why clincher more pricey???

[Jules]

I've been following this thread with the interest of a racing cyclist trying to get the last possible ounce (or joule) of (legal) performance out of my machine.

 

 

 

I have a question for the gurus. We have been talking about the effect a weight saving on the rim has on performance while accelerating. And the conclusion is that it does make a difference. But what about riding at a constant speed?

 

 

 

It seems logical to me that even while riding at a constant speed, a point on the rim is constantly accelerating and decelerating as it turns. It'll be going much faster than the bike when it passes the brakes and it'll be doing 0kmh for a split second when it touches the tar.

 

 

 

With this in mind, I ask whether a weight saving at the rim will be beneficial for a cyclist riding at a constant speed.

[Christie]

A lighter rim will need more energy to ride at a constant speed, if you add up all the decimals.

[Tumbleweed]

 

I drink Windhoek Lite.

 

?

 

i thought you drank beer?

[Wyatt Earp]

. The spoke holes are chamfered' date=' not square.[/quote']

 

Now that's a new one, square spoke holes ???

 

JB when are you opening your wheel company, you could out do all the big boys

 

Slowbee

 

Well done on your desert dash.

[jmaccelari]

 

 

 

 

 

I have a question for the gurus. We have been talking about the effect a weight saving on the rim has on performance while accelerating. And the conclusion is that it does make a difference. But what about riding at a constant speed?

It makes no (zero) difference at a constant speed.

 

It seems logical to me that even while riding at a constant speed' date=' a point on the rim is constantly accelerating and decelerating as it turns.

[/quote']

Again we turn to the reference frame (look at the post on the centrifugal

force). A constantly turning wheel is not accelerating in the rotational

reference frame. No acceleration = no force = no energy. If the wheel

were accelerating in the rotational reference frame the velocity at the

rim would be increasing and thus so would the bike's speed.

 

 It'll be going much faster than the bike when it passes the brakes and it'll be doing 0kmh for a split second when it touches the tar.

 

 

The outer perimeter of the tyre has to do exactly the same speed as the

bike when it touches the tar' date=' or else it would slip since it would be

going either faster or slower than the bike. In normal riding, no slippage

occurs so it must be travelling at the bike's translational velocity.

 

 

Also the parts of the wheel at the rim have to all travel at the same

 

speed or the wheel would be ripping itself apart (connected bits travelling

 

at different speeds do not stay connected for long). This is true for each

component at each radial distance (the angular velocity of all the bits

everywhere in the wheel is the same, or the wheel rips apart).

 

 

 

(The "confusion" here is that the reference frame you have chosen is the

 

relative to tar one. It is correct that the relative velocity against the tar of

the tyre at contact is zero, but it is twice the speed at the top - at the

brakes. However, we cannot practically use this reference frame for

investigating the rotational kinematics of the wheel. We could if we

wanted to, but the equations would be horrifically complex - even to JB)

 

(A way to illustrate this is that if we take a point on the rim, there is

always a point directly opposite it travelling in the opposite direction,

so the acceleration of these these two points as we see it in the

cartesian reference frame "cancel" each other out - this is where the

centrifugal and centripetal "forces" - known for this very reason as the

"fictional forces" - come into play)

 

(Mathematically, we say the angular velocity of the wheel at the contact

point - tar - is:

omega = v(trans) / r(contact) * sin(theta)

Where in this case theta is the tangential angle (90deg), so

sin(theta) is 1. You will note that when we have v(trans) we have

omega. At no stage does this change and we have zero or twice the

speed unless we are accelerating or decelerating the entire wheel in

one piece)

 

 With this in mind, I ask whether a weight saving at the rim will be beneficial for a cyclist riding at a constant speed.

None. Zip. Zero. Nada.

 

All energy at a constant speed goes into overcoming wind resistance,

friction and potential energy (as well as powering your body).

 

 

 

An aside:

 

To do a "Bornmanism", I will ask - what is "beneficial"? To some it is

an energy saving, but we have seen at least one person who is

prepared to go for heavier wheels and use more energy, because

he prefers the rolling smoothness (inertia) of his heavier wheels. There

is a level of trade off and personal preference that comes into play.

