Nope, but we can reasonably assume that the total piston area is larger on the Super Duty series calipers than on the F150 calipers.
Larger brake boosters (or hydroboost) allow for larger master cylinder diameters increasing volume without increasing pedal travel.
It has nothing to do with control, it has everything to do with a light weight which provides insufficient traction weight on the axle to give the traction necessary to utilize the brakes full capacity. Proportioning and ABS both work to limit the applied brake force to a level that the traction can support, in effect reducing the braking capacity. When added weight adds traction, more of the available braking force can be utilized.
there is no point in increasing the total piston areas. see below.
- to keep the relative ratios fairly constant.
by definition, if the slave area doubles, the master must also double or the pedal travel will double. the latter is clearly impracticable - there is very little latitude to vary these ratios, and certainly not by a factor of two.
then, with respect, you don't understand what "control" means.
that's adding weight to increase tire contact area, not braking capacity.
they don't "decrease" braking capacity, they decrease skidding. skidding isn't braking capacity.
KE = 1/2 M.V^2
if you add weight, you increase the amount of braking required.
The master can double as readily as the slaves can.
Of course I do.
Added weight increases traction (both tire contact area and tire contact pressure) which allows the use of more of the brakes capacity. Insufficient traction prevents the use of the brakes full capacity.
They decrease the braking force applied and thus the percentage of the braking capacity available in order to prevent skidding when there is insufficient traction.
When that increased weigh has also provided the traction necessary to utilize more of your existing braking capacity it does not increase your stopping distance. When you already have sufficient traction to utilize your full braking capacity, additional weight increases stopping distance.
I guess what really stops you is the pressure, in pounds or whatever, that you apply to the pads. Not PSI, but total pounds, assuming the pad areas are not changed. One somewhat larger piston can put out as much pressure as two pistons of somewhat less diameter each. Or have I missed the point.
I can remember when some cars had two and four piston calipers. I believe that they did not always offer an advantage over a single piston. Will have to refresh on this...have slept since then.
ok, so we double the slave area, then we double the master area because we need to keep the pedal travel the same, then what have we achieved?
indeed, but to what purpose? provided ratios remain the same, there are significant reasons not to do so - do you know why?
let's come back to that...
the brakes' "full capacity" is locked solid. what you really mean is to the full capacity of the tire traction. tire compounds being constant, traction is maximized by maximizing contact area. in that regard, you should lower your tire pressure before you increase weight.
abs doesn't decrease pressure, it pulses it so that high pressure is applied intermittently [in electronics, it's called "pwm", pulse width modulation].
proportioning valves decrease pressure, but as above, pressure over the locking value is ineffective. proportioning, done right, /increases/ braking capacity.
which is a matter of control. when i say you don't understand control, that's not a pejorative, it's just that you're not fully aware of all the factors. if you're talking a big rig trailer that is skidding and not stopping when unladen, that's because of poor control and excess tire pressure relative to the load. if the brake anti-lock/proportioning system was better, it wouldn't skid. if tire pressures were actively managed, as some modern vehicles now are, again, it wouldn't skid. both are control issues, not "insufficient weight" issues.
Providing better distribution of the clamping force on the rotors.
Increasing the piston sizes allows more force to be transmitted at a given hydraulic pressure, much like increasing voltage allows more power to be transmitted at a given current. Keeping pressures within the limits of standard components is beneficial. The larger volume of fluid to be displaced won't have any notable effect since the fluid is essentially non compressible, this isn't like air brakes and the reasons for relay valves.
Full braking capacity is not "locked solid", that is a static state. Full braking capacity is the maximum energy dissipation rate the brakes are capable of while the rotors are still turning.
You need to increase tire pressure as you increase the loading on them (per tire manufacturers).
ABS PWM relies on the time constant of the pistons and calipers to average the PWM to a lower effective pressure. The same in electronics where PWM signals are averaged by a low pass filter such as a capacitor, or by rotor inertia in a PWM motor drive.
