## Brakes

So. I noticed my rear brake wasn’t really doing anything. If I squeezed really hard, I could slow down, but not enough to stop going down a steep hill. No big deal—it’s the front brake that really matters, right?—but just for safety’s sake I went to the bike store one day and they replaced the pads so the brake works again. And, hey—it really works! I hadn’t realized how effective the brake can be when it’s fully operational. Good to know.

1. jd says:

Rule #65. velominati.com

2. Guive says:

This reminds me, I need to get my brakes fixed.

3. J Storrs Hall says:

Next time you go down a steep hill and brake with only your front brake, let me know ahead of time so I can send flowers.

• Martha (Smith) says:

+1

• Nah, the physics of bicycles is such that the rear brake can never really provide strong stopping power, as seen in this demonstration: https://youtu.be/QCHsNoBEdrM?t=115

obviously the rear wheel can’t do much braking if it’s not on the ground ;-)

it can provide good speed moderation, but when you need to stop, virtually all the stopping power is dissipated in the front brake regardless of whether you mash both or just one brake.

• Incidentally, for those who love some mechanics, here’s the breakdown on how that works:

The center of mass of the bike+rider is obviously somewhere up around the height of the seat. However the point of action of the reaction force between the road and the tires is down at the level of the road.

If you’re braking, it means you’re decelerating relative to the road, so in the frame of reference of the contact point between the front tire and the road (which is an accelerating frame of reference that’s accelerating backwards) the forces on the system are the force at the contact point of each tire, and the inertial force applied to the center of mass. The inertial force has two components, one from gravity towards the center of the earth, and one from the acceleration of the reference frame (equal to -ma where a is the acceleration of the reference frame itself).

Now, if we’re going to have a stable stop, we shouldn’t be rotating around the contact point of the front tire, so we look at the total torque around that point, where the contact force doesn’t enter because the lever arm is zero.

Obviously, the center of mass inertial force provides a positive torque that tends to send the rider over the handlebars, and the gravitational force provides a counterbalancing torque that keeps the rider from accelerating over the handlebars… but let’s look at the reaction force for the back tire.

Taking a lever arm from the front tire to the back tire, if the road is relatively straight, then the reaction force that keeps the tire from sinking into the ground actually provides a positive torque that tends to send the rider over the handlebars, and the drag force caused by friction plays no role in the torque because it goes in the same direction as the lever arm (along the lever). This suggests that the torque tending to make you go over the handlebars comes to a minimum just as the back tire leaves the ground.

So in a hard braking situation, it’s always the case that the torque induced by the system brings you to near the point where your rear wheel leaves the ground, at which point it exerts zero braking force.

If you brake with the rear wheel only, the bike still pitches forward, and as it does the rear wheel becomes less and less stuck to the ground, so its braking force decreases (friction force is proportional to normal force with the road), eventually it starts to slide which reduces the friction force even more (static friction is greater than sliding friction).

What this means is: the rear brake is more forgiving because it naturally feeds-back to a stable low-friction sliding, but it’s also incapable of producing much braking power.

• jd says:

But you need that rear brake in slippery conditions. Too much input to front, especially in corner (which should avoid braking in anyway), can cause front wheel slide out.. Rear wheel skid not as dangerous as front wheel skid. If surface is slippery you need both

• Jackson says:

Front brakes may be more effective at transferring energy from the rider to the ground, but they are also much more effective at transferring the rider himself to the ground. Although since I myself roll a bit when I hit the ground I have some sort of built in ABS, so maybe you’re right about front brakes after all.

• Andrew says:

Jackson:

I go at such slow speeds that this particular failure mode will not be happening. And on the rare occasions that I need to brake so hard that my rear wheel does lift, I’m aware of what’s happening and can adjust accordingly.

• Doug Davidson says:

If anyone here is a fan of MotoGP, I encourage you to check out Andrea Dovizioso’s braking technique… it is almost like they use (front) braking to turn the bike.

Also Brembo has great stats graphs on braking times…

• jim says:

I propose that if the rear wheel locks when the brakes are applied its stopping power is equal to or greater than the front wheel, excepting the effects of the riders weight distribution.

• As I said below, stopping power here means the energy dissipated per unit time, and the rear brake actually has *zero* stopping power once the rear wheel locks, as all the energy is dissipated at the contact point between the road and the tire.

