Bremsentuning? | Posts 80+

Quote:

Quote: By the way, I also tried sudden, full braking maneuvers, so that the pressure increase was definitely faster than with a gradual, full brake application by an elderly person, even with a brand new braking system. The result was always the same: no squealing, no ABS activation. Well, that sounds a bit strange. The brakes definitely have a problem.

If that is really the case, I would first replace the brake pads/shoes all around .

Bertil wrote:

ulf wrote: And so, my question remains: Why doesn't the full hydraulic pressure of the emergency braking system in my Polo cause any tire squealing on a dry highway, nor does it activate the ABS? On any road surface? So, also on wet surfaces or snow/ice?

When there is poorer grip and sufficient pedal pressure, the ABS works normally, meaning that it (apparently) keeps the wheels from locking up by allowing them to rotate slightly, the pedal vibrates, and the pump hums.

Quote:

So, that sounds a bit strange. The brake really has a problem.

. . . but I think it feels quite normal, as long as it's not about maximum delay until the locking point on a dry surface.
The brakes were also not flagged as an issue during the vehicle inspection.

Do you have any suggestions on where I could start my search?

Could an ABS malfunction limit the braking pressure significantly before wheel lock-up, even on a dry surface, without the pedal vibrating? If so, I would try removing the main fuse and attempting a brake lock-up again (with a completely de-energized ABS, the speed signal is missing because the transmission sensor is omitted).

What are some ways to identify glazed surfaces when dismantling them?

In a "full emergency braking" situation, either the ABS system must engage, or the wheels will lock up.
"Definitely getting winter tires."
or you might have a stuck can lid (brake caliper), brake fade, air in the system, Qualifire slicks, or seized calipers/pistons/frame.
"But with a 280 disc, I can't imagine that being normal."
I believe that the SBB (Swiss Federal Railways) are most likely finished, because they are exhausted.

Burnt-out SBB (Swiss Federal Railways) brake pads: You can stick the screwdriver into the soft material and easily scrape it off, just like charcoal.
"Verglaste" means "glass-hard" on the surface.

.. and I don't think my legs are so weak that I can't generate enough pedal force for a full stop.

So, I once used a braking reaction simulator to measure the force with which I press the pedal during an emergency braking situation.

The device indicated 75 kg as the pedal pressure .

'I was able to get my Leon approved, even with 225 tires on 8' wheels, but only because the aftermarket body kit (BKV) was within the ABS regulation range.'
Which tire brand do you use? There are also significant differences in traction between different brands.

One more thing:
Ulf, did you depress the clutch pedal during your attempts?
When not engaged, the motor still has enough power that the wheels will not lock up.

Best regards, Jochen.

Jochen_145 wrote:

Which tire brand do you use?

There are also significant differences in traction between brands.

Quote:

In the tests, the factory-installed tires were: 195/55 R 15 Firestone Firehawk.

Ulf, did you press the clutch during your attempts?
If it's not disengaged, the engine still has enough power that the tires won't lock up.. . but that's probably only if it's resisting stalling at a speed below the idle speed?
Since I was doing the tests above 150 km/h, I was in 5th or 6th gear, and at that speed, even the LLR (Low-Level Torque) doesn't provide much torque to the wheels. However, when braking, the engine was definitely always above the LLR speed, so it was in overrun mode.

Without auxiliary brakes, both the vehicle and the engine's rotation need to be slowed down. However, in higher gears, the difference in RPM per km/h is relatively small, so "the slight inertia of the rotating engine components" shouldn't overwhelm a 288 mm brake, especially if it's used to decelerate both the vehicle and the engine?

In addition, it is my opinion that it would be highly problematic if achieving the maximum braking force, i.e., the blocking limit, is only possible if the driver, during a panic braking situation, is still calm enough to think about disengaging the clutch. }

ulf wrote:

... but I think it feels quite normal, as long as it's not about maximum braking force right up to the point of locking up on a dry surface.

It feels normal, but it's not normal!

Quote:

The brakes were also not flagged as an issue during the vehicle inspection.

Yes, I've experienced something like that twice before. After the technical inspection, I drove home and bumped into the wall in the garage... Once, a brake hose burst... the second time, it was a seal in the hydraulic brake booster. So much for the technical inspection .

Quote:

Do you have any suggestions on where I could start my search?

"It's tricky... I would start by replacing the brake pads and rotors all around. It's quite expensive, but probably unavoidable."
I'm also leaning towards glazing.

Quote:

Could an ABS malfunction limit the braking pressure significantly before wheel lock-up on a dry surface, without the brake pedal vibrating?

"So, normally I would say: no."

Quote:

Then I would try pulling the main fuse for the pump and attempting another ABS activation (with a completely power-less ABS, the speed signal is missing because the transmission sensor has been omitted).

