Comparing hub/disk brake vs rim brake. Power, reaction, and skidding

Question

I'm not a bike enthusiast but I've commuted quite a lot with bikes. I don't always understand the physics but believe I can extract well what happens per experience. I could be mistaken though. I've been looking for an answer for these questions and I couldn't find an explanation for the particular form of questions I will raise. Also, it is surprising how little agreement there is, even when explaining the physics behind each opposing argument.

Let's assume with two bikes.

• One uses rim brakes
• One uses a rear hub brake, specifically a pedal brake like the Dutch bikes use, a "back-pedal" or coaster brake.

Assuming we don't use the front brake and the tires are the same and it is dry and the pads are clean, which one:

• provides better braking power or smaller braking distance
• will immobilize the wheel first. Which one will manage to provide the best braking force before immobilizing the wheel.

In both case, the pedal is at the same alignment, but if necessary provide an explanation when the pedals are horizontal and when they are vertical. For rim it doesn't matter but for pedal braking it does matter. (see secondary question)

I'm looking for answers with physics-based explanation if possible.

Why do I ask this?

I've driven rim brakes all my life. I would never rely on the rear brake only. In fact my experience is that without front brakes the braking is too weak and has the highest chances to skid. In fact it happens quite easily. My bikes always had good brakes configured relatively close to the rim.

Recently I rode a bike with pedal/coaster braking, specifically one from a rental service in Antwerp called Swapfiets. I'm worried about pedal braking and I tried how effective it is by trying maximum force in a safe place. To my surprise, the braking force was really high and almost comparable to using both rim brakes. In fact it was almost better. What surprised me the most, was that though the force was so much higher, the wheel would not skid. I understand that the bikes can be different, but thickness and age looked similar.

My personal experience is that rim brakes will immobilize very easily resulting to a skid. It's like the braking system loses an intermediate applied force and jumps directly to so much force that the wheel will stop spinning. I couldn't do this with the pedal brake while honest to god I felt I will jump in front of the bike because of momentum.

Also, it seems to me that the pedal brakes can vary from almost zero brake to full depending on the position of the pedals. This scares me, because the hand levers apply immediate force on sudden reactions. I always had them configured close to the rim, so with little squeeze I would get quite quickly the force applied. It seems to me that this feeling is not possible with pedal and you constantly need to consider the position of your pedals when your route becomes suspicious.

Final part: When using only rear braking, would you agree that there is a feeling of the bike lifting the rear? Based on physics this wouldn't happen, but your body will move forwards unless you hold tight the steering bar and don't let your elbows and shoulders give in. I know it's stupid but my question is whether human reality and imperfection, the bike does lift. It feels more with the rim brake compared to the hub.

Answer 1

One aspect which I feel hasn't been sufficiently addressed yet:

My personal experience is that rim brakes will immobilize very easily resulting to a skid.

This largely matches my experience. The underlying problem is that rim brakes have rather poor modulation capability. They can be very strong, but then they're suddenly too strong and lock up the wheel. In particular, as soon as the wheel is locked up the static friction is substantially higher than the dynamic friction used during normal braking. Which results in a hysteresis: even if you quickly reduce brake pressure, it likely won't get the wheel rotating again because the pads stick to the rim, as it were.

Coaster brakes avoid this effect because they use greased metal surfaces, which don't really have static friction at all. That also means they require much more force to achieve enough dynamic friction, but because legs are stronger than fingers one can still achieve enough braking force on the rear wheel. Nevertheless the wheel won't lock up unless you do some rather brutal back-kicking – and then there's no hysteresis.

I'd note that disc brakes generally also have better modulation than rim brakes. They can still lock up in static friction, but are much easier to get rolling again.

An entire separate thing: I wager that the Swapfiets you rode actually did not brake as well with the coaster brake as you think. There are two reasons why you might have perceived it so:

1. When using brakes from levers on the handlebars, your arms are already active. The arms are what actually slows down your body (the heaviest part of the rider-bike system). On the Swapfiets you start out in a very upright position where there is almost no force on the arms. From that position even rather gentle coaster-braking feels like a bigger effect than the same decelaration would feel on a bike where you already start out with more loaded arms.
2. You likely didn't go fast on the Swapfiets. Starting with a low speed obviously means you can more quickly come to a standstill, which also can suggest better braking than is actually happening.

One thing the Swapfiets really has going for it is that it has a long wheelbase, slack seatpost, and a low center of mass. All of these reduce the weight-to-the-front redistribution effect, so you may indeed find that you get better deceleration with its coaster brake than with the rear brake alone on a more sporty bike.

Answer 2

There's probably a lot going on when a bicycle rider is braking.

