Rides4Beer

I can feel the difference between my gravel bike (on road tires) and my road bike, mainly when accelerating. Part of it is the 4lb weight difference, but then there's also the gearing, carbon layup/compliance, and aero. At cruising speeds, they feel about the same. The heavier gravel bike actually climbs better, but that's the gearing.

wphamilton

At the risk of seeming superficial, F=MA, change in weight is proportional to the change in acceleration, whether you can produce a large force or small one. The assumption that most people make about this is that people who produce more power have greater mass.

The smaller rider will see a greater impact on his acceleration from 5 pounds difference, because 5 pounds is a greater percentage of his total weight. That result is not really related to his power output, except to the extent that higher power means bigger rider. As you point out, that's not always true though.

livedarklions

I was assuming all other things, including rider's weight, to be equal. While there may be a weak correlation between size and leg power, I don't think anyone would assume that people who weigh the same or nearly the same produce the same force.

As you say, F=MA, but I think you have the implications of that backwards by talking in proportions instead of absolute values.

A=F/M. So if the weight of the two riders and the reduction of weight is the same, then the solution for acceleration has the more powerful rider with the higher F numerator divided by the same M denominator than the less powerful rider. In other words, the more powerful rider gets the bigger speed boost from the same percentage of weight reduction.

Did I get this wrong? I have absolutely no confidence in my physics abilities.

wphamilton

Naw, you can talk about (delta M) and (delta A) in the same way.

to clarify, it's mass plus delta mass and acceleration plus delta acceleration. example, If your mass increases by 10%, your acceleration decreases by 10%. 10% of 100 pounds is only 5% of 200 pounds, so the lighter guy loses twice the acceleration of the bigger guy with twice the total weight.

ThermionicScott 

Weight doesn't matter, but it makes a big difference. Got it.

Eric S.

I have no proof but aside from going uphill I think there is a rider/bicycle weight relationship that makes a difference. I kind of don't like my lightest bike sometimes. I'm 6' 2" and ~200lbs; I prefer my heavier steel road bikes over the lightest one.

livedarklions

Sort of misses the point of having M and delta M the same for both riders but varying the force in my example, doesn't it?

You aren't really claiming that it's likely that two 200 pound guys will each produce the same amount of force, are you? Even if they're both muscular, the distribution of that muscle will make a significant difference.

If M and delta M are the same for both riders, and F is higher for one than the other, the one with the higher F will have the bigger increase in A. The change in A will be of the same proportion for the two riders (let's say X percent) but that's X percent of a higher value for A than that for the weaker rider--in other words, a greater increase in m/sec. A 2% increase in speed is a lot larger if you were going 17 mph with the heavier bike than if you were going 10.

Of course if the two equal weight riders are significantly apart in speed to start with (say 24 mph vs, 10 mph), air resistance becomes a factor, but let's not go there.

wphamilton

I don't know what your point is. If one guy puts out 300 watts, and the other 150 watts, and they both add the same percentage mass, they will both have the same percentage change in acceleration. Does that help?

See above. It makes no difference in the change in acceleration per change in mass.

Bandera

I'm still sticking with: No.

-Bandera

livedarklions

And if it's the same percentage change in speed, the one who is moving faster will have the bigger change in absolute terms. A 2 percent increase over 20 mph is twice as big as a two percent increase over 10 mph. Kapish? The stronger rider gets a bigger benefit from the weight reduction.

wphamilton

Even that's not necessarily true, depending on the specifics of what the weights are and what his power is.

But more to the point, you are adding an extraneous variable. does weight matter on the flats - this isn't asking if weight can be overcome at some level of power. If you go that way, what about a bad drive train, what about wind, or other extraneous variables?

At higher powers you get more speed, more acceleration. You can have that, but I don't think it's relevant.

livedarklions

Relevant to what? OP just asked if it matters or if it's a "major downer." If the question isn't about speed or effort, what exactly do you think it's about?

I'm just playing with the variables you brought up when you raised Newton's second law.

TimothyH

This is easy.

Just head over to the touring forum and ask the same question.

They will know whether weight matters on the flats.


-Tim-

AlmostTrick

Of course weight matters even on the flats... it doesn't keep moving all by itself without any input from the rider.

Try this exaggerated test: Pull an enclosed trailer while empty. (enclosed to maintain the same aerodynamics) Use an electric motor to get up to X speed. (to eliminate acceleration) Accurately measure speed and power output over a flat course.

Then repeat while adding weight inside the trailer. If weight really doesn't matter like some here are saying, you should be able to keep piling it on (hundreds of pounds!) and see no increase in power needed. Does anyone here really think that would happen?