Again, these devices are limiting the brake force to match the available traction, so yes, they are reducing the braking capacity from it's maximum value.
Again, it's not at all about control. ABS and proportioning maintain control by reducing the braking forces to less than the maximum values which the brakes can sustain, the values that would cause a skid due to insufficient traction. A vehicle will not be able to achieve the maximum braking that the brakes are capable of unless it has sufficient load to achieve enough traction. This is why for many vehicles the stopping distance decreases as you add cargo weight up until you reach maximum traction at which point further weight will increase stopping distance.
The main advantage of multi piston calipers is to better distribute the clamping force across the pads. A caliper piston can't really be lager diameter than the pad is wide, and most pads are not round which leaves the overhanging areas not backed by the piston to flex and be less effective. Multiple pistons can better cover the area of the pad and allow the whole pad area to work more evenly.
I've been told, although don't have enough experience to have felt it first hand, that for the same piston area, multiple piston calipers tend to flex less (makes sense) and are therefore easier to modulate. Not as big a deal with ABS on pretty much everything from the factory today... but back in the day, being able to modulate your brakes was a Good Thing. And it still theoretically makes it easier for your ABS to keep things under control.
increasing piston size doesn't do that easily. piston count, as per your response to hls, is a better solution.
it only transmits more pressure if the ratio between master/slave changes. if that doesn't change, and as we've discussed repeatedly above there are substantial limitations to that, then it can't.
yup. that's one.
right. so what else? there's one particular practical issue that's very important. apart from cost, weight, etc.
sorry, it is. if the brake can't lock the wheel, it's ineffective.
and dissipation is caused by? and what energy are we dissipating?
and [theoretically] decrease it as you reduce the load again! there are practical limitations of course, but anyone who goes off-road and scrambles loose surfaces knows this first hand.
no, the pressure starts high, just like a non-abs brake, then gets cut by the abs system momentarily before reapplication. it doesn't "average" a low pressure, it averages a lower braking force.
you misunderstand pwm. put a capacitor across the poles of a pwm-driven dc motor and tell me what happens. seriously.
preventing lock-up is increasing braking capacity, not reducing it. by definition.
it's not about control but abs/proportioning maintain control??? you're confused.
any brake that works properly should be able to lock its wheel. whatever conditions that wheel is experiencing. ensuring it /doesn't/ lock is absolutely /all/ about control.
nope. see above.
nope, it's because the control systems don't work well at low loads.
Which is what I originally indicated about dual pistons.
Sorry, you are very much mistaken.
Example with some arbitrary values:
100# of force applied to a 1 sq in master cylinder = 100 PSI fluid, applied to a 1 sq in slave cylinder = 100# force on the caliper.
200# of force applied to a 2 sq in master cylinder = 100 PSI fluid, applied to a 2 sq in slave cylinder = 200# force on the calipers.
Twice the force transmitted, at the same hydraulic pressure. Obviously master cylinder bores are much smaller than slave cylinder bores, but that doesn't change the fact that increasing both bores allows more force to be transmitted without increasing hydraulic pressure.
Simple 1:2 ratio:
100# of force applied to a 1 sq in master cylinder = 100 PSI fluid, applied to a 2 sq in slave cylinder = 200# force on the caliper.
200# of force applied to a 2 sq in master cylinder = 100 PSI fluid, applied to a 4 sq in slave cylinder = 400# force on the calipers.
A locked brake is dissipating no more energy than an un-applied brake.
Brake friction converting the momentum of the vehicle (and thus rotation of the brake rotor) into heat.
I don't find many loose surfaces around here, I'm quite good at getting stuck in teflon-like clay mud around here. Airing down doesn't help much on that.
Semantics, now you're relying on the elasticity of the tire tread to average the near lockup of the rotor.
I don't misunderstand PWM and I said nothing about putting a capacitor across a motor, though that is common for RFI suppression.
No, it is limiting the braking capacity to match the available traction, which is often less than the maximum capacity of the braking system.
