JBerman

So I know a lot of people are concerned with bike weight, and I was curious how much it matters, what the increase in speed actually equates to, and so on.

I know my bike for example is rated as sub-18 lbs, however, fully loaded with all its gear and such it's 24 lbs (water is heavy, plus the 2 25oz bottles themselves, cages, bike computer, bike bag with spare tube, CO2 inflator and 16g CO2 cartridge, tire levers, bike multitool, phone in the bag, keys for house, etc).

Myself, I'm 174 lbs. Even taking my weight into account, is there some formula that says x amount of weight loss yields more speed (or rather, less power required for same threshold)?

Just curious if an 18 lb bike is theoretically really much faster than a 22 lb bike. Is a 15lb bike that much faster than an 18lb bike. etc

FBinNY 

There's always been debate about how much weight matters. Many do as you do, and consider the percentage difference in total weight, but that only relates to a small part of the story.

At constant speed on level ground, weight doesn't matter (in theory) because all the work is being done to overcome wind drag. In climbing, most of the work is done lifting the total weight, so in theory 5% more weight needs 5% more strength, or a 5% lower gear.

But, as I said, that's only a part of the story. Theory is fine, but bicycles are not like theoretical wheeled vehicles. The bike doesn't move at uniform speed, but instead is constantly being accelerated in small (tiny) bursts with every pedal stoke, and slowing down in between. By the same token, we constantly rock the bike side to side when riding and that also takes a bit of work. How much work this involved is hard for me to say, but anyone who has ridden light and heavy bikes knows how different they feel. That difference is reflected in total effort, speed and times on longer rides.

To get a sense of what I'm describing, buy a 10# sack of potatoes, and attach them to the bike or carry them in a back pack. Go ride, including some hills, and you'll see that that 5% weight difference makes much more than a 5% difference.

That said, there is a point of diminishing returns, so at some point the differences in bike weight will (seem to) matter less, and other factors like feel and comfort may matter more.

10 Wheels

For me it was in the gears.

Compact Double for acceleration.

Top Speed my Triple was Best.

datlas 

It matters, but VERY little.

caloso

It doesn't really matter on a flat road with a steady effort. Speed on a flat course is a function of power over frontal area; speed on a climb is a function of power over weight.

But lighter bikes accelerate and turn quicker under the same power, which may mean the difference between getting dropped or hanging on, or the difference between coming around in a sprint or bogging down. So even for crits or flat RR's, racers want a light bike.

Abe_Froman

This is a nifty little tool: Bike Calculator

Based on my amateurish understanding of physics...everything it calculates seems reasonably accurate.

Long story short if you want to avoid a few days of research and/or going back and forth on the forums: effect on speed both on flats and hills is measurable. It's probably measurable, but functionally negligible on flats unless you are literally a pro racer, assuming we're talking a few pounds and not 50+. On hills it makes a bigger difference. Will 5 lbs make a big difference on a hill? The bike calculator says 5lbs will make a difference of ~0.2mph on a 5% grade, which sounds about right to me. The calculator says the change in speed is about half that on flats.

As a test: leave your 2 water bottles, keys and phone at home for a ride. That's ~5lbs.

JBerman

Just out of curiosity, when thinking of acceleration and not so much turning, is there an equal difference of bike A of 20lbs and me at 175lbs vs bike B at 15lbs and me at 180lbs?

Is all always about bike weight and not so much total weight?

JBerman

Very cool, thank you

Seattle Forrest

In Florida, no. In the North Cascades, yes.

Abe_Froman

Yes and no.

Weight on the bike that doesn't move/turn on the bike relative to the bike works exactly the same way as your body weight. Examples: frame, cables, housing, brakes, etc.

Wheels and drivetrain parts - these essentially count double for weight. You have to overcome inertia to move their weight forward along with the rest of the bike, but you ALSO have to overcome their rotational inertia. This will be most pronounced in the wheels as they obviously turn the fastest, but the effect will also present itself in the cranks, chain, cassette, etc.

