Steep hill -- short hard braking or drag the brakes

[ShinySideUp]

The only answer I've been able to find that answers this question relates to motorised vehicles using low gears with engine braking, something that doesn't apply to bicycles.

I often go down steep hills and always wonder whether to drag the brakes all the way down or to pull the brakes hard and let them off to cool. If I start at a certain speed at the top of the hill and end up with the same speed at the bottom then theoretically exactly the same amount of energy has to have been dissipated but in practice might this be different. I tend to favour the short, hard braking method but can't really explain why.

Any physicists here?

 

[RebornRider]

This is something I started wondering about after moving to Denver Colorado and driving in the Rocky Mountains. Some of the long highway descents were steep enough that shifting to a lower gear (manual transmission) wouldn't hold a steady speed. The brake lights of the cars ahead of me showed that many drivers were applying the brakes all the way down. You could smell cooked pads! Like you, I preferred to let the speed rise until any more would be too fast for conditions, then use maybe 50% effort braking to get a bit below the speed limit. Repeat as needed.

But I've never seen a well-controlled test comparing the two methods for brake temperature and brake component life.

 

[LJR69]

No brakes, trust in gravity and your will to live, scream ‘Geronimo!‘ all the way down





message sent from Royal Berkshire Intensive Care Ward

 

[Mikerb]

In a car using a lower gear plus intermittent braking is the best plan. In an auto...if its smart....it does that automatically.
On a bike intermittent hard braking using both brakes is better than dragging the brakes because it is too easy to overheat the rotor/pads.

 

[Slapbassmunky]

The wannabe professionals and keyboard warriors will tell you to brake, let go, brake let go.

In reality you'll drag the back brake to control your speed and keep the back end stable/planted. Then grab the front brake to really scrub the speed when you need to.

 

[RebornRider]

I think there are two different interpretations of the OP's question: which method offers better control versus which method will result in longer component life. I'm voting for the component life interpretation.

As a wannabe professional and keyboard warrior who took his most recent physics class 40 years ago, what I remember matches what shiny side posted.

• brakes slow you down by converting kenetic energy into thermal energy
• assuming the initial speed at the top of the hill and the final speed at the bottom of the hill are the same in both cases (the cases being drag the brakes to hold speed constant versus stab the brakes occasionally to stay alive), the total amount of kinetic energy converted to thermal energy will be the same for both cases

Dragging the brakes all the way down will slowly increase component temperatures because there is no cooling off period. The temps will rise until the rate of heat in from braking equals the rate of heat out from the brake's ability to transfer heat to the environment.

Stabbing the brakes will dump a bunch of heat into the components in a short time, resulting in a steep temperature rise, but then there is a cooling off period when no heat is being added to the components. Rather than the slow steady temperature rise of the drag method, I'm guessing the stab method causes the temps to oscillate up and down, but the average of the highs and lows will slowly increase if the cooling off period isn't long enough to reject all the heat from the most recent stab.

Which of these is harder on the components? I'd like to know for sure. Some brake manufacturer must have done a well-controlled experiment. Oh, Google...

 

[Doomanic]

Components are easy and cheap to replace, teeth not so much.
You need to control your speed, not doing so is a sure fire way to get a ride in the disco-bus.

 

[RustyIron]

Any physicists here?

More importantly, are there actually any physicists who ride well? If you want to know how to ride down a hill, then let's leave the physicists at their chalk boards and discuss it among ourselves.

If the question is about how to brake while descending a hill, then it's too general and there isn't a single answer. I'll offer a couple different scenarios with different solutions. Assume the hills are really steep and have problems that necessitate you manage your speed.