Nope, you're confused. If you do not have the traction to allow the use of the brakes at their maximum capacity, you are operating at *reduced* braking, even if you want to twist semantics to claim that ABS or proportioning are maximizing the (available before skidding) braking.
The subject of quite a bit of debate, since a locked wheel with the vehicle moving is not a desired condition.
Yep, ABS is certainly about control, but that control is all about limiting the brakes to less than their maximum braking capacity to compensate for inadequate traction.
Sorry, inadequate traction = less than the brakes maximum braking capacity.
Nope, it's because an inadequately loaded axle has little traction and the brakes on that axle can provide little braking capacity as a result.
The control systems also suck eggs when such a lightly loaded vehicle with stiff suspension hits a slight bump while braking moderately hard. This is / was a significant safety hazard with some trucks, including ones I owned, where you had to watch the road surface and stop braking at each bump, lets the POS ABS freak out and cause you to loose all braking and greatly increase your stopping distance. Many NHTSA incident reports are directly attributed to this ABS fault.
you're not reading what i said. if you want to double the output force on an hydraulic system, you can increase the piston size, but you will double the input travel. this is immutable and there is no way around it.
right, but i didn't say that.
not the momentum, the kinetic energy. momentum is m.v. ke is 1/2.m.v^2
i'm not, the abs system is. and it's not relying on the tire elasticity, it's relying on the physical inertia of the moving parts.
oh, please. and don't waffle, do what i said so you can see what happens.
which is /increasing/ the braking capacity if it's preventing lock-up - skidding tires don't offer as much traction.
no, the "less than" is the applied hydraulic force relative to the maximum, not the braking force.
abs' sole function is CONTROL.
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it is CONTROL that allows you to maximize braking without skidding.
any "debate" is whether a driver can control maximum braking better than a machine, not whether a braking system should be powerful.
no, it's to maximize available braking and CONTROL. this is why abs was invented - drivers are typically BAD at CONTROL.
no, CONTROL
no. and you contradict yourself below.
so we're back right where we started - if you're experiencing poor braking, it's because of poor control, not poor physics.
You're not reading what I said. If you want to transmit more force without increasing the hydraulic pressure, you increase the bore on both sides of the equation which increases fluid flow without changing travel and allows you to transmit more force at the same pressure. This is little different than stepping up voltages in order to transmit more power at lower currents.
You said that full braking capacity was a locked state, which it is not. Braking is dissipating energy, a locked state is "holding", not "braking". A brake will hold when it has dissipated all of the energy, or caused another component to dissipate that energy such as locking the wheel and letting the tire skid.
Sorry, not a physics major.
It's a complex system, multiple components are performing that averaging.
I clearly said PWM signals are averaged by a low pass filter such as a capacitor, or by rotor inertia in a PWM motor drive. Perhaps you are misreading and thinking the capacitor applied to a motor drive, it did not, that was other PWM applications. Capacitors are still common across motors for RFI suppression however.
No, it's *reducing* braking capacity to match available traction capacity. If the traction was up to par the brakes would be operating at full capacity. There is no way around it, ABS or proportioning causes the brakes to be applied at less than full capacity.
The brake is the pads and the rotor, if the braking force has to be reduced to prevent a failure outside of the brake, i.e. a traction failure of the tire, it is still reduced braking force.
It is the control that automatically reduces braking force to prevent skidding. This is not any different than traction control which reduces engine power to prevent skidding from excess drive power, "anti-braking".
No, I've read plenty of debate over whether the ability to lock a wheel at speed is a relevant qualification for a brake system, since such a condition is not desirable.
No it's about limiting braking to match the available traction thus aiding a poor driver in maintaining control.
Sorry, in conditions of inadequate traction such as an unloaded pickup truck, whether controlled by a competent driver or automatic by ABS, the braking force is reduced to match the available traction.
No, and there is no contradiction below.
