14

I heard from many sources (some of them on this site, e.g. on answers to this question here) that you should be alternating the use of your front and rear brakes. Most people give as a reason that the brakes can cool better that way.

I have been mostly doing the same, but I never really understood if it actually helps and/or if there is a good explanation for it.

The way I see it: when descending we transform all our potential energy (at some altitude) first to kinetic energy, then to heat in the brake pads/rims/disks.

Does it actually matter if we first heat up the front brake a bit more, then the rear brake, then front again etc. compared to just braking evenly with both of them?

  • 3
    I've always tended to alternate for safety reasons. If you begin to loose braking power you'll notice it sooner, and will likely have sufficient braking left to come to a complete stop. – Daniel R Hicks Jun 14 '17 at 0:21
16

I looked at modelling this and it's surprisingly complicated. There are quite a lot of time constants and heat flows to consider. If I had a bunch of logging non-contact thermometers I'd consider an experiment.

In practical terms you might want to think about the failure modes if both brakes are working equally hard: If you need to slow down more but both sets of pads/rims/discs/brake fluid are already hot you have less in reserve than if one is cooling while the other is heating -- that last bit of braking might be enough to tip you over the edge.

For a finite length of descent, getting one brake hot enough to lose significant amounts of heat early in the process can help, but heat transfer is linearly proportional to temperature difference, which will counteract the effect of having one brake hot early. The front brake will get more airflow and so cool faster than the back brake; but if you need to stop you'll need the front brake to work well (and predictably), so running downhill with the front brake lightly on probably isn't a good plan.

For the same total heat input per brake, if the peak temperature of the braking surface is higher it will lose heat into the air faster, potentially (and this is the hard part to model) reducing the transfer to the rest of the system (tubes for rim brakes, fluid for hydraulic discs). In addition this can only increase the heat lost to air from the working face of the pads as a hot pad touching a hot rim can't give up heat, but a hot pad close to a hot rim with air blowing though the gap can. This is more applicable to rim brakes as the airflow over the pads in a disc sysyem is tiny -- clearances are smaller and there's often a mechanism blocking the airflow. Most pads have poor thermal conductivity meaning little or no heat out the back (sintered metal disc brake pads are an exception).

  • 2
    I like the explanation about alternating brakes being more failsafe. – fgysin Jun 13 '17 at 11:16
  • Nice answer. I also considered modelling this and gave up after realising how unknown constants there are! Do you have a reference for the claim about poor conductivity in brake pads? I guess this means that Shimano's finned pads aren't that useful? – Will Vousden Jun 13 '17 at 15:04
  • @WillVousden they're sintered metal pads - the probable exception to my "most" pads. Rim brake pads are basically rubber, which has a thermal conductivity of ~0.2 W/m.K or plastic (similar TC so we can assume the same for organic disc pads). The thermal conductivity of aluminium is 200 W/m.K; even steel is 50 W/m.K. Little technical information on sintered pad compositions is given, but figures I've seen for sintered copper range from 2 to 50 W/m.K The low end is 10x that of organics, and the high end >100x; fins on sintered metal pads are probably quite useful though not a game-changer. – Chris H Jun 13 '17 at 15:14
  • 2
    @fgysin similarly on short sharp urban descents (I live in a hilly city) I tend mainly to use the back brake for not going too fast (and use of that timed to the features of the road) and keep the front for actual stopping. Weight well back of course. – Chris H Jun 13 '17 at 15:17
  • You're missing the other part of this. Heat melting the glue on tubular tires (where the advice originated) or blowing up the tube in clinchers from the heat. – tpg2114 Jun 14 '17 at 2:26
6

Braking with both brakes at the same will make the braking more effective and shorter, leaving plenty of time in between for the system to cool down.

Brakes should be applied for a short period only at maximum intensity until your speed is reduced to the desired level.

Having brakes applied and dragging over long periods will keep speed down but only at the expense of considerable temperature rise of the disk, the rim and if it applies of the brake fluid. There will be a risk of failure if the brake fluid reaches boiling temperatures. This is true with any vehicle.

