Comments on brakes

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DJ & Mace -
It's been a long, long time but what NT says is correct IMHO. Counter-intuitive, but correct. See Coefficient of friction - Wikipedia, the free encyclopedia and http://www.collegeboard.com/prod_downloads/ap/students/physics/info_equation_tables_2002.pdf

It is counter-intuitive, but the key (to me anyway) is that a Coefficient of Friction (Cf) has no units. If it has no units, then it is independent of pressure or area or any other measurable feature.

The temperature vs. Cf curve of any given pad compound is unique to that compound. Rarely are they a linear relationship. Page 6 here: http://catalogs.wilwood.com/_pdf/_pads/BrakePadCatalog.pdf shows some curves for the various wilwood pad compound offerings. Note wilwood's recommendations in the pages above though, it's not a simple thing to pick the best pad based just on those curves.

NT -
Using the area of one side on a 2-sided caliper & all of a single-sided really seems wrong.
I know that it does. It was something that I struggled with for quite a while and I still can't say that I understand it well enough to explain it. It has to do with comparing the areas of the opposing pistons of a fixed caliper vs. the piston(s) on a single side of a floating caliper.
 
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I see the larger pad thing as having too factors. First, The pads can't be too small or they simply cannot do their job of turning kinetic energy into heat. Second, if they are too big, they will just flex and distort, so they need to be supported by the caliper in order to show any benefit from being larger. Then there is the question of their efficiency at a given temperature. There may also exist some thermal relationship between the ability of the caliper and pads to dissipate heat and the rotor's mass for a given material, but that's beyond my pay grade.
 
Thermal Effects

If you're lucky enough to catch a wheel-well cam at a race like Martinsville you'll get to see the rotor turn bright red just before the corners. That kind of heat would boil the brake fluid in an instant. A lot of time & energy goes into keeping the heat out of the caliper pistons & body. The pad tends to act a bit as an insulator, but high-end calipers will have heat shielding at the most important areas and some of them have rather exotic multi-piece piston designs. The bridge over the rotor's OD is the first place where the shielding is needed as the air being pumped through the rotor's vents blasts the caliper bridge with the hottest air.

Pad flex is a bugger. Can make the backing plate thicker, but then you either have a wider caliper (wheel fitment problems) or you have a thin wear portion (short pad life). The approach used by most is to place the pistons in the location where they'll distort the backing plate the least.
Sample heat Barrier piston design:
heat barrier caliper piston.webp
 
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The first time I saw the rotors glowing red while watching the races from the infield I thought there was a problem, but it was "business as usual". Remarkable that the rotors can handle that much heat for the whole race and not come apart.

Do they use carbon brakes? I thought they provided more friction the hotter they got. Some motorcycles had 1 steel and 1 carbon rotor because the steel had initial bite and then the carbon would kick in once it got hot.
 
From what I understand carbon-carbon brakes are used in F1, but are not legal in NASCAR.
 
Porsche's carbon-ceramic rotors are getting "affordable": & they don't fade according to the reviews I read, & they're guaranteed for 1-200K mi IIRC.
 
I have embarked on a project of CAD (SolidWorks '05) modeling the wheel hub, spindle, knuckle, steering arm (the one that I have is a Mini), and rotor. Eventually I'd like to also have the FJ60 and the 4rnnr calipers also modeled. Modeling the calipers accurately, even if only on the exterior, will take some time. So far I have the wheel hub done and am starting on the spindle.

The idea being that I can then look to see what other rotors & calipers might fit, like those from the racing aftermarket or from the Taco and the Tundy. I also plan to possibly use the FJ calipers on the rear of my Baja project, so having a model of that caliper will come in handy down the road.
 
Well I guess until my 40 gets fast enough to have the brakes fade then I will look to upgrade from there. Cool info on the Porsche, thanks.
 
NT -
Using the area of one side on a 2-sided caliper & all of a single-sided really seems wrong.

I know that it does. It was something that I struggled with for quite a while and I still can't say that I understand it well enough to explain it. It has to do with comparing the areas of the opposing pistons of a fixed caliper vs. the piston(s) on a single side of a floating caliper.

Got to thinking on this a bit more. The reason is rooted in comparing fixed calipers to floating calipers. If you just took the total piston area of each you'd have a gross mismatch. By using only one side of a fixed caliper you get a nearly apple to apples comparison.

You could use all of the piston area in a fixie, but then you'd have to explain that the backside of the piston bore(s) in a floater are technically also 'pistons' (since they effectively operate on the outboard pad) and then double the floater's piston area. Conceptually that might be more difficult to grasp, dunno.....

I'll venture that for the sake of simplicity it was decided at wilwood to just use one side of the fixie.
 
