One of the evergreen questions we get from readers and viewers concerns our testing regimen. “How do you get that 0–60 time?” “Do you use a dragstrip?” “Who drives?” “What equipment do you use?” “Do you ‘correct’ for weather?” Beyond these basics, many ask why numbers so often vary from one vehicle-testing outfit to another. Below, we answer all of these questions and probably even answer some you didn’t know you had.
The Basics
When: Only after each vehicle is “checked in” does a wheel turn in the name of testing. First, we top off the fuel with the manufacturer-recommended grade of gasoline. This is typically the same one the EPA has used and published for their fuel-economy estimates. This is also the grade of gasoline shown in our charts on the Recommended Fuel line. However, in Southern California, “Premium” grade means 91 octane is the best we can get from commercial pumps, not 93. This sometimes means not meeting manufacturer-supplied acceleration estimates, but not often. Occasionally, when a vehicle like a Nissan GT-R or McLaren 720S hasn’t been engineered to run optimally on 91, we’ll allow a can of octane boost to be added, or if a racetrack dispenses higher-octane gasoline, we’ll use that for lapping. For electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs), we also top off the battery packs with an electrical source a couple hundred yards from the test surface. For hybrids, we attempt to charge the batteries as best we can by driving a few miles in charge-optimized or charge-retaining mode, if available.
Next, we ensure the tires are inflated to the manufacturer-recommended pressure, often stipulated on the doorjamb or under the fuel door. Occasionally these displayed values are for the maximum number passengers and max cargo load (not what we’re looking for), so we often dive into the owner’s manual to locate “normal” cold-inflation pressures. Because hot-lapping a race track is a “special circumstance” with different safety concerns and goals compared to our routine testing, we use the manufacturer-recommended hot-tire inflation guidelines. This way we can check the pressures during the event to ensure we’re getting the most from the car. As a safety precaution, we ensure all the lug nuts are at their manufacturer-recommended tightness with a professional torque wrench. Underhood, we check oil and coolant levels and for obvious signs of “that ain’t right.” Finally, we jot down the vehicle’s VIN, odometer reading, tire specifications, and other various attributes/available settings for posterity. Finally, we put each vehicle on a set of calibrated Rebco racing scales and record corner weights, their sum, and the percentage front/rear distribution. This is what is quoted in our articles and shown on our specifications panels as Curb Weight.
Who: Instrumented testing is different from road testing. It requires years of track experience, specialized skills, and instinctive muscle memory to extract the best performances from all the different types of vehicles we test: compact cars/hatches, sedans, wagons, crossovers, sport utilities, pickup trucks, and yes, sports cars, supercars, and an occasional hypercar. There are currently three staffers (each with more than 20 years of testing experience) who are qualified to perform Motor Trend’s instrumented testing: technical director Frank Markus in Detroit (weather permitting), testing director Kim Reynolds, and road test editor Chris Walton, the latter two both in Southern California. Associate road test editor Erick Ayapana is in the process of training up (and is showing great “feel” and talent) to fill in should any of our regular test drivers be unavailable.
Where: We test in dry, temperate, still weather for the most part. Our Southern California home usually cooperates on the first two fronts, and when winds kick up, running in two directions and averaging the acceleration results mostly cancels the wind effect. Some testing takes place in extremely cold, hot, or humid weather, and the effects of these parameters are compensated for with our weather-correction protocol (more about which follows). Next, we use a flat, level surface with consistent pavement. For straight-line testing, this means at least a half mile for ample shut down and braking. For handling tests, the ideal minimum “black lake” size is 300 by 800 feet. We try to use the same venue whenever possible for ultimate repeatability. Today, Auto Club Speedway/Dragway in Fontana, California, is used for the vast majority of our 250-plus annual tests, and Mojave Desert–adjacent Hyundai-Kia California Proving Ground and Honda Proving Center, plus FCA Arizona Proving Ground are used for our Of The Year programs. In the Detroit area, we use Milan Dragway and occasionally General Motors’ Milford Proving Ground. In a pinch, we’ll use local dragstrips, and we make it a rule to always run in the direction that is opposite to the race direction. Why? Because dragstrips aren’t the real world. The launch area is often “rubbered in,” and as with much of the race surface, it’s also coated with a non-representative traction aid. And believe it or not, street tires actually do not hook up well on the race-prepped surface and lose traction more easily than on plain old pavement.
