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Brake proportioning

Brake proportioning

(OP)
The wife's car (Nissan Cube) has had issues from practically day one... but this particular thread is about brakes.

At some point in the past, the steering wheel took on a real shudder during braking... fine, warped rotors. But why did they warp? While doing some maintenance work (read, replacing the failed CVT that nearly killed us... twice), they dealer was kind enough to notice the shimmy and turned the rotors. A month later it was back. We decided to live with it.

Took it to a local shop (for a SECOND broken flange on the tailpipe, which left the muffler hanging) and they noticed the shimmy, too (as if you could miss it!). The front disc pads were down to the last millimeter or two, but the rear drum pads were practically untouched. Mechanic said it appears as if the brakes are running about 90/10 rather than a more pad friendly 70/30 split on braking power... which would explain why the front rotors overheat and warp so easily. So, new rotors/pad are on order, and I threw in some new drums/pads for good measure because the price was so good (about $300 for everything).

My question is, how can I change (fix/repair?) the proportioning on this car (and on an acedemic note, why isn't it set properly from the factory)? On the NSX, it was a set of handles in the boot, but I'm learning with most cars it's generally a setup that requires replacement of the master cylinder if you want to change it out (assuming you can get a master cylinder with a different proportion... such as the Altima).

Could this simply be an issue with the rear drums not being adjusted properly? Don't most modern drums have auto-adjusting pads that ratchet with every pedal press, ensuring after a few initial presses (after initial assembly/replacement) that every succeeding press will apply proper pressure for the life of the pad?

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Brake proportioning

Not likely to be adjustment. A quick google found this thread Link which indicates Altimas have a brake proportioning valve as follows:
ALL 93-97 SE altimas (with or without abs or whatever the case)have a brake bias of 65% to the front and 35% to the rear.
All 93-97 Models with ABS have 65% front and 35% rear brake bias
All 93-97 Models Except SE Without ABS have 68.7% front and 31.3% rear brake bias
For 98-01 it gets simpler:
All 98-01 SE models have 62.5% brake bios up front, 37.5% to the rear
All 98-01 alti's except SE models have 65%front, 35%rear brake bias


If the Cube has the same system, your best bet might be the BPV from a 98-01 Altima SE.
If the BPV can be dismantled it is possible to alter the bias by changing spring, shimming etc.

Other ways to shift bias to the rear include:
Harder front pads
Softer rear linings
Larger diameter rear wheel cylinders
Different master cylinder - smaller rear piston, larger front or both. (This won't work if a BPV is fitted)

je suis charlie

RE: Brake proportioning

I don't think it's that simple.

In most cases, the basic brake force proportioning (and I am talking about braking force "at the tire contact patch") is set not by the proportioning valve, but rather by the sizing of the wheel cylinders, diameter of the brake rotors, etc. Basically with the same hydraulic pressure applied all around, the front brakes will give you the 65% of total braking force (or whatever it is) and the rear brakes will give you the remaining 35% because the rear caliper pistons are smaller and the rotors in the rear are smaller so the caliper has less leverage on it, etc. It's a little different with drum brakes but the same principle applies; the basic brake force distribution is set by the design of the brakes with the same hydraulic pressure applied to all brakes.

The proportioning valve only comes into effect beyond a certain pressure threshold. Because the weight distribution as seen by the tires is thrown further and further forward with increasing deceleration, at a certain point it is necessary to limit the pressure being supplied to the rear brakes so that the rear brakes do even less (in proportion) than their sizing alone would give. The proportioning valve just limits the maximum brake pressure above a certain threshold. It should have no effect during normal driving unless the driver is a maniac.

But then the complication ... Drum brakes have springs in them which pull the shoes away from the drum when the brake is not being applied. Disk brakes don't. Disk brakes will (almost) respond to the slightest bit of brake line pressure (there is some preload necessary to overcome the seals, but it's really, really low). Drum brakes don't do anything until enough brake line pressure is applied to overcome the springs. Someone who normally brakes very gently could very well be only using the front brakes.

