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FixedGearFever - :: View topic - Disc vs deep dish wheel
i really disliked rear disc. but i move the bike around a lot when out of the saddle, and i didnt like the way the disc fought with me. Stifnes, i think goes to disc, but not really noticable.
my bos rates rear disc as the BEST... horse for courses mate. And deeps can be trued
whats it for?stiffness of that level will only really matter on big bankings and standing starts with BIG hp riders. never used or ridden hed stuff, but the stuff i have worked with has seen poor manufacturing tolerances.
zipp have addressed some of their major issues and seem a lot less prone to cracking on the tyre bed now. built as a 24 rear with tied and soldered 2 cross, awesome all rounder. Just make sure you get the new rims if you head that way, as i stated the old rims cracked on the tyre seat just by looking at them!!
Joined: Sep 19, 2004 Posts: 159 Location: wales Home Track:
Posted: Sat Jul 25, 2009 7:44 am Post subject:
i,ve been using front and rear mavic io copys like them alot front wheel with tub is 1280 grams and rear with tub and cog 1480grams yea ther a little heavy on the scales but feel really stiff on the bike and do not notice the extra weight
I actually don't know about the weights or how the weight is distributed on the zipp or hed wheels but I can contribute a little physics knowledge, knowing that having weight at the outer and of a ring(or rim) will cause a lot more momentum, so it is both positive and negative, it will be both hard to accelerate and decelerate, a disk with equal weight will be easier to accel and deccel but will not carry speed as well. the other thing is the weight difference does have to be pretty significant for the momentum to be the same.
using:
I = 1/2 m r^2: for a disk and
I = 1/2m(r^2 +R^2) for a ring where r is the inner radius and R is the outer radius
If you look at the second one, both radii are squared, added together then multiplied by them mass where as the disk is just the mass multiplied by the one radius squared. that isn't going to give you a number in watts or anything of the sort but it will give you a ration that you can compare to each other.
In reality it depends on what you want, if need to accelerate and decelerate a lot I suggest the lower momentum (I) if you want the constant speed I suggest the higher one.
Weight distribution in the wheel does hardly matter. The "rotating mass" myth is just a myth.
Whatever the weight of your wheels, you're always able to accelarte them without effort in a fraction of a second from 0 to e.g. 45 km/h: position the crank horizontal, place your foot on the pedal, press the foot down and lift the rear wheel - the rear wheel will spin very fast in just a quarter crank rotation.
As long as you have reasonable rim and tyre weights, it just doesn't matter. And for the scientists among us, the're invited to do the math and calculate what it means when a rim is e.g. 100 g heavier. In another forum, someone calculated it and found out, that 100 g additional rim weight is similar to having a frame that is 190 g heavier (when accelerating, after acceleration, 100 g additional rim weight is like 100 g additional frame weight).
This means, as long as you're lucky enough not to have rims that are 1 kg heavier, it probably won't matter (as it doesn't really matter whether your frame is a few 100 g heavier).
To continue with the physics presented by goliath.
Moment of inertia is, for angular work, like MASS for regular force and acceleration.
It's a measure of how difficult it is to get a certain shape rotating about an axis.
Torque is the force at a distance that is trying to make the object spin with an angular acceleration.
Using Goliath's equations, and a mass, M of 1, and an outer radius R of 1 for the disk, while for the deep dish, we'll use an inner radius of 1/2 (0.5) and an outer radius of 1.
So, for disk, M = 1/2 x 1 x (1²) = 1/2 = 0.5
For the deep dish, M = 1/2 x 1 x (.25 + 1) = 0.75
We assume that the mass is distributed evenly in each.
The above shows that in this simplistic case, the deep dish wheel that weighs as much as the disk will take 50% more torque to turn it at the same angular acceleration.
Now if we change this slightly to a more realistic case where the dish weighs 3/4 of the disk, the two moments of inertia are almost the same and both wheels would take the same torque (force) to move them to the same acceleration.
The interesting thing is that the limiting case of the narrowest deep dish gives a physical result that is not intuitive.
For a very narrow dish, having THE SAME MASS AS THE DISK (not ever likely), the narrow dish will have double the moment of inertia over the disk. Now, more realistically, the narrow rim having a mass of 1/4 of the disk will have half the moment of inertia of the disk, and so on.
We'll leave the aerodynamics to real physicists. _________________ And if we'd get up off our knees,
Why then we'd see the forest for the trees.
Too Much Math Guys,
I've had problems with my 36 spoke training wheel, (coming out of true, breaking spokes), and took it to Jim Obrien in Concord, NC. When I told him my problem he said "Let's build you one like we used to build Marty Northstein". In other words, just use larger gauge spokes.
He built it and I just finished a hard training day at Dick Lane. It felt as stiff as my Zipp 950 disk. And I could not tell the difference with acceleration. It's just a Fuji rim and a regular 36 hole high flange hub.
Recently I also read through the following website with has much very good data and conclusions. At the end of the website they have a data table where they measured various wheel stiffness. Check it out.
While the Damon Renard lateral stiffness tests have lots of good stuff in them, I wonder how much any of that applies to modern composite wheels made in 2009. His site hasn't been updated since 2001 if we can believe the date on the web page. _________________ And if we'd get up off our knees,
Why then we'd see the forest for the trees.
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