Why do bigger rotors stop you quicker? (physics)

rhialto

yeehaamcgee wrote:

Gwaredd, Some would say that haivng a more powerful rear brake puts you in the same intelligence camp as the OP's dad.

I realize that we are getting off topic (although I bet the OP left the building ages ago), but why, yeehaa? When I am pointed down, I am very light on the front brake, preferring to scrub speed with the rear. When I am reducing speed on relatively flat sections, I use both brakes evenly, at least I think I do. Why would one want a more powerful front brake?

Anonymous

Because the front brake is the one that will have most traction, and most weight over it. it is the brake that will decelerate you and stop you most efficiently. This is why motorcycles will have two very very large discs at the front, and one tiny one on the back. this is why a Car has a similar front-weighed braking system.
Braking with the rear is a hang-on from what we were taught as toddlers learning to ride - don't use the front brake it will flip you over. Well, it's nonsense.

Skippy2309

physics....

front brake is best for braking, no matter which brake you pull the same thing will happen the bike slows but your body keeps moving, shifting weight to the front wheel, there fore the grip on the rear wheel is decreased as there is less weight over it causing it to lock and skid.

no way of escaping it, no matter what your doing if what your on stops your inertia keeps going forwards...

Gizmokev

supersonic wrote:

But you have the mass of the bike and rider driving the wheel around. The mass of the rotor is neglible compared to this. I think people are assuming just a spinning wheel rather than the system as a whole.

Yes it is neglible compared with the weight of the bike and rider added. I would suggest removing all of that and treat it in its purest form. Irrespective of what is added to the system to break it into its simplest form is the easiest way of understanding the physics of what is going on.

The extra heat generated comes from the increased velocity through the disc (kinetic to heat) by virtue of the extra distance from the axis of rotation.Assume the frictional force remain constant but it is only the disc size that increases. That force will not be able to grab any harder than on a smaller disc and will in fact be less able to grab the disc due to the increased velocity/momentum of the wheel/disc. What I now see is that as well as better heat dissipation a larger disc will give you greater control of your braking but not an increase in your braking force so long as everything else in the brake system remains constant.

supersonic

But for a given brake lever effort, you do get more energy converted to heat with a larger rotor, and larger torque applied to the hub. Putting the brakes on a dyno shows that, and agrees perfectly with theory.

Grab a piece of string in your fingers tightly. Pull slowly. Feel any heat? Now with the same finger pressure, pull it faster. A lot more heat! Yet the pressure is the same. The same would happen if you tried to grab the rotor between finger and thumb.

hobbitsharley

dependant on weight distribution when you brake hard your rear wheel unweights locking up giving you less control as everyone knows a rotating wheel has more grip and traction than a sliding one (hence abs) there are exceptions to all these rules and statements and my motorcycle is one of them due to its physical weight and distribution of its portly weight, it has a larger rear brake disc compared to the front and more braking force can be applied to the rear and braking is led by the rear over the front - this does not mean the front is poor its just that trying to stop a 600lb motocycle with most of its weight over the rear wheel with just a single front brake is like trying to stop an elephant with a stick - as i said its the exception to the rule and every other bike i have owned has had the front brake as the most powerful be it single disc or twin set up

Anonymous

Sounds like a monumentally bad design from Harley Davidson (assuming from your image that that's what the motorcycle is).
Of course that doesn't surprise me in the slightest.

supersonic

http://www.hayesdiscbrake.com/hayes-u/rotor/

Gizmokev

supersonic wrote:

But for a given brake lever effort, you do get more energy converted to heat with a larger rotor, and larger torque applied to the hub. Putting the brakes on a dyno shows that, and agrees perfectly with theory.

Grab a piece of string in your fingers tightly. Pull slowly. Feel any heat? Now with the same finger pressure, pull it faster. A lot more heat! Yet the pressure is the same. The same would happen if you tried to grab the rotor between finger and thumb.

Yes I agree with you.....but where is your extra heat coming from....It is coming from the increase in speed of the string....not an increase in brake/finger pressure.

supersonic

The same way a larger rotor has more rotor pass through the pads in a revolution. Each rev takes the same time... so the rotor is moving faster at the edge than a smaller one.

Gwaredd

yeehaamcgee wrote:

Gwaredd, Some would say that haivng a more powerful rear brake puts you in the same intelligence camp as the OP's dad.

I knew this would have the purists screaming 'Nooooooooo!!!!'

2 Reasons:

1) The reason I mentioned. Coming from motocross, you very rarely use your front brake hard unless you're coming to a complete sudden stop. You usually trail brake into corners using the rear & lightly dragging the front to scrub speed.This is something I have carried across to mountain biking. Road bikes are very different, hence the two huge discs.

