Brake Tuning: Improve Your Car's Stopping Power

Hello,

My Polo 9N originally came with slightly larger brakes than the versions with up to 74 kW, which means that at least 15-inch wheels must be used (minimum size for versions up to 74 kW: 14 inches).

Now, one might think that these things would provide exceptionally good braking performance. However, when riding on dry pavement at a higher speed (enough that you don't immediately come to a stop), even applying the brakes forcefully doesn't activate the ABS.
To even get the ABS to engage, you really have to stomp on the brakes with full force, like you would in a panic stop, for example, when you're driving at 160 km/h behind a bend on the highway and suddenly come upon the end of a traffic jam.
Conversely, this means that with less force applied to the brake pedal (due to lack of physical strength or an initial misjudgment of a dangerous situation), braking power is wasted because even the narrow 195-size tires aren't reaching their grip limit!

My 96kW "naked" Polo is certainly lighter than most of the 74kW TDI Polos, which come with the usual extra weight due to their features (the catalog states a weight range of approximately 160 kg for the unladen weight, but the minimum weight of the 96kW TDI is only about 40 kg higher than the 74kW model).
When I try to imagine the maximum braking power of an average-weight 74kW TDI with even smaller brakes, for example, at a speed of 160 km/h, compared to my car, I almost feel sick .

What is VW thinking, installing such poorly-looking brakes?
Could the factory-specified combination of disc and pads (friction coefficient?) be so unfavorable that high braking forces cannot be generated?
Could the effect be improved, for example, by installing different linings (without invalidating the operating license)?
Or is it possible that the geometry (friction surfaces and diameter) of the brakes simply doesn't allow for higher braking performance, so that even the large brakes of the 9N model are, in terms of weight, simply undersized, regardless of the friction material used?

Oh yeah: the friction surfaces of my brakes are neither rusty, nor oily, nor excessively worn.
Tread depth is okay, estimated mileage is approximately 20,000 km.

Hi,

@Ulf:

Quote:

But if you're on dry pavement and accelerating at a higher speed (so that you don't come to a complete stop), the ABS doesn't even engage. To even get the ABS to activate slightly, you really have to stomp on it with full force

.
I've noticed that too. I've seen it in several Volkswagen Golf IV Variant models (5 in total), as well as in 3 Ford Focus models. (More specifically, all the Golf Variant and Focus models I've tested so far have exhibited this.)
When braking from 180 km/h by continuously pressing the brake pedal, there comes a point where you apply full force to the brake pedal, and the ABS still doesn't engage.
Anyway, at first I typed "brake fluid," then "discs and pads," and now I'm typing "at a loss."
You can do this 20 times in a row, and it always yields the same result. What's strange is that you can hear a slight squealing of the tires and also see a very faint black mark on the asphalt. Nevertheless, you don't hear or feel anything from the ABS at all! Furthermore, I'm surprised that it's the same with the Focus. So, I can't really believe it's a design flaw. On the other hand: From a safety perspective, it's _DANGEROUS_.

Regards,
Sure, here is the translation of the text from German to English:

"Micha"

I know that problem, but in a different way:
"On our Audi A4 (1996, 110hp TDI), you have to press the brake pedal very hard, like an ox, especially when it's wet. In contrast, on the Golf Variant (1998, 90hp TDI), the ABS activates even with less force applied."

My girlfriend usually drives the Audi, but when she occasionally drives the Golf, the passengers, including the driver, are thrown forward into their seatbelts every time she brakes. I've already installed sport brake pads on the front axle of the Audi, but the effect on the braking force is hardly noticeable. I've been considering convincing my girlfriend to replace the front brake calipers with ones that have larger pistons, but that would mess with the brake balance and is probably not the right approach.

I've also considered the vacuum pump from the brake booster. When I have the chance, I'll try fitting the one from a Golf into my Audi. A larger master cylinder is also a possibility, if I can find one.

It can't be possible that the cheaper Golf brakes better than the Audi.

Best regards, Rudi.

Hi,

I'm not sure if the performance of a brake should really be judged by whether or not it can trigger the ABS at 180 km/h. If it can almost reach the point of skidding before the ABS engages, that's actually probably okay, maybe even better.

On the Ibi, the front ABS activates on dry roads at slower speeds (less than 50 km/h) only when braking very hard. However, the perceived braking power is still enormous, which I believe is likely related to the very unbalanced weight distribution. Other smaller TDI models (like Ulf's Polo...) probably have the same problem.

Specifically for the Ibi, the weight distribution is 66% at the front and 33% at the rear. When braking sharply on the highway, the vehicle tends to sway quite a bit... In fact, the rear wheels can hardly contribute any braking force at that point, so the ABS system kicks in, although you might not notice it much.

