brake torque : pressure relationship

[charliefarlie]

Hi - can anyone of you clever chaps please tell me the telationship between hydraulic brake pressure and braking torque for a defender front axle . A graph would be good but a factor I can multiply pressure by to get torque would be good too .  For example 50 bar pressure results in 2500 Nm torque ! Thanks 

[lo-fi]

Wikipedia has this to offer:

"The dynamic friction coefficient "µ" for most standard brake pads is usually in the range of 0.35 to 0.42. This means that a force of 1000 Newtons on the pad will give a resulting brake force close to 400 Newtons"

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

You'll need to create a calculation which take into account piston area, pad area, disk diameter and that coefficient. Be warned that this is an extremely simplistic model, and pretty much good for nothing more than curiosity's sake. 

 

And welcome!

Edited May 15, 2018 by lo-fi

[landroversforever]

What are you trying to work out.

I don't think the calculation is going to do anything meaningful  .

[Escape]

Pad area doesn't directly affect braking force. Larger pads are fitted to get acceptable wear and better heat dissipation.

Force on the pads is determined by the pressure in the system multiplied by the piston area and number of pistons. With the above friction coefficient you get the force on the discs. To get braking torque you need to multiply with the radius from the axle center to the center of the force applied by the  pad. The middle of the pad would be a conservative estimate.

Also curious what you're trying to work out?

Filip

[Snagger]

14 hours ago, Escape said:

Pad area doesn't directly affect braking force. Larger pads are fitted to get acceptable wear and better heat dissipation.

Force on the pads is determined by the pressure in the system multiplied by the piston area and number of pistons. With the above friction coefficient you get the force on the discs. To get braking torque you need to multiply with the radius from the axle center to the center of the force applied by the  pad. The middle of the pad would be a conservative estimate.

Also curious what you're trying to work out?

Filip

Resultant pressure on the pads is also going to depend on pad area - the force is going to he hydraulic pressure x piston sectional area.  Pad pressure will be that force divided by the pad area.

I can see how increased pad area helps with reducing wear, but I can't see it being irrelevant to braking capacity.  Sure, it won't be a linear relationship between area and braking effort, but there will be a relationship.  As for cooling, a bigger pad will have greater heat sink, but their composites don't absorb heat as much as the discs, which is where most of the energy goes.  However, bigger pads mask more of the disc and would thus reduce disc cooling.  The disc temperature is going to be a function of how much energy you put into it, and I can't see how pad size will affect that for a given retardation rate.

[Carloz]

On 18-5-2018 at 9:03 AM, Snagger said:

Resultant pressure on the pads is also going to depend on pad area - the force is going to he hydraulic pressure x piston sectional area.  Pad pressure will be that force divided by the pad area.

I can see how increased pad area helps with reducing wear, but I can't see it being irrelevant to braking capacity.  Sure, it won't be a linear relationship between area and braking effort, but there will be a relationship.  As for cooling, a bigger pad will have greater heat sink, but their composites don't absorb heat as much as the discs, which is where most of the energy goes.  However, bigger pads mask more of the disc and would thus reduce disc cooling.  The disc temperature is going to be a function of how much energy you put into it, and I can't see how pad size will affect that for a given retardation rate.

Brake capacity and brake force are different things.

 The first one is about how much energy can be handeled and the 2nd is the force which can be handeled. Those two are not direct connected because you can have a brake with a huge capacity but with little brake torque/force and vice verca.

[Snagger]

I understand that - capacity is more about resistance to fading, ie the ability to be repeatedly used for heavy braking, while force is what can be done in a single moment.  But my comment above is that they cannot be treated in isolation; one will influence the other and the deign has to take account of that.

[Escape]

On a basic level, friction is independent of area, it is just force * friction coefficient. So increasing pad size for the same clamping force, will not increase braking. Of course this only holds in a well designed and balanced system and for a relatively small increase (or decrease) in pad size. If the friction surface becomes too small for the force applied, friction will no longer depend only on the surface effects, but the pads will dig into the disc (stick-slip), causing much greater wear and with less linearity (deformation). As for heat transfer, bigger pads will transfer less heat to the piston and brake fluid, so have less chance of boiling the fluid. Again, for a small increase in pad size, the effect on the cooling surface of the disk will be limited.

But this is taking it a bit beyond the original question and I don't think we really disagree, just different viewpoints.

Charlie, a bit of feedback as to what you're after exactly?