2012 Jeep Wrangler: Big Tire Track Test
December 30, 2011
We just came back from the test track, where we put our 2012 Jeep Wrangler Sport through its paces on its new LT285/70R17 BFGoodrich Mud Terrain T/A KM2 off-road rubber.
Before I tell you how it did, here are few facts to consider as you make your predictions:
- It stands 1.9 inches taller on account of the larger rolling radius of the BFGs. But the Wrangler's center of gravity increase has to be something less than 1.9 inches higher because...
- It weighs some 200 pound more, and, with the exception of the high-mounted spare, this weight increase is low down. Each wheel and tire assembly is 40 pounds heavier.
- It rests on a track width that's more than 4 inches wider at the center of the tread, and that track width increases to something over 6 inches if you measure from outer tread rib to outer tread rib. (We still have to confirm these estimates with real measurements.)
Here's how things shook out.
0-60 (seconds): Stock = 7.1; Big Tires = 7.5
1/4-mile (seconds @ mph): Stock = 15.4 @ 89.3; Big Tires = 15.8 @ 85.6
Not unexpected with what amounts to taller gearing. Comparing actual speed to VBox GPS speed, we measured the difference in effective gearing at 13 percent taller, which more or less matches what I came up with by comparing tire dimensions.
Interestingly, this makes it possible to complete the 0-60 speed run in 2nd gear. On real roads, the close ratio 6-speed manual now drives like an ordinary 5-speed, with 6th gear a useless lug-fest. It still feels stronger than any unmodified 2011 Wrangler would have, on account of the 83 additional horsepower churned out by the new 3.6-liter V6 engine.
30-0 (feet): Stock = 35; Big Tires = 33
60-0 (feet): Stock = 140; Big Tires = 136
Here it seems the extra tire width (and probably a stickier rubber compound that is less concerned with low rolling resistance and fuel economy) is more than offsetting the increase in rotational inertia. This matches up with the subjective brake feeling on the road, which comes across as no more perilous than before.
100 ft radius (lateral g): Stock = 0.63; Big Tires = 0.65
Once again the extra track width, extra tire tread width and presumably stickier rubber more than offsets any increase in weight and center-of-gravity height. Indeed our re-tired Jeep feels very well mannered (for a solid-axle off-road machine) on cloverleaf freeway off-ramps. It goes down the freeway a bit less nervously and somewhat straighter than before, too.
600 ft x 6 gates (t/c off, mph): Stock = 55.4; Big Tires = 53.0
Here the very dynamic nature of this maneuver finally reveals a weakness. The extra "stick" we observed around the skidpad may or may not be overwhelming the stock tuning of the stabilizer bars and shock absorbers, but the extra unsprung mass certainly isn't helping keep the tires planted over the cone #3 bump.
On top of that, the outside edges of the open lug BFG Mud Terrain KM2s are designed to be most proficient at clawing through mud and gripping rocks and sand. Catching a slaloming Jeep in quick transitions and balancing it on a knife edge is not their forte. In deference to that fact Monticello didn't beat a dead horse and repeat this test too many times in order to preserve the outer tread lugs for their intended off-road use. All-Terrain T/As would have done a bit better here, I'd wager.
Idle (dbA): Stock = 49.4; Big Tires = 43.8
WOT (dbA): Stock = 81.0; Big Tires = 78.7
70 mph cruise (dbA): Stock = 73.2; Big Tires = 72.0
At first, this data made no sense at all. And, frankly, it still might not make any sense. But it does match up to the subjective feel on the road. Yes, there is more tread noise, but it's not as bad as I expected and it's still not the loudest component of the overall sound; it's not the peak the meter is measuring -- that's the trouble with meters and peak-only measurements.
Why are the peak measurements (and subjective impressions) lower, especially at idle? The only theory I can come up with has to do with the Wrangler's newfound additional ground clearance of about 1.9 inches. The component of noise that's reflected back at the cabin by the ground will surely be dissipated by this larger air gap. But by 5.6 dbA at idle? I'm not so sure.
Got another theory? Let us know. In the meantime we're going to repeat this measurement to see if it's a real thing or not.
Dan Edmunds, Director of Vehicle Testing
Photo by Rex Tokeshi-Torres