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Solid Axle Versus IFS: Which One's Better?

Posted in How To on May 1, 2012 Comment (0)
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“Solid front axles are for rock donkeys!” “IFS stands for ‘inferior front suspension.” We’ve heard these standby beliefs bantered back and forth more times than we can count, so we thought we’d spend some time exposing some myths and sharing some truths.

If you’re looking for a clear-cut winner, this story’s subheading should be a giveaway. There isn’t one. Each style of front suspension has strengths and weaknesses, and it’s up to you to decide what to go with.

For this story, we’re sticking with 4WD solid front axles and A-arm-style IFS. The Ford Twin Traction Beam front suspension has its own merits and demerits, but there’s not enough room for a three-way comparison. This beautifully-constructed TTB buggy (below, right) made a big splash at King of the Hammers last year.

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Myth one:
You Can’t Go Fast With a Solid Front Axle

When the Jeepspeed racing series began, many thought the term “Jeepspeed” was nothing short of an oxymoron. It turned out that the only things Jeeps needed to go fast were the right build and the right mindset. Both 2WD and 4WD frontends are used by Jeepspeed racers. This custom 2WD frontend was built by Tony Sato at Currie Enterprises, and is found under the neon-green XJ seen flying in the following photo.

How’s this for going fast and flying high? The Nissley Racing XJ Cherokee enjoyed several seasons of Jeepspeed racing before recently going to a new owner. Two of our four favorite trucks from last year (see “Looking Back,” January 2012) were also high-flying solid axle trucks, 4WD no less. Since both wheels are locked together, what affects one side will affect the other. “Driving a Jeepspeed, you have to be ready for each bump,” Chris Nissley told us. “A hit to one side can pull the whole front end to that side.”

Myth two:
A-Arms are Worthless on the Rocks

It turns out that almost every swipe against A-arm IFS has to do with a factory-delivered suspension. Sure, there are more moving parts in an A-arm system, but what if those moving parts are designed around performance instead of around cost spreadsheets? Shannon Campbell answered that question with a convincing win at 2011’s King of the Hammers. By the time you read this, he may well have won yet again.

Getting significant suspension travel out of a 4x4 A-arm system has historically been challenging because the stock front differential was so wide that the A-arms and CV shafts couldn’t be built to a significant length without incurring a silly-wide track width in the process. Campbell’s KOH buggy uses a Currie F9 IFS differential. The Currie F9 IFS center section measures a mere 14.5 inches wide from axle face to axle face. The narrow IFS differential means longer CV shafts and longer A-arms are possible while maintaining a reasonable track width, all with rock-and-whoop gobbling wheel travel.

Ground Clearance

Ground clearance is determined primarily by tire size, not by suspension type. Portal axles (think Unimog or H1 Hummer) are the exception to this, but that’s another story.

There is, however, a fundamental difference in the type of ground clearance you get with a solid-axle versus A-arm frontend.

It’s true that the differential hangs down on a solid axle. However, the differential also goes up and down with the suspension. As such, you can count on the same amount of ground clearance as your suspension cycles.

In contrast, the differential is mounted to the frame with an A-arm system. As the suspension cycles up and down, the ground clearance changes. These fundamental differences will influence what lines you pick on the trail, as well as influence driving style.

IFS Weaknesses

Remember the part about most IFS weaknesses resulting from factory designs? Here are some examples.

Many IFS differentials have aluminum housings. These tend to be weaker and less rigid compared to their cast-iron counterparts. A housing that flexes often allows the ring and pinion to become misaligned and possibly break while the flexing is going on. The other potential problem with an aluminum differential housing is fragility in the face of sharp rocks.

The second photo shows two other IFS problems. CV boots are prone to trail damage and wear from age and everyday driving. In this case, a factory Toyota sway bar end link failed at the ball joint. The loose end of the broken link swung down and mangled the CV boot. The other IFS problem in the photo is the dinky diameter of the factory tie rod.

Ball joints can also be a weak point for 4WD IFS vehicles. Fullsize IFS trucks tend to see more of a problem with this than their smaller counterparts, but ball joints can fail and allow knuckles/wheels/tires to leave the front corners of the truck.

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Solid-Axle Strengths

While IFS weaknesses are usually factory-installed, the same can be said about solid-axle strengths. By and large, solid front axles have a lot of built-in strength right from the dealer lot.

