Pursuit-Rated, Police-Package Chevrolet Tahoe 4WD

(Ed. Note: See the Jan-Feb 2015 issue of

Police Fleet Manager

for complete coverage of the NextGen 2015 Tahoe: new engine, new exterior, new interior. This article deals with the mid-year release of the police package for the 4WD Tahoe.)

 

The Chevrolet Tahoe has been available with a pursuit-rated, 2WD police package and with a non-pursuit-rated, 4WD special service package every year since 2005. Until now, however, the 4WD Tahoe has not been available with the high-speed, police package. It has only been the 2WD, rear wheel drive Tahoe.

The special-service package 4WD Tahoe SSV is basically a de-contented retail Tahoe with few police-specific, heavy-duty parts. The powertrain, drivetrain, ride height, suspension, brakes, wheels, tires and 98 mph top speed are all the same as the retail Tahoe. Chevrolet will continue to offer the non-pursuit 5W4 Tahoe SSV. It has more aggressive, On/Off Road tires and aluminum wheels. It also has the maximum trailer tow package.

The pursuit-rated 4WD Tahoe PPV is an entirely different vehicle. It is essentially the 2WD police package version with a 4WD transfer case. Both have the same performance brakes; the same V-rated Goodyear Eagle RS-A tires, and the same 17-inch steel wheels. Both have the same track-tuned suspension, same lowered ride height, same interior content and the same options. The 4WD police version is limited to 121 mph, while the 2WD police version has a top speed of 139 mph.

 

How 4WD Works

The pursuit-capable Tahoe PPV and the special-service package Tahoe 5W4 use exactly the same 4WD drivetrain, hardware and software. The 4WD system on the police package Tahoe PPV is not new. It has been used on the special-service package 4WD Tahoe 5W4 for a decade. It has also been used on hundreds of thousands (some company officials estimate over a million) of retail Chevrolet Tahoes and Silverados and GMC Yukons and Sierras.

The police-package and special-service package Tahoe both use a truck-based 4WD system. The transfer case in AUTO mode works very similar to a car-based AWD system. They both activate a clutch to send torque to a second axle either when slip occurs or pre-emptively when slip is anticipated.

The main distinction between this truck-based system and a car-based system is in the transfer case operation. This truck transfer case has a 2HI, AUTO, 4HI and 4LO. The 4HI and 4LO are available modes that allow the Tahoe to navigate off-road conditions that a standard AWD system cannot.

 

The 4WD system manages the clutch pressure to divide the torque between the front and rear axles. A clutch pack in the transfer case applies progressively more or less pressure to send progressively more or less torque to the front wheels. In AUTO, it is always partly ON (engaged) and never fully OFF (disengaged).

In AUTO mode, the front axle is always engaged so the clutch in the transfer case is ready to immediately send torque to the front axle. It is easier on the driveline to remain partially engaged rather than to have the mechanical and hydraulic stress of a sudden engagement or a sudden disengagement throughout the life of the system.

Driving in AUTO mode affects the fuel economy because the front axle is engaged all the time. It is the drag from the spinning front axle that reduces the fuel economy in AUTO mode by about ½-mpg. In 2HI mode, the 4WD system disconnects, or completely disengages, the front axle for improved fuel economy. With the option of selecting 2HI, the Tahoe achieves the maximum fuel economy since the front axle and driveshaft are disconnected from the drivetrain. Otherwise, the system is always engaged, i.e., the driveshafts to both axles are always spinning.

 

Driver’s Actions

The AUTO system sends a constantly varying amount of torque to the front axle, as dictated by the driver’s actions, and based on many inputs. The system’s primary inputs are throttle position, throttle acceleration, engine rpm, trans gear state, lateral acceleration, and steering wheel angle.

Simply put, the sensors and software for the ABS, traction control, and stability control are all involved in the AUTO system. During lower speed maneuvers with high steering input, the system also provides slip correction during aggressive cornering.

For all practical purposes, if the rear wheels ever turn faster than the front wheels, the AUTO system increases transfer clutch pressure to send more torque to the front axle. As the rear wheels begin to turn at speeds closer the front wheels, the clutch pressure is reduced to send less torque to the front axle.

