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question on frame size(9 posts)

question on frame sizejrogers
Sep 30, 2002 6:48 PM
I am not that good at road bike sizing and I have a feeling that I am going to have to mail order any bike that I get for cross. I have a 54cm Cannondale right now and it fits okay. I am a bit over 5'9" (maybe by 1/3 an inch or so) and my inseam is about a 32". Taking, for example, an Empella Bonfire, what size would I be for general road and trail riding?
There is a 53 cm (measured c to top) with a 52cm TT, a 54 with a 52.5 cm TT, a 55 with a 53 cm TT, a 56 with a 54cm TT and a 57 with a 54.5 cm TT. I think the last one is closest to what I have now. Simply, what bike would be the right size in your opinion? They all seem to have very short top tubes and it is throwing me off
empella frame sizebuffalosorrow
Sep 30, 2002 8:59 PM
1)what is the top tube length on you cannondale?
2)you should make a descision on wether a indentical top tube or shorter top tube will suit you better.
3) from my estimation the empella frame measure c-top of seat tube which extends well past top tube
4) some claim to size down 2cm for cyclocross if you cann. measures 54cm c-c, perhaps a 52cm empella c-c
5) how I would size an empella: I ride a 52- 53cm c-c road frame depending on tubing and top tube length, that would translate to a 50-51cm c-c (55cm empella c-t w/53cm tt?) my colnago measures 51cm c-c and 53.4 tt, my singlespeed measures 50cm c-c and 54cm tt. All fit fine with saddle adjustments.
6) seems to me if you ride the 54cm c-c cann. the 52cm c-c / 56c-t emplella, might be the size for you.
7) I come with no guarantees.
re: question on frame sizecxer
Oct 1, 2002 5:28 AM
The 56 or 57cm Empella will be comparable to your 54cm Cannondale. Cross bikes have slightly shorter top tubes compared to road bikes- but you need that off road.
Remember the CX bike will be taller than your road bike because of the higher bb.
Top tube length is a ***BAD*** way to determine fit. (long)flyweight
Oct 1, 2002 9:41 AM
People seem to think that top tube length is the most important aspect of determining bike fit. They also, incorrectly, assume that a longer top tube yields a roomier cockpit and a longer reach to the bars. People who believe this probably should have paid more attention to their high school geometry class! At first it seems to make sense that a longer top tube would yield a longer reach to the bars but in reality it fails to compensate for the seat tube angle and the location of the bottom bracket relative to the saddle.

Proper bike fit is controlled by 4 measurements:
1) seat height
2) saddle set back (how far back the saddle is from the bottom bracket)
3) front center (how far forward of the bottom bracket the bars are)
4) handlebar drop (how for below or above the bars are relative to the saddle)

Simply relying on top tube length or
length from bars to saddle isn't accurate because it
doesn't account for the location of the bottom
bracket. Using saddle height and setback gives you a
dimension that is independent of the geometry of the
bike and allows you to replicate your ideal position
on just about any bike. Relying on top tube length and
bar to saddle length means you would have a different
position on every bike because of differences in top
tube length and seat tube angle. In other words, you'd
be forcing yourself to accomodate the bike rather than
having it be the other way around.

For example, let's say you have two frames. The first,
Bike A, has a 56cm top tube and a 74 degree seat tube.
The second Bike B, also has a 56cm top tube but a 72
degree seat tube. For sake of simplification assume
both bikes have identical saddle heights, the saddle
is clamped in the middle of the rails, and stem
lengths are identical. If you use a known set back
measurement then the distance from saddle to the bars
is actually shorter on the bike with the 72 degree
seat tube than on the the bike with the 74 degree seat
tube. The shallower the seat tube the further forward
the saddle must sit in the in order to maintain the
same set back measurement. Now let's say you don't use
a known set back measurement and instead use a known
saddle to bar measurement as you state above. This
means that on the 72 degree angle bike your saddle
will sit further behind the bottom bracket spindle
than it would on the bike with the steeper 74 degree
angle. This in turn would place a greater demand on
your quads and lower back muscles which may or may not
be the ideal position for you. Sketch these examples
out on a sheet of paper and you'll see what I mean.

So, you start with the saddle height and set back.
Once you've got that dialed in you still need to make
sure that you're not overly stretched out or crunched
up. This too should be determined using numbers that
are completely independent of the geometry of the
frame so that your position can be replicated from one
bike to another. To measure this number start an
imaginary point directly above the bottom bracket
spindle and at the same height as the saddle and
measure a line parallel to the ground to the point of
the brake lever hoods (this is better than measuring
to the stem clamp because the amount of forward sweep
in a handlebar differs from one bar to the next, thus
impacting overall reach) This is the horizontal reach.
The final number is from the horizontal reach line
straight down to the brake hoods. This measures
vertical drop distance between your butt and hands.

