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Help on sizing?

2K views 15 replies 7 participants last post by  saagusta 
#1 ·
I am going to purchase a MOTOBECANE FANTOM CX (yes please don't try and talk me out of it, its what I want) I just have a question about what size to get as this is my 1st cyclocross bike. I will be using this in some of my local races. I am 5'8 with a 30in inseam. Not sure if I should get a 54cm or a 52cm. I have read that for a cyclocross bike you will want a little more clearance for dismounting. Can I get some help. Here is the sizing guide.
Motobecane USA | Cyclocross Bicycles | Track Bicycles | Cross Bicycles
 
#3 · (Edited)
Any idea what top tube length on a road bike works for you? If so, do you know the STA on said road bike?

I would size based on the combo of ETT and STA, which basically gives you "reach" to the bars.

Purely anecdotal, but I am 5'6" with a 28in. inseam and would probably do the 49cm. but the 52cm geo is pretty darn similar. 4mm longer ETT, but 1/2 degree slack STA, which basically creates a wash reach wise. The standover is 2cm higher on the 52, but frankly I don't pay much attention to Standover, as it has little bearing on comfort while riding. BB drop is the same on all, so saddle height will be equal among models, so the same distance to "jump" during remounts.
 
#4 · (Edited)
I don't see stack, reach, or head tube height listed on that geometry chart; those are key, along with effective top tube, to determining size.

FWIW, the eft TT os the 52 is 529, and the eft TT of the 54 is 548. I would think the 529 may be a bit on the small side, and the next size up too large. (I'm 5'7" with a 33.5 inseam. Typically use a 535ish eft TT). There are many other factors that determine the correct length, including femur size, torso length, etc, etc. Your torso is longer than mine, but I don't use an offset seat post, so that allows me to get away with a bigger size.

Not trying to talk you out of buying a bike online, but I suggest checking bike Nashbar and Performance Bike. Both have frequent sales on bikes, their stuff is good, and you should be able to get much better information on the bikes. Plus, you can call them and have this discussion, too. And both take returns no questions asked.

Is the price actually 1695? That's way overpriced for aluminum frame with mostly sora components. The comparable one from Nashbar is almost ridiculously less. ($549.00 plus an additional 15% off today). You can get a Cannondale Disc 105 components for 1695.00 And it will come with a full fitting. (Yeah, I know, you said you are not changing your mind. But you can do so much better). I own a CAADX 105, great bike. And their Sora option is much less.

Nashbar - Cyclocross Bikes
Cyclocross Bikes, Triathlon Bikes & Time Trial Bikes
CAADX - Cyclocross - ROAD - BIKES - 2014
 
#6 ·
I don't see stack, reach, or head tube height listed on that geometry chart; those are key, along with effective top tube, to determining size.

FWIW, the eft TT os the 52 is 529, and the eft TT of the 54 is 548. I would think the 529 may be a bit on the small side, and the next size up too large. (I'm 5'7" with a 33.5 inseam. Typically use a 535ish eft TT). There are many other factors that determine the correct length, including femur size, torso length, etc, etc. Your torso is longer than mine, but I don't use an offset seat post, so that allows me to get away with a bigger size.
But you'll see the STA between the 52 and 54 are different. You can't look at ETT alone (as it seems you are already aware). So although the 548 ETT on the 54 may seem long, it will ride "shorter" because the STA is significantly more slack (by 1.5 degrees, which gives you approximately 1.5cm less reach given the same saddle position relative to the BB).

Again, I think it comes down to the OP knowing the ETT and STA of their current, well fitting road bike for us to help pinpoint a size recommendation.
 
#12 ·
Good, what make and model? If you can find the geometry chart for your current road bike, compare the ETT and STA of your road bike to the geo chart for the Moto's. Match as closely as possible, understanding that a degree difference in STA is going to impact your reach by about 1cm of ETT. STA goes more slack (most likely the case), look for longer ETT.

The idea that CX bikes fit that much different from road is pretty old school. Most folks these days match their reach as close as possible and go from there.
 
#13 ·
It's all preference. I like a similar sized frame with a shorter stem, but I long a longer fit on my road bike then some people prefer.

Like the above person said, I'd say get in the ball park as the road frame and go from there.
 
#14 ·
op, let's keep things simple. Go with the 52. The reach between the 52 and 54 is going to differ by only about 4mm (because the STA is 1.5 degrees steeper in the 52) and you get added benefit of a standover height that's not one inch taller than your inseam. Plus, if you plan on actually riding it as a cross bike (ie not as a road bike), I prefer a bit smaller,less stretched position than road bike.
 
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