Brake rotors - different weights for those from different vendors / product tiers

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1998_K1500_Sub

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I made mention earlier [1] of how the brake rotors from different vendors / product tiers can weigh MUCH differently, but I didn't have my notes handy write I wrote that post. I couldn't quote my findings.

Today I found my notes. This topic is worth a new thread IMHO.

Background: I had had a pulsation-while-hot front brake situation on my Suburban. Upon inspecting the rotors, I noticed that one was noticeably less massive (bigger vent gap) then the other.

I figured it was time to get replacements and shopped for some of the more expensive rotors I could find [2], figuring one can't buy quality without spending money.

While replacing the rotors, I pulled out my refrigerant scale and weighted the old and the new. These measurements were made on Jun30, 2017 @ 231,950mi on the Sub. Here are the numbers:


15.4# OLD Front Left
12.1# OLD Front Right

16.5# NEW Front Left
16.5# NEW Front Right

Look at the differences above in their weights [3]. Why the disparity?

First, I learned that one of the Suburban's OE rotors had been replaced by the Chevrolet garage years prior (when my father owned the vehicle). The service ticket showed a GM replacement rotor (8-digit p/n), but of course GM makes a lot of "serviceable" replacements and perhaps this replacement was a cheapie.

In addition, I know that both rotors had been turned at least once.

The OLD rotors' difference of about 27% (15.4# / 12.1#) is quite marked. It means, in any braking scenario, the lighter rotor's temp rise would be 27% greater than the temp rise of the heavier.

Notice the NEW rotors are 7% and 35% heavier respectively than those they replaced, and in total there's 20% more iron to soak up the heat from braking [4].

FWIW


Notes:

[1] Brake Vibration @ https://www.gmt400.com/threads/brake-vibration.58230/post-1242928

[2] Raybestos p/n 580438PER, $83/ea on RockAuto as of this writing. These also appear to be marketed as ACDELCO p/n 18A258SD $76/ea. These rotors have some funky design etched into them which, according to the inventors' patent, is simply a design with no claims of utility, see https://patents.google.com/patent/USD640620S1 and then take the option to download the .pdf Otherwise, the Raybestos p/n 580438 might the same rotor w/o the funky design and at lower cost, $42/ea RockAuto.

[3] Mass, and not weight, is what really matters, but I'll use weight (lbs) because it's a common term... and anyway, weight and mass are simple multiples of each other when measured at a specific elevation (e.g., sea level).

[4] This is a bit of an mis-statement because some of the total mass is in the hat.
 
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Frank Enstein

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Frank has a high frequency vibration ( 5-10Hz) sometimes when stopping from 55+ mph. I doesn't show up until around 35-45 mph and is gone from 20 mph and down.
I will do the rotors when I do the hub/bearing assembly due to the all day nature of changing either one separately.

I will look into the mass of the available rotors and purchase the most massive ones. Thanks for the heads up!
 

1998_K1500_Sub

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I will look into the mass of the available rotors and purchase the most massive ones. Thanks for the heads up!

Whatever information you can glean, please report it.

The best information is that which you measure yourself, but mfgr's specs aren't a bad substitute when you can find them... and provided they don't include the weight of the cardboard box :)

I made the same measurements on my 1998 Honda Accord in 2017. Here are the numbers [1]:

12.6# OLD Front (both sides, which were OE rotors @ 106,500mi)
13.2# NEW Front (both)

7.6# OLD Rear (both sides, which were OE rotors @ 106,500mi)
7.7# NEW Rear (both)


Notes:

[1] Replacement rotors were Raybestos 96710FZN (rear) and 96711 (front)
 
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df2x4

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This is cool, and something I've never thought about before. Thanks for posting!
 
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