Right. If I put on panniers with heavy stuff in them, the stuff might add enough weight, and the added drag might not slow the bike down as much as the weight speeds it up. On the other hand, changing to heavier tires should not make me descend any faster.

But I'm not so interested in descending. I'm interested in how best to improve speed on flat ground because it factors out terrain. Most of what you can do to get faster on the flat will help for climbing. Improving downhill speed isn't so important. Besides, I'm already a fast descender.

I'm also a fat descender.

Oopx fast descender.

Very correlated, those two, in both physics and spelling.

Some "proof" that weight does not matter on flat, steady speed roads - the Lotus superbike from 1996 which set many a world record in the day was a portly 8.35kg (18.41lb) which even then was heavy for a track bike (even road for that matter). But this is a track -no sidewinds, hills and acceleration save for the initial ramp up.
https://www.bikeradar.com/us/road/ge...ort-110-41260/

The #1 factor in flatland speed: body position. The bike itself generates very little in the way of drag. The big bag of meat on top of the bike creates a whole lot of drag. The trick is finding a position that has an aero benefit, while being comfortable. Hands on the hoods with forearms parallel with the ground is fast-- but I feel the fatigue from it in a matter of minutes.

If we're talking about steady-paced riding on flat ground, basically wrong. There are technically some subtle physics differences, but nothing that's measurably impactful.

Gravity pulls harder on heavy objects than light objects.

When you go downward through Earth's gravity, you receive power from that gravity. If your bike+rider system is heavier and gravity is pulling harder on it, then you get more power. So you go faster, just like you would if you added power by pedaling harder. (Assuming that whatever you did to add weight didn't increase other drag forces).

The benefits of extra weight on the descent generally don't make up for the penalty of that weight on the climb, though.

It could, if those heavier tires did not increase the rolling resistance.

All other things being equal, a wheel that's heavier at the rim will be accelerated slower by gravity downhill (due to the mass being rotating), but have higher terminal velocity (just like all added mass).

How those kinds of positions feel varies a lot by the particular handlebar arrangement. Especially when you get into the phantom-aerobar-esque realm of gripping the hoods from the inside.

Hang on, [MENTION=406505]HTupolev[/MENTION]. The rate of gravity is constant, irrespective of a body's mass. I know at least that much. In a vacuum, a feather and a bowling ball and a penny fall at the same rate. The bowling ball falls harder because of its mass, but it does not fall faster.

You can also notice this if you go snow skiing with a significant other or friend that is a lot smaller than you are. Again, there are a lot of factors in play (ski length is a big one), but generally speaking a bigger skier will get pulled down the slop just that little bit faster.

And if you really want to see some long debates about the physics of gravity, object mass and wind resistance, check out some of the WW2 airplane forums where they argue this stuff ad nauseam.

[MENTION=152773]noglider[/MENTION], on the few occasions that I get to chug along on a flat stretch without interruption, I have actually noticed that my heavier bike seems to maintain speed a bit easier (in terms of my perception) than my lighter bikes, especially if it is windy. Once I get the momentum up on the heavier bike, which definitely takes more effort, it just seems to plow along with less sensitivity to outside forces.

You're not riding in a vacuum. You're riding in air.

If you have two bowling balls with the same exact shape, and one is a real bowling ball while the other is as light as a feather, the former will fall a heck of a lot faster in atmosphere. The aerodynamic CdA of the two balls is the same, but the heavier one is getting pulled down with much more force.

I fully agree with this, but it seems to contradict what you said previously. Or maybe I misunderstood you.

What does it contradict with?

You said More mass doesn't make things fall faster. It does if drag is increased less than mass is increased but the increase in mass alone doesn't make falling happen faster.

I also said:

OK, good, we agree on that.

So can we get back to flat ground? You say a lighter bike (or rider) makes a smaller difference than we expect. I can accept that, but I can't accept that it makes no difference. As I said, it gives me a 2 or 3 mph advantage.

Any reasonably differences in bike weight won't be the primary culprit in a 13mph vs 15mph difference on flat ground. My road bikes vary over a 13 pound range in weights, and they all perform much closer to each other than that when riding steady on the flats. Is your fit/posture about the same on both bikes, and are you running the same tires set up in the same way?

Fair questions, and the sample size is low enough that I can't prove anything. Still, I'm having a hard time believing that weight doesn't matter on flat ground.

I think isolation of factors is a bigger problem than sample size, in this case.

15mph versus 13mph is about 15%, which is an absolutely gigantic performance difference to exist between two bikes. Like, even when I'm climbing steep hills, I'm only about 6-7% faster on my 20lb road bike than on my 33lb gravel bike, at similar effort.

bluehills proof above with the record-breaking lotus superbike seems pretty convincing to me.

Also I somebody else (maybe you?) said before something about, sure there are micro-accelerations every pedal stroke, and sure, lighter bikes will micro-accelerate easier and heavier bikes will take more effort to micro-accelerate, but heavier bikes (+loads) maintain more momentum through while pushing through the same air resistance, so heavier bikes need less micro-acceleration.

So your choice is heaver=less, harder micro-acceleration vs lighter=more, easier micro-acceleration, and it's practically a wash.

Maybe there is a hair advantage to the lighter there, on the same principle that spinning is better than mashing, but that's gotta be miniscule.

Surely there have to be experimental results out there showing indoor track speed results with constant wattage over varying bike weights

Well obviously the critical difference is that roadies remove their valve stem caps

Nice reference

That was me who brought it up, and I theorized that his feeling that it was harder would be due to accumulated fatigue from heavier power strokes, along with the other accelerations.

It could be that just having a different power stroke is fatiguing in itself, if you have trained extensively and are used to different mechanics. On a long flat time trial I suspect it would be beneficial to have heavy wheels, for the reason you mention.

OP has it right

I lived in a small town in the finger lakes area in central NY. I used to commute at 20-25mph going to work and around 15-20mph on the way back (it was a hilly town.

Now, I moved to a small city in Ontario (approx. half a million people) and my commute speed immediately dropped to 10-15mph. The city is almost completely flat, but there are tons of obstacles. I can't go fast on the bike paths because the pavement is awful. I can't go fast on the shared paths because there are pedestrians. I can't go fast on the roads because there are traffic lights and traffic jams.

In an ideal environment, it will make no difference.

I train in an ideal environment – velodrome. When doing mass start races, we use the lightest tire and bike available (within UCI regulations). When doing a time trial, we put a big heavy solid disk wheel on the bike because mass does not matter in these conditions (flat steady state). To some degree, having a big ole flywheel on the bike helps to keep our speed consistent. I do about 25 miles in an hour time trial – but I’m more worried about aero than weight at those speeds.

OK then, so why does having a light bike help in an un-ideal environment? I rode to work today on my racing bike. I concede that I'm not controlling variables, but I was faster than ever today.

A lighter bike requires less power to accelerate, and mass-start races aren't steady-paced rides. If you intend to hang with a pack, you can't just *not* accelerate when that pack accelerates, because drafting is so crucial.

If you're in a group that's going 25mph, and in 5 seconds you all accelerate to 30mph, 2 extra kilograms on the bike is going to cost you around 10 extra watts during that acceleration. For 5 seconds of intense effort, you're having to do something on the rough order of 1-2% more power than you'd need to do on the 2kg lighter bike. It's certainly less significant than a lot of other differences you see between bike setups, but it's a cost that it doesn't hurt to not pay nonetheless.