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Recreational Cyclocross and Gravelbiking This has to be the most physically intense sport ever invented. It's high speed bicycle racing on a short off road course or riding the off pavement rides on gravel like :The Dirty Kanza". We also have a dedicated Racing forum for the Cyclocross Hard Core Racers.

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Old 11-14-12, 02:48 PM   #1
pdxtex
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How to fit a cross bike????

so i just picked up a 2001 GT ZRX off of craigslist. great price and nice build, on paper. im having a hard time getting the thing to fit right and wonder how cross geometry differs from road bikes. i get the premise of a more slack seat tube but it just seems to fit wonky. my back feel great, and the reach to the bars is excellent, but my knees feel squashed in front. the seat height is approximately right, maybe a little higher but the seat feels like it needs to go back. perhaps move it back and get a shorter stem? its an odd sensation to explain but it seems like the top half of the bike is sized long, while the back half is too short!
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Old 11-14-12, 03:07 PM   #2
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not familiar with the specific bike , got a Link and so forth?

Actual races are all about acceleration out of the start and the corners .
that is where the steeper seat post comes in.. not comfort..
out of saddle sprinting to next corner.

you may want another type for the road commute , perhaps ..
seek out long setback seat posts , shorter reach stems..
and all that ..


I take it you did not test ride it enough before taking it home.
Don't like it? stick it back up on CL.
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Old 11-14-12, 03:24 PM   #3
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Steeper seat tube also keeps it out of the way of the rear tire and allows the chainstays to remain shortish.

I'd start with a setback post and change one thing at a time after that if needed, allowing time in between to adapt to the change.

Even if you're able to exactly duplicate your road position the bike will still feel different. I always consider how the bike handles when making changes for fit too.
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Old 11-14-12, 03:59 PM   #4
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I tend to think of fit and geometry as (mostly) independent. I figure out where I want the bars and saddle relative to the bottom bracket and then I make it so. Of course, the geometry doesn't always make that easy. With a steep seat tube angle I need a setback seatpost. With a short head tube, I need a stem with a lot of rise. It's all about function, but if the fit you want doesn't correspond to the bike's geometry you can end up with something that looks wonky (if you care about that sort of thing).

As a general rule, the seat tube angle directly maps to saddle position adjustment. For every one degree of increase in seat tube angle, you have to move the saddle back about one centimeter to get the same position relative to the bottom bracket. Of course, that doesn't help if you don't know the seat tube angle of both bikes you're comparing. What you can do, though, is just set the two bikes next to each other with the bottom brackets side-by-side. Then adjust the saddle height (factoring in any difference in bottom bracket height) before sliding the new bike's saddle forward or back to match the bike whose fit you like.

Handlebar height and reach is a slightly different beast and should definitely be done after saddle position is fixed. Head tube angle has almost no effect on handlebar reach, so you can ignore that. Obviously you're adjusting for comfort at this point. You can match the bar position the same way as described above for the saddle to get the same fit. If you're racing, you might want the bars slightly higher and/or closer than you would for the road. Personally, I like a relaxed position, so I use the shorter/higher position on my road bike too.
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Old 11-14-12, 05:50 PM   #5
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assuming bike A fits me, what if i dropped a plumb line straight down from the seatpost cradle of bike A and measured the distance from where the line hits the seat stay to the bottom bracket and then matched this dimension on bike B. would that work as an initial starting point?

concerning test riding this bike before i bought it, nay, didnt test it. i knew the frame was my size but it was covered in mud, had two flat tires and no pedals. it cleaned up super nice and has a full 105 kit and thomson post! score.
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Old 11-14-12, 06:21 PM   #6
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Quote:
Originally Posted by pdxtex View Post
assuming bike A fits me, what if i dropped a plumb line straight down from the seatpost cradle of bike A and measured the distance from where the line hits the seat stay to the bottom bracket and then matched this dimension on bike B. would that work as an initial starting point?
Sounds right. Different saddles may make a small difference in where they support you, but what you described will at least get you close.

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Originally Posted by pdxtex View Post
i knew the frame was my size but it was covered in mud...
...as a CX bike should be.

Bringing it out to PIR Sunday?
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Old 11-14-12, 06:27 PM   #7
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Sounds right. Different saddles may make a small difference in where they support you, but what you described will at least get you close.



...as a CX bike should be.

Bringing it out to PIR Sunday?

Nah, Im a poseur! I ride it to work right now. but i wanted canti brakes for the rain. i live right next to leif erickson though so im going to start riding that however.
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Old 11-14-12, 06:54 PM   #8
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It's a small step from Lief Erickson to the Crusades.
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Old 11-14-12, 08:19 PM   #9
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This Leif Ericson?
http://en.wikipedia.org/wiki/Leif_Ericson

these Crusades?
http://en.wikipedia.org/wiki/The_Crusades

[just kidding]
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Old 11-20-12, 12:23 PM   #10
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Do this

then read this
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