• jim says:

“the rear brake actually has *zero* stopping power once the rear wheel locks..”

Hmmm…I was going to take a stupidbow and concede the point but…

but the more I think about it, I’m not sure you’re right. The rear wheel stopping power when the rear is locked depends heavily on the riding surface and tread geometry. On clean rough concrete, it would have a lot of stopping power but on a smooth hard surface with a sandy or small pea gravel cover hardly any.

Having smashed a few helmets myself by going over the front end, unless there’s some new technology that I’m not ken to that prevents lockups on the front, from a practical standpoint the best policy is to control your speed to the point that the front brake isn’t critical.

I will concede, however, having ridden only mountain bikes (rarely over 25 mph), it’s possible that the much higher speed attainable on a road bike (easily 40mph, right?) changes the equation big time. Which is a great reason to stick to mountain bikes.

• Yes the rear *wheel* stopping power can be nonzero but it’s the tire dissipating the power, not the brake. skid too long and you blow out a tire.

consider dropping down off a hill to a busy intersection. if you don’t stop (or maybe turn right in emergency) you will be killed by cross traffic. learn to brake using the front brake, brake early so you don’t have to brake quite so hard, and keep your brakes in good working order.

• Terry says:

There is an interesting distinction here. You seem to be focusing on energy dissipation and breaking down energy dissipation between the brakes and the tires. If a brake locks, it no longer dissipates energy and the tires have to do the work.

I am focused on the distribution of the horizontal force between the road and the front wheel and the back wheel. In order for the brake to do any work, the force has to get to the brake, and it does this by resisting the torque generated by the force of the road on the tire. If there is no force between the road and the tire, the brake can’t dissipate any power.

• Terry says:

But of course, in your comment above you made the points about the weight being thrown to the front wheel in great and accurate detail.

• Terry says:

Pretty sure this isn’t right.

The force between the pavement and the tire is equal to the coefficient of friction times the vertical force on the wheel. As you brake, the vertical force on the front wheel increases and the vertical force on the back wheel decreases. Therefore the force between the pavement and the back wheel is less than the force between the pavement and the front wheel (ignoring the effects of the rider’s weight distribution as you note).

For some intuition, think of the situation when you brake so hard that the rear wheel is just beginning to lift up from the pavement. Then, there is no vertical force on the rear wheel and therefore no frictional force between the pavement and the wheel. All the stopping power is on the front wheel. (Even more obviously, there is no stopping power on the rear wheel when the rear wheel actually does lift up.)

• Anonymous says:

Quote from above: “Nah, the physics of bicycles is such that the rear brake can never really provide strong stopping power, (…)”

My intuition and experience is, and has always been, to mostly break with the back brake i think. This may come from having ridden bikes with no front breaks, and no way to break with your hands via handlebar brakes for that matter. I have mostly ridden bikes with what i think is called “coaster brakes” in English (in my native language Dutch it is called a “terugtraprem”, which roughly translates as “back-pedal-brake”).

But even when riding bikes with handlebar/front- and back brakes, i (think i) still mostly brake with either both brakes at the same time, or mostly the back brake. I think it comes from the idea/intuition/feeling/experience that skidding/slipping when braking too much with the back brake is better than being thrown over the handlebar when braking too much with your front break.

Perhaps the physics concerning how to optimally brake with regard to the forces, and math, and all that stuff might not be the most important thing to take into account concerning performing the actual action of braking when on a bike.

I did some searching and found this video that talks a bit about front vs back braking on mountain bikes, and how it could relate to simply a preference, and/or if you are riding on a flat road or a declining one, etc.: 01.31 – 02.28 minutes https://www.youtube.com/watch?v=qy9JaA-OHlM

• The key here is the term “stopping power”. In physics, unlike statistics, power is a real quantity ;-) specifically the amount of energy converted (to heat in this case) per unit time.

So, it’s perfectly fine to modulate your speed with the rear brake, but if you actually need to keep from plowing into a tree or going off a cliff or out of a driveway into the street where a truck will hit you, you need to stop *fast* and that means dissipating a lot of kinetic energy into heat in a short time. The rear brake inherently can’t do this anything like as effectively as the front brake for all those reasons.

By all means though, watch where you’re going, apply brake *early* so you don’t need as much power, and use the back brake to get a more stable stop if it’s sufficient. When braking especially while going downhill, move your body back towards the rear wheel and down towards the seat to try to minimize the effect that makes you fly over the handlebars.

