Hydralic Road Brakes

John.T

diy wrote:

I'd like to see the evidence of brakes over heating. While its true that MTBs run larger discs, the wheels are smaller, the bikes heavier and demand is higher due to the steepness of descents and tightness of trails.

The bikes weight is a small proportion of the full weight bike and rider. The speeds are considerably higher and descents often longer. As the kinetic energy of a moving body increases with the square of the speed much more heat is produced by road bike brakes. I would be happy to use hydraulic discs on my winter bike but would not want them in the Alps for instance. Cable would be fine with the right pads.

desweller

John.T wrote:

Not sure if anyone has mentioned it yet (not read the whole thread) but the biggest down side for hydraulic discs on a road bike is heat build up causing the fluid to boil. This is more likely than on MTBs as the discs are smaller and the potential energy is much higher due to greater speeds. They don't help matters by fitting the front calliper behind the fork out of the wind.

Evidence? Bearing in mind that the same fluids are used in automotive braking systems.

You're far more likely to get good old brake fade before you can generate enough heat to boil brake oil.

philwint

fast as fupp wrote:

ok

my discs are set up ok on my mtb with one set of wheels (knobblies) i change wheels for slicks and lo and behold they drag like fupp.

imagine this scenario with multiple wheel changes in a road race?

Odd - are they the same hubs?

ugo.santalucia

DesWeller wrote:

John.T wrote:

Not sure if anyone has mentioned it yet (not read the whole thread) but the biggest down side for hydraulic discs on a road bike is heat build up causing the fluid to boil. This is more likely than on MTBs as the discs are smaller and the potential energy is much higher due to greater speeds. They don't help matters by fitting the front calliper behind the fork out of the wind.

Evidence? Bearing in mind that the same fluids are used in automotive braking systems.

You're far more likely to get good old brake fade before you can generate enough heat to boil brake oil.

That's what I always thought too... how come it's good for cars but not for bikes?

Strith

Because this is the internet

dodgy

Cars have vented, drilled and hollow discs. They're also much thicker and a much larger diameter, there's also 4 of them and a order of magnitude more fluid in the system. The effect being there's always cooler fluid mixing in from further away from the heat source.

There's definitely at least one documented case of a rider boiling brake fluid on a blog somewhere.

But these problems are surmountable, the case on the blog probably happened due to early prototypes where the engineer hasn't fully accounted for brake fade in the design. Even us lot on this forum know about the demands of a road bike compared with MTBs, I'm sure these horror stories will be quite rare now.

dodgy

PS Here's the blog posting I was going on about. http://www.bikerumor.com/2012/02/14/roa ... they-work/

John.T

ugo.santalucia wrote:

DesWeller wrote:

John.T wrote:

Not sure if anyone has mentioned it yet (not read the whole thread) but the biggest down side for hydraulic discs on a road bike is heat build up causing the fluid to boil. This is more likely than on MTBs as the discs are smaller and the potential energy is much higher due to greater speeds. They don't help matters by fitting the front calliper behind the fork out of the wind.

Evidence? Bearing in mind that the same fluids are used in automotive braking systems.

You're far more likely to get good old brake fade before you can generate enough heat to boil brake oil.

That's what I always thought too... how come it's good for cars but not for bikes?

Read dodgy's post and blog. The brake manufacturers all agree that fade and fluid boil are the 2 main problems. Better pad materials will eventually overcome the fade problem but this will probably make the fluid boil one worse.
Good braking technique will help just as much as it does with rim brakes (which are only a large disc brake really) but you can never stop some people dragging their brakes.

nicklouse

dodgy wrote:

PS Here's the blog posting I was going on about. http://www.bikerumor.com/2012/02/14/roa ... they-work/

reminds me of lots of Dutch cars in Yorkshire. crashing on one long hill.

fast as fupp

philwint wrote:

fast as fupp wrote:

ok

my discs are set up ok on my mtb with one set of wheels (knobblies) i change wheels for slicks and lo and behold they drag like fupp.

imagine this scenario with multiple wheel changes in a road race?

Odd - are they the same hubs?

all shimano

wheezee

You'd have to have everybody on board In a race. You wouldn't want half the riders being able to stop on a sixpence, while the rest can only slow down a bit, on a coin several metres in diameter.

nicklouse

fast as fupp wrote:

philwint wrote:

fast as fupp wrote:

ok

my discs are set up ok on my mtb with one set of wheels (knobblies) i change wheels for slicks and lo and behold they drag like fupp.

imagine this scenario with multiple wheel changes in a road race?

Odd - are they the same hubs?

all shimano

quite normal as hubs are used the position of the disc moves. adjust your caliper.

fast as fupp

i did. but i cant see team (insert team name here) relishing the prospect of stopping to adjust their calipers after wheel changes in the tdf etc.

diy

From the bikerumour article

As we began our descent, it took only a few seconds to get up to speed. Being my first time on this road, I kept light pressure on the levers, dragging my brakes to keep my speed around 30mph on a very curvy, steep road.

Coming around a corner, I pulled my brakes a little harder and the levers went to the bar. I held them there. I was still accelerating. In the span of one second, I realized that a) I had no brakes, b) if I kept descending it was only going to get worse, so c) I better crash now and minimize the damage...The brake fade that led to my accident is something I’ve experienced on my mountain bike on several occasions with multiple brands of brakes.

This says quite a lot about the cause of the crash:
- dragging your brakes at 30mph will cause even mtb brakes to boil (and I reckon a motorbikes brakes would fair no better). The rider has experience of cooking mtb brakes too, so you wonder if he actually knows how to brake properly.

Anyone with experience knows that you brake, release, brake, release or alternate front to rear to allow cooling. Second the brakes don't suddenly go, you feel them getting softer. I have done similar descents on my MTB on grass where you descend approx. 800ft in about 3/4 of a mile. You can hit 50mph with a little pedaling at the start. There is no way I would drag my brakes, even with 203/180mm rotors. You brake, release and brake again.

Looking at the rotor suggest that there may have even been glazing of the surface, which will also reduce effectiveness.

desweller

Maybe we'll get disc brake calipers with fins on! I don't know about you but to me that sounds feckin' cool.

ugo.santalucia

I have just fitted organic pads to replace the worn sintered ones... So much nicer, great feel and modulation, don't need hydraulics anymore... 8)