In the roughly similarly built Polo 6N (7 cm shorter, 2 cm less wheelbase) with the AEX engine (which was quite a bit lighter, probably 200 kg or more), the handling felt much more balanced and stable. However, I never drove it so fast that I needed to brake from speeds of 170+ km/h.

Best regards,

Jan.

We've discussed something similar before. Back then, I also complained about the harsh braking performance of my Golf, as Ulf and the others describe, which, especially at very high speeds and in combination with the somewhat unstable suspension, can slightly increase your heart rate.
At that time, I managed to acquire a V6 brake system at a relatively low price. It has 312mm discs, which means 16 inches. That wouldn't be a problem. However, the brake caliper is the same as the one I have. The difference is that the brake disc has a larger surface area. I also had the chance to drive a V6 (or rather, brake it!), and I was also disappointed (okay, it's not really comparable, I admit, it's just too heavy).

I then, and I mean Rainer was one of the people who suggested it, decided to go with Zimmermann brakes all around. I "got rid of" the V6 stuff from egedöns and have been very happy with the braking performance ever since. Unfortunately, I can't provide any specific measurements, but some smaller tests have given me more confidence. I hope this isn't just subjective.

...
After replacing the rear brake discs and pads, I was finally satisfied with the braking performance of my G4.
"It's a great design, and even with a light touch, you can easily access the cheap ABS plastic for installation."

Jan6K wrote:

Other smaller TDI engines (like Ulf's Polo...) probably have the exact same problem. Specifically, for the Ibi, it means: 66% of the mass at the front, and 33% at the rear. When braking sharply on the highway, the vehicle tends to sway quite a bit...

Hi Jan,

When I accelerate hard in my Polo at high highway speeds, there's no noticeable swaying – at least, I haven't experienced anything like that so far.
Perhaps I was too preoccupied with being startled that the ABS wasn't activating.

Quote:

If they manage to get right up to the point where the tires start to slip, that's actually okay, maybe even better.

I don't see it that way: how do you know how much more deceleration is possible without locking up, and therefore you're wasting it(!), if neither the ABS kicks in, nor do you apply enough pedal pressure to cause a brief lock-up (and then slightly release)?

Bertil wrote:

After I replaced the rear brake discs and brake pads, I was finally satisfied with the braking performance of my G4. It's really appealing, and even with a light tap, you can feel the cheap ABS plastic ready to be installed.

Hi Bertil,

What were your concerns about the old brake pads and rotors on the rear?

And what is the size of your front windshields?

ulf wrote:

... What were your concerns about the old brake pads and rotors on the rear?

Nothing!
That's exactly what happened. I've experienced this several times where a faulty brake would start working properly again after replacing the rear brake components.

Quote:

And what is the size of your front windshields?

280 x 22 (15")
All G4 TDI station wagons have a 5-bolt pattern with 15" brakes.

Hi,

Quote:

"When I accelerate hard in my Polo at high highway speeds, there's no noticeable swaying – at least, I haven't experienced anything like that so far."

It could also be related to the tires... back then, they were the cheap ones from Firehawk (or whatever they were called). I haven't experienced that with the Contis yet, but I also haven't had to do any emergency braking at 200 km/h with them. In any case, I definitely notice that the rear of the car gets very light (figuratively speaking...).

Best regards,

Jan.

Bertil wrote:

ulf wrote: ... What were your concerns about the old brake pads and rotors on the rear? Nothing! That's exactly what happened. I've experienced this several times where a faulty brake would start working properly again after replacing the rear brake components. Bitte gib mir den deutschen Text, den du übersetzt haben möchtest. All G4 TDI station wagons have 5-hole designs with 15" brakes.

Well, this is another interesting thing... Do you have any explanation for why your car brakes better now, especially at the front, where most of the energy is "dissipated"?

Could it possibly be related to a misadjustment of the handbrake lever mechanism (e.g., the cable lever not being fully engaged when the brake is released), or is the adjustment somehow different after the replacement, other than the retracted piston?

Or are the rear brakes the only ones that work better with the new parts?
That wouldn't be very useful for my Polo: My rear axle, which is hardly under any load, only provides a minimal braking effect, even at the point where it would normally lock up (tested using the handbrake).

According to the records, my front windows are 288 x 25 mm in size, which is slightly larger than those of the G4 Kombi.

ulf wrote:

According to the documents, my front windows are 288 * 25 mm in size, which is slightly larger than those of the G4 Kombi.

"Same here. However, I don't feel that the system can't handle the braking power well, but rather that the vacuum pump doesn't provide enough assistance. This is supported by the fact that with a similarly sized system in the GIII GTI 2.0, you could practically throw out anchors. When I was driving the G III with 4 people (with a weight of 1,110 kg), it should be roughly equivalent in weight to a G IV with only the driver inside (1,301 kg). However, the GIII braked much better, even though it only had 205 tires (G IV: 225 tires)."

