Are the Black Magic rear shoes/drums a significant braking upgrade similar to the BMB front pads/rotors? I know the fronts do most of the braking effort, but I'm curious if upgrading the rears would have any decent effect. I'm not interested in doing a rear disc conversion.
Black Magic drum brakes upgrade worth it?
- Thread starter GASnBRASS
- Start date
I've often asked for an independent test of stock brakes vs ..... but no one has had the balls to do it. JP Mag started one a few yearsago but the series quit before posting any real results. After 40 years of motor sports my best conclusion, you want better brakes, you cannot increase the brakes performance to keep up with the increase of leverage a 33/35/37 tire gains. If you grow tire size exponentially, you will decrease braking performance exponentially.
Everyone understands the rear end ratio changes to go from 30s to 33s... but there is complete ignorance concerning braking.
Everyone understands the rear end ratio changes to go from 30s to 33s... but there is complete ignorance concerning braking.
Theoretically, you could improve the performance of the rear brakes simply by bypassing the proportioning valve. The proportioning valve reduces pressure to the rear brakes in a hard stop to prevent the rear axle locking up before the front. That said, you probably don’t want to bypass it, as locking up the rear before the front will cause instability in heavy braking, and could lead to a spinout and possibly even a rollover if you leave the road.
For the same reason, you likely wouldn’t want to significantly upgrade the rear brakes on a lifted TJ without very good front brakes. Lifting the Jeep and installing larger tires increases the front/rear loading ratio in a hard stop, so more relative force is required in the front axle. It happens to work out that upgrading the front brakes and not the rear when lifting a Jeep and putting on larger tires works out to be an adequate (perhaps not ideal) solution to account for the increased load on the front axle during braking.
The ideal method would likely involve installing an adjustable proportioning valve and tuning it so that the front locks up just before the rear. But that is more involved than most people on this forum are willing to do.
For the same reason, you likely wouldn’t want to significantly upgrade the rear brakes on a lifted TJ without very good front brakes. Lifting the Jeep and installing larger tires increases the front/rear loading ratio in a hard stop, so more relative force is required in the front axle. It happens to work out that upgrading the front brakes and not the rear when lifting a Jeep and putting on larger tires works out to be an adequate (perhaps not ideal) solution to account for the increased load on the front axle during braking.
The ideal method would likely involve installing an adjustable proportioning valve and tuning it so that the front locks up just before the rear. But that is more involved than most people on this forum are willing to do.
No, they are not a performance upgrade. They are a solid set of replacement parts to bring your rear brakes up to proper working condition, nothing more.
The problem with spending any time addressing most of anything you say is you are a drive by poster. You lob out bullshit, it gets refuted and you never come back to explain that you understand and appreciate the correct info, or post up correct info to refute the counter claim.
What testing would satisfy you?
Only by you. I and thousands of my customers implicitly understand that going from being unable to lock up 33's to now being able to lock up 35's, 37's, and 40's is in fact an upgrade.
The proportioning section of the TJ combination valves actually reduces pressure relative to the front across the entire pressure range. The rest is I wouldn't like to see any rear lock up on a SWB TJ.
Contemplate for a moment that making the fronts work better may be a reason NOT to upgrade the rears. If we operate from the position that rear lock up in a lifted rig is very dangerous at freeway speeds at other than straight line stopping on lower traction surfaces, then the opposite may be true. As we upgrade the fronts, that increases weight transfer to the front which reduces the weight on the rear axle. Less weight on the rear axle means the rear brakes can be induced to lock up easier.
The problem with adjustable prop valves is they lack knee point tuneability which would let you tune the break over point on the pressure graph up or down to do what you explained in the first part which is reduce the likelihood of lock up in a panic stop.
The better way is to make a spring loaded adjuster that replaces the piston in the front part of the combo valve. You can add preload that way to move the knee point around and keep the stock relationship while improving rear brake response.
I've yet to see an adjustable that does anything but limit overall pressure.
Almost true. I do have a billet reservoir cap that serves no useful purpose other than looks. I also have pre machined unit bearings for the folks that don't want to grind down the flange to get a rotor to fit. Baffles me that folks want to pay money for something they can easily do with a flap disc in 3 minutes.
Thanks for the straightforward reply. I'm replacing my complete brake system during my rebuild (new MC, prop valve, lines, etc) and was curious if the rear set did more than stock, but as was mentioned earlier you don't want TOO much braking from the rear.
Can I also ask you, do the boosters fail with any sort of regularity? Mine's 22 years old already and I'm leaning towards replacing it before the Mopar boosters go the way of the dodo.
I've worked on lots of TJ's. I've seen and replaced about 4-5 boosters over the years. If I had the wherewithal, I'd replace it if for no other reason that they will go away at some point.
John Scoggins
TJ Enthusiast
Along with what Mrblaine has explained about the combination valve, another very important function of the combination valve is to ensure that the rear brakes received fluid pressure before the fronts do (pretty much at the same time) to prevent the ass end of the jeep (or any vehicle) from moving around and getting out of control even during regular braking.
I built a test bench with a TJ master and combo valve, 4 pressure gauges, and a linear actuator. If the pressure goes to the rears first (I've also read the same explanations) there is such a small difference in time that it is indiscernible. Knowing how the internal passages are routed, I also don't see how a straight through T for the front calipers can get fluid pressure even a millisecond later than the rears. That function may be built into the master and my autopsy of several TJ masters doesn't show anything but the typical arrangement of springs, push rods, and piston seals.
John Scoggins
TJ Enthusiast
On the test bench, were the four gauges connected directly to the combination valve, (ie with no distance of brake line)? If so then I would expect not to see any difference in pressure rise vs time between the front and rear pressure readings, there is no distance of brake line for the pressure wave to travel through. Then again the TJ is so short it might just not need that function....
Did you ever graph the results, would be cool to see?
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I don't believe there is any delay in the transfer of hydraulic pressure, that's why it is used to actuate wheel cylinders. That and like I said, the rear portion of the combo block is nothing more than a connecting T for the front brake circuit. If there was a pressure wave delay, it would certain affect the rears first since they are further away.
John Scoggins
TJ Enthusiast
Copy that, I edit my earlier post to suggest that the TJ is so short it might not need that function of a combination valve,
John Scoggins
TJ Enthusiast
In other words, the TJ combo valve does not have the "Metering valve stem" section in the following pic, it just connects the two front calipers together.
The hyd brake system is more similar to an electric circuit than a garden hose like I think you are using to base the delayed response of the rear due to longer lines. There is no air back filling the lines when you release the brake pedal. There is in your garden hose. That is the delay in pressure when you turn the spigot on. Brake fluid is completely filling the brake lines at all time so as soon as you apply pressure at one end it is the same throughout the entire system like when you turn on the light switch. There is no delay as the electrons fill the wire to the lightbulb. It is immediate.
Correct, there is no metering valve function in the TJ combo block. This is not one from a TJ but they are identical.
John Scoggins
TJ Enthusiast
Cool, thanks
PS I noticed it is a Fiero's combo valve, worked on a few of those "X-body in reverse" cars back in the day.
PS I noticed it is a Fiero's combo valve, worked on a few of those "X-body in reverse" cars back in the day.
Length makes no difference. Say you had a hammer and 2 different punches...one being 6" long and one being 10 feet long. If you hold one end of each punch against a surface and smack the other end with a hammer...is the impact on the tip of the 10 foot punch going to be delayed compared to the 6" one? no. As long as the lines are full of fluid, length doesn't matter.