OT- Force at the brake caliper

dkmc · Sep 7, 2009

Interested to know some numbers ie; what is the actual range of clamping force -at the brake caliper- from say, a small car to a full size pickup truck?
Is this data in print someplace?

dk

metlmunchr · Sep 8, 2009

dkmc said:
Interested to know some numbers ie; what is the actual range of clamping force -at the brake caliper- from say, a small car to a full size pickup truck?
Is this data in print someplace?
dk

Dan, when I worked for Michelin we had Motor Vehicle Manufacturer's Association standardized spec sheets on all sorts of vehicles. Detailed down to the part numbers, wire diameter, free length, number of coils, and spring rates of springs used on a given model depending on its options. Tons of info on most every mechanical component of a vehicle, but never did see anything like what you're wanting. IIRC the brake info covered caliper piston dia, rotor dia and thickness, swept area, master cyl bore dia, etc. But nothing on the clamping force or any ratio between applied force at the pedal and developed force at the caliper.

Phil Burman · Sep 8, 2009

Depends on what you mean by clamping force.

Here is a simple calc of the force applied to the disc by the pads, substitute you own values and recalc according to the formula used. I take no responsibility for the correctness.

Phil

braking distance - L = 160 ft
brake from 60 to 0 mph - V = 88 ft/s
vehicle mass - m = 1,500 lbs
tire radius - Rt = 10.0 inch
brake caliper on radius - Rc_ = 3.0 inch
coeff. of friction (pad/steel) - Coef = 0.9

average deceleration - a = 24.2 ft/s^2
average stopping force - Fs = 1,134 lbs
shear force on brake caliper - Fc = 3,781 lbs
clamping force on disc - Fd = 4,201 lbs

a = V^2/(2*L)
Fs = m*a/32
Fc = Rt/Rc_*Fs
Fd = Fc/Coef

Forrest Addy · Sep 8, 2009

Do you need to know for some reason or are you just curious? Either way the question is open ended. The clamping forcee will vary with the weight of the vehicle, size of the brake cylinders, braking effort distribution, decelleration required etc. And that's about all you'll get out of me. Your question will need a LOT of research and experimentation to answer. There's probably an SAE or ASTM paper somewhere that addresses this but you'll have to pony up a membership fee to gain access to it.

If you are really curious you'll have to gather cylinder diameter data and make a pressure gage with suitable fittings to be installed in the brake line of whatever test vehicles you can round up. I seem to recall a typical modern brake system maxes at about 1200 PSI in a hard stop. Go from that to the disk brake cylinder dia and the number of cyls per side. Mash the brake pedal and monitor the line pressure for verious kinds of stops. Do the math and you'll have the brake clamping pressure for all the good it will do you.

The clamping pressure for internal drum brakes is trickier. The shoes and internal linkage are self-actuating to a degree. Drum brake cylinder pressure will not be a reliable indication of the actual radial force of the shoes against the drum.

Here's a link:

http://www.stu-offroad.com/suspension/bpress/brake-pressure.htm

and a Google page:

http://www.google.com/search?hl=en&ie=ISO-8859-1&q=brake+line+pressure&btnG=Search

Limy Sami · Sep 8, 2009

Hang about guys, granted I'm only a simple metal basher, buy how can the vehicle weight, friction area etc etc etc have anything to do with the caliper force?

Caliper force ( as requested by the OP) can only be a product of line pressure multiplied by caliper piston area.

The line pressure is a product of force applied to brake pedal X mechanical advantage, X piston area.

FWIW a servo doesn't increase line pressure, only reduce pedal effort.

Anyway, one raggy assed metal basher retires and awaits to be shot down in flames.

Why am I getting the feeling this will run and run?

Phil Burman · Sep 8, 2009

Basically because one can either measure the result (hydraulic pressure) or one can calculate what is needed (weight, friction etc). The first method requires a car fitted with a pressure gauge together with a calculator the second only requires a calculator. Of course the first method will be more accurate.

Phil

PS: to caclulate from the foot to the caliper piston requires a number of difficult to find variables. From the tire/road contact point to the caliper piston has easily findable variables.

Limy Sami said:
Hang about guys, granted I'm only a simple metal basher, buy how can the vehicle weight, friction area etc etc etc have anything to do with the caliper force?

Caliper force ( as requested by the OP) can only be a product of line pressure multiplied by caliper piston area.

The line pressure is a product of force applied to brake pedal X mechanical advantage, X piston area.

FWIW a servo doesn't increase line pressure, only reduce pedal effort.

Anyway, one raggy assed metal basher retires and awaits to be shot down in flames.

Why am I getting the feeling this will run and run?

