Braking Forces

[nikpro]

Right, this has done my head in and given me brain ache!

Had a discussion the other day around improving barkes on a car and I was told my theory was wrong.

Here was my arguement:

The owner of the car wanted to upgrade the front brakes on his vehicle. He intended changing just the front calliper to a '4 pot' system from a 'two pot' system.

When I checked the specs of the calliper the surface area of the 4 pistons was identical to the surface area of the two pot system but the pads of the 4 pot system were slightly larger.

I argued that without changing anything else this would not improve the braking efficiency of the vehicle as the pressure acting on the pads was the same in both callipers. The fact that the pads were larger made no difference as the pressure acting on the brake disk was reduced due to the larger surface area.

He was argueing that there would be more braking force as the pads were larger?

Who was right?

[Snake the Sniper]

One advantage of 4 pot set ups can well be the slightly larger pads, but also the more even pressure across the pad. Ok, the difference will be small, but how noticable will depend on how adequate the stock brakes are. Most people would get 90% of the benefits from just changing the pads to a more fast road type, rather than the usual cardboard ones.
Oh, the 4 pots may also be lighter, giving a reduction in unsprung weight which is useful.

[nikpro]

Thanks for the reply - I understand about a four pot brake pad being larger but surely if the pistons have the same surface area the pressure the pad exerts on the disc will be less since
Pressure=force/area

[Snake the Sniper]

I'm not 100% sure how the pressure will vary from the 2 pot set up, just that what pressure there is will be more evenly distributed across the pad. Some 6 pot callipers have all 3 pistons as different sizes to maintain an even pressure under braking, as the leading edge of the pad can pull itself in.

Oh, also since the pad is bigger, there will be more area further away from the hub centre, which even though the pad pressure may be lower, could well give more braking effort due to the lever arm effect.

Edited by Snake the Sniper on Sunday 21st December 13:59

[Sam_68]

The other thing that has a major influence is the shape of the pad.

The 4 pot caliper may well have the same pad area, but if it's a long, thin pad where a greater part of the pad's area sweeps the outer part of the disc, it will brake a lot better, because the speed of the disc surface relative to the pad will be much faster.

...and a 4-pot caliper can maintain more even pressure between the disc and the pad, especially with longer shaped pads, so increasing efficiency considerably.

[spend]

Surely the greater piston area the greater force your foot effort will generate on the pad, extra pistons generally allow greater area to exert pressure & force = pressure x area. You may have more pedal travel so you have to press longer thats all AFIK.

[nikpro]

If the piston area increased then, yes the pressure applied to the pads would also increase and, all else being equal the braking would improve.
My arguement is that if the piston area is the same, the force applied to the pads is the same and therefore the braking is the same.
This has been confirmed via Internet search!
I understand pad application on multi pot callipers might be slightly better but this is cancelled out by the slightly lower force applied by the multi pots added hydraulic frictional losses.
Usual advantage of multi pot callipers is to increase piston area within the swept area of brake disc
Thanks for help.

[Snake the Sniper]

spend said:

Surely the greater piston area the greater force your foot effort will generate on the pad, extra pistons generally allow greater area to exert pressure & force = pressure x area. You may have more pedal travel so you have to press longer thats all AFIK.

But that's the point here. The 4 pot calliper has the same piston area as the 2 pot. Hence the discussion about swept area/swept shape. As SAM68 and I have said, if more of the pad is further out from the hub, then although the pad pressure is the same, the braking force will be greater. That is a large part of having bigger disks, as opposed to just having a larger area.

[nikpro]

Snake the Sniper said:

spend said:

Surely the greater piston area the greater force your foot effort will generate on the pad, extra pistons generally allow greater area to exert pressure & force = pressure x area. You may have more pedal travel so you have to press longer thats all AFIK.

But that's the point here. The 4 pot calliper has the same piston area as the 2 pot. Hence the discussion about swept area/swept shape. As SAM68 and I have said, if more of the pad is further out from the hub, then although the pad pressure is the same, the braking force will be greater. That is a large part of having bigger disks, as opposed to just having a larger area.