Nope, in the specific case I cited above, it is a defective ABS system that is presenting a hazard by not being properly matched to the vehicle and/or having inadequate sensors such that it could differentiate the momentary wheel lock due to the tire coming off the ground in a bump from an actual traction loss / skid event. When that defective ABS system saw that fraction of a second lock when the wheel was off the ground, it went into full panic mode causing a near total loss of braking for an extended period of time which in many cases caused accidents that would not have occurred had the system worked properly or not been operating.
absolutely incorrect. if the bore area ratios on both sides of the equation are the same, the force ratios are the same. by definition.
you can only "transmit" more output force with the same input force if you monkey with the ratio, and that's impracticable.
that's your analogy, not mine. and they're not equivalent.
you didn't go here, so i'll tell you - it's brake line internal diameter. they need to fine enough for the surface tension of air bubbles to occupy the whole tube, not simply float through it. without that, brake systems are next to impossible to bleed.
i didn't say "any locked state". i was talking about the locking of a brake after passing through peak threshold. and that varies with the energy of the vehicle.
no, in order to lock, it has to pass through the peak threshold.
its /local/ energy.
red herring.
mmm. so why are we arguing? i'm not trying to tell you you're wrong, i'm trying to tell you how to get it right.
any "complex system" comprises simple components. figure out the simple stuff and you can get a handle on the complex.
but pwm signals are /not/ averaged through a low pass filter - unless you have some kind of funky audio application, and even then, the people that say it is probably don't understand what's really going on. the reason i said to use the capacitor on a pwm controlled motor is so you'll see why capacitor "averaging" is untrue.
this is becoming pointless. i'm saying the flag is red white and blue, you're saying it's purple green and black.
there is no such thing as "par" on braking - it constantly changes per conditions. control, whether from the driver or from an abs system, matches the two.
but at maximum braking [i.e stopping] capacity!!!
but it's increasing braking! again.
increasing braking.
increasing traction where it would otherwise be lost.
exercising it and having capacity for it are two totally different things. you need excess braking capacity for safety because, as any truck driver should know, if you experience overheating and fade, you'll need it.
by increasing braking...
no, it's limited by poor control. the driver doesn't control the proportioning, the manufacturer does. and i'll bet he's also not controlling the tire pressure. his only bet is if there's an abs system on there, and most production versions are cheap enough that at low loads/low tire contact, they don't modulate [CONTROL] finely enough.
there is. but you need to understand to understand.
a "defective abs" is neither the basis on which to argue or the foundation of a subject understanding. it's simply a defective abs.
"jim beam" wrote in message news:_rednUALVcPjnXDQnZ2dnUVZ snipped-for-privacy@speakeasy.net...
The total force exterted on a brake pad is a function of piston area exposed to the brake fluid times the brake fluid pressure. Brake caliper pistons are usually (always?) round. So the maximum area of the piston is limited by the diameter of piston that will fit between the wheel hub and the inside of the actual wheel (along with other factors....). Two pistons side by side can have more piston area than the largestest possible single piston that weill fit in the space (even if they are slightly samller in diameter than the single piston). Plus they better spread the load over the pad. I used the word "assume" becasue I don't know the relative sizes of the F150 and F250 pistons. It is possible that the F150 has one large piston that has as much or even more area than the two pistons used by the F250 calipers - but I doubt it. Putting it simply - Two 1.8" diamter pistons (side by side in a caliper) can exert more force on the brake pad than a single 2" diamter piston and they better spread the load on the pad (in this case about 60% more force).
