Weak brakes

[JohnK]

Over the years, I've heard people complain about their Caterham's brakes being weak – and the usual advice is, “Don't worry about it. Just go around the corner faster.” However, this is not much comfort when you find yourself approaching the rear of a city bus faster than you intended.

 

Now a brake system's job is to turn the car's kinetic energy (the car's weight times the square of it's speed) into heat, and it should do this quickly and with a modest amount of pressure on the brake pedal. The design of a complete braking system can be done with nothing more complicated than the Physics and Algebra one is supposed to have learned in high school and the dimension of all the bits involved; i.e., how the braking system will behave can be shown to result from the dimensions of the different brake system parts and the weight and dimensions of the car the system is fitted to*. So using paper and pencil, you can create a complete picture of how you'd like the car to be able to stop, and using that information, order what you need to accomplish that behavior. (I have a spreadsheet if anyone's interested.)

 

However, one thing that cannot be gotten from the above and that greatly influences how the brakes will perform has to do with something called “bedding in the brakes”. If the brakes are not bedded in, it will be difficult for the driver to stop the car either easily or quickly – i.e., the brakes will be weak. This can be especially difficult to recognize and solve in a Se7en.

 

When the friction material that's part of each brake pad assembly (friction material plus backing plate) is manufactured there are compounds contained within the pad friction material that make the friction material slippery – as manufactured, the pad doesn't create the friction it is supposed to when pressed against the rotor/drum. These compounds can be driven out of the pad by the driver progressively heating up the brakes in a series of stops done as a series of progressively higher speeds and greater pedal forces on the brake pedal. This progressively cycles the pads through hotter and hotter episodes, with cooling off periods in between all of which allows the compounds to vaporize into the atmosphere (and not deposit on the brake's rotors/drums). **See below, as copied directly from an old Wilwood web page.

 

The goal here is gradually creating more and more heat in the pad in order to progressively drive the problem compounds out of the pad. As this occurs, i.e., while the driver is following the sequence of bedding in the pads, the pads will begin to generate more and more friction as more and more of the problem compounds leave the pad, and the pads will progressively generate more and more friction and hence more and more heat. The driver should be able to feel this as it takes place over the sequence of brake applications – it should become apparent that it takes less and less pressure to stop the car and that the sensitivity at the brake pedal becomes greater and greater.

 

However – for cars that don't weigh much but have relatively big brakes, such as Se7ens and race/sports cars, it can be difficult to get the temperature of the pad up to a temperature high enough to drive the problem compounds out of the pad.

In a 3,200 lb car running disks/rotors which together weigh 25 lbs, slowing quickly from 90 to 20 mph, will create a rise in temperature of 400 degrees F in the disks. This is enough heat to get the pads into the temperature range where they will generate their specified amount of friction and which should begin to drive off the problem compounds in the pads.

The same setup in a 1,650 lb car (like a Se7en) will only create 200 degrees F in the same weight disks/rotors, which is generally not enough heat to get the pads to the temperature where they generate much friction, and not enough heat to drive out the problem compounds we're talking about.

 

When I realized this by paying attention to the numbers I calculated (*Puhn), I began doing sequences of stops from 100 mph and using as much pressure as I could generate on the brake pedal, and leaving 1 to 2 minutes cool down between stops - not touching the brake pedal during that time. I also set the brake balance bar to bias the force as much as I could to the front brakes, planning to attend to bedding in the rear pads later. Even so, this only gets the disks/rotors to around 270 degrees.

 

However this produced OK results. After a couple of such runs it required much less pressure on the brake pedal to slow the car, and even at lower speeds, the brakes were much more responsive and provided a much better feel. All of a sudden I had braking power where before there was very little. I expect to repeat this a number of times more to get the pads bedded, the problem being finding a place where I can get the car to 120 mph or so – without getting busted.

 

 

- But don't start the process at high speed because the generation of heat needs to be gradual. I'll guess that in a light car one just has to be be a bit more aggressive on the pressure end of things.

 

My own take is that the compounds you want to get rid of come off gradually, and if you try to do this all at once they will stay on the pad surface, and especially if you let the pads cool when they're pressed against the rotor/drum it will trap the compound in the pad and on the rotor/drum surface.

