Heavy rims

[Willehond]

Everyone adds their bit of pseudo-bike-science' date=' which makes some sort of sense if you read it really really fast, but somehow it all just smells a bit off.  A bit like BS methinks[/quote']

Well I am a few months away from a MSc in Mechanical Engineering. So I do like to think I know a little about "science"

Well, I got my Engineering degree 15 years ago so I like to think that I do too, but after reading this thread apparently not.  Looks like physics has changed a lot since then. Maybe my degree expired?

I got my undergrad only two years ago and I am shocked at how quickly I have forgotten some basic principles I learned in first year. A quick flick through the physics textbook reminded me that its still there ... somewhere ... I think

Sounds like you guys haven't gotten to the chapter on sarcasm yet?  No hang on, they don't teach you that at Engineering school right?

 

Sorry, got to run.  Enjoy the mental masturbation boys!

[Minion]

 

Wow' date=' Newton has had a couple of new laws ripped into him this week.  Everyone adds their bit of pseudo-bike-science, which makes some sort of sense if you read it really really fast, but somehow it all just smells a bit off.  [/quote']

You cut me deep, real deep.

 

Anyway, to quote the very first words said to us by a former physics lecturer:

"Newton was an arsehole"

 

[parabola]

 

 

 

 

 

I came across this in one of my textbooks and though it is not entirely relevant, it proves two interesting points.

 

1. That heavier wheels do not make you go faster downhill (see the red box)

    

2. That the fastest way down a hill would be spherical wheels! It has to do with the way the rotating object stores its energy. A sphere converts 71% of its potential energy at the top of the hill into translational energy (the energy which makes it go foward). A hoop, which is the closet approximation to a bicycle wheel and the slowest of them all, converts only half of its energy into translational energy.

    

   This explains the point of deep section wheels to me, they convert more energy into translational energy than a normal bicycle wheel.

So to answer Woofies initial question about "would heavier rims make you go faster" and the answer is NEVER unless your pedalling stroke was an uneven atrocity. In which case it would be more beneficial having professional training to smooth out your pedalling motion.

 

 

parabola2009-11-27 12:14:08

[Johan Bornman]

A very nice way to describe and experience that for yourself is as follows.

 

Get onto your rotating office chair. Put an old-style laptop in each hand, bit don't forget to disconnect them first. Lift your feet, extend your arms and have someone twirl you around. You'll go at X-speed. Pull your arms in and you'll go at 2X speed. Extend your arms again and you'll slow down.

Pulling your arms in approximates the sphere, where the mass is close to the axle.  Puttin your arms out, approximates the situation with the weight at the circumference of the circle.

 Edit: In spite of this elegant experiment, remember that it has bugger all relevance in cycling where the mass of the wheel is insignificant in relation to the mass of your body and your bike.

Accellerating a bicycle wheel of average or superior absense of weight has ziltch effect on the overall accelleration of the bicycle or its average speed.

 

 

Johan Bornman2009-11-27 12:17:10

[leet22]

Hmm cant we have a yes or no answer

[Johan Bornman]

I have been wondering for some time now about how a heavy rim would work on a very flat course instead of using a light one.
Surely the heavier one will carry more momentum?

For instance on a TT bike where the profile is very flat. Wouldnt it be of an advantage then?

Have any of you DIY guys every made a deep section rim your self out of an old rim and carbon fibre or fibre glass?

How funny would that be' date=' rocking up at a TT race with a old lejuene with the down tube shifters moved to the TT bars and your homemade deep sections that used to be alex rims.
Funny enough, I think it would actually work as well and if the course is flat I think you could have some fun.
I actually have a old lejeune that is lying in my friends wendy house......
Do you think that is a worthwhile project?



[/quote']

 

 

Leet wants a Yes/No answer so here goes:

 

 

I have been wondering for some time now about how a heavy rim would work on a very flat course instead of using a light one.
Surely the heavier one will carry more momentum?

For instance on a TT bike where the profile is very flat. Wouldnt it be of an advantage then?

