Lowering Suspension - Laying FramePosted in How To: Suspension Brakes on August 1, 2002 Comment (0)
OK, so we're not really going to be "laying frame" like our friends at Sport Truck magazine. We just wanted our 4x4 to take advantage of a lower center of gravity. We wanted increased climbing abilities. We wanted better handling and sidehilling. We wanted driveline angles that let U-joints live longer than 1,000 miles. We also wanted to get the truck in the dang garage!
Have you ever had a great idea that turned out less than great in practice? Well, welcome to our life. We were oh-so happy when we finally got our Bronco up a couple more inches in order to fit the 37-inch tires we salivated for. Plus, it looked cool too. Driving it on the daily commute and the occasional trail, however, uncovered some negatives about our plan. Nothing major, mind you, it's just that we thought the truck could work better with a little less altitude-as long as we could keep the bigger tires.
What we came up with isn't rocket science. It's not really even mechanical engineering. But it is a little bit more than just fender trimming. We watched what works, talked to other wheelers, and, when the ideas seemed sound enough, decided to give it a go. Read on to see what we figured out.
The Way We Figure It
This exercise began with our driving a '79 Bronco around on a 4-inch lift with 35-inch BFGoodrich Mud Terrains. With the installation of extended radius arms and a pair of lift blocks in the rear we ended up with about 6 inches of total lift. We were stoked that we could now fit a set of the new 37-inch BFGs (see the "Seventeen" sidebar). It's not that this setup worked poorly. On the contrary. The taller tires helped us cruise over the rocks easier than before. We just felt really high and tipsy. Wondering if a lower center of gravity could make our wheeling better, we talked to our off-roading buddies and came up with the following list of many pros and some cons about a lower truck.
*Less side-to-side tipsiness. (Keep the mass low to act like ballast. Heavy axles, tires, and wheels can help.)
*Less side-to-side head whip. (For an analogy, it's the short-building-versus-a-tall-building-in-a-strong-wind-storm scenario.)
*Increased climbing abilities. (Again, keep the mass low where it can transfer to the tires, not 3 feet behind them.)
*Flatter cornering at speed. (Of course it's not quite racecar handling.)
*Shorter braking distances.
*Less wheelhop without rear blocks.
*Better driveshaft angles. (Less trips to the Spicer catalog.)
*Truck is not as affected by crosswinds.
*Might require sheetmetal trimming. (So what? It's probably smashed anyway.)
*Body may need better protection. (Whose doesn't these days?)
*Less clearance. (That's what today's big tires are supposed to correct.)
*Less visibility in traffic. (OK, we're stretching a bit here.)
Think about any form of motorsport. A low center of gravity is always important for better performance. Stock cars sit low and have heavy rollcages, Indy and Formula 1 racers would probably wear lead underwear if they could, and drag racers get better traction by controlling the weight transfer onto the rear wheels. It's true that 4x4s need a certain amount of ground clearance for the trail, but maybe off-roaders should think more about the effects of the vehicle's center of gravity on weight transfer and how it applies to performance.
It was easy enough to lose the lift blocks to lower the rear a couple of inches, but the front suspension would require some shorter springs. We replaced our 4-inch front springs with a set of Wild Horses 311/42-inch lift, progressive coils. Since the Wild Horses springs are rated for the lighter weight of an early Bronco, they settled in for a reduction in height of 2 inches to match the rear drop. As an additional benefit, the truck rides much better with the softer springs up front. The progressive design, however, still offers plenty of spring rate for the heavy hits.
This amount of drop reduced the operating angle of the rear driveshaft's CV-joint by two degrees. Our rocker panels were lowered a couple of inches so some sort of body protection will be added in the future.
The new 17-inch tires and wheels are the hottest thing going for off-roaders presently. Bred from race-inspired technology, the combination of BFGoodrich's excellent 37x12.50R17 Mud Terrain T/A tires and Weld Racing's forged 17x8 Typhoon wheels balanced easily and was super smooth at speed whether on pavement or fireroad.
Visually, it's a sleeper combo since the tire doesn't appear to be as large as its sidewall specs state, but believe us, those extra inches really help keep you off the rocks (not to mention all the nice additional room now around the brakes!). The tire's sidewall height actually matches what a 35-inch tire on a 15-inch wheel would offer, which translates into a better-balanced, large tire. There are, however, some things to keep in mind when rockcrawling with this combination as compared to the 35s-on-15s arrangement.
We used to run our Load Range C 35-inch BFGs at 10 pounds of pressure on the trail to get a nice footprint. The 37-inch BFGs have a stiffer Load Range D rating and at 10 psi they burped air way too easily, leaving us without enough pressure to retain the tire on the bead. Even when the almost deflated tire stayed on the bead, the taller wheels tended to hit rocks, which tore up and severely bent the rims.
We found that raising the pressure about 4-5 pounds over what we ran in our 35s kept the air in the 37x12.50R17 tires much better without any detriment to trail performance.