 

If the bedding is done incorrectly it will require using emery cloth to take 0.010” off the pads and sanding the rotors/drums with Maroon 3M abrasive pads to provide a clean surface to start the process over again (use a 3' length from a roll of 3” wide 80 grit emery cloth clipped to a board to take the pads down 0.010” – this takes some work/patience).

 

Getting this right can be hard. Carroll Smith wrote in Prepare to Win, page 81-82 that, even with a professional driver and having a race track at hand to set up the car, this can be difficult – to the point where Smith always went to races with a set of pads that had been bedded in correctly in his brief case, just in case.

 

 

Notes:

 

Before I went through all of this I changed from 9.75” front disks to 11” rotors, thinking that this would solve the problem – but if you follow the above, the larger rotors only made it worse. The larger diameter disks are even harder to get hot since they give the pad more leverage on the greater diameter rotor.

 

Going to higher-friction, more aggressive pads seems like the fix here (which I also tried) but the benefits of such pads are gained by shortening the life of the rotors a lot, and these pads also need to be bedded in correctly as well. If you do the arithmetic the numbers show that the track-day grade Wilwood BP-20 pads are capable of standing the car on its nose (actually, just locking up the wheels), without shortening the life of the rotors like the next grade up.

 

*”Brake Handbook” by Fred Puhn is the reference I used, which I recommend. There are likely a number of others that give .

 

**Quote from http://www.wilwood.com (from an early web page):

“Bedding is a “real conditions” heat cycle and the final step in preparing the pads for service. All pads, even OE stock replacement parts, will benefit from a proper bedding cycle. Bedding can be done either in the vehicle, or on a special bedding dyno that can realistically duplicate the torque loads, pressure, and temperature that will be realized in the vehicle.

 

The bedding process is the final “heat cure” for the pads. This final bedding cure differs from an oven heat cure in such that the oven heat cure does not include the pressure, torque, and elevated surface temperatures that are necessary to properly condition the pad for service. New pads must be gradually brought up to temperature and then slowly cooled. If the pads are put into hard service right from the start, damage from fractures or accelerated deterioration due to extreme temperature variations between the surface and the body of the pad can occur.

 

Once the brake system has been tested and determined safe to operate the vehicle, follow these steps for bedding of all pad materials.

 

1. Begin with a series of 8-10 light stops from approx. 30 MPH down to 15 MPH allowing 20-30 seconds for cooling between each stop.

2. Progress to series of 8-10 moderate stops from around 45 MPH down to 30 MPH allowing a 20-30 second cool down period between each stop.

3.Proceed with a series of 8-10 hard stops from 55-65 MPH down to 25 MPH allowing 20-30 seconds of cool down time between each stop.

4. Drive at a moderate cruising speed, with the least amount of brake contact possible, until most of the heat has dissipated from the brakes. Avoid sitting stopped with the brake pedal depressed to hold the car in place during this time. Park the vehicle and allow the brakes to cool to ambient air temperature.

 

During the bedding process, a more positive feel from the brakes should develop. This is an indication that the bed in process is working. If any level of brake fade is observed during the hard stops, it may be an indication that the brakes have been more than adequately heated. Begin cooling the brakes with light driving and without brake contact immediately.

[coffee break]

John,

Thanks for the science lecture.

Well done!

Carl

[WestTexasS2K]

Great Info John. What about just driving with the brakes applied to build up the heat? Is it not enough pressure and torque? I usually do this when I first get in the car because the first few stops are poor because the brakes are cold. Once I drag them for a block they are ready to go. Maybe the problem was I wasnt getting a good break in done on the pads. I did 10 cycles of 70 -0 moderate pressure and let them cool down. Im thinking it wasnt enough.

[JohnK]

Hi Loren,

. Good observations and conclusions on your part, and I've tried just what you describe about warming up the brakes because I also saw that as a very sensible approach. And this should work because most pads increase their friction when they get hotter, but only in the short term. If you're driving on the street, the brakes will have time cool off and lose their friction. On the track you'll keep them hot. But as I've learned, bedding the pads in successfully will give you much much greater stopping power - and that power (bite) will be there without having to get the brakes warmed up.