 

Of course, we would all notice a 1kg wheelset (Zipp 202s) versus a

3kg wheelset (el cheapo Pick 'n' Pay brand). But when we get to 200g,

other factors become relatively more important.

 

My tuppence.

jmaccelari2010-01-10 00:24:50

[jmaccelari]

 

 

 

 

A lighter rim will need more energy to ride at a constant speed' date=' if you add up all the decimals.[/quote']

Answered to Jules question.

 

To re-iterate:

Unless there is a force applied (i.e. some acceleration), no energy

is input into the system.

 

So, if we have an (ideal) wheel weighing 2kg and one weighing 1kg,

the 2kg wheel will take twice the energy to spin up to the same

speed as the 1kg wheel. As soon as both wheels are spinning at

that speed (and we have no friction or air resistance) they will both

continue to spin at that speed until we apply a force (braking or

more acceleration).

 

So, in fact, a lighter rim uses LESS energy - not more to accelerate up

to a certain speed.*

 

Once at that speed, both wheels will also use the SAME energy to stay

at that constant speed - zero (ignoring air resistance and friction).

 

Newtons First Law says a body in a state of constant motion will stay

at that speed unless a force ("acceleration") is applied. Consequently

if the body is moving at a constant speed, there is no force applied and

no energy being input into the system (if we ignore friction and air

resistance) - this is a consequence of Newtons Second Law: a body

can only accelerate if a net force is applied.

 

*We have shown and debated ad nauseum that this difference is small

and may be imperceptible**. But as we know, little bits all add up.

 

**The original statement was not whether it was perceptible, but

whether there is a greater energy saving by taking rotational weight off

the wheel than off the frame, to which the answer is a resounding "YES".

It may be small, but there is ALWAYS a saving - no matter what wheel

JB is holding in his hand.

jmaccelari2010-01-09 23:21:41

[jmaccelari]

 

 

 

 

. The spoke holes are chamfered' date=' not square.[/quote']

 

Now that's a new one, square spoke holes ???

 

I think that Johan means the profile is not square, i.e. it's not

just a simple drilled hole but one that has been bevelled to some

extent to round the corners (or allow countersinking). The hole itself

is still round!

 

BTW, bits do exists to drill square (and pentagonal, hexagonal and other

shaped) holes!!!

 

http://demonstrations.wolfram.com/DrillingASquareHole/

 

They're based on something called a Reuleaux triangle...

jmaccelari2010-01-10 02:59:20

[Wyatt Earp]

Jmac

 

 

 

Quite aware of holes in different shapes and sizes

 

It's always an issue of semantics for JB, so my bit of sarcasm and also just saying that I think this thread was sown up a page or two back.

[jmaccelari]

 

Jmac

 

 

 

Quite aware of holes in different shapes and sizes

 

It's always an issue of semantics for JB' date=' so my bit of sarcasm and also just saying that I think this thread was sown up a page or two back.[/quote']

Went straight over my head, that one!

 

Busted!!!

 

[Johan Bornman]

Sorry about the delay. ....

 

More irrelevant sarcasm deleted.

 

OK. I assumed 36 (18 of each) spokes' date=' since this was not specified. I also
assume nipples (36) on the inside of the rim.

 

[/quote']

 

Don't assume. I said ask and yo didn't ask that question. It is a 32-spoke wheel.

 

You didn't give me the
tyre/tube weight' date=' so I measured one of my own (0.280kg).

 

[/quote']

 

My omission, I'll go with the assumption.  But you have to ask if it is a wire bead or Kevlar bead, tubbie or clincher.

(Note that I don't have the exact geometry of the hub' date=' so I calculated
the maximum and minimum values, which are both so small we can
actually ignore the hub entirely. The value given is a ratio approximation
from measurements off one of my Shimano hubs.)

[/quote']

 

Exact geometry - my point exactly. You didn't ask. Do ask, because it is important. I've only answered what you asked.

 

I(spokes) is about 5.53x10-3 kg/m^2
You were given I=m*L^2 / 3

 

Nice guess' date=' but you overlook the fact that the spokes are longer than the internal radius of the rim minus the outer radius of the hub. This implies what?