 

 

No it won't be of any advantage. You still have to store the momentum to start off with and will waste it by braking once you've finished your ride. Therefore you want a wheel that's as light as possible. However' date=' heavier wheels come with aerodynamic advantages which over longer distances, outweigh the disadvantage of weight.

 

So, if your heavy wheels are very aero and the distance is long, it will improve your time, otherwise not.

 

QUOTE=Woofie']

 

How funny would that be, rocking up at a TT race with a old lejuene with the down tube shifters moved to the TT bars and your homemade deep sections that used to be alex rims.
Funny enough, I think it would actually work as well and if the course is flat I think you could have some fun.
I actually have a old lejeune that is lying in my friends wendy house......
Do you think that is a worthwhile project?

 

It will  be funny and lots of people will make circular movements with their forefingers pointing to their heads. However it may or may not (sorry Leet, can't give you a definite here) be a worthwhile project. It all depends on how crazy you want to appear.

 

 

 

[bikemonster]

...This explains the point of deep section wheels to me' date=' they convert more energy into translational energy than a normal bicycle wheel.
So to answer Woofies initial question about "would heavier rims make you go faster" and the answer is NEVER unless your pedalling stroke was an uneven atrocity. [/quote']

 

Parabola, BS.  Was it you who got all sniffy and said that you're nearly a MSc a few posts back, or was that somebody else?  Me, I'm an economist by training, but I'm pretty sure I'm getting the physics right here.

 

Deep section wheels do not convert more energy blah blah blah.

 

Deep section wheels are AERO wheels.  Moost of this discussion has ignored aerodynamic effects.  Deep section wheels offer an advantage DESPITE their mass disadavantage, and the aero advantages are greater than the mass disadavantages.  Most of the discussion in this thread has ignored aero effects, but it is aero effects that have led to BS statements along the lines of "Heavy wheels are great for descending".  (Note, previous statement was intentional BS for illustrative purposes only.)

 

Pedal strokes are utterly irrelevant.  EVERYBODY's pedal stroke is an uneven atrocity because of the fundamental design of a 2-piston crank.  However smooth you think you pedal, you are delivering pulses of power to the rear wheel of your bike.  On top of that, unless you are riding a fixie, which will fee back momentum from the "flywheel" heavy wheel, even that non-existent benefit can't be gained.

 

Summary: Lighter is better than heavier.  ALWAYS!

 

James

 

[parabola]

 

 

...This explains the point of deep section wheels to me' date=' they convert more energy into translational energy than a normal bicycle wheel.

So to answer Woofies initial question about "would heavier rims make you go faster" and the answer is NEVER unless your pedalling stroke was an uneven atrocity. [/quote']

 

However smooth you think you pedal, you are delivering pulses of power to the rear wheel of your bike.  On top of that, unless you are riding a fixie, which will fee back momentum from the "flywheel" heavy wheel, even that non-existent benefit can't be gained.

 

Thanks for pointing that out, with a freewheel you would be pulsing power, I suppose I was thinking more along the lines of a fixie, but now that you mention this important point, that means the flywheel effect would make it redundant. So mythbusters make sure your heavy rimmed bike is a fixie!

 

 

This thread started as a disscussion and the ideas presented are for merely for that, I'm not stating facts but just trying to figure out an answer. Hell I even stated I've never even ridden a road bike, but I now know the purpose of deep section rims. I still stand by my point that a deep section rim would convert more energy into translational energy than a normal bicycle wheel, but it may be that the difference is negligible.

 

Anyway I will keep my crazy thoughts to myself and accept the fact that I suck at physics.

parabola2009-11-28 04:08:21

[mountain_lion]

 

Summary: Lighter is better than heavier.  ALWAYS!

 

<cut>

 

unless you are riding a fixie' date=' which will fee back momentum from the

"flywheel" heavy wheel, even that non-existent benefit can't be gained.

[/quote']

 

Which is what I have been saying from page one!

(although the way I put it was a bit confusing... )

 

[bikemonster]

Here is what this thread has needed from about halfway down pg 1...and the one thing we can all agree on:

 

 

 

http://languagelog.ldc.upenn.edu/myl/llog/duty_calls.png