A cyclist has to do battle with two primary forces, air resistance and gravity. Since air resistance increases exponentially with speed, it becomes of little consequence to the hill climbing equation because of the drop in speeds as the incline increases. Scientifically speaking, drafting behind other riders while climbing provides little to no advantage. I came across some research done by David Swain, Medical & Science in Sports & Exercise, entitled, Cycling Uphill and Downhill. I've included a portion of this article to give you some juicy scientific data if you are so "inclined."
There is another side to the climbing coin though. Being successful at climbing has much to do about mentally preparing for a climb, staying focused and being relaxed. Although physical drafting is negligible, "mental drafting" is huge. Imagine that the rider ahead of you is doing all the work and all you have to do is keep relaxed, control your breathing and maintain a constant tempo. NEVER panic. Remember the best climbers want to disrupt the rhythm of the climb. Knowing this, you can shake other riders on a climb faster than on the flats by changing the pace for a bit. Another tip...be sure you are close to the front of the pack (not in the wind) before you hit the climb. If you drift back you'll be drifting back in the pack.
Don't climb the hill before you have to.
Excerpt from "Cycling Uphill and Downhill" by David Swain Uphill
As indicated in the following equation (DiPrampero et al., 1979), there are three primary forces to be overcome in bicycling: rolling resistance, air resistance and gravity:
W = krMs + kaAsv2 + giMs
where W is power, kr is the rolling resistance coefficient, M is the combined mass of cyclist and bicycle, s is the bicycle speed on the road, ka is the air resistance coefficient, A is the combined frontal area of cyclist and bicycle, v is the bicycle speed through the air (i.e. road speed plus head wind speed), g is the gravitational acceleration constant, and i is the road incline (grade; however, this is only an approximation, as the sine of the road angle to the horizontal should technically be used).
On modern bicycles with narrow, high-pressure tires, rolling resistance is negligible. Since the power required to overcome air resistance is proportional to the bicycle speed cubed (if there is no wind, and s = v), an exponential increase in power is needed as the cyclist attempts to increase speed.
Going uphill adds gravity to the forces that must be overcome. Since the cyclist has a finite power supply, he or she must necessarily slow down in proportion to the steepness of the hill, if steady-state aerobic metabolism is to be maintained. While this effect of hills is obvious, more subtle effects of this shift in forces have a dramatic impact on the competition.
Consider the effects of gradually increasing the road incline above zero while holding power constant. Since the air resistance component is proportional to speed cubed, only a relatively small decrease in speed is needed to offset an increase in incline at first. As the incline continues to increase, relatively greater decreases in speed must occur, given the curvilinear nature of the speed cubed relationship. Thus, a steep hill is required to substantially slow a competitive cyclist. The precise percent grade which has a meaningful impact cannot be specified, however, as this depends on the sustainable power level of each individual cyclist.
Given that the appropriate steepness has been reached, a major change in race dynamics occurs. As gravity supersedes air resistance as the primary force that must be overcome, drafting becomes a relatively less useful tool in the competition. At very slow speeds (on the order of 16 km·hr-1 or less) air resistance is negligible, and drafting becomes nearly meaningless.
This shift in forces causes the peloton of a road race to break up, as those cyclists who have the highest aerobic power can outdistance less aerobic competitors who are now deprived of drafting derived assistance. An interesting aspect of this shift is to note that smaller cyclists generally excel on hill climbs, while larger cyclists are generally better on solo efforts on level ground. As a consequence of scaling geometry (Astrand and Rodahl, 1986; Schmidt-Nielsen, 1984), mass increases with the cube of height, while surface area only increases with the square of height. This means that, although larger cyclists have a greater total frontal area to push through the air than smaller cyclists, their advantage in mass (and power generating capacity) is even greater. As a consequence, large cyclists have a higher ratio of power to frontal area than smaller cyclists, giving large cyclists an advantage in overcoming air resistance (Swain et al., 1987), as observed in time trials. It is not surprising that Miguel Indurain, the pre-eminent time trialist of the 1990's, was larger than most of his rivals.
However, since air resistance is negligible at slow climbing speeds, all cyclists are at a similar energy cost, relative to their body weight. Small cyclists excel at hill climbing because they generally have greater relative aerobic power (VO2max in ml·min-1·kg-1 ) than do large cyclists. This is also a consequence of scaling geometry: relative to body mass, smaller organisms have greater alveolar and capillary surface areas in the lungs, greater capillary surface areas in the muscles, and greater cross-sectional area of arteries for the delivery of blood. Examinations of elite endurance athletes from a variety of sports, including cycling, have revealed that VO2max scales with the 2/3 power of mass; that is, if a 60-kg elite athlete has a VO2max of 80 ml·min-1·kg-1, then a comparably trained elite athlete who weighs 100 kg would be expected to have a VO2max of only 68 ml·min-1·kg-1 (Astrand and Rodahl, 1986; Swain, 1994). Thus, at the elite level, one expects superior performance from smaller athletes in aerobic events where power requirements are approximately proportional to body mass--such as distance running and uphill cycling. (Technically, the power requirement scales more closely to mass, i.e. M0.76-M0.79 than does the power supply, M0.67; Swain, 1994). It is not surprising that the pre-eminent climber of the 1990s, Marco Pantani, is one of the smallest men in the peloton at only 55 kg. Anything that can be done to reduce the weight of a cyclist and bicycle, without compromising the cyclist's aerobic power, will improve hill-climbing performance.