Scenario 1:
Let her rip and brake when you can obtain extra traction from trail features. Maybe there's a little hump that will give you a little air, or at least lighten your bike a little. When the bike comes down, the suspension will compress and there is a short period where the tires are being pushed into the dirt a little harder. Use this opportunity to grab a handful of brakes. Get off the brakes as the suspension rebounds. Similarly, while there might not be actual turns with berms, there might be tiny features where you change direction a little, compressing the suspension a little. Again, used this opportunity to hit the brakes. Sometimes the texture of the dirt under your tire will dictate when to brake. Loose over hard can be challenging. But if there's a patch of sand or clean rock approaching, use it for braking. On the flip side, when your suspension is unweighted or when the terrain doesn't offer great traction, brake less.

Scenario 2:
The terrain is super steep, you need to manage speed for what's ahead, and the traction is sketchy. In such a situation, you might not want to get too much speed because you might not be able to scrub it off in time for whatever problems lie ahead. In these cases, sometimes it's helpful to drag the brakes, feathering each as traction allows, and maintain a moderate speed. Pay attention to the sound and feel of your tires, so you can let up a little before you break traction.

Scenario 3:
Super steep and loose or powdery. Brake hard with the rear, where the wheel is rolling slower than you're traveling, or maybe it's even locked up some of the time. Use your butt to control the direction of your bike, as if you're skiing down the hill. Use the front brake for speed control, but don't let it lock up or tuck in.

 

[RebornRider]

One of the common themes I see here is the complete disconnect between guys who are competent riders of EWS downhill tracks and approach every topic from that perspective (I could names names, but you already know who they are), and those who are working their way through Intermediate trails (me, and probably many others). There is room for both types, I hope, but y'all should be mindful of the difference.

Condescension never improves a discussion.

 

[ShinySideUp]

I should probably be more specific. When I say going downhill, I mean going down a generally straight, normal tarmac road so the handling issue is considerably less important than the fact that one is just trying to keep the speed down.

I can see that it rather depends on how long the hill is. I might drag the brakes if it was a light hill and only a few metres long but on a long, steep hill I would tend to brake hard then release. Intuitively I feel the intermittent braking would be less wearing on the components in the long run as, as has been stated above, there is not the constant increase in temperature that would result from brake dragging for a long period which may well contribute to brake fade, a brake condition you would not want to experience; I know, I had it happen once when I was driving HGVs.

Whatever one is riding, this is still a physics question, most specifically thermodynamics.

 

[Slapbassmunky]

I should probably be more specific. When I say going downhill, I mean going down a generally straight, normal tarmac road so the handling issue is considerably less important than the fact that one is just trying to keep the speed down.

Transitioning between runs, or out on a ride with your buddies, etc? You'll still end up dragging your brakes to steady your speed. No one wants to follow someone who's speed is going up and down like a yo-yo, and your super tacky tyres will thank you. Appropriately sized modern disc brakes with good old sintered pads can cope with a lot of abuse before problems arise.

From a physics point of view, kinetic energy is kinetic energy. It has to go somewhere (heat) and you'll be descending the same hill whatever technique you use. Terrain and who you're riding with will arguably have the biggest impact on how you go about it.

Without engine breaking our choices are limited.

 

[Rusty]

I prefer to keep my rotors nice and straight so use the brake, let go, brake, let go method mostly. Not saying that I wont drag them occasionally, but always drag both front and rear to be more balanced. Occasionally on some really steep, scary transition roads, the build of the road means speeds increase super fast and the corners are sketchy so I will drag front brake then rear brake - alternating between them to keep speed down, but also to allow some cooling to stop totally screwing up my rotors.
Where I ride, I regularly ride in excess of 50kph on the trails I ride and in some events I used to do had been over 100pkh on smooth gravel or sealed sections. I probably replace pads every couple months so always have some spares and some good second hand pads in my kit, but really hate replacing rotors too often as $100 each end makes that an expensive proposition. I also use resin pads to give me the feel and control I want as well as being kinder on the rotors.

Oh yer - I'm also 108kg so carry a lot of momentum on the downs.

 

[Slapbassmunky]

I prefer to keep my rotors nice and straight

What rotors are you using? I've seen floating ones turn a lovely blue hue without warping. I've found resin pads will destroy themselves/fade long before decent rotors start to suffer, that goes for bikes with and without proper engines. Have you tried decent sintered pads?

