Question About Disc Brake Rotors

Joined
Mar 8, 2012
Messages
16,900
Location
Colorado Springs
and some Bilstein B6's! They are one of the best things on my wife's GM SUV. Same price as OE (a little more on the rear shocks, same on the front struts), but 4X the quality.
Yeah. Bilstein doesn’t bother with cars like that. I have them on BMW. They don’t even make B4 for it. But on new one it seems Toyota is using Bilstein on XSE version. But there are complaints about ride, which means they slapped harder shocks and left everything else the same.
 
Joined
Mar 8, 2012
Messages
16,900
Location
Colorado Springs
Oh man, you have to admit the vehicle would've looked awesome. Lol.
Nah just rotors. You cannot slap Brembo calipers on that knuckle. I toyed with idea to slap calipers from Tundra etc. But then larger press area, probably resulting in soft pedal, then I would need master cylinder from some other car etc. It ain’t BMW that comes with coding option for Brembo and off we go.
 
Joined
Dec 17, 2009
Messages
1,277
Location
Wash, DC
On a 2022 Chevy Tahoe police vehicle, it has 6 piston Brembos with 16" rotors in front.
The run-of-the-mill Tahoe and other large GM SUVs have puny looking brakes.
In theory, larger brake rotors matched with larger area brake pads make for better stopping power and shorter stopping distances.
Even the undersized brakes have good stopping power until they overheat.
Brake size is mostly about heat capacity.
I would hope any vehicle has brakes at least large enough for one hard stop from full speed on flat ground.
 
Joined
Mar 8, 2012
Messages
16,900
Location
Colorado Springs
The run-of-the-mill Tahoe and other large GM SUVs have puny looking brakes.

Even the undersized brakes have good stopping power until they overheat.
Brake size is mostly about heat capacity.
I would hope any vehicle has brakes at least large enough for one hard stop from full speed on flat ground.
That is what many manufacturers design. Brakes sufficient for 2-3 hard stops. Problem is also choice of wheels. Many appliance vehicles come with smaller wheels to keep cost of ownership down.
That is good thing, not so good when designing brakes.
But they use cheaper materials too etc.
on other hand my aftermarket ATE rotors Made in China on BMW have 25k, 400mls of track time, vanes discolored into brown/yellow and they still don’t vibrate, work like a clock.
 
Joined
Feb 27, 2009
Messages
8,049
Location
down in the park
I have not found this specifically addressed on here . . . . The question: What factors are used to determine the size (thickness as well as overall diameter) of brake rotors on vehicles. I see cars like Mercedes sedans and others in that size class with massive diameter rotors and caliper assemblies front and rear. On the flip side, I've seen trucks in the Dodge Ram 1500 class (Ford and Chevy/GMC as well) and many other sedans that appear to have very small brake rotors and caliper assemblies.

In theory, larger brake rotors matched with larger area brake pads make for better stopping power and shorter stopping distances. If that is the case, then there appears to be a lot of vehicles with undersized brakes. I know there's got to be a logical engineering answer to this . . . what am I missing ??


Thanks in advance

It's all about cooling capacity, which all depends on the needs placed on the system by use and weight of the vehicle. Differences are small if the speeds are low when the brakes are applied, and the brakes aren't used repeatedly in short order.
 
Joined
Feb 27, 2009
Messages
8,049
Location
down in the park
I think as time has gone on, rotors have gotten larger and larger. The only downside I can think of is unsprung weight. On a 2022 Chevy Tahoe police vehicle, it has 6 piston Brembos with 16" rotors in front. 16". Not a misprint. Anyone remember in the 90's when a 16" rim was "optional" lol And now the rotor is that size? I forget off the top of my head but my 2007 BMW has 13.7" rotors in front and 13.2" in the rear, which at the time was pretty large

rotors got larger because the useage changed. ESP and traction control can use the brakes aswell as the driver can, and then some cars apply brakes when steering aswell to create a yaw moment. This means tiny brakes could be hot even before the driver brakes for a corner.
 
Joined
Jul 10, 2022
Messages
365
swept area is the stat you need to figure. Outside diameter is not the only number.
Great point, just as like with a cd number for a car, never do you see the frontal area in the marketing materials. My Lexus has a .27 cd and if it had the air suspension, .26. That's super low. But the frontal area is not small. That happened to be 1/4 criteria the engineers were given in the 80's, when Lexus was created. Top speed was another which is amazing. They're now limited to 130 mph on most models, but the original was capable into the 140's....
 
Joined
Jul 10, 2022
Messages
365
A simple first step is to determine kinetic energy of various vehicles:

KE = 1/2mv2


Take a 2020 Ford F150 with a loaded travel trailer at a combined 16,000lbs (nearly maximum GCWR) traveling 83mph (maximum speed for most trailer tires).

That combo has about 4 million joules of kinetic energy that the brakes would need to dissipate.

Now look at a 2020 Mercedes Benz C63 AMG. Only weighs 4,400lbs, but has a maximum velocity of 175mph.

The C63 generates a maximum of slightly over 6 million joules of KE for the brakes to deal with.

This is obviously super simple and there are many more variables, but it does outline the relationship between mass and velocity when it comes to energy.

FWIW, our demo F150 has 350mm rotors and the C63 AMG has 360mm rotors.
It's funny how everyone here knows a C63 or S63 is a real AMG. But there are many who see C43 or M240i (I'm making it up I don't follow anymore) and think these are AMG or M because of trick badging. I remember seeing a t shirt: C43, Do you AMG bro?

Also what I find interesting is which car mfgs still put fixed calipers on cars. It tends to be Porsche, then Italian, then even Tesla and Rivian etc. Even Kia on that sport model. But look at a S63 or M5, floating in the rears. Cost cutting is truly amazing. 1/3 of my cars has fixed calipers on all 4, and it's not the BMW!
 
Joined
Jul 10, 2022
Messages
365
rotors got larger because the useage changed. ESP and traction control can use the brakes aswell as the driver can, and then some cars apply brakes when steering aswell to create a yaw moment. This means tiny brakes could be hot even before the driver brakes for a corner.
imho this lends to the phenomenon where my RWD BMW has rear pads that wear at 2X the rate of the fronts. Normally, just common sense and physics, front brakes do more work than the rears. Even upon reset, the computer shows the life of the front pads as double the rears. The car is a manual and it's so annoying to have the brakes grabbed in the rear for how ever many ms it is, as a hill holding feature. Maybe moot but I wonder if a 2022 manual car has that, it's terrible.
 
Joined
Mar 8, 2012
Messages
16,900
Location
Colorado Springs
It's funny how everyone here knows a C63 or S63 is a real AMG. But there are many who see C43 or M240i (I'm making it up I don't follow anymore) and think these are AMG or M because of trick badging. I remember seeing a t shirt: C43, Do you AMG bro?

Also what I find interesting is which car mfgs still put fixed calipers on cars. It tends to be Porsche, then Italian, then even Tesla and Rivian etc. Even Kia on that sport model. But look at a S63 or M5, floating in the rears. Cost cutting is truly amazing. 1/3 of my cars has fixed calipers on all 4, and it's not the BMW!
Many manufacturers are using floating in the back. I saw that Lamborghini Urus or whatever the name, with floating in the back. Many Tesla’s have that too. KIA has that on Stinger bcs. when you are new kid on the block, you better have all stuff sorted out. Still didn’t help them.

However, I am not sure end result matters for 99.99% of drivers.
 
Joined
Mar 8, 2012
Messages
16,900
Location
Colorado Springs
imho this lends to the phenomenon where my RWD BMW has rear pads that wear at 2X the rate of the fronts. Normally, just common sense and physics, front brakes do more work than the rears. Even upon reset, the computer shows the life of the front pads as double the rears. The car is a manual and it's so annoying to have the brakes grabbed in the rear for how ever many ms it is, as a hill holding feature. Maybe moot but I wonder if a 2022 manual car has that, it's terrible.
That is bcs. all electronic nannies. It is big problem since 2004 when all these DSC features were introduced. Get ProTool and go to DSC coding and you will see list of like 30 different functions. But really big one is vectoring or electronic differential function. It absolutely obliterates pads on track and affects life of pads on street especially if vehicle is pushed.
Here are my rear rotors after 120mls of track driving. I can’t code out this function as it would affect torque distribution between front and rear. Look discoloration from heat:

66074311-AFDE-40A7-AB6E-FDC1F5C4A676.jpeg
 
Joined
Jul 10, 2022
Messages
365
Many manufacturers are using floating in the back. I saw that Lamborghini Urus or whatever the name, with floating in the back. Many Tesla’s have that too. KIA has that on Stinger bcs. when you are new kid on the block, you better have all stuff sorted out. Still didn’t help them.

However, I am not sure end result matters for 99.99% of drivers.
When I was a kid, I learned the hard way on a pad replacement job....it's tempting to try to push the pistons in, on fixed calipers, without removing them. It's very possible, even with a screwdriver. But when I bent them on a friend's Porsche and had to pay for the caliper, started to make sense to remove it and push them in the right way....on a Lexus, Toyota wants the caliper bolts replaced, and they are pricey.
 

TiGeo

$50 site donor 2023
Joined
Nov 5, 2009
Messages
4,184
Location
Richmond, VA area
I have not found this specifically addressed on here . . . . The question: What factors are used to determine the size (thickness as well as overall diameter) of brake rotors on vehicles. I see cars like Mercedes sedans and others in that size class with massive diameter rotors and caliper assemblies front and rear. On the flip side, I've seen trucks in the Dodge Ram 1500 class (Ford and Chevy/GMC as well) and many other sedans that appear to have very small brake rotors and caliper assemblies.

In theory, larger brake rotors matched with larger area brake pads make for better stopping power and shorter stopping distances. If that is the case, then there appears to be a lot of vehicles with undersized brakes. I know there's got to be a logical engineering answer to this . . . what am I missing ??


Thanks in advance
As long as your ABS can activate, you have adequately sized rotors. Larger rotors are for heat sinnk/disapation as you are seeing on performance vehicles for repeated brake application/reduce fade. Your tires are what is controlling stopping distance (assuming brakes can lock up/ABS activate). I have a massive aftermarket "BBK" (big brake kit) on my Sportwagen and have some great data for you on this (focus on the stopping distance 60-0, not the time). Comparing 312mm singe-piston sliding caliper stock brakes to the 350 6-piston BBKs, same tires (close enough)...stopping distance? The same.. Change tires to summers? Shortened. Road and temps were for all intents and purposes "close enough" (the Dragy temp isn't always accurate) for this test so variables controlled to the extent I could. If I would have done repeats the BBK would have kept the distance same/similar each time while the stockers would have faded into the distance.....
BBK before after.JPG

AS_summer_brake.JPG
20220423_162008.jpg
 
Last edited:
Joined
Jun 15, 2003
Messages
38,594
Location
ME
All German cars come with large brakes. It is cultural bias during design. 70% of autobahn doesn’t have speed limit. You can make anything go fast eventually. But take into consideration that vehicles you are describing are governed to 155mph and tuned can easily shoot over 200mph. You need something to stop that ballistic missile in case there is traffic ahead etc.
But then when you go to order brakes for a VW Golf there are like 2-3 different size diameters. They size "appropriately."

Half-ton trucks with small, visible rotors behind huge rims are all bling. Those same rotors fit under the 16-inch steelies for the fleet special models. Look up the load ratings, once you fill the extended cab with people there's enough weight for maybe a cooler in the bed.

Pedestrian safety regs require a higher hoodline which looks dumb without bigger fenders which looks dumb without bigger tires. At some point (brake size) it's something only car nerds care about and the product keeps selling.
 
Joined
Mar 8, 2012
Messages
16,900
Location
Colorado Springs
I was going to ask what happened to the van. Is that why you have the Pilot now?
Nah. My in laws left it with us. Most uncomfortable vehicle I had it in two decades.
Ordered new Sienna. I need minivan and it is hybrid. I like mpg. Rest of the vehicle is, well, Sienna.
 
Joined
Mar 8, 2012
Messages
16,900
Location
Colorado Springs
But then when you go to order brakes for a VW Golf there are like 2-3 different size diameters. They size "appropriately."

Half-ton trucks with small, visible rotors behind huge rims are all bling. Those same rotors fit under the 16-inch steelies for the fleet special models. Look up the load ratings, once you fill the extended cab with people there's enough weight for maybe a cooler in the bed.

Pedestrian safety regs require a higher hoodline which looks dumb without bigger fenders which looks dumb without bigger tires. At some point (brake size) it's something only car nerds care about and the product keeps selling.
I don’t think it is just car nerds. My wife never knew brakes like on Tiguan exist until she drove Tiguan. All her previous experience was Asian vehicles. Now, she would never go back to Nissan or Toyota and brakes, handling are primary reason. I guess what you saying explain people’s unwillingness to buy anything outside their comfort zone, so they don’t know for anything better.
 
Last edited:
Joined
Jan 18, 2020
Messages
2,485
Location
United States
It's funny how everyone here knows a C63 or S63 is a real AMG. But there are many who see C43 or M240i (I'm making it up I don't follow anymore) and think these are AMG or M because of trick badging. I remember seeing a t shirt: C43, Do you AMG bro?

Also what I find interesting is which car mfgs still put fixed calipers on cars. It tends to be Porsche, then Italian, then even Tesla and Rivian etc. Even Kia on that sport model. But look at a S63 or M5, floating in the rears. Cost cutting is truly amazing. 1/3 of my cars has fixed calipers on all 4, and it's not the BMW!
fixed calipers in the rear are not cost effective or worth the manufacturer’s time. when drum parking brakes left the german brands threw in the towel. fixed rear needs a separate floater epb or super expensive integrated fixed caliper.

a brand new urus uses 1 piston rear and it stops no problem
 
Joined
May 25, 2005
Messages
2,480
Location
USA
Yeah they do. Japanese vehicles come with rotors exactly for their intended purposes, outside of that it is only downhill.
Toyota Prado I have in Europe has extremely undersized rotors. Sienna I had same. My in laws Pilot same way.

German vehicles on other hand have autobahn cultural bias. It is all about brakes! My 328 that weigh 3570lbs has more rotor surface area than both Prado and Sienna. Same goes for my Tiguan.
Honda was notorious in the late 90s early 2000s for having warped rotors. Every early honda that I owned did this. I did quit using the cheap tire shop and went to discount tire which helped as they use a torque wrench not an impact gun.
 
Top