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Old 19th November 2022, 05:25   #1
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Why do brakes groan when on a slope?

I've noticed this happen in multiple cars, including mine. A video explains the sound much better than I could, so here it is at the 4:00 mark



Why do brakes make that groaning sound when one is on an incline? I believe this happens in both forward and reverse directions though I may be wrong about that part.
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Old 19th November 2022, 13:08   #2
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Re: Why do brakes groan when on a slope?

Brake dynamics engineer here

Brakes groan due to a phenomenon called stick slip which has something to do with the tribology (contact between disc and pads) and the the response of the chassis parts and bushings. This leads to results in self excited vibrations.

The reason why this is becoming more common is because of the choice of friction materials for brakes. Cars getting faster+heavier+regulatory requirements+competition to achieve best stopping distances all have led to the need for a certain type of friction material.

Please note that this phenomenon or brake squeal which also occurs due to broadly similar reasons has no bearing on safety or stopping distances. In fact the better the performance, higher the likelihood of these issues cropping up (in fact this is a bigger headache for luxury car makers and they spend millions to figure out a good compromise between NVH comfort and performance).

High speed judder on the other hand is a cause for concern something that will need looking into. But that is another lesson for another day.
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Old 19th November 2022, 13:16   #3
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Re: Why do brakes groan when on a slope?

Quote:
Originally Posted by ashivas89 View Post
Brake dynamics engineer here
Great to have someone on this topic.

My question- do brake shims/ spacers help arrest this noise? Will adding more than 1 shim change anything?
Also, do brake shims in any way, positively or negatively impact braking efficiency?

Thanks in advance.
Regards,
Saket
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Old 19th November 2022, 13:29   #4
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Re: Why do brakes groan when on a slope?

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Originally Posted by saket77 View Post
Great to have someone on this topic.
Always happy to contribute

Quote:
Originally Posted by saket77 View Post
My question- do brake shims/ spacers help arrest this noise? Will adding more than 1 shim change anything?
Also, do brake shims in any way, positively or negatively impact braking efficiency?
Primarily shims are used in this context to either tune the contact patch between the pad and the disc or to enable optimal placement and movement of the pads within the caliper housing. This requires a lot of tuning (100s of man hours simulating+testing on dynos)and they are specific to each problem encountered. So they help, if tuned as mentioned.

There is never a one size fits all solution and aftermarket shims are definitely a hit or a miss.

Performance in terms of brake pedal feel might take a marginal hit if a thick elastomer layer is used, for example. Stainless steel shims don't register.

Absolute performance in terms of stopping distance is never an issue. However, in high performance cars, materials have to be chosen carefully to account for the high thermal demands of performance brakes.
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Old 20th November 2022, 21:58   #5
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Re: Why do brakes groan when on a slope?

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Originally Posted by ashivas89 View Post
Brake dynamics engineer here

Brakes groan due to a phenomenon called stick slip which has something to do with the tribology (contact between disc and pads) and the the response of the chassis parts and bushings. This leads to results in self excited vibrations.

The reason why this is becoming more common is because of the choice of friction materials for brakes..
Thanks. But what is the reason brakes make these noises on steep inclines, but not during hard braking?

Jeroen
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Old 20th November 2022, 22:08   #6
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Re: Why do brakes groan when on a slope?

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Originally Posted by Jeroen View Post
Thanks. But what is the reason brakes make these noises on steep inclines, but not during hard braking?

Jeroen
Great question.

Primary reason: The relationship between the coefficient of friction and relative slip velocity between the pad and the disc. If you plot these to in a graph you will find that the friction value changes quite rapidly at very low velocities (vehicle speeds corresponding to slopes, creeping at traffic lights etc..) and remains constant with respect to slip velocity at higher speeds. The sharply falling friction value at low speeds is required to excite these vibrations. This is the main cause behind stick-slip.

Another reason (which only supports reason 1): The friction characteristics required to excite these vibrations more often than not occur when the brake pads are exposed to cold temperatures and humidity (soaking during car washes) and also if there is mild corrosion on the disc. High speed braking largely gets rid of all these factors.

Last edited by ashivas89 : 20th November 2022 at 22:17.
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Old 20th November 2022, 22:55   #7
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Re: Why do brakes groan when on a slope?

It sounds very similar to the noise pretty much all automatics make under normal breaking conditions too.
Actually the brakes in automatics are usually noisier than their manual counterparts and seem to be more audible at lower speeds
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Old 20th November 2022, 22:57   #8
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Re: Why do brakes groan when on a slope?

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Originally Posted by Mortis View Post
It sounds very similar to the noise pretty much all automatics make under normal breaking conditions too.
Actually the brakes in automatics are usually noisier than their manual counterparts and seem to be more audible at lower speeds
This is because creeping at very low speeds is much easier in autos than manuals, and done more often. Perfect "boundary conditions" for these kind of friction induced vibrations.
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Old 20th November 2022, 23:12   #9
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Re: Why do brakes groan when on a slope?

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Originally Posted by McLaren Rulez View Post
I've noticed this happen in multiple cars, including mine.
...
Why do brakes make that groaning sound when one is on an incline?
Indeed, at one time this question had flummoxed me, and I had posted a thread here (Grinding noise from the brakes). The noise can be heard in this video (use headphones):

Took me a long time to understand the reason, which is exactly what @ashivas89 replied...
Quote:
Originally Posted by ashivas89 View Post
Brakes groan due to a phenomenon called stick slip...
Thanks for the detailed explanation, @ashivas89.

