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Another sizing question(16 posts)

Another sizing questiongregdogg62
Apr 29, 2003 6:21 PM
I have been trying to determine what size CX frame size I need and am having a little trouble. I am 5'- 9.5" with a 31" inseam and I ride a 54 c-c road bike with a 55 cm top tube with a 120 stem. On my road bike I have about 2 cm of standover. Here are my concerns. How much stand over should I have? Should my top tube reduce? By how much? I will do some racing, but will also use the bike for winter training. I have been looking at the Gunnar Crosshairs and it seems that I would need the 50 cm size. Does this make sense? It seems like I am at the small end of their sizing, which seems strange compared to where I normally fall in a road bike sizng chart. http://www.gunnarbikes.com/mainframe.htm
Thanks in advance
re: Another sizing questionatpjunkie
Apr 29, 2003 7:12 PM
you are doing fine. First shop by TT length. If a 55 TT works on your roadie it will work in cross. Second find a bike with that TT length and adequate standover. ST lengths will vary due to BB height etc... Stem you want to go shorter and taller.
for example I ride a 61 Merckx road bike with a 58.8 TT and a 130 x -5* stem.
My Specialized CX is a 58 with a 59 TT and a 110 x +5* stem.
My Ridley Cross (Y2K old geometry) is a 57 with a 59 TT and a 110 x +5* stem. This bike has slightly more S/O than the S-Works.
My Specialized is about 2.85 or so cm S/O while my Ridley is 3.5c or so. I could go to a 120 or 115 stem on both but I like the higher/tighter riding position for cross. (I could also go to a 120-125 on my roadie but I like the stretch and my road frame is .2 cm shorter than my cx's). usually cx stems are 1 -2 cm smaller and 0-+5 in angle.
none of this is in stone, much is to personal pref. I like 'jewel' room S/O, some go fairly tight, some like equal length stems. usually though total reach is about 1-2 c shorter than road and can be made up by combining TT and stem. I don't like long stems on cx as it slows steering, (it does stabilize though) but twitchy steering seems to help on tight cx track vs 50 mph road descents.
My guess offhand is you should find a 52 c-c cx bike with a long TT, Lemonds are good for this for example. if you can find a 54 TT and a 110-115 stem you should be pretty much dialed. remember bigger tires add to total S/O so check BB heights and S/O with what tire measured. Hope this didn't confuse.
good luck
One tiny flaw to bear in mind...flyweight
Apr 30, 2003 8:39 AM
There's one minor flaw in going by top tube length: it assumes that a longer top tube will yield a more stretched out cockpit. This isn't always the case. My Colnago has a shorter top tube than my Gunnar but has a more stretched out cockpit (assuming I were to use the same size stem on both bikes). Your saddle needs the same amount of setback from the bottom bracket spindle no matter what frame you chose. This means a bike with a shallower seat tube angle will put more of the top tube behind the bottom bracket potentially yielding a shorter reach while also requiring you to slide your saddle further forward or to use a no offset post such as a Thomson.

How roomy a bike is depends on both the top tube, seat tube angle, and the amount of set back the rider needs.
re: Another sizing questionDan Cas
Apr 30, 2003 6:24 PM
Against the grain,I like a cross bike that is at least as big as my road bike,maybe bigger.The key number for me is the front center-no matter the seat tube angle,your saddle should be in the same place as your road bike,and the front center determines where the bars will be in relation to the front axle.

For example,on a 73 head angle,I will not use a stem longer than 11 cm because its too easy to take a header.

I like the bigger triangle of a larger bike too for shouldering,and I like a longer wheelbase for stabilty.I never feel like I want a quicker bike off road.I think standover is a non issue-if you are stradling the top tube,you ain't goin' anywhere.

I think it takes a couple of bikes for you to learn what you like.

Dan
re: Agree on front center issues but.....atpjunkie
Apr 30, 2003 6:48 PM
you must have not yet slipped on a mount or dismount yet.
our cx courses out here have extremely tight hairpins and never any long speedy downhills. A quicker bike tends to get ya around faster especially in traffic.
re: Another sizing questionflyweight
Apr 30, 2003 8:59 AM
Gunnars do run a bit towards the low and long and finding the right size isn't always easy when just looking at the numbers. I'm 5'10" and have a 32" inseam and ride a pair of 54cm Gunnars. On both I use a healthy amount of headset spacers, a riser stem, and have the seatpost up near the minimum insertion line. I could easily ride a 56cm Gunnar but opted for the smaller frame so I'd have more standover since I use one of them as a MTB. I've done several long rides on a 56cm Gunnar and it fit me great and would probably be a better choice if I were doing more road riding or 'cross racing. I also work part-time for the largest Gunnar dealer in the country so I've fitted and sold my fair share of Crosshairs.

