Downhill braking

IanEdward wrote:

Quote:

As an aside, my regular riding buddy is a fast decender, and recently swapped  back to his rim braked, carbon rimmed Colnago, after a couple of years on a disk braked Roubaix.

We did the Rococorba recently and I was glad of my rim brakes, very little scope for letting brakes off to cool and it's consistently steep in the middle. I don't know if I'm 'fast' but I do like to play with the road, taking racing lines where possible etc.

For long steep descents with a lot of braking I'm not sure I'm sold on hydraulics yet, I'd rather brake how and when I want to rather than have my braking style dictated by the needs of my brakes (seems a bit arse-about-tit if you ask me). To me, gentle feathering and dragging seems more appropriate for road bikes rather than pulsing or braking hard and late.

That being said, I was beginning to wonder about heat build up in my aluminium rims, given that I was running latex tubes, but I believe that's more of an issue with carbon rims?

From the odd person who I've met in Tenerife who were using carbon wheels, all swapped out to aluminium wheels from the hire centre due to concerns about excessive heat in the rim.

I would employ hard on off braking on my motorcycle coming down a mountain pass. Dragging brakes compounds the heart build up. Hydraulic brakes, sintered pads allow for heavy braking, rapid deceleration. Pulsing brakes is a smooth but firm braking, then release and repeat, its not a ABS style pulsing. Coming down a mountain can take upto an hour with lots of hairpin bends to contend with speeds easily reaching 50mph if you like to descend quickly

Boatsie wrote:

Short hard braking then releasing and repeat is a lot less system wear plus the reduced chance of brake fade. My point of view is from truck yet the same physics is such with bicycles. I've had brake loss and once brakes heat beyond friction point they simply disappear and to me such was surprisingly fast. I don't have much experience with bicycle steep descents with braking requirements because most of my descents had been a torqueless top cog absent of tight corners nor need to slow. But pulse braking reduces wear and tear of the braking system according to the monitored trucks I drive and their maintenance schedules as to the best of my knowledge, I'm only a driver that usually does much less driving and just loads them in. Be careful if dragging because once beyond friction temperature range you'll be accelerating.

that's were ceramic rims and pads come into their own, you can brake hard over and over from high speed with not much if any 'fade'.

Robert Hardy wrote:

It seems to me that continuous light breaking controlling the speed of descent to a near constant will cause fewer problems than occasional heavy breaking, The energy that is dissipated ultimately as heat will be the same in both cases, but heavy breaking will result in much higher brake component temperatures during the period of braking, whilst continuous breaking will result in a lower equilibrium brake component temperature to be established.

That will actually create more heat. Hard short braking will work much better and produce less heat. It give the rotor fluids time to cool

Mybike wrote:

Robert Hardy wrote:

It seems to me that continuous light breaking controlling the speed of descent to a near constant will cause fewer problems than occasional heavy breaking, The energy that is dissipated ultimately as heat will be the same in both cases, but heavy breaking will result in much higher brake component temperatures during the period of braking, whilst continuous breaking will result in a lower equilibrium brake component temperature to be established.

That will actually create more heat. Hard short braking will work much better and produce less heat. It give the rotor fluids time to cool

A lot of truth in what Robert says.
The total amount of heat generated will be equal to the total amount of gravitational potential energy, and that will be a linear function of mass x height (as in altitude) differential. Whether you add it up in large but less frequent dollops, or eke it out in one continuous stream, it's the same mass x the same height differential.

However, that all ignores wind and other resistance, which is a non-linear function of speed. So cadence braking, by having intervals of higher speed, should dissipate more energy in resistance losses leaving less for the brakes. At the limit, you would not be braking at all and all the energy loss would be wind/rolling resistance, but death might overtake you first.

Then you have to consider how the heat flows. It's generated at the frictional surfaces, and flows from there. The trick is to keep it away from heat-sensitive things, like the brake fluid, by dissipating it to the air. I'm guessing that with cadence braking the rotors and pads reach a higher peak temperature, causing them to dissipate heat at a faster rate, and then continuing to cool during the rest periods. So they act as a heat sink that is continuously refreshed before each successive injection of heat, so that less heat ever reaches the brake fluid. But I'm yet to convince myself.

Could make for a good test with two matched riders on the same descent, one dragging the brakes, the other using cadence braking to achieve the same average speed, the bikes rigged with sensors and data capture. Any engineering students out there?

Sriracha wrote:

Mybike wrote:

Robert Hardy wrote:

It seems to me that continuous light breaking controlling the speed of descent to a near constant will cause fewer problems than occasional heavy breaking, The energy that is dissipated ultimately as heat will be the same in both cases, but heavy breaking will result in much higher brake component temperatures during the period of braking, whilst continuous breaking will result in a lower equilibrium brake component temperature to be established.

