Disc brakes road bikes

Discuss light weight issues concerning road bikes & parts.

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

by RollingGoat

adriano wrote:huge pic.

http://i.minus.com/i0NF23fnLCQ28.jpg" onclick="window.open(this.href);return false;



I'd stay away from WW rotors on the road. Did you see the bikerumor article? TRP and Volagi are blaming it on the use of WW rotors that don't dissipate heat well enough.
I'm so rubbish at this.

Illuminate
Posts: 404
Joined: Mon May 24, 2010 4:18 am
Location: Australia

by Illuminate

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


Own calculations


Can you please post these?

Thanks,
I

User avatar
adriano
Posts: 83
Joined: Tue Jun 02, 2009 5:37 am

by adriano

i havent had problems, but the most i can descend around here is 400 feet. as cool as these are, im not opposed to going back to th stock bb7 rotors or some dh tested stuff.

Epic-o
Posts: 621
Joined: Tue Nov 20, 2007 7:18 pm

by Epic-o

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


Own calculations


Can you please post these?

Thanks,
I


Pretty simple

Without considering wind drag and assuming constant wheel angular speed, emergency braking (around 0.6g) results in 540N in the contact between the ground and the tire (F=m·a). Considering that the weight distribution is 70-30 and the wheel radius is 0.35m, the braking moment is M=F·w·Radius=540·0.7·0.35=110Nm

For the standing start, a 600Nm peak torque in the crankarms is 600·(drivetrain ratio)=600·13/52=150Nm in the freehub pawls

User avatar
slyboots
Posts: 445
Joined: Sun May 10, 2009 3:31 pm
Location: Russia, Moscow

by slyboots

I disagree with your calculations.

1. Braking.

Emergency braking means there is no such thing as weight distribution - around the tipping point at 0.6g all the weight is on front wheel, so it means torque is 540*0.35=189 Nm

2. Pedaling
When was the last time when you saw Chris Hoy doing a full gas standing start on a 24-spoked road carbon wheel with a freehub? A 36-spoke track wheel with a fixed gear is more like it...

So for a road wheel a more extreme real-life scenario is climbing a very steep short hill, let's take a 20% one for example. Let's assume the same 90 kg rider (+7 kg bike) who is putting out 700 Watts during this short effort. He is going up at 3.57 m/s, wheel circumference is 2.1 m, so wheel angular velocity is Va=2*pi*3.57/2.1=10.676 and torque is 700/Va=66 Nm

So, braking torque is roughly 3 times higher than pedaling torque.

airwise
Posts: 1049
Joined: Tue Jul 27, 2010 8:31 pm

by airwise

Fascinating maths. Now I'm 70 kg. I averaged 60 nm up a 13 minute climb last week with peaks over 80 nm. I didn't get anywhere near 700 w. when I do I find it easy to break traction on the rear wheel Much harder to do so under braking. Somehow this doesn't seem to fit in with the maths. Ideas?

User avatar
slyboots
Posts: 445
Joined: Sun May 10, 2009 3:31 pm
Location: Russia, Moscow

by slyboots

You must be talking torque at cranks, not rear wheel.

Epic-o
Posts: 621
Joined: Tue Nov 20, 2007 7:18 pm

by Epic-o

slyboots wrote:I disagree with your calculations.

1. Braking.

Emergency braking means there is no such thing as weight distribution - around the tipping point at 0.6g all the weight is on front wheel, so it means torque is 540*0.35=189 Nm

2. Pedaling
When was the last time when you saw Chris Hoy doing a full gas standing start on a 24-spoked road carbon wheel with a freehub? A 36-spoke track wheel with a fixed gear is more like it...

So for a road wheel a more extreme real-life scenario is climbing a very steep short hill, let's take a 20% one for example. Let's assume the same 90 kg rider (+7 kg bike) who is putting out 700 Watts during this short effort. He is going up at 3.57 m/s, wheel circumference is 2.1 m, so wheel angular velocity is Va=2*pi*3.57/2.1=10.676 and torque is 700/Va=66 Nm

So, braking torque is roughly 3 times higher than pedaling tortue.



