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Go Back JeepForum.com > Models > Jeep Grand Cherokee & Commander Forums > WJ Grand Cherokee Forum > Another Quadra Drive Vari-Lok thread....

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Unread 02-07-2013, 06:40 AM   #16
lazyWJ
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Quote:
Originally Posted by rm2001wj View Post
To have it make more sense, it probably would be better to say "to a single wheel that has traction." However, there is indeed some finessing in all of this. By design, the gerotor pump is limited as to the amount of pressure it can apply to the clutch pack. That limit can be reached when the torque is well below the maximum torque produced by the engine. Under operating conditions where the engine is not making maximum torque, it is possible that nearly 100% of engine torque can be sent to the single wheel that has traction.
Good point, the engine could overwhelm the clutch in the system. But then is the torque actually produced if there is slippage, either in a tire or clutch? If my understanding is right, you can only have torque if there is "resistance". Maybe that's why they use the wording "developed torque" in a lot of their descriptions? Anyway, you're definitely right about the limits of the system on that point.

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Unread 02-07-2013, 07:02 AM   #17
rm2001wj
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Since there is always going to be some small amount of friction when things are slipping, I guess the engine has to develop at least a little more torque than that required to keep the rotating parts in the engine and drivetrain moving.
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Unread 02-07-2013, 08:34 AM   #18
jahcure
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It would be interesting to tune it via the bleed holes and the metal relief valve if there was a way to prevent the check valve from releasing the built up pressure my guess is the the diff would remain locked and then make it bleed off pressure when lock diff is no longer required
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Unread 02-07-2013, 10:17 AM   #19
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Quote:
Originally Posted by lazyWJ View Post
Either there is a wide variance in performance, or a wide variance in expectations. .
I think expectations and driver behavior are the two greatest variables in the utility of the quadra-drive system.

I'll try to organize my points for readability:
  • The system is dynamic, which means it is made to operate in motion and it clearly has a "tuned" range of speeds (axle RPM) at which its performance is optimal. This means that changing tire size will necessarily change the effective behavior of the system because larger tires mean lower axle RPM and a greater amount of axle torque required to move the vehicle because of the effective gear ratio change caused by the greater tire circumference.
  • Jeep's engineers have directly made it clear in the "Chat with Jeep Engineers" section of this forum, that their design perspective is maximum availability of a feature to the 'lowest common denominator' of driver. They have even gone so far as to outright state that they regret this means sometimes the hardcore Driver (or the wannabe Driver) doesn't always get exactly the performance they want if tuning for a Driver would mean compromising the safety or feature accessibility for a less dedicated vehicle operator.
  • Jeep's engineers are also concerned with warranty expenses and vehicle longevity (brand value) - note that they installed Quadra-Drive on their heaviest vehicle with their most powerful engine in the line.
  • Because manufacturers are necessarily concerned most with the original buyer of the vehicle and how it performs for that buyer right off the showroom floor, and this began life as a pricey luxury SUV, they designed the system more for Vail and less for Moab. In showroom condition, I strongly suspect a WJ Overland outperforms a TJ Rubicon on slushy/snowy/icy roads.
  • Since the vari-lock system consists of both a clutch pack and a pump which pressurizes the clutch pack, both components transfer torque until either both wheels are operating at the same speed or the pressure relief valve opens. How does a pump transfer torque? It's not much different from your brakes - you pump an incompressible liquid with your foot, "clutches" grab the brake disc and stop, your pump (the brake pedal) stops moving. The same thing occurs inside the Vari-Lock carrier - the clutch packs get pushed together, the pump comes up against an incompressible liquid - except there's a relief orifice to prevent binding and hysteresis so it only presents resistance that varies with speed, not the "firm" resistance of a brake pedal - and there's a relief valve to prevent destructive amounts of hydraulic pressure from building up if the vehicle is stuck, one wheel has massive traction and you're just leaning on the gas pedal. (It also protects your diff from exploding when an axle shaft breaks).

So, since we know the system is dynamic and necessarily operates best in motion, was designed and optimized to offer as much benefit as possible to the most ignorant of drivers in the most common environments someone might drive a brand new luxury SUV, and because it is a dynamic system it is sensitive to tuning, it was necessarily tuned to a stock vehicle (duh) which means that adding weight to the vehicle, changing the tire size of the vehicle, etc are very likely to shift the behavior of the system.

With that understanding it's reasonable to tune our expectations around the system.
  • It requires wheelspin to reach full operation. Not awesome for 0.01mph rock crawling.
  • It isn't, and was never engineered to be, the most capable 4x4 system in the whole wide world. It was engineered to be lagom equipment for the buyer of a luxury SUV to get around as many typical environments with as few levers and knobs and special driving techniques as possible. We all dream of conquering Everest in our WJ's but virtually every mile they're (collectively) ever driven in the real world is either on pavement or on icy/snowy pavement or on the sort of dirt track that, at worst, would be 2wd in dry weather.

For what my own experience is worth, I find that the less I actually try to manually illicit operation from the quadra-drive system, the better it works - basically just putting myself in the mindset of a novice driver and pointing the steering wheel and applying throttle tends to make the system work best. I still follow best practices for overall vehicle control, but try to rely less on the sensation of feedback one usually expects to feel from a locker or other traction aid. The system usually ends up finding traction, even if it seems counterintuitive to deliberately ignore a system.
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Unread 02-07-2013, 10:44 AM   #20
lazyWJ
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Very well put, and great info! I especially agree with your point about driving like a novice and getting the most from the system. My immediate reaction whenever I detect wheel spin has been to let off the throttle, which in a sense counteracts the system. Just this morning as I was leaving the snowbank I park in very night I was thinking about how just letting the wheels spin and staying gently on the throttle would predictably and effectively activate the quadra drive system, and drive away happily. The downside to that is wheel spin usually causes an irreversible loss of traction as the terrain is disturbed, suspension unloaded etc, but certainly outperforms an open diff system. As you say, the system only works when there's motion, and relative wheel spin and that's not ever going to change.

The piece I want to focus on is where the system is designed for the lowest common denominator. Obviously compromises are made to accommodate on road drivability, road manners, noise, warranty and so on. Is it possible to give back a little bit in some of those departments in order to gain a little in off road performance. There is always a trade off. Thanks for taking the time to post that up.
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