• Anonymous says:

Quote from above: “By all means though, watch where you’re going, apply brake *early* so you don’t need as much power, and use the back brake to get a more stable stop if it’s sufficient. When braking especially while going downhill, move your body back towards the rear wheel and down towards the seat to try to minimize the effect that makes you fly over the handlebars.”

Thank you for the reply. Here’s another source about this topic, that mentions an “ideal” way of braking somewhere in the comments: https://www.quora.com/Why-is-the-front-brake-more-effective-than-the-back-brake-on-bikes

“The ideal way to stop your bike, of any variety, is to rely mainly on the front brake, steadily increasing the pressure you put on the brake as you go, and to use the back brake towards the end of the stop. You will find steadily increasing your front break instead of jamming it on will help keep your back tyre on the ground (two tyres are better than one!). The back brake towards the end will help you stop quicker but also helps to stabilise the bike.”

• jd says:

All of this depends on surface condition and whether one is travelling in straight line. Take it from an ex-bike racer – input too much front brake on a slippery surface on a bend in the road at your peril.
Front brakes can stop you.
Rear brakes can help save you some road rash.
Blog posts about brakes are cool.

• Anonymous says:

“Blog posts about brakes are cool.”

They are! And so are the comments to that blog post.

It also reminds me of the time when i did some “soul searching” (after dropping out of high school, and not wanting to work crappy jobs anymore for a while). I (think i) was about 19/20 years old, and wanted to go ride a bike for a while.

At the time, i just happened to came to know of “Camino de Santiago” https://en.wikipedia.org/wiki/Camino_de_Santiago and i thought it could be a nice route to cycle, and be a goal/destination to ride to.

I bought a very cheap second-hand bike with handlebar brakes (i think for about the equivalent of 100-150 US dollars), bought a small tent that was so small that if i stretched out my hands i could easily touch the sides and ceiling of the tent, mounted a rack for saddle bags (if that’s the correct word) to put all my stuff in, and on i went.

I cycled from The Netherlands to Santiago de Compostela (Spain) in about 3 weeks (you could do it much faster, but i took my time and sometimes stayed in a cheap hotel from some comfort). I had to go through mountain passes on the border from France to Spain and going downhill on some of those roads was scary!

I also may have escaped a small (?) disaster because my tire burst going downhill fast but luckily just at the point where the road flattened out. I still have a nice image in my head of me changing the inner tube of the tire with lots of green hills and mountains surrounding me.

I wanted to end this comment, and story, by saying “good times, good times” but then i thought about how lonely it was at times, and how i found myself crying all alone in the middle of a free camping site somewhere in France (at least i think it was France) wondering what the h#ck i was doing.

Regardless, i made it to Santiago, and perhaps i achieved some other things as well. I think i lost about 30 pounds during to trip to name just one thing. At this moment in my life, about 20 years later, i think it’s still one of the few things that sort of “stick out” in my mind. I think this might be because it may be one of the few things in which i achieved the goal/finished something.

I reason that it’s not always necessary to reach a goal, that it may sometimes even be impossible to know if you reached a goal, or that the road to something may be the goal in and of itself. However, perhaps it’s sometimes nice and/or useful to reach a goal/destination nonetheless :)

• Terry says:

Anonymous,

Great story. Surprisingly candid.

• Terry says:

“I bought a small tent that was so small that if i stretched out my hands i could easily touch the sides and ceiling of the tent”

Look at Mr. Anonymous money bags here! He had a tent that was bigger than he was!

When *I* went long-distance bicycling, I used to sleep in a Hefty garbage bag. And I liked it! Some days I couldn’t afford a garbage bag, so I just stretched out my underwear a bit.

Mr. Anonymous Rockefeller Scrooge McDuck!

• Anonymous says:

“Look at Mr. Anonymous money bags here! He had a tent that was bigger than he was!”

Haha!

I bought the small tent because of the size and weight. That needed to be as small and light as possible. I think it cost me the equivalent of around \$350 back then. I think (some) options for a bigger, and heavier tent would actually have been much cheaper. Anyway, i am not a tent-expert.

