Comments on brakes (1 Viewer)

[ntsqd]

I got to thinking that I've made comments about what I learned in two years of designing racing brake components and several decades of working on racing vehicles in many different threads, and perhaps I should put them all in one place instead. This thread is an on-going effort to do just that. I won't pretend to know all of the answers, but hopefully those that do know the answer will post it.

For the sake of keeping it easy to find a particular topic I'll request that specific need questions be confined to those threads rather than here. Perhaps, with enough input from others we can turn this into a sticky regarding brake system design.

 

[ntsqd]

Types of Leverages in a Brake System

There are two leverages with a modern brake system; hydraulic and mechanical. The mechanical is the ratio of pivot points (& foot pad) distances. The hydraulic is the ratio of piston bores. The total system leverage is the mechanical ratio multiplied by the hydraulic ratio.
If you go smaller on the master cylinder or bigger on the calipers you increase the hydraulic leverage ratio. If you go bigger on the master bore size or smaller on the caliper pistons you decrease the hydraulic leverage.

 

[ntsqd]

Total System Leverage - Changes and their effects

OK, now the flip side of all of this. When you decrease the total system leverage the pedal feel gets firmer. When you increase the total system leverage the pedal feel gets mushier feeling. If you go too far in getting more total leverage the pedal will start to feel like you have air in the system. It isn't air that you're feeling, it is flex in all of the components. ALL of them. The firewall flexes, the calipers flex, the steel tubes expand, the hoses expand. Literally everything in the system grows a little and the cumulative growth is enough volume to make the pedal move more than you'd like. And because the system leverage is so high it doesn't feel like you're pushing very hard to do this. Most systems have an upper pressure limit of 1200-1500 psi, yet you're generating that pressure with less than 100 lbs of force on the pedal.

The important thing to realize here is that a firm pedal might feel good and inspire lots of confidence in the brakes of a vehicle, but may actually perform worse than stock if the total leverage is too low. The trick is to find the balance between a firm pedal feel and actual increased brake effectiveness.

 

[ntsqd]

Rotor Diameter

There is a leverage in a brake system that doesn't affect how the pedal feels, but has a huge effect on how well the brakes work. The diameter of the rotor. Torque is a force applied at some distance from the point of rotation. Foot - pounds, so many pounds applied one foot from the axis of rotation. Inch - pounds, Inch - ounces, etc.

So if you increase the distance but keep the force the same then you've increased the torque. That's a larger diameter rotor. There is a limit imposed by the wheel as to just how large you can go in rotor size. Generically it is 3" smaller than the rim size. A 12" rotor is as large as will fit in a 15" wheel. Could go to the 13" rotor in a 16" wheel, etc. I believe this is what is really driving late models to have larger rim sizes (excluding those awful "dubs").

Rotor width really only plays a part in how fast the rotor can shed heat. By itself does not increase braking effectiveness or shorten stopping distances. What it does do is increase the rotor's ability to dump the heat generated into the air, which increases the system's resistance to brake fade.

 

[Tinker]

Had an old friend/customer stop by yesterday for a windshield who's been upgrading & trading 4xs for a long time. Said he's always been frustrated upgrading Heep brakes because he's never found the proper balance between upgraded calipers, rear disc conversions, & upgraded MCs, & the prop valve only does so much. I told him lots of folks here have gotten close enough.

So what would be the "ideal" 60/62 setup with 4runner fronts: what rear calipers (I share your aversion towards single-piston, floating-frame calipers) & what MC?

 

[S&S1stCruzer]

I purposely brought 17x8 wheels to see if I could fit a 13" rotor upfront at some later date in the future. Maybe this thread will get me closer to that.

 

[Vicm]

So if you increase the distance but keep the force the same then you've increased the torque. That's a larger diameter rotor. There is a limit imposed by the wheel as to just how large you can go in rotor size. Generically it is 3" smaller than the rim size. A 12" rotor is as large as will fit in a 15" wheel. Could go to the 13" rotor in a 16" wheel, etc. I believe this is what is really driving late models to have larger rim sizes (excluding those awful "dubs").

How is increasing the rotor diameter going to increase the torque without moving The pads outward.
What about the effect of pad size (as in friction surface area) on stoping force.
???
Please explain and thanks for this post.
Vic

 

[ntsqd]

Hydraulic Leverage

Something that just came up in a thread. Caliper piston area means nothing until you consider the whole hydraulic part of the brake system. If you increase the caliper piston area by 15% and then also increase the master cylinder bore by 15% you've effectively done nothing to change the way that the system performs.

One thing to consider about opposed piston calipers is their stiffness vs. their piston sizes. A caliper body with small pistons will be stiffer than the same body with larger pistons. Two things at work in this, the smaller pistons exert less force because of the smaller size, and the smaller piston bores mean that there is more metal in the body, e.g. the holes aren't as big so they are harder to distort.

