Bike weight? what "real world" gain

Triathlon Forum
21

Gotta disagree to some extent with some posts. Bike weight is always important, but a light bike is only an advantage when acceleration is of concern, and that means going uphill where you have to overcome gravity’s acceleration, and on technical courses where braking and getting back up to speed is critical. In both cases, it helps to have a fairly rigid bike as well. I have no use for light noodles.

On the other hand, I’ve always liked a heavier bike in windy conditions where decelleration is crucial. I have a 22 pound Zipp, and when it’s windy I can put big time into friends who ride light bikes. We have a TT series where it gets windy a lot, and I’ll hear guys say "I dropped from 26 mph to 18 in about 2 pedal strokes. With my Zipp’s weight and aerodynamics, I may only drop to 23 or so. Sure, it’s harder for me to accellerate, but I don’t have as much accelleration to do.

If you ride in varied terrain and want to maximize your performance, you need a couple of rigs. A light, rigid bike for hilly technical courses, and an aero monster for everything else. Some courses are so steep and technical that the extra couple of pounds really do become detrimental.

Same goes for wheels. I have an older HED disk that weighs about 40 pounds, but it is definitely faster on my Zipp than my brand new Zipp 909’s. Sure, it takes me 5 extra seconds to get it rolling, but once it’s rolling, especially on a flat course, I ain’t slowin’ down!!!

Light is not always best. It is when it’s hilly and or technical, but I’ll actually add weight for flat windy courses.

Bzzt. Acceleration is defined as a change in velocity over a change in time. “Gravity’s acceleration”? Sorry, but if you are going a constant velocity uphill, you are not accelerating.

So you are saying that, faced with a sudden headwind, a rider whose total weight is, say 85kg, slows down from 26mph to 18mph, while another rider (you), whose bike weighs, say 2kg less, slows down only to 23mph? Riiight…

Your credibility is in serious question.

Take a physics course nimrod, and you’ll find out that the force of gravity accelerates all objects at 32 ft/s2, or 9.8m/s2. To go uphill, you have to constantly accelerate, even to hold a constant velocity. Simple physics pal. Think, study, learn, then post. It’s not that tough.

Second, take a reading course. I said my bike was heavier, not lighter.

Your mental abilities are in serious question.

You are kidding, right? Acceleration is delta V/delta T. If something is going a constant velocity, there is no acceleration.

As for my reading abilities, they are intact. I erred in writing (not reading) that your bike is lighter, not heavier. Do you still claim that your bike loses only half the speed as a bike that weighs a bit less? That’s crap.

22

My gosh. it’s really not that hard. Put it this way, if you could eliminate friction and wind resistance, once you are up to speed on a flat surface, you will maintain that velocity (objects in motion stay in motion unless acted upon by an outside force). Do that on an uphill and what happens, you quickly slow to a stop, then roll backwards at an ever increasing velocity. That’s acceleration, and you have to fight it every pedal stroke.

THAT’S WHY GOING UPHILL IS HARD!!!

My last post on this. I am right, you are wrong. Check with your local physics professor before you post back, or you can simply learn from this physics professor…

Or simply check the slide on this website called “Grade resistance” and you will see what I mean

http://www.ctre.iastate.edu/educweb/ce355/Lecture%20Notes\Fourth%20Presentation%20for%20CE355%20Spring%202005.ppt

23

Ken

Professor V is right. It is one of the first things you learn in Physics 101. Stop pedaling on a hill and you will roll backwards. That’s gravity. It is an accelaration force. Simple really.

Mike P. who got an A in physics 101

24

It’s pretty interesting how overhyped weight and rotational weight is in the cycling industry
Forgetting acceleration there, my solo-riding friend?

25

Nope. Go run some models on analyticcycling.com. Compare acceleration times for a bike carrying an extra pound on the wheels as opposed on the rider.

I never said weight doesn’t matter, just that it matters far, far less that one might think.

26

Nope. Go run some models on analyticcycling.com. Compare acceleration times for a bike carrying an extra pound on the wheels as opposed on the rider.

I never said weight doesn’t matter, just that it matters far, far less that one might think.
Nah, I trust you. I switched to wheels that were about 1/2 lbs lighter midway last season. Same guys still beat me.

27

Actually, here are some numbers:

5k climb (6% grade) @ 250 watts:
Rider 1 (75 kg total weight) finishes ahead of Rider 2 (76 kg total weight) by 10.98 seconds. That’s a gain of only 11 seconds for dropping about 2.5 pounds/2000 grams.

28

"It is an accelaration force. Simple really. "
That’s great. What other kinds of force are there? (F=ma)

29

Very interesting, thanks!

I think the question to answer would be, ‘from 30 km/h to 50 km/h w/ a given wattage’, who gets there first, and are lighter wheels more beneficial than a lighter frame/rider?

Anyways, that’s the fun about road racing - the strongest guy w/ the stiffest frame and the lightest wheels doesn’t always win, if he’s up against somebody smarter!

30

"You are kidding, right? Acceleration is delta V/delta T. If something is going a constant velocity, there is no acceleration. "
Yes and therefore the sum of all forces acting on the body is zero. That’s not the same as no forces acting on the body.

