# Effective top-tube length.



## Guest (Feb 14, 2011)

How exactly does the effective top-tube length of the Tarmac or Allez differ from my road bike that has a fairly parallel to the floor top-tube? Does it bring the rider closer or farther from the handlebars? I ride a 50cm with a 51cm top-tube, and was wondering if I would be better off with the 52cm or the 49cm? I plan on replacing my current frame and will do a test ride, but I wanted to get some opinions from those who know before I start looking. Thanks.


----------



## PJ352 (Dec 5, 2007)

Mevadus said:


> How exactly does the effective top-tube length of the Tarmac or Allez differ from my road bike that has a fairly parallel to the floor top-tube? Does it bring the rider closer or farther from the handlebars? I ride a 50cm with a 51cm top-tube, and was wondering if I would be better off with the 52cm or the 49cm? I plan on replacing my current frame and will do a test ride, but I wanted to get some opinions from those who know before I start looking. Thanks.


Because the sloping top tube measurement of compact geometry bikes is pretty meaningless in regards to fit, manufacturers now use a virtual (or effective) measurement as illustrated below.

If you're now riding a bike with a 51cm horizontal top tube, assuming it fits you well, you'll want to stay as close to that number as possible, so a 49cm Allez or Tarmac (having an effective top tube of 518mm's) would be preferable to the 52cm. Also, keep in mind that differences in seat tube angles affect the effective top tube length, and (when comparing two frames) has to be accounted for. 

Just a FYI. This recommendation is based only on the information you've provided, regarding reach. There are other important criteria in determining frame sizing needs, such as head tube length and standover, so take those into consideration as well.

View attachment 223712


----------



## Guest (Feb 14, 2011)

Thank you for the reply. What you have said is fairly close to what I thought, but I just wanted to make sure so I can go into the store with some frame geometry knowledge. I measured the seat-tube length on my current bike and looked at the numbers of the Specialized, and found the 49cm to be closer to my current bike than the 52cm. I know I have some play as well depending on how high my seat-post can go, stem length, and the zero set-back seatpost I use. My inseam is a measley 30 inches, so the 49cm would probably fit me better than the 52cm in terms of stand-over height, but my biggest concern is reach, and I am currently using a 100cm stem with the 51cm top-tube. This combination works very well for me. Thanks for your assistance.


----------



## PJ352 (Dec 5, 2007)

Mevadus said:


> Thank you for the reply. What you have said is fairly close to what I thought, but I just wanted to make sure so I can go into the store with some frame geometry knowledge. I measured the seat-tube length on my current bike and looked at the numbers of the Specialized, and found the 49cm to be closer to my current bike than the 52cm. I know I have some play as well depending on how high my seat-post can go, stem length, and the zero set-back seatpost I use. My inseam is a measley 30 inches, so the 49cm would probably fit me better than the 52cm in terms of stand-over height, but my biggest concern is reach, and I am currently using a 100cm stem with the 51cm top-tube. This combination works very well for me. Thanks for your assistance.


Depending on the differences in seat tube angles (if any) the 49cm with an ETT of 518 _might_ require that you go to a 90mm stem, but not knowing the geo of your current bike, I'm just guessing. 

Also, given your inseam, a 52cm Allez with a standover of 777mm's won't work, and even the 52cm Tarmac with a standover of 755mm's is pretty close. From this standpoint, the 49cm's are better bets.


----------



## RkFast (Dec 11, 2004)

Coming from a traditional frame, if you will, I compared the geometry numbers of the old bike to the Tarmac I ordered, finding the frame size where the numbers most closely matched my old bike.


----------



## Guest (Feb 14, 2011)

Thanks for your replies. I am going to head to the store here in the next few days, and test ride a few of the already built Allez frames.


----------



## RkFast (Dec 11, 2004)

Definitely get fit properly and get your new bike inseam measurements and all that. But like I said last night if your old bike did fit you well, compare the geometry between your old frame and the one you want to "validate" the size youre getting and overall, go for the frame where the numbers are overall closest to the bike you have now. It will be pretty easy to do as there should be one frame size with small differences to what you have now and the others will have different measurements a good few CMs off.


----------



## roadie01 (Apr 13, 2010)

IMHO I would read up on the XY fitting method and try to replicate this for all of your future bike fit needs.


----------



## Guest (Feb 17, 2011)

Thank you all for your help. Unfortunately the Specialized Allez really isn't what I am looking for. The bike is nice, but I feel like I would be stepping sideways and maybe down a little from what I currently own. My Fuji is weighing in at 16.4lbs right now, and the bike store rep told me the lightest I could get with the Allez would probably be 17-18lbs. He then proceeded to put me on a Giant TCR Advanced 2 w/105 that was selling for $2,000. Absolutely beautiful ride, but I am not sure I am ready to spend that much or buy a carbon frame.


----------

