How much seatpost showing?

[KeatonR]

Just curious -- how much seatpost does everyone have showing on their cross bikes? I ask b/c I'm trying to decide between two frame sizes. For road bikes, the trend seems to have been lately to go with a smaller frame (and this have ~5 inches of post showing), but of course compact geometry has contributed to this.

On cross rides, I see equal numbers with 2-3 inches showing and 4-6 showing.

[va_cyclist]

Are you asking for aesthetic reasons, or structural integrity, or some other reason?

[KeatonR]

To help me get a sense of whether to go with a smaller or larger frame -- I'm between two sizes ... Thx.

[va_cyclist]

I'm no expert, but the amount of seatpost showing isn't usually on the list of criteria for good bike fit. Top tube length, neutral knee position, reach, those things matter. Get all that set correctly, and your seatpost ends up wherever it will for proper leg extension. More to your point, I've heard it said that if you comfortably fit either of two frames, go with the smaller one because it will be stiffer & lighter.

[Tom boyd]

I have always heard the most important measurement is top tube. Get the smallest frame you can with a top tube that fits. Simple matter of frame weight.

I do not believe the "amount of seat post" showing is really an issue. At least I have never bought a frame with that in mind.

[dlbcx]

Only thought on seatpost that I have is not to use a superlight or carbon post, if you intend to race cross. The post has to absorb all of the remounts and crashes so it needs to be strong. I have been using Thomsons and have had pretty good luck with them.

[EXCALIBUR]

It is best to go with the smallest frame that will allow for proper leg extension. The smaller frame will give you better handling.

[Lectron]

You’re not giving us much data to go on here.
What’s your overall reach? Do you prefer riding with a long or short stem?
Are there any (big) differences on the wheelbase between the two seizes?
What’s the difference in top tube length ? And if the bigger has a slacker STA you’ll
probably eat in a bit on that length.

If you’re, like you put it, between to seizes, and those frame seizes are pretty
close to each other. I would go for the smaller one.

[mgwadz]

It is best to go with the smallest frame that will allow for proper leg extension. The smaller frame will give you better handling.

..but a smaller frame could make the bike hard to fit your shoulder into

an additional concern comparded to fitting a road bike.

marc

[mgwadz]

Just curious -- how much seatpost does everyone have showing on their cross bikes? I ask b/c I'm trying to decide between two frame sizes. For road bikes, the trend seems to have been lately to go with a smaller frame (and this have ~5 inches of post showing), but of course compact geometry has contributed to this.

On cross rides, I see equal numbers with 2-3 inches showing and 4-6 showing.

I don't think there is a consistent recommendation on how to size a CX bike relative to your road bike. traditionally, it seems the idea was to go 1-2cm smaller on a cross bike. I've read other expert sources say to size it the same. the old rule of thumb might have been updated as CX courses have gotten faster and more road-like.

one rule of thumb i've heard is that there should be about a fistful of seatpost showing on a roadbike.
depending on how you decide to size your CX bike (and assumming comaprable toptube configuration) , that would imply the same as that, or 1-2 cm more. sloping top tube would change all that though, and I think a setup for larger people will show more post than bikes for smaller people.

my cx bike happens to be about 1 cm shorter top tube (and shorter C to T measurement) and has a sloping top tube, so it has a more seatpost showing.


marc

[KeatonR]

You’re not giving us much data to go on here.
What’s your overall reach? Do you prefer riding with a long or short stem?
Are there any (big) differences on the wheelbase between the two seizes?
What’s the difference in top tube length ? And if the bigger has a slacker STA you’ll
probably eat in a bit on that length.

If you’re, like you put it, between to seizes, and those frame seizes are pretty
close to each other. I would go for the smaller one.

I'm 5'11", but with shorter legs (30" pants inseam, probably 32-33" actual inseam). My road bike size is 56, so I test rode the the 55 cx bike, but it actually felt big, so today I'm going to look at it with the smaller one, 52, side by side. 52 sounds a lot smaller than 55, but in actuality the frames don't seem to be too different on paper: the top tube for the 52 is 54 (21.26") vs. 56 (22.05") on the 55. The standover on the 52 is 79.5 (31.3") vs. 80.3 (31.61") on the 55. The wheelbase is listed as 1004 vs. 1015, which I'm guessing means 39.5" vs. 39.96".

