Brake discs rotational? Appears not

[mickday]

I replaced my rear discs on Saturday with parts supplied direct from MB

I noticed that both disks were exactly the same part number and as such were both the same and the internal vanes were identical. I also noted that the ones I took off were also both the same.

I have also checked the ones currently on the front and both OSF and NSF are both identical, this also seems to be the case for the new ones I have yet to fit

I would have expected that each side would be different so that the rotation of the vanes relative to the motion of the car would result in best cooling for the discs on both sides?

Seems that one side will be more effectively cooled than the other?

 

[Blue Moon]

I had exactly the same questions when I replaced my discs and pads last week on the C55. Same part, same rotation etc. So one wheel directs heat away from the bell, and the other directs heat toward the bell.

Phoned MB dealer parts department who confirmed both the horrendous MB supplied part price, (luckily I purchased from ECP ) and the fact that the discs are not rotational. Strange but true

 

[MancMike]

Same with the discs for my W204. Seems strange/wrong but in both cases air is getting flung outwards, not pulled in.

 

[John Jones Jr]

. So one wheel directs heat away from the bell, and the other directs heat toward the bell.

Interesting. Are you sure? Basically what you are saying is your car requires handed brake discs.?

 

[hotrodder]

Vented brake discs work like a centrifugal fan i.e. air is drawn in the centre and thrown outwards regardless of whether we're talking straight/radial vanes, pillars, curved or angled vanes. While the later two are directional designs and will flow more air when fitted in the correct orientation it's not gonna make a massive amount of difference unless it's a track day toy or you live in the alps. Especially with the rear discs which contribute very little to stopping a front engined car.

Half the stuff car manufacturers do with their "sport" brakes is marketing BS/about looks... look at any real racing car and (while there may be a few exceptions with very light stuff that doesn't have much power) you'll never see cross drilled discs on the front for example. This is because pad outgassing is pretty much a thing of the past. The back brakes that do very little are often cross drilled purely to save weight. Gooving does a better job of 'cleaning' pads and improving bite without making the discs as prone to cracking

 

[carat 3.6]

Half the stuff car manufacturers do with their "sport" brakes is marketing BS/about looks... look at any real racing car and (while there may be a few exceptions with very light stuff that doesn't have much power) you'll never see cross drilled discs on the front for example. This is because pad outgassing is pretty much a thing of the past. The back brakes that do very little are often cross drilled purely to save weight. Gooving does a better job of 'cleaning' pads and improving bite without making the discs as prone to cracking

Spot on.

 

[Deleted member 37751]

Did someone mention brakes?

 

[markjay]

Found this:

"When a vented disc is spinning, it pulls air from the center of the disc, through the channels (which are called vanes), and out the rim of the rotor. The vanes in some Mercedes-Benz rotors are also curved. That's because the engineers who design rotors know that curved vanes move cooling air more efficiently than straight vanes, which are less expensive to manufacture. This cooling air moves in a radial direction. We can view the spinning rotor as an efficiently designed air pump."

It seems that curved vanes are more effective regardless of the direction of rotation.

I suppose that direction is a relatively insignificant factor, and the benefit so small that it does not justify the additional cost of having 'right' and 'left' discs?

 

[whitenemesis]

Where does the air enter the disc at the centre to be able to be flung out at the rim?

 

[streethawk]

Where does the air enter the disc at the centre to be able to be flung out at the rim?

From the inside diameter of the hub where inside disc face meets hub diameter.

 

[whitenemesis]

Obviously the two part discs would have greater airflow?

How important are the disc backing plates, could greater cooling be provided by removing the backing plates?

 

[carat 3.6]

Obviously the two part discs would have greater airflow?

How important are the disc backing plates, could greater cooling be provided by removing the backing plates?

Some backing plates, (w124 being a good example) are shaped to duct air into the center of the disc.

The touring car brakes in post 6 have no back plates, but do have a carbon fiber duct system that takes air from the front bumper and forces it through vanes of the disc.

Remember also that the bigger the disc is with more surface area, the cooler it will run. More vanes would also help.

 

[Mr Kripling]

Interestingly, my old Passat W8 had different part numbers for the left and right front brake discs. The curvature of the vents was pointing towards the rear of the car for both.

 

[markjay]

Interestingly, my old Passat W8 had different part numbers for the left and right front brake discs. The curvature of the vents was pointing towards the rear of the car for both.

This makes sense, and my assertion was that MB decided the delta of the cooling benefit from directional vanes did not justify the cost, presumably.

 

[MancMike]

It's interesting that although the curve that appears correct will vent a little more air than the one that appears to be in the wrong direction, but is that way because the costs outweigh the negligible benefits of handed discs, - the correct looking swirl direction is on the driver's side.

So does the slight extra weight of the driver put slightly more work on the driver's side brake? - and so it makes most sense to have the "correct swirl" on that side for any slight tangible benefit, or is it just coincidence?

 

[Lit91]

So does the slight extra weight of the driver put slightly more work on the driver's side brake? - and so it makes most sense to have the "correct swirl" on that side for any slight tangible benefit, or is it just coincidence?

Great point, but you have to remember that most of their production would be geared towards the LHD market, so in this case they would expect the driver to be sitting on the left...

Would be funny if the LHD market got the opposite set to the UK, meaning that you could make them handed by buying one drivers side disc for RHD & one drivers side disc for LHD

 

[MancMike]

That's a good point, I could be wrong about which side has the "correct" swirls, I was going from memory, but went with it for question clarity.

 

[st13phil]

So does the slight extra weight of the driver put slightly more work on the driver's side brake? - and so it makes most sense to have the "correct swirl" on that side for any slight tangible benefit, or is it just coincidence?

I believe you're over-thinking this...

 

[markjay]

It's interesting that although the curve that appears correct will vent a little more air than the one that appears to be in the wrong direction, but is that way because the costs outweigh the negligible benefits of handed discs, - the correct looking swirl direction is on the driver's side.

So does the slight extra weight of the driver put slightly more work on the driver's side brake? - and so it makes most sense to have the "correct swirl" on that side for any slight tangible benefit, or is it just coincidence?

The great mystery of 'pulling to the left' finally resolved!