The Benefits of Lightweight Parts
If you haven't heard (you must have been living on Mars to have missed it) reducing the weight of your bike by spending most of your paycheck on trick little bits every now and then is a raging trend. Manufacturers were quick to spot this, and, clever folks that they are, started making parts lighter and lighter(and even lighter yet in some cases). Whether or not this was a good thing to do depends on a delicate balance between how much faster you can go after you put the trick parts on, how long the trick parts last, the costs in medical bills if the trick parts snap off rudely dumping you on your nose, and how much cred and envy you get from your cycling mates for merely owning the things. The question remains, how much good is taking weight off your bike likely to do for you out on the trail?
I've figured out what should happen when you shave weight off of your bike. There are quite a few ways reducing the weight of a bike can actually help you, though most are tediously small contributions. The big potential advantage of weight reduction is its effect when you are climbing hills. When you climb you increase your potential energy. The amount of work you do against gravity depends on how high you climb and how much you weigh. The rate at which you do this work is the power you put out. The more time it takes to climb the hill, all else being equal, the less power you put out. It's useful to predict the changes in your performance at a constant power output. Saving weight doesn't make the motor put out more horsepower! This means that the comparison we want to make involves a given rider, climbing a hill at a constant effort, and riding two identical bikes except one is heavier than the other. What differences would we expect in speed up the hill, the time saved due to the weight reduction, or the distance gap the lighter bike and rider would have if they were racing each other.
It may surprise you to learn that anyone as involved in the technical side of cycling as I am would bother with this. After all, everyone knows that it works right? My tendency is not to buy into this sort of common knowledge easily. I need to prove it, even if I spend hours and hours pondering the obvious for no apparent benefit. True science is like that.
I am deeply indebted to R&D machinist, Jensen, for pointing out an error in an early calculation that would have completely changed the conclusion of this article and made me look pretty foolish. True science is like that.
So I will make a simple prediction of the benefits of the weight saving efforts that are driving much of the innovation in the sport today. This effort is humble in its scope, so I'd prefer to not be reminded too many times of all the things I'm sweeping under the rug. For example, I know, and everyone else that has taken an interest in elementary physics knows, that it is the mass of the thing we are really reducing. Grams are a unit of mass; pounds are a unit of weight and the whole thing is a jumbled up mess. But "low mass" hasn't the marketing ring to it that "light weight" has, and I get tired of getting blank stares when I talk about mass, so I'll contribute to the extension of the confusion.
First of all I'd like to go on record as saying - shaving off weight is not much good really. You knew I was going to say that though, didn't you? Think about it. If you aren't trying to make a living at racing, the benefits are fairly intangible, just for fun, and frequently only for your ego. Set aside everything but the actual, measurable performance advantages of light weight parts which I might get to if I don't run out of space lecturing. If you're able to (choose one):
1) hammer your cycling mates at will
2) keep up easily with your cycling mates
3) barely keep up with your cycling mates
4) barely keep sight of your cycling mates
5) none of the above (you don't have any mates)
by going faster up hills, it's not doing you that much good (like food, water, warmth, shelter, and beer do for you). It actually may be contributing to making you a snob! (If you selected #5 above you might want to consider spending the money on more directly social activities).
Secondly, if you are serious, I mean really serious, about all of this you'll rid yourself of excess weight first. Then, and only then (unless you are as old and desperate as I am) will you shell out for the trick parts (I have it all over you here - I test them - it's my job). Everybody knows deep down that there is much more personal benefit in doing this than in laying out the cash for the new parts. Well, maybe the magazines haven't figured it out yet, at least they're holding back if they have.
Now let's get on with the prediction. You want to rip past the folks you ride with even though you are hopelessly off the back on the climbs now. You don't have the time or the patience for any more training than the hour a week you already put in. How much do you have to spend on light weight parts to make this happen?
I've included the following handy scientific table to give you an idea. If you look at the table closely you will see that there is some funny business in the column labeled "distance gap". The distance advantage is the same in all cases independent of the speed you are traveling or the slope of the hill. Trust me when I say that it all works out in the end; its part of what happens when you assume a constant power output.
But here's the good news. Let's say that you are lucky enough to only weigh 150 pounds, and that you are wealthy enough to own a 25 pound bike. You're doing okay really; you sure you want to mess around with the lightweight stuff? We'll also assume that you are staring at a steep 1 mile climb from the back of the group. Suppose you are capable of making enough power to propel yourself up this hill at 5mph. It will take you 12 minutes to make it to the top.
Now, let's say you went out and got some light weight parts, enough to shave off 795 grams. This saves you a whopping 1% of the combined mass of you and your bike and costs enough to buy a lot of premium beer. Assume you are at that familiar position in the back of the group again, hoping for the best. After the investment you will be able to make the climb at 5.05 mph, expending the same power as before. It will take you 11.88 minutes to finish the effort, and you'll have stayed over 52 feet closer to your mates at the top!
52 feet! It may not put you out of sight off the front, but it is a lot. And how much did it cost? Well, taking 795 grams off of a 25 pound bike is not easy or cheap. Think of 8 parts you can shave down 100 grams, starting with decent stuff in the first place. Now think of how much that sort of thing can cost! Maybe the extra training isn't such a bad idea. 795 grams is a little less than 2 pounds. Who doesn't have this much to spare somewhere other than their quads, heart, and lungs?
Come to think of it, maybe you should spend the money on the parts if it keeps you away from the beer?