Hydraulic Brakes - Why motorcycle brakes more powerful than bicycle brakes?

Question

I was wondering why motorcycle brakes have so much more stopping power than bicycle hydraulic brakes (Than can be found on mountain bikes for example).

The rotor has a bigger size which gives better leverage for the brake pads and thus more stopping power.

But I don't understand the following:

If the moment (force times lever length) exerted on the brake lever (by the rider's hand) is the same (is this a correct assumption) and the total amount of lever travel is the same... the amount of clamping force the hydraulic brake's caliper's pistons exert on the pads must be equal (I think)(since the same amount of energy is used at the lever.

Or is the free travel (the distance the pads have to travel before they hit the disc) much smaller on a motorcycle disc brake resulting in a shorter (free throw) of the lever which means there is more percentage of the lever travel that can be used to 'build pressure' meaning a smaller piston can be used which would give more clamping force at the pads?

The brake pads are much larger on motorcycles but then again because of the increased surface area one would have to exert more force on the bigger brake pads to get the same pressure/area.

Are my conclusions incorrect?

• Is the pad material a lot softer on the motorcycle brake pads?
• What design changes would enable higher clamping force with the same lever throw/force exerted on lever (if any design changes could accomplish this)?

Another example (light motorcycle vs heavy motorcycle): my motorcycle (Suzuki gn250, 140 ish kg wet weight) has a hydraulic front (disc) brake. A Honda Goldwing (very heavy motorcycle , 380 kg wet weight) also has a hydraulic front (disc) brake. What would be the main differences that enable the Goldwing's front brake to have so much more power?

Another question: The Goldwing has 2 discs/2 calipers in the front. There is more pad material indeed but the same thought (more surface area of pads means more force needs to be exerted in total on the brake pads in order to get the same force/area on the contact patch between pads and disc surface). Is even softer material used on the brake pads for 2 rotor front brake systems? Or am I missing something?

Some disc brakes have 2 piston calipers, some have 4 piston calipers. It is suggested that the 4 piston caliper variants have more stopping power.. Is it true that the 4 piston calipers have more stopping power in general? If so, what is the reason behind this?

Thank you!

Answer 1

Disk brake efficiency is often tied to what is called swept area. This is the surface that is covered by the brake pad as the wheel rotates. I suspected that disk brakes are more effective with a larger swept area, which is true but not for the reason one might expect. It is allowed that a larger swept area provides for greater mass to be cooling when not under the pads, but that's not the primary factor.

Bicycles are frequently designed to be as light as possible, for reasons related to limited human power. This means smaller diameter disks, smaller pads and smaller swept area. It matters not how much power the hand can apply to the lever if there is insufficient material to accept the force. The smaller diameter means a smaller (shorter) lever arm when speaking of mechanical advantage.

Motorcycles have substantially more power and can afford to have a few more pounds of rotating weight, including doubling up the number of disks. This increases the friction area providing for more powerful braking. Accordingly, larger diameter means a longer lever arm as well.

The reverse is true, however. A lightweight bicycle and rider need not have toss-you-over-the-handlebars braking, even though they do, when operated poorly by the rider. Motorcycles can also perform this acrobatic feat, but with the mass of a motorcycle and rider, more braking is necessary.

For the same reason, four pads on a disk are going to apply more braking force than two. Calipers in disk brakes operate by floating against the surface of the disk, reducing almost to zero the distance traveled by the pad when braking. This means that the majority of travel of the hydraulic system is placed into friction. Old-style drum brakes still in use on the rear wheels of some vehicles have to be adjusted to keep the shoes clear of the braking surface, "eating up" some of the force by traveling to contact the drum.

Pad softness/hardness/compostion usually means something in terms of pad life and damaging wear to the disks. A soft pad may provide great, grippy braking and wear out in half the time of a harder pad, while the super hard pads will still grip well when properly bedded, but may chew away disk material in short order, requiring expensive disk replacement. Foreign material between the pad and disk will accomplish the wear without providing braking benefits.

It's advised to use the manufacturer's recommendation for pads on a specific disk. When replacing a disk with a more exotic version, pad recommendation is always required.

In response to the commented question about insufficient material: Your hand can apply enough force to brake a heavy motorcycle and if you're on a bicycle, your hand does not have less force as a result. Applying more force than the bicycle brakes can accept results in something else going wrong. The calipers could be forced apart, the hydraulic tubing used could rupture, or your muscles will just hurt more than they should. I had hydraulic brakes on a tandem cycle and the response and power was wonderful and they were rim brakes, not disks.

I don't have an answer to the question about doubling pad area, as the physics involved in calculating such a complex system is beyond my understanding. With hydraulics, the increased area of the pistons in the calipers would mean a corresponding increase in the travel of the hand brake master cylinder.

Answer 2

Swept area INCLUDES all material of the rotor, not just the pad surface area. The entire rotor itself acts like a heat sink.