JBerman

Is low bike weight and low rider weight equal? Or does bike weight still have an advantage? Such as my example above. Me at 175lbs on a 20lb bike vs me at 180lbs on a 15lb bike. Is this the same?

Abe_Froman

P.S. you should probably know I don't actually own a road bike, but the science behind them fascinates me lol

JBerman

Makes sense. I noticed that calculator gave the same results based on bike weight and rider weight whether 15lbs bike or 20lbs bike and long as total weight remained the same so I wasn't sure. Thanks!

JBerman

Haha that's ok you are still welcome here based on user name alone.

Seattle Forrest

It's very similar. There are some subtle differences in the real world. But there are a lot of variables, too.

It's easier to carry a 15 pound bike up the stairs or out to your car than a 20 pound one. In some situations, it's easier to move it under you, like when you go to extremes to avoid a pothole.

Losing weight is hard (that's why it's a multi-billion dollar industry) and often impermanent; your bike will never gain 5 pounds back after binging. Losing weight tends to improve peoples' health, but it means some amount of muscle loss.

5 pounds is a pretty small difference and we're kind of majoring in the minors here.

Abe_Froman

Yup. Oh, I meant to add that the difference between the static and rotating parts only presents itself when accelerating. Zero difference at a steady speed. You can have a 1lb bike and 199 pound you, or 150 you and 50 lb bike with 30lb wheels and it won't make any difference at all if you're already at speed.

Abe_Froman

Yay! Sausage for everybody lol

rpenmanparker 

It matters to me. A lot. Ultra-light bikes are a joy to ride. It doesn't matter why. They just are.

JBerman

5 pounds was kind of my focus though, as the price difference between bikes that weigh 5 lbs less than others is quite significant. 20lb bikes are common. 15lb bikes cost LOTS more typically.

JBerman

Awesome, I've never had sausage before from the king! (wait, that sounded awful).

therhodeo

Whats this nonsense now?

JBerman

Very true. Coincidentally, today, I met my weight loss goal, LoseIt badge and all haha (was 207 and today I'm 174). I'm sure I lost some muscle which is an unfortunately part of weight loss.

79pmooney

That one doesn't have a straight answer. Your power will almost certainly be different at the two different weights. Your ability to replicate that power over and over again will also differ. I might well have had more power at my best at 150 but on that 20th surge of the race, I could still be there at 145. 150 was above my best best possible racing weight.

But as said above (curse this forum format; that when I answer a post I don't see the thread so I have to take notes in advance or open another tab. I cannot remember who said this (calso???)) light wheel and bikes mean that in a race you might just make it onto that wheel and a little heavier almost make it. The difference between making it and almost can be 10 minutes. I rode the race of my life on very light wheels and tires. The pace was record breaking fast. I was the last to make the cut on the climb 15 miles from the finish. Beat the guy who fell off my wheel by that 10 minutes. Dropped my chain on each of the next two hills, using everything I had to get back on. Finished with that group, same time, under the old course record and in the money. No way that would have happened on heavier wheels. No way. There was no extra reserve to draw from. (And it took me two full days just to come back into my body. Good thing I worked for our club's sponsoring bike shop. I was paid two full days for being useless.(

I saved those really light wheels for the big races. If I had ridden them for say one of my really long solo training rides that summer (up to 175 miles), they might have made that 10 minute difference, maybe, but that would have been almost entirely because they were a lot nicer rolling tires than my training tires. (10 minutes on a 14 hour day. Big deal!)

Ben

Seattle Forrest

If you don't spend most of your miles going up or down hill, then the 5 pounds aren't worth paying an extra $1,500 for. But whenever you compare two bikes, there are differences beyond the weight. The 15 pound bike will be nicer than the 20 pounder in a lot of ways that aren't really related to weight. Better shifting, etc.

JBerman

Absolutely true. Good point!