I wrote this because, after all the problems I've been having and all the different things I've tried and finally learned, I developed the suspicion that most people driving Se7ens of any sort put up with really lousy braking - because it's so difficult to properly bed in pads on a light car (and this especially includes race cars which can be even lighter than a Se7en). (When reading my post keep in mind that, as has been the case with my car, I changed things quite a bit* from what was shipped, but in this area I think the basics apply across the board.)

 

It all comes down to the fact that you have to really heat up the pads a lot in order to drive off whatever stuff is in the pads that prevents them from developing the friction that they're supposed to deliver. At the end of the process, after you do a succession of many progressively harder stops from higher and higher speeds, when you do a final bonsai stop from high speed, an IR gun-type thermometer should show at least 300 degrees F when pointed at the caliper. Wilwood provides stuff to people who are mostly driving cars that weigh over 3,000 lb and our cars are less than half of that. With so little weight, it just doesn't take much to slow the car down, and the pads just never get hot enough. Plus that, bedding in the pads takes a long sequence of progressively harder stops to build up conditions gradually in order to drive off the volatile compounds. And with a light car, nothing's happening here either.

I talked with Wilwood today and confirmed my suspicions. First of all it was recommend that I use lighter duty pads which also happen to bed in easier, (I was running BP-20s and will switch to BP-10s) because 'track-day' pads are spec'd for heavier cars generating lots of heat. For my car following many progressively harder stops, the tech suggested 100 to 50 mph stops hard enough where you could smell the brakes, allowing 10-15 SECONDS of cooling and then repeating a 100 to 50 mph stop with 10-15 sec cooling a second and then a third time. Take a shot of the caliper temperature with an IR thermometer and verify that you have 300+ degrees F, and then move the car to some place without touching the brakes, giving it 30 minutes to cool, and during this time periodically rolling the car forward to move the pads from where they were resting on the rotors, and then letting the car rest overnight. If you read what Carroll Smith wrote on this it's pretty much the same, and can be just as difficult to achieve as it sounds.

 

. But, if you get this all together, you'll have brakes that'll stand the car on its nose (actually, just lock up all 4 wheels) with relatively modest force on the pedal, and the car will be a LOT more fun to drive.

 

I'm struggling with figuring out where I can do this. There's a drag strip on the other side of town that may allow something like this,and I'm going to see if I can find a rural airport that'll allow me to make a few runs on a runway. I'm guessing that, after lots of preliminary, progressive stops, a single heavy pull down from 130 mph will do the trick.

 

If you've glazed the pads, take 0.010” off each of the pads with 80 grit emory cloth and use maroon 3M pads on the rotors. I did this just to be sure I was working with good surfaces.

 

 

*I found out that Wilwood recommends Coleman Machine in MI as a supplier of rotors and used them to produce front and rear rotors in cast iron for my S2K. In particular the fronts were cut from heavy duty blanks (not really what you would call "blanks") that could sink a lot of heat and lighter rotors for the rears that don't do such heavy duty. These have to go through much different dynamics than the steel rotors fitted to the 'standard' S2K. I also fitted radial mount 'Powerlite' calipers on both ends: driven by 0.625" front and 0.750" rear master cylinders pushing 1 3/8" front slave cylinders and 1" rear slave cylinders. If anyone is interested I'll post the full specs, which I did according to what Fred Puhn laid out in his (out of print) Brake Handbook. Puhn provides the math to figure out what to expect. On my car , with big heavy cast iron rotors weighing 26 lbs altogether, a stop from 90 to 20 mph raises the temperature of the rotors less than 200 degrees F.

Edited July 28, 2015 by JohnK

[BruceBe]

Lighter cars typically have proportionally smaller brakes (calipers, pads, rotors). That simple fact needs to be considered when vendors provide sweeping statements about their compound being designed for X-thousand pound cars. And while the amount of work required to slow a car is proportional to the mass of the car, it is also proportional to the *square* of the velocity.

