why clincher more pricey???

[Johan Bornman]

He we go' date=' I cut the relevant bit out for you:

 

Cut cut cut cut

The best answer is to ride the two types of wheels of the same profile (full carbon clincher and alloy/carbon clincher) back to back so that you have just the rim weight to compare as that was what the original question was.

 

I have the full carbon clincher and can get the same with alloy rim to compare (I also have the same wheel with the tubbie version of the rim if someone wants to compare that) then you just need a bike with a power meter and go and do a race and see for your self.

 

I am sure when teams like CSC, Rabobank and Astana are looking at equipment that is what they do and also why they dont ride heavy wheels.

[/quote']

 

You havent cut out the relevant bits.

 

You are back to perceptions. I've showed you what the percentage difference is and if you insist that you can feel it, well so be it. We can't agree on that.

 

It is no use bringing in the fact that professional teams use this or that wheel. We're looking at the physics here. Nobody said weight doesn't matter. Weight is weight and my point is that because bikes accellerate so slowly, it doesn't matter where that weight is.

 

 

[Jules]

 

 

JB, your calculation does not factor in the difference in speeds of the nipples and I can prove it.

 

 

 

Let?s assume that we have two bikes. One weighs 91,6kg. The other weighs 91,2kg. We accelerate them to 30kph.

 

 

 

Let?s calculate their kinetic energy as per your formula:

 

 

 

Bike 1: 1/2 x 91,6 x 30squared   = 41 220

 

Bike 2: 1/2 x 91,2 x 30squared = 41 040

 

 

 

The difference? 180 units of energy or 0,43 percent.

 

 

 

Sound familiar?

 

 

 

You have agreed that where the nipple is will affect how much energy is required to accelerate a wheel.

 

 

 

However, I have just shown that your calculation does not take the difference of speeds of the nipples. It only factors in the difference in weight.

 

 

 

Again I ask you to repeat your calculation assuming wheels of the same weight, but with nipples at the perimeter and at the centre.

 

 

 

The weight difference of 400g is a red herring!

 

 

[CScriba]

wow and here i thought it was just gonna be a simple little answer to a quick question haha.... so now the outcome...is it better to spend the extra 2k on full carbon clinch or go for ones with alu side walls?? ( without any big arguments ) http://smileys.smileycentral.com/cat/36/36_6_2.gif

[Jules]

I still think go for alu. Unless you're in the TDF or you really have money to burn, it's not worth the extra cash.

[CScriba]

ja...il rather spend the money saved on a nice set of tyres or sumtn haha

[Thug]

I think that Johan's going to have to bring his abacus/scientific calculator with him on Sunday and explain in detail as we're cycling up an 8% gradient ..... 

[Christie]

Agree 100% with JB's position. "Saving mass at the rim" is a big cycling BS story - probably thought out by some marketer trying to sell light rims in the distant past.

 

The 0 - 30km/h acceleration the calc is based on is MASSIVE in cycling terms, Boonen & Petacchi top out at 70km/h in a sprint, so a 30km/h acceleration from 40 to 70km/h would only happen once in a race. Most accelerations would be smaller.

 

Had a good laugh at Edmund Bourke's quote, hehe. His PhD was not for engineering, thats for sure

 

Mass is mass, does not matter where you take it off, because the acceleration on a bicycle is so small. The pros ride over huge mountains - less mass anywhere off the bike is good if you have got 2 or 3 climbs of 20km long to race over.

 

Kiwi, you posted the full force equation. If you substiture values into this equation for wheels of different Ir (rotational inertia) but the same mass, and calculate acceleration, it will show that Ir  has a very very small effect on everything. 

 

edit: Also not stated, is that Ir only appears in terms of equations related to acceleration. When riding at a constant speed, like what happens in most races, (even up a steep hills) lower rotational mass does not make you faster at all. To the contrary, an argument could be made that more mass at the rim would help preserve momentum better, (think flywheel) and actualy require less energy (probably a tiny tiny bit less if you do the sum) during a race ending on the same place it started, compared to the extra expense on the 20 or so  accelerations.

 

I'm waiting for someone to bring up another marketer's favourite - micro accelerations

Christie2008-11-28 11:42:06

[GIRR]

Would it be possible using those % energy saving values to show what the difference in watts would be in accelerating to those speeds for the different wheels.  Those % energy values confuse me a bit and don't really hold any meaning for me that I can compare something to.

[Kiwi]

Edmund R. Burke Ph.D

 

Much like his mentors and associates, Dr. Burke gained worldwide renown. Professionals in the fields of exercise physiology, nutrition and medicine are familiar with his exceptional work ethic and reputation as an author, scientist and educator. His 39 page curriculum vitae speaks for itself as it reveals some of his outstanding career accomplishments, including over 1,000 articles and many book chapters and books that he wrote and published in the areas of health, sport science and applied physiology. He served as Coordinator of Sports Sciences for the U. S. Cycling Team leading up to the Olympic Games in 1996 and was a staff member for the 1980 and 1984 Olympic Cycling Teams.

 

Some of his books: http://www.pickabook.co.uk/typesearch.aspx?Type=authorisbn&Code=9781583331460&Key=Edmund%20R.%20Burke

 

Also:

 

"Kiwi, you posted the full force equation. If you substiture values into this equation for wheels of different Ir (rotational inertia) but the same mass, and calculate acceleration, it will show that Ir  has a very very small effect on everything. "

 

Different wheels with different Ir are different because their mass is different. Looking at the table I posted you can see a lighter wheel has a lower Ir and a heavier wheel such as the mountain bike wheel at the other end of the table has a higher Ir.

