Aspire Master Cylinder & Brake Booster on a Festiva ?

[FestYboy]

auto's had different front rotors

woot, i own this page

[getnpsi]

im junking my 1997 aspire and it looked to have a newer MC vs the rest of the parts on the car. It's all being transferred to the festiva regardless of how they perform for peace of mind.

[Safety Guy]

Here's the way I look at the "manual vs. auto" Aspire rotor/caliper upgrade:

I bought two sets of Raybestos PG rotors; auto and manual thickness. I weighed both pairs together and found the autos were 3 pounds heavier per pair. That's 1.5 pounds per side.

Since the swept area and the pads are the same, the braking is probably about the same, all other things being equal (I could be wrong here, I suppose) when the rotors are not stressed by heat from heavy braking.

On really heavy braking, the thicker auto rotors should not get as hot as quickly, providing better braking. If you live in a mountainous area, pull a trailer, haul heavy loads regularly, or just want the thickest rotors, then do the auto swap. If you regularly push your brakes to the limits, then the thicker rotors are probably less likely to warp as soon, too.

If you just want a significantly superior upgrade from Festiva brakes and you want to keep your front wheel unsprung weight as low as possible, then go with the manual Aspire rotors. I figure that if you were choosing wheels based on weight (all other things being about equal), you'd go for the ones that are 1.5 pounds lighter, wouldn't you?

BTW, the only differences are the rotors (thicker) and the caliper mounts (longer to fit the thicker rotors). The actual pads, calipers and knuckles, etc. are the same on both. That's the way I understand it, anyways.

There is also the cost of manual vs. auto rotors. The PG Plusses I bought were within about one dollar of each other from Rockauto. Some other brands or lines may be significantly more expensive, though. I've seen some (ATE grooved) where the auto rotors were around $50 each vs. $25 for the manuals.

Finally, there is the question of a possible Kia Rio brake upgrade, which I believe has yet to be fully explored. Rotors and pads on '00 to mid-year '02 Rios are the same as auto Aspires. After that mid-year change, they went to different (bigger, probably) rotors, so there may be a way to swap in Rio stuff to make your front brakes even bigger. Whether this is really necessary or not is questionable at this time. Those who run larger tires may find it a good possibility. Also, Kia Rios are obviously newer than any Aspires, so newer parts are nicer! If they fit right, that is. The possibility of Rio knuckles and calipers, etc. fitting onto Festivas (and upgrading Aspires, for that matter) is a tempting prospect!

Since I run skinny 165/70-13 all seasons on my '93 GL ATX, I chose to go with manual Aspire rotors.

Karl

[Safety Guy]

Oh, and as for the M/C debate, I have no opinion yet. I did put a new M/C in my car when the old one died. I think someone has used a 626 or 929 M/C before, so that's interesting.

I'm waiting for Jim to come back and provide his educational slant on this issue!

One thing I'm interested in for my '92 "Speedstiva" buildup is to investigate getting rid of my brake booster entirely and going "manual" on my brakes. That's read "no power assist." I mainly want to loose weight from the front end, and I'm willing to go to some lengths to do it! But will it work okay or not? Hmm...

It would be nice to say of Speedstiva: "She aint' got no 'power nothing,' except for her 'power engine.'" 8)

Heck, my '76 Chevette didn't need no stinking power assist brakes!

Karl

[muscle_Car1]

^^^ I'll have to explain Pascal's Law to you later, when I have a bit more time...

What about it?

The stock festiva master cylinder has a bore of 3/4 or .75in
The aspire w/o ABs has a cylinder bore of 13/16 or .8125

so that's a increase of 1/16 or .0625 in surface area.

So wouldn't a larger bore size equal more fluid in the cylinder? That in turn would flow a larger volume of fluid with the same effort.

To keep the braking system in balance like the factory did, you cant just increase the surface area of one pistons without increasing the other.

The pressure would remain the same but the force extracted would be greater.

[FB71]

OK, Pascal's Law... hydraulic force is equal to the input force divided by the surface area of the actuating piston. Surface area is equal to (pi*r sqrd). So, surface are of the Festy mc is roughly 1.766 inches, and the Aspire MC is 2.0729 inches.

Now, let's assume (I know..) the the force applied to each piston is constant between the two. Meaning, the same brake pedal is used, as well as booster, and the same person applying the same amount of pressure to the pedal. Now, for the example, let's use an easy number for the pressure applied, say 10lbs.

In the Festy MC, 10lbs is divided by the surface area of 1.766in to equal 5.6625lbs/in
the Aspire mc divides 10lbs of force by 2.0729in to equal 4.8242lbs/in

Now, I don't have the aspire caliper piston dia in front of me, so I'll guess it's around 2 inches. Therefore, to determine force at the pads, you multiply the MC pressure by the surface area of the caliper piston (pi * 2 sqrd = 12.56 sqr in). So;

Festy MC = 5.6625 lbs/in * 12.56 sqr in = 71.121 lbs/in force at the pads
Aspire MC = 4.8242 lbs/in * 12.56 sqr in = 60.5920 lbs/in force at the pads

Therefore, the Festiva MC applies more force to the pads than the Aspire MC, given the same input force

However.....

because the Festy MC is smaller in dia, it does move less fluid than the Aspire MC, given the same stroke length, as you pointed out. This causes the pedal to require excessive movement to actuate the brakes. It also reduces pedal "feel", or feedback to the driver. If we were interested soley in stopping force, the Festy MC would be the ideal choice. But, as drivers, we look for some information to be returned to use about the braking event (feedback). Therefore, increasing the MC piston dia, which lowers the force applied, also reduces pedal travel, and "pushes back" against the driver's foot a bit more, which we use to determine the effectiveness of the braking event, even though it does require us to apply a bit more force to the pedal to equal the force developed by the Festy MC, about 1 lb.

[25Horseplay]

^^ I knew that! :shock:

[muscle_Car1]

How much additional distance is required to do the same work? As you only have what at most 6inches of travel to use.
In real world testing I found it to be quite noticable.

Not as "mushy" <-- big technical term there pedal feel for the first bit of travel then ramps up to real firm feedback close to the end of travel.

[Silv3rBull3t]

Very worth it... For the $25 it cost me it was very worthwhile.

[kzbill]

I like Jims answer, cause he is right about the hydraulics. I used a new Festiva MC with the Aspire booster. I set the freeplay between the MC and the Booster actuator rod to the Aspire Spec. The pedal travel is fine with the stock Festiva MC but the brakes require a much lighter Foot pressure, truly like Power Brakes. I did the aspire brake swap on both ends and am extremely pleased with the results. I used MTX brakes and the stopping power is much greater. Using the Festiva MC also meant no change in brake lines.