And then the even greater complication: ABS. A lot of vehicles with ABS have no proportioning valve. The front/rear brake force (to the tire contact patches) is set by the caliper/drump sizing with the same pressure applied all around but it uses the ABS hardware to sort out the proportioning. One way is to not actually do any proportioning, but simply go into ABS activation when the tire runs out of grip. Another way is to use the ABS hardware to actually apply the brake, since in this day and age of ABS/ESP/traction control, the ABS hardware has the authority to apply individual brakes without driver input.

I doubt if the real issue is the proportioning. The real issue is more likely that the front brakes are simply undersized (read: cheap).

RE: Brake proportioning

I agree the front brakes seem to be inadequate regardless of bias issues. Drum linings will always outlive disc pads especially when they are only doing half the work.

A quick check on rear braking is to jack a rear wheel off the ground and turn it by hand while your assistant gradually presses the brake(you may need the engine running to power the booster). You will be surprised how much torque you can apply with gloved hands at the tyre tread. You might like to repeat to compare a front wheel.

You could test the brake bias by performing some emergency stops with an observer each side of the road to confirm which wheel is locking. Start with enough pedal pressure to lock a wheel and repeat with increasing pressure until all 4 wheels are locking (if possible). You will get a feel for how much imbalance exists.

je suis charlie

RE: Brake proportioning

(OP)
Brian, the wife and I aren't gentle brakers... each for different reasons. wink

I should have noted we're on the second set of rotor pads (nearly gone) for the front and the car has around 65k miles... the drum pads are original and have hardly been touched, so they're just not being used. I agree that the braking system seems wholly inadequate for this car during anything other than grandmother braking (it IS essentially a Nissan Versa under the skin), but it appears to be something a bit more.

I'm only familiar with rotors (having rebuilt, restored, etc. a slew of caliper systems over the years), so while I understand the basic operation, I have never actually had hands on one. Am I correct in thinking the drums have a ratcheting mechanism that will auto-equalize pressure with each pedal press (similar to the mechanism on calipers that slowly screws the piston out as the pads wear)? If not, could this imbalance have been caused by the drum pads not being adjusted close enough to the drum itself? If it's as simple as that, I can easily readjust to the appropriate clearance while I'm working on all four corners.

Just looking for anything that could be tweaked in the system to get a more appropriate braking operation (and stop ruining the front rotors!).

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Brake proportioning

Drums normally have an automatic adjustment mechanism that operates when the brakes are applied when driving in reverse. Normally you never have to touch them. This adjustment won't affect how much pressure is actually applied to the brake shoes ... only how much movement the shoes have before something happens (i.e. how far the brake pedal goes down before something happens).

The test described above, to turn the wheel by hand with the car off the ground when applying the brakes (may need engine running to have the power brakes and ABS operating normally) should tell you if the rear brakes are being applied at all.

The brake shoes for the drum have much more surface area than the disk brake pads and it's normal for them to last far longer.

If you are on the second set of front brake pads in 100,000 km plus you are warping rotors, driving patterns likely have something to do with it. I usually get 150,000+ km out of a single set of front pads.

RE: Brake proportioning

It's not uncommon, at least for me, for the front pads and rotors to wear significantly faster; I think it's to do with the fact that most modern cars are front wheel drive, which puts both car and engine inertia on the front pads. On one car, I don't ever recall getting pads for the rear wheels, but got at least 2 sets of front pads


re. your front rotors; our old Honda Pilot seemed to have rotor shimmy on a regular basis. We had the rotors turned several times, and the shimmy would invariably start up after about 5k miles. Never found the cause, and we had it traded in in 2009, and the new Pilot doesn't have this problem.

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RE: Brake proportioning

When disc brakes first appeared, there were indeed mechanisms that screwed some part of the piston out, or something similarly complicated, that had to be reset in some way when pads were changed, and most of the mechanisms were not very reliable.