 

Ask.

 

I(nipples) is about 12.531x10-3 kg/m^2
You were given I=m*r^2

 

Oop! You did it again.

 

Your son assumed that the nipples are perfect little tubes. They are in fact real nipples that are lillipop shaped. Not only their shape affects their weight bias but also the large blank bore on the small side.

 

You need some more data points. Ask.

 

I(rim) is about 197.759x10-3 kg/m^2
You were given I=0.5*m*(r1^2 + r2^2)

 

 

I(tyre) is about 114.849x10-3 kg/m^2
You were given I=0.5*m*(r1^2 + r2^2)

Is this a tubbie or a clincher? You need another few data points here methinks.

 

 

Total is about 330.677x10-3 kg/m^2. To get this we simply "integrate".
This means in plain' date=' simple English that we add all the little bits up...

[/quote']

 

I think you're adding this up too quickly. Wait for the other data points.

If you need more info' date=' I'm here, armed with tape measure  scale and baited breath.

[/quote']
Surely you mean "bated"? "Baited" could be a little smelly.

 

English is not my first language but I'm used to this type of ridicule. I imagine lots of my African countrymen are subject to similar sarcasm.

 

 

[epoh]

 

 

 

[Tumbleweed]

Sorry about the delay. ....

 

?

 

More irrelevant sarcasm deleted.

 

?

 

shame. why? didn't get yours in first?

 

 

 

edited for grammar. post read like my first language wasn't english either. johan, how the hell did you help the independent group compile a style guide - like you told me you did - if your grip on eeeeenglish isn't so good?

 

 

 

*apologies for the smilies in a serious tech fourm. am blaming myprodol holy roller2010-01-10 09:03:03

[GoLefty!!]

actually the term is "baited breath" or simply "baited" implying that you have nibbled on something used to lure you into a debate, fight, skirmish etc etc.

 

 

But we're getting into semantics here and that is detracting from the intention of the thread.

We reached a conclusion, so perhaps it is time to lock the thread and move on.

 

debating for debating's sake serves for nothing more than penis waving. I don't know where you will go to settle that one as gravity and MOI of a penis can vary greatly depending on the state of excitement, height above sea level, air pressure, water quality etc

 

[jmaccelari]

 

Carp ... crap ... crap

Johan - how can a guy like you be so dense?

 

I gave you the figure. Give me any measurements and I'll generate

another number. Use any number (or humungous, complex JB-ish

hand holding numbers) you like. The physics ain't gonna change, boy.

 

Now matter how much you'd like it to.

 

I didn't formulate the equations. Some guys a lot more intelligent

(and less childish) than you did. Are you now going to prove Newton

wrong?

 

What is your point? You're now just arguing for the sake of arguing.

 

You were wrong to start off with. You blew your mouth off at Kiwi

without understanding the issue and you're making a complete **** of

yourself...

 

Again - what is your point?

 

[Tumbleweed]

actually the term is "baited breath" or simply "baited" implying that you have nibbled on something used to lure you into a debate' date=' fight, skirmish etc etc.But we're getting into semantics here and that is detracting from the intention of the thread.[/quote']

 

 

 

actually it isn't, http://www.phrases.org.uk/meanings/bated-breath.htmlbut you're right, we digress...

[Johan Bornman]

Carp ... crap ... crap

Johan - how can a guy like you be so dense?

I gave you the figure. Give me any measurements and I'll generate
another number. Use any number (or humungous' date=' complex JB-ish
hand holding numbers) you like. The physics ain't gonna change, boy.

Now matter how much you'd like it to.

I didn't formulate the equations. Some guys a lot more intelligent
(and less childish) than you did. Are you now going to prove Newton
wrong?

What is your point? You're now just arguing for the sake of arguing.

You were wrong to start off with. You blew your mouth off at Kiwi
without understanding the issue and you're making a complete **** of
yourself...

Again - what is your point?
[/quote']

 

My point shall reveal itself, don't you worry about that. Just do your calculations. What other measurements can I make for you? As you said yourself, you don't need thousands of data points to do this properly. Lets complete the job.