Some sizing/geometry confusion

heavymental

I'm a little bit confused on several issues regarding sizing so I'm hoping to get a bit of clarity.

First off, I'm 5'8 but with long legs compared to my body.

I currently ride a CAAD2 Cannondale R600 thats a few years old. From my measurements its 53cm along the top tube (from centre of headtube to centre of seatube). The seat tube is 55cm from the centre of the BB to the top of the collar. Thats around about 52cm from the centre of the BB to the centre of the top tube. But as its ever so slightly sloping I'm not sure if that is a useful measurement?

I have a 120mm stem on at the moment but I have to reach slightly forward to get onto the hoods so I tend to ride with my hands slightly back from the hoods and reach forward to change gear and brake. I'd therefore like a similar/slightly shorter length in the top tube...I don't want it longer anyway.

Now, I want to buy a new frame and have been looking at some fancy frames like a nice Time and a couple of Look frames. Trouble is, the top tubes seem quite long on the models I've looked at. For example I was looking at a Look KG381i (ended now) on ebay but for a 53cm frame the top tube is 54.8cm long. Thats going to be a bit long for me unless I get a stumpy stem but even on a 49cm frame the TT length is given as 53.9cm. Infact...for that model, between the sizes of 49 and 53 there is less than 1cm difference on the length of the top tube. Why is that?

Basicly I hadn't thought there was so much variation on geometry. Is it just a case of looking for the length of top tube I need? That seems to be the defining measurement really but most bikes are listed based on what the length of the seatube is. Seems a bit strange to me.

I realise this is a bit of a ramble but its late and my brain hurts!! :roll: can anyone shed a bit of light on whats the best way to shop for a new frame. Obviously custom would be the way forward but I'm not considering that at the moment.

Smokin Joe

If you get a compact frame then seat-tube length is all you really need to worry about, the vertical dimension can be solved by adjusting the seatpost.

On the other hand, if you get something the same as you have, why not fit a shorter stem? 110mm will make quite a difference.

aracer

You're working on the right lines, as the most important measurement on a bike is the top tube length - head tube length is kind of important too, though can be compensated for with spacers / stem angle. Seat tube is pretty much the most meaningless measurement, particularly with the advent of sloping geometry, as you just need a longer / shorter seatpost (though too long a seattube may give you standover problems). If only companies would spec bike sizes by TT length!

One issue you may have with your measurements is that it sounds like you're measuring the actual top tube length along the tube. With a horizontal tube this is fine, however as soon as you get onto sloping geometry it's not really useful, as what you actually want is the theoretical length of the top tube if it was horizontal. Measure this from where the tt intersects the head tube to the point at the same height on your seatpost - ie your ruler/tape measure is horizontal. You'll find that's a bit longer than what you measured before, so may make the difference to what you can fit on.

Of course the other issue is that you've got quite a long stem there - for that frame size it would be more normal to have something like a 100, and a 90 would also probably be fine. What it all means is that you can probably get a frame slightly bigger than you think (though I wouldn't recommend anything with a longer horizontal tt than you already have, I suspect you'll find that's actually ~54cm). Oh, and don't get a frame just because it's nice if it doesn't fit.

bigdawg

Im similar size 5ft7 long legs shorter body...

standard sizing I ride a 52cm cannondale but have a 90mm stem on it, can sit all day on the hoods and t he drops.

Ive also got a giant TCR in a small, again with a 90mm stem very comfy (although I do have virtually a foot of seat tube showing..

heavymental

Smokin Joe wrote:

If you get a compact frame then seat-tube length is all you really need to worry about, the vertical dimension can be solved by adjusting the seatpost.

On the other hand, if you get something the same as you have, why not fit a shorter stem? 110mm will make quite a difference.

I guess you mean Top Tube there do you Joe? (good to have you back by the way!)

The geometry on my Cannondale is actually fairly traditional....I mean the top tube doesn't slope much at all really so I think the top tube is probably like I said....53cm. Although I guess the way to tell is to take a spirit level and take the measurement from the intersection on the head tube to the point on the seatpost where the tape is horizontal. Might give me a bit extra on the measurement I guess. I'll check again tonight

I guess I can replicate the position of my Cannondale by matching the distance along the seat tube to the centre of the stem clamp. But it then comes down to the other aspect which is aesthetics. Persoanlly, I'd prefer to have a fairly small frame with plenty of seatpost and a reasonable (100mm) reach on the stem purely from a looks point of view. Theoretically I could get the same position on a much bigger frame if I had a tiny stem and no seatpost showing but it doesn't look great does it.

I'm still a bit confused about the geometry on some manufacturers though. Why does the top tube length shrink in smaller increments than the seatube? Do they think that its just the legs that tend to be shorter the smaller you are rather than the whole body being shorter?

Monty Dog

Look frames do have pretty long top tubes, particularly in the small sizes, and the Giants aren't too far behind. IME Many Italian makers have a better understanding of the geometry for smaller frames, rather than making an 'average' frame smaller. For a 52cm 'traditional' frame, you'd expect a toptube of somewhere between 53 and 55cm, but from the sound of it, you may be better looking at something like a 48 or 50cm frame with a toptube in the region of 51-52cm. However, from what you say, you may benefit from some expertise in helping setting up your position. It may be worth spending on getting a proper 'bike fit' done and then base your frame selection on what you really need - that should give you a pretty good 'short-list' of your needs and the frames that match. In the short term, fitting an 11 or even a 10cm stem may help with your reach problem.

heavymental

Thanks for all the advice. I guess really I just have to look at frames that have a top tube measurement in the range you suggested, Monty. 51/52ish. Seat tube isn't really an issue. I might see if I can hop on a jig at some point and get some guidance from someone who knows frames but I think thats a good start. Looks like the Look is out of the window unless I can get a titchy one.

Before I ditch the idea though. Heres the measurements....

Current top tube = 53cm
Look top tube = 54.8cm
Current stem = 12cm

So if I bought the Look frame and knocked 1.8cm off the stem length I'd be in the same position as I am on the Cannondale? I might have less seatpost out but essentially I'd be in the same position? I could then knock a further cm off the stem to put me in a slightly better, less stretched position. Should I therefore still consider it or am I just better off getting a shorter top tube and working around that?

I'd be interested to hear what other people around my height ride too.

Smokin Joe

Hi HM, yes I did mean top tube. :oops:

Ta for the welcome back, I left this place for a while after the changeover, but missed it to much to stay away.

heavymental

Good to have you back Joe...means I'm not the only pembs face on the forum!

Ok....so I've done a bit of research on some frames and their geometry...here's my findings....

..................................... Seat Tube c to c Top Tube c to c
Time VXR...................... 47............. 53
Pinarello f4:13............... 46............ 52.4
Eddy Merckx classic.... 48/50.......... 52.5/53.5
Cervelo R3................... 51............. 53
Ambrosia Guido........... 52................. 53.4
Look KG381i................ 49 and 50... Both 53.9
Look 585 origin............ 45............... 53
Giant TCR.................... 46.5................. 53.5
Spesh Allez Elite.......... 43(!)................... 53.7

De Rosa only give the size as top tube length.....on the Parkers site at least.

So, to get roughly the same top tube length I'm looking at frames ranging between 43 and 52cm's. Confusingly the Spesh is listed as a 52 even though the seat tube is 43cm c to c!

Anyway...dunno what I've really concluded....I spose just that the seat tube size is only a guide to sizing and I'll have to have a good look at top tube length before I go buy something.

edited to try and make things line up!

heavymental

Anyone got any more views on this?