If at all possible I'd try to ride both a 50cm and a 52cm. I'd bet that the 52cm will fit better. You say your road bike has a 55cm top tube and a 120mm stem. I'll assume it has a standard 74 degree seat tube angle and I'll assume your stem is a 90 degree model with an average amount of spacers (the more spacers the less reach your stem has). The 52cm Gunnar has a 55.5cm top tube but also has a 73 degree seat tube angle. Without seeing you in person and based solely on the numbers you provide, I'd suggest a 52cm Gunnar with a 100-110mm stem.

Before going for a test ride I'd suggest you measure your saddle height AND your saddle setback AND your saddle-to-bar reach. When you get to the shop have them adjust the bike to match those numbers. Adjusting saddle height alone would probably put the saddle further behind the BB compared to your road bike which would result in the bike feeling more stretched out.
good call on the seat tube angleatpjunkie
Apr 30, 2003 11:00 AM
forgot that part of the equation. my bikes are all very similar so I tend to overlook.
seat tube angle and TT lengthatpjunkie
Apr 30, 2003 1:19 PM
also relies on HT angle. So if ST and HT angles are different but relative (same degrees of diff) relativeTT length is same. In contrast a bike with a steep HT and slopy ST would have a longer 'overall reach' than a bike with similar angles.
Yep.flyweight
Apr 30, 2003 1:26 PM
You used to see bikes that had a steeper than average HT angle and a shallower than average ST angle (Merckx used to be this way). This meant that as you raised the saddle and stem the cockpit would stretch out assuming you didn't move the saddle. A bike with 74 HT angle and a 74 ST angle won't have this problem.

All this is why shopping for a new frame by simply looking at the numbers on a sheet of paper is really not an easy thing to do. We get plenty of people in our shop who complain about a bike they bought via mail order or e-Bay (and this isn't meant to bash either of those) that should have been perfect for them based on the numbers on paper but doesn't ride nearly as well as their old bike. Also why it's good to go to a dealer that knows what they're doing - whether they use a fancy "system" or simply an eyeball and piece of string.
What is an acceptable saddle height?buffalosorrow
Apr 30, 2003 12:58 PM
Depends who you talk to....flyweight
Apr 30, 2003 1:20 PM
There are several theories on how to arrive at ideal saddle height. The only certainty is that no one theory is perfect for every rider and this no one theory is perfect. In a nutshell, "ideal" saddle height is the position that proves to be most efficient without causing injury. You should be high enough that when you ride your knees don't stick out to the sides but not so high that your hips rock in an effort to reach the pedals in the 6 o'clock position. In general, I see far more people with their saddle set too low than I do people who have it too high.
re: Another sizing questionortman
May 5, 2003 8:48 AM
Hi flyweight,
If you don't mind me asking, where is the largest Gunnar dealer in the country? I might try to stop in if it's anywhere near where I live (Portland, OR). I have a 54 cm Gunnar Crosshairs with a 9 mm, 75 degree stem and am wrestling with the question of getting a 52 cm. I use the bike for commuting, have plenty of SO ht, but sometimes feel like I'm reaching a little too much. I'm 5'9.5", 32" (82 cm) inseam, 24" (61 cm) trunk, 25.5" (65 cm) arms. Inseam is not a pants measurement, but was done the "Fit Kit way", with a level pulled up into crotch while in bare feet and measured to floor. It seems if I had a 52 cm I could use a little longer stem. What do you think? Thanks.
re: Another sizing questionflyweight
May 6, 2003 8:36 AM
It's American Cyclery in San Francisco. However, Portland is home to the 2nd largest Gunnar dealer which is River City Bicycles. Richard Schwinn has a tight relationship with both our shop and River City. I was at the Seattle Bike Expo in February and Richard was actually manning the River City booth.
re: Another sizing questionortman
May 6, 2003 1:27 PM
Thanks, if I'm ever in S.F. I'll have to stop in. I have been to River City, it's a great store. Regarding Richard Schwinn, I've corresponded with him via email. It's cool to be able to communicate personally with someone of his stature.

Any opinion on my frame size and my measurements? Thanks.
re: Another sizing questionflyweight
May 6, 2003 2:24 PM
I'd stick with the 54cm. I'm 5'10" with a 32" inseam and I ride a pair of 54cm Crosshairs. I find they're on the small side and I can just as easily and comfortably ride a 56cm Crosshairs.
re: Another sizing questionortman
May 5, 2003 9:00 AM
Hi greggdogg,
Is your inseam measurement a pants length measurement or was it done with a book or level pulled up into crotch? I'm your height, have a 32" inseam (or 33", if I pull up hard while measuring), and am riding a 54 cm Crosshairs with a 9 mm 75 degree stem. It does seem a little long to me, but the input I've received from many bike dealers who sell Gunnars is that a 52 would be too small. I'm getting a professional bike fit this week so I'll have a better idea of the correct frame size soon.