2) I needed a new rear disc after a pad failed on a long descent scoring the disc. My mate had a spare 203mm disc & CRC had the 203mm adaptor on sale @ £6.00 which was cheaper than a new 180mm disc @ £18.00. No brainer.

Just to make you go apoplectic, I go through my rear pads more than my front at a ratio of 3:1

supersonic

This in turn creates a force opposing the rotation, which multiplied by the lever radius provides more torque about the hub, therefore more decelleration for a given pad pressure.

Anonymous

Gwaredd, there is no reason to use the rear brake more than the front in motocross. Same principles apply.

Gizmokev

supersonic wrote:

The same way a larger rotor has more rotor pass through the pads in a revolution. Each rev takes the same time... so the rotor is moving faster at the edge than a smaller one.

Which is what I said initially.....but extra heat does not make for extra brake force...the extra heat is from the extra velocity of the disc due to its increased size.....when you apply an equal frictional force to the disc a larger disc will pass through the pads easier than a smaller one due to its increased speed/momentum. Try grabbing a fast moving kids roundabout to get the idea.

Lets remember here as well that deceleration is not linear but inversely proportional to speed.

So a with 2 set ups side by side the only difference being the disc size the smaller disc will decelerate quicker and come to a complete stop faster for a given pad pressure.

It would be interesting to do this in my garage but I cant be arsed setting it all up.

Skippy2309

yeehaamcgee wrote:

Gwaredd, there is no reason to use the rear brake more than the front in motocross. Same principles apply.

I was actually told to use the rear when cornering on a motorbike... as the front wheel will only do one thing at a time and will wash out braking on a bend. So its probably a learnt thing.


Personally I always go front brake and try not to hit the brakes when i am leaning a bike in a corner or turning.

Anonymous

Gizmokev wrote:

Try grabbing a fast moving kids roundabout to get the idea.

Yep, grabbing a roundabout on the outside makes it easier to stop than grabbing it in the middle.
Since we're on a mountain bike forum, and aren't in the crudcatcher we presumably (hopefully) have at least one bike each at out disposal.
So, do this for me, before you post anything else.
Spin up a wheel on your bike, and try and stop it by grabbing the outside edge, which would be the tyre.
Now do the same thing but this time try grabbing it nearer the center.
Ignoring everything else in this thread, this blows your roundabout theory out of the water.
And don't go shouting your mouth off, go and DO IT. Then you can say you're doing science.

Skippy, several things are taught just for "safety" reasons, because it's easier than teaching people to do the right thing, correctly.
The front wheel braking thing is a perfect example. It's actually far more useful to teach people to learn to get a feel for the front brake. It's all that will stop you in a hurry anyway.
Same with the "don't brake whilst cornering" thing, on both motorcycles and cars.
Actually, you CAN, but you need to know what you're doing, and how to not overdo it. There's even a name for it in racing circles, "trail braking".
The MotoGP yesterday had a perfect example of braking hard in the middle of a corner, when Stoner thought he was going to run into the back end of his team mate right at the end of the race. Cranked right over, on a fast bend, he hits the brakes, and he's fine.

rhext

You'll all using the wrong branch of science to answer this question. It's nothing to do with physics or engineering: it's psychology.

My physics degree would not stop me from being adamant that the moon is made of green cheese and is held up by skyhooks if one of my know-it-all offspring tried to teach me about gravity. It may be true that the Great Wall of China does not have a catflap every mile, but I don't see any reason why my kids need to know that.

hobbitsharley

yeehaamcgee wrote:

Sounds like a monumentally bad design from Harley Davidson (assuming from your image that that's what the motorcycle is).
Of course that doesn't surprise me in the slightest.

its only the softails that seem to suffer this design issue - even from inception they can ruin pads within 3K miles and you will be replacing rears before fronts they also use different compounds front and rear to try and balance them out as well as thickness

mine is more extreme due to the apes and lowered at the rear

as for trail braking the rear in corners - should always scrub speed in a striaght line and then corner is what i was taught and trail brake thru corners to keep the balance

hobbitsharley

yeehaamcgee wrote:

Sounds like a monumentally bad design from Harley Davidson (assuming from your image that that's what the motorcycle is).
Of course that doesn't surprise me in the slightest.

its only the softails that seem to suffer this design issue - even from inception they can ruin pads within 3K miles and you will be replacing rears before fronts they also use different compounds front and rear to try and balance them out as well as thickness

mine is more extreme due to the apes and lowered at the rear

as for trail braking the rear in corners - should always scrub speed in a striaght line and then corner is what i was taught and trail brake thru corners to keep the balance

Anonymous

hobbitsharley, I have to say, I'm finding it very hard indeed to visualise a bike so odd in shape that the front wheel has so little weight over it that it cannot create sufficient traction to be an effective brake. Once the bike is decelerating, the weight transfer to the front wheel would just reinforce it. I think Rhext is right. I think it's psychology.

