Braking g's Comparison (edit 6/23/05, 14 additions)
03-12-2005, 08:05 PM
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Braking g's Comparison (edit 6/23/05, 14 additions)
When comparing the braking performance of various cars, I find it much easier to relate to deceleration g's rather than the stopping distances reported in the various car magazine tests. It doesn't help that they report stopping distances from 60-0 mph, 70-0 mph, 80-0 mph, or 100-0 km/hr, etc., which then makes direct comparisons almost impossible.
This simple formula converts stopping distance to g's, where the initial speed V is in fps and stopping distance SD is in feet.
g = V<sup>2</sup>/(64.4 x SD)
The deceleration rate at 1.0 g is 32 ft/sec<sup>2</sup>. Once you the know the deceleration g capability of a car, you can calculate the stopping distance from any speed with,
SD = V<sup>2</sup>/(64.4 x g)
Using the Enzo as the benchmark, the following results are from a not so random sample of car tests. When someone reported the stopping distance from two different speeds (R&T uses 60-0 and 80-0), I used the higher speed. The deceleration g is calculated, and the 80-0 mph stopping distance is the reported value if available, calculated if not. Obviously these tests were done on different days, on different tracks, with different tires, etc; but I still find the results interesting.
G's ....... SD
1.14 .... 187 ft ..... Ferrari Enzo
1.13 ..... +2 ........ Chevrolet Corvette Z06
1.13 ..... +2 ........ Porsche GT
1.11 ..... +5 ........ Suburu WRX STI
1.10 ..... +7 ........ Mitsubishi Evolution
1.10 ..... +7 ........ Lamborghini Gallardo
1.09 ..... +9 ........ BMW M3
1.09 .... +10 ....... Dodge Viper
1.09 .... +10 ....... Maserati Quattroporte
1.08 .... +12 ....... Caterham Seven Superlight R
1.07 .... +13 ....... Ford GT
1.06 .... +14 ....... <b>Audi RS6</b>
1.06 .... +14 ....... Porsche Boxster S
1.06 .... +15 ....... Jaguar XJR
1.06 .... +15 ....... Cadillac CTS-V
1.05 .... +16 ....... BMW M5
1.05 .... +16 ....... BMW 745Li
1.04 .... +18 ....... Ferrari 612
1.03 .... +20 ....... Honda S2000
1.03 .... +20 ....... Porsche 911 Turbo
1.03 .... +20 ....... Porsche GT3
1.02 .... +22 ....... Porsche GT2
1.02 .... +22 ....... Lotus Elise
1.02 .... +22 ....... Saab 9-3
1.01 .... +24 ....... <b>Audi TT</b>
1.01 .... +24 ....... BMW 745
1.00 .... +26 ....... Lamborghini Murceilago
1.00 .... +27 ....... <b>Audi D3 A8L</b>
1.00 .... +27 ....... Lexus IS300
0.99 .... +28 ....... Acura NSX
0.99 .... +28 ....... <b>Audi D2 S8</b>
0.99 .... +28 ....... <b>Audi B5 S4</b>
0.99 .... +28 ....... MB S55 AMG
0.99 .... +28 ....... Mini Cooper S
0.99 .... +29 ....... Mercedes Benz CL600
0.99 .... +30 ....... Volvo S40 T5
0.98 .... +31 ....... VW Phaeton 4.2
0.98 .... +31 ....... Lexus SC430
0.97 .... +32 ....... Infiniti M45
0.97 .... +33 ....... Mercedes Benz E500
0.96 .... +35 ....... Ford SVT Mustang Cobra
0.96 .... +35 ....... Toyota Camry
0.96 .... +36 ....... Jaguar XJ8 L
0.95 .... +38 ....... <b>Audi D2 A8</b>
0.95 .... +38 ....... Mercedes Benz E320
0.95 .... +39 ....... Jaguar S-type 4.2
0.93 .... +40 ....... Aston Martin DB9
0.94 .... +40 ....... Chrysler 300C
0.94 .... +40 ....... <b>Audi C6 A6 3.2</b>
0.94 .... +41 ....... Aston Martin Vanquish
0.93 .... +43 ....... Acura RL
0.93 .... +43 ....... Bentley Continental GT
0.93 .... +43 ....... BMW 330i
0.93 .... +44 ....... BMW 530i
0.92 .... +45 ....... Jaguar S-Type
0.91 .... +48 ....... Nissan Maxima
0.91 .... +48 ....... <b>Audi B7 A4 1.8T</b>
0.90 .... +50 ....... Range Rover
0.90 .... +51 ....... Mercedes Benz E350
0.90 .... +51 ....... Lexus GS430
0.89 .... +53 ....... Saturn LW300
0.89 .... +53 ....... Volvo S80
0.89 .... +54 ....... <b>Audi B6 S4</b>
0.88 .... +55 ....... <b>Audi C6 A6 4.2</b>
0.87 .... +58 ....... <b>Audi A4 3.0</b>
0.87 .... +59 ....... Kia Amanti
0.87 .... +60 ....... Mercedes Benz S500
0.86 .... +61 ....... Cadillac STS V8
0.85 .... +66 ....... Mini Cooper
0.85 .... +66 ....... VW Passat
0.84 .... +67 ....... Acura TSX
0.84 .... +69 ....... Subaru Legacy 2.5GT
You might be tempted to blame Audi's generally lackluster performance (except for the RS6) on the fact that our cars are nose-heavy. Note, though, that the equally nose-heavy Suburu WRX and Mitsubishi Evo finished near the very top of the list. The Porsches had rather mediocre performance (except for the GT), even though their rear weight bias gives them the capability to be among the best.
Frankly, it looks like it's the rare auto manufacturer that knows how to optimize braking ... and then does it.
This simple formula converts stopping distance to g's, where the initial speed V is in fps and stopping distance SD is in feet.
g = V<sup>2</sup>/(64.4 x SD)
The deceleration rate at 1.0 g is 32 ft/sec<sup>2</sup>. Once you the know the deceleration g capability of a car, you can calculate the stopping distance from any speed with,
SD = V<sup>2</sup>/(64.4 x g)
Using the Enzo as the benchmark, the following results are from a not so random sample of car tests. When someone reported the stopping distance from two different speeds (R&T uses 60-0 and 80-0), I used the higher speed. The deceleration g is calculated, and the 80-0 mph stopping distance is the reported value if available, calculated if not. Obviously these tests were done on different days, on different tracks, with different tires, etc; but I still find the results interesting.
G's ....... SD
1.14 .... 187 ft ..... Ferrari Enzo
1.13 ..... +2 ........ Chevrolet Corvette Z06
1.13 ..... +2 ........ Porsche GT
1.11 ..... +5 ........ Suburu WRX STI
1.10 ..... +7 ........ Mitsubishi Evolution
1.10 ..... +7 ........ Lamborghini Gallardo
1.09 ..... +9 ........ BMW M3
1.09 .... +10 ....... Dodge Viper
1.09 .... +10 ....... Maserati Quattroporte
1.08 .... +12 ....... Caterham Seven Superlight R
1.07 .... +13 ....... Ford GT
1.06 .... +14 ....... <b>Audi RS6</b>
1.06 .... +14 ....... Porsche Boxster S
1.06 .... +15 ....... Jaguar XJR
1.06 .... +15 ....... Cadillac CTS-V
1.05 .... +16 ....... BMW M5
1.05 .... +16 ....... BMW 745Li
1.04 .... +18 ....... Ferrari 612
1.03 .... +20 ....... Honda S2000
1.03 .... +20 ....... Porsche 911 Turbo
1.03 .... +20 ....... Porsche GT3
1.02 .... +22 ....... Porsche GT2
1.02 .... +22 ....... Lotus Elise
1.02 .... +22 ....... Saab 9-3
1.01 .... +24 ....... <b>Audi TT</b>
1.01 .... +24 ....... BMW 745
1.00 .... +26 ....... Lamborghini Murceilago
1.00 .... +27 ....... <b>Audi D3 A8L</b>
1.00 .... +27 ....... Lexus IS300
0.99 .... +28 ....... Acura NSX
0.99 .... +28 ....... <b>Audi D2 S8</b>
0.99 .... +28 ....... <b>Audi B5 S4</b>
0.99 .... +28 ....... MB S55 AMG
0.99 .... +28 ....... Mini Cooper S
0.99 .... +29 ....... Mercedes Benz CL600
0.99 .... +30 ....... Volvo S40 T5
0.98 .... +31 ....... VW Phaeton 4.2
0.98 .... +31 ....... Lexus SC430
0.97 .... +32 ....... Infiniti M45
0.97 .... +33 ....... Mercedes Benz E500
0.96 .... +35 ....... Ford SVT Mustang Cobra
0.96 .... +35 ....... Toyota Camry
0.96 .... +36 ....... Jaguar XJ8 L
0.95 .... +38 ....... <b>Audi D2 A8</b>
0.95 .... +38 ....... Mercedes Benz E320
0.95 .... +39 ....... Jaguar S-type 4.2
0.93 .... +40 ....... Aston Martin DB9
0.94 .... +40 ....... Chrysler 300C
0.94 .... +40 ....... <b>Audi C6 A6 3.2</b>
0.94 .... +41 ....... Aston Martin Vanquish
0.93 .... +43 ....... Acura RL
0.93 .... +43 ....... Bentley Continental GT
0.93 .... +43 ....... BMW 330i
0.93 .... +44 ....... BMW 530i
0.92 .... +45 ....... Jaguar S-Type
0.91 .... +48 ....... Nissan Maxima
0.91 .... +48 ....... <b>Audi B7 A4 1.8T</b>
0.90 .... +50 ....... Range Rover
0.90 .... +51 ....... Mercedes Benz E350
0.90 .... +51 ....... Lexus GS430
0.89 .... +53 ....... Saturn LW300
0.89 .... +53 ....... Volvo S80
0.89 .... +54 ....... <b>Audi B6 S4</b>
0.88 .... +55 ....... <b>Audi C6 A6 4.2</b>
0.87 .... +58 ....... <b>Audi A4 3.0</b>
0.87 .... +59 ....... Kia Amanti
0.87 .... +60 ....... Mercedes Benz S500
0.86 .... +61 ....... Cadillac STS V8
0.85 .... +66 ....... Mini Cooper
0.85 .... +66 ....... VW Passat
0.84 .... +67 ....... Acura TSX
0.84 .... +69 ....... Subaru Legacy 2.5GT
You might be tempted to blame Audi's generally lackluster performance (except for the RS6) on the fact that our cars are nose-heavy. Note, though, that the equally nose-heavy Suburu WRX and Mitsubishi Evo finished near the very top of the list. The Porsches had rather mediocre performance (except for the GT), even though their rear weight bias gives them the capability to be among the best.
Frankly, it looks like it's the rare auto manufacturer that knows how to optimize braking ... and then does it.
03-13-2005, 07:22 AM
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And may I ask where your car fits in, with your brakes?
Especially when you already start at mid-pack with the S8 on the list? It'd be instructive to see the difference between track pads and street pads too, I think. 
03-13-2005, 11:41 AM
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The tires are normally the limiting factor, except ...
At 80 mph, I'm traveling at 117 ft/sec. Assume I'm driving along at 80 mph, minding my own business, when a herd of deer starts crossing the road in front of me. What has to happen?
Even if it takes only 1/2 second to move my foot from the gas pedal to the brake, I've already traveled 58 ft, and nothing's going to change that. This isn't included in measurements of stopping distance, but I use it as an example to show that things are happening very fast. Next ...
Now that I've got my foot on the brake, I have to apply enough pedal pressure to go from coasting to decelerating at 1.0 g (32.2 ft/sec<sup>2</sup>), the limit of adhesion of the tires with ABS active. Assume that this requires me to move the brake pedal 5" to achieve 1500 psi of hydraulic pressure at the brake calipers. If all of this takes another 1/2 second, I've traveled 29 ft (at an average of 40 mph or 59 ft/sec) just to get to full braking. I can reduce this with,
1. <a href="https://forums.audiworld.com/a8/msgs/75190.phtml">Brake Pads</a>: Compared to the 0.35 cf Audi brake pads, the 0.50 cf Ferodo DS2500 would require only 1050 psi to get enough friction to lock the tire and activate ABS. This would require 1.5" less pedal travel, saving 0.15 sec or 9 ft of stopping distance. The 0.62 cf Ferodo DS3000 racing pad would be even quicker.
2. Master cylinder movement: <a href="https://forums.audiworld.com/a8/msgs/38595.phtml">The Block</a> eliminates 2" of brake pedal travel, saving 0.2 sec or 12 ft of stopping distance. This removed by far the largest source of "compliance" from the braking system.
3. Approx 5% larger front and rear rotors: Like higher friction brake pads, this also reduces the hydraulic pressure required to lock the tires and reduces the amount of time it takes to activate ABS.
4. Braided SS brake lines: This removes another small amount of compliance from the hydraulic system and also slightly reduces the amount of time it takes to activate ABS.
5. Stiffer caliper: The Alcon TA-6 is a more rigid caliper than the OE Brembo, also removing compliance.
6. Brake Bias: The optimum brake bias is 2.2 for the S8 (see <a href="https://forums.audiworld.com/a8/msgs/36145.phtml">Weight Distribution revisited</a>). If brake bias is too high, for example, you have to continue increasing pedal effort, after the front brakes go into ABS, to get the rears into ABS. This also takes additional time; or worse yet, if you assume you're braking at maximum once the front brakes are into ABS, you don't continue increasing brake pressure, and you're wasting stopping power of the rear brakes. Whether street or track, I run higher cf pads in the rear to get as close to a 2.2 brake bias as possible.
Items 1-6 are not additive, but the 12 ft reduction in stopping distance from just The Block is the equivalent of a 0.06 increase in braking g's. So to answer your original question: How did all this affect stopping distance with my S8? I don't know, I never did a braking test, but I definitely wouldn't want to go back to OE brakes :-)
Even if it takes only 1/2 second to move my foot from the gas pedal to the brake, I've already traveled 58 ft, and nothing's going to change that. This isn't included in measurements of stopping distance, but I use it as an example to show that things are happening very fast. Next ...
Now that I've got my foot on the brake, I have to apply enough pedal pressure to go from coasting to decelerating at 1.0 g (32.2 ft/sec<sup>2</sup>), the limit of adhesion of the tires with ABS active. Assume that this requires me to move the brake pedal 5" to achieve 1500 psi of hydraulic pressure at the brake calipers. If all of this takes another 1/2 second, I've traveled 29 ft (at an average of 40 mph or 59 ft/sec) just to get to full braking. I can reduce this with,
1. <a href="https://forums.audiworld.com/a8/msgs/75190.phtml">Brake Pads</a>: Compared to the 0.35 cf Audi brake pads, the 0.50 cf Ferodo DS2500 would require only 1050 psi to get enough friction to lock the tire and activate ABS. This would require 1.5" less pedal travel, saving 0.15 sec or 9 ft of stopping distance. The 0.62 cf Ferodo DS3000 racing pad would be even quicker.
2. Master cylinder movement: <a href="https://forums.audiworld.com/a8/msgs/38595.phtml">The Block</a> eliminates 2" of brake pedal travel, saving 0.2 sec or 12 ft of stopping distance. This removed by far the largest source of "compliance" from the braking system.
3. Approx 5% larger front and rear rotors: Like higher friction brake pads, this also reduces the hydraulic pressure required to lock the tires and reduces the amount of time it takes to activate ABS.
4. Braided SS brake lines: This removes another small amount of compliance from the hydraulic system and also slightly reduces the amount of time it takes to activate ABS.
5. Stiffer caliper: The Alcon TA-6 is a more rigid caliper than the OE Brembo, also removing compliance.
6. Brake Bias: The optimum brake bias is 2.2 for the S8 (see <a href="https://forums.audiworld.com/a8/msgs/36145.phtml">Weight Distribution revisited</a>). If brake bias is too high, for example, you have to continue increasing pedal effort, after the front brakes go into ABS, to get the rears into ABS. This also takes additional time; or worse yet, if you assume you're braking at maximum once the front brakes are into ABS, you don't continue increasing brake pressure, and you're wasting stopping power of the rear brakes. Whether street or track, I run higher cf pads in the rear to get as close to a 2.2 brake bias as possible.
Items 1-6 are not additive, but the 12 ft reduction in stopping distance from just The Block is the equivalent of a 0.06 increase in braking g's. So to answer your original question: How did all this affect stopping distance with my S8? I don't know, I never did a braking test, but I definitely wouldn't want to go back to OE brakes :-)
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03-13-2005, 08:15 PM
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I would love to see how Stoptechs with aggressive pads and new stick tires would do.
Today, I am about to blaze through the iPASS gate on the exit from the tollroad. There are 2 lanes and iPASS users can just drive through either. There is a slow line moving through the non-pay side, and no one on the pay side, so I slow a bit and plan to go right on through. All of a sudden a big range rover cuts over top the cones directly in front of me and making almost a 90 degree turn even shuts off any area to the right of the lane. I stand on the brakes and stop about 2 feet short of his wife's door.
We are both glad I had Stoptechs on the TT, because even if they only stop 2 feet better than stock, that was a very good 2 feet today.
The idiot got a long blast of the horn.
We are both glad I had Stoptechs on the TT, because even if they only stop 2 feet better than stock, that was a very good 2 feet today.
The idiot got a long blast of the horn.
03-13-2005, 10:27 PM
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And except for the RS6, the TT is the best performing Audi.
The others would have completed that stop against the Range Rover's door, or worse. Even if you actually need those two extra feet only once every decade, when you do need them, you're sure glad they're there.