bigger rear disc in rear than front a problem I assume?

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newguy

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I'm running into a dilemma, really bummed because I want to keep my 15" cop wheels for my 74 dart, but I am doing an HDK k member and need to order front brakes and it seems the only ones that will fit for sure are 11" front disc, and those will be smaller than the rear disc that are 11.2" on the explorer rear. I am not a brake guy, but it always seems rears are always smaller than the fronts. Is this going to be a major issue? I am trying to avoid 17" wheels as they never look right to me on the older muscle cars, too little sidewall. I also want the ride to be softer with the larger sidewall, this cars is going to be a street cruiser, nothing more.

so......will I be okay with the backwards disc sizes, or am I doomed to do the 12.19" front disc kit and go aftermarket 15's, or ?
 
My Thoughts..............

If the co-efficient of friction is greater in the rear than the front, the rears may lock up before the front and you'll spin out. I am not sure what your situation would be with the combo you are suggesting, but it would make sense to me, that an adjustable proportioning valve may allow you to dial down the rear breaks to avoid this. Just my 2c:)
 
depends on caliper and swept area as well

if they are tiny 1 pot calipers on the back that use only the middle 66% of the rotor swept area i see less of a problem

brake bias installed set it so the back are almost turned off
do some brake tests on a private road
stop adding in rear brake when all 4 skid at the same time
and then take a smidge out so you still have some balance when its wet

but yeah i agree with poster above you need the fronts to lock first not the rears..
 
What's the piston/puck size front and rear.

Smaller pistons rear is like running smaller diameter wheel cyls.
Proportioning valve adds overall adjustability with weight load changes, etc .
 
Last edited:
inertia said:
What's the piston/puck size front and rear.

Smaller pistons rear is like running smaller diameter wheel cyls.
Proportioning valve adds overall adjustability with weight load etc .
Click to expand...
Finally, someone understands and offers to chime in.
You could have rear rotors twice the size of the front and it wouldn't matter as long as the caliper size is proportioned right.
A common guideline for a 4 wheel disc system in a well handling car is a 2 to 1 brake bias, front to rear.
You want twice the caliper bore surface area in the front compared to the rear. Rotor size does matter some but it is the clamping force of the caliper that determines the actual braking capability.
 
Kern Dog said:
Finally, someone understands and offers to chime in.
You could have rear rotors twice the size of the front and it wouldn't matter as long as the caliper size is proportioned right.
A common guideline for a 4 wheel disc system in a well handling car is a 2 to 1 brake bias, front to rear.
You want twice the caliper bore surface area in the front compared to the rear. Rotor size does matter some but it is the clamping force of the caliper that determines the actual braking capability.
Click to expand...

Pad composite and surface area are also adjustable .
 
The 11" wilwood is a four piston caliper that are 1.75", the explorer would be stock single piston which I think is 1.875"
 
We're it me, I'd put it together with an easily accessible proportioning valve, - go from there.
Good luck, you'll be okay .
 
It'll help to know what size tires you're running on the rear compared to the front. Pro-street Mickey-T steamrollers on the rear will be less likely to skid than pizzacutter polyesters on all 4 corners.
 
Kern Dog said:
Rotor size does matter some but it is the clamping force of the caliper that determines the actual braking capability.
Click to expand...

Rotor size is your lever arm. Like a 6" ratchet compared to an 8" ratchet, there is a noticeable difference.

It all works together and a bigger piston may have a greater impact than a small increase in rotor diameter, not really certain. Just saying that I think rotor diameter matters more than "some".

I've got a huge spreadsheet that I use to calculate brake torque, but never played with rotor diameter to see how much of an impact. But I do see many big rotor setups with smaller piston area making far more torque than a big piston and smaller rotor setup.
 
jos51700 said:
It'll help to know what size tires you're running on the rear compared to the front. Pro-street Mickey-T steamrollers on the rear will be less likely to skid than pizzacutter polyesters on all 4 corners.
Click to expand...
Agree. I had tiny stock Girling dics on the front, and JFZ's on the rear, and it stopped great..... cause I had tiny front tires and 14x32 slicks in the rear. (Without a proportioning valve, just a line lock).
 