 

[urastus]

Where I ride it is generally steep and rowdy. I grab brakes and compress at the same time, when I can (a patch of smooth ground). I compress to maximise braking and not lose traction, but I'm sure we all know that.

If I have to brake in rough stuff because there is bigger rough stuff - especially steep washouts; I'll trail brake and let go before the washout and pump through the washout. I have a lot of steep sided washouts.

I've also found that I prefer more aggressive braking on the rear, more controllable on the front, because it's also loose where I ride. My brakes are the same front and rear, just the pads are different; metal rear, resin front. This seems to make a big difference.

I have an ancient mtb setup for urban use - I do the same with this. We have very steep sealed roads - I squash the rear wheel and brake hard and late. It usually freaks people out who think I'm not going to stop in time. My imagination does go through the scenario of "what if my brakes failed here"

 

[steve_sordy]

The physics question. The key piece of information missing is the time taken for the descent. The start and finish speeds may be the same, but the time taken can be different. If the time is shorter, then the average speed will have been higher. Kinetic energy rises with the square of the speed. So a descent that is faster will have absorbed more energy and generated more heat. Therefore the slower descent will be better for reduced energy intake by the brakes. Frequent braking and release will reduce the speed and also give time for heat dissipation.

Where I ride the most, the long steep stuff is sand over leaf mould, often the other way around, or a multi-layer sandwich. It is loose whatever the weather but much worse when it's dry, like right now. I have learned the hard way that if I brake for more than a fraction, the wheels lock up and the trail surface starts to build up under the tyres and forms a raft. When that happens the direction of travel, while still down, can be to the left or right, sometimes both! The only thing I have found that works is to cadence brake front and rear at the same time. I have tried braking alternately, thinking it might be better; it feels better but I have been too busy to measure the result. Frequently, there is an obstacle at the bottom of the descent, so I have to come to a full stop, or as near as.

 

[ShinySideUp]

I prefer to keep my rotors nice and straight so use the brake, let go, brake, let go method mostly. Not saying that I wont drag them occasionally, but always drag both front and rear to be more balanced. Occasionally on some really steep, scary transition roads, the build of the road means speeds increase super fast and the corners are sketchy so I will drag front brake then rear brake - alternating between them to keep speed down, but also to allow some cooling to stop totally screwing up my rotors.
Where I ride, I regularly ride in excess of 50kph on the trails I ride and in some events I used to do had been over 100pkh on smooth gravel or sealed sections. I probably replace pads every couple months so always have some spares and some good second hand pads in my kit, but really hate replacing rotors too often as $100 each end makes that an expensive proposition. I also use resin pads to give me the feel and control I want as well as being kinder on the rotors.

Oh yer - I'm also 108kg so carry a lot of momentum on the downs.

I have found that if I brake for too long my front rotor warps a bit and rubs a little until they've cooled down; probably cheapo brakes but they work for the most part. I too am a tad over 100Kg so they have a lot of work to do.

 

[RebornRider]

The physics question. The key piece of information missing is the time taken for the descent. The start and finish speeds may be the same, but the time taken can be different. If the time is shorter, then the average speed will have been higher. Kinetic energy rises with the square of the speed. So a descent that is faster will have absorbed more energy and generated more heat. Therefore the slower descent will be better for reduced energy intake by the brakes.

This discussion is going to drive the hard core riders crazy!

I don't think you are correct. If we assume for simplicity that the bike (with rider, obviously!) starts at the top of the hill with no speed, and comes to a complete stop at the bottom of the hill, all of the kinetic energy of the bike+rider comes from the difference between the bike's initial and final potential energy (no pedaling, just coasting, to keep it simple). Potential energy has been converted to kinetic energy, and the kinetic energy has been converted to heat energy. So the total amount of kinetic energy that has to be converted to heat comes from the decrease in potential energy caused by going from the top of the hill to the bottom. Time is not a factor.

