Comparing geometry on two bikes

daniel_b

Morning all,

On my Scott the geometry is as follows:

54cm frame
545mm top tube
73 degree headtube angle
Head tube length 156mm
74 degree seat tube angle

I run this with a 20mm setback seatpost, the saddle slightly (maybe 1cm) further back than central, and a 90mm upturned stem with one spacer above.
I have long legs versus a short torso, so the saddle to handlebar drop is still fairly healthy.

On the Bike I am thinking of buying the dimension are as follows:
55cm frame
548mm top tube
73 degree headtube angle
Head tube length 147.5mm
73.5 degree seat tube angle

So the only difference that could have a biggish impact is the more acute seat tube angle, but will 0.5 degrees make a difference over what 60 (including the seatpost) cm?
And the head tube length, but I wonder if I can nullify that just by not running any spacers ontop of the stem, as I know you are meant to do with a carbon fork.
I assume it will put me further away from the front of the bike, and therefore I might either just need a shorter stem, or to adjust the saddle position.

Have I missed anything obvious?

Man Of Lard

It will set you back (seat tube + seat post) x (sin 74.0 - sin 73.5) - which is 0.002 x the combined length of the seat tube & post. (0.25mm setback per 100mm or there abouts).

a shade under 2mm in 750mm combined length.

Of course it would drop you by a small amount too for the same combined length of seat post & tube - around 6mm (assuming the combined length is 750mm)

mfin

Maths is wrong above.

sin(0.5deg) x 750mm is about 6.5mm (NOT 2mm)

Anyway, on the new bike if you have a layback 20mm post then you'll have your saddle about 6.5mm more forward and the bike will be about 3mm shorter to the bars best guess.

OP, your assumptions are backwards, you say that the new bike has a more acute seat angle, it doesn't, it's slacker. Plus, weirdly you say you can nufflify the head tube length by not running any spacers when the new bike has a SHORTER head tube, so you might run more. (I say more, but that is affected by any difference between the two bikes Bottom Bracket Drop and headset stack height).

Anyway, what might be more useful to you is it would seem you will be alright on it, and I'm building in some tolerance here as you can't be too fussy as you don't mind running an upturned 90deg stem. I reckon you'll make it fit and be happy enough.

daniel_b

Cheers chaps,

I took acuter to mean less of an angle, ie 73.5 versus 74.0 - wrong use of acute?

You misunderstood the bit about spacers - I said I would get around it by not running any spacers on top of the stem.

Currently I have 2 underneath the stem, and one on top as per best practice - assuming the fork on the Btwin has an allow steerer, there will not be the requirement to have a spacer on top, and so I can run 3 spacers underneath the stem, and thus have the topcap securing straight onto the stem.

mfin

Yeah wrong use of acute.

I see what you mean about 'above' now, not many people talk about that as steerer tubes are cuttable. Spacers also come in different heights, so '2' or '3' doesn't mean a lot, they could be 2mm spacers, could even be up to 15mm ones or something. So, how tall are these or what's their total height? (This is a double check that they aren't massive in case they add up to a lot and the new bike has to do more... plus, note that headset conical top caps and bb drops can effect all this again).

daniel_b

From memory, I think there is probably something like 3-4cm underneath, and 1cm ontop.

It's not that I am not fussy, but with a short torso and 34" inseam, unless I have a frame custom made, I need to make the bike work for me. Oh and I have a slightly dodgy back, although not too bad these days.
Stems have the option of being upturned, so they were clearly designed to do so.
I have around a 3" drop from saddle to bar height which I think it still pretty normal, as I have loads of seatpost showing.

Here we go - this is when it still had an inline post on,it's probably back around 3cm since then with the saddle position and setback seatpost:

mfin

If there's more BB drop on that new bike then you're in danger of running too many spacers there. I'd be careful with this. I would not buy until you know the depth of both the conical headset cap on that Scott and the one on the new bike, and know the BB Drop of each bike. (If the new bike has less BB drop, then you will need to add the same amount in spacers again just to cancel this out). If the new bike had more BB drop then you might be ok, but if they are the same for example then I would say you are right on the limit of fudging this new bike to work, you're already fudging the scott really (although pretty safely)

Man Of Lard

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19715412#p19715412]mfin[/url] wrote:

Maths is wrong above.

sin(0.5deg) x 750mm is about 6.5mm (NOT 2mm)

Nope, your vision is excluding the whole picture: at 74 degrees, it is set back 750 x sin 74
If the angle changes to 73.5, it is then set back 750 x sin 73.5

So the difference in setback is 750 x (sin 74 - sin 73.5) which is not the same as sin 0.5

sin 73.5 = sin(74 - 0.5) = sin 74 cos 0.5 - cos 74 sin 0.5

mfin

No, you're wrong, and if you wanna do it your way you'd have to take the angles away from 90, the set back at 74 would actually be sin(16)x750mm for example, or it's cos(74)x750mm same thing.

But, it's all pointless, approximately it is 6.5mm worth of difference, not 2mm.

750 x (cos 73.5 - cos 74) = 6.28mm , so if you want to argue about 0.2mm now as if it matters then go ahead.

mfin

(By the way, I know even this is a simplified method if you want to work out something like this exactly using geometry, ie you'd work it out for half the angle difference then double it at the end so the 750mm remains constant, but its easily close enough for use in bike fitting).