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Road Frame Size(8 posts)

Road Frame Sizeazdesertrat
Jan 28, 2003 10:00 PM
Question: Am I better suited on a 56cm frame or should I sacrifice standover clearance to get more spread out on a 58? Here's the details: I am new to road biking (long time mtn biker)and am looking for my first road bike. Using the Greg Lemond (Colorado Cyclist) and the Zinn fitting guides my inseam puts me on a 56cm frame but my torso and long arms put me on a 58. I am 5'11" 195 lbs with 33" inseam and will be riding 50 -100 miles/week and occasional century rides.
the 56C-40
Jan 29, 2003 6:21 AM
Based strictly on your inseam, the 56cm, measured center to top, not center to center, would be the most appropriate size. If you look at brands measured c-c, then a 55cm would be the closest (but slightly larger) equivalent. Unless done very carefully, inseam measurements tend to inaccurate (toward the short side).

If you carefully measure your saddle height from the center of the bottom bracket to the top of the saddle, along the center of the seat tube, the appropriate frame size can be extrapolated. With a 33 inch (84cm) inseam, a typical saddle height would be 29 inches (74cm). Subtract 18-19cm and you get a frame size of 55-56cm (center to top).

Unfortunately, frame size is just part of the equation. You also need to figure out the best seat tube angle for your femur length. The (possibly) good news is that you don't have much choice. Almost every stock frame on the market in the 56-58cm size range has a 72.5-73.5 degree STA, with the majority being 73 degrees.

Since you have a long torso and short legs, you should look for frames that have a longer than average TT length. The TT length of frames with different STA cannot be compared directly. Subtract 1.2cm per degree from the TT length of the frame with the numerically smaller STA. For example, if a frame has a 72.5 STA and a 56cm TT and another has a 73.5 STA with a 54.8cm TT, they are actually the same length after you subtract 1.2cm from the 56cm TT length.

The other thing to keep in mind is that a 2cm larger frame usually only has a 1cm longer top tube. You will run out of standover clearance much more quickly than you gain TT length.
C-40, when are you going into the fitting business?! =)jtferraro
Jan 29, 2003 6:54 AM
Really? You're the guy and you have my vote!

-Jeff
How 'bout me...asphalt assault
Jan 29, 2003 8:22 AM
...I'm 5'9" w/29 or 30" inseam... does it sound like I'd be in a 54cm? I was figuring a 56 because every roadbike I'v "tried on" at my local shops felt "right" in the 56cm size.

The reason I ask is because GT gave me a ZR 1.0 frame for a Zaskar (MTB) frame I broke. They told me they were sending me a 54 (it's all they had) but sent a 52...and I KNOW that's going to be too small.

Do you know much about the ZR series frames? Do they run small or large? Are they any good? I'm primarily an MTB guy
but a road bike would be a very nice endurance training tool to have around.
do a search...Fez
Jan 29, 2003 8:57 AM
for starters, you need to measure your true cycling inseam. since an inch = 2.54cm, an approximation is not sufficient. and you need to measure it all the way, simulating saddle pressure (different than what size pant you wear).
How 'bout me...Shihtzu on EPO
Jan 29, 2003 9:50 AM
I'm 5'9'' n ride a 54
maybe...C-40
Jan 29, 2003 11:25 AM
A true 56cm c-t frame will have a standover height that's about 31-3/4". If your cycling inseam is 30 inches or less (not your pants inseam), there is no way that you could stand over a 56cm frame.

GT has a website with their geometry info. If the 52 is measured c-c, it might be useable. Once again, saddle height will tell more than inseam.
another viewtarwheel
Jan 29, 2003 7:18 AM
I am the exact same height and inseam as you and about 5 lbs. lighter.. I have two bikes, one a 56 c-c and the other 57 c-c. I think you could fit either size frame, depending on your riding style. If you prefer to ride with higher handlebars, say with a 0-2" drop from the saddle height, I would recommend the 57 frame. However, if you can ride comfortably with a larger drop (2" or more), go for the 56. Personally, I like my handlebar about 1" below the saddle height. On my 56 frame, that means I have to use a lot of spacers and riser stem. I also have the saddle moved all the way back on the rails and a fairly long stem (12 cm). My 57 frame uses less spacers, a shorter stem with less rise, and has the saddle positioned centrally.

Most of the fitting formulas assume that riders are comfortable with a large drop, so they err on the side of recommending smaller frames. However, if you are more comfortable riding with higher bars, get the larger frame -- since most new bikes have threadless stems, making it difficult to adjust handlebar height without using lots of spacers and riser stems. Standover height is no big deal -- typically cycling shoes and cleats will add at least 1 cm to your inseam length. In nearly 30 years of cycling I have never had a problem resulting from lack of standover clearance riding 57 frames.

Another factor that could complicate matters is the seat tube angle of the frames you are looking at. Keep this in mind: A steep angle (74) effectively lengthens the top tube because you have to move the seat back to position knees over the bottom bracket. A slacker angle (72-73) effectively shortens the top tube because you can move the saddle forward more to keep knees over the BB. My 56 frame has a 74 seat angle/ 55 c-c top tube and fits almost the same across the top as my 57 with a 72.5 seat tube angle/57 c-c top tube.