Bike weight (Page 2)

For rolling terrain, shedding a pound or two should yield tremendous results, which includes both the rider and the ride.  The predominance of my events take place on hilly courses.  Aerodynamics play an incredibly important role, but you can't deny the importance of junk in the trunk when you're at the bottom of a hill.  At typical climbing speeds, aerodynamics become irrelevant and its all about moving your weight up that incline.  Trimming 3lbs over a 40k, rolling hill course "should" yield a dramatic time savings when all other factors are kept the same.

There's a lot of "shoulds" in that post.

Seriously, I'd like to see some data 'cause I'm on the fence about this. Not talking about even a 10% reduction in weight, but say 200lbs vs 197lbs or a 1.5% reduction. Is the gain in efficiency worth, say, the $2000 you might pay to get a lighter frame and components?

Wouldn't the extra weight benefit you on the downhills? A loop course with 5,000 feet of climbing also has 5,000 feet of decents.

Yes, its true that what goes up must come down, but you don't have a 100% exchange in the energy.  When you climb a hill, you're basically storing energy for the decent.  However, upon the decent, you have the additional force of wind resistance, terminal velocity, etc.  So, its not an equal trade off.

Besides, there is not a 100% transfer of energy from rider to bike.  Does the amount of energy applied to the pedals remain constant when climbing versus decending?  Does it change based upon the relative speed of the rider and the relative effort applied due to speed and gearing?

I'm getting a headache.

It's also not an equal trade off, because even though the distances may be equal, the time spent descending is far less than the time climbing.

Or, assuming someone's doing a 1 hour 40k TT, it's an improvement of 0.28%.  And that's a with over 11 lbs of weight change, which isn't realistic unless you're taking off the rider instead of the bike.  Light bikes are nice, but on the typical flat TT course, they don't make you significantly faster.

Yeah but I made a couple of points earlier:

1) If you've got weight to lose off  your bidy, save your money and do that first; 2) Less weight on a flat course doesn't make that much of a difference, but as soon as you add hills to the mix, all bets are off. And that being said, even on a flat course, if there are lots of turns, less weight accelerates quicker than more weight.

Check my logic to see if it makes sense.  Assuming a loop course - climbs average 3%, downhills average 3%.  The course is composed of 1/3 uphill, 1/3 flat and 1/3 downhill. 

Keeping all other factors the same as the original example:

or 0.047 miles per hour.

Here's a better model taking into account the time difference:

So an 11lb lighter bike on a rolling course would save ~48 seconds over a 40k TT, all else being equal.

Your average of the speed on the three parts of the course to get an average speed for the entire course is incorrect, because it assumes you spend the same amount of time on each part. That's not right (you obviously spend more time climbing than on the flat, and more time on the flat than on the downhill. Consider the following:

30 mile course, 10 mile uphill at 10mph takes 60 minutes, 10 mile flat at 20mph takes 30 minutes, 10 mile downhill at 30mph takes 20 minutes.

If you average the three speeds, you get 20mph. BUt the total time is 1hr50min for 30 miles, or 16.36mph.