Acceleration Testing
How: We typically switch off traction-control systems and experiment with launch-control systems when so equipped. We do not pull fuses to deactivate any such systems if there is no standard way to do so. We run from zero to the maximum practical speed increment above the quarter mile. As does the NHRA, we subtract a “1-foot” (about 11.5 inches in reality) rollout from the launch to replicate dragstrip time measurement equipment. Dragstrips from coast to coast and the NHRA started the whole quarter-mile acceleration craze, and these remain the best practical and legal way for most owners to test their own cars. We want our numbers to match those acquired in this way. We experiment with launch techniques (brake torquing automatics to fully energize the torque converter, varying the launch rpm and amounts of wheelspin on manuals, etc.), then shift as quickly as possible while depressing the clutch and lifting off the throttle in three-pedal manuals. If there’s a sport/sport plus mode, drag race mode, sport drive, or any other performance-enhancing setting, we’ll start with those and work backward to see if it indeed helped. They often do not. We don’t “speed shift” manual transmissions. To date, only two manufacturers (select Chevrolet and Porsche vehicles) have engineered a “no-lift-shift” protocol that allows the driver to keep the throttle fully depressed while momentarily pressing the clutch and shifting to the next gear at or near the tachometer’s redline. We do use these manufacturers’ standard performance-enhancing feature.
We record data to an SD card with Racelogic’s Vbox satellite data-acquisition system, sampling at either at 20Hz or 100Hz (meaning 20 or 100 data points per second). Because the test driver can view/review each quarter-mile pass and iterate techniques to arrive at the best result, the number of runs varies. In the end, we typically select a vehicle’s single “best” acceleration performance (or best pair of passes on a windy day), but sometimes that’s a judgment call. Why? One pass might have the quickest 0–60 time and another might have the fastest quarter-mile time or speed. They’re usually the same run. However, there are times when we need to pick one over the other, and we never blend two different runs.
Weather Correction
Why: In an attempt to ensure fair comparisons between cars with internal combustion engines tested in the high-desert heat of summer and the dense cold of a Michigan winter, we record ambient weather conditions using a Computech RaceAir system. With that data tied to each vehicle, we then use the Society of Automotive Engineers’ SAE J1349 procedure as a guide to correct all acceleration results to standard operating conditions: 77 degrees F (25 C), 29.2348 inches mercury (Hg) barometric pressure (99 kPa), and zero percent relative humidity. This procedure also levels the field for multiple cars tested on a given day that might start out cool and humid but become blazing hot and dry for the 10th car tested. Some of our competitors use this same correction method, some do not, and many others do not use a weather correction at all. Other than car-to-car variations, this is the main reason published test numbers often vary for a given model of vehicle. It’s worth noting that the correction factor is reduced for turbocharged engines, for hybrids, and turbocharged hybrids because electric motors and turbochargers are not affected much by swings in barometric pressure (turbos reach a preset boost pressure regardless of intake air pressure). Because supercharged engines tend to add a fixed level of boost, they get the full J1349 correction. So far, pure battery electric or hydrogen fuel cell cars have no correction applied to them; although we know that they’re affected by hot ambient temperatures, we don’t yet know (nor does anybody else) how to reliably correct for it.
Example 1: Two identically equipped 2018 Dodge Durango 4 R/T sport utilities separated by 2,000 miles and just 7 pounds in curb weight were tested a week apart in December 2017: one in sunny Southern California (72.1 degrees F, 6 percent relative humidity, 28.70 inHg) and the other in chilly Michigan (29.3 degrees F, 74 percent relative humidity, 30.21 inHg). Because the Durango R/T’s naturally aspirated engine loves cool, dense air, the Michigan test vehicle was much quicker than California tester—until we applied the J1349 standard. Here, the correction factor added time to the Michigan data and subtracted from the California data. Look below for how the final computed results agreed.