Since we're comparing braking methods, the bike speeds at the top and the bottom of the hill don't matter, as long as they are equal in both cases. The total amount of kinetic energy that must be converted to heat is determined by the change in potential energy, which is the weight of the bike+rider times the height of the hill. Time doesn't matter.

I'm pretty sure about this, but I admit it's been several decades since I did this kind of stuff as homework problems!

 

[RustyIron]

I should probably be more specific. When I say going downhill, I mean going down a generally straight, normal tarmac road

The physics question. The key piece of information missing is the time taken for the descent.

Braking while on a straight, downhill road isn't normally the sort of thing I concern myself with, but I can't resist thinking about math problems. My intuition tells me that the brakes do less work when you let the bike pick up speed, brake hard, and then let it pick up speed again. Here's why...

A large amount of the energy required to move the bike forward is to fight wind resistance. The formula for the amount of drag is complicated, but it varies by the square of the wind velocity... or bike velocity in stationary air. Consider that 14^2=196, and 20^2=400. The amount of drag at 20 mph is more than DOUBLE the amount of drag at 14 mph. So the faster you allow the bike to go, the more energy is used up pushing air, and the less work the brakes need to do.

A fun experiment can be to start at the top of a hill, and let the bike go. Drop the saddle, get down low, and be slippery through the air like an Su-27 Flanker. See how fast you're going? Now sit up straight like you're mama told you to do at the dinner table. Watch your speed drop like a rock. By increasing your drag coefficient, you're controlling speed.

 

[ShinySideUp]

Braking while on a straight, downhill road isn't normally the sort of thing I concern myself with, but I can't resist thinking about math problems. My intuition tells me that the brakes do less work when you let the bike pick up speed, brake hard, and then let it pick up speed again. Here's why...

A large amount of the energy required to move the bike forward is to fight wind resistance. The formula for the amount of drag is complicated, but it varies by the square of the wind velocity... or bike velocity in stationary air. Consider that 14^2=196, and 20^2=400. The amount of drag at 20 mph is more than DOUBLE the amount of drag at 14 mph. So the faster you allow the bike to go, the more energy is used up pushing air, and the less work the brakes need to do.

A fun experiment can be to start at the top of a hill, and let the bike go. Drop the saddle, get down low, and be slippery through the air like an Su-27 Flanker. See how fast you're going? Now sit up straight like you're mama told you to do at the dinner table. Watch your speed drop like a rock. By increasing your drag coefficient, you're controlling speed.

So keeping ones speed as high as is safe to do so while braking hard then letting the brakes cool while the bike accelerates back to the safe speed seems to be the most efficient method while at the same time being as friendly as possible to the cycle's components; at least so it would seem so far in this discussion.

 

[Rusty]

What rotors are you using? I've seen floating ones turn a lovely blue hue without warping.

Icetech mostly - depends on what comes on the bike and how long until I replace Sram with Shimano.

I've found resin pads will destroy themselves/fade long before decent rotors start to suffer, that goes for bikes with and without proper engines. Have you tried decent sintered pads?

Yes, have tried pretty much every type out there, as well as some custom made ones from the guy who used to do my brakes on my motorcycle race bikes. Always go back to resin for the feel I prefer.
A few years ago I had a GT something with those crappy shifter/brakes where you knocked the brake lever up or down to shift. At a P2P event called the Gentle Annie I destroyed the rotors due to excessive heat. That was 120km (?) event with only a couple hundy km on the new bike prior. It was a gravel road, but the grade was so steep over the hump that they sealed just that section to allow road transport. One cornerI used a car as a berm after brake fade made it impossible to slow for the corner.

 

[steve_sordy]

This discussion is going to drive the hard core riders crazy!