31

"Actually, here are some numbers:

5k climb (6% grade) @ 250 watts:
Rider 1 (75 kg total weight) finishes ahead of Rider 2 (76 kg total weight) by 10.98 seconds. That’s a gain of only 11 seconds for dropping about 2.5 pounds/2000 grams."

First, when you compare 76kg to 75kg you are talking about 1000g not 2000g.

Yes, at 250W, rider 1 (75kg) is 11seconds faster than rider 2 (76kg.) It’s all relative, but I wouldn’t term the difference in time as “only 11seconds”. It’s 11 free seconds for only 1kg less weight. And when racing, 11 seconds could mean alot.

edit: missed your earlier post. Anyway, time savings are all relative.

32

What about this: is the weight of your pedals, shoes & cleats more important than the rest of the bike because you are actually lifting that weight with each stroke?

33

"You are kidding, right? Acceleration is delta V/delta T. If something is going a constant velocity, there is no acceleration. "
Yes and therefore the sum of all forces acting on the body is zero. That’s not the same as no forces acting on the body.

I never said there weren’t any forces acting on the body. What our esteemed professor originally said was " Bike weight is always important, but a light bike is only an advantage when acceleration is of concern, and that means going uphill where you have to overcome gravity’s acceleration". This is not accurate, despite what those who say they got an ‘A’ in physics 101. Gravity is a force, which if unopposed will result in an acceleration (just like any other force) in the direction of the pulling object. When one rides a bike uphill, one is opposing the force of gravity with a greater force, resulting in a net velocity upwards. If the pedaling force is constant, the velocity is constant, and no acceleration takes place.

Had the original quote been “…that means going uphill where you have to overcome the force of gravity…”, I wouldn’t have argued, of course. But the prof used the term “acceleration” incorrectly.

The prof went on to claim some huge change in velocity when hit by a sudden gust of wind; the claimed change was far greater than would be explained by the small difference in weight between two bikes. He hasn’t responded to the challenge of that claim. Inertia (the force that keeps a bicycle moving) is proportional to the mass of an object; reduce the mass by 2.5% (as in my example of the difference in weights of his and his friend’s bike/rider mass) and the inertia changes by 2.5%, not the large amount (from 26mph to 23mph vs 18mph) he claimed.

34

I normally stay out of discussions fueled by people armed with their freshman physics textbook, but the exchange was so pleasant and friendly I just had to join in:

Stated so politely by Prof V:

//Take a physics course nimrod, and you’ll find out that the force of gravity accelerates all objects at 32 ft/s2, or 9.8m/s2. To go uphill, you have to constantly accelerate, even to hold a constant velocity. Simple physics pal. Think, study, learn, then post. It’s not that tough.

Second, take a reading course. I said my bike was heavier, not lighter.

Your mental abilities are in serious question.//

As Ken attempted to point out, to use the terms constant velocity and constant acceleration together is somewhat incorrect. to go uphill at a constant velocity (assuming a constant grade, air resistance, etc.) you need to apply a constant force. When this force is balanced by the component of gravity acting parallel to the road surface you have no net acceleration and travel at a constant velocity. When the force you apply is less than the force of gravity (and air resistance, rolling resistance, etc.) you’re fighting against you’ll go backwards, when it’s more you’ll speed up (accelerate!).

When Mike stated

//Professor V is right. It is one of the first things you learn in Physics 101. Stop pedaling on a hill and you will roll backwards. That’s gravity. It is an accelaration force. Simple really.

Mike P. who got an A in physics 101 //

He was trying to convey this concept. Yes, stop pedaling and you will go backwards (until you crash). This is because you stopped applying a force that was acting against the force of gravity. Yes, gravity can accelerate you (so can any force! that Newton guy was pretty bright, eh?), but no, Prof V was definitely not correct.

Jeff M. who taught physics 101.

35

Ken

Had you said that in your original post I would have agreed with you. Probably professor V would have also. The A in Physics part was tounge in cheek. Not trying to slam you or anything. I don’t care one bit one way or the other about this argument (acceleration vs constant speed vs whatever). It is really only important to physics professors and students (at least long enough to get through the course). Personally I forgot 99% of what I learned in physics. I really did get an A though! Second highest grade in the class behind a Chinese student! Lots of good that did me though. Now I can’t even work my way through a bike forum physics post! Take care.

Mike P.

36

If you are doing an uphill race or a race with lots of mountains where seconds mean places, 500 g matters. Otherwise it doesn’t matter.

Wheel weights - this is the definitive study I’ve seen on the subject.

http://www.biketechreview.com/archive/wheel_theory.htm

37

Ken

Had you said that in your original post I would have agreed with you. Probably professor V would have also.

Didn’t I?

" Acceleration is defined as a change in velocity over a change in time. “Gravity’s acceleration”? Sorry, but if you are going a constant velocity uphill, you are not accelerating. "

As you said, it’s not very important.

38

I rode a lemond road bike that weighs 23 lbs for about a year, then bought a dual . well i took 2 minutes off my sprint distance bike leg first time out, and I know i’m faster on that bike.

imagine my surpise the first time I stepped on a scale holding it , and saw that it also, weighs 23 pounds the way I have it set up.

I 'm sure the stiffness of the frame, the better bearings in the wheels , crank, hubs, and the aero tubes are what make the difference.

now, if I could just make it lighter :slight_smile: