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PostPosted: Thu Jun 06, 2013 1:33 pm 
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Location: Lancaster, UK
I already run discs on my commuter and take it from me, the amount of braking power you can generate on a skinny road tyre is a lot more than people think. People keep saying that the tyre contact patch is the limiting factor, in their arguments against the need for discs, but this really is a red herring and should be dismissed. Discs are about modulation, power and consistency. On any technical descent they are significantly faster (and safer) than rim brakes. If you don't want to try/test discs that's fine. I for one, will be making the switch on the 'good' bike asap.


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PostPosted: Thu Jun 06, 2013 2:11 pm 
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so trying to summarize:

+Wider contact patches to allow for better application of Braking Forces to the ground without skidding. <---- good thought, esp because these early disc systems are generally going to be a little heavy (especially for this group), might as well embrace that weight with a sturdier, more versatile build.

+Weight reduction. <--- I think this will take Alchemy, Tune, Extralight or somebody like that to start building disc hubs. Right now were dealing with MTB hubs. also:

Has anyone messed with the brakepads that guys on Kickstarter was pushing a while back? Silicon/ Carbon Composite or something of that effect? Supposed to we way lighter and have virtually zero heat buildup?

It would be interesting to see if those:

A) work

B) can be made affordably enough to be adopted by the big names which will pour in the R&D $$$...

C) can downplay the fears of getting cut to death by a rotor in a crash (they are supposed to be pretty pliable if force is applied in ways other than braking)


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Posted: Thu Jun 06, 2013 2:11 pm 


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PostPosted: Thu Jun 06, 2013 3:25 pm 
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Location: Netherlands
I'd need to say that they are here to stay, and not just a temporal trend. I'd need them to be standar from about 4-5 years, not gonna buy them just released. I'd need them to be optimised for road frames, and frames and forks optimized to the, I'd need them to have light carbon clincher rims, which is kind of complicated, and i'd need them to have an overall bike weight not too higher than rim brakes, which won't happen i think, so i will not ever have discs in a road bike i think. time will tell. I'd need to already have a di2 groupset, as that would be my share of not needed things to still buy... just like discs.

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PostPosted: Thu Jun 06, 2013 4:41 pm 
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Location: NoVA/DC
Regarding the SICC rotors and pads, the stopping power was a let-down for most. There are ways to fix that, check out HuckingKitty's forum for that, awesome stuff there.

Sent from my EVO using Tapatalk 4 Beta


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PostPosted: Thu Jun 06, 2013 5:01 pm 
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thisisatest wrote:
Regarding the SICC rotors and pads, the stopping power was a let-down for most. There are ways to fix that, check out HuckingKitty's forum for that, awesome stuff there.

Sent from my EVO using Tapatalk 4 Beta



Thanks for that! If the concept is shown viable, I'm pretty sure some big name can run with it and create a rotor/ pad material combination that works alot better. kinda like carbon rims/ braking advances over the past several years.


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PostPosted: Thu Jun 06, 2013 5:06 pm 
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cula_ru wrote:
hey. Moment of Inertia has a square factor on the formula so check your numbers now. ;)


I already have. Have you? I didn't think so.

It is true that rotational inertia of mass at the periphery of wheel is proportional to the square of the radius: I = m x R^2. But you can't compare translational inertia to rotational inertia, but it uses different units. You have to calculate the equivalent inertia, knowing how the translational speed is related to the rotational speed. You can do this with the kinetic energy equation.

The total energy of motion of a spinning wheel is its translational kinetic energy (Kt = 1/2 x m x V^2) plus its rotational kinetic energy (Kr = 1/2 x I x w^2)

Thus, total kinetic energy:

K = Kt + Kr = [1/2 x m x V^2] + [1/2 x I x w^2] = [1/2 x m x V^2] + [1/2 x (m x R^2) x w^2] = 1/2 x m x [V^2 + R^2 x w^2]

But if the wheel rotates without slipping on the ground, the rotational speed w will be equal to the translation velocity V divided by the radius (w = V/R). Substituting this in give:

K = 1/2 x m x [V^2 + R^2 x (V/R)^2] = 1/2 m x [V^2 + R^2 x (V^2)/(R^2)] = 1/2 x m x [ V^2 + V^2] = 1/2 x m x [ 2 x V^2]

Rearranging, this gives:

K = 1/2 x (2 x m) x V^2

In other words, the equivalent inertia of the mass at the periphery of a rotating wheel is twice its mass.

Mass at a smaller radius has an even smaller affect on inertia.


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PostPosted: Thu Jun 06, 2013 6:29 pm 
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thisisatest wrote:
I meant power, so I typed power.
...
As far as people dragging their brakes all the way down a mountain, that is why thermal mass and ventilation/dissipation are the big issues, not power.


I don't think you understand the what power is, because the rate at which the thermal energy from braking is absorbed and/or dissipated is power. The energy dissipation of brakes is measured in Watts, just like pedaling power. Brake power is the rate at which kinetic energy of motion is converted/dissipated as heat. If a brake does not have a sufficient power rating for the length of braking, it will overheat, and that is an apparent concern for disc brake makers in attempting to develop road-specific disc brakes.