Exactly; but in this case the disc remains the same diameter and although the pad is larger the pressure on the disc must be less

as previously stated:

Pressure = Force/Area

We know the force is the same in both scenarios (Piston area is the same and master cylinder is unchanged)but the pad area has increased, therefore pressure must drop. It surely stands to reason that the effectiveness of the larger pad is negated by the decrease in pressure (accepted it might not be identical but I don't think it's worth the £400 upgrade considering pad choice is very limited for the 4 pot calliper compared with the existing 2 pot)

[Snake the Sniper]

nikpro said:

Snake the Sniper said:

spend said:

Surely the greater piston area the greater force your foot effort will generate on the pad, extra pistons generally allow greater area to exert pressure & force = pressure x area. You may have more pedal travel so you have to press longer thats all AFIK.

But that's the point here. The 4 pot calliper has the same piston area as the 2 pot. Hence the discussion about swept area/swept shape. As SAM68 and I have said, if more of the pad is further out from the hub, then although the pad pressure is the same, the braking force will be greater. That is a large part of having bigger disks, as opposed to just having a larger area.

Exactly; but in this case the disc remains the same diameter and although the pad is larger the pressure on the disc must be less

as previously stated:

Pressure = Force/Area

We know the force is the same in both scenarios (Piston area is the same and master cylinder is unchanged)but the pad area has increased, therefore pressure must drop. It surely stands to reason that the effectiveness of the larger pad is negated by the decrease in pressure (accepted it might not be identical but I don't think it's worth the £400 upgrade considering pad choice is very limited for the 4 pot calliper compared with the existing 2 pot)

I think in this case the "upgrade" isn't really much of an upgrade at all, merely a way to waste £400. The first thing I always do if I think the brakes need upgrading on a car is change the pads and maybe the discs for better versions. Unless you plan on doing lots of track/race work or you've upped the power by a fair bit, then better pads and vented/drilled discs will normally suffice. There are some vehicles where uprating the callipers is a good idea, but mainly for the weight loss. My 7 for example, stops quite well on the standard sierra brakes, but they are quite heavy.

[leorest]

Snake the Sniper said:

spend said:

Surely the greater piston area the greater force your foot effort will generate on the pad, extra pistons generally allow greater area to exert pressure & force = pressure x area. You may have more pedal travel so you have to press longer thats all AFIK.

But that's the point here. The 4 pot calliper has the same piston area as the 2 pot. Hence the discussion about swept area/swept shape. As SAM68 and I have said, if more of the pad is further out from the hub, then although the pad pressure is the same, the braking force will be greater. That is a large part of having bigger disks, as opposed to just having a larger area.

...or put another way...
The mechanical advantage (think levers) of the calliper on the wheel is defined by the distances from the centre(s) of the pistons to the centre of the wheel (and the rolling radius, though that isn't changing in this example) If the total area is the same then the four pot calliper will have smaller pistons which allows their centres to be nearer the edge of the disk. I thought that was going to be easier to explain in words. Obviously I'm wrong. Ignore me if I've confused matters LOL.

[spend]

I guess if the piston areas are equal there is no straightforward answer, completely dependent on comparing the piston - pad - disk footprints and how the clamping force is exerted (including the float I suppose). Generally it will be pretty obvious that multi-pistons will put more force on the outside of the disk, exactly where you would want it?

[Snake the Sniper]

The mechanical leverage will depend on the calliper. Some move the piston centres out, some don't. Depends how it's been designed. It may be a lower profile calliper with the centres just the same. It may move them out a bit. Either way, a new pad will be stiff enough for it to not make a lot of difference.

Oh, just thought of another advantage. The new calliper may well be stiffer, which can give a more consistent pedal under heavy braking.

Edited by Snake the Sniper on Sunday 21st December 17:42

[Snake the Sniper]

spend said:

I guess if the piston areas are equal there is no straightforward answer, completely dependent on comparing the piston - pad - disk footprints and how the clamping force is exerted (including the float I suppose). Generally it will be pretty obvious that multi-pistons will put more force on the outside of the disk, exactly where you would want it?

Ideally, all on the outside edge to gain the maximum lever length. But then you get brake drums!

[Avocet]

Snake the Sniper said:

I'm not 100% sure how the pressure will vary from the 2 pot set up, just that what pressure there is will be more evenly distributed across the pad. Some 6 pot callipers have all 3 pistons as different sizes to maintain an even pressure under braking, as the leading edge of the pad can pull itself in.

Oh, also since the pad is bigger, there will be more area further away from the hub centre, which even though the pad pressure may be lower, could well give more braking effort due to the lever arm effect.

Edited by Snake the Sniper on Sunday 21st December 13:59

I think the differeing piston size is to exert greater presure at the back edge of the pad - which will be working at a higher temperature than the front edge and thus, more prone to fade.

