Luis50

I'm 168 cm tall, 80 cm inseam & 58 cm trunk. Fast group rides & maybe a race or two down the road. I'm thinking a 50 cm...what do you think?

Steev

Try plugging your measurements into this calculator.
https://www.competitivecyclist.com/za...LCULATOR_INTRO

AEO

52 and 50 are the same thing because of the seat tube angle and head tube angles.

at your size either 50 or 52 will work.

ljrichar

Not sure how you came to that conclusion.



At your size, I'm thinking a 50 or maybe even a 48. You won't really know unless you test ride them. I'm 5'6.5" and thought the 52 was still little big but I'm looking for an aggressive fit.

AEO

https://www.bikeforums.net/road-cycling/484169-52cm-50-cm.html

post 13.

DaveSSS 

There's not a lot of difference between the 50 and 52, but there is some. The difference in the STA is only .5 degree, which does NOT cancel out all of the decrease in the reach on the smaller size. The 52cm has about 6mm more reach and a 5mm taller head tube.

The 52cm should fit fine, unless you want the bars extremely low. Even then, the 50cm only offers another 5mm of drop.

DrPete 

So you're giving advice for someone else's 4-figure investment based on an anonymous opinion on the internet that's not even yours?

The 50 and 52 have a 1-cm difference in top tube length, and being that most competent fitters will tell you that top tube length is the most important dimension on a frame, it is a significant difference.

+1 for the CC fit calculator, but precision in measuring is key. It's very easy to get a bogus frame size if you get the measurements done wrong, especially with the arms.

admcptch

I got a 52 (as of last weekend) and I'm 5'9". The top tube length fit better for me as my arms aren't quite as long as the rest of me. Even on the 52 I'm probably going to need a shorter stem or my seat moved forward.

DaveSSS 

The TT length means nothing without the STA to go with it. The two work together to create the reach of the frame.

I don't think much of the CC fit calculator either. Too easy to make errors in the measurements and the results don't list a STA to go with the TT length. No fit calculator has any way of knowing if the rider can tolerate a 5cm drop or a 12cm drop. That could make a big difference in the frame size chosen.

Jynx

I agree that is is not a good idea to rely on someone else's info who just spent a lot of money but you are leaving out the differences in seat tube angle and headtube angle that would change the reach when you set both frames with the same amount of setback relative to the bottom bracket. If one has an extremely steep seat tube and one has a relaxed seat tube (even though they have the same toptube) the reach will not be the same when both bikes are setup. The steeper seat tube would use a more setback post to have the same saddle setback which would increase reach.

I didn't look at the specifications of these two frames but it is possible the reach difference is only 5mm or so even though they have a 10mm difference in top tube. Also I wouldnt even consider 10mm a large difference in top tube length. Stems can easily cover that gap. The other geometries are more important in this case.

rollin

Sta?

Jynx

Seat Tube Angle.

Luis50

My present ride is a Ridley Damocles. 54.5 cm top tube; 73.5 sta; 73 hta. The seat is jammed all the way forward & I have a 70mm stem...not good. I'm too stretched out. Will never be right on this frame mainly because of the tt length. My arms are 56 cm. The rest of the numbers are in the op. I'll get a test ride on both eventually but I'm buying from out of state and don't want to bug the lbs too much. I should pay for a fit so I don't mess up like I did with the Ridley but I'll take the free advise from you guys also so let's hear it.

AEO

AnthonyG is the go to guy for small bikes on this board, so I'll take his word for it.

dallasmarlow

i have this bike and i am about the same size of you. i ride a 54cm (it's probably a bit big on me), caad's run kinda big. if i were going to do it again, i would have gone with the 52cm.

DaveSSS 

Your numbers don't add up unless you have unusually short arms. There are so many ways to measure arm length (none accurate) that I wouldn't hazard a guess there.

FWIW, I'm only 1cm taller with 3cm longer legs (83cm inseam, 73cm saddle height), so I have a shorter torso. If I had your frame, I'd have the saddle slammed back with the nose 6-7cm behind the BB, a 100-110mm stem and short reach bars. I'd be very cramped if the saddle was 2-3cm further forward and the stem was 20-30mm also shorter.

I ride a 51cm LOOK 585 that should have only about a 5mm shorter reach, with a 53cm TT, but steeper 74.5 degree STA.

Luis50

You're right. My arms are 64 cm according to the comp. cyclist method. I've considered the short reach bars in hopes of being able to get in the drops easier. I'm still leaning towards the 50 cm because the Ridley has shown me that it's easier to make a bike bigger than to shrink it.

DaveSSS 

Th 50cm would make sense. It decreases the reach by 6mm and the head tube only decreases by 5mm.

I'd also measure the total head tube length with the headset and spacers on your current bike and be sure that the TT length is adequate.

Another thing that adds to a reach problem is Shimano shifters that have more brake hood reach than SRAM or Campy. From all reports, the new 7900 is even longer. A step in the wrong direction, IMO.