I don't think you are correct. If we assume for simplicity that the bike (with rider, obviously!) starts at the top of the hill with no speed, and comes to a complete stop at the bottom of the hill, all of the kinetic energy of the bike+rider comes from the difference between the bike's initial and final potential energy (no pedaling, just coasting, to keep it simple). Potential energy has been converted to kinetic energy, and the kinetic energy has been converted to heat energy. So the total amount of kinetic energy that has to be converted to heat comes from the decrease in potential energy caused by going from the top of the hill to the bottom. Time is not a factor.

Since we're comparing braking methods, the bike speeds at the top and the bottom of the hill don't matter, as long as they are equal in both cases. The total amount of kinetic energy that must be converted to heat is determined by the change in potential energy, which is the weight of the bike+rider times the height of the hill. Time doesn't matter.

I'm pretty sure about this, but I admit it's been several decades since I did this kind of stuff as homework problems!

Working out how fast a falling (or rolling) object will be doing at the bottom of a drop/slope is a classic 1st year A level maths or physics question. Potential Energy is the driving force and is a maximum at the top and a minimum at the bottom. That loss has been converted to kinetic energy, which in turn will have been converted via friction and wind resistance into heat.

So, we agree, but only because you made the time constant by your conditions (no pedalling). @RustyIron also makes a good point about wind resistance absorbing some of the energy.

In practice, we each have our own tolerance for what is a safe speed and we will brake to achieve it. My braking when descending a steep tarmac hill will include sitting up to slow me down. I try to avoid tarmac wherever possible, especially on steep hills!
PS: My tolerance is 30mph. I can't help imagining the effect of my tender body hitting the tarmac at that speed.

 

[RustyIron]

Working out how fast a falling (or rolling) object will be doing at the bottom of a drop/slope is a classic 1st year

We need to get some of those first year math students to join the forum. They can tell us what tires to run and what pressures. I enjoy pondering the variables, even though they're mostly inconsequential.

Back when I was a young pup when dinosaurs roamed the Earth, my best friend's dad was a truck driver. He said that when going downhill, it's best to ride the brakes. He wasn't the sharpest knife in the drawer, but it was his job, and I never had reason to doubt his wisdom... until today. I did a little googling on the subject, and came up with this video that says it's best to use the brakes off and on. If it's not a subject that interests you, don't bother watching it because it isn't directly applicable to MTB's.

 

[ShinySideUp]

We need to get some of those first year math students to join the forum. They can tell us what tires to run and what pressures. I enjoy pondering the variables, even though they're mostly inconsequential.

Back when I was a young pup when dinosaurs roamed the Earth, my best friend's dad was a truck driver. He said that when going downhill, it's best to ride the brakes. He wasn't the sharpest knife in the drawer, but it was his job, and I never had reason to doubt his wisdom... until today. I did a little googling on the subject, and came up with this video that says it's best to use the brakes off and on. If it's not a subject that interests you, don't bother watching it because it isn't directly applicable to MTB's.

I was an HGV driver before I retired. Many years ago I found out what brake fade was and what it felt like as I drove down a steep hill with a load of steel on the back. I initially dragged the brakes to keep my speed down but quite quickly i found I was pressing harder and harder on the brakes to less and less effect until they eventually gave up the ghost no matter how much pressure I applied. With a roundabout and a supermarket coming up rapidly, the only thing I could do was take my foot off the brakes and wait until the last second before stamping on them and hoping for the best. The best turned out to be a safe stop just in time and since then I've always started a downhill at a slow speed, intermittent braking, exhaust braking, electric retarder -- basically anything that would keep my brakes cool and my speed down. Of course there is only the one option on a cycle hence my original post.

 

[Alexbn921]

Cycling your brakes will reduce the average heat put into the rotors and decreases wear. If you are going down a grade that is long and steep enough that your brakes will over heat, you have to to cycle them. On the road it is almost always enough traction to control your speed as you see fit.

If you are on dirt and it's a fall line you might not have a choice and then you need to increase the thermal capacity of your system. Smooth is fast and dragging takes traction away from cornering.

 

[skinipenem]

Traction. Intermittent purposeful braking unless, maybe, you're riding a perfectly even grade and surfaced asphalt road with no turns. Dragging brakes reduces suspension activity and unsettled bike balance as well brake fade.