 

Also - the performance characteristics of pads can vary immensely across vendors and compounds. Some vendor compounds require very little bedding for material transfer and out-gassing. Others are a PITA, where you're either wasting precious track-time, or becoming a menace on public roads. Many pads have a "green fade" during the bedding process - it is very discernible, and indicates that the bedding process is more-or-less complete.

 

For mixed/street use, don't get a pad that is designed for high-temperature track use, as the performance of the pad will be poor/dangerous when touring the car. The BP-10 vs. BP-20 comment above is more directly related to temperature range, than weight of car. The BP-10 will perform better at a lower temperature.

 

For motorsport use, at least one vendor provides factory-burnished pads that require no bedding. We have used these pads extensively in Grand Am Rolex GT, and several track/race Sevens under our care use them as well. These Sevens *all* have uprated calipers/rotors - the brakes are always ready to throw you through the windscreen at the first brake zone. And our clients don't waste an entire track session "bedding" the brakes.

 

Lots of good discussion above.

 

Cheers,

-Bruce

[JohnK]

Thanks for sharing your experiences. What you say is consistent with much of what I've learned in this part of my car-building adventure, and much of it is restated in the most current information I've found on the Wilwood site. Also, an understanding of some of what you write about can be gotten in revealing detail from a text covering brake systems, such as Brake Handbook by Fred Puhn.

 

I think your comments might be especially relevant to someone who has an existing system and who's been attracted by some of the marketing to consider ways of making their car stop better. Reading through Wilwood's section on Pads (for example http://wilwood.com/Pdf/Flyers/fl384.pdf , http://wilwood.com/PDF/Flyers/fl227.pdf , and http://wilwood.com/BrakePads/BrakePadsApp.aspx) show that they are certainly making an effort to keep people from shooting themselves in the foot if they're considering different pads. If you take the time to read the material and think about what you'll be doing with the car, their recommendations make sense for the majority of situations.

 

--------------------------------------

 

What I've written about addresses a special case that I've been wrestling with and applies to Se7ens in general. So it may be useful information to folk out there who have reason to fit 'grippier' pads, and may shed light on why their car's brakes feel weak in the first place, or are unsuccessful in getting different pads to work as they intended.

 

The thing that makes Se7ens unique is that, for something that's driven on the street, they're very light. And as everyone who owns a Se7en knows, that means they can be tossed about on the road as if they were purpose-built race cars. When doing such driving one wants brakes that give appropriate performance, and that means 'grippy' pads that don't fade when pushed. If you're feeling that you need more/better brake performance, upgrading pads sounds like a straightforward thing to do.

 

But in the case of Se7ens, lightness creates a problem. All of the aftermarket pads I know of have to be bedded in. If they're not bedded in they'll never deliver their advertised performance, and bedding in the pads is particularly difficult when the car is light. Thus you can install a set of pads and find yourself scratching your head wondering why they're downright awful when they're supposed to be much better than what you replaced.

 

Here's how it works.

At its heart, the bedding in process involves getting the pads hot through a series of progressively harder applications of the brakes at progressively higher speeds. This drives off the problem compounds within the pads that are created when the pads are manufactured. If the pads are new and you use them before they're bedded in, you'll experience what I've heard called "green fade": the pedal is hard, and high, all the parts of the system are all working as they should, but the car doesn't stop at all well. Bedding in makes green fade go away.

 

Getting the pads hot. You're shooting for something over 300 F here, measured at the caliper - measure with a gun-type IR thermometer. If you've got a car that weighs 3,200 lbs or so, getting the pads to this temperature is not a problem. If you've got a 1500 lb car, getting the pads this hot is a problem. The physics that applies to brake system design gives a good approximation of how much heat is generated when you stop a car, and what the temperature rise will be in the car's rotors+drums (i.e., the elements that the pads act on to turn the kinetic energy of the moving car into heat so that the car stops). Looking at some values here: given two cars running brake systems which both run a set of rotors weighing 26 lbs, a 3,200 lb car braking from 90 mph to a complete stop dumps enough energy into its set of rotors to raise them 428 F. Doing the same thing with a 1,500 lb car raises a set of rotors that weigh the same 201 F. As far as pad heat range goes, this is next to nothing.