Gizmokev

yeehaamcgee wrote:

Gizmokev wrote:

Try grabbing a fast moving kids roundabout to get the idea.

Yep, grabbing a roundabout on the outside makes it easier to stop than grabbing it in the middle.
Since we're on a mountain bike forum, and aren't in the crudcatcher we presumably (hopefully) have at least one bike each at out disposal.
So, do this for me, before you post anything else.
Spin up a wheel on your bike, and try and stop it by grabbing the outside edge, which would be the tyre.
Now do the same thing but this time try grabbing it nearer the center.
Ignoring everything else in this thread, this blows your roundabout theory out of the water.
And don't go shouting your mouth off, go and DO IT. Then you can say you're doing science.

I did that before posting anything in the first place to make sure my theory was right. My argument assumes the brake force or frictional force is the same throughout. I was using the roundabout as an example to illustrate angular momentum. Grab that roundabout at the edge and see how quickly you get ripped off your feet. Now reduce the diameter of the roundabout and you wont because the angular momentum is much reduced. Same applies to the disc. I have no idea as to the weight differences between disc sizes but angular momentum equates to mass x velocity x radius. Deceleration is directly linked to angular momentum so a smaller disc has a lesser mass, velocity and radius than a larger disc so its angular momentum is less and so it is easier to decelerate. The faster things move the harder they are too stop.

Yes Sonic is right the torque increases for a given brake force as you venture further from the axis of rotation but no-one has factored in momentum increasing as well.

hobbitsharley

yeehaamcgee wrote:

hobbitsharley, I have to say, I'm finding it very hard indeed to visualise a bike so odd in shape that the front wheel has so little weight over it that it cannot create sufficient traction to be an effective brake. Once the bike is decelerating, the weight transfer to the front wheel would just reinforce it. I think Rhext is right. I think it's psychology.

sent you pm from EBC

rhext

Gizmokev wrote:

yeehaamcgee wrote:

Gizmokev wrote:

Try grabbing a fast moving kids roundabout to get the idea.

Yep, grabbing a roundabout on the outside makes it easier to stop than grabbing it in the middle.
Since we're on a mountain bike forum, and aren't in the crudcatcher we presumably (hopefully) have at least one bike each at out disposal.
So, do this for me, before you post anything else.
Spin up a wheel on your bike, and try and stop it by grabbing the outside edge, which would be the tyre.
Now do the same thing but this time try grabbing it nearer the center.
Ignoring everything else in this thread, this blows your roundabout theory out of the water.
And don't go shouting your mouth off, go and DO IT. Then you can say you're doing science.

I did that before posting anything in the first place to make sure my theory was right. My argument assumes the brake force or frictional force is the same throughout. I was using the roundabout as an example to illustrate angular momentum. Grab that roundabout at the edge and see how quickly you get ripped off your feet. Now reduce the diameter of the roundabout and you wont because the angular momentum is much reduced. Same applies to the disc. I have no idea as to the weight differences between disc sizes but angular momentum equates to mass x velocity x radius. Deceleration is directly linked to angular momentum so a smaller disc has a lesser mass, velocity and radius than a larger disc so its angular momentum is less and so it is easier to decelerate. The faster things move the harder they are too stop.

Yes Sonic is right the torque increases for a given brake force as you venture further from the axis of rotation but no-one has factored in momentum increasing as well.

You'd have a point if the disk was the main store of the kinetic energy you're trying to get rid of. But it's not!

Anonymous

Gizmokev, the larger disc has a larger mass than a smaller disc, but this pales in comparison to not only the wheel, to which it is attached, but also the bike.
The wheel will be several orders of magnitude heavier than the disc (the term "a drop in the ocean" springs to mind) and the bike+rider will be several orders of magnitude heavier AGAIN.

I want a video of you stopping a child's roundabout by grabbing it near the centre, then a stopping it by grabbing it at the edge, because I do not believe you have done the practical experiment.

supersonic

The momentum increase is negligible compared to the overall mass of the system and the extra torque the larger disc can provide. I am sure it can be worked out. A 203mm disc provides roughly 27% more braking 'force' than a 160mm rotor for a given lever input on the same system. This can be quite a sharp decelleration, a retarding force in the order of several dozens of newtons. The extra mass of the rotor being a little further out pales in comparison.