Here is an article from Bicycle Retailer And Industry News about the Magura's concern about disc brake power/heat, and why they think present designs might not be ready for road bikes yet:

http://www.bicycleretailer.com/product-tech/2013/01/03/mag-road-disc-brake-safety-focus-new-magura-study#.UbCvIxfD-9I

Here's a technical article from Velonews, also discussing the technical issues that have not been fully addressed for road disc brakes (including power/heat):

http://velonews.competitor.com/2012/08/bikes-and-tech/technical-faq/technical-faq-throwing-ice-water-on-road-disc-brakes_235280


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PostPosted: Thu Jun 06, 2013 6:36 pm 
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mjduct wrote:
so trying to summarize:

+Wider contact patches to allow for better application of Braking Forces to the ground without skidding. <---- good thought, esp because these early disc systems are generally going to be a little heavy (especially for this group), might as well embrace that weight with a sturdier, more versatile build.


Wider contact patches won't help increasing braking rates. The limit of bicycle braking is not tire traction, it is the tip-over point (the braking rate at which the rear tire lifts off the ground, and the bike starts to go end-over-end). Bikes have relatively high centers of gravity and short wheelbases, so the tip-over point is only in the range of 0.5 - 0.6 g, whereas tire traction limit is closer to 1.0 g.

Try this - riding in a straight line on smooth, flat, dry pavement, hit your front brake hard, until either: 1) the front tire skids; or 2) the rear tire lifts off the ground. I've done this test many times, and I've never gotten the front tire to skid, but I've gotten the rear tire off the ground many times.


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PostPosted: Fri Jun 07, 2013 10:08 am 
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I think the heat issue is a bit of a misnomer, with much speculation.

Here are a couple of more recent articles that discuss the fluid diferences from the big manufacturers

http://www.bikerumor.com/2013/04/11/tech-speak-brake-fluid-break-down-and-implications-for-road-disc-updated/

The shimano rep (Nick Murdick, Lead Multi Service Technician) actually gives a quite detailed answer re heat in the brakes.



And here is another showing some of the detail development that SRAM did with their road disc group

http://www.bikerumor.com/2013/04/15/sram-hydro-r-hydraulic-road-rim-disc-brakes-unveiled-details-first-rides/

I run a 180mm disc on the front and a 160mm rear, not for more braking, but better modulation and greater safety margin if i ever do those alpine decents like Stelvio etc. The truth is though, I've never locked a front brake, even in the rain, sliced anyone up, been able to brake so hard in a pack that I cause accidents etc.

Except for some minor gripes (a small amount of weight and some aero :roll: issues), they are simply better, wet or dry.

But if they are not for you, then so be it. But at least try :beerchug:


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PostPosted: Fri Jun 07, 2013 10:33 am 
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Joined: Tue Jul 27, 2010 8:31 pm
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I've descended this twice on a cross bike fitted with discs without encountering fade or any problems, speed topping out at 87kph.

Image

If you can find more of a challenge then I'd be interested. The fears seem to be completely unfounded IME.


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PostPosted: Fri Jun 07, 2013 11:38 am 
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Joined: Tue Apr 22, 2008 4:31 am
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Looks fun to go down - what is the layout like ? Would be a challenge to go up !!!!


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PostPosted: Fri Jun 07, 2013 1:10 pm 
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I had to climb it to go down - but certain pitches are really too much when you get above 2600m. It's a great ride though and I was massively impressed with the performance of the discs.


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PostPosted: Fri Jun 07, 2013 3:14 pm 
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WOW, nice ride. The bike rumor guys who blew up his hybrid cable/hydraulic setup did about everything wrong that you could do. I haven't heard of similar experiences from riders who have decent setups and decent technique.


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PostPosted: Fri Jun 07, 2013 4:17 pm 
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Posts: 473
You just buy some chromoly tubed bike, TIG some tabs on that shizzle and let the good times roll

Image
2013-3-20_18.30.33 by Quattrings, on Flickr

Of course it's not as efficient as a proper road bike but it's just fun to ride, easy on the eyes and a doodle to keep clean.

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Posted: Fri Jun 07, 2013 4:17 pm 


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PostPosted: Fri Jun 07, 2013 5:04 pm 
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Posts: 91
MichaelB wrote:
I think the heat issue is a bit of a misnomer, with much speculation.

Here are a couple of more recent articles that discuss the fluid diferences from the big manufacturers

http://www.bikerumor.com/2013/04/11/tech-speak-brake-fluid-break-down-and-implications-for-road-disc-updated/

The shimano rep (Nick Murdick, Lead Multi Service Technician) actually gives a quite detailed answer re heat in the brakes.



One issue raised with disc brakes is that the are heavier than rim brakes. It has often been claimed that road bicycle disc brakes can be smaller and lighter than MTB disc brakes, because it is claimed that the demands of braking are less on a road bike than an MTB. But in the article referenced above, the rep. from Magura points out that the brakes on road motorcycles use larger (and heavier) rotors and calipers than do off-road motorcycles. I suspect the same will be true for road bicycles as well, if they are built to take the power that long, steep descent sometimes demand.

Although there are several anecdotes here from riders who have used road disc brakes on long steep descents with success, that doesn't necessarily prove that they are ready for general use. Manufacturers have to consider the entire range of potential users before they release a product; even if many customers are only 150 lb, the bikes still have to be made to withstand the 250 lb. riders who may buy the same bike - including those 250 lb. riders who may drag their brakes all the way down the steep descents.


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