At full throttle and lower speeds, about one-half of the engine’s torque can be sent to the front wheels, while at full throttle and higher speeds, about one quarter of the engine’s torque can be sent to the front wheels. That is, the AUTO mode defaults nearly all the power to the rear axle, but can put vary up to one-half the torque to the front axle or any combination in between, monitored many times a second.

 

Vastly Improved Performance

The heads-up comparison between the 4WD Tahoe PPV and the AWD Ford PI Utility conducted by the Michigan State Police proves how much the Tahoe performance has improved for 2015.

In the past, the Tahoe PPV has had (by far) the slowest acceleration of any police-package vehicle. Now comparing the new direct injection engine to the standard equipment engine in the AWD PI Utility, the 4WD Tahoe is now ¾-second faster to 60 mph than the AWD PI Utility. The Tahoe is also now more than two seconds faster to 100 mph. Around the Grattan road racing course, the PI Utility has typically been two seconds faster than the Tahoe, which is about 10 car-lengths. Now the two are virtually tied, separated by just 0.06-second over a 2-mile race track.

The MSP tests also show that the 4WD Tahoe PPV does not give up much, if anything, to the 2WD Tahoe PPV. The 2WD Tahoe is one-half second faster to 60 mph and 1.5 seconds faster to 100 mph. However, put the acceleration, the same braking, and the same handling all together on the road course where the 4WD version is better able to put the power to the pavement, and the 4WD Tahoe has 0.1-second faster lap times.

The MSP tests also prove the 4WD/AWD drivetrain offers clear benefits even on the hot, dry pavement. In addition to the 4WD Tahoe being quicker around the road course than the 2WD Tahoe, each year, the AWD Charger V8 has been faster than the RWD Charger V8. Better traction on grassy medians, wet roads, sandy beaches, and gravel roads are all in addition to obviously better performance in snowy conditions.

 

 

Brakes at Michigan State Police

The 4WD Tahoe failed to complete the vehicle dynamics portion of the MSP annual testing on the first attempt. Over the years, exactly the same thing has happened to both Dodge and Ford. None of this has been driver error, but instead eventually attributed to a mechanical or electronic issue on vehicles still under development. The trouble with the 4WD Tahoe involved its aggressive brake pads.

 



All brake pads are temperature-dependent. Some brakes have friction materials that work the best at routine patrol temperatures of 350 degrees F, and not as good at pursuit temperatures of 800 degrees F. More aggressive, higher-performance friction materials work better at 800 degrees F than at 350 degrees F. This is normally not a problem since on the same vehicle brakes heat up and cool down at the same rate.

During testing at the Grattan road course, the vehicles turn right for most of the course. The weight transfers forward during braking and the weight transfers left due to body roll while turning right. The left front brake gets the hottest, and at racing temperatures, has more braking grip than the right front brake. When the brakes were applied at the end of the 3,200-foot straight with the Tahoe going 115 mph and hugging the left side of the track for a right-hand turn, the Tahoe pulled left and went off the track.

Upon inspection, the differences in operating temperature (left to right) were visually confirmed: the left pad was bright and clean while the right pad had a different appearance. However, MSP drivers formally retested the 4WD Tahoe PPV a month later and the vehicle completed all 32 laps at Grattan without incident.

According to GM Fleet officials, no changes were made to any aspect of the braking system (pads, rotors, calipers, ABS logic) before the retest. Again, the 2WD Tahoe PPV and 4WD Tahoe PPV both use exactly the same brakes, and the 2WD completed the 32 laps on the first attempt.

There were changes made to the brakes between the older 2014 Tahoe PPV and the NextGen 2015 Tahoe PPV. “A more aggressive friction material was needed to offset the more powerful EcoTec3 V8,” said Dana Hammer, Manager of Law Enforcement Vehicles with GM Fleet & Commercial Operations. This newer, more aggressive pad is made to work better at higher temperatures and also to be more durable.