This is good reading. I didn't know the 4-runner calipers had larger pistons than the 40/60/62 stuff. I'm looking for a way to better balance the front to rear disc brakes. I'm going to try a set of the 4-40mm bore calipers on my 40 which has the rear GM floating calipers and see what kind of difference it makes.
Right now, even with an adjustable proportioning valve I don't like how much brake I'm getting in the rear. Also, I'd like a better pedal feel, not so firm, and the bigger pistons might help with this as well.

Thanks for all the great info.

Keep posting everyone!
 
Bigger calipers are not a good idea or a bad idea. Just a different idea.

Bigger pistons increase hydraulic leverage, giving a softer pedal, longer travel, lower pedal effort.
Inverse is true for smaller pistons.
 
Interesting Option for reducing rear brake bias with GM Metric calipers

AFCO (afabcorp.com) offers a version of the metric GM caliper (D154 pad) with a 2-1/4" bore instead of the standard 2-1/2" bore. That is a 19% reduction in rear piston area. Comes at a price though, at the time of this posting they are listed at ~$85 each.

R/H p/n: [FONT=Arial, Helvetica][SIZE=-1]7241-9005
L/H p/n:[/SIZE][/FONT][FONT=Arial, Helvetica][SIZE=-1]7241-9006[/SIZE][/FONT]
 
My inclination is to say that it's an AFCO-only piece - which has the obvious availability downsides. I've never seen a 2-1/4" 'metric' caliper listing, but that doesn't mean it doesn't exist.
 
Great idea for a thread, i think by reading this i may come closer to solving my brake bias issue with my 4 wheel disc setup on my mini truck and saved me from making the same mistake on my 60 build.
Thanks.
 
Brake Boosters

A part of the system that hasn't been touched on yet are brake boosters. I know very little about them beyond their basic function. The one thing that I have learned about them is that each application can have it's own, unique boost curve. The relationship between pedal effort vs. force on the master cylinder's piston is not linear and can be tuned by the mfg to produce a certain feel to the brake pedal. This curve is the usual difference in pedal feel between the 'sport' version and the 'grocery getter' version of the same basic model.

So a booster from an 80 series may bolt into a 60 series without modification (example; no idea if this is true or not), but it's boost curve could be be totally different and that I know of, there is no way to know this from a visual inspection.
This is one way that two identically assembled custom brake systems could have noticeably different pedal feels. I've no information that Toyota did this, but it stands to reason that identical appearing boosters from different applications could have different boost curves.

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Vacuum boosters work by applying atmospheric pressure to one side of the diaphragm and engine vacuum to the other. There is a valve in the booster that vents the vacuum on one side allowing atmospheric pressure into the chamber on that side. How much atmospheric pressure is allowed in depends on how far the pedal is pushed (see the boost curve comments above). Since engine vacuum can only ever only be 14.7psi maximum (& extremely rarely is it that high) the surface area of the diaphragm is directly related to how much boost is possible. Double diaphragm boosters (two diaphragms placed front to back) are used where a single diaphragm of the necessary size won't fit or where more boost than is possible with a single diaphragm in needed.

When the boost demands go high enough a vacuum booster can't do the job. Hydraulic boosters, or "Hydra-Boost" systems can. The factor that really limits the vacuum boosters is their low maximum operating pressure. With a Hydra-Booster the operating pressure is what ever the power steering pump can put out. Instead of trying to make more out of not much (vacuum) the booster designers are given too much pressure to work with and have to figure out how to reduce it. This is a good place to be.

Hydra-Boosters mostly came about with the rise in popularity of diesel pick-up engines. Since diesel engines do not have vacuum (no throttle plate) some other brake boosting method needed to be devised. Hydra-boost was their answer, and it is a good one.

Later model Hydra-boosts will have an aluminum cylinder offset but parallel to the main body of the booster. This is an accumulator and it holds some PS fluid at the pump's pressure. Like a vacuum booster so long as you never fully release the brake pedal you can modulate the pedal and still have boost.
 
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Proportioning Valves

I thought that I had posted something about Proportioning Valves, but in looking for that post I didn't find it. So time to write something about them.

There are two kinds of proportioning valves, adjustable and fixed. Most cars & trucks come with fixed p-valves. Some, like Toyota FJ62's and Mini Trucks, have a load sensing adjustable valve. Most adjustable p-valves are racing oriented aftermarket parts.

It is the racing type valves that I am most qualified to discuss, although not by a lot. These valves are nearly all based on the Kelsey-Hayes proportioning valve developed for early disc braked cars. Most pressure regulators offer a set output pressure regardless of input pressure so long as it is greater than the output setting. Welding gas regulators and fuel pressure regulators work like this. They output the set pressure regardless of the input pressure, even if it is varying while in use, unless the input pressure is below the setting.

P-Valves do not work like this. They output a percentage of the input pressure. If the input pressure rises, so does the output pressure. The K-H design outputs ~57% of the input pressure. So if the m/c makes 100 psi the p-valves output is ~57 psi. If the m/c generates 1000 psi then the p-valve's output is 570 psi.

The adjuster does not change the output pressure ratio. What adjuster does is change the "knee point" of the valve. Proportioning valves output the same pressure as the input pressure when that pressure is below some given limit. Once the pressure rises above that limit the output becomes ~57% of the input. The adjuster changes what that limit pressure is.
 
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Rear Brakes and Rear Brake Bias

In a high speed, panic stop or nearly something like that weight shifts forward onto the front axle. So the front brakes need to do more of the work than the rears because they have a lot more traction. The flip side is that with the weight off the rear tires they can't do a lot of braking without locking up.

This is all dynamic though. It won't stay that way for very long. As the rig slows the weight starts coming back onto the rear brakes and they can start to do more braking. So a properly tuned LSPV is a very good thing since it has the potential to offer maximum possible braking effectiveness in all situations. I've no idea if the range of adjustment of the LSPV on the 62 series version can actually do this.
My experiments with the one on my completely stock braked, but on 33-10.50's Mini lead to it being set to give full rear brakes all of the time. The only time that it could use less is when the shell is off and there is nothing in the bed.

Since any speed results in some weight transfer during braking there needs to be what I call a "baseline bias" meaning that the rear brakes, at best, will never equal the front brake's effectiveness. The less speed that the rig sees overall allows the front and rear caliper pistons areas to become much closer to equal. A trail only or comp rig can probably use the same size caliper at all corners.
A rig that sees highway speeds can't have this nearly 50/50 bias as that will result in the rears locking up in panic stops, or even something considerably less than that.

Classically adjustable proportioning valves cut the output pressure to 57% of the input pressure above their knee point. Using that as a guide I suspect that the rear piston area could be as small as 57% of the front piston area. I've not done any experiments on this, so it is only theory at this time.

One way to put some baseline bias in the brake system is to use rear calipers with less piston area. Since clamping force is pressure multiplied by area, reducing the piston area reduces the clamping force, which reduces the rear braking power.

Ideally the rear calipers would have *just* exactly the right amount of piston area so that at the greatest possible loading of the rig it would see maximum rear braking power, and then an LSPV would reduce rear line pressure proportionally with lighter loadings.

Alternately an adjustable proportioning valve could be used, the operator would have to keep track of the settings and adjust it frequently, or live with reduced rear braking power and leave the adjustment at some low setting.

Another way to deal with the rear wheels wanting to lock up is with a pressure delay valve. Currently the only part that I know of to do this is the Dan Press Ind. "Lock Resistant Brake System", a pressure delay valve that can be added to the rear brake plumbing. What this valve does is slow the pressure rise to the rear brakes. So when max pressure would result in locked rear tires this valve has slowed down the delivery of full pressure. By the time that full rear pressure can be handled without locking up the valve has allowed the pressure to rise to the maximum.
 
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p-brakes

The mechanical brake might have started out in the early days of hydraulic brakes as an "Emergency Brake", but I agree with Mark W, today they are a Parking Brake at best.

My disillusion with "line locks" stems from the way that they are designed. To hold pressure in the system there must be electrical power applied. Loose that power and the braking function disappears.

Contrast that with the more expensive (though I don't really understand why) Mico-lock design. These use electrical power to engage the pressure holding mechanism, but the pressure itself is what maintains the seal. Stepping on the pedal without the electrics being turned on releases the brakes. This is a much better concept and design.

My disillusion with hydraulic parking brakes on the whole is that it is the nature of hydraulics under pressure to leak down over time. With no active method of maintaining that pressure (metered accumulator, pump, etc.) line pressure will slowly fall. How fast it falls depends on the unique system. I've read of guys saying theirs will hold overnight or longer, but you can not expect that to be the case every time. I highly discourage anyone from expecting them to hold position if they plan to climb under the truck. I have seen line locks unexpectedly fail to work, I just do not trust them. If you need to climb under the truck while needing the brakes to hold position then use the line lock to hold position until someone else can sit in the driver's seat with their foot on the pedal.

My friend Lars wrote this tech sheet: Rear Disc Brakes - Cadillac/GM calipers | ClassicBroncos.com Tech Articles
He has since given up on the GM Hydro-Mechanical calipers and gone to the Explorer caliper and p-brake system. The basic problem with the GM p-brake calipers is that there are no parts for the adjuster mechanism, and most of them are too rusty to work correctly. After all of that Lars managed to coax the Cad calipers to function properly for not much more than a year. The Explorer system has been trouble and more importantly user interaction free for over 2 years and counting.

If you insist on using a hydro-mechanical caliper I'll suggest looking into the Varga. I think these are used on some late model Fords, but I've no idea what. I know of a very, very custom Early Bronco somewhere in BC that has been using them for about a year and the reports back on them is good. I've not seen the calipers, I've no idea what is involved in making them work.

[originally posted: https://forum.ih8mud.com/60-series-...bout-rear-disc-conversion-2.html#post4505556]
 

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