this is a link from Sheldon Brown's bike shop:
http://www.sheldonbrown.com/harris/terry/

Which Wheel Size?
Like Terry, some other manufacturers like Trek® and Cannondale® make small bicycles using 26" wheels front and rear; but Terry also uses 24" in the front and 700C in the rear. Why?
This seems to be a muddled marketing decision rather than an elegant engineering solution on our competitors' parts. A 26" wheel offers some advantages, but not the way it's used in the frames we see from our competitors. Everyone has the same goal when making a smaller bicycle for the small rider: make the top tube short enough! If you use a 700C wheel on the front of a bicycle, you can only shorten the top tube so much (to about 53 cm) before the front wheel overlaps with the pedal, often called toe-clip overlap. A 26" road tire is about 2 cm smaller in radius than a 700C tire, so you can shorten the top tube just as much (to about 51 cm) before there is overlap. A 24" road tire is about 5 cm smaller in radius than a 700C tire, so the top tube can be shortened to about 48 cm with no problems.

We use 24" in the front on our smaller road bikes (48.3 cm and under) so we can have more latitude for good design. The answer to the next question explains this in more detail.


Top-Tube Length
Other manufacturers have fairly short top tubes on bikes with 26" and 700C front wheels. How is this possible?
It's possible because their designs are compromised. Ours are not. How do you make a bicycle with a top tube under 51 cm if you use a front wheel larger than 24" and you want to avoid toe clip overlap? There are a couple of ways to do this.

First, use a steeper seat angle (75+ degrees). The steeper this is, the shorter the top tube will be if nothing else changes. The problem with this? You will be too far forward of the pedals, unable to establish the proper relationship between your knee and the pedal spindle. You won't be as efficient a rider, and knee problems may result. You'll spend a lot of time wishing you could push back more on the saddle.

Second, use a shallower head angle (71-degrees) and more rake (6.5+cm). The problem here is that the bicycle will have rather sluggish handling.

Neither of these is a solution because it limits the design. Head angles, rake, and seat angles should lead a design not follow it. Head angle and rake are chosen to achieve the desired handling of the bicycle: is it for racing, touring, criteriums? Seat angle should be chosen to give the rider the best position on the bicycle.The 24" front wheel lets us build a bike that fits without compromise. It would be an insult to our customers to offer anything less.

Aren't 24" tires hard to find?
Not that we know of. They are widely distributed and we always keep a good stock here for dealers and consumers in need.

Won't I go slower with a small front wheel?
No. Even though the wheel turns more rapidly than a larger wheel, it has less mass, so its momentum is about the same.