I agree that the lever arm of a larger rotor does have an impact but if a tiny caliper is pinching a huge rotor, I don’t see the rotor size slowing the wheel any faster. A tweeker with a 3 foot breaker bar won’t crack a nut loose quicker than I could with a standard ratchet.
There is a principal known as “the point of diminishing returns”.
 
Have you considered the wheels you want to run, you mentioned overpriced (imho) Wildwoods, and they are noted for wheel interference, at caliper, and often centerhole .
Just an fyi after all this work.
You wouldn't be the first person to install W/woods, then find they need bigger wheels, and of course, tires and $$$.
Good luck.
 
newguy said:
The 11" wilwood is a four piston caliper that are 1.75", the explorer would be stock single piston which I think is 1.875"
Click to expand...
Before I optimized my combination, I had Cordoba 11.75” front discs, Dr Diff/Cobra 11.7” rear discs but the caliper sizes were vastly different. Fronts were the 2.75” single piston design used from 1976 onward. The rears were 1.5” single piston. I was at a 3.3 to 1 ratio and while the car stopped well, the rears barely did anything. I swapped to 13” Cobra rotors and smaller front calipers along with a proper booster and now the car feels right. I can actually get the fronts to skid. The car stops better than newer cars I’ve driven.
 
Current tires are 235/60-15's rear and 185/60-15 in front on steel cop wheels, but the tires are around 10 years old and will need to be replaced (and may change sizes) if it ever drives again. I have been thinking about minitubbing the car and going 15x10's in back, but decided to put that off until the motor swap is done (the reason I went with the hdk k member, hence the need for mustang II brake kit).

I appreciate all the help as I need to install the k member/brakes/coil overs before I start mocking up the motor tranny
 
Go back to rear drum. Stay Mopar. 10" & 11" available. A lot less problematic.
Brakes need air circulation, hence the need for larger wheels. Looks or performance. Just another choice to make.
 
newguy said:
I'm running into a dilemma, really bummed because I want to keep my 15" cop wheels for my 74 dart, but I am doing an HDK k member and need to order front brakes and it seems the only ones that will fit for sure are 11" front disc, and those will be smaller than the rear disc that are 11.2" on the explorer rear. I am not a brake guy, but it always seems rears are always smaller than the fronts. Is this going to be a major issue? I am trying to avoid 17" wheels as they never look right to me on the older muscle cars, too little sidewall. I also want the ride to be softer with the larger sidewall, this cars is going to be a street cruiser, nothing more.

so......will I be okay with the backwards disc sizes, or am I doomed to do the 12.19" front disc kit and go aftermarket 15's, or ?
Click to expand...

Are you sure the 12.19" Wilwood kit won't fit your cop wheels?

Because the 12.19" Wilwood kit for the Mopar spindle lists a 15" wheel as the minimum.

MoparLeo said:
Go back to rear drum. Stay Mopar. 10" & 11" available. A lot less problematic.
Brakes need air circulation, hence the need for larger wheels. Looks or performance. Just another choice to make.
Click to expand...

Lol. The rear Explorer disks are not "more problematic" than a set of Mopar drums. Ridiculous.
 
Not sure, but keep hearing people having problems with 15" wheels and the 12.19" kits, and I don't know anyone that has a wilwood setup I can test it on. I really want to keep the wheels, I'll even buy some aftermarket 15x10s cop style wheels if I do minitub the car.
 
newguy said:
Not sure, but keep hearing people having problems with 15" wheels and the 12.19" kits, and I don't know anyone that has a wilwood setup I can test it on. I really want to keep the wheels, I'll even buy some aftermarket 15x10s cop style wheels if I do minitub the car.
Click to expand...

Have you asked @HemiDenny? I'm sure he has an idea of what some of his customers are running for brakes and wheels.

The other thing is, I don't think the Wilwood hubs will clear the factory cop wheels to begin with, regardless of the rotor size. The hubs used with the Mopar Wilwood kit definitely won't, the hub diameter is significantly larger than the hub bore on the cop wheels. Not sure about the MII kit. Do you have the part # for the Wilwood kits that you want to use?
 
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