Hopefully, your brakes are not assisted by a hydraulic pump... (but that's rather rare these days).


Okay, one more thing:
How old is your brake fluid?

Bertil wrote:

I also suspect glazing.

I just tried to find possible causes for glazing on the coating, but I haven't really become any wiser - except for indications of severe overheating.
Since I try to avoid braking as much as possible and let the car coast whenever I can, I'm wondering how my brake pads could still develop glazing – and, more importantly, how to identify glazing on the pads (when they're removed).

The approximately 4 test braking maneuvers were not always brought to a complete stop, but at best reached around -80 km/h. However, even the first test was unsatisfactory, as the Polo had never experienced "glazing" of the brake pads before. Could the pads already have been glazed at that point? If so, what usage errors caused it, and how can I avoid them in the future?

Quote:

Hopefully, your brake is not assisted by the hydraulic pump... (but that's rather rare these days).

Then it would probably make a regular buzzing sound before the ABS kicks in? -> No, there's no buzzing sound.

Quote:

How old is your brake fluid?

It was about 2 years old during the tests, but now it's almost 4 years old. I'll have it changed soon...

During the tests, the factory-installed tires were: 195/55 R 15 Firestone Firehawk.
Hmm, there's no tire here known for its particularly soft compound and therefore high grip.

. . but probably only if it resists stalling at low engine speeds?
However, the motor has a certain amount of inertia, so it takes time to lose its speed, and it continues to drive the wheels for a while.

I wouldn't bet on being able to lock the tires by now if the clutch isn't fully engaged.
With my old Fiesta, I hadn't yet learned to use the 'blip' technique when downshifting. If I released the clutch too quickly, the engine would dictate the wheel speed, causing them to either squeal or slip.

It wasn't until I used racing brake pads (Ferodo ID 451) that I was able to effectively reduce the engine speed using the brakes.

OK, without the torque converter, both the motor rotation and the vehicle need to be slowed down, but in higher gears, the difference in rpm per km/h is relatively small, so 'that little bit of inertia of the rotating motor masses' probably shouldn't overwhelm a 288 mm brake, especially if it's used to decelerate both the vehicle and the motor?

However, it further hinders the ability to slow down.

By the way, it is my opinion that it would be extremely problematic if achieving the maximum braking force (the blocking limit) is only possible if the driver, during a panic braking situation, is still calm enough to think about disengaging the clutch...

No, I don't see that, because that's how it's taught...


I've done a little searching online, but I haven't found exactly what I'm looking for yet.
Perhaps someone here knows more about it.

Given the maximum braking power, shouldn't the principle be analogous to that of a motor?

So, the formula would be: Power = Torque * RPM * Constant.

The maximum torque transmitted by the wheels must remain constant.

According to this, there should be a correlation between the wheel speed and the maximum transmittable braking power.

At low wheel speeds, the wheel is more likely to lock up because the maximum transmittable torque is exceeded more quickly than at high wheel speeds.

However, the maximum braking performance is not speed-dependent.

@Ulf

Can you potentially lock up the wheels if you're going too fast at low speeds?

Best regards, Jochen.

ulf wrote:

... I've just been trying to find out possible causes for glazing defects, but I haven't really become any wiser – except for some indications of severe overheating.

Curing through aging.
"This is actually a normal part of the process. The constant cycle of heating and cooling makes the surface harder, which speeds up the glazing process. Then, you won't need to use as much heat for the glazing. However, it's difficult to estimate how far along this process is with your specific materials."

Quote:

Since I try to avoid braking as much as possible and let the car coast whenever I can, I'm wondering how my brake pads could be glazing over.

Well, not really... but depending on the type of tires, it can definitely happen. I also experienced something similar with the original tires on my Golf. They weren't worn out, but they were just ineffective. But it wasn't nearly as bad as what you're describing here.
"Breaking in" new brake pads is crucial for their longevity.

Quote:

and, most importantly, how can you identify it from the (finished) surface?

That was already written in the partial clause:
Burnt-out SBB brake pads: you can stick the screwdriver into the soft material and easily scrape it off, like charcoal.
"Glazed" surfaces are "glass-hard" on the surface.

Quote:

The approximately 4 test braking maneuvers were not always brought to a complete stop, but at best reached a speed of -80 km/h. However, even the first test was unsatisfactory, as the Polo had never experienced "glazed" brake pads before. Could the pads have already been glazed at that point?

I would actually say no. However, it looks like that's the case.

Quote:

If so, what user errors were involved, or how could I avoid them in the future? . .

"Properly" bedding in new brake pads and rotors is crucial, as the line between achieving optimal performance and experiencing premature wear is very thin.