See here for more physics on forces:

http://www.ne.jp/asahi/julesandjames/ho ... k_release/

Anonymous

hobbitsharley wrote:

yeehaamcgee wrote:

hobbitsharley, I have to say, I'm finding it very hard indeed to visualise a bike so odd in shape that the front wheel has so little weight over it that it cannot create sufficient traction to be an effective brake. Once the bike is decelerating, the weight transfer to the front wheel would just reinforce it. I think Rhext is right. I think it's psychology.

sent you pm from EBC

Yes, a page from EBC discussing crap design (more powerful brakes on rear on big custom bikes), and the rider's decision to use the rear brake.
Ultimately meaning nothing, except that it's expected that riders and designers of custom bikes don't really know what they're doing.

Gizmokev

It is also worth noting that the bigger the disc the better cooling as previously stated. This is because of the larger surface area which allows easier heat dissipation. The brake disc gets cooled by its surrounding environment versus the rate of the increase of heat due to friction from your braking force, and so you'll see its much easier for smaller disc to get overheated rather than having a larger surface area for heat to expand out more and be cooled much easier. As you brake hard you have an amount time before you completely come to a stop. The more time the more heat you occur. So as this heat is increased by the time it takes for you to come to a stop without "totally locking them up" the discs and pads are hotter, rather than if you had a larger rotor which will get rid of the heat better. So sure with small discs you can lock them up instantly because the brake rotors are cool at that point. But try having your full braking strength when your rotors get hot and you will get surprised. As heat increases so friction coefficient decreases so a larger disc will give better braking not by virtue of its size but by virtue of its better heat dissipation properties.

So as you are going around your local trail dabbing the brakes here and there heat needs to be got rid of and that is where the bigger rotor comes in.

A lot of that has been plagiarised from a physics forum

rhext

yeehaamcgee wrote:

hobbitsharley, I have to say, I'm finding it very hard indeed to visualise a bike so odd in shape that the front wheel has so little weight over it that it cannot create sufficient traction to be an effective brake. Once the bike is decelerating, the weight transfer to the front wheel would just reinforce it. I think Rhext is right. I think it's psychology.

If you had a chopper, with the front wheel right out in front, heavy engine plus rider effectively over the rear wheel, that would rely on rear wheel braking.

mossychops

Gizmokev wrote:

I did that before posting anything in the first place to make sure my theory was right. My argument assumes the brake force or frictional force is the same throughout. I was using the roundabout as an example to illustrate angular momentum. Grab that roundabout at the edge and see how quickly you get ripped off your feet. Now reduce the diameter of the roundabout and you wont because the angular momentum is much reduced. Same applies to the disc. I have no idea as to the weight differences between disc sizes but angular momentum equates to mass x velocity x radius. Deceleration is directly linked to angular momentum so a smaller disc has a lesser mass, velocity and radius than a larger disc so its angular momentum is less and so it is easier to decelerate. The faster things move the harder they are too stop.

Yes Sonic is right the torque increases for a given brake force as you venture further from the axis of rotation but no-one has factored in momentum increasing as well.

Man, that's twice you have been told off now for not doing your homework. LOL.

Edit: And another rollocking while I was writing this.

supersonic

Gizmokev wrote:

It is also worth noting that the bigger the disc the better cooling as previously stated. This is because of the larger surface area which allows easier heat dissipation. The brake disc gets cooled by its surrounding environment versus the rate of the increase of heat due to friction from your braking force, and so you'll see its much easier for smaller disc to get overheated rather than having a larger surface area for heat to expand out more and be cooled much easier. As you brake hard you have an amount time before you completely come to a stop. The more time the more heat you occur. So as this heat is increased by the time it takes for you to come to a stop without "totally locking them up" the discs and pads are hotter, rather than if you had a larger rotor which will get rid of the heat better. So sure with small discs you can lock them up instantly because the brake rotors are cool at that point. But try having your full braking strength when your rotors get hot and you will get surprised. As heat increases so friction coefficient decreases so a larger disc will give better braking not by virtue of its size but by virtue of its better heat dissipation properties.

So as you are going around your local trail dabbing the brakes here and there heat needs to be got rid of and that is where the bigger rotor comes in.

A lot of that has been plagiarised from a physics forum

Much of it is right - larger rotors will rid of the heat better and reduce chance of fade. But it still takes more lever effort with a smaller rotor for a given torque and therefore decelleration. Larger rotors are used for both these reasons.