Since you mentioned the Polo with the 14" brake system. Our 9N has FS-III brakes with 256x22 rotors in the front and drum brakes 200x40 in the rear, despite the optional ESP system. Last summer, we drove the Polo with 2 people and approximately 80 kg of luggage on the highway, going downhill and hitting the speed limiter (190 km/h on the speedometer). When I then needed full braking power according to Murphy's Law, I felt like I was watching my entire life flash before my eyes ...

ulf wrote:

... Well, this is another interesting thing... Do you have any explanation for why your car brakes better now, especially at the front, where most of the energy is "dissipated"?

If you don't bring the ABS (Anti-lock Braking System) into operation when braking, you're wasting a significant amount of braking power.
The HA (hydraulic actuator) is the only braking component in a vehicle that has an adjustment mechanism to compensate for wear. If this mechanism malfunctions, the entire braking system becomes unbalanced and can lose up to 30% of its total braking performance.
Those who use the search function will notice that we've already had this discussion before. Back then, I already tried to explain why that is.

Quote:

Could it possibly be related to a misadjustment of the handbrake lever mechanism (e.g., the cable lever not being fully engaged when the brake is released), or is the adjustment somehow different after the change than it was before?

No.

Quote:

, besides the inserted piston?

... and what is the purpose of this mechanism?
Then you have your explanation!

Quote:

Or are the rear brakes the only ones that work better with the new parts?

That explains itself.

Quote:

That wouldn't be very useful for my Polo: My rear axle, which is hardly under any load, only provides a minimal braking effect, even at the point where it would normally lock up (tested using the handbrake).

That was exactly my problem. After replacing the (actually good) brake pads and rotors, the braking feel was very harsh after the break-in period (200 km). Now, when I pull the handbrake, it feels like a bolt is shooting through the rims.

Quote:

My front

windows, according to the documents, are 288 * 25 mm in size, which is slightly larger than those on the G4 Kombi.}
I just double-checked the measurements... I also calculated 288 * 25... According to the documentation (AKTE), the G4 model has both 280mm and 288mm brake options.

Roger wrote:

Last summer, we drove on the highway in a Polo with 2 people and approximately 80 kg of luggage, going downhill at full speed (speedometer reading 190). When I then had to brake hard due to Murphy's Law, I felt like I was watching my entire life flash before my eyes...

Oops.
What saved you?

Bertil wrote:

In addition, the HA (hydraulic actuator) is the only braking component in a vehicle that has an adjustment mechanism to compensate for wear. If this mechanism does not function correctly, the entire braking system becomes unbalanced and can lose up to 30% of its total braking performance. Those who use the search function will find that we have already had this discussion here before.

Back then, I already tried to explain why that is the case.

Quote:

Hi Bertil, I can't find anything about it - I'm probably using the wrong search terms. Do you have another link?

besides the inserted piston?

... and what is the purpose of this mechanism?
Then you have your explanation!
I think it's related to the parking brake adjustment mechanism.

But what this has to do with the hydraulic }braking force is not clear to me...

ulf wrote:

... I can't find anything about it - I'm probably using the wrong search terms. Do you have another link?

I'll have to look for that first.

Quote:

I think it's related to the parking brake adjustment mechanism.

No, that's not the case (at least not primarily)!

Quote:

But what this has to do with the hydraulic braking force is not clear to me.

Ulf... just think about it!

If you have too much free play in the rear axle (HA), the valve assembly (VA) brake will engage before the rear axle brake. This results in a loss of valuable pedal travel and braking force. In the worst case, the rear axle brake may not engage at all.
The adjustment mechanism on the HA (rear axle) is designed to synchronize the engagement points of the brakes. If this mechanism is out of adjustment, you will lose a significant amount of braking power.
It is important that a braking system responds uniformly across the entire clamping range. This is achieved through the adjustment mechanism.
Unfortunately, this mechanism doesn't compensate for frosted or deteriorated (foggy) windshields. The only solution is to replace them. That's the whole problem.

Bertil wrote:

I'll have to look for that first.

Found ... https://community.dieselschrauber.org/en/viewtopic.php?t=9982

@isar/ Rudi

Quote:

In the case of the Golf Variant (98 model, 90PS TDI), the ABS system activates even with less force applied.

Then it's a Golf 3, with drum brakes in the rear and a really bad brake balance.
I also command a ship, and I've come to the following conclusion:

It only brakes in the front.
This creates such immense force on the front tires that they want to grip and stop every time.

My ABS is working overtime, and the tractor is contemplating whether it should actually slow down.

Modern cars pull you into the seatbelt much more noticeably before ABS or similar systems even kick in.

m;