Nowadays, all of that is done by a square section seal ring that (reliably, mostly) pulls the piston back from the pad's backing plate when the hydraulic pressure goes down.

Your brakes may indeed be undersized and/or unbalanced, but the first sensible thing to try in response to short pad life (while of course correcting collateral damage by truing the rotors, etc.) is rebuilding the calipers with new hydraulic seals and new dust seals. It's not difficult. It is messy. It is easy to screw up if you're in a hurry, as are most flat rate mechanics.

Mike Halloran
Pembroke Pines, FL, USA

RE: Brake proportioning

(OP)
Mike,

I expect the new brake components will be in before this weekend, so hopefully I can find the time to take care of the install. Once I'm in there I can better assess the condition of everything. I have no doubt the drums could use some cleaning, so it'll likely be some brake cleaner, adjustment, then lube for a handful of components back there. I used to rebuild my own calipers, but it's a dirty job and the bleeding process is a royal pain, so these days it's usually easier for me to pay a shop to do it (should it come to that).

I also have a ball joint on order to fix the driver's lower control arm... only $40 for the entire arm (including joint), but the driver's side requires jacking up the engine to remove one of the bolts (thanks Nissan!), so I'm opting for just replacing the ball joint itself at $10. That won't show up until next week sometime, so it'll have to wait (and doing it while the brakes are off would have been the perfect time, too). <sigh>

I'll post what I find...

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Brake proportioning

(OP)
After more research, I may be chasing the wrong dragon (the proof will be available in a few more weeks)...

From what I've read (and in talking with some other gearhead friends), a bad ball joint can cause the steering wheel to shake. Since the shaking doesn't happen with every application of the brakes, only at random times, that suggests it's not warped rotors causing the issue (I would expect warped rotors to be a constant cause of wheel shake).

Should this turn out to be true, it renews my faith in the stock braking system. The front rotors/pads still need replacement (this weekend), but I'm crossing my fingers the ball joint is the real cause of the shake issue and next weekend's replacement resolves the issue for good.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Brake proportioning

I recently replaced 6 tires on my truck and four on a trailer at the beginning of an interstate trip to pick-up some equipment. As the technician was lowering the truck after completing the job, I asked him why the front left tire had scalloped so badly. He said "Do you want me to check?". I said "That would be nice." Turned out that the ball-joint was completely worn out with over 1/4" of play in it.

RE: Brake proportioning

Rebuilding most factory-style brake calipers is pretty trivial. But unless they are actually leaking, and as long as they will push back into their bores without difficulty, they are probably fine.
I hope the ball joint turns out to be the issue.
A comment on drum brakes - the further out of adjustment the shoes are, the further their return springs must be stretched before the shoes reach the drum.

So, adjustment will have some effect on the braking balance. Maybe not very much, though.

Jay Maechtlen
http://www.laserpubs.com/techcomm

RE: Brake proportioning

Front end shimmy when applying the brakes might be due to a bad front wheel bearing.

RE: Brake proportioning

(OP)
Rotors/pads replaced... 3 hours for the left side, 1 for the right (guess I got the sequence down... truth be told, I didn't have to run inside and up/down the basement stairs getting tools I forgot i needed... 50 times). Car still stops, so that's good.

I had ordered a ball joint to replace, but once I got in there I noticed the control arm bolts were NOT hidden by the engine (as in the Versa). I'll eat the $11 on the ball joint, and I ordered both left/right lower control arms to replace them wholesale... $80 for both.

On my test ride I felt zero shaking, but the shimmy comes/goes, so that's not a conclusive test. I'll know more if/when SWMBO comes home and complains about it.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Brake proportioning

(OP)
SWMBO says no steering wheel shake since putting on the new rotors/pads (control arm / ball joint replacement scheduled for this coming weekend). I'm not sure if I like that, as it really only offers two choices: 1) I moved something around enough to (temporarily) stop the shaking... not likely, but possible, or 2) warped rotors were the actual cause of the random shakes (and random leads me to believe that's not it).

