Questions About Performance of Stock Brakes and Rear Disc Swap

[Sid_RallyX_82]

So, our Festivas don't a proportioning valve, which, with the spring resistance in the drums, means the front brakes will apply first, which is good for trail braking. How differently does the car handle after the rear disc swap?

[Greywolf]

Don't know - I haven't got there yet although it is in the plans. You want to ask advanced dynamix about that...

[w4rkry]

I think the general consensus is that you don't want rear disc, they lock up too quickly with the lack of rear weight. I think Charlie has even ran without rear brakes at all before.

[theastronaut]

Charlie has stated that he's recorded better lap times with no rear brakes. I'm running 100% stock brakes except for a Miata booster/master cylinder and the rear brakes may be benificial on an autocross course to make the car rotate better. Depends on your application. I know that I wouldn't want any more rear braking power than I have now, it'll already rotate at low speeds, and I'd hate to hit the brakes at higher speed and have it try to swap ends from too much rear brake bias.

[FestYboy]

So, our Festivas don't a proportioning valve, which, with the spring resistance in the drums, means the front brakes will apply first, which is good for trail braking. How differently does the car handle after the rear disc swap?

A well sorted drum system will apply just before or at the same time as the front. Having the front apply first increases the likely hood of rear lock up due to weight transfer. A good system will make the whole car sit down.

[Sid_RallyX_82]

A well sorted drum system will apply just before or at the same time as the front. Having the front apply first increases the likely hood of rear lock up due to weight transfer. A good system will make the whole car sit down.

I don't know of any way to make a system do that without a proportioning valve

[ryanprins13]

I don't know of any way to make a system do that without a proportioning valve

I didnt quite understand your original question so i didnt say anything, but why do you not want to use the proportioning valve?
I would guess at most there has been maybe 2 or 4 people who have done a rear disc swap.

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[FestYboy]

I don't know of any way to make a system do that without a proportioning valve

It's all about the shoe to drum clearance...

[Sid_RallyX_82]

It's all about the shoe to drum clearance...

But wouldn't the springs in the drum setup retain the shoes until the front Pistons have expanded to engage the pads? I imagine that it would be a reasonable task force to have all clearances minimized before actually driving the car, but what about on the 3rd lap in a race? Surely the braking attitudes would change with use, right?

[FestYboy]

But wouldn't the springs in the drum setup retain the shoes until the front Pistons have expanded to engage the pads? I imagine that it would be a reasonable task force to have all clearances minimized before actually driving the car, but what about on the 3rd lap in a race? Surely the braking attitudes would change with use, right?

You need to remember that you're using a hydraulic system... And that the square seals in the front pistons are what gives a gap to the front pads... Which is not adjustable. The gap to the rear drums however is...

So, we apply pressure to the system via the master. Since it's hydraulic, all points of the system experience the same pressure at the same time. Therefore, the rear wheel cylinders will see no delay when the master is depressed. The perceived delay has everything to do with the difference in take up between the front and rear, and the volume of fluid required to "fill the gap" front and rear.

Compared to the hydraulic pressure available from the master, the drum shoe springs are worthless. The only reason for them is to retract the shoes once hydraulic pressure is released or parking brake cable is released.

[Sid_RallyX_82]

I didnt quite understand your original question so i didnt say anything, but why do you not want to use the proportioning valve?
I would guess at most there has been maybe 2 or 4 people who have done a rear disc swap.

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I just don't want to go through the effort of adding in a proportioning valve and tuning it.

You need to remember that you're using a hydraulic system... And that the square seals in the front pistons are what gives a gap to the front pads... Which is not adjustable. The gap to the rear drums however is...

So, we apply pressure to the system via the master. Since it's hydraulic, all points of the system experience the same pressure at the same time. Therefore, the rear wheel cylinders will see no delay when the master is depressed. The perceived delay has everything to do with the difference in take up between the front and rear, and the volume of fluid required to "fill the gap" front and rear.

Compared to the hydraulic pressure available from the master, the drum shoe springs are worthless. The only reason for them is to retract the shoes once hydraulic pressure is released or parking brake cable is released.

I understand Pascals Law of confined incompressible liquids and the concept of displacement. My concerns are
1) the pad to disc clearance, which would need to be filled before there is enough resistance to overcome the shoe springs
And 2) when the shoe liners wear, the breadth of the shoe-drum clearance falls in the hands of the star adjuster and how finely it can adjust, which I assume its increments are fairly wide

But if everyone agrees stock drums are better than a disc swap, I'll keep with that

[ryanprins13]

I just don't want to go through the effort of adding in a proportioning valve and tuning it.



My concerns are
1) the pad to disc clearance, which would need to be filled before there is enough resistance to overcome the shoe springs
And 2) when the shoe liners wear, the breadth of the shoe-drum clearance falls in the hands of the star adjuster and how finely it can adjust, which I assume its increments are fairly wide

Ok, i didnt realize the oem one wouldnt work.
Concern 1: the shoe springs are pretty light springs and are most likely being moved while the front piston is moving.
An easy way to check what would happen is to remove front caliper and rear drum. Have someone else very very slowly start pushing the brake pedal, you watch the front caliper and someone else watches the rear shoes and see what happens.

Concern 2: its not a star adjuster on threaded rod like trailer brakes or something. Its a self adjusting mechanism with very, very fine teeth. Lots of fine adjustment there i believe. I dont have a photo unfortunatly, but there was a for sale thread for an adjuster recently which had a photo.

*edit* found the thread from febuary but it was a photobucket photo and is gone.
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[w4rkry]

Ok, i didnt realize the oem one wouldnt work.
Concern 1: the shoe springs are pretty light springs and are most likely being moved while the front piston is moving.
An easy way to check what would happen is to remove front caliper and rear drum. Have someone else very very slowly start pushing the brake pedal, you watch the front caliper and someone else watches the rear shoes and see what happens.

Concern 2: its not a star adjuster on threaded rod like trailer brakes or something. Its a self adjusting mechanism with very, very fine teeth. Lots of fine adjustment there i believe. I dont have a photo unfortunatly, but there was a for sale thread for an adjuster recently which had a photo.

*edit* found the thread from febuary but it was a photobucket photo and is gone.
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PM me Ryan and I can recover it really quick, while it's relevant haha

[ryanprins13]

PM me Ryan and I can recover it really quick, while it's relevant haha

Not sure its really helpful but here it is

https://fordfestiva.com/forums/showthread.php?t=60067

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[Sid_RallyX_82]

Ok, i didnt realize the oem one wouldnt work.
Concern 1: the shoe springs are pretty light springs and are most likely being moved while the front piston is moving.
An easy way to check what would happen is to remove front caliper and rear drum. Have someone else very very slowly start pushing the brake pedal, you watch the front caliper and someone else watches the rear shoes and see what happens.

Concern 2: its not a star adjuster on threaded rod like trailer brakes or something. Its a self adjusting mechanism with very, very fine teeth. Lots of fine adjustment there i believe. I dont have a photo unfortunatly, but there was a for sale thread for an adjuster recently which had a photo.

*edit* found the thread from febuary but it was a photobucket photo and is gone.
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Festivas have a metering valve (stops rear brakes from locking under hard braking), not a proportioning valve (uses a kind of pressure gate).
1) good idea. Wouldn't have thought of that.
2) oh, I've haven't had to change my rear brakes, so I've never seen what it uses