Disc brakes road bikes

Discuss light weight issues concerning road bikes & parts.
Epic-o
Posts: 621
Joined: Tue Nov 20, 2007 7:18 pm

by Epic-o

RollingGoat wrote:....


I have witten the equilibrium equations and I'm right. You can see the calculations here:

http://cds-0.blogspot.fr/2012/03/rim-an ... tical.html

by Weenie


RollingGoat
Shop Wrench
Posts: 284
Joined: Thu Feb 26, 2009 1:18 am
Location: Boston

by RollingGoat

You're missing the component of the braking force perpendicular to the seatstay for the disk brake.
I'm so rubbish at this.

Epic-o
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Joined: Tue Nov 20, 2007 7:18 pm

by Epic-o

The angle between the speed direction of the disc in the point of contact with the pads and the seatstays is very small. Anyway, I have made the calculation for that case too and the moment M is Mbrake(L1+L2(cos(alpha)-1))/(L1+L2) with alpha the angle between the speed direction of the disc and the seatstays, so even smaller than for the case with the disc force acting parallel to the seatstays.

Are you convinced now?

lcoolb
Posts: 313
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Location: Jakarta, Indonesia / The Hague, Holland

by lcoolb

Guys no offence but can you hold this discussion with PM's, please?

RollingGoat
Shop Wrench
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Location: Boston

by RollingGoat

No, because a) you're missing reaction forces for both cases and b) the speed direction of the disk is very nearly perpendicular to most seatstays, and is usually completely perpendicular for the chainstays.
There's a professor at my school who's written a few papers on bicycle dynamics. I'll email him and see what he thinks.
I'm so rubbish at this.


bones
Posts: 414
Joined: Sun Aug 23, 2009 10:38 am

by bones

This is actually a very, very good thing. 2012 is already 25% over now. Nobody even expected the UCI to consider discs this year. However, the article states that the UCI equipment commission will review disc brake usage for road competition this summer for possible approval as early as 2013!

:D

arbitrage
Posts: 15
Joined: Fri Aug 20, 2010 3:49 pm

by arbitrage

My apologies if this has already been covered, but I was thinking about this during today's ride:

We've discussed how disc brakes shift the braking force from the rim to the hub and how rims may get heavier/need more spokes. But how does this force compare to the power put through the hub during a sprint? A pro sprinter will pump out some serious power through the hub which is then carried through the spokes to the rim. In a sense, this is the opposite of slowing with a disc brake. How do these forces compare in magnitude?

I have no engineering background, but would love to hear what you guys think.

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ergott
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by ergott

Braking far more powerful than any sprint.

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slyboots
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Location: Russia, Moscow

by slyboots

First, pedaling force comes through rear wheel which currently has more spokes to share the load than the front one.
Second, pedaling force is several times less than what can come from braking. Just compare what time/distance you'll have to travel to speed yourself up from 0 to say 40 km/h with how fast you can slow down from 40 km/h to full stop by braking.

edit: it's obvious that when you speed up you work against the air and when you slow down it helps you, so it's not a perfect example, but it's still holds true since the difference will be far larger than what can be accounted to aerodynamics...

Epic-o
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Joined: Tue Nov 20, 2007 7:18 pm

by Epic-o

Emergency braking of a 90kg rider is 100Nm of torque in the disc fixing points
Chris Hoy sprinting with a 52/13 ratio puts 150Nm of torque in the freehub pawls

For a strong amateur cyclist, the sprinting torque in the freehub pawls would be 70-80Nm

arbitrage
Posts: 15
Joined: Fri Aug 20, 2010 3:49 pm

by arbitrage

Cool--thanks for the details. Out of curiosity, where did you find these numbers?

So it's possible that rear wheel lacing won't have to change considerably. Front wheels will definitely need some beefing up, but maybe only to a level found on rear wheels today.

Epic-o
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by Epic-o

arbitrage wrote:Cool--thanks for the details. Out of curiosity, where did you find these numbers?


Own calculations

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Berk
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by Berk

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BeeBee30
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by BeeBee30

OMG I want one of those :shock:
Ti or dye!

The Weenie formally known as CAADHEAD

by Weenie


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