Below are calculation results that show what's happening during braking to a stop from different speeds, and calculations that show braking to reduce speed such as you might do at the track.

-----------------------------

To accomplish the bedding-in process for a Se7en, the car needs to be treated as teams do when they bed in the brakes of their race cars. Carroll Smith has a complete and clear description of bedding brakes in his Tune to Win on pg 81. Willwood describes much the same thing but for ordinary (i.e., heavy) cars in http://wilwood.com/Pdf/Flyers/fl384.pdf. In both cases it involves a non-trivial process, and in the case of a Se7en, involves speeds that make it necessary to be at a track go get it done. Smith cautions that the processes is difficult enough that a professional driver using practice time at a race track can fail to get a set of pads bedded properly, to the point where, as team manager, on race-day Smith would always carry a set of pads that he knew were bedded in properly in his brief case, just in case.

 

Net, for a light car, the bedding-in is harder to do relative to a production car. So whatever level of performance you need from your brakes, you have to first bed them in.

 

Regarding pad friction compounds, consider some choices offered by this particular manufacturer.

 

Looking at

http://wilwood.com/Images/BrakePads/compound_chart-2.jpg

you can see that, for e.g., the BP-10 compound gives you good friction at low temp, an even change of friction over its temperature range, and you will see fade if you push hard. In comparison the BP-20 compound costs you a bit of cold friction but gives much better friction when used hard and a higher temperature maximum. Both compounds are very similar in other respects

 

http://wilwood.com/Images/BrakePads/compound_chart-1.jpg

Shows that all of these compounds start with much higher friction and deliver very high friction at very high temperatures - but Wilwood tells you that these are for competiton only and will shorten the life of the rotors significantly.

 

When you add to this sort of thing to a very light car, i.e., a purpose-built road race car or a Se7en, you have to ask if you're going to be in the temperature ranges that the pad is designed to work in. You can run the race pads and get really great friction right off the bat and never have to worry about fade, but they're likely to be working below the range where they're happy and it'll cost you frequent rotor replacement. I'll guess that these are a bear to bed and bedding applies to the rotors as well as the pads.

 

Looking at the calculation results below, a Se7en will have to be driven really hard to generate the temperatures where a competition pad is suitable. The numbers also say that, just the bedding-in requires that you get the car to at least 125 MPH, and even then the temperature is not all that high.

 

Calculations -------------------

Temperature generated in a 26 lb set of rotors by a:

1,500 lb car coming to a complete stop from 90 mph = 200 F

3,200 lb car coming to a complete stop from 90 mph = 428 F

 

For a 1,500 lb car, same rotor set:

 

MPH................Degrees F

135 to 0..........452 (=452.00-0.00)

135 to 50.......390 (=452.00-62.00)

 

130 to 0..........419 (=419.13-0.00)

130 to 50.......357 (=419.13-62.00)

 

125 to 0.........388 (=387.51-0.00)

125 to 50.......326 (=387.51-62.00)

 

120 to 0.........357 (=357.13-0.00)

120 to 50.......295 (=357.13-62.00)

 

110 to 0.........300 (=300.09-0.00)

110 to 50.......238 (=300.09-62.00)

 

90 to 0............201 (=200.89-0.00)

90 to 50..........139 (=200.89-62.00)

 

Physics:

Kinetic Energy in lb-ft =

( Weight of Car in lbs x (Speed of Car in MPH)squared ) / 29.9

Temperature Rise of Rotors+Drums in degrees F =

Kinetic Energy in lb-ft / ( Weight of Rotors+Drums in lbs x 77.8 )

 

 

The Brake Handbook by Fred Puhn spells out the physics and usage. While the material here is dated with respect to current product offerings, physics is physics.

The problems I faced were a bit different since I started with a blank sheet of paper, necessary because the brake system that showed up with my kit was laughable.

 

I set out with the goal of building a brake system that could haul the car down very quickly, and that it should do so with modest force on the pedal so that I would have good controllability. The system should not fade if put to heavy use, and the system needed to be sized to fit the car; I wanted the setup to work on a car with 13" wheels, which put some contraints on what I could do.