I have considered that both brakes were actuated during the emergency braking, intelligent riders doesn't brake only with the front caliper. Your assumption of all the weight on front wheel depends of many factors (COG height, inertia and the braking moment in each wheel) so I wouldn't say that my calculations are wrong.

Chris Hoy numbers are an extreme example, obviously he's not the average cyclist

You have also to consider that pedalling loads are continuous and braking ones aren't. Anyway, some simplified calculations to give a first order of magnitude

airwise
Posts: 1049
Joined: Tue Jul 27, 2010 8:31 pm

by airwise

Ah yes I am. But I recall my friend measuring around 10% less if that with similar peaks using a Powertap. He weighs less than 60 kg

User avatar
slyboots
Posts: 445
Joined: Sun May 10, 2009 3:31 pm
Location: Russia, Moscow

by slyboots

Epic-o wrote:I have considered that both brakes were actuated during the emergency braking, intelligent riders doesn't brake only with the front caliper. Your assumption of all the weight on front wheel depends of many factors (COG height, inertia and the braking moment in each wheel) so I wouldn't say that my calculations are wrong.


Well, you can brake with both brakes, but emergency braking = braking at or very close to tipping point = all weight is on front wheel. And you can't really brake at 0.6g with 70/30 weight distribution on a road bike. Maybe if you go all the way behind the saddle, but I still doubt it.

BTW, these are just some real life examples of the loads. I don't know torque limit manufacturers design their road wheels around. Maybe they can withstand 150 Nm, maybe not.

User avatar
slyboots
Posts: 445
Joined: Sun May 10, 2009 3:31 pm
Location: Russia, Moscow

by slyboots

airwise wrote:Ah yes I am. But I recall my friend measuring around 10% less if that with similar peaks using a Powertap. He weighs less than 60 kg

Using 67 kg (rider+bike), same 20% slope and 300 Watts you'll have torque of 45 Nm at rear wheel. So, yeah, kind of hard to believe in 60 Nm average.
Just so you know, at 60 Nm at pedals and 70 rpm cadence, one is producing 440 Watts irregardless of his weight.

airwise
Posts: 1049
Joined: Tue Jul 27, 2010 8:31 pm

by airwise

I could send you the files. Cadence was around 40 as the average gradient was 17% with pitches at 30%. It's the Bastille at Grenoble. Gearing was 36 x 25.

User avatar
slyboots
Posts: 445
Joined: Sun May 10, 2009 3:31 pm
Location: Russia, Moscow

by slyboots

airwise wrote:Cadence was around 40

It changes things considerably: 60 Nm and 40 rpm equals 250 Watts, which is quite reasonable.
And with 36x25 torque at rear wheel was 42 Nm.

PS. climbing at 40 rpm for 13 minutes - you must be tough. :thumbup:

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

by bones

Let's see. The UCI has already been secretly approached by SRAM who secretly have a fully functioning road hydraulic system already developed. Oh and the UCI is going to informally review this summer disc brake usage for road cycling competition starting as early as 2013. Shimano, at least, have a functioning cable-actuated road disc brake. No word on if they are secretly developing a road hydraulic disc brake system. Umm, let's see. Campagnolo has eschewed disc brakes, and of course they would. Since they don't develop any MTB components, they don't even have a MTB disc brake system. So, they are not in a good position.

Frame manufacturers are already incorporating disc brakes into future models. Specialized has a road disc brake road bike, secretly of course. This should be introduced as part of their 2013 ine. I guess they weren't too happy with Volagio beating them to this market given that the Volagio founders were past Specialized employee nobodies.

2013 is going to mark the beginning of the end of caliper brakes on road bikes.

Post Reply
  • Similar Topics
    Replies
    Views
    Last post