Last edited by SS-Traveller : 20th November 2022 at 23:15.
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Old 21st November 2022, 10:12   #10
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Re: Why do brakes groan when on a slope?

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Originally Posted by ashivas89 View Post
The friction characteristics required to excite these vibrations more often than not occur when the brake pads are exposed to cold temperatures and humidity (soaking during car washes) and also if there is mild corrosion on the disc. High speed braking largely gets rid of all these factors.
Thanks for the explanation

A layman's example here, seems like the same logic is at play.

I was accustomed to these "vibrations/noise" as the staircase in our house has a rounded wooden railing. Rounded is important because as I am coming down my entire hand's surface area is gripping the railing similar in concept to how the caliper grips the flat disk via flat brake pads.
Coming down and then gripping the railing harder to slow down for the last steps it would make a slight squeal and then the vibration would start and as I pushed more(accelerated) just for fun, the vibrations were significant enough to visibly shake the unhinged end of the railing.
Exactly like you said this was easy to happen during rains due to the absense of dust and presence of moisture both in the hand(brake pad) and the railing(brake disk) aiding grip and it doesn't happen at all during dry and dusty environs as the hand just slides over without much grip.

So experiencing the same in cars wasn't surprising at all.
Hope this crude example makes sense and please do advice if it isn't correct on the principle.

Last edited by shancz : 21st November 2022 at 10:14. Reason: better info
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Old 22nd November 2022, 18:03   #11
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Re: Why do brakes groan when on a slope?

Interesting one!

Let me share my observations on brake groan or brake chatter. This issue is pronounced in my experience in two scenarios. When the speed is extremely low and the weight is biased towards one side i.e. front or rear. This sound emanates almost entirely from the front disc brake of the car as opposed to the drum brakes.

Now, I've observed this phenomenon on both my motorcycle and car. Case in point, when the motorcycle is washed and where I live there is slope to cover to access the main road, and when you engage the front brake, the front brake groans. What's interesting is while the rear is almost silent when both the brakes are simultaneously applied, the front groans and moans. You can feel the front slip-grab-slip-grab along with the groan sound. The loaded front end of the motorcycle, along with inadequate CoF ultimately results in this sound.

This slip-grab happens so quick, perhaps even in milliseconds, you don't feel the judder yet you hear the sound and experience a sore brake feel.

The above phenomenon happens even in my car after wash. The reason I've highlighted my motorcycle as an analogy is to try to dissect it in a simple way.

Once the rotors and pads are warmed up, they tend to do their job well and there is absolutely no squeal, groan, moan of any sorts. So, even temperature has a role to play in how well a brake performs.

Now, there are different aspects as to why this "groan" can pronounce itself in an unusual way and a lot of factors come into play, condition of brake pads, the condition of rotors, how sticky/grabby the caliper pistons are, how dirty are the pads themselves of accumulated brake dust, there are so many variables, but that's for another day, perhaps.

The bottom-line, weight, temperature and pressure all alter a given friction coefficient for a given circumstance. And as simply and neatly said in the First Law of Thermodynamics - Energy cannot be created or destroyed, it can only be changed from one form to another.

Cheers!
VJ
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Old 23rd November 2022, 18:27   #12
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Re: Why do brakes groan when on a slope?

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Originally Posted by ashivas89 View Post
Brake dynamics engineer here
Slightly off-topic but since you are here and we are discussing brakes, I thought why not pick your brains on this one?

Always wonder what kind of forces are generated in disc brakes. For instance, what is a typical clamp load on a disc for a typical family sedan? Also what kinds of pressure are generated in the brake lines when brakes are pressed hard?

Last edited by vb-san : 25th November 2022 at 15:30. Reason: Typos
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Old 25th November 2022, 22:16   #13
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Re: Why do brakes groan when on a slope?

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Originally Posted by JediKnight View Post
Slightly off-topic but since you are here and we are discussing brakes, I thought why not pick your brains on this one?

Always wonder what kind of forces are generated in disc brakes. For instance, what is a typical clamp load on a disc for a typical family sedan? Also what kinds of pressure are generated in the brake lines when brakes are pressed hard?
A family sedan weighing around 1.5 tonnes would need anywhere between 6-8 kN (one maruti 800 or thereabouts) of clamping force (depends on the disc size, I have assumed upper C2 segment disc sizes here) under ideal conditions at the front brakes to achieve a garden variety deceleration of 0.2g.

Brake master cylinder pressures for actual braking action can start from 5 bar and go up to 150 bar (even higher in some cases).That is, 5 bar when you just about feel the initial bite while pressing the pedal to 150 bar when you're standing on it.

Last edited by ashivas89 : 25th November 2022 at 22:19.
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Old 26th November 2022, 10:19   #14
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Re: Why do brakes groan when on a slope?

Easiest way to fix brake related sounds is to get rid of the 'Speaker' , watch this video to know more.



Given the automatic use of brakes by systems like ESP, you would observe all kinds of sounds if you drive something like a BMW aggressively on the highway and it's all perfectly normal.
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