Carl Darnell · Sep 8, 2009

dk, all you have to know to get the clamping force is the diameter of the master cylinder piston and diameter of the caliper piston/s. Determine the area of each and that is the ratio of force. Then the force on the rotor is the ratio you just figured out.

As you can see the force varies as the pressure your foot applies to the master cylinder changes.

The power brake booster has to be figured in there somewhere and that may be the bugger in the mess.

You could do it with a line pressure gauge and figure the psi on the area of the piston/s in the caliper and that may be the most accurate way to find it.

moonlight machine · Sep 8, 2009

On the old turbo charged indy cars, lola, reynard etc. you could see line preasures up to 1800 psi....each caliper has 8 pistons pushing on 4 pads. 4 pistons @1.255 dia and 4 @1.020 dia. its way to early for me to do the math..............

Phil Burman · Sep 8, 2009

Don't forget the brake pedal lever or the front brake bias in the hydraulic distribution system and of course what force will you apply with your foot.

Phil

Carl Darnell said:
dk, all you have to know to get the clamping force is the diameter of the master cylinder piston and diameter of the caliper piston/s. Determine the area of each and that is the ratio of force. Then the force on the rotor is the ratio you just figured out.

As you can see the force varies as the pressure your foot applies to the master cylinder changes.

The power brake booster has to be figured in there somewhere and that may be the bugger in the mess.

You could do it with a line pressure gauge and figure the psi on the area of the piston/s in the caliper and that may be the most accurate way to find it.

Hdpg · Sep 8, 2009

Surely the numbers for fluid pressure, cylinder area and stuff like that must be must be floating around somewhere in cyberland; I expect they are needed for programming the ABS computer.

thruthefence · Sep 8, 2009

here's a spreadsheet that may be helpful.

it allows the length of the pedal to vary, the master cyl size, # of caliper pistons, ect.

http://74.125.47.132/search?q=cache...xls&cd=1&hl=en&ct=clnk&gl=us&client=firefox-a

if the link doesn't work, search "brakemath.xls"

Phil Burman · Sep 8, 2009

I don't think it includes a calc for servo assistance. At least I couldn't find it.

Phil

thruthefence said:
here's a spreadsheet that may be helpful.

it allows the length of the pedal to vary, the master cyl size, # of caliper pistons, ect.

http://74.125.47.132/search?q=cache...xls&cd=1&hl=en&ct=clnk&gl=us&client=firefox-a

if the link doesn't work, search "brakemath.xls"

Carl Darnell · Sep 8, 2009

Yep, line pressure and master/slave areas will tell what you want.

dave5605 · Sep 8, 2009

Here is some info on racing brake technology.
http://books.google.com/books?id=yU...num=6#v=onepage&q=brake line pressure&f=false

Its a portion of the HP Book titled 'Brake Systems OEM and Racing Brake technology.

and then there is this for designing race car brakes
http://www.hotrodheaven.com/tech/brakes/brakes1.htm
and more reading...
http://www.thebrakeman.com/valvetechii

I'd hazard to guess that the clamping force is typically in the range of 300-500 under normal driving (in typical vehicle), higher in a panic stop, and is in the 1000+ range for high performance brakes used under extreme conditions.

However all that doesn't take into account the friction characteristics of the pads and rotors under varying conditions

USMCPOP · Sep 8, 2009

A bit of discussion of clamping force about halfway down this page:

http://www.import-car.com/Article/4..._brake_system_performance_and_appearance.aspx

Phil Burman · Sep 8, 2009

But what about the vacuum boost?

Phil

Carl Darnell said:
Yep, line pressure and master/slave areas will tell what you want.

Phil Burman · Sep 8, 2009

Just realised that this is the clamping force applied by all 4 brake units (assume all brakes are locked-up).

Phil

Phil Burman said:
Depends on what you mean by clamping force.

Here is a simple calc of the force applied to the disc by the pads, substitute you own values and recalc according to the formula used. I take no responsibility for the correctness.

Phil

braking distance - L = 160 ft
brake from 60 to 0 mph - V = 88 ft/s
vehicle mass - m = 1,500 lbs
tire radius - Rt = 10.0 inch
brake caliper on radius - Rc_ = 3.0 inch
coeff. of friction (pad/steel) - Coef = 0.9

average deceleration - a = 24.2 ft/s^2
average stopping force - Fs = 1,134 lbs
shear force on brake caliper - Fc = 3,781 lbs
clamping force on disc - Fd = 4,201 lbs

a = V^2/(2*L)
Fs = m*a/32
Fc = Rt/Rc_*Fs
Fd = Fc/Coef

Limy Sami · Sep 8, 2009

Any help?

http://www.kfz-tech.de/Engl/Bremskraftverst.htm

OT- Force at the brake caliper

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