That will actually create more heat. Hard short braking will work much better and produce less heat. It give the rotor fluids time to cool

The total amount of heat generated will be equal to the total amount of gravitational potential energy, and that will be a linear function of mass x height (as in altitude) differential. Whether you add it up in large but less frequent dollops, or eke it out in one continuous stream, it's the same mass x the same height differential. However, that all ignores wind and other resistance, which is a non-linear function of speed. So cadence braking, by having intervals of higher speed, should dissipate more energy in resistance losses leaving less for the brakes. At the limit, you would not be braking at all and all the energy loss would be wind/rolling resistance, but death might overtake you first. Then you have to consider how the heat flows. It's generated at the frictional surfaces, and flows from there. The trick is to keep it away from heat-sensitive things, like the brake fluid, by dissipating it to the air. I'm guessing that with cadence braking the rotors and pads reach a higher peak temperature, causing them to dissipate heat at a faster rate, and then continuing to cool during the rest periods. So they act as a heat sink that is continuously refreshed before each successive injection of heat, so that less heat ever reaches the brake fluid. But I'm yet to convince myself. Could make for a good test with two matched riders on the same descent, one dragging the brakes, the other using cadence braking to achieve the same average speed, the bikes rigged with sensors and data capture. Any engineering students out there?

Is that like how a cup of tea will cool quicker if you wait for it to cool down first and then add milk rather than adding milk and then waiting for it to cool?

Mybike wrote:

Robert Hardy wrote:

, The energy that is dissipated ultimately as heat will be the same in both cases, but heavy breaking will result in much higher brake component temperatures during the period of braking, whilst continuous breaking will result in a lower equilibrium brake component temperature to be established.

That will actually create more heat. Hard short braking will work much better and produce less heat. It give the rotor fluids time to cool

Mybike, how would it create 'more' heat to drag the brake? You need to show your working here. I've read the cycling weekly article where someone makes such an assertion but without data.

Ignoring differences in aerodynamic drag, the amount of heat generated is the same. The temperatures reached will vary based on how effectively the disc dissipates that heat. The heat loss will be more rapid at a greater temperature difference between disc and surrounding air, but it isn't obvious from that which is a better braking strategy.

Dragging a brake can be done on a rear and front brake, whereas sharp braking tends to be more powerful on the front. So dragging gives you more disc to dissipate the same amount of heat. 
 

If you want graphs, there are some here, but it doesn't resolve the question. We should also consider why if matters- if you have clean fresh brake fluid and properly sized rotors your brakes should survive the descent.

https://res.mdpi.com/d_attachment/proceedings/proceedings-02-00215/artic...

Drinfinity wrote:

Mybike wrote:

Robert Hardy wrote:

, The energy that is dissipated ultimately as heat will be the same in both cases, but heavy breaking will result in much higher brake component temperatures during the period of braking, whilst continuous breaking will result in a lower equilibrium brake component temperature to be established.

That will actually create more heat. Hard short braking will work much better and produce less heat. It give the rotor fluids time to cool

 

Mybike, how would it create 'more' heat to drag the brake? You need to show your working here. I've read the cycling weekly article where someone makes such an assertion but without data.

Ignoring differences in aerodynamic drag, the amount of heat generated is the same. The temperatures reached will vary based on how effectively the disc dissipates that heat. The heat loss will be more rapid at a greater temperature difference between disc and surrounding air, but it isn't obvious from that which is a better braking strategy.

Dragging a brake can be done on a rear and front brake, whereas sharp braking tends to be more powerful on the front. So dragging gives you more disc to dissipate the same amount of heat. 
 

If you want graphs, there are some here, but it doesn't resolve the question. We should also consider why if matters- if you have clean fresh brake fluid and properly sized rotors your brakes should survive the descent.

https://res.mdpi.com/d_attachment/proceedings/proceedings-02-00215/artic...

 

Dragging the brake will. Always create heat when you off the brakes it gives the fluid rotor time to cool. Sure it will maybe reach a higher temp for a short while but it will also cool down when release Unlike dragging the brake which will just build up higher temps and not cool down
https://en.m.wikipedia.org/wiki/Brake_fade

CXR94Di2 wrote:

Hydraulic brakes far superior. I tend to brake hard then release to let brakes cool and brake again. Hydraulic brakes allow good feel and modulation of braking effort. If you hold and drag the brakes everything will begin to cook. I upgraded to Hope 4pot calipers, with longer pads. I use this technique for descents like off Mt. Teide, Ventoux etc. Also dont let speed get too high to start with, so shedding speed is easier

One of the most ignored and/or under appreciated aspects of descending (when you're not racing) safely, it's easier to control and gives you more thinking time from a lower speed, if you know you've got some steep sections coming up or maybe simply a hairpin then reducing/getting your speed under control BEFORE you reach 'terminal velocity' is pretty damn important. Sitting up on the tops can also help to reduce speed rather than being in the dops, even opening a jersey/jacket, not going to be huge but it all adds up when you want to keep the speed down.

Also picking the right line as best you can (taking other traffic into account) gives you more control/time to brake, scrubbing off your speed in panic because your line is wrong/going too fast/tyres losing traction under braking (which can happen in the wet easily even with carbon wheels and rim brakes) too often doesn't end well.

Some riders and pretty much all bog standard drivers find it quite hard to go wide out to take turns/bends but having that more open angle means you can see further, be apply the brakes for that smidge longer (than taking the inside line) and less chance of going straight on if you do get it wrong.

I've come down in the alps on std ally's and ceramic rim brakes, the latter are brilliant if you do have a lot of high speed braking points over a relatively shorter distance, do they do ceramic disc brakes for bikes?