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Discussion Starter #1 (Edited)
4 lug disc brake kit from ssbc or lmr

Not 100% if this belongs here or not but my question is has anyone run the SSBC 4 lug rear disc brake conversion or the Lmr.com 4 lug rear disc brake kit?

Just put 4 lug 31 spline axles in last year not really looking to get rid of them just yet.

Thanks for the input as usual
 

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I believe those kits are just the turbo coupe rear disc conversion with 4 lug discs.

Problem with that kit is they use the 45mm rear calipers and you basically have to use that with the upgraded Lincoln 73mm front calipers to keep brake bias in check. If you use that kit with stock front calipers you will have way to much rear brake bias....Yes, even with a proprtioning valve.
 

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I run that kit with cobra calipers and 13 inch disks.I'm using my stock booster with a 93 cobra master cylinder. No issues at all.
He wants to stay 4 lug so the cobra 13" setup will not work for him.

I would have to do the math but I'm pretty sure even with the 13" cobra setup in front it's still to much rear bias to be "optimum". The 45mm rear caliper is just to big to use with any of the conventional front brake kits besides the single 73mm front caliper setup.
 

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Discussion Starter #5
I mean if 4 lug truly isn't going to work bias wise and what not that is fine. I could go 5 lug that just ups the budget a lot. I really wanted to run those rovo durbans 18" on my 92 fox but I guess the back spacing and what not isn't correct. I am already 31 spline rear so cheap axles are a no go, and spindles for front are getting harder to find locally here but I will keep an eye out.
 

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I need to add that before i went to the cobra brakes I used stock calipers with the ssbc kit. I also used a adjustable proportioning valve to keep the bias in check.I did numerous test stops and the rears did lock up but only after the car was almost at a complete stop. I understand what lt1haha is saying but these were my results.

Both of my mustangs have the turbo coupe rear disks and they both started out the same using the stock front calipers with a prop valve. My 86 has the cobra disks up front now and my 93 gt has the baer 4 lug big brake kit with dual piston calipers.
 

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I believe those kits are just the turbo coupe rear disc conversion with 4 lug discs.

Problem with that kit is they use the 45mm rear calipers and you basically have to use that with the upgraded Lincoln 73mm front calipers to keep brake bias in check. If you use that kit with stock front calipers you will have way to much rear brake bias....Yes, even with a proprtioning valve.
But that setup is factory stock on the 1993 Cobra using the 60mm front fox calipers and 45mm rears. I've seen many copy that exact setup without much complain as to too much rear bias. The base model 92-93 Saleen setup did use the 73mm front calipers, with 45mm rear calipers stolen from the 91-92 Lincoln Mark 7.

I can't imagine a 10.5" rotor and 45mm caliper is generating more rear brake torque than an 11.65" rotor and 38mm caliper. Can't ignore the mechanical advantage of 1" greater rotor diameter



Anyway, reading into the OP's question, it sounds like he's looking for a way to add rear disk brakes, but keep his recently purchased 4-lug 31 spline axles. Yes, there is a way to do it, but he'll need to purchase additional parts on top of the two kits he mentioned, which are basically 93 cobra/87-88 t-bird rear disk setups.

North Racecars Brake Solutions

Basically he'll need to spring for the $170 to buy a different set of brackets to move the rear calipers inboard 3/4" and run his current 31 spline axles. This will also allow him to run wider rear wheels than he could if ran the setup as is and pushed it out 3/4" each side.

There is also a way to modify the existing brackets by cutting them and mounting them inboard of the axle flange.

Of course, then he'll need MC, booster, and prop valve mods, not to mention adapting the parking brake
 

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I can't imagine a 10.5" rotor and 45mm caliper is generating more rear brake torque than an 11.65" rotor and 38mm caliper
Well I went ahead and did the calculations. Calculations were done using 60lbs of leg force making 509 PSI on the rear line

With the 10.5" rotor and 45mm caliper (1.77") brake torque is 2318.346 inch pounds

With an 11.65" rotor and 38mm caliper (1.49") brake torque is 1847.123 inch pounds.

As you can see, piston area has a much greater effect on torque than the size of the rotor/lever.

Just because the 93 cobra used stock fox front calipers with the t/c rear disc still does not mean the system isn't rear biased. ESPECIALLY when your really pushing the car on something like an autocross course and braking in anything other than a straight line. Particularly the reason Saleen decided to use the 73mm upgrade as it made the brake system better balanced and offered additional braking force overall.
.
 

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Well I went ahead and did the calculations. Calculations were done using 60lbs of leg force making 509 PSI on the rear line

With the 10.5" rotor and 45mm caliper (1.77") brake torque is 2318.346 inch pounds

With an 11.65" rotor and 38mm caliper (1.49") brake torque is 1847.123 inch pounds.

As you can see, piston area has a much greater effect on torque than the size of the rotor/lever.
Did you forget to account for the fact there are two 38mm pistons? The Cobra brakes (13" with dual piston PBRs) have greater piston area and greater rotor diameter, so I'm not sure how they could end up with less brake torque.

And I also don't understand the comment that piston area has a much greater effect than the rotor size; to determine torque, you multiple area and effective rotor diameter, so they have an equal effect.
 

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Did you forget to account for the fact there are two 38mm pistons? The Cobra brakes (13" with dual piston PBRs) have greater piston area and greater rotor diameter, so I'm not sure how they could end up with less brake torque.

And I also don't understand the comment that piston area has a much greater effect than the rotor size; to determine torque, you multiple area and effective rotor diameter, so they have an equal effect.
No, as it was a comparison between the cobra REAR rotor/caliper and the turbo coupe REAR rotor/caliper. NOT the twin piston front calipers.

What I mean by that is that the larger rotor diameter of the cobra rotor (11.65) Vs the smaller t/c rotor (10.5) can't, even with the additional leverage of the larger rotor, manage the same brake torque as the t/c setup with the bigger piston even though it has a smaller rotor. As per the discussion above.
 

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Sorry, I obviously didn't have my reading comprehension hat on. :p
 

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Well I went ahead and did the calculations. Calculations were done using 60lbs of leg force making 509 PSI on the rear line

With the 10.5" rotor and 45mm caliper (1.77") brake torque is 2318.346 inch pounds

With an 11.65" rotor and 38mm caliper (1.49") brake torque is 1847.123 inch pounds.

As you can see, piston area has a much greater effect on torque than the size of the rotor/lever.
.

That's interesting data.

Makes sense that they put that setup on heavier cars (turbo coupe) and a 5-lug similar version on the 91-92 Mark 7.
 

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Rear SVO calipers are 54mm not 45mm. Also SVO rear rotors are 11.375" and the 93 cobra/TC rear discs are 10".

I have North Racecars 4 lug rear discs on my car and the 73mm calipers didn't work out for me. Too much pedal travel for my taste and had to really back off on the rear pressure with the adjustable proportioning valve. Switched back to the 60mm front calipers and the braking feel was improved and was able to dial in more rear bias. I think the pad size is the same for the 73mm and 60mm front caliper and with the the same rotor size I doubt there is a measurable difference between the 2. From what I read the only thing you gain going to 73mm caliper is slightly better pad wear.
 

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I would say you're right on with all of that. Especially the part where a smaller piston caliper would make no measurable difference.

☝😖
 

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I have North Racecars 4 lug rear discs on my car and the 73mm calipers didn't work out for me. Too much pedal travel for my taste and had to really back off on the rear pressure with the adjustable proportioning valve. Switched back to the 60mm front calipers and the braking feel was improved and was able to dial in more rear bias. I think the pad size is the same for the 73mm and 60mm front caliper and with the the same rotor size I doubt there is a measurable difference between the 2. From what I read the only thing you gain going to 73mm caliper is slightly better pad wear.
In all fairness, without a properly sized MC, your results were to be expected. Increasing the front caliper piston surface area by nearly 50% is going to give you a soft pedal with a lot of travel without a corresponding increase in MC piston diameter. It's the same concept as if you put a larger bore MC on a car without changing any of the calipers resulting in a firmer pedal with a short travel (but requiring more effort)

I don't disagree with your conclusion though that the 60mm front and 45mm rears is the way to go here.
 

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I don't disagree with your conclusion though that the 60mm front and 45mm rears is the way to go here.
I respectfully disagree but to each their own. No doubt that the 73mm calipers is a better balanced setup. I did not like it with the stock calipers in front as threshold braking was nearly impossible without premature rear lockup. If your not planning on doing any racing, then keeping the stock fronts won't be nearly as menacing a setup.

HOWEVER, if you would decide that you want the 73mm front calipers it can be done VERY cheaply. What I did was go to Autozone and get both calipers for the appropriate Lincoln (I forget what year/model but it's easily found). Then buy a master cylinder for an 85 SVO (there's other models that use the same one which is also easily found). Switch it all over then bring your stock calipers back in and get your core return money back.

You will have to buy the appropriate size adapters for the MC that have the appropriate female threads for your brake lines. I got these from NAPA.

A well balanced brake setup for under $200.
 

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In all fairness, without a properly sized MC, your results were to be expected. Increasing the front caliper piston surface area by nearly 50% is going to give you a soft pedal with a lot of travel without a corresponding increase in MC piston diameter. It's the same concept as if you put a larger bore MC on a car without changing any of the calipers resulting in a firmer pedal with a short travel (but requiring more effort)

I don't disagree with your conclusion though that the 60mm front and 45mm rears is the way to go here.
When redid the brakes I'm 99% sure I went with a 94 GT MC which is 1 1/16", this was almost 9 years ago. A SVO MC is 1 1/8". From reading of other people's experiences going to 73mm calipers, I figured the 1 1/16" bore would be acceptable but it didn't work for me. I wanted to keep the newer style MC is the reason why I didn't go with the SVO MC. It's also possible I got the wrong size MC since I didn't measure it before installing it.

I did a few track days with NASA after the install and anybody that I told about 73mm calipers all said the same thing, go back to the 60mm calipers with the TC rear discs.
 

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When redid the brakes I'm 99% sure I went with a 94 GT MC which is 1 1/16", this was almost 9 years ago. A SVO MC is 1 1/8". From reading of other people's experiences going to 73mm calipers, I figured the 1 1/16" bore would be acceptable but it didn't work for me. I wanted to keep the newer style MC is the reason why I didn't go with the SVO MC.
Strictly speaking from a perspective of hydraulic systems, I could see how you still had a soft pedal. Just doing some napkin calcs, the 93 cobra setup with 60/45 and 1" bore works out to a ratio of about 17.5:1. If you keep everything else the same (booster, pedal arm ratio, calipers, etc) then changing the MC to the 1 1/16" bore gives you areound 20:1, which is on the soft/long travel side of things. An SVO MC works out to about 18:1 so that would be pretty close to the same exact feel, but it's the old style MC which i personally wouldn't want to run, but probably would be the one to try.


It's been years since I'd run any of these setup as I've had a mix of 4 and 5 lug setups along the way and really don't remember feel/balance/etc enough to comment. Current setup is SN95 11.65" rears and 13" cobra front rotors with c5 vette calipers. I certainly like my setup now.


I respectfully disagree but to each their own. No doubt that the 73mm calipers is a better balanced setup. I did not like it with the stock calipers in front as threshold braking was nearly impossible without premature rear lockup. If your not planning on doing any racing, then keeping the stock fronts won't be nearly as menacing a setup.
I'm actually confused as to what brake setup we are talking about? What MC did you use?
 

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I was talking about the 60mm fronts with 45mm tears as far as premature lockup.

It's been so long ago I don't remember what MC I had with the stock 60mm fronts and 45mm rears.

I know for sure when I had the 73mm fronts and 45mm rears I was running a SVO 1 1/8 bore MC. Balance was much improved with this setup Vs the stock front 60mm calipers especially under threshold braking/ race conditions. However, even with the 1 1/8 bore MC pedal travel was a little long for my liking.
 
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