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Look 381i frame sizing differences?(8 posts)

Look 381i frame sizing differences?Jay H
Aug 7, 2003 7:38 AM
Looking at the frame sizes for a 49cm and a 50cm, the only difference appears to be standover height and effectively the seattube measurement. Both top tubes are 539mm with a head and seat angle of 72.5/72 respectively. I'm wondering if both these frames are slightly too large for me. I ride a 49cm Lemond Zurich with a 74° seat tube and a top tube of 520mm. (forget the head angle). The stem/fork is fairly standard.

Would this frame be radically too big, I would like something with a longer cockpit but not sure if the 2+ cms might be a bit too long. I could get a shorter stem but that may affect my center of mass and steering feel.

re: Look 381i frame sizing differences?Horace Greeley
Aug 7, 2003 8:57 AM
Keep in mind for every degree the STA is relaxed (i.e. less than 74 degrees), there is an effective reduction in TT length by approximately 1.6 cm (assuming KOP remains the same). I ride a C'dale 54 cm (C-T, which is effectively smaller) and a 54 cm KG381i (C-C), and I've reduced my effective top tube length slightly (while keeping KOP the same).

Hope this helps.
re: Look 381i frame sizing differences?Jay H
Aug 7, 2003 10:20 AM
Wouldn't relaxing the STA from say 74° to 72° increase the effective top tube length? (assuming the same saddle position) and all other geometry?

close. but not quite right...C-40
Aug 7, 2003 11:37 AM
Every degree of seat tube angle changes the effective top tube length by the amount: saddle height x (cosA-cosB), where A and B are the two seat tube angles. The amount will vary between 1.0 to 1.5cm, depending on the frame size. For this small size frame, the amount is much closer to 1cm per degree.

If you run the rumbers for a 68cm saddle height, the difference is 1.7cm for a 1.5 degree STA difference. Subtracting 1.7 from 53.9 equals 52.2cm. Thus, the TT lengths of the two frames are nearly identical, after the saddle has been moved to the same position relative to the BB.
close. but not quite right...Jay H
Aug 7, 2003 11:51 AM
The Look has a STA of 72 whereas my Lemond has a 74, wouldn't this make the eff. TT length 1.7cm greater? As the slacker STA is pushing the center of the saddle back and away.

What are the pros and cons of having a relatively shorter stem, any perceived differences? I would be getting a non-setback seatpost and alighning my saddle based on KOP then looking at getting a stem in the right length and angle.

Aug 7, 2003 12:00 PM
Some folks have a hard time understanding this concept. You are correct that you would need a "straight-up" (Thomson or similar) post to move the nominal saddle position forward 2cm. With this type of post, you will have nearly an identical nominal saddle position as the Lemond with a 74 STA.

Isn't it obvious if the saddle is moved forward 1.7cm to achieve the same KOP as your other bike, that 1.7cm should be subtracted from the TT length of the LOOK? If not, trust me, as an engineer, CAD drafter and experienced cyclist, this is correct. You should not need any different stem length than your current bike, since the "effective" TT lengths are within 2mm of each other.
Aug 7, 2003 3:29 PM
Make sure you can move the saddle back 1.7cm!

I was interested in buying a LOOK frame. I did the Serotta fit and I asked the guy to set the Serotta fit-cycle's seat angle to 72 degrees. My femurs are short, and the saddle was already moved way forward even with a 74 STA. Having a slack angle of 72 STA forced the saddle to pinned at the most forward position to get the correct KOP.

To C-40 and VWJay H
Aug 8, 2003 4:12 AM
Well, a quick check of my old Lemond shows that it is a setback seatpost and my saddle is in the fairly neutral position, i.e. has room to move back and forward.

If I get a non-setback seatpost, I should have much more than 2cm of adjusting room to play with. All the new bikes seem to use threadless forks (my Lemond is not) so changing the stem should be easier.