[GreenV8S]

I don't think fade is the issue. The pads have a natural tendency to twist due to the offset between the friction surface and the backplate, this twisting moment increases the clamping load on the leading half of the pads and reduces it on the trailing half. The better multi-piston callipers have a smaller piston on the leading edge to counteract this effect.

[nikpro]

leorest said:

or put another way...
The mechanical advantage (think levers) of the calliper on the wheel is defined by the distances from the centre(s) of the pistons to the centre of the wheel (and the rolling radius, though that isn't changing in this example) If the total area is the same then the four pot calliper will have smaller pistons which allows their centres to be nearer the edge of the disk. I thought that was going to be easier to explain in words. Obviously I'm wrong. Ignore me if I've confused matters LOL.

Not sure if I agree with this?

The force exerted by the piston is even all around it's edge - so why is it measured to the centre of the piston and not to it's edge?

But, More so - the pad is stiff enough to transfer this force all over its surface otherwise the pad wear would be uneven?

How many brake pads do you replace where they are worn just where the pistons push on the back of them?

I think we are now looking to deeply into it!

I am quite satisfied that if the area of the pistons in a four pot calliper = the area of the pistons in a two pot calliper and nothing else changes on the braking system then there will be no significant improvement in braking.

I do accept there may be minor differences but the whole idea of a brake upgrade IMO is to improve braking force along with the other differences. (ie more surface area to the pistons and greater mechanical advantage by increasing disc size.)

[leorest]

nikpro said:

leorest said:

or put another way...
The mechanical advantage (think levers) of the calliper on the wheel is defined by the distances from the centre(s) of the pistons to the centre of the wheel (and the rolling radius, though that isn't changing in this example) If the total area is the same then the four pot calliper will have smaller pistons which allows their centres to be nearer the edge of the disk. I thought that was going to be easier to explain in words. Obviously I'm wrong. Ignore me if I've confused matters LOL.

Not sure if I agree with this?

The force exerted by the piston is even all around it's edge - so why is it measured to the centre of the piston and not to it's edge?

But, More so - the pad is stiff enough to transfer this force all over its surface otherwise the pad wear would be uneven?

How many brake pads do you replace where they are worn just where the pistons push on the back of them?

I think we are now looking to deeply into it!

I am quite satisfied that if the area of the pistons in a four pot calliper = the area of the pistons in a two pot calliper and nothing else changes on the braking system then there will be no significant improvement in braking.

I do accept there may be minor differences but the whole idea of a brake upgrade IMO is to improve braking force along with the other differences. (ie more surface area to the pistons and greater mechanical advantage by increasing disc size.)

My point is that the centre of the piston(s), expressed as a distance from the centre of the hub, is likely to be very close to the effective centre of applied force and for a simplified model of what is going on (levers) I think this suffices.
Put another way... If the smaller pistons allow the force to be applied further out the leverage will be greater and that is all I'm trying to convey.

[GreenV8S]

nikpro said:

The force exerted by the piston is even all around it's edge - so why is it measured to the centre of the piston and not to it's edge?

But, More so - the pad is stiff enough to transfer this force all over its surface otherwise the pad wear would be uneven?

How many brake pads do you replace where they are worn just where the pistons push on the back of them?

I think we are now looking to deeply into it!

The position of the center of the piston tells you where the center of effort is and hence the average mechanical leverage that the pads have (distance from the axle centre line.

Pads do flex especially under heavy braking and this results in an uneven pressure distribution and reduced effectiveness. This would only show up as uneven pad wear if you spend a lot of time braking hard, but if you're ever looked at the pads after a hard track day you'll find they show uneven wear. Since this leaves the pads with a curved friction surface, the brakes can feel spongy under light braking until they're had time to wear 'flat' again in normal use. At that point you would normally throw them away rather than carry on using them, though.

[RT106]

Don't think anyone's actually answered the original question...

Pad area makes sod all difference to the maximum achieveable braking effort, but a greater area will probably result in less fade. Decent multi-pot calipers are all about maximising the effectiveness of the pad/disc contact area to minimise localised fade, moving the lever-arm as close to the edge of the disc as possible and (in the case of most OE brakes) removing the potentially problematic sliding calipers.

Having said that, I've never understood why people spend/waste so much money on uprated brakes for slowly driven track-cars. As said above, decent pads and discs are usually all you need, especially as the factor that really limits how quickly you can stop is friction between the car and the road.