 

After a lot of trials and head scratching, my "aHah" came about when I did the math to calculate how much energy the car was going to develop at speed and the rise in temperature that that energy would generate in the rotors when I used the brakes. Then I was informed by Wilwood that the pads needed to get to over 300 F , minimum, before they would bed.

 

Disclaimer: I have no relationship with Wilwood or any other brake system manufacturer. My personal preference leans toward Brembo, but at Wilwood I found the components that fit my needs at a price I could manage.

[coffee break]

Great stuff, it makes me wonder what the F1 teams do. I would guess they have a computer controlled machine to tune them.

[BruceBe]

John - just noticed that your note has incorrect units for energy. Lb-ft is a measurement of torque, not energy. Not sure if you're pulling these formulas directly out of Puhn's book - could be a typo on his part.

 

Cheers,

-Bruce

[JohnK]

Great stuff, it makes me wonder what the F1 teams do. I would guess they have a computer controlled machine to tune them.

 

Me too. I'd love to know the properties of the materials they use for their pads and rotors nowadays. Is there something beyond carbon rotors and ceramic pads nowadays?

[JohnK]

John - just noticed that your note has incorrect units for energy. Lb-ft is a measurement of torque, not energy. Not sure if you're pulling these formulas directly out of Puhn's book - could be a typo on his part.

 

Cheers,

-Bruce

 

 

Lb-ft is indeed a measure of torque. It's also a perfectly good unit with which to measure energy. (My high-school physics teacher would be right proud of me - and Puhn too.) Also, in the context of brake system design, it's a very natural unit to use when you're sizing the components so that they accomplish what you want re stopping the wheel and are adding into the model equations the coeffecients of friction of the different bits, and when you've got enough torque to overcome the forces on the tire's contact patch, and so on.

Edited July 10, 2015 by JohnK

[BruceBe]

Lb-ft is indeed a measure of torque. It's also a perfectly good unit with which to measure energy. (My high-school physics teacher would be right proud of me - and Puhn too.) Also, in the context of brake system design, it's a very natural unit to use when you're sizing the components so that they accomplish what you want re stopping the wheel and are adding into the model equations the coeffecients of friction of the different bits, and when you've got enough torque to overcome the forces on the tire's contact patch, and so on.

 

Respectfully, that is not the case. Torque, measured in ft-lbs (or Newton-meters, etc) is a measurement (or unit) of angular force. It needs to go through an angular displacement (e.g. 30-degrees, 20 radians, etc.) before that product may be equated to energy. Recalling your HS physics, Work (Energy) = Force x distance. In the angular case, torque must be applied through an angular *displacement* to do any work (energy).

 

To simplify things, Puhn will incorporate "magic" coefficients here and there to keep the equations more intuitive, and sometimes to ease unit conversion.

 

Cheers,

-Bruce

[slomove]

I guess you are both right

 

John actually said that lb-ft (i.e. ft*lbs) is a measure of torque as well as a unit to measure energy which is correct.

 

Bruce said, torque is only a measurement of angular force (measured in ft-lbs) and to be multiplied with angular displacement to get energy which is also correct. However, since angular displacement is dimension-less (radians do not have a dimension) the resulting energy is still measured as ft-lbs which is what John said.

 

For the easier case of linear motion, the energy is the simple product of force over distance (in case of changing forces the integral of force over distance) and also measured in ft*lbs

 

I hope I have now confused everybody.

 

GB

Edited July 31, 2015 by slomove

[BruceBe]

I guess you are both right

 

John actually said that lb-ft (i.e. ft*lbs) is a measure of torque as well as a unit to measure energy which is correct.

 

Bruce said, torque is only a measurement of angular force (measured in ft-lbs) and to be multiplied with angular displacement to get energy which is also correct. However, since angular displacement is dimension-less (radians do not have a dimension) the resulting energy is still measured as ft-lbs which is what John said.

 

For the easier case of linear motion, the energy is the simple product of force over distance (in case of changing forces the integral of force over distance) and also measured in ft*lbs

 

I hope I have now confused everybody.