 

Rotational Inertia and Mass for Various Wheels

 

 

Wheel	Details	Ic (kg m^2)	Mass (gm)
Wire Spoke	Rear, Std Rim, 700, track, 36 spokes, w/o tire, w/ axle, nuts	0.0528	1177
Wire Spoke	Front, Std Rim, 700, 32 spokes, w/ tire, tube, rim strip, axle, skewer	0.0885	1264
Wire Spoke	Rear, Std Rim, 700, 32 spokes, w/12-21 cassette, tire, tube, rim strip, axle, skewer	0.0967	1804
Specialized tri-spoke	Front, 700, w/ tire, tube, axle, skewer	0.0904	1346
Specialized tri-spoke	Rear, 700, w/ 12-21 cassette, tire, tube, axle, skewer	0.1032	1771
Specialized tri-spoke	Front, 650, w/ tire, tube, axle, skewer	0.0683	1207
Mavic	Front, Std Rim, 650, 28 Bladed Spokes, w/ tire, tube, rim strip, axle, skewer	0.0632	1179
MTB	Front, 32 Spokes, w/ tire, tube, rim strip, axle, skewer	0.1504	1847

I'm not suggesting anyone should believe me, this is something very easy to find out for yourself. Strap on some wheels in the 1800 gram range, do a race, then strap on something in the 1400 gram range and do a race and you can feel the difference. As I suggested do the above with a power meter and you can measure the difference.

[Kiwi]

Would it be possible using those % energy saving values to show what the difference in watts would be in accelerating to those speeds for the different wheels.  Those % energy values confuse me a bit and don't really hold any meaning for me that I can compare something to.

 

Yes, exactly. That's what I am saying!

[Christie]

Ir (rotational inertia) is influenced by mass and the distance from the center of rotation

 

but:

 

Ir makes very little difference on a bike, because the acceleration is small and only a very small % of the ride.

 

If you had 2 sets of wheels, of the same mass, but with differt Ir (like nipples at the hub vs. nipples at the rim) the difference would be so small it won't mean anything.

 

I do not doubt that Ed Bourke has a lot of knowledge, but if he was not joking when he made the statement you posted, it is obvious that that knowledge is not about physics or engineering. The statement is complete BS, according to the equation you posted 

[Kiwi]

I think we have lost track of what the original question was: <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

just a quick Q...why are most carbon clinchers so much more expensive than the tubbies??? i want clinchers but the price differences between them and tubbies get me wondrin if it worth it gettin tubbies rather than clinch...anyone know why exactly??

 

And I was say it is because they are more complicated to make and weigh less. I'm saying the 100 grams saved on each wheel at the rim (carbon rim with alloy braking surface vs. full carbon rim and braking surface) is significant and will make a difference to your ride.

 

I totally agree that the weight savings of lighter or fewer nipples or nipples moved to the hub end of the spoke is negligible.

 

 

Kiwi2008-11-28 12:20:36

[Christie]

Easy calculate average power: P(ave)=dW/dt

So if the units of energy in JB's extreme calc of accelerating 30km/h in 30 seconds is Joule, the average power over the 30s for bike A is 1350w [edit] 1373w, and for bike B (with 200 grams less on each wheel) is 1368w.

Sorry, my bad, took 40500 for bike A, instead of 40500+720

 

Christie2008-11-28 21:17:23

[linnega]

 

Easy calculate average power: P(ave)=dW/dt

So if the units of energy in JB's extreme calc of accelerating 30km/h in 30 seconds is Joule' date=' the average power over the 30s for bike A is 1350w, and for bike B (with 200 grams less on each wheel) is 1368w.

[/quote']

Isn't it the other way round - heavier bike needs 18w more to get the same acceleration over 30s?  Sounds a small but noticeable number over hours, but with maybe 10-15 minutes maximum accelerating over a 2.5 hour race, it is neglible.

 

[Kiwi]

So that acceleration was done using 18w less. OK, most accelerations wont be as strong as that but over a 100km race think about how many accelerations a rider does, over the tops of hills, closing gaps, matching the pace of the riders around them etc. <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

How many watts or kilojoules less would the rider on the lighter wheels expend? If you have a power meter it would make some interesting data.

 

If you look at the races where you haven't made it across the crucial gap or you just couldn't quite come around a rider in the sprint to get that podium placing etc. and you think about what if you had those extra joules of energy still in your legs? I'm saying that?s where something like lighter rims could seal the deal.

 

[linnega]

For me this is entirely academic as not making a gap is 99% in my head when I even have the ability to physical make the juncture.  Most of the time I couldn't be bothered to stick my head in the wind let alone try and come around someone.

 

If you assume that top end energy expenditure is worth say 3x more than your average, expending 1.3% (18w / 1368w) more energy for 5% (10min/150min) of a ride is a difference in energy expenditure over the entire ride of 0.25%.  Still looks pretty neglible, and your success or lack thereof is far more likely to be determined by other factors (if you fart hard enough you may get enough additional acceleration to gain the missing 18w!).

 

I know where you are coming from Kiwi - it may once in a lifetime make that small difference for an athlete competing for top honours.  Chances are though that it won't for even top athletes and for the majority of us the chances are even slimmer.  That is why I can't understand spending much more than R2000 on a set of wheels, and you have an excessive number of people buying wheels for more than my bike cost including two sets of wheels.