Join Date: Dec 2003
Location: Inside the beltway
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These are shots in the dark, time for a little high school trigonometry to shed some light-
Knowing the top tube length and stem size is not enough to go by- different frame geometry on 2 different bikes can throw the seat-to-handlbar dimension off by several centimeters. You really want to focus on the setback measure itself as more important to replicate how you fit on the bike compared to your road bike, or make an informed decision to alter your cross position relative to riding on the road. Assuming you have dialed in your road bike the way you like it with the right top tube/ stem length combination, you can replicate that on a cross bike with different angles/dimensions. If you are a new rider, or don't really have a clue what your ideal dimensions are, then it's REALLY worth it to spend some money on a professional fitting from a GOOD fitter (these may be hard to find in some places- ask around). The measure you are looking for is the amount of setback on the new bike compared with your familiar road bike. Setback is the horizontal distance between the center of your seatpost (on top of your seat, not at the top tube) and the imaginary vertical line that passes through the center of your bottom bracket. In other words if your seat tube angle were 90 deg. then setback=0. The more shallow your seat tube angle, the more setback you have. The setback measure will affect the way you pedal the bike, how you sit in relation to the bottom bracket, and how (which) your muscles are used. Some people like to have their cross bike pedal the same way as their roadie- in this case buy a frame where the setback is equivalent. Some cross riders (esp. racers) like to move their position forward for more power in a short race. In this case, buy a frame that has less setback- you can calculate it based specs for the seat tube length, and ST angle. If you want to replicate the seatpost-to-handlebar on the new bike (Poprad) with something you know works for you then you also have to take the setback into account. Anyway to calculate setback you need a calculator with trig functions. Then plug in this formula:
Setback= Sin(90-Seat Tube angle) * seat_Bottom Bracket distance
Seat_Bottom bracket distance is NOT your seat tube length, it depends on how much seatpost you have sticking out. As an example, assume you have 12 cm of seatpost showing on a 49 cm seat tube, your saddle top is 4cm above the top of the seatpost, and the seat tube angle is 72 deg (I think that's what the Poprad spec is?)
Setback= Sin (90-72) * (49+4+12cm) = 0.31 * 65cm = 20.1 cm
Now to compare your road bike, assume your roadbike is 52 cm, with 9 cm seatpost showing, and the same saddle (4cm high), with a seat tube angle of 73 deg.
Setback= Sin (90-73) * (52+4+9) = 0.29 * 65 cm = 19.0 cm
Just to show how the geometry works, you can see that there is about 1 cm difference in setback between the two bikes. So to maintain the same Seat_to_handlebar distance, you will want a top tube that is 1 cm shorter than your road bike. But this is not necessarily the most important dimension, as you can alter this by using a 1 cm shorter stem. Assume your seat_to_BB distance will be the same, or sometimes a few mm's less on the cyclocross bike for the calculations. Keep in mind that your choice of seatpost will also affect the setback measure. Most seatposts attach to your seat rails a cm or so back from the center of the post, and you can purchase "setback" seatposts that are a lot more than this, so take that into account also- the calculations above assume 0 setback on the seat post, and the same seatpost used for both bikes. Anyway, maybe this can help sort out a wild-ass guess as to which frame size to order- consider the following-
Longer stem length = less responsive steering (this can be good or bad), not usually much of a problem unless you are toward the extremes (< 10 cm or > 13cm) Longer stem also puts your weight further forward, giving you less traction on steep muddy inclines, more difficult to stick your ass way off the back on steep descents, but makes your bike a little easier to hold a line through sharp slippery turns.
Less setback= more high end power- good for short races, but less efficient for long rides- this is where you start to see some differences between "cross" bikes for touring and "cross" bikes oriented toward racing.
So with a given frame you can obviously play with different combinations of frame size, stem length, and seatpost choice (setback) to get what you want. You can also look at different cyclocross frames with different geometries- much more important consideration than Aluminum vs. Steel, which manufacturer, etc. as this will determine how the bike fits and rides in the real world.
If you start obsessing over this stuff, you start to see the compromises when you buy a stock frame. You can make a range of frame sizes "fit" the same, so that they feel the same to you on a stationary trainer. But in the real world, riding on the road/trail you will notice the tradeoffs between ideal fit and ideal handling as different combinations with the same "fit" dimensions will ride differently. When you realize this you then start to lust after your dream bike custom made just for you! I think it's worthwhile for every serious rider to spend a few bucks on a professional fitting at some point- it really makes buying bikes much easier once you have more than a good guess at what you should look for given your dimensions and riding style. I've built a couple frames myself, but havent found perfection yet- still working on it. Don't stress too much on the decision, but maybe this helped a little. Now go find your old math teacher, apologize for falling asleep, show her your new campagnolo tatoo, and thank her for trying.
Last edited by ZenNMotion; 07-28-05 at 01:45 PM.