Solid front axles start with a housing that’s got either a cast-iron centersection (Dana style) or a stamped steel housing with a dropout third member (Toyota style). Either of these two styles is decently flex and rock resistant.

At the knuckle, U-joints don’t have fragile factory boots so they’re stronger in the face of trail debris and flailing sway bar link ends.

We’re showing custom aftermarket tie rods here, but most solid front axles come with tie rods much, much thicker and stronger than their IFS equivalents.

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Ultimate IFS?

Besides Shannon Campbell’s Class One 4x4 buggy and his KOH winner, there’s one other unfettered IFS rig that comes to mind. Everything on the front of Mark McMillin’s 2003 F-150 Supercrew 4x4 was designed and built for no-compromise performance. As might be expected, there are no factory Ford parts involved.

A Chrisman front differential is found between the A-arms, complete with a 10-inch ring gear. As you’d expect, the CV shafts are completely custom.

The A-arms are built from scratch, the lowers created from TIG-welded 4130 plate and the uppers from CNC-machined aluminum.

The steering system uses a custom-built Saginaw steering box with a ram assist. These control a custom “swingset.” The burly centerlink manipulates a pair of equally-strong tie rods.

The factory frame is long gone in front of the firewall. All of this IFS trickery hangs from custom tube work. The front suspension cycles 20 inches of Fox-controlled suspension travel.

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A Word about Un-Sprung Weight

Un-sprung weight is vehicle weight which is not supported by springs. The more un-sprung weight you have, the more difficult it is for your shocks to control. Wheels, tires, brakes, and solid axle housings are all examples of un-sprung weight. On an A-arm system, about half of the lower A-arm also qualifies as un-sprung weight. By their very nature, solid front axles have more un-sprung weight than A-arm systems.

Bolt-On IFS Performance

If you can’t build a fully custom IFS system, what do you do? There’s good news in the form of bolt-on long-travel 4WD IFS suspension systems from a number of companies. Who are they? An incomplete list includes All-Pro Off Road, Brenthel Industries, Camburg, Dixon Bros. Racing, JD Fabrication, Mazzulla, PRG Products, Pure Performance, and Total Chaos Fabrication.

Bolt-on IFS long-travel kits typically bolt to stock A-arm mounting points. They’re different from lift kits because their goal is to increase wheel travel, rather than to provide lift height. Long-travel IFS suspensions replace the stock control arms with longer, stronger ’arms. The added arm length equals added wheel travel. Extended CV shafts span the new distance and retain full 4x4 function. Stock front differentials are retained.

In many cases, upgraded steering kits are available from these same companies, replacing the weak stock steering parts with stronger aftermarket offerings. You’ll find IFS long-travel kits to be quite a bit more expensive than drop bracket lift kits. If you’re just after tire clearance or are on a tighter budget, a drop bracket IFS lift kit is the answer.

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Bump Travel

A solid axle inherently can’t achieve as much bump travel as an A-arm system. Remember that with a solid axle, the wheel centerline is in the same plane as the axle housing. The axle housing can only travel so far upward before it hits the frame rails or oil pan.

On the other hand, an A-arm system doesn’t have this restriction, and the wheels can travel much further upward. Of course, you do have to be careful that your frame doesn’t hit the ground at full bump!

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Speedy Solid Axles

A speedy solid axle rig doesn’t have to be completely custom built, but it still has to use the right combination of aftermarket parts. Long-arm suspension kits, such as Off-Road Evolution’s Double Throw-Down system seen on this ATX Wheels JK Wrangler, offer greater strength and increased wheel travel thanks to re-configured suspension mounts, and longer, stronger suspension links.

If there’s no long-travel suspension kit for your solid axle rig, builder parts are readily available to help you create your own. Long-travel leaf springs are also available from companies like Deaver Spring.

No go-fast party, solid axle or A-arm, is truly complete without a set of bypass shocks. Bypass shocks offer a level of control that other shocks can’t touch. The added control means added safety and confidence. This translates into higher velocity and more fun.

Bottom Line

Need rugged simplicity? A solid front axle wins this one. Want more strength for less coin? A solid front axle wins this, too. Want ultimate go-fast off-road performance? It’s time for a modified IFS.

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