Help with Fred's SVO

[Chalky]

So Fred and I have been crunching the numbers and we've come up with a better option. We're going to go with DBA T2 slotted rotors, "they're Australian mate!!!",with HPS pads. The HP Plus are great but very dusty.

I sent a message to MM about the MC size and they said not to mix and match brake systems. Someone obviously missed the fact that the GT & Cobra use the same rear calipers with a 38mm piston. The only other difference, from the point of the hydraulic system, is the dual 40mm piston in the Cobra versus the twin 45mm pistons in the GT's PBR calipers.

Therefore, it makes sense, to me, to use the same MC bore size as the 2000 GT which, according to my research has a 1" bore. So it looks like the '93 Cobra 1" bore MC is the winner. Any thoughts???

[vrinner]

MM sent me this info on my race car when I asked a question about MC piston sizes and how to properly balance the brakes, truly facinating stuff...hopefully it can help you....

> Mr. Rinner,
>
> Your SVO comes with huge 54mm rear calipers and 11.25" rear rotors. It
> is the caliper piston AREA multiplied by the effective rotor diameter
> that determines the brake torque generated for a given brake fluid
pressure. Your stock SVO has a front brake system with 73mm calipers and 11"
rotors. This makes the front and rear brake torque
percentages:
>
> Front:
>
> 73mm diameter = 4,185mm^2
> Effective rotor diameter = 9.0"
> 4,185 x 9.0 = 37,665 units of front torque
>
> Rear:
>
> 54mm diameter = 2,290mm^2
> Effective rotor diameter = 9.45"
> 2,290 x 9.45 = 21,640 units of front torque
>
> Front brake torque percentage = 37,665/(37,665+21,640) = 64%
>
> The SVO Mustang should really have a front brake torque percentage of
> 66-67%, but since the entire brake system was pulled from a Lincoln,
> that
was the closest SVO could achieve.
>
> When you installed the Brembo front brakes, your front brake torque
> percentage changed to 54%. This is too little front brake percentage
> for the weight distribution of your car. If you then install the Cobra
rear brakes that have 11.65" rotors and 38mm calipers, your front brake torque percentage changes to 69%. This is much closer to the target percentage of 64%.
>
> It is very important to realize that the brake torque is proportional
> to the rotor diameter and to the piston AREA, so this means it is
> proportional to the piston diameter SQUARED. If you change from the
> SVO
rear brakes to the Cobra rear brakes, the rear brake torque drops a lot since the piston AREA has gotten a lot smaller, even though the rotor diameter has increased slightly.
>
> If you car has an adjustable brake bias valve, that will only slightly
> compensate for the incorrect front to rear brake torque ratio. The
> brake bias valve is used to compensate for the change in weight
> distribution that occurs during braking from weight transfer. It has
nothing to do with the static brake torque percentages I've been explaining in this e-mail. Some of these valves have a knob on the end and others have a lever, but they are the same thing.
>
> If you car has a brake balance bar, that is completely different. This
> is a bar that goes between the brake pedal pushrod and the pushrods to
> TWO different m/cs. This can be used to adjust the front to rear brake
bias. It is very unlikely that your car has this in it.

[Chalky]

Hi Vince, thanks mate, that's interesting reading but doesn't really answer my question. I'm trying to determine the best MC choice, 1" or 15/16", for a twin 45mm/ 10.9" front and 38mm/11.65" rear set-up and I can't seem to work the math out or get a straight answer.

Using the info above I calculated the front brake torque to be 72%.

[vrinner]

Hi Vince, thanks mate, that's interesting reading but doesn't really answer my question. I'm trying to determine the best MC choice, 1" or 15/16", for a twin 45mm/ 10.9" front and 38mm/11.65" rear set-up and I can't seem to work the math out or get a straight answer.

Using the info above I calculated the front brake torque to be 72%.

Here is another reply I got from MM saying that the below is the most accurate way to get the right percentage formula...

If you want to know the actual brake torque at each wheel, it can be determined with the following formula:

Brake torque at wheel (ft-lbs) = caliper piston area (in^2) * brake fluid pressure (psi) * 2 * cf of brake pad * effective rotor diameter (") / 12

So...based on that info it's way beyond my brain to figure out, I think you will get closer with the first formula. I think you mentioned you spoke with MM. Did you talk with Jack...he seems to know a lot about figuring out the right setup for the brakes.

[Jcamper]

Hey Chalky, ran through the various setups:

With a stock SVO brake setup the ratio of overall brake piston area (6475 mm^2) to master cylinder bore area (641 mm^2) is 10.1. 64.6% front brake bias.

With a stock GT brake setup including 1" master cylinder bore the ratio is 4315 mm^2/(506.7mm^2) = 8.52. This means the pedal will travel less in relation to brake cylinder area. The real downfall to this setup is that it puts you at 74% front brake bias.