To give you an idea, i did a quick search and i think the tent i bought and used was something similar to this (i think the video is depicting a 2-person tent that is way bigger, i had the 1 person tent): https://www.jack-wolfskin.com/gossamer/3000433-4502.html

• “rely mainly on the front brake, steadily increasing the pressure … as you go”

that’s good advice. especially about the steady increase rather than jamming the brake on. Stopping fast is an important skill. The quicker you try to stop, the less the rear brake can do anything. If you’re maximizing your braking power, the rear wheel will be barely in contact with the ground at all, so it’s useless at that point.

Rear brakes are fine for fine-tuning speed, but basically useless for hard braking.

As mentioned by others elsewhere, it’s also a bad idea to try to break hard during a turn, the sideways and backwards forces are both contributing to the friction requirements, and you will slide more easily in a turn.

Basically, practice braking hard using the front brake so you get to know how the dynamics of a stable braking process feel, you’ll better avoid accidents caused by either too fast braking (flip over handlebars) or too slow (slam into car/truck/wall)

• jd says:

Anonymous – for some reason I don’t have a “Reply to this comment” link for your below story. Sounds like a great adventure. Sometimes those experiences of grinding it out for a goal are real character building experiences… especially when it involves had physical effort and uncertain outcomes. I bet those mountain passes were beautiful as well.

I raced elite am road bikes, with some pro races, for many years in my 20’s. Some of my goals I reached and some I did not. Sometimes I think, well that was a lot of sacrifice for missing some main goals. But it was beautiful too.

I too can relate to scary descents. I come from ‘flat land’ as well. I remember one stage race in particular where we had a ‘hair raising’ descent on stage 2; so much so, that it was actually marked out with a big warning in the race bible. I happened to be in a break with a fellow from Guatemala, who descended like a maniac. Apparently Guatemala is mountainous and his bike handling skills were superb. I distinctly remember my rear wheel skittering around several switchback hairpins as I followed him down. As a flatlander, my bike handling was clearly lacking! I felt relieved to arrive at the bottom of the mountain still on my bike.

Cycling is a great sport for “soul searching”, character building, adventure. ..oh yeah, and fun!

• Anonymous says:

1) You wrote: “Anonymous – for some reason I don’t have a “Reply to this comment” link for your below story.’

The “reply to” option only goes on for so long, which might be a good thing otherwise i think reading comments becomes difficult. What i do in such instances is go up until you reach the first comment with the “reply to this comment” option.

If you reply to that one, your comment turns up just below the comment you actually (probably) want to reply to. This works for me, and if you quote something from the post you want to reply to, it is usually clear to which post you are replying and your reply is depicted just below the comment you are replying to (unless someone comments as you were writing your comment).

2) You wrote: “Sometimes I think, well that was a lot of sacrifice for missing some main goals.”

Perhaps the goal you have in mind at one time may not be the most important goal in the end. Perhaps by missing a goal, a different and more important goal reveals itself.

4. Paul Alper says:

Hard to believe Andrew, a statistician, wrote, “And, hey—it really works! I hadn’t realized how effective the brake can be when it’s fully operational. Good to know.”
A sample of size one, “effective” is undefined, “works” is vague, and “the brake can be” is obscurantist jargon. Unsaid is whether he has any ties to the replacement pad industry.
P.S. Because of the recent snow storms and the subsequent thawing, instead of a braking problem on a hill, the issue is avoiding mammoth pot holes.

• Martha (Smith) says:

Picky, picky, picky!

• Anoneuoid says:

I disagree, it is simple applying Bayes’rule to an otherwise surprising a priori prediction. What other explanation do you have for the brake performance improving immediately after getting the new brakes, exactly as predicted?

• Ricardo says:

Experimenter bias, he might have applied more force this time, leading to a better breaking as expected. A blinding control is needed, where someone the breaking pads are changed without Andrew knowing…

5. Thanatos Savehn says:

I’m glad I didn’t respond immediately. I thought this was about optional stopping.

6. jrc says:

That is how I felt when I got new glasses. I had no idea I had been seeing so poorly. I think it has something to do with the neuro-biology of a human frog in a slowly heating life pot, but I am also pretty sure I haven’t had my brain removed… or maybe that does explain a lot.

https://en.wikipedia.org/wiki/Boiling_frog

• Anonymous says:

Bad news. You’re a brain in a vat or maybe a philosophical zombie worse yet.

