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Was looking at these on-ne dirty disco's and figured I would need a size 54 as my current road bike (spesh roubaix elite) is a 54, but then I realised the stem on the road bike is fairly long. Do you typically run shorter stems on a CX bike and if so would I need a longer top tube CX bike to get the same fit?
I'm not sure you are trying to replicate the fit of your road bike 100% with a CX bike. The angles are different and the BB normally higher, therefore its going to feel different.
But [i]generally[/i] then if your road bike it 54 then a CX should be that size, [i]or[/i] smaller. This is why going to a LBS is so useful.
For reference I run a 110mm stem at -6 degrees on my mountain bike, and a 100mm at +6 degrees on my cross bike
The overall reach of a CX bike is 'generally' shorter than a road bike. My road and CX bikes have almost the same effective TT, even taking into account head and seat angles & relative positions. My road bike has a 120mm stem, my CX bike a 100mm stem.
I would imagine if you set up your CX bike with an overall reach the same as your road bike, it's going to feel massive & a bit like a barge to manoeuvre off road.
[EDIT] that's how mine felt when I was experimenting with stem length/heigh/position on the bike. The bars are only slightly closer to the seat height than on the road bike.
All down to personal taste but my cx top tube and stem are both 10mm shorter than my road bike, and bars higher relative to seat. I find this gives a bit more control over the rough stuff.
depends
do you want the same fit on your CX bike as on your road bike?
some prefer a slightly shorter reach for more control off-road, racier types prefer the fit to be about the same
also some people go for a slightly smaller effective frame size for the reasons above
Can you compare top tube or effective top tube measurements.
I certainly can't see you wanting a cross bike bigger than your road bike assuming your road bike is the right size.
How long is your current stem. I would have thought some thing around the 100mm-120mm length would be fine on a cross bike, afterall it wasn't that long ago that everyone used this size on mtb.
My cx bike has a more upright position than my road bike. Cross riding is generally not as fast as road riding so the loss in aerodynamic efficiency matters less also when I'm climbing I prefer the more upright position as I find it easier to breath.
this is the geometry of my roubaix:
Size 54 cm
Stand Over Height 760mm
Seat Tube Length (center-to-top) 450mm
Top Tube Length (measured horizontally) 548mm
Head Tube Length 165mm
Head Angle 72.0°
Seat Angle 73.5°
Wheelbase 1000mm
Chainstay Length 415mm
Front Center 595mm
Fork Offset (Rake) 49mm
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how does this compare to the dirty disco? they look fairly similar in a 54? Slightly longer wheelbase on the disco, not much in the top tube, head tube is shorter on the disco
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so do you think a 54 dirty disco would be the best fit for me, or should I go smaller?