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Technical > Four Wheel Finesse

February 2005 Issue

Putting the Hammer Down

When, Where and How To Pull the Trigger

by Jim Allen

Anyone who reads this series knows that I advocate a thoughtful and measured approach to four-wheeling. Finesse! Still, one of the major four-wheeling credos dictates, “as slowly as possible, as fast as necessary.” That latter part is what we’re going to talk about today.

Aggressively Finesseful?

First off, realize there are finesseful ways to wheel aggressively. When you attack an obstacle with a clear plan and enough energy to clear it, but with no more energy than is needed, that’s finesse. Still, there’s a fine line between an aggressive but calculated attack and mindless vehicular abuse. From the outside, to a casual observer, both approaches may look like a wild ride to the gates of hell by a crazy-eyed, adrenaline-addicted lunatic. While the lunatic approach occasionally succeeds, most often the difference between the two is the successful traverse of a difficult obstacle.

The Vehicle: Abuse or Hard Work?

Part of finessing difficult terrain is knowing how much hard work your rig can take. I deliberately avoid the word “abuse” in the context of navigating a difficult obstacle. If you make a logical and thoughtful decision about your rig’s capabilities and hit the obstacle aggressively, that’s just hard work. Hammering it against the obstacle with no calculation is abuse. Even with prior thought, your rig may still come out second best. It’s the same sort of decision an athlete makes when subjecting his body to competition.

Primary Tool: Momentum

The primary tool of an aggressive approach is momentum. Momentum (or inertia, the two terms are similar) is a simple aspect of physics that says an object in motion tends to stay in motion. This means that momentum can be used for substitute traction. Less traction is required to keep a vehicle in motion than to get a vehicle into motion. With a roll, you may successfully clear something you couldn’t from a stop or at a crawl. Even at low speeds, the inertia of a 5,000-pound vehicle is formidable, but like a hammer that’s too big for the arm using it, it can sometimes be difficult to control.

Aggressive Driving in Rocks

Start by carefully picking a likely line and making an easy try. That will give you many clues about what will happen when you do it harder. You’ll see what articulation, weight transfer, and tire-loading/unloading issues you’re facing and whether you can hold the chosen line. It’ll also give you a feel for the angles, especially those that could put you into a rollover or endo situation. All these observations are hints as to how good or bad the line. Feel free to try another one.

Once you’ve done some feeling around, ramp up the energy level and give it a shot. Stay with it until you’re clear it’s a no-go. One of the biggest mistakes is lack of commitment, when the driver backs out too early. But that’s better than the other common mistake: not knowing when to back off, which often results in damage or accidents.

Generally, avoid tire-spinning on rocks. Most times, more traction is found by minimizing tire slippage. That means a controlled application of throttle and the use of momentum. Even small amounts can help. One of the common rock-crawling uses of “Mo” is jamming a tire against a rock or ledge to momentarily increase ground pressure and tire grip. Time an application of throttle to that momentary increase in grip, and you can jump up stuff you can’t crawl.

Aggressive Driving on Soft Ground

Let’s get the tire-spinning business out of the way first. Yes, sometimes a judicious amount of wheelspin is what gets a rig across mud, because it slings the mud out of the tread. Okayfine! Spinning also can be counterproductive, so it’s a judgment call. Vary wheel speed to what is needed. Too much, and you’ll dig a hole to China. If you aren’t moving or if you’re moving too slowly, try decreasing wheelspin. Sometimes that will get you moving faster. Counterintuitive, but it often works. You can always increase wheel speed with more throttle or shift up a gear.

The two primary reasons for aggressive driving on soft ground are lack of traction and sinking into it, which increases the need for traction. Even a tire sunk just three inches into mud or sand generates significant amounts of rolling resistance, which increases traction needs. If you are sunk to the axles or chassis, the equivalent of pavement traction is often necessary to get out.

With some speed going in, you will essentially hydroplane across the top of the soft ground. Spinning tires can aid that process to a degree. The problem is that often you don’t have much steering control, especially if you are locked up at both ends. Getting across the mudhole is no victory if you careen into a tree on the other side. How much speed you can use depends on the situation. Diving into a three-foot-deep mud abyss with a lot of speed can tear up a vehicle. Speed is most useful in sand, which is usually flat. In dunes, take care not to drive off a sand cliff.

Strap or Winch Aversion

Some competitive drivers would rather take blows from a blunt object than use a strap or a winch cable. That’s fine in competition but on a regular trail run, there’s often more to consider. Bear in mind that when your battered and broken rig is hemorrhaging on the rocks, it’s your trail mates and buddies, dragging the carcass off the trail, who’ll end up depriving themselves of four-wheeling fun. That means the risks of aggressive driving are not just about you and your wallet. When the terrain puts your rig at the ragged edge of its capability, consider the effects of a breakdown on your buddies, and the run in general, before you say no to a strap.

Things That Can Bite You in the Butt

Spin-n-Grip

A spinning tire suddenly gaining grip can be quick death for the weakest parts of your drivetrain. Be judicious, therefore, in situations where traction rapidly comes and goes.

Tire Killers

There are lots of tire-killing terrain hazards out there: loose rocks and scree, chop-suey mud (mud with lots of sticks and wood in it), logs and downed limbs, etc. Charging into potentially hazardous terrain with too much speed or spinning tires can result in sliced sidewalls. Factor these items into your aggressive-driving calculations. Tip: aired-up tires are more resistant to cuts.

Axle Wrap

Closely related to Spin-n-Grip is axle wrap. It’s mostly a problem for leaf springs. Axle wrap torque-twists the axle opposite the trust of the tire and pushes the snout of the axle up. That can change the driveline geometry to varying degrees. Increased angularity puts more torque load on the drive-side trunnions and yokes, and it can bind and break a u-joint in extreme cases. If traction is suddenly released by tire slip, the axle snaps back into place with a big bang, which is harmful to the suspension and driveline. You’ll know this problem by the thumping and banging coming from under your rig, and it often creates a wheel-hop situation. It will happen most often on rocks with steep climbs, where loss of traction can be sudden. In severe cases, better move your wheeling difficulty to lower levels until you get your rig fixed-or limp home broke.

Wheel Hop

Wheel hop will make your rig bob like an over-caffeinated bunny. It’s often a symptom of axle wrap. Sometimes it comes from a particular combination of tire pressure, suspension design, and ground surface. It can cause driveline damage if it gets out of hand. Often a quick change in throttle position (usually less pedal) will make it go away, and you can finish your attack on the obstacle. If not, a change in tire pressure (up or down) might be necessary. Some drivers report that a change of shock setting (for adjustable shocks) helps.

Front-Axle Fragility

In many cases, the front axle u-joints or CV-joints (Birfields for Japanese-rig wheelers) are the weakest link in the drivetrain. When the time comes to put the hammer down, having the wheels as close to straight as possible (less angularity on the axle joints) makes it less likely that you’ll break them. Torque loads build with increased angularity.

 

 

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