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PostPosted: Sun Feb 09, 2014 12:34 am 
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Joined: Tue Dec 02, 2008 4:21 am
Posts: 277
The infamous fit question....

I've been riding/racing road for a number of years but looking to get into cross. Been offered a really sweet Stoemper Ronny single speed CX bike, would really like to make it work but concerned it is too big, so thought I'd pose the question to those of the Internet.

I ride a 52 cm road bike - 49 cm seat tube, 53.7 top tube, 110 head tube w/o spacers to a 110mm stem, saddle height from BB is 67 cm. (compact geometry)
The CX frame measures - 52 cm seat tube, 54 cm TT, 120 head tube (traditional geometry)

I've always heard go a size down from your road when fitting a CX bike but others have said just stick with your road geo more or less and adjust with stems. I'm less concerned with the reach/stack of the CX bike it's the stand over and using only about 12 cm of seat post + saddle since I've got shorter legs. But the other thing that makes me think it'll work OK is my road bike is compact geometry while the Stoemper is traditional with a horizontal top tube.

Thoughts?


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Posted: Sun Feb 09, 2014 12:34 am 


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PostPosted: Sun Feb 09, 2014 12:36 pm 
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Joined: Sat Jun 19, 2004 11:08 am
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I don't see an issue with it. The size down thing is kind of an urban myth. The only difference between my cross and road bikes are the saddle on the cross is a few millimeters lower for easy of mounts and dismounts.

http://www.youtube.com/watch?v=ZTTOBnu7w-k

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PostPosted: Sun Feb 09, 2014 4:51 pm 
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My CX bike has .5mm shorter top tube due to geometry differences between different manufacturers. I also run slightly wider bars for leverage and stability when I go gravel riding (the county puts down rocks, and I feel better with my weight outward a bit more).


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PostPosted: Sun Feb 09, 2014 5:02 pm 
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Joined: Fri Apr 20, 2012 9:04 pm
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From my research, the guidance for sizing down probably comes from the fact that the stack height at the front it typically higher (due to increased fork bridge height and typically a fairly generous head tube length). This combined with a smaller bottom bracket drop makes the stand over height greater.

In short, if it is too big, it will probably be the clearance when standing over the top tube which is probably more important on a CX than a road bike.


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PostPosted: Tue Feb 11, 2014 2:41 am 
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Joined: Sun Nov 15, 2009 12:27 am
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I've heard the one size down a lot but I prefer the same size as my road bike, tt and seat height are pretty much same as my road bikes the only difference is about 1.5cm higher bar height which I prefer on the cross bike. I've had 2 bikes that I went down one size and while I was able to get the setup right I had to use a long steam and more spacer which made the front end flex more and always felt cramped on a smaller frame. If you have short legs keep an eye on stand over height because while the seat tube and top tube might be the same as your road bike stand over height could be higher because of higher bb and cross tires. If longer legs and long arms like me it's not as noticeable but I have to pay more attention to the tt length otherwise I end up with a long stem.


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PostPosted: Sat Feb 15, 2014 2:09 pm 
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Joined: Mon Jul 26, 2004 8:57 pm
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Location: Ireland
Just remember, if your road bike has a head tube of 110 and you ride with no spacers and the cx bike head tube is 120 there is a chance of you having your bars up to 3 to 4cm higher due to the length of cx forks being around 400mm towards 370mm for road forks

fork + Headtube = Total height
370 + 110 = 480mm
400 + 120 = 520mm

Difference = 4cm#

However you also need to take b/bkt height into account but it's usually only a few mm.


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PostPosted: Sat Feb 15, 2014 4:48 pm 
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Joined: Tue Dec 14, 2010 9:18 pm
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Don't forget wheelbase either. I was advised to go down a size and sometimes get caught out by my toe hitting the front wheel in sharp turns.


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PostPosted: Sat Feb 15, 2014 6:01 pm 
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CrossRob wrote:
Don't forget wheelbase either. I was advised to go down a size and sometimes get caught out by my toe hitting the front wheel in sharp turns.



That was one of the problems for me on a smaller frame, had to be very careful in slow sharp turns.


