Check out the geometry for the Trek Crockett, one of their new 'cross designs.
What we want to look at is the "wheelbase" under "K". See how the difference between the very largest frame, the 61cm, and the smallest, the 50cm, is only 4.2 centimeters? That's just over an inch of extra wheelbase for two bikes designed to fit a human height discrepancy of over a foot. Between the sizes we're only talking about .8cm, so there really isn't a big difference. In general, bike wheelbases are fairly static because the rear wheel has to stay within a certain distance from the bottom bracket so that the gears work as the component manufacturer intends (usually 440mm. You can go longer but you'll run into problems if you go smaller than 380mm). Bikes with really long wheelbases and really short wheelbases also run into issues with the wheels not having enough clearance in certain spots or aftermarket fork options being limited, or there could be incompatibility with roof racks. It wouldn't surprise me if the UCI has a rule about wheelbase too; they have a rule about everything.
If you want faster handling, look for a bike with a steep head tube angle (74 or greater) and/or a shallow fork rake (44mm or less). These factors have a bigger effect on handling than wheelbase.
If you want faster handling, look for a bike with a steep head tube angle (74 or greater) and/or a shallow fork rake (44mm or less). These factors have a bigger effect on handling than wheelbase.
The way a bike designer makes a bike fit a larger person within the component-guided wheelbase constraints involves messing with angles. As the top tube grows to fit the larger person's trunk, the head tube angle steepens and the seat tube angle lessens. You can see this in the chart above. The frame is effectively growing without sacrificing maneuverability. The side benefit of this design method is that larger bikes don't use much more material than the smaller ones, thus saving weight and material costs.
Another reason you don't want to size down is that your body needs to breath. On a small bike you have to hunch in, which constricts your lungs and limits their capacity. You won't notice this on a short ride, but once you warm up the natural tendency is to stretch out. In order to do this on a small bike you have to lengthen the stem, but running a super-long stem will affect the bike's handling by putting your body weight ahead of the front axel. Also, your chances of going end-over-end increase with more weight in front and you have to buy a bigger, heavier stem, which will also put more leverage on the fork's steerer, increasing the likelihood of a break, and yadda yadda yadda.
OH, but here's one more reason not to size down: in cyclocross you have to jump on and off your bike a lot. Every time you jump on the saddle you're stressing the seatpost. The longer your seatpost, the more points of failure there will be, and the greater leverage on the frame at the head of the seat-tube. You can see where I'm going with this. Snap.
The bike companies are aware of people's desire to size down, and have made some responses. One sneaky trick is to just take a 56cm or 57cm frame and label it a 55cm. This is what All-City does and Trek used to do. The bike shop worker should be in on the con and can play with the customer's desire to size down while still putting them on the right bike. The problem, of course, is that the myth continues to be propagated.
Giant and a few other companies use an alphabetic sizing system (XXS-XL). A bike shop worker will know what numbers the letters correspond to, and if they're on point they can steer the customer in the right direction without confusing them with effective top-tube lengths and so on.
Giant and a few other companies use an alphabetic sizing system (XXS-XL). A bike shop worker will know what numbers the letters correspond to, and if they're on point they can steer the customer in the right direction without confusing them with effective top-tube lengths and so on.
Don't buy the bike size your brain has reasoned to be the best for you, buy the bike that fits your body the way it is. If you know your roadbike size, stick with that and try to get a bike with the geometry closest to it.
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| My man and former world champ, Sven Nys, didn't size his bike down when he won Cross Vegas last year |
Hey, as a former GB cyclo-cross rider, you seem to be overlooking seat tube and headtube changes, which are very evident. Plus, of course the shorter top.tubes too. In old money, I rode a 56cm customer.steel frame by Dawes. The top tube was 54cm. They didn't slope the top tube deliberately so you could carry it more easily..
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