With the cobra (40mm) front calipers and 38mm rear calipers it puts you at a very good 68.9% front brake bias. However, since there is less overall caliper cylinder area, you would want to move to the 15/16" master cylinder to get a 3647mm^2/(445.35mm^2) = 8.19 ratio. So pedal will travel less than either stock SVO or stock GT comparatively. To get back to the same brake area/master cylinder ratio as stock SVO (helps with heel/toe I would imagine), you would need a 21.44mm master cylinder bore diameter (or .84 inches).

Remember that overall brake torque at the contact patch will be affected by both the swept area of each different style brake cylinder/pad, the diameter of rotor it is acting upon, the overall diameter of the wheel/tire package, and the brake pad compound. For our purposes I would guess that you will be pretty close in terms of overall wheel/tire diameter due to packaging constraints in the stock wheelwell.

For overall brake torque I think new front brake rotors are like 13"? So 3" greater than stock SVO, so about 25% more torque just based on that. Since the newer brake system has about 33% less area in the front caliper pistons, you should be pretty close to same amount of brake pedal pressure for overall contact patch torque applied.

I am no brake system expert, but I did stay at a Holiday Inn Express last night. J

[MikeFleming]

I am no brake system expert, but I did stay at a Holiday Inn Express last night. J

Expert status right there!

[Chalky]

"I am no brake system expert, but I did stay at a Holiday Inn Express last night." What is the commute to Vancouver getting to be to much??? Thanks for the answer, but punch this into you calculator and tell me what you get....

2x45mm piston, 10.9" rotor on front

1x38mm piston, 11.65" rotor on rear.

The constraint comes down to cost, $100 versus $350 for calipers and needing to stay within a stock wheel, 16".

[Jcamper]

"I am no brake system expert, but I did stay at a Holiday Inn Express last night." What is the commute to Vancouver getting to be to much??? Thanks for the answer, but punch this into you calculator and tell me what you get....

2x45mm piston, 10.9" rotor on front

1x38mm piston, 11.65" rotor on rear.

The constraint comes down to cost, $100 versus $350 for calipers and needing to stay within a stock wheel, 16".

Ok, I was lying about the Holiday Inn Express.

Looks like that gives you a 74% front brake bias. You would need to swap master cylinders, going to the Cobra M/C gets you close to the same M/C to caliper cylinder area ratio as the SVO (9.69 compared to the original 10.1). This keeps pedal travel from being totally out of whack, and means that you will have as close to stock pedal pressure as possible. The overall pedal pressure for equivalent force at the wheel cylinders will be within 4% of the stock SVO.

Due to the smaller volumes at the master cylinder and wheel cylinders, the internal pressure will be quite a bit higher than the stock SVO system (my calcs say around 47%) during braking, but you probably don't care about that. Hmm, need to fix my original post, wasn't accounting for that. J

I think I see front SVO calipers on rockauto? Am I wrong, they don't look too bad?

[Chalky]

Ding,Ding!!!!! Winner!!!!!

[Jack Hidley]

Chalky,


The stock SVO brake bias is 64%. The combination of parts you've selected will have a bias of 70.8%. That is a big change. The brake balance will not really be able to be adjusted correctly. The owner will need to adjust the proportioning valve one for particular friction condition (high grip pavement, low grip wet ground, etc) and just leave it there. When driving the car on higher or lower grip surfaces, either the front or rear brakes will lock up first.

When sizing the m/c the pedal ratio and power booster assist also need to be taken into account. From 1979-2004 there are five different pedal ratios used. This makes it invalid to compare one car to another to determine the correct m/c size, without also taking into account the pedal ratio differences. The same also applies to the power booster situation. Which power booster was he planning on using?

BTW, when calculating brake torque, the effective rotor diameter needs to be used. This is always smaller than the actual rotor diameter. To calculate the effective diameter, use the actual diameter and subtract the radial height of the brake pad.

[Jcamper]

Great info jack, do you know what the various pedal ratios are? J

[Jack Hidley]

From 1979-95 all power brake Mustangs have a 3.5:1 ratio. The 1999-2004 V6 Mustangs also have a 3.5:1 ratio. The 1996-98 V6 have a 3.92:1 ratio. You CAN NOT use the power boosters, brake pedals or pedal boxes from these cars. The booster pushrod won't line up properly, causing bad things to happen.

The 1996-2004 Cobra and GT have several different pedal ratios, but since these cars use hydroboost, the m/cs, pedals and pedal boxes are also different and can't be mixed and matched with the vacuum boost brake parts. The m/c can't even be bolted to the vacuum booster for instance.

In addition the hydroboost units do not have a 1:1 motion ratio, like the vacuum boosters do. This makes it really dangerous to look at the m/c sizing on a hydroboost application and try to scale this for a vacuum boost application.

[Jack Hidley]

Chalky,

Your PM box is full.

[Jcamper]

From 1979-95 all power brake Mustangs have a 3.5:1 ratio. The 1999-2004 V6 Mustangs also have a 3.5:1 ratio. The 1996-98 V6 have a 3.92:1 ratio. You CAN NOT use the power boosters, brake pedals or pedal boxes from these cars. The booster pushrod won't line up properly, causing bad things to happen.