 

[Slapbassmunky]

What do you do if the trail is so steep you've got no choice but to drag the brakes. I'm talking off piste alpine runs etc. Let off the brakes for a second and you're in a lot of trouble very quickly.

In this situation the rear will cook very quickly, and yet the front brake which is used heavily but Intermittently will generally stay at acceptable temperatures.

 

[B1rdie]

Don’t worry, bosch has worked on it, and with their new abs break for bikes, all you need to do is pull the levers when you start the ride at the trailhead and then release at the end.
They will soon implement the system, with the voice activation, so all you need is to say “ boscha I wanna go fast” …
I only drag the breaks when breaking in new pads or cleaning the rotors if they get noisy due to moist or dust.

 

[KnollyBro]

What do you do if the trail is so steep you've got no choice but to drag the brakes. I'm talking off piste alpine runs etc. Let off the brakes for a second and you're in a lot of trouble very quickly.

In this situation the rear will cook very quickly, and yet the front brake which is used heavily but Intermittently will generally stay at acceptable temperatures.

Get a larger diameter and thicker front rotor as 70% of the braking is done in the front. I have cooked enough rear rotors using Shimano IceTech with metalic compound pads that I know it doesnt help to drag your rear brake. When it gets real steep, we tend to lock up the rear wheel and surf down (keep the front wheel rotating!). This of course only works on loose loam and would be suicide on rock slabs as your rear wheel slips out .

 

[RickBullotta]

More importantly, are there actually any physicists who ride well? If you want to know how to ride down a hill, then let's leave the physicists at their chalk boards and discuss it among ourselves.

If the question is about how to brake while descending a hill, then it's too general and there isn't a single answer. I'll offer a couple different scenarios with different solutions. Assume the hills are really steep and have problems that necessitate you manage your speed.

Scenario 1:
Let her rip and brake when you can obtain extra traction from trail features. Maybe there's a little hump that will give you a little air, or at least lighten your bike a little. When the bike comes down, the suspension will compress and there is a short period where the tires are being pushed into the dirt a little harder. Use this opportunity to grab a handful of brakes. Get off the brakes as the suspension rebounds. Similarly, while there might not be actual turns with berms, there might be tiny features where you change direction a little, compressing the suspension a little. Again, used this opportunity to hit the brakes. Sometimes the texture of the dirt under your tire will dictate when to brake. Loose over hard can be challenging. But if there's a patch of sand or clean rock approaching, use it for braking. On the flip side, when your suspension is unweighted or when the terrain doesn't offer great traction, brake less.

Scenario 2:
The terrain is super steep, you need to manage speed for what's ahead, and the traction is sketchy. In such a situation, you might not want to get too much speed because you might not be able to scrub it off in time for whatever problems lie ahead. In these cases, sometimes it's helpful to drag the brakes, feathering each as traction allows, and maintain a moderate speed. Pay attention to the sound and feel of your tires, so you can let up a little before you break traction.

Scenario 3:
Super steep and loose or powdery. Brake hard with the rear, where the wheel is rolling slower than you're traveling, or maybe it's even locked up some of the time. Use your butt to control the direction of your bike, as if you're skiing down the hill. Use the front brake for speed control, but don't let it lock up or tuck in.

Actually, yes. Matt Miller is a PhD and a very accomplished MTB rider/racer. He's coincidentally done a lot of research into braking performance, optimization, and analysis:

BrakeAce | Own the Mountain

 

[KnollyBro]

Actually, yes. Matt Miller is a PhD and a very accomplished MTB rider/racer. He's coincidentally done a lot of research into braking performance, optimization, and analysis:

BrakeAce | Own the Mountain

Sounds interesting but $1600 for sensors to learn how to brake better sounds a little steep ! I would give it a try but in general, I just want to ride safely and crash less. challenge myself and enjoy riding my bike. I am not out to win any races.