7. Peter F Chapman says:

We live in the UK and a few years ago we took a holiday in Majorca where we hired bikes locally. My wife was in training for a charity cycle ride through Brazil, whilst I was simply on holiday. We were cycling along a gravel path full of potholes, which required me to concentrate on the ground. I looked up to see my wife had stopped and that I was about to plough into her so I instinctively engaged the brake that would have been the rear brake in the UK – ie the one on the left handlebar. Unfortunately, in Spain the rear brake is on the right handlebar – so you can easily guess what happened next.

• zbicyclist says:

Varies by country. In the US, the front brake is on the left handlebar, rear on the right. Meaningless non-standardizaation.

From the late, great Sheldon Brown:
https://www.sheldonbrown.com/brakturn.html#whichside

‘In countries where vehicles drive on the right, it is common to set the brakes up so that the front brake is operated by the left lever.
In countries where vehicles drive on the left, it is common to set the brakes up so that the front brake is operated by the right lever.

The European Union has adopted this as a standard, even though only the United Kingdom and Ireland are left-side driving countries. The standard is not universally observed; a reader has written in to say that the left lever usually controls the front brake in Denmark.
The theory that seems most probable to me is that the national standards arose from a concern that the cyclist be able to make hand signals, and still be able to reach the primary brake. This logical idea is, unfortunately, accompanied by the incorrect premise that the rear brake is the primary brake.’

For real fun, consider a bunch of cruise ship tourists renting bikes in Helsinki and finding out the tour company bikes have coaster brakes, which most of us hadn’t seen in many decades. I blame this confusion for the crash ahead of me that led to one crashee breaking a finger.

8. Terry says:

Just for the record, I’m all in favor of working brakes too.

9. J Storrs Hall says:

The limits to your braking power can be approximated by the slope of a prolate cycloid formed by your front tire and your center of gravity, at the angle the CG makes. For low and far back, this can be 45 degrees short of vertical, and you can brake at up to about 0.6 Gs. If you’re standing on the pedals and putting weight on the handlebars, down to about 0.3.
If you are going down a steep hill, subtract the angle of the hill from your CG angle over the wheel AND subtract the slope of the hill from the slope of the cycloid.

10. A 10 year old says:

“Prolate cycloid” – Cool! That’s some kind of transformer, right?

11. Dzhaughn says:

The middle brake has not worked on my last 3 bicycles, I never bothered fixing it.

12. Jim Hatton says:

Warning: I had a back brake on my mountain bike that was barely engaging – misadjusted by me. Three quarters of the way downhill I had burned out my front brake pads and warped the front disc badly. I was lucky I didn’t get pitched over the handle bars. I walked home. It was so bad the bike repair person didn’t want to see it, though he fixed it later.

• Bombing hills will do that to your brakes. having the rear brake probably wouldn’t have helped much, especially on hills it has even less ability to make a difference. What would help is taking some breaks where you go slow or even stop to let your brakes cool. At least you didn’t overheat rims and blow a tire while bombing.

• Dalton says:

Disc brakes don’t overheat rims. And some bikes are made for bombing down hills. I like those bikes.

• yes, I was contrasting the failure mode of disc vs traditional rim brakes… glad no one got hurt. better to shred your disc brakes and come to a safe stop than shred your rims and tires and get hurt.

• jd says:

Disc brakes.
But anyhow, I don’t remember anyone blowing a clincher tire on a mountain descent when racing road due to heating a rim… i guess that’s possible.. not sure. Used to be that the worry with increased heating of rims was melting the glue on a tubular and rolling it off the rim in a corner.

• There’s a balance between the cooling airflow caused by descending at high speed, and the power you dissipate in the rim when you brake hard. Also modern tire materials are probably much more heat resistant as you mention, particularly racing tires.

Most dangerous point for overheating a tire is probably a fast descent where you have to come to a near stop at some point to do a tight turn. The rim will be at its hottest, and the turn will give the tire a lot of shear forces. Blowing a tube is also possible just due to melting the tube at high temps.

• Dalton says:

Rim heating due to braking is also more of an issue with touring bikes/riders and tandem bikes/riders than racing bike riders. The bike part of the equation is the increased mass which requires more energy in the form of friction to reduce the acceleration due to gravity. The rider part of the equation is that both touring cyclists and tandem riders tend to more concerned with high-speed descents and are more likely to make frequent and liberal use of rim brakes.