  • 1
    If you go in with a certain amount of kinetic+potential energy, and come out with a certain amount of kinetic+potential energy, the difference has gone to heat in the brakes (or tyres+road if you lock the wheels, which would be bad). Braking early increases the amount of time for heat to dissipate. Braking at maximum intensity makes assumptions about the road surface that are probably unwise -- certainly some of my trickiest (road) descents have been prone to dusty surfaces or wet leaves, or of course the dreaded oily patch. – Chris H Jun 13 '17 at 15:23
  • 1
    When you go faster, you lose more energy to air resistance and there is less left for heating the brakes. – ojs Jun 13 '17 at 17:36
  • 2
    @ojs air braking is a very good point, and yet another variable to consider. We start having to consider what factors limit speed at which points in the descent – Chris H Jun 13 '17 at 17:55
1

On paved roads (road bike, rim wheels) I always brake front and rear simultaneously and repeatedly. This allows a better braking by having a stronger braking (front and rear) and allowing the rims to cool down. Especially with carbon rims or on alpine roads this is actually obligatory. Without the time to cool down for the rims the brake pads will be too hot and not be braking any more and the rims will be also to hot and may led to some tube explosion (latex) or rims damage (carbon). Braking simultaneously and therefore having a stronger braking allows you to be faster before braking and to have a "late" braking.

This is my 25 years experience of biking (and racing) on all types of roads in Europe (including the Alps etc...)

0

Heat and temperature are related but not the same.

It takes a pretty long decent to over heat brakes.

By pumping the brakes more heat is released to the air. Even on disc brakes a small gap is way bigger than no gap. It does not take much air flow to carry off the heat. On a hot day even the slightest breeze makes a difference. Radiant heat will find its way out with no air flow.

Under harder braking you will generate less heat as it wears the brake down more and that absorbs some energy.

As the brake gets hot it has less friction so more of the kinetic energy goes to heat then temperature.

Alternating the brakes allows you to pump the brakes and maintain a constant speed. You could pump both at the same time for equal cooling.

A slower decent will have less temperature because the brakes have more time to cool. And the heat is generated at a lower rate. There might be window were medium braking is more than light but if you slow it down then you can pretty much always control temperature. I loaded truck will creep down a long hill.

If you feel a brake starting to fade (less friction) stop and let it cool off.

On rim brakes you can heat up tires.

On disc brakes you can boil the brake fluid. Naturally a bigger disc will have more cooling.

I will use the rear on a longer decent as the primary to have some reserve in the front.

Don't go into an aero position so more air drag.

  • Wow down vote I am not complaining but I have a degree in chemical engineering. We study heat and also aerodynamics as need that for fluidized catalytic beds. I was not making it up. – paparazzo Jun 18 '17 at 20:47
0

As a kid, my family used to go on cycle touring holidays around Wales. Once, on a very long downhill, several miles, my front tire exploded from overheating - from rim brakes, obviously.

Don't know if this is a common occurrence, but it might explain the alternating thing.

We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

  • It exploded from overheating, and the alternating thing allows one brake to cool rather than dragging them all the time, which increases the temperature. Alternating means your off-brake will be off and cooling not dragging. – Criggie Jun 17 '17 at 5:03
-2

Whether you use both brakes evenly or alternate, you have no idea whether one of them is getting hotter than the other.

You cannot control a variable that isn't monitored.

If you had a display showing brake temperature, then the best way to brake would be whatever manner keeps both brakes at the same temperature. This would be almost certainly be achievable with a multitude of braking patterns, alternating and not.

  • That would only be best for temperature management, not necessarily overall, as other factors come into play. – Chris H Jun 13 '17 at 15:24
  • @ChrisH That's what the question appears to be about. I'm answering it. People believe that some protocol for braking will manage the temperature better, but without any basis (they have never monitored the temperature while braking). – Kaz Jun 13 '17 at 15:38
  • 1
    That's not quite my reading of the question -- I see it as about more than just temperature management. But you make a good point and it wasn't my downvote. I've just ordered a very cheap IR thermometer with a view towards an initial test. – Chris H Jun 13 '17 at 15:46
  • @ChrisH Basically, if we keep the bike at around a certain speed of descent, that corresponds to a certain wattage of heat being dissipated by the brakes: energy is mgh (mass, gravitational constant, height). Divide by time, we get intensity. No matter how we do the braking, if we maintain that speed we get the same dissipation. If we keep the brakes at a constant temperature and equal between them, that seems ideal. If they are identical hardware (same surface area, material), they dissipate the same then. – Kaz Jun 13 '17 at 20:59
  • Identical hardware in non-identical airflow won't behave the same, but otherwise you're right. That's very similar to my comment under Carel's answer but I only consider mgh+0.5mv² at the start and end while your approach should cancel out the effects of air braking. – Chris H Jun 13 '17 at 21:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.