 

GB

 

In my view, the use of ft-lb as a unit of energy can tend to cause confusion, and potential errors in calculation. The use of ft-lb as a unit of energy, which is generally not a popular engineering convention, will tend to confuse a scalar (energy), for a vector (torque). While torque and energy can share units, that is where the similarity ends. From a units perspective, it is interesting to visualize torque as Joules/radian. And when it comes to brakes, we are both balancing *forces*, and converting kinetic *energy*. Definitely don't want to munge all of that together.

 

Cheers,

-Bruce

[slomove]

Totally agree that ft-lbs (while valid) would be an uncommon measure of energy. I was just talking about the principle.

I come from a metric background and Nm is the common unit of torque as well as energy, although then usually called Joule (with 1 Joule = 1 Nm). This terminology is probably also to reduce confusion.

[JohnK]

Respectfully, that is not the case. Torque, measured in ft-lbs (or Newton-meters, etc) is a measurement (or unit) of angular force. It needs to go through an angular displacement (e.g. 30-degrees, 20 radians, etc.) before that product may be equated to energy. Recalling your HS physics, Work (Energy) = Force x distance. In the angular case, torque must be applied through an angular *displacement* to do any work (energy).

 

To simplify things, Puhn will incorporate "magic" coefficients here and there to keep the equations more intuitive, and sometimes to ease unit conversion.

 

Cheers,

-Bruce

 

To simplify the issue ( and not spend the evening re-teaching a section from the average Physics text ) I'll quote a page from my favorite reference book, "BOSCH Automotive Handbook 5th Edition."

From the section 'conversion of units', page 25:

Units of energy

(units of work)

Unit.....|....J..... |....... kW . h..... | kp . m........| PS . h.............| kcal..................| ft-lbf..........| Btu............. |

1 J = ...|....1.....|... 277.8 . 10-9..| 0.10197 | 377.67 . 10-9 | 238.85 . 10-6 | 0.73756 | 947.8 . 10-6|

 

Nett, you can express energy/work in a number of different units. I'll guess that Puhn chose lbf-ft because it simplifies the math he defined the brake system's operations in, and made understanding how you design a system to deliver a certain amount of stopping power easier to understand. He starts out by showing the difference in energy in a car at the top of a hill and then at the bottom, which is easy to understand in terms of lbf-ft, and the same unit makes sense when describing the force you can get from a caliper acting on a rotor - it follows nicely and doesn't involve a lot of unit conversions.

Edited September 5, 2015 by JohnK

[Kitcat]

My solution to this whole thing would be to push harder on the brake pedal:).

[BruceBe]

Totally agree that ft-lbs (while valid) would be an uncommon measure of energy. I was just talking about the principle.

I come from a metric background and Nm is the common unit of torque as well as energy, although then usually called Joule (with 1 Joule = 1 Nm). This terminology is probably also to reduce confusion.

 

Agreed, and, we probably wouldn't be having this conversation at all, if we had a specific SI unit name for Torque :-)

 

Cheers,

-Bruce

[JohnK]

To complete the thread, see

http://www.usa7s.net/vb/showthread.php?7421-Ithaca-Lemonade&p=101568#post101568

which got stuck somewhere else.

[BruceBe]

This gets back to the "compound" of the friction material. A NAPA street pad is not designed to operate at 800 degrees, so it will "bed" at much lower temperatures. Putting a pad on the car that is designed to operate at a temperature the car will never see on the street will yield low coefficients of friction.

 

With regard to motorsport pads - Performance Friction motorsport pads are factory burnished, requiring no bedding *at all*. We run them exclusively on our clients' track Caterham Sevens - great pedal feel, initial bite, and overall brake force. We put new pads on immediately prior to a race, and the brakes are good-to-go before the end of the pace lap. Pagid pads *do* require bedding, but, it is not a protracted process.

 

Cheers,

-Bruce

[MichaelD]

Is there a part number for those Performance Friction pads for a Caterham?

 

I have been having Porterhouse make me copies if the no longer available Hawk Caterham pads. I use regular brakes & roters not the uprated brakes & roters.