1010shane wrote:
For a given route, how would one go about calculating its "average gradient"? I know that this is a relatively simple question but I'm not quite sure what the answer is.
If, for example, I rode up a 10 mile climb that was exactly 10% grade the entire time, and then rode back down it to get back to where I started, pushing 200 watts the entire time, would this take the same amount of time as riding a 20 mile loop that is perfectly flat, maintaining 200 watts? For the sake of this question I guess you need to ignore the fact that drag and internal friction differ at different speeds. Is average gradient of a route as simple as rise/run?
Average gradient is somewhat useful for a climb, especially if it's fairly consistent. But it's utterly meaningless for a route, as others have suggested.
Distance and elevation is a more sensible way to describe a route. It doesn't tell you how steep the climbs and descents are but it gives you a good idea of the impact they will have on the ride regardless, since a short steep climb of a given elevation gain, is somewhat equivalent to a longer shallower climb of the same gain in terms of total effort and time - they're not equal, to be clear, but it gives you some sense of scale.
Any condition that slows your speed, whether headwind or climbing, will not be compensated by the equivalent tailwind or descent. There are a number of reasons for this, but rather than explain why, it may be best for you to think through a few examples to understand this. For example, if the first half of a route is uphill and is steep enough to halve your speed compared to riding on the flat, you would have to complete the second half instantaneously to manage the same time for the distance as if it were flat. Clearly this is impossible, as it is for any hill steeper than this. There is no magic happening as you reduce the steepness that makes the descent suddenly adequate to compensate once the climb speed rises above half the flat speed. The critical thing to spot is that the slow speed is happening for a much larger proportion of the ride than the high speed and has far more impact on the average than a much shorter stint at high speed. This is even more the case with wind, since the amount of headwind you experience is actually far greater than the amount of tailwind, since you spend far more time in it. Therefore wind is far more negative than positive on a looped course. The same is not quite true for elevation since a given amount of elevation equates to the same energy regardless how long it takes you to supply or harvest it.