If the difference in weight is only a pound or two, of course the additional power needed will be small, but it is still there. You can say you wouldn't notice the difference, or that it doesn't matter much, but to say there is no difference is inaccurate.

bikingtotown

Really depends how far you are going and how much weight. I used to commute alot on bike and had to travel with all kinds of things and I wouldnt want to ride too far with alot of extra weight. You're gunna tire out faster with extra weight no matter what unless its downhill whole way. so usually if you have a heavy load and a ways to go you should try to just roll and pedal just enough to keep yourself coasting. So I'm gunna say yes the weight is the reason i get exhausted while riding.

Bandera

Actually a Lack of Fitness is "the reason" why one gets "exhausted while riding", not the "weight" of the machine and/or stuff carried on it.
More saddle-time is required for Endurance, for which there is No Substitute.

-Bandera

xroadcharlie

Increasing weight increases rolling resistance almost proportionately. And rolling resistance may account for 25 - 50% of power requirements to maintain speed on a flat on a windless day. If we add 4 kg to a 80 kg bike and rider, and we generate 150w of power, we need and extra 2 - 4w of power with a bike that's 4 kg heavier.

To put things in perspective, The rolling resistance of touring tires can easily vary by 25%. Under the same conditions, Just by switching to a different tire, power requirements increase by 9 - 19w.

We need 5x more power to move the same bike with different tires, then we would by adding 4kg to the bikes weight.

livedarklions

Right, there's a lot of things that are going to matter more than minor differences in the weight of bikes, including the weight of some of the stuff we carry. I've seen posters on the forum describe radical gram shaving in one thread, and then describe how they carry ludicrous amounts of water in another thread without any sense of irony.

bikingtotown

I should really read the original first poster. He seemed to just be talking about the weight of the bike. I thought it was more about carrying stuff total weight. Sounds light enough though I'm sure it will fly. I'm not sure about KG.

jamesdak 

It's never a simple matter of weight alone, LOL! Too many other factors figure in. Today was a prime example. I took out the old SL tubed Ribble bike. It weighs 19 lbs, 7 ozs and has on some 40mm deep and light carbon fiber tubular wheels. I did my normal base 25 mile route. I worked really hard today but the flex of this 1985 race built frame under my 175 lbs meant that no matter how hard I tried I couldn't hold the higher speed I wanted to. Ran 19.1 mph avg and it was work. It just wasn't there in this bike and it's current configuration. Yet two days ago I took out the PDG Series Paramount made with oversize Tange Prestige and rode the exact same route with another 3 mile diversion and finished with a 20.2 mph avg. Yet this bike is over 2 1/2 lbs heavier with a wheelset over a pound heavier and not as deep. Both bikes are 53/42T with identical 8 speed cassettes. The fit is the same in terms of saddle height, setback, crank arm length, reach, handlebar width, etc. The heavier bike is consistently one of my fastest. It also is one of my better climbers which totally goes against what the experts will say. A well designed bike can help offset a "weight penalty" IMHO.

BirdsBikeBinocs

What he said^^^^^^^^^^^^^^^^.!!!! The Schwinn I sometimes ride is 40lbs. I changed the original tires over to a small knubby tread that weighs much less than the original tires. Wow. I could not believe how much more effective my pedal stroke was. And the rolling resistance was much lower. Just incredible. And that's why many with experience recommend riders to go with light rims, light wheels. I'm convinced. It was this that taught me .... Weight matters.

Easier, faster acceleration is better in every situation on a bicycle. IMO. Better in the hills and better on flat terrain.

One day I'm going to do a test for myself and compare the roll of my Schwinn to my Specialized. I am almost certain the Scwhinn wins that race. One half pedal stroke and see how far I glide. As mentioned the Scwhinn is 40lbs. and the Specialized is 22lbs.

Will the heavier bike always win that simple test.??

63rickert

Imagine the bike is a hammer. Imagine the saddle is the hammerhead. Now do you want to sit on a 11kg hammer or a 7 kg hammer?

smarkinson

Given the choice between two otherwise identical bikes I'd pick the lighter one.

MikeyMK

Yes, I think it's a question of how much it matters.

If you're 200 lbs with 1.5in tyres on flat lands I really don't think swapping your cro-mo frame for a carbon one is gonna have much effect, all else the same.

However, changing other thing might work better (tyre tread, handle bar position, etc).

You'll probably gain more by locking out a suspension fork, you might find steel more comfortable, but ultimately the lighter the better if it's otherwise like-for-like.

Cars work better when they're lighter, I got a power:weight ratio of over 200bhp per ton out of a car with a little 1.6 litre engine and that went like a Scalextrix car!

livedarklions

That's obvious, but a pretty silly hypothetical. Either the components or the frame would have to be different in order for there to be a significant weight difference. But to play along, how much more would you pay for the identical but lighter bike?