Rider Weight/Bike Weight Question

More than once, I have read something along the lines of "If you want to ride faster, forget about the weight of your bike and lose some weight from your gut." I get the idea, but here's my question: I have a 26.5 pound touring bike. Over the past month, I've lost 7.5 pounds of body weight--down from 175 to 167.5. Now, assuming that I'm using the same wheels, tires and other components, would a 26.5 pound bike climb, accelerate, or generally go as fast a 19 pound bike would if I were carrying my previous body weight? The total weight package would be the same--194 pounds--in each case, and, for the sake of this question, the vo2max and other physical capabilities would be the same.

I know there are many factors involved, but are there any biomechanical/physics formulas that actually look at this issue?

PS: The more I look at this question, the more I think I need to get a life.

 

I never said a thing...

 

Obviously it's hypothetical, since in losing that weight you've also increased your cardiovascular ability (assuming that you did it by excercising and a sensible diet rather than by eating nothing and sitting in a sauna) so therefore you'll go faster anyway. But as far as the question you ask, one of the main factors is rotating mass - extra weight on your belly is the same as weight strapped on the frame, but extra weight on the wheels has an exaggerated effect. Therefore if you assumed that you took the weight off the bike proportionally from all parts, then some of that weight would come from the wheels, so you'd get more performance benefit from lightening the bike. Although as I say, that's modelling the weight you lost as, say, a 7.5lb (3.5kg I believe?) mass in your back pocket rather than as an inefficiency in the engine.

 

You also mentioned climbing, if you were to compare 2 cyclist both weighing the same at 165lbs (75kg) but one has a 26lb(11.8kg) bike and the other has a 17lb(7.7kg) bike, the one with the lighter bike will out perform the other. this is all due to drag, resistance and coefficiency. Honestly its blah, blah, blah to me. Bottom line is yes a lighter bike will help. Although on flats one doesn't matter from the other, its all about the rider.

 

Your body weight and bike weight combine to total a weight you have to cart up a hill against gravitiy and on flats to accelerate from one velocity to another. It doesnt matter if the weight is on you or the bike, its the total that matters. Less is always more!!

Just to make it complex, its the rotating mass on a bike that is critical (tyres, rims, spokes, pedals, shoes etc) rather than static mass (frame, saddle, bars etc).

 

Get a lighter bike. Everything else aside, it is a great excuse to buy new gear and that is always a good thing.

 

The effect of rotating mass is on acceleration so whether the weight loss is on wheel rims or body frame shouldn't matter if going at a constant pace even up hills. If acceleration is involved then weight loss on wheels rims/tyres counts for much more than anything else (pedals, chainwheel etc) because of their greater diameter - most light wheels publish rotational weight on their websites.

As mentioned before loss of body weight implies more aerodynamic body and there's statistical relationship between frontal surface area -> body surface area -> weight, height.

 

For steep climbing, say 10% grades and above, I figure the power required for a given vertical ascent speed is going to be roughly proportional to the total mass of bike and rider. Aero drag isn't a big factor for most of us on a sustained 10% grade....it certainly isn't for me at any rate!

This makes a steep constant hill an easy way to measure power output and test your progress. I've read the best pros can sustain 0.5 m/sec vertical for 20-30 minutes....that would correspond to 5m/sec speed up a 10% grade, or 18 kph (11 mph).
So, power to the ground for a guy like Lance (around 80kg bike and rider) would be 80x9.8x0.5 n-m/sec, or 392 watts. Applying a .95 factor for bike efficiency, and another 50 w for aero drag means he would be putting around 460 watts to the pedals.

If I'm good for half of that maximum sustained output, say 230 watts, it means I can climb the long 10% mountain grade at about half the speed that Lance does, or 5.5 mph.

All this math explains why I need the triple, and he can go with a 39/23!

 

Rotational weight on a touring tire is typically a lot more than on a regular road bike. Count that, and a lighter road bike will climb easier.

As for the life part, welcome to the world of cycling. Outside it, most say we don't have lives. ;-)

 

Like a number of respondents have said, a change of wheelset is probably going to make more difference to your bike than any small stationary weight change, but at the end of the day it's not the metal it's the meat.