 

City Brake Package

Brake performance is almost never a problem in actual patrol, even with police departments that have frequent pursuits or that run a lot of traffic enforcement. On the other hand, excessive brake pad wear or poor pad life is a near-constant complaint. Brake friction materials are a complex compromise. Just like tires, you can’t have aggressive performance and long wear. The OE brake pads, developed to pass both the MSP and LASD race track testing, are obviously geared more toward pursuit performance and less toward long wear.

The Police City Brake Package (1LR) is a solution Chevrolet offers both as an OE option and as replacements parts. The 1LR brakes are designed to give improved pad life under most conditions, i.e., the 1LR pad material wears better at lower (patrol not pursuit) temperatures. However, the pad “is not optimal for track performance.” The brake performance is likely to decline with repeated high-speed stops. The benefit for those vehicles not involved in this kind of driving is longer pad life in urban scenarios.

The friction materials in the Police City Brake Package pads are more aggressive, more performance-oriented than the retail Tahoe brake pad, but have a better pad life than the Tahoe PPV brake pad. The Police City Brake Package pads are developed to work better at mid-range brake temperatures typical of patrol. The Tahoe PPV with these Police City brakes still retains Chevrolet’s “pursuit-capable, police package” rating.

 

Mid-year 2015i Tahoe

All versions of the Tahoe PPV and Tahoe SSV have had a couple of mid-year running changes. Called the 2015i (interim) changes, these include the use of a black wheel center cap instead of the chrome center cap. The chrome lug nuts are retained. This black center cap is probably the fastest way to identify a 2015i Tahoe.

 

Another mid-year change is the move from 3G to 4G LTE connectivity with data encryption. All the Tahoe models built after October 2014 have the mid-year upgrade. The visual cue for the 4G LTE is in the shark fin antenna. The smaller, black shark fin goes with the 3G service. The larger, body color shark fin means 4G LTE. Built-in WiFi allows Internet access for up to seven devices.

The cell phone connectivity remains Bluetooth. OnStar is standard hardware on the Tahoe. OnStar is needed for Bluetooth capability, since Bluetooth uses the OnStar hardware. You cannot delete the OnStar option since this will also delete the Bluetooth capability. Bluetooth works without activating OnStar. OnStar is not active until the officer or fleet manager initiates the set-up process from the vehicle.

The radio is used to pair the cell phone to Bluetooth for hands-free operation. Simply scroll the addresses using the radio channel tuner. The Tahoe uses the base-level Bluetooth, i.e., it does not display or verbalize text messages like some higher-level Bluetooth versions.

 

Driving Impressions

We put 1,200 miles on the 4WD Tahoe PPV in January, just in time for blowing and drifting snow and wet, slush covered roads. During the entire two-week evaluation under winter driving conditions, we ran the Tahoe in 4WD in either AUTO or 4HI the whole time. We averaged 14.4 mpg. This compares to 17.8 mpg under clear, late-fall road conditions.

We crossed some muddy, snowy medians and answered calls for service driving on snow- and slush-covered asphalt and gravel roads. The Tahoe itself is confidence-inspiring, the 4WD ability just adds to that, and the pursuit rating brings the package all together into an excellent vehicle.

The AUTO system works so well under adverse conditions that we quickly found the tires to be the weak link. Of course, traction control, ABS, stability control—in fact any aspect of traction—is only as good as the tire’s grip on the road. The Goodyear Eagle RS-A is an excellent V-rated, All-Season tire.

However, for winter driving, a winter tire is needed. It cannot be any tire that fits the rim—it needs to have the speed rating to match the vehicle. The Tahoe PPV runs 121 mph. Most snow tires are speed rated to just 99 mph. Goodyear offers an excellent V-rated winter tire specifically for the Tahoe PPV, the Eagle Ultra Grip GW-3. Some departments in the northern climates run the Ultra Grip year-round.

 

Local politics aside, the Tahoe PPV has the highest customer loyalty of any police vehicle. Once the police department starts to use the Tahoe for patrol, it is extremely rare for the department to ever go to another vehicle. All that room. All that cargo. And now, all that torque and all that pursuit-rated 4x4 traction.












Published in Police Fleet Manager, Mar/Apr 2015

Rating : Not Yet Rated


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