[Lectron]

I'm 5'11", but with shorter legs (30" pants inseam, probably 32-33" actual inseam). My road bike size is 56, so I test rode the the 55 cx bike, but it actually felt big, so today I'm going to look at it with the smaller one, 52, side by side. 52 sounds a lot smaller than 55, but in actuality the frames don't seem to be too different on paper: the top tube for the 52 is 54 (21.26") vs. 56 (22.05") on the 55. The standover on the 52 is 79.5 (31.3") vs. 80.3 (31.61") on the 55. The wheelbase is listed as 1004 vs. 1015, which I'm guessing means 39.5" vs. 39.96".

Ok. Then shouldering is not an issue since they both have nearly the same standover height.
I prefer a few cm shorter saddle-handlebar distance on the CX, say around 2 cm, and also less handlebar drop.
A long wheelbase will as good as always feel slower, so I’d try to fit in to the smallest frame here.
I recon the 52’s toptube is not much shorter than on your racing bike, if shorter at all, so it sounds to me that might be the right one.
The SeatTubeAngle, I guess, is the same based on that there is an 11mm difference in wheelbase and 20mm difference on the toptube.
Those 9mm is no more than what’s probably covered by the HeadTubeAngle.
Well. For what it’s worth. My recommendation would be the smaller frame,
unless that means a whole lot of spacers under a very long stem. (Spacers bring the handlebar closer to you due to the HTA)

[KeatonR]

I guess the strange thing I'm having trouble getting my mind around is being almost 6' yet riding a 52 -- but as a few pple have mentioned, the geometry of each bike is different and you really gotta straddle it to know. I'll report back after I do so ...

[MattC]

IF you are almost 6' a 52 is going to end up to small unless it is like a 56-57cm top tube

[Lectron]

I guess the strange thing I'm having trouble getting my mind around is being almost 6' yet riding a 52 -- but as a few pple have mentioned, the geometry of each bike is different and you really gotta straddle it to know. I'll report back after I do so ...

Sorry there. I read 5’1 not 5’11”. That’s about 3.5 cm shorter than me and I’m on a 58 frame.
54 toptube can be to short then unless the STA is very steep and you can make use of a (very)setback seatpost.
Try them both out, but it’s most likely the 56 toptube will be the best fit.

[ZenNMotion]

You're fixating on the wrong dimension- forget the seatpost already. Look at the top tube length first. How long a stem do you need to fit both frame sizes properly? Longer stems mean that steering is a little slower, shorter is a little quicker. There are lots of exceptions, but most people prefer the handling of a bike with a 100-120mm stem. Also look at the head tube length. The smaller frame may have a head tube that is too short to comfortably fit a more upright cyclocross position without a lot of stem spacers- in fact you may not be able to dial in to your ideal position at all even if the top tube is about right but the head tube too short. Fitting a cross bike is really organized around your center of gravity. Bigger frame, longer top tube, shorter stem = a CG that is further back. Nice to get your butt further off the rear of the seat on downhills, but less responsive handling with more weight over the rear wheel while seated. These are all shades of difference, not huge choosing between two close sizes of the same frame. In the end, most people tend to get used to what they ride, and assuming that it doesnt make them hurt from ill-fitting, that's what they tend to like. Think about how you want to ride most of the time. If you are buying the frame mostly for racing cross, go with the smaller, if its an all rounder for commuting, offroad, some racing maybe go for the larger size for comfort. But forget about the seat post showing thing-
By the way, the difference between a 52 (TT?) and a 56 TT is not minor. 4 cm is a lot. You must be getting one of those damn compact frames that come in only 3-4 sizes. If you are comfortable on your road bike, go with the frame that is closest to that.
FWIW, I'm 5'10.5" Road bike 56x56, 120 stem, fits well for agressive crit racing.
CX #1 53.5TT, 130mm stem
CX#2 54TT 120mm stem
CX#3 55TT 115mm stem. These all work for me, but each fit just a little differently. The best fast CX race fit is CX #1, but not comfortable for long training rides. Best for longer training/long day ride adventures is #3. #2 is a second pit bike, best of the three for wrestling through mud. Even though the combined top tube + stem on all of these is very close, they feel different because of geometry and steer tube length differences. Manufacturers often don't include these in geometry specs, and people don't pay much attention to them but they say a lot about how the bike will fit and handle.

[KeatonR]

OK, consider the seatpost thing forgotten, and thanks for all the info --

[KeatonR]

Came home with the 55. It still feels big, but I've come to realize that it's b/c there are serious spacers in the stem, and the stem itself rises quite a bit (and the handlebars are wide) ... Long story short, I took one look at the 52 and knew it was too small, mainly in the TT dimension. So thanks to everyone for the patience ... I could still stand for a bit more standover, but the TT is perfect on this ride and this one's a keeper. I'll be swapping out the stem, tho.