You're correct - a rear brake will always be less effective than a front brake, all other things being equal. As you brake, your weight transfers forward which adds weight to the front tyre and lightens the rear tyre. This is why the rear tyre skids much easier than the front.

A dutch rental bike will probably be stout and heavier than the bikes you're accustomed to, and will probably have a different riding position. So its different.

The act of using a coaster brake, driving the rearmost pedal downward, will encourage the transfer of weight rearward. This helps reduce the likelyhood of rear-tyre skidding.

Coaster/rear pedal brakes are incompatible with derailleur gears, so you have to be single-speed or fit the bike with an internally geared hub. This takes up space where the coaster brake's internals would be, and somewhat compromises the function of both.

The best braking performance comes from the brake pads gripping on the rim/disk/cones, and not from the tyre sliding on the road surface. Locking up the brakes is therefore worse, so if you're skidding relax the brakes a little. This is very hard to do when everything's going wrong.

Answer 3

Not a physicist, but I'll try to explain in simple terms.

Stopping is the property of tire grip with the surface. Better grip results in shorter stopping distance. So the goal of breaking is not to loose tire grip. Speaking simplified, when wheel rotates freely, there is no traction between tire and the surface. When you apply brakes you slow down wheel rotation and tire starts rubbing against the surface, that creates friction which results in stopping. But when you slow down wheel too much, vehicle moving force overcomes tire rubber friction properties and wheels starts skidding. The shortest stopping distance is achieved by being as close to tire slipping point. So in conclusion, the quality of the brakes depend on how well they can control wheel rotation for tire to be as close to slipping point.

For a bicycle ability to control wheel rotation with brakes depends of braking surface, braking pads, force applied to brake lever and total cyclist weight. Since weight is almost constant with any brakes, we can ignore it. By changing other parameters you can better control wheel rotation.

• Coaster brakes: you can change nothing about it, other than loosening up hub to make brakes feel spongier which would allow to apply braking more gradually.
• Drum bakes: only thing you can change is drum size.
• Rim bakes: steel, aluminum, carbon rims and various brake pad material combinations can create perfect breaking and some even make their own pads to achieve it.
• Disc brakes: hydraulic brake levers allow precisely control force applied and different disc rotor size and brake pad material can additionally change braking characteristics.

In my experience, I rode the same bake types that where good on one bike and poor on another, but in general more expensive brakes had better braking feel.

Answer 4

The important thing here to note is that all brakes can roughly be divided into two classes: disc and drum brakes.

Bicycle rim brakes are a special case of disc brakes. In this case, the entire rim functions as the brake disc, and since the rim can have even a millimeter of non-trueness, the brake pads need to have a large clearance, which means rubber pads are used as opposed to organic or metallic pads in other types of disc brakes. Because the rim is close to the road, it easily gets wet, and the rim being large doesn't heat rapidly, and the types of brake pads suffer in friction coefficient when wet, so wet braking with rim brakes is poor.

Drum brakes are notoriously hard to control. In many cases, they can be considered on/off brakes: either you don't have braking or the wheel is skidding. Especially on bicycle rear brakes, where even slight braking results in skid, the brake is usually either fully on or fully off. So coaster brakes, being drum brakes, should be avoided.

Drum/coaster brakes will immobilize the wheel first, mainly because they are hard to control. However, this doesn't mean they are superior. The superior brake would be one that with maximal force would barely skid the wheel, but the control from zero to this maximal force would be entirely linear. Drum brakes are far from linear.

As for better braking power, the wheel skidding can happen with any type of brake especially if used on the rear wheel, so all brakes have equal power with one exception: with rim brakes you have to wait for two wheel revolutions when wet to skid the wheel. So in wet rim brakes lose in braking distance, but in the dry all brakes are equal.

I would say that rim or disc brakes are the superior type of brake. In the wet, disc brakes are infinitely preferable (because of braking and because wet conditions eat brake pads and rims on rim brakes), but in the dry, rim brakes can have a lower total maintenance cost over the long term especially if the cyclist has wheelbuilding skills.

Drum/coaster brakes excel in having the longest life if left unmaintained, but eventually they require maintenance too, which may mean you have to replace the entire wheel hub (since it may be hard to find someone who can service drum brakes), which usually means you will have to build an entire new wheel since needed spoke lengths may differ, and this may mean you have to replace internal hub gears on the rear hub too at the same time. Reusing rim might be a possibility, though.

Answer 5

• Ceteris paribus any brake which can brake a given wheel to a skid is able to deliver the same amount of deceleration.
• Typically, both back pedal drum brakes and rim brakes can achieve that with the back wheel.
• Things that can differ are tire rubber, air pressure and weight distribution.
• Do not rely on a single brake.
• Emergency braking relies mostly on the front wheel.
• On slippery ground, be gentle on the front brake. When the front wheel slips, you fall.