Cyclocross - CX bike sizing based on my track bike size
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11-19-08, 11:47 AM
So I'm almost ready to pull the trigger on this: http://www.bikesdirect.com/products/windsor/cyclo.htm
But I'm trying to figure out whether to get a 54 or 56cm and the main sizing I have to go off is what I ride in track bikes since that's all I have. I'm 5'10.
What I've found with my track bike setups is I ride comfortable around a 53-55cm c-c seat tube and 55ish cm top tube.
KHS I rode a 53cm: http://www.khsbicycles.com/PDF/KHS_Fram ... _Track.pdf
Alien I ride a 55 c-c (57 on the site): http://alienbikes.com/gear.html
CX Bike geo. has slacker angles (73 ST/72 HT) which I know effects the sizing of what bike I need, but not positive in what way: http://motobecane.com/ftx_geometry.html In addition the 54cm is c-t so it'd be more like a 52 c-c, and the 56 more like a 54c-c.
I'll be racing it cross by next season, but before that I'm going to do road training / possible road racing with it. I'm poor and can't afford a real road bike yet haha
11-19-08, 12:00 PM
Imagine your body in the position of riding a bicycle. Now image a frame fitting into the space around you. From where the bottom bracket is you could draw an imaginary line straight up to the proper height of your saddle and then back several centimeters to where you normally like your saddle positioned. For this example assume that the front half of your bike is all fine and comfortable. All you need to understand is there's a certain height you like your saddle at and a certain distance behind the bottom bracket that feels "right" for your saddle to be.
Now think about the seat tube that actually leads from the bottom bracket to your seatpost (with it's offset - or not - clamp). If your seat tube was a degree steeper then you'd just move the saddle back a centimeter to end up with that same "correct" position behind the bottom bracket that feels right to you. If the seat tube angle was a degree slacker, you'd move the saddle a centimeter forward on the rails to get the same position.
Now hopefully you can see how this will affect the all-important effective top tube length you'll be looking for. On a bike with a slack seat tube angle (relative to whatever bike you already have that you know you're *super* comfortable on) you could go with a longer top tube and end up with the same fit. On a bike with a steeper seat tube angle, you might end up going for a smaller nominal size. Of course there's always that 90-120mm range of commonly available stems to fine tune things but that's my logic for how seat tube angle affects bike fit.
One other thing to consider, I like tall head tubes on 'cross bikes to make thing easier on my back. Might be worth taking a look at head tube lengths as well.
11-19-08, 12:07 PM
Do you have a photo of your current track bike? If your current setup has a lot of seatpost, and a lot of drop from saddle to bars, I'd suggest the 56. On the cross bike, you want the bars higher up (in other words, a longer headtube), and the longer toptube doesn't equate to a longer reach because you'd be using a lot less seatpost.
If, on the other hand, your track bike has a more conservative road-style setup, that might indicate getting the 54, because you'd want to match your current dims as closely as possible.
11-19-08, 12:20 PM
If your seat tube was a degree steeper then you'd just move the saddle back a centimeter to end up with that same "correct" position behind the bottom bracket that feels right to you. If the seat tube angle was a degree slacker, you'd move the saddle a centimeter forward on the rails to get the same position.
Now hopefully you can see how this will affect the all-important effective top tube length you'll be looking for. On a bike with a slack seat tube angle (relative to whatever bike you already have that you know you're *super* comfortable on) you could go with a longer top tube and end up with the same fit. On a bike with a steeper seat tube angle, you might end up going for a smaller nominal size.Not saying this discussion is wrong, but just to add a perspective: Different bikes are built for different purposes.
The person designing a track bike uses a steep seat tube angle because it is designed around a rider having a super-aggressive position, with a nearly horizontal torso. A cross or road bike has slacker seat tube angle because the rider will be seated more upright. And yet slacker for a cruiser bike, etc etc.
In other words, there is no single "correct" position of the saddle w.r.t. the bottom bracket. Most people would do well simply to use a standard seatpost, with a standard saddle clamped right in the middle, and focus on getting the saddle height and crankarm length correct.
IMHO, YMMV, yadda yadda.
11-19-08, 01:04 PM
Ya ya, as far as I know Andre's track bike is a "street" fixed gear, so I'm basing some assumptions on that. Can't cover everything in one post.
11-19-08, 01:14 PM
As requested, a picture of my track bike in track setup. That's what I raced on last season, seemed to work fairly well for efficiency and power... though it's the fitting I've derived from my own tinkering, not a pro fit. Nevertheless, I did a century on the same bike swapping the bars out for bullhorns.
Oh yeah, I rode that bike as an all-arounder - track, street, road rides of 40 or more miles, etc.
11-19-08, 01:29 PM
IMO a slightly larger frame would give you a more relaxed position, which is what you would want for a cross bike.
11-19-08, 01:38 PM
FWIW check out Tim Johnson's setup. Note that his top tube is slightly sloped; with a horizontal top tube he'd have a bit less seatpost showing. Also, he has a modest drop from saddle to bars, even though he doesn't use hardly any spacers or stem rise. IMO it's right on the money.
11-19-08, 03:12 PM
So, steepening the seat tube puts the bars / reach farther away while slackening it brings them closer, meaning that a longer top tube on a slacker frame will fit more closely to a slightly shorter top tube on a steeper frame, yes? That's what I gather. In that case I'm strongly leaning towards the 56.
11-19-08, 03:46 PM
Based on that image it looks to me like you want the larger frame. But ther eare some on line calculators out there to help you out. You will need some one's help to measure you. Measure yourself for a road bike and then consider getting a cross bike a bit smaller. My cross frame is the same size as my road frame but I adjust the bars higher and the stem 2cm shorter.
TT measurment is the most important.
11-19-08, 08:37 PM
I just ordered the 56cm in white. Stoked! I think I may try to race the last CX race of the year down here in Decemnber if I can get some good practice on the bike in beforehand.
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