Once you have these 4 numbers you can replicate your
position perfectly on any frame regardless of geometry
(of course you'll need a good carpenter's level and a
plumbline). You can also discover that just because a
bike has a given top tube length doesn't always mean
it will be the ideal fit for you.

None of this is new adn I didn't come up with any of it. Faliero Masi, Max Testa, Ernesto
re: question on frame sizejohnnyd
Oct 1, 2002 10:07 AM
I still think the original question has gone partially unanswered and it is the same question I have. I understand the point that you need a shorter seat tube because the bottom bracket is higher, but what is the word on top tube length. It seems like this really varies--from the Empella you mentioned with a shorter top tube than seat tube, to a Surly Cross Check, where on a 54 cm seat tube the top tube is 56 cm.

Should the top tube on your cross bike be shorter, longer, approx same length as your road bike frame and how does shorter/longer affect your ride. I'm trying to buy a frame over the internet and need help sizing too.

johnnyd
re: question on frame sizeclimbo
Oct 1, 2002 10:59 AM
it all depends on what you want as to ST and TT lengths. Remember, all frames are measured differently. The "53" Empella is actually a ST of 49cm C-C with a 52cm TT, as they measure to the top of the seat collar for the sizes. You want to make sure you compare the same center-to-center measurements.

In general you size down based on ST and get about the same TT length. Any adjustment after that it's all personal choice.
re: question on frame sizeflyweight
Oct 1, 2002 1:41 PM
"Should the top tube on your cross bike be shorter, longer, approx same length as your road bike frame and how does shorter/longer affect your ride."

Again it all depends.

Let's put it another way. Which gear is harder to push: a 11 tooth or a 13 tooth cog? Most people would say an 11 but that may not be true. Gears are a combination of a cog AND a chainring. A 53x13 is going to be much harder to push than a 39x11!

Top tube lengths are the same way. By themselves they are completely meaningless statistics. Top tube length only means something in relation to seat tube angle and seat tube length (and to a much smaller degree, head tube angle). What you really need to do is figure out where your seat and handlebars need to be with respect to the bottom bracket. Then see what size frame best accomodates those numbers. The most practical way to do this is to sketch out your current frame then draw in the vertical line up from the bottom bracket and add in the set back and front center. You can then see how much top tube is behind the bottom bracket and how much is ahead and compare this to another frames top tube and seat tube numbers.

Generally speaking, most people prefer a shorter reach to the handlebars on cross bike. However this does NOT always mean having a shorter top tube. For instance, my Colnago cross bike has a longer top tube than my road bike but the reach to the handlebars is actually less than on my road bike. How is this possible? Simple: the road bike has a steeper seat tube angle than the Colnago. This basically puts more of the top tube ahead of the bottom bracket thus making for a longer reach.

If this all sounds rather confusing that's because it is. This is also what sets apart the truly great framebuilders from guys who can simply weld tubes together!
Creative measurementsvarmit
Oct 2, 2002 3:53 AM
I have also been looking at getting a new cross frame to replace my current C'dale cross bike. At first I was comparing the published dimensions. Then I took actual measurements on my old frame and compared them to the spec sheet. Not a single measurement was close. Most measured 1 - 2 cm. less tham the specs. These measurements didn't match the next size smaller frame specs either. The old bike fits me fine, but I am wondering how to extrapolate this fit to some other manufacturer's "imaginary" dimensions. I understand "nominal" dimensions and tolerances, but this seems beyond these criteria. I assume that other bikes are measured just as strange?
this is were custom frames are helpfull?buffalosorrow
Oct 2, 2002 4:23 AM
It is not just the frame that you are paying for it is, or should be the competent experience and time of a frame builder that understands the specific geometrics and how they can benefit the rider.
Yes like flyweight one can take the time to understand, reseach, learn here these applications. And yes they are important. But it is hard to want a good fitting bike, and being unexperienced in the the topic makes it even harder. Like all new topics, I like to learn. Is there a really nerdy book on this subject? Or are these mathmatical secrets? I have lucked out with my two cross frames and am struggling with my vintage road bike, comfort has never been just right. And now from reading above, I am getting a grasp on why.
I enjoy this discussion.