EDIT:
I once had an instruction manual from a brake pad manufacturer:
Bedding in new brake pads:

Proper bedding-in of new brake pads is essential for their long lifespan and the correct braking performance. If possible, avoid installing new brake rotors on your vehicle during the bedding-in period of the new pads.

If different brake pad material was used on the old brake discs previously, it is recommended to clean the discs with sandpaper. The brake pads must fit freely in the brake caliper without sticking.

After starting, brake several times at a slower speed to create an 80- to 90-percent contact area between the pads and the brake rotors. After a few brakes at a higher speed, the pads should be cooled down during normal driving. Subsequently, the pads can usually be loaded.

Hi!
Okay, something completely different: Did you happen to get any grease on the glass while working on the suspension? Like, did you over-grease the brake pad guides or the wheel studs, or something like that?


Best regards.
Dirk.

Bertil wrote:

ulf wrote: Since I don't brake aggressively, but rather let the car coast as often as possible, I wonder how my brake pads could still develop a glazed surface. Well, not really... but depending on the type of pads, that can definitely happen. I also experienced that with the original brake pads on my Golf. They weren't worn out, but they were just ineffective. But it wasn't nearly as bad as what you're describing here.

What type of brake pads did you "successfully" replace your original ones with?

Perhaps my description sounds a bit too dramatic. The fact that I can't activate the ABS on dry pavement is true. However, during full braking, I don't feel that it's doing significantly too little. I will soon test the braking distance from a real speed of 100 km/h on a dry surface and compare it to average car values of around 38-40 meters (?).

Quote:

"Proper" bedding-in of new pads and rotors. The line between this and "driving them again too soon" is very thin. EDIT: I once had an instruction manual from a brake pad manufacturer: . . .

Thank you, that will certainly help some of the readers as well .

Quote:

But you didn't somehow get any grease on the brake discs while working on the suspension, did you?! Like, too much grease on the brake pad guides or the wheel bolts, or something like that?!

To prevent rust, I always apply a little grease/oil to the wheel hubs and bolts during the seasonal wheel change (not a thick layer "in advance"), and I'm very careful to make sure nothing gets on the friction surfaces - but basically, I can only rule out your point 99%. If this stuff, thanks to capillary action, were to travel unexpected distances, you might be right that small amounts could end up on the outside of the brake discs...

ulf wrote:

Which types of vehicles did you successfully replace the original brake pads on?

I can't say for sure, but I think it was LUCAS.

Quote:

Perhaps my description sounds a bit too dramatic. The fact is that I can't activate the ABS on dry pavement.

Just a thought.
You are conducting this test with an empty vehicle. Your permissible gross weight is likely to be 300-400 kg higher. Additionally, you are probably allowed to tow an unbraked trailer weighing 600-750 kg.
If a brake reacts like that under these conditions, I would say it's overloaded.
However, in your (i.e., empty) case, the brakes would need to be able to "overbrake" the wheels slightly, and the ABS would have to intervene. The maximum braking force is always based on the maximum load condition.
At least during the initial braking, the ABS should need to modulate the brakes for 1-2 seconds. This applies even on dry surfaces.

Quote:

However, I don't feel like the braking power is significantly lacking during full emergency stops.

I even believe you.
Unfortunately, you tend to get used to such a "weak" brake.

By the way, the initial braking force and its effect are the most important factors for a short braking distance. In other words, the energy dissipated during the first third of the braking process is the most crucial.

Okay, I just removed the VA-SBB (factory-installed) brake pads. They are Ferodo 4307: is Ferodo generally known for having moderate grip?

I noticed no perceptible signs of grease or oil on the friction surfaces, and I was able to slightly indent the lining support plates with a screwdriver when applying more pressure.
However, I still processed the entire surface of each piece with a medium-grit sanding block, which resulted in noticeable sanding dust (indicating a dry surface, free of oil or grease in the material). This should have removed any potential thin layer of glazing... ?

During installation, I thoroughly cleaned the track guides and applied copper grease.

Let's see how the brakes perform on the next trip...

@Ulf

What is the free play of your handbrake lever?

Bertil wrote:

@Ulf What is the free travel of your handbrake lever?

7 cm until the pads engage, measured right at the front of the release button.

Quote:

Let's see how the brakes perform on the next trip...

After braking, it's exactly the same as before.

Build something new; everything else is just nitpicking.
"Hey, does the brake start making noises as soon as the pads get hot?"

If so, then all the more so.

It's probably thanks to your generally cautious driving style that you haven't damaged the brake discs with the Ferodo pads.

Regarding Lucas Klötzen products, I have installed them a total of 3 times because there were no other options available. I have also filed a complaint 3 times.