Killer SUA - I
Building a Killer Spring Under Suspension - Part I. Short Cuts
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SUA Part I: | Page 1 | Page 2 | Page 3 | RE YJ 4.5" Lift Kit |

By: David Gray - 8/2000

Introduction: Evolution vs. Incrementalism


The first rung up on this evolutionary ladder are the first modifications to make vehicles a bit more capable, more able to meet and beat some of the challenges that bested them on the first trail ride.
They seek more ground clearance, more traction, and maybe some insurance in case things go wrong. They are still convinced that they are not trying to build "one of those ridiculous monster trucks" and are probably still using their 4x4 as a daily driver. They decide a lift kit and some bigger tires are what they need. After a visit to their local 4x4 center, they drive away with a mild 2-3" lift, and some new, beefier tires (32" is a popular size at this level), a new winch on the front, and about twice the confidence they had the first time around. They can now drive around the streets with a bit more swagger, waving at other 4x4s, eager to tackle the trails they had trouble with before. They conquer these trails, but soon find that the 4-wheeling bug is not that easy to cure, and start to seek tougher trails. They may have found their local 4x4 club by this point, they are regularly bringing home a couple of 4x4