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PostPosted: Mon Feb 17, 2014 11:33 pm 
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Joined: Tue May 23, 2006 4:33 am
Posts: 732
When you're talking a small frame, there are all kinds of particular considerations that don't necessarily apply to bigger riders. That's true with all bikes, but especially with cross.

First, you have to throw away many of the rules you'll hear from other bigger ridders -- like pedal/front wheel overlap, stand over height, etc. You'll never get stand over height in a small frame; just plan on tilting the frame over slightly when you stop so you lower the top tube and can get both feet down. But honestly, you never have to have both feet on the ground, do you? One foot on a pedal, bike tilted over and one foot on the ground. Similarly, you'll get a slow long bike if you try to avoid toe clip overlap (sorry for the old fashioned term) on a small frame. Everything on a small bike is just too tight. Same for insisting on shallow seat angles -- you'll have a steeper seat angle and less setback, which you'll have to handle with a setback seat post if necessary, but you probably have shorter thighs anyway and don't need the same setback as bigger riders anyway. And every centimeter of setback gets you more performance gain than it does on a bigger frame anyway. Same for bottom bracket drop -- don't go to a low bottom bracket simply because you think you need top tube clearance. Instead, choose bottom bracket drop to suit the kind of riding you do and your riding style.

Overall, as a smaller rider you'll need greater flexion at the waist to get a low position. You run out of head tube length on a short frame anyway, but the better position for a shorter cross rider is to go to a slightly longer stem and extend your torso forwards rather than just down. You'll get the same aero position but with a position that acts a little bit more like being on aero bars. Use narrower bars (you can be a good candidate for 38 cm c-c bars like 3T Ergonovas), don't go deep with them, and don't think you need to be on a short stubby stem. One problem on a small bike is getting your weight balance right, both for handling and for personal power generation, and being slightly stretched out is a plus.

Shouldering your bike? Hard or impossible with tiny bikes. There's no triangle in front to stick your shoulder into. So you have to pick up the bike differently. It's inefficient to pick it up in your hands all the time, but plan on putting your shoulder into the triangle at the seat lug and hold the opposite side of the bars rather than the front wheel. You can help by pointing the frame downwards a bit more than taller riders do -- not great but once you get used to it, it works well. Frames with sloping top tubes might seem contradictory for small riders, but they let you put the bike higher on your shoulder so you can handle the frame better. Again, don't worry about stand over height on a small frame.

Your frame size might be a candidate for 650 wheels. I'm always loath to suggest them because then nothing is compatible and gearing and everything else is suddenly different. The smaller wheels do give you a slightly easier lift and carve some room in your frame design, but honestly, I'd think about joining a circus first. We short riders aren't served by 650 wheels unless we're just touring and that kind of thing.

The basic point is that you need to break rules in your frame design and equipment choices. I'd say to go as small as you can for your saddle height (67 cm from bottom bracket axle to top of saddle?). A 48 would work, a 50 would be OK, a 52 is probably too big. I'm on a 50 with a 70.5 cm saddle height and could have gone a size smaller if I could have gotten one. Big riders are focused on top tube length but you'll have your top tube defined by your overall frame size. Use seat post setback, choice of saddle, and stem length to set your horizontal dimensions on the bike. Again, if you make the top tube long you just ruin the overall handling of the bike, to no good end.

Small bikes are a compromise. That's all there is to it. But you can race them really well if you don't stick with design rules that don't make sense for small riders.


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PostPosted: Fri Feb 28, 2014 7:18 am 
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Joined: Wed Feb 12, 2014 11:56 pm
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I was concerned about seatpost length aesthetics when i sized down from a specialized 52 road to a 49 cm specialized crux. It's such a roadie thing to consider and I wish I didn't give so much consideration when choosing a size. I just feel too cramped on the 49 cm and i'm running a 130 stem (vs 120).


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Posted: Fri Feb 28, 2014 7:18 am 


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PostPosted: Sat Mar 15, 2014 10:51 pm 
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Joined: Wed Jun 27, 2007 12:18 am
Posts: 186
This may help, it's an article I was interviewed for last fall.
http://georgiacx.com/cyclo-cross-bike-fit/


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