I'm still replacing the control arms, so maybe I'll be only partially lucky and solve the problem without ever knowing the true cause.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Brake proportioning

I often use a dial indicator to check rotor runout when installing new rotors, lest my knowledge remain of a most meager and unsatisfactory kind. It doesn't take much of a burr or ding on the hub or rotor to create rotor runout, and invite lug looseing. A 1 inch long chunk broken off the end of fine flat file is pretty good at cleaning and deburring the rotor and hub surfaces even working between the studs. (And I use a micrometer to check even new rotors for thickness variation < 0.001" and whether the thckness is full OEM spec) . Serviceable mag base 1" travel dial indicators can be had new for pretty short money ( less than $40 US bux) .

Some intriguing tapered shims used to be available to compensate for outright machining errors, or to do a super nice job.
About $15 bux apiece from these guys.
www.nucapperformance.com.

RE: Brake proportioning

(OP)
I didn't mic the rotor, but I made sure to really clean the hub and get rid of the mass of rust that had built up with a wire brush. It took quite a few minutes (and a very tired arm/wrist) "tapping" on those rotors to free them... they may be floating rotors when the car is first assembled, but they quickly become captive versions with all of that rust build-up. I applied a very thin coat of high-temp grease to the interface to prevent that build-up.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Brake proportioning

I spin the rotors by hand, using the caliper as a tool rest, like in a wood lathe, and using a big cold chisel as a cutter, to remove the crusted rust outside of the pad path (to make clearance to get the calipers or the pads off). A similar technique should help, next time you want to de-crust the hubs.

I'd use Never-Seez to provide for future separation. Graphite grease might be okay.


Mike Halloran
Pembroke Pines, FL, USA

RE: Brake proportioning

(OP)
I had no issues getting the pads/calipers off of the rotors.. it was separating the rotors themselves from the hubs that caused some serious banging. It's difficult not to just beat the tar out of it with the 10-pound engineer's mallet, but that might screw up the alignment. So I tapped tapped tapped for 5 minutes each until they came off.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Brake proportioning

Do they look like this ? (And I expect they do)
http://www.paulstravelpictures.com/Nissan-Cube-Fro...

If so, the rotor is clamped between the wheel and hub. Not floating at all. Pretty typical of the passenger car disk brakes I hang around with. And most of the pokey old motorcycles too.

RE: Brake proportioning

(OP)
My bad... did a little more research on definition of floating versus fixed (don't remember where I got the impression it was a floating type). These are fixed.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Brake proportioning

Calipers can be "fixed" usully with opposing pistons.

Some calipers float, and can get away with piston(s) in just one side.

RE: Brake proportioning

If your calipers have pins make sure they are lubricated and don't bind.

RE: Brake proportioning

Do modern cars with drum brakes still use residual pressure valves? Looking at the rear brakes again, if the M/C has integral RPV's it's possible it is leaking by. Then it would take much more pedal travel to engage the shoes.

RE: Brake proportioning

Not mentioned anywhere so far - what sort of bedding-in procedure for the original and new pads has been employed? "Warped rotors" frequently catches the blame for what was really uneven pad material transfer.

Probably a lot less common is the possibility that the thicknesses of the rotor friction surfaces is not constant around the entire circumference, with implications of uneven expansion. I have a couple sets of name-brand front rotors for a Nissan Maxima where this thickness variation is visible to the naked eye that caused severe brake pedal shuddering during and after attempting to bed the pads.


Norm

RE: Brake proportioning

(OP)
We're now a month+ in on the new pads/rotors and nary a wiggle has shown up. I'm wondering if something odd happened at the factory to cause the issue. As long as it stays away, I'm happy.

Dan - Owner
http://www.Hi-TecDesigns.com

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