My original comment was only in the context of carrying a load on a trailer (without trailer brakes) versus carrying a load in the bed of the truck. You do understand that right? Most modern vehicle have brakes powerful enough to lock the tires under most conditions at least on the first stop (except that most vehicles with ABS prevent this from happening). So for most vehicles, the making braking force is limited by the tire / road friction available. This maximum braking force is a function of the frictional coefficent between the tire and the road, the tires contact patch, the normal force pressing the tires down on the road and other minor factors. The amount of braking force needed to stop the vehicle is a function of the mass to be stopped and the desired deceleration rate.With a load in the back of the truck, the normal force applied to the tires is directly related to the mass to be stopped - right? If you have more weight in the bed, you have a higher normal force on the tires and therefore you have a higher maximum braking force available. When you have a trailer, without trailer brakes, the situation is different. The load on the trailer only partially contributes to the normal force on the tires that provide the braking - depending on how the tailer load is distributed, maybe only 10% or 20% . The load on the trailer still contributes 100% to the mass to be stopped. So although you have the same total mass to be stopped with part of the load on the trailer instead of in the bed of the truck, you have less maximum braking force available to stop it. Therefore, it is possible (very likely) you cannot stop the vehcile plus trailer as qucikly as you could stop the vehicle alone if the weight was all carried in the bed of the single vehicle. OK?
Sorry, you are absolutely incorrect. I said nothing about changing the ratio, I said transmit increased force without changing the fluid pressure. If you want to transmit more force without changing the bore size on both ends you will have to increase the hydraulic pressure.
100# of force applied to a 1 sq in master cylinder = 100 PSI fluid, applied to a 1 sq in slave cylinder = 100# force on the caliper.
200# of force applied to a 1 sq in master cylinder = 200 PSI fluid, applied to a 1 sq in slave cylinder = 200# force on the calipers.
200# of force applied to a 2 sq in master cylinder = 100 PSI fluid, applied to a 2 sq in slave cylinder = 200# force on the calipers.
As you can clearly see, increasing the bores on both sides allows more force to be transmitted (not multiplied) without increasing the line pressure.
They are exactly equivalent, this is fact.
And this relates to the topic how? Moving more fluid due to larger bore sizes doesn't automatically equate to larger line diameter, we aren't moving 4' stroke cylinders on an excavator here, the total volume is still very low.
And as soon as it is locked, it is "holding", not "braking" and energy dissipation (if any remains) has moved elsewhere, such as grinding tread off tires.
Pass through and no longer be "braking".
Then why are you making factually false claims such as those around hydraulic line pressure vs. bore size and force transmitted?
Give me a PWM application where the end result is not averaged by some form of low pass filter. Even PWM drive for lighting dimming relies on human eyes as the low pass filter.
Yep it's pointless. You don't want to admit that inadequate traction forces a limitation in brake capacity to prevent skidding.
I said "par" on traction. If traction was always up to "par", the braking would not have to be reduced from maximum.
Braking reduced to maximum traction capacity.
No, it's reduced braking to match available traction.
Reduced.
Preventing the loss of traction does not equate to increasing traction. The available traction does not change at all, engine power is simply reduced to prevent exceeding the limits of the traction. Increasing traction would require adding a traction modifier such as spraying racing "traction compound" on the road ahead of the tire.
And a runaway ramp...
By limiting braking to match available traction.
Braking force is still reduced to match available traction.
Nope, there isn't.
It points out that those marvelous control systems are not as infallible as many believe.
that's a frequently regurgitated myth. the original concept behind single piston was to make it easier to achieve negative scrub radius on the steering axle because you can dish the wheel more.
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manufacture of singles is a little more complex than opposed pistons because they have to be one piece [opposed calipers are two pieces bolted together] and you have to machine past the "claw". that machining requires a long bit, and that means inherent tool stability problems.
what the industry has found subsequent to use though is that their simplicity and lower component count makes for higher reliability. that's why you now find single piston now on positive scrub steering axles, and even rear axles.
so, yes, you'll get 62% more output force for the same hydraulic pressure.
but as i keep telling pete c, the brake pedal will have to move further unless you also increase the input piston size. if you don't, you'll be pressing that pedal 62% further. how high is the seat in your cab?
is this a coulomb friction coefficient ed? yes or no. [careful, that's a loaded question.]
no, not ok. just like pete c, what you're experiencing is not "better braking", it's better control. apparently this is a really difficult concept.
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