| CA Durango No Correction | MI Durango No Correction | CA Durango J1349 Correction | MI Durango J1349 Correction | |
| ACCELERATION TO MPH | ||||
| 0-30 | 2.2 sec | 2.0 sec | 2.1 sec | 2.2 sec |
| 0-40 | 3.3 | 3.1 | 3.3 | 3.4 |
| 0-50 | 4.6 | 4.3 | 4.6 | 4.7 |
| 0-60 | 6.6 | 5.9 | 6.5 | 6.4 |
| 0-70 | 8.7 | 7.8 | 8.5 | 8.5 |
| 0-80 | 11.1 | 10 | 10.9 | 11 |
| 0-90 | 14.1 | 12.7 | 13.9 | 13.9 |
| 0-100 | 17.8 | N/A | 17.5 | N/A |
| 1/4 TIME | 15.0 sec | 14.5 sec | 14.9 sec | 14.9 sec |
| 1/4 SPEED | 92.7 mph | 95.6 mph | 93.0 mph | 92.9 mph |
Example 2: Naturally aspirated, a 2017 Chevrolet Corvette Grand Sport was tested on a 76-degree day in the high desert, high 56 percent humidity with a barometric pressure of 28.64 inHg. (This one receives full weather correction.)
| No Correction | J1349 Correction | |||
| ACCELERATION TO MPH | ||||
| 0-30 | 1.7 sec | 1.6 sec | ||
| 0-40 | 2.3 | 2.2 | ||
| 0-50 | 3.2 | 3.1 | ||
| 0-60 | 4.1 | 3.9 | ||
| 0-70 | 5.1 | 4.8 | ||
| 0-80 | 6.5 | 6.2 | ||
| 0-90 | 7.9 | 7.5 | ||
| 0-100 | 9.4 | 9 | ||
| 1/4 TIME | 12.5 sec | 12.3 sec | ||
| 1/4 SPEED | 113.2 mph | 115.1 mph | ||
Example 3: A twin-turbocharged 2017 Alfa Romeo Giulia Quadrifoglio was tested on a hot 91.2-degree day in the high desert, 23 percent humidity with a low barometric pressure of 27.26 inHg. These conditions, applied to the above Corvette, would result in a fairly substantial correction factor. Turbos, however, receive “half” the usual weather correction (as the turbo with a blow-off valve naturally compensates for barometric pressure but not for the heat).
| No Correction | J1349 Correction | |||
| ACCELERATION TO MPH | ||||
| 0-30 | 1.6 sec | 1.5 sec | ||
| 0-40 | 2.2 | 2.2 | ||
| 0-50 | 3.0 | 2.9 | ||
| 0-60 | 3.9 | 3.8 | ||
| 0-70 | 4.9 | 4.8 | ||
| 0-80 | 6.0 | 5.9 | ||
| 0-90 | 7.3 | 7.2 | ||
| 0-100 | 8.8 | 8.7 | ||
| 1/4 TIME | 12.1 sec | 12.1 sec | ||
| 1/4 SPEED | 117.9 mph | 118.5 mph | ||
Example 4: Finally, a 2017 Toyota Prius Prime, tested on a mild 78.3-degree day with 17 percent relative humidity and 28.74 inHg barometric pressure. Plug-in or not, hybrids receive very little correction because a good portion of their power comes from batteries that feed electric motors. In this case, the plug-in Prius Prime derives 95 of its 121 combined horsepower from an internal combustion engine.
| No Correction | J1349 Correction | |||
| ACCELERATION TO MPH | ||||
| 0-30 | 3.2 sec | 3.2 sec | ||
| 0-40 | 5.0 | 4.9 | ||
| 0-50 | 7.3 | 7.2 | ||
| 0-60 | 10.1 | 10.1 | ||
| 0-70 | 13.6 | 13.5 | ||
| 0-80 | 18.0 | 17.9 | ||
| 1/4 TIME | 17.5 sec | 17.5 sec | ||
| 1/4 SPEED | 79.0 mph | 79.1 mph | ||
Brake Testing
With the Vbox recording, we accelerate to just above 60 mph, hold that speed, then brake as hard as possible without slipping a tire. (Every new vehicle has antilock brakes, so that usually means simply mashing the pedal.) The distance measurement starts when the car decelerates through 60 mph and ends at a complete stop. Consequently, what we’re reporting is the car’s pure braking ability without the human variable of brake-response time. We do several stops—some in rapid succession in case the brake system functions better hot than cold—wait to see a trend or a plateau, choose the best one, round to the nearest foot, and report it. For some high-performance cars, we also perform a 100–0-mph brake test so that we can calculate a theoretical 0–100–0-mph time, which is always an interesting metric.
