's Forum Archives - General

Archive Home >> General(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 )

i know, if i have to ask...(disc wheels)(14 posts)

i know, if i have to ask...(disc wheels)jp2
Nov 22, 2002 11:18 AM
here is my dilema. I have a zipp 950 disc. I hate tubulars(hate is a strong word, let's say dislike). I like to train on the wheels i am going to race on. I don't want to have to carry a tubular along with clincher supplies if I don't have too. Do I sell the zipp and get a H3D clincher, with the wieght penatly of a whole(sarcastic) 300 grams and therefore once a gain be tubular free, or do I stick with the status quo and not train on the wheels I will race. This is one of those, let's just throw some money around questions.
what do you dislike about tubulars? nmtrekkie1
Nov 22, 2002 11:32 AM
what do you dislike about tubulars? nmjp2
Nov 22, 2002 12:06 PM
glue, even if it is only once every 2-3 years. it gets messy when you don't practice.

having to carry a spare, they are not small even when folded up.

i have never found them to have the mystical feel so many others have found.

i have always found clinchers to be more than adequate in race situations.
what do you dislike about tubulars? nmJBergland
Nov 22, 2002 12:22 PM
"have never found them to have the mystical feel so many others have found.

have always found clinchers to be more than adequate in race situations."

Based on that alone, I'd suggest selling your current disc and buying a clincher disc.
re: i know, if i have to ask...(disc wheels)Lactate Junkie
Nov 22, 2002 1:32 PM
Personally I would keep the disk and only use it for racing. If for no other reason than it is too expensive a piece of equipement to wear out JRA. In the land of wheels 300g is a ton, 3/4 of a pound in one wheel!! On top of that the 3D is slower both aerodynamically and because it will be heavier.
Once you've spun it up, weight is weightKerry
Nov 22, 2002 3:06 PM
The weight of a wheel is no different than any other weight once it is spun up, and the energy to spin it up is minimal. If the guy wants to train on his racing wheel and doesn't ride tubulars, the question is answered. On flat roads, the penalty of weight is minimal.
Is that true?androssmazor
Nov 22, 2002 3:15 PM
Once a wheel is spinning..each part of the wheel is constantly changing the speed at which it is traveling. The part of the wheel that is touching the ground is not moving, and the top of the wheel is moving twice as fast as you are. I know this is obvious, but wouldnt it seem to suggest that the weight of a wheel is important even when the wheel is spinning because it's constantly be accelerated.
It is simple physicsKerry
Nov 22, 2002 3:45 PM
Since the wheel is rotating at constant speed, the kinetic energy of rotation is 1/2mV^2 (1/2 of the mass times the square of the rotational velocity). Likewise, the KE of the entire mass (bike, wheels, rider, etc.) moving down the road is 1/2mV^2 (in this case forward velocity). For a bike, the rotational velocity and the forward velocity are the same, so if you treat the wheels separately, their KE is mV^2 (2*1/2mV^2). Any given point on a rotating object is NOT "constantly be(ing) accelerated" but rather is in contstant motion. And before someone brings this up, any variations in speed (say from uneven pedaling) are canceled out. An increase in speed of a heavier wheel requires more energy, yes, but that energy is returned as the wheel is slowed down. Think of the wheels as flywheels - a heavier flywheel takes more power to spin up but the power to keep it moving is only friction and aero drag, there is no mass related term. And that heavier flywheel has more energy to give up as it slows down.
Wheel Design ?Scot_Gore
Nov 22, 2002 3:57 PM
Will a wheel that has more weight toward it's center spin up easier, and conversly return less energy and slow down faster when you coast? Since the further out the mass is, the higher velocity you have to bring it up to on the downstroke and the less it has to give back on the glide.

Makes sense to me. What do you think Kerry or others.

You've got itKerry
Nov 23, 2002 12:42 PM
Two wheels, of equal weight, the one with the lighter tire/rim combo will spin up with less energy. The physics I was quoting assumed a mass concentrated at the outer circumfrence. That said, much of the weight savings in wheels has been in lighter hubs, as there is only so much you can do to lighten a rim, particularly an aluminum rim. Hubs are much lighter than they used to be, and this accounts for a good fraction of the reduced weight we see advertised. Plus, wheel suppliers are now claiming that wheels don't need QR skewers - otherwise why would they advertise the weights minus the skewers? :) While the hub (plus cranks and pedals) are also rotating mass, their angular velocity is so low compared to the velocity of the tire/rim that it's not worth talking about in the big scheme of things.
You know nothing about physics!Alexx
Nov 23, 2002 7:40 AM
You are confusing linear and angular velocities. Unless you are accelerating a wheel, it's angular velocity is constant-and that's all that matters for torque. comparing this to linear velocity (with respect to the ground) is like comparing apples to oranges. In the case you mention, there is no angular acceleration, and no linear accelaration.
You mis-read my postKerry
Nov 23, 2002 1:04 PM
Kinetic energy is kinetic energy whether it is a result of linear or rotating motion and mass. If you go through the math, you can determine the power required to spin a wheel up (really the rim/tire weight) vs the power to simply bring that same mass up to a linear speed. You'll find those two power requirements to be the same (the acceleration case to which you refer). Once at speed, there is no change in KE, and therefore no power needed to keep the wheel spinning, and therefore there is no difference if a mass is rotating or not. That is what I said. Once the bike is up to speed, the power to move it down the road is a function of friction, total mass, and wind drag. It makes no difference whether some of that total mass is rotating or not, which is just what I said and just what you said. But thanks for that nice post title.
Training on a disc?JS
Nov 23, 2002 9:39 PM
For what possible reason?
Nov 24, 2002 2:13 PM
Theres a number of reasons behind this:
1) You look like a poser.
2) You look like a poser.
3) You look like a poser.
4) A crash will be very costly and make you look like a poser.
5) You look like you try to pay money to go fast.

Just kidding. I don't believe in people being posers but the thing is aero wheels are a lot of money and don't handle well in cross winds. So if you hit a pothole the wrong way, crash, get hit by a car, flat and not stop in time its gonna be very costly. Then because they don't handle well in cross winds increases your chances of crashing. So you should train just enough to be comfortable on them... say the week before a race. Other than that hang them up on the wall and use them as eye candy.