The science of disc brakes

butcher of bakersfield

A question out of curiosity more than anything else. I dare say many on here have been brought up on disc brakes too and won't have any idea of what I'm talking about. Or maybe I don't know what I'm talking about and just have an active imagination. But that's the question:

Forgetting about stopping power for a second, do disc brakes brake differently in comparison to traditional calipers?

The reason I ask, is being relatively new to disc brakes, I sometimes find them quite volatile. A lot of terrain has now become a no-no for using brakes, where I was comfortable using calipers. In my mind I attribute this to the brake stopping the center of the wheels, rather than the outer, making the braking sharper due to the smaller braking surface area and the wheel more likely to lock up, sometimes even with soft touches depending on the surface. I find you have to be a little more sensitive with the brakes because of this, having less margin for error, whereas calipers were always quite forgiving. Is there a science behind this, or am I just talking crap?

MountainPete

The pads on disc brakes move very little when you squeeze that lever - it's all to do with how they generate so much stopping power. As a result, there isn't much difference between 'slowing down' and 'grinding halt'. You just have to get used to them and be a bit more careful!

Tank-slapper

Perhaps Wikipedia can provide you with an answer:

http://en.wikipedia.org/wiki/Bicycle_brake_systems

butcher of bakersfield

MountainPete wrote:

The pads on disc brakes move very little when you squeeze that lever - it's all to do with how they generate so much stopping power. As a result, there isn't much difference between 'slowing down' and 'grinding halt'. You just have to get used to them and be a bit more careful!

I'm sure that is part of it, but I can't help but think the position of the brake is almost like a change of gears. With discs, I feel like the wheel is locking from the center, if that makes sense. A change to the centrifugal force or something. I don't know.

Atz

It's basically down to the fact the disc brakes are better at stopping you. If you're used to hauling on the lever like a madman to slow you on descents with V brakes then you're going to find discs a bit of a revelation.

supersonic

We know larger discs improve power - so that seems to oppose what you are saying in some respects as a rim is much larger still!

The main advantage of hydraulic disc brakes is effieciency of energy transfer through the hydraulics and pads on steel rotor. V brakes, rubber and aluminium don't seem to be quite as good, but the large rim means good power can still be had - in the dry, exceptional power.

if your disc brakes are on/off, grabby and lack modulation, or bite very early, you could try a different brand (lever and caliper mechanics also have a play on modulation) or a smaller rotor.

butcher of bakersfield

Atz wrote:

It's basically down to the fact the disc brakes are better at stopping you. If you're used to hauling on the lever like a madman to slow you on descents with V brakes then you're going to find discs a bit of a revelation.

It may be the case, but I'm not buying it just yet, not until I see the evidence.

What I'm thinking at the moment is, because the disc is much smaller than the rim, and hence it's spinning faster (depending on how you look at it, I can't think of the techy term), braking for a shorter time will have the effect of pulling the brake for longer on a rim brake. So small touches can equate to long touches on a rim brake, and it's these kinda short touches where I'm near soiling myself with discs. As though discs have made intricate braking defunct.

I'm hoping this makes sense, or at least strikes a chord with someone :?

supersonic

Angular revolution speed is the same - relative rotor surface speed is less the smaller the rotor is.

The smaller diameter the rotor the more lever force is required for the same stopping 'power'.

butcher of bakersfield

supersonic wrote:

We know larger discs improve power - so that seems to oppose what you are saying in some respects as a rim is much larger still!

The main advantage of hydraulic disc brakes is effieciency of energy transfer through the hydraulics and pads on steel rotor. V brakes, rubber and aluminium don't seem to be quite as good, but the large rim means good power can still be had - in the dry, exceptional power.

if your disc brakes are on/off, grabby and lack modulation, or bite very early, you could try a different brand (lever and caliper mechanics also have a play on modulation) or a smaller rotor.

Thing is, I'm not sure it's an issue of power at all. And I don't think my brakes are grabby. It's something I've felt with any bike I've ridden with discs....admittedly not many.

I imagine hydraulic Vs have a shed load of power, but I wonder if they'd feel the same as discs....I'm unsure.

supersonic

They will feel different as use rubber pads! But as above, lever and caliper bore ratios all have an effect on feel and power.

I have ridden disc brakes with lots of modulation, ones with no bite - they can vary quite a lot.

Atz

You need to learn how to modulate disc brakes differently to the older brakes. My road bike with conventional rim brakes is a different experience to the discs on my off road bikes and I have to adjust accordingly.

butcher of bakersfield

That's interesting. I'll have a look into the modulation. My mind is convinced that what I'm experiencing is something to do with the brake being in the center of the wheel though...

llamafarmer

I'd imagine what also plays a part is the relative power loss through materials flexing. In theory the reaction forces on the rotor cancel each other out and don't try to bend it laterally, unlike the reaction forces on a v brake, which are trying to push the fork legs or seat stays apart. V Brakes use a cable that can stretch and a long aluminium cantilever which will flex, transmitting the force into the fork/stays, which can also flex.

Being so close to a nice solid hub, all bolted together with a skewer means disc brakes have a much more solid platform from which to work and the hydraulics mean no power loss through cables.

Hydraulic discs do feel grabby to start with, but once you get used to applying it, the extra power is a really good thing! One finger braking ftw

supersonic

Hydros do not have to be grabby though.

nicklouse

if your brakes are grabby it is normally because of too much power from too big a disc brake.

unless the brakes are hayes.

joshtp

in one way, i know what you mean, on a slimy slick rock, or a sheet of ice, disks will lock easyily, however, it is simply a case of feathering it slowly and "feeling" the bitepoint, and then using it gently, on v's you can just grap it a bit more, it wont feel as delicate, disks however bite a bit harder and more sudenely, but again, it depends on make and model, personaly, i rend to brake very little, only before corners and things for which i am going way too fast, and when i do brake instead of feathering, i brake violently and very late, so for me a brake with a wooden feel is no problem, it actually soots me to have a brake with a definate bite. its quite rare for me to feather my brakes, i like to brake with "authority" bringing the bike down to the speed at which it needs to be going in an instant, and if at all posible, i like to avoid using the brakes at all.

bike-a-swan

if you had a rotor the size of the wheel, would it have the same difference in 'feel' that you suggest? i'm with llamafarmer on the materials being the cause

nicklouse

a rim mounted rotor was developed for trials bikes. so the wheel was either locked or free to move. (rear wheel only).
no modulation at all.

ride_whenever

it's mainly because of the non-compression of the fluid in comparison to the compression of brake outer.