Settle an argument?
 

My friend contends that a 180 lb rider on a 20 lb bike is the same as a 185 lb rider on a 15 lb bike and I say not even close, 5 lbs of rider weight is far less significant than 5 lbs of bike weight. I also contend that a 16.5 lb bike with 1500 gram wheelset is going to be an advantage over a 15.5 lb bike with 1750 gram wheelset.

Why would rider weight matter less than bike weight? Makes no sense.

Wheels spin, maybe.

How about a 100 pound rider on a 100 pound bike?

https://analyticcycling.com Knock yourself out.

Well, technically your friend is correct, from a purely International Bureau of Weights and Measures perspective.

If you are saying a 20lb bike is harder to move than a 15lb - I would agree.

....and what about the riders? is the 185lb chap an Olympic class athlete and the 180 lb a 5ft 1in couch potato or vice versa?

I think 5 pounds is 5 pounds, everything else being equal. Now, is it harder to lose 5 pounds or spend the extra money to get a lighter bike? Separate question.

I would have agreed with your friend. I wasn't going to worry about dropping grams off my bike until I got down to my own ideal weight. But I've been riding an old, heavy bike for a few years now. Since I started riding, I have dropped 40 pounds off myself with no appreciable improvement in performance (at least based on average speed). I do have more endurance now than when I started and I can attribute that to better conditioning and weight loss.

I just got a new bike. It's MUCH lighter than the old one. On the first day out, my average speed improved by 4 mph over what I was doing at the end of last season. And that is on the first ride of the year. I know it's not entirely the lighter bike. I went from a heavy, upright, flat-bar hybrid to a carbon-fiber road bike with very nice components. I'm sure the more aggressive riding position is helping me put more power through the pedals and my more aero position is helping me not lose all that power through wind resistance. But the new bike INSTANTLY made me a much better rider.

Gravity, friction, rolling resistance, what ever factor is in the equation, does not care where the weight is. Unless you are an elite racer 5 lbs is not going to make much difference. The mind set, talent, fitness and skills of the riders involved is the difference in average cyclists.

If the 2 riders are the same fitness level, same body fat percentage, etc., I'd guess the heavier rider has an advantage because he's propelling a smaller percentage of additional weight.

I wrestled in High School.

I was at 148 lbs. It was a big difference wrestling with guys say a couple of weight classes lower (130 pounds?), or a couple of weight classes higher (165 lbs?). And, the heavyweights were interesting.

So...
The big question is whether the 185 lb rider is actually stronger than the 180 lb rider.

Even if it is the same person, did the person lose muscle mass going from 185 to 180, or gain muscle going from 180 to 185?

Keep in mind, simply walking, that 185 pound person is getting more of a workout than the 180 lb person.

As far as rotating weight. It will depend a bit on whether it is at the hub or rim. There was discussions about rotating weight decades ago. I don't know what the current status is. Perhaps it is less important for steady state riding, and more important for accelerating.

For a high quality bike, there is also a trade-off between low profile light wheels and heavier high profile aero wheels. With the difference being in part dependent on the terrain.

We have a winner!

Vs. a 105 lb rider on a 105 lb bike (who is running Shimano 105, the other guy is running Ultegra).

I think you are correct, 5 pounds off the bike is not something to sneeze at. Certainly it will make easier effort.

This I don't know, but probably

9.8 m/s^2

Both fall over at exactly the same rate.

I agree with this. If the five pounds goes to muscle, it makes a huge difference.

Which side is the butter on?

"Vs. a 105 lb rider on a 105 lb bike (who is running Shimano 105, the other guy is running Ultegra)."

That is called the 105 formula. 105+105+105=X
There is no known answer to this day and mathematicians the world over have been working the problem since the 90's. On the same level as the Beal problem, except there is no prize money if solved.

For your second example, that's 450 grams less bike vs 250 grams more wheel weight.

That's 200 grams net difference. Just one full water bottle, 22 ounces, is 650 grams. You can tell the difference when your bottle is full or empty?

Lightweight wheels feel faster, and usually have a little quicker steering response. They probably spin up faster, but that means they also lose speed faster when coasting.

Peanut butter is more likely to land face-down than butter. Regardless of how light the wheels are. Oh, and the answer is 42.

ROTATING weight is more important than the total bike weight. Other than that, my 200-ish pounds could stand to lose 40 pounds, and have more effect than my bike losing 20% of it's non-rotating weight.

Fitness levels being equal (that is, watt output per lb of mass), the 185 lb rider will be slightly more powerful than the 180 lb rider. If the bikes being ridden equalize total weight, then the heavier rider will have the advantage.

that's an impressive improvement man. How do you measure mph?