The 1996-2004 Cobra and GT have several different pedal ratios, but since these cars use hydroboost, the m/cs, pedals and pedal boxes are also different and can't be mixed and matched with the vacuum boost brake parts. The m/c can't even be bolted to the vacuum booster for instance.

In addition the hydroboost units do not have a 1:1 motion ratio, like the vacuum boosters do. This makes it really dangerous to look at the m/c sizing on a hydroboost application and try to scale this for a vacuum boost application.

Makes my amateurish calculations pointless now, doesn't it?

J

[Chalky]

Jack, emptied the PM box. Message away.

[Chalky]

Ok, so what's the situation if we decide to go '03 Cobra on all 4 corners but would like to keep the stock SVO pedal and booster???

[Jack Hidley]

Chalky,

With the 2003 Cobra front and rear brakes, the front brake bias will be 71.9%. If you install the 1994-98 PBR front calipers, these have smaller 38mm pistons and the brake bias would be 69.4%. I would use the smaller calipers as the brake bias will be much closer to optimum for an SVO. To tell if the calipers are the correct ones, see the photo below. They should have ribs on the outside of the piston bore, like the red caliper.

http://www.pix8.net/pro/pic.php?u=11130S5RAi&i=1117626

There are several models of PBR calipers that are similar. Mach 1, Bullit, etc. These all have the larger 40.4mm pistons.

With the Cobra rear brakes, you are going to have to get new rear axles for the Cobra brakes. You'll need to use axles from a 1994-98 V6 Mustang. This assumes that the SVO in question still has a 7.5" differential in it. You could use the SVO axles, but you would need to build spacers to move each caliper outboard 0.50". Using the SVO axles will severely limit your wheel selection in the rear, and keep you from using the same wheel and tire at both ends of the car.

If you use the larger PBR Cobra calipers (40.4mm), I would definitely use the 1" bore m/c. If you use the smaller PBR Cobra calipers, you could use the 15/16" or 1" m/c. If the car is used for track use, I would recommend the 1" m/c. This will make the pedal stiffer and higher effort which will make the brakes more confidence inspiring on track. If the car is more of a street car, I would use the 15/16" m/c to lower the pedal effort.

I would install the above combination, properly bed the brake pads and see how the pedal effort is. If the effort is too high, you can switch the brake booster to an SN95 model to reduce the effort about 25%. The only SN95 boosters that will fit are the 1993 Cobra/R (direct bolt in), 1994-95, or 1999-2004 V6. The later two of these will require the holes in the firewall to be elongated as the SN95 bolt mounting pattern is different that the Fox mounting pattern.

Reagrding my comments in post #32 above, I want to clarify that it is only the 1996-98 V6 vacuum boosters that can't be swapped into the other cars. These boosters have a dog leg shaped input shaft that requires the unique use of the matching pedal box and pedal.

[Chalky]

Jack, thanks again for your reply. More research and number crunching tonight and some more baseline stuff to add, as I think I missed giving you some of the info. The rear end in the car is a '95 GT 8.8 with stock width 95 axles and 95' 10.5" solid rotors. Sorry for the confusion there and i think the M-2300-M bracket kit will provide all the parts I need for the rear as the calipers are the same. The front will be modified SVO control arms with SN-95 balljoints and '94 spindles attached to a rebuilt original '86 SVO rack. The front struts will be Koni 8741-1211S attached to MM CC plates.

From what I've read the 94-8 PBR's with the 38mm pistons are listed as Cardone 18B4655 (L) and 18B4654 (R). I think I'll go with the 15/16" 95 Cobra MC even though it will need machining to fit the SVO brake booster. I like a solid pedal, but not a rock. If it's too soft more my taste, I can always switch it for the 1" '93 Cobra unit.

[Jack Hidley]

Chalky,

The SN95 Cobra and GT/V6 applications use the same 38mm rear caliper, but with different caliper hangers and caliper brackets. The M-2300-M kit will only come with the correct caliper bracket. You will also need to buy the correct caliper hangers. See the photos below for some of the differences. The rotor groove in the GT/V6 caliper hanger is too narrow for the thicker Cobra rotor and the hanger does not form a complete rectangle. Some people machine open the groove on the hanger. This can work, but then the pads wear with a lot of taper due to the reduced hanger stiffness.

http://i248.photobucket.com/albums/g...r_brackets.jpg

http://i123.photobucket.com/albums/o...GtCalipers.jpg

To install the 15/16" m/c in your car, you'll need the MMBAK-9 plus one Weatherhead 7818 fitting. If you swap to the 1" m/c, you'll need the MMBAK-17 and the Weatherhead fitting. Unfortuantely, those two m/cs have exactly reversed outlet sizes. We can supply the Weatherhead fitting.

[Chalky]

Must be my lucky day!!! I was going through a box of stuff looking for something else and came across these and then saw the pics you posted. They look the same to me so I guess that's 1 more thing I don't have to buy. Now I just need to find the appropriate axle mounting brackets and we're in business. Thanks again Jack.