Bias-ply tires filled with original new-old-stock 1963 air purchased in cans from Layson or Mitchell for the low, low price of just $345/can!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Now yer talkin' Toyota prices...

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

how has the force increased in the front? I understand that more of the front's braking ability is available when the rear does not lock-up; but is realistic to say that an inverse proportion exists?





Also, what does it mean to modulate?

Thanks,
Seth

When applying the brakes, weight is transferred from the front to the rear. The front settles down and the rear lifts as weight is transferred. It's the reverse of what happens during a hard launch such as when drag racing. That's the primary reason that front brake wear typically about 3 times faster than the rears.

To modulate means to vary the pedal pressure to prevent lockup. It's similar to what ABS does automatically.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

Other way round. When applying the brakes, weight is transferred from the rear to the front. That's why in the cartoons you watched as a kid, the rear wheels would fly up off the ground when somebody screeched to a halt. From the rest of your post, you know that and just maccidentally istyped

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yeah, what he said. My brain was still in a fog at 5:30 this morning and I got it backwards.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

Right, I understand that a large portion of the car's weight is transferd to the front during braking... But as it reads (the first post that I quoted) , it sounds like decrease in wheel cylinder size in the rear brakes, somehow enables the front caliper to exert more pressure to its friction surface. This could not be true considering the front and rear systems are independent of one another, eh?

The force applied in a hydraulic system is computed by multiplying the line pressure by the surface area of the piston. If you reduce the surface area you reduce the force applied for any given line pressure. By reducing the force to the rear for a given line pressure you can have higher line presure before lockup which allows the front brakes to apply more stopping force before the rears lock up. The limitation is usually with the rear brakes.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

OK, so you are able to apply more pressure with your foot; not as a function of inverse proportions.... thank you for clarifying what was already stated (thanks Dan)

It's a rather delicate balancing act to get it right. You can reduce rear braking force to the point that they aren't doing any work at all; obviously that would be counter productive. The most effective setup would be for the rear to lockup just a little before the fronts. Sliding tires contribute very little to slowing the car but sliding front tires don't provide any steering force either.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

...and this is done via a proportioning valve?

That's part of it. The reason a proportioning valve is needed is that the front disk brakes and the rear drums operated in significantly different manners. Disk brakes are linear in application; that is, when line pressure is doubled braking force is approximately doubled. Drum brakes, as used on Mopars after around 1960, are non-linear. They use the motion of the drum to increase braking force so that a doubling of line pressure will approximately quadruple the braking force. The front, or primary, shoe contacts the rotating drum and is pulled in the direction of rotation. The motion is transferred through the adjustment link to the rear, or secondary, shoe.

The obvious question then becomes, "why are disks so much better?" The answer is complicated and and to some, not entirely clear. Drum brakes are very efficient at exerting stopping force through the above described servo effect but there are several drawbacks. First of all, they are heavy. Because of the direction of force application, drums must be thick and beefy to withstand the pressures without splitting apart. Secondly, they are more subject to fade. As kinetic energy (motion) is converted to heat, the drums expand outward requiring more pedal travel to sustain braking force. The shoes expand too but not as much. Also, drum brakes are more susceptible to wet fade and dirt contamination. Fianlly, and this is probably the clincher, they are cheaper and easier to install on an assembly line.

Then, what aren't rear disks more common? The primary reason is the problem of an emergency brake. One could rig up a lever mechanism to apply the brakes as is done with rear drums. That is exactly the system that SSBC uses for their conversions, but that tends to warp the rotors, especially if they are hot. Warped rotors reduce braking efficiency and cause an annoying pulse in the brake pedal. We could go back to a driveline mounted brakes such as Chrysler used before the early 60s. The problem with those is that if a rear wheel is lifted the braking effect is lost and the vehicle is free to roll off a jack The common solution is to build a small drum into each of the rear brakes. The disk are used as service brakes and the drums as parking or "emergency" brakes. That works well but then the factory has to install 6 brake assemblies per car instead of 4. It also increases unsprung weight in the rear which adversely affects handling.

For these reasons, the hybrid front disk / rear drum has been the standard for most manufacturers until about the last 10 years. Since then the manufatures have increasingly switched to all disk systems.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

<Drum brakes are very efficient at exerting stopping force through the above described servo effect but there are several drawbacks. First of all, they are heavy. Because of the direction of force application, drums must be thick and beefy to withstand the pressures without splitting apart.>

I would argue the point that drum brakes are heavier then disc. It depends on the design. Take a 72 "A" body with drum front brakes, and a 74 with disc brakes. Weigh the complete brake front brake system, everything between the control arms. The disc system is heavier. Don't remember how much, but I did this about 10 years ago. Now, of course, with aftermarket pieces, the disc could be lighter.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Actually I think you are probably right about that. What disk brakes do give you though is reduced rotating mass and, if they can be moved inboard as on some Jaguars, they reduce unsprung weight as well.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

I'll just throw my 2 cents in on braking. I have converted three of my old A-body cars to the latest front 10" drum set ups from post 72 cars with very respectable results. These have the advantage of larger surface area, finned drums, the larger upper and lower ball joints and you can retain your SBP 14" rims. My subjects include a 63 Val Convert, a 65 Barracuda, and my wife's 65 Dart Wagon. The stopping power is amazing and heat fade has never been an issue. All of these cars have 8.75 rears that didn't need BBP conversions and proportioning has never been an issue. I have done three disc brake conversions with wonderful results but for many reasons (some admitedly financial) I can really recomend this route!

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. . . there was good sport at his making.