 

[ntsqd]

How is increasing the rotor diameter going to increase the torque without moving The pads outward.
What about the effect of pad size (as in friction surface area) on stoping force.
???
Please explain and thanks for this post.
Vic

The caliper pads have a very specific radial location on the rotor, they have to move out if you go to a larger rotor. Some sort of bracket to move the caliper would need to be developed. With the right tools that's not as hard as it might sound, but to do it in a home fab garage is a pretty advanced project.

 

[ntsqd]

Pedal Feel is Subjective

Pedal feel is very subjective. What feels good and is "right" to one person may be "horrible brakes" to another person. This is a big factor that is not often mentioned in the brake related threads that I've read on all sorts of forums and lists.
Some folks really like the 4rnnr calipers with the stock master. Others prefer it with one of the various larger bore masters. Neither is incorrect, it's up to the driver to put the combo together that works for them. Due to driver preferences, this sort of adjustment is not all that uncommon in race cars.

A lesson that I keep re-learning relates to my '84 4WD Mini truck. AFAIK it has the smallest bore stock master ever fitted to those trucks, and the brakes are always sort of a question mark in my head. Yet the truck actually stops really well. It's just that the pedal isn't as firm as I'd like. And that is the key. Have to separate out how the pedal feels from how the brakes actually work.

 

[Bret]

Since this looks like a place to put it...

the 4 Runner calipers have 4-40mm pots per caliper roughly 100 sq cm for both calipers combined

My 76 FJ40 had 2-35mm pots and 2-28mm pots per caliper roughly 63 sq cm for both calipers combined

For the master cylinders 3 sizes come up
7/8 close to 3.87 sq cm
15/16 close to 4.45 sq cm
1" close to 5.06 sq cm

There is a pretty good increase in the area between the 40 calipers and the 4-runner so it isn't a stretch that the calmping pressure would be greater with the 4-runner and it is spread out on a larger pad

the increase isn't as significant as the pad increase so it would seem that even with the larger master you should still get a "little" more pressure at the caliper with the same amount of pedal pressure (just more travel)

I don't know how the volume of the rears plays into it or the difference in pad surface area so maybe someone can check the math and fill in the holes.

 

[ntsqd]

Coefficient of Friction

I know it doesn't seem right, but the coefficient of Friction (Cf) is unitless and is a simple multiplier. Cf is not a number that is calculated, in all cases it has to be found thru experimentation and only applies between two specific materials.
In this case it multiplies the caliper's clamping force, the surface area of the pads has no bearing on the friction. (Pad area & thickness of a pad mostly determine pad life span.)

Say the pads have a Cf = .5 That means that what ever the clamping force generated by the caliper is, the "dragging force" that the pads have on the rotor is 1/2 of it.

Another thing that I'm still not too sure of, but the math has always worked out correctly, is to only consider the piston area of one side of an opposed piston caliper. With a single piston, floating caliper the full area of the whole piston is used.

 

[Tinker]

So anyone recall what the piston sizes are in the 60/62 caliper? and do they mount the same as Bret's '76 40? And did it have the 20mm vented rotors?

Just selfishly looking for smaller calipers for my FF that will fit like a 60/62: 90mm mounting hole spacing (12mm X 1.25 bolts) & 20mm thick rotor.

NT -
Using the area of one side on a 2-sided caliper & all of a single-sided really seems wrong.

 

[Bret]

best I can measure the rotors with the wheels on is close to 22mm, these are not new rotors sooo.. new thickness may be different. i have 60 knuckles so the bolt pattern should be the same for the fronts on 40-60 and 4 runner on the calipers.

 

[Mace]

the surface area of the pads has no bearing on the friction. (Pad area & thickness of a pad mostly determine pad life span.)

????

that is like saying that that airing down a tire has no impact on traction...

 

[djawahir]

I agree with Mace on this one. More surface area always = more friction.

Are you trying to correlate something to pedal feel?

 

[Tinker]

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

 

[Cruzerman]

I believe that the friction does have nothing to do with surface area. But the coefficient of friction changes with not only the type of materials, but other factors such as temperature (see wikipedia article linked above). As the heat goes up, the coefficient of friction goes down. Thus hot brakes don't work as well as cold brakes, which explain brake fade going down steep hills, etc. So larger brakes don't work better because they have more friction, but because they can dissipate the heat better. There are also other factors such as wear, etc.

 

[Tinker]

Bret -
The pistons on my 62 front calipers measure the same as your '76 40's. The Mountain rotors are 20mm thick, though. I'd guess you have a ridge on yours so maybe that's what you're measuring. Or the aftermarket rotors could very well be thinner (the Wal-Martization of America).

Still looking for calipers with pistons either larger than the '93 4runner's 4 40mm or smaller than the 60/62's 2 35mm & 2 28mm but with the same mounting holes (12mm 90mm apart) & throat (for 20mm rotor

 

[FJ40Jim]

There is one caliper that came on all all 76-90 FJ40/55/60/62.
Also the same as minitruck/4runner 1979-1987.

All 76-90 Cruiser front rotors are ventilated, w/ a minimum thickness of 19mm.