Handling Tests
Best: The best way to explore every nuance of a sporting car’s dynamic behavior is to run it on a closed race circuit with a variety of curves of different radii, hills, and plenty of runoff. When possible, we rent racetracks and measure lap times, corner speeds, V-max, lateral g loads, and segment times. For the past nine years, we’ve used Laguna Seca Raceway in Salinas, California, for the track portion our annual Best Driver’s Car contest. Many Motor Trend editors are above-average drivers and can certainly approach a car’s limit on a track in relative safety, but we don’t imperil ourselves or the often-pricey hardware to find or even exceed limits. As a result, we never claim our laps as those of record, unlike some other enthusiast publications. (We’ve seen 4-second lap time deltas between pro drivers and those auto scribes in the same car on Virginia International Raceway, for instance.) For our lap times, we employ a professional development and race car driver, Randy Pobst, to wring every last hundredth of a second from cars on a track to find their true limits. We might also like to think we could test which set of skis is the quickest down a mountain, but we’d still hire Lindsey Vonn to really find out. Like Ms. Vonn, Randy’s expertise is the result of thousands of hours of practice doing the real thing. Along with Randy, we’re also frequent fliers at Willow Springs International Raceway (and Streets of Willow) in Rosamond, California, and have on occasion used Chuckwalla Raceway and Thunderhill, as well. We’ve also started a tradition of having the California Highway Patrol shut down a 4.2-mile portion of State Route 198 for Motor Trend’s Best Driver’s Car evaluations. Lastly, after thorough DOJ background checks (yes, seriously) we were given access to the pristine 15,000-foot runway at Vandenberg Air Force Base for our annual World’s Greatest Drag Race.
Better: When space permits, we feel the best repeatable assessment of a car’s handling is our unique, self-developed figure-eight, which consists of two circles with 200-foot diameters, their centers placed 500 feet apart. For practical reasons, we drive around the outside of these circles (unlike the 200-foot skidpad—discussed next—where we straddle the line). By the way, the lateral acceleration we compute recognizes whatever actual corning radius the car is following.
The vehicle achieves steady-state lateral acceleration through the outermost half of each circle (which we isolate and report as our lateral acceleration figure), but the transitions from steady-state accelerating out onto the straight and then braking into the next circle are far more telling of the car’s dynamic behavior. We report the overall time of the best lap along with the average combined acceleration vector (acceleration, braking, and lateral g) experienced around the approximately third mile course.
Good: When space is too tight, we measure lateral acceleration by negotiating a fixed 200- or 300-foot-diameter circle as quickly as possible while keeping the center of gravity on the measured line. Our GPS equipment measures and computes the average lateral acceleration maintained around the entire circle. We run two laps in each direction, averaging the best left and right lap to account for differences in right/left weight balance with the driver aboard.
Special tests: For special events such as our Best Driver’s Car tests, we concoct a variety of ingenious tests that focus in tight on different aspects of dynamic handling. Among them: pitch and roll angles using ride-height sensors, chassis slip angle from a yaw gyro, and steering wheel angle using a rotary potentiometer. We used to run slalom or lane change tests during the course of a special handling events, but as a rule these tests tend to be highly reliant on driver skill and unduly influenced by factors like wheelbase, overhangs, vehicle width, etc.
Other Tests
Our technical team has an unquenchable thirst for knowledge and is constantly scheming up new ways to report meaningful data on new cars. We’re looking for a good way to measure and report sound-level data. To date we’re unsatisfied with the results of current dBA readings, but there’s motion on that front, so stay tuned. We also frequently measure ride quality where appropriate. We’ve even measured driver heart rates, anxiety level, and even facial expressions while hot lapping to determine which car was hairiest at the limit. You never know what we’ll test next.
The post Vehicle Testing, the Motor Trend Way appeared first on Motor Trend.
from Motor Trend http://ift.tt/2beRK9h
No comments:
Post a Comment