of bottom bracket to top of seat tube.
2. Centre of bottom bracket to top of top tube.

Would it matter if it was a horizontal top tube or not?

Cheers 😉

centre of BB to centre of seat tube.

although if I’m buying I tend to look at top tube length far more.

wot wwaswas said (if he’d said top of seat tube or centre of top tube – I don’t care which); I’d only really be interested in the TT length assuming otehr stuff was “typical”

I tend to look at the top tube length but with shorter stems being designed for that’s less easy than it used to be.

Seat tube measurements appear to be more variable as they are measured as you describe and sloping top tube will also cause more complications.

Seat tube measurements were fine when most frame had a horisontal top tube

Option 1 if it was described as 60.5cm c-t, the c-t is centre to top (of the seat tube). The other possibility would be c-c which is centre to centre, i.e centre of BB to centre of top tube, measuring along the centreline of the seat tube.

Top tube is what matters most. Seatposts are adjustable

doesn’t compact/semi compact geometry screw it all up anyway?

and by that i mean: my 60cm frame doesn’t measure 60cm, but a horizontal line from the top of the head tube intersects the seat-angle at a point 60cm from the BB.

centre of BB to centre of seat tube.

idiot!

centre of BB to centre of Top Tube as per scaredypants.

Top tube is what matters most. Seatposts are adjustable

Well, yes, this is mostly an aesthetic thing. Okay, the background is that someone ordered a custom fillet-brazed frame from me, and he provided the measurements, including “frame size 60.5cm c-t” – so I built a frame which measures precisely 60.5cm from the centre of the BB to the top of the seat tube. The actual top tube is 35mm down from this, to give room for the collar and the fillet.

Now he’s trying to argue that no-one measures frames this way, and I should have made it 60.5cm to the top of the top tube, with the seat tube extending some arbitrary amount beyond that.

So wondering what everyone else thinks – I’ve already checked, and with the odd exception of Surly I can’t find any other manufacturer or framebuilder on the planet who measures frames this way.

In reality, all he’d need to do is put the seatpost up a bit and use a slightly more angled-up stem. The TT length and all other angles are as specified, of course…

Ah the joys of a custom frame eh.

c-t is top of seat tube, no doubt.

As you say, no one meaures top of top tube.

Yup, you are right, it’s either to the top of the seat tube (C-T) or to the centreline of the top tube (C-C). Never the top of the top tube.

A= 1. Centre of bottom bracket to top of seat tube.

Or, where the top of seat tube would be if level, in the case of a compact frame.

I’d say you’re right. Only Colnago ever measured to top of top tube that I knew of, wasn’t aware that Surly did too. That’s a detail that should have been mentioned when ordering.

How do you define a frame size – if, for example, someone had a frame that was 60.5cm c-t (road frame), where would you measure from?

Effective top tube and a quick look at the seat angle to see if it’s silly steep or shallow then a mental calculation of the +/- on the stem length this might affect. (and a quick look at the head tube to see if this will negate any differences in seat angle as raising the bars effectively shortens the ETT and dropping lengthens it slightly)

If you know your ideal ETT with your preferred stem length you can instantly tell from a geometry chart if a bike/frame will fit. You could always ignore that and go with the moronic meaningless anecdotal ” I’m 6ft and ride a 57/medium” posts you get from ‘experts’ on Internet forums.

Just read the post about your clients seat tube measurement. Unless he stated a specific c to c or c to t length then tough shite.

Cheers everyone – it’s one of those things that makes you doubt your own sanity, you know? 😉

Anyhow, I’ve agreed to build him another frame for the cost of materials…

Anyhow, I’ve agreed to build him another frame for the cost of materials…

blimey.

what’s the ‘duff’ one look like? Wouldn’t fit me but I’m interested.

i think you’re right.

but – does that help you?, he wants something else!

Anyhow, I’ve agreed to build him another frame for the cost of materials…

Scurries round to the bin behind Ben’s shop

Traditionalish road frame – 700c wheels, 130mm spacing, fillet-brazed throughout, with down-tube gear bosses, pump peg, chain peg, set up for over-bb cable routing, double bottle bosses, eyelets for mudguards and rack.

In British Racing Green powdercoat.

that’ll do. Try not to scratch it as you put it in the bin

A pic:

In Columbus Zona, weighs around 1.9kgs.

so it’ll be identical but with the TT a couple of cm lower?

[edit] it does look nice 🙂

No, it’ll be identical, with the top tube now where the very top of the seat tube is, the seat tube extending another 15mm up, and a longer head tube to match.

Effectively 20mm bigger than this frame, if you measure the seat tube.

c-t = centre to top of seat tube.

TBH, this is a deffo “checkfor the future” point, as he was clear in his mind (and didn’t think he needed to explain) and you were clear in yours (and didn’t need to explain).

I prefer Mr Smith’s approach, though have yet to really get my head around the best angles for me. So I’m still in internet expert territory.

I guess it’s a learning experience re: dotting i’s and crossing t’s 🙁

I’ve sent him another drawing, with every possible dimension marked – just to be really, really sure this time 😉

A project for the holidays is to get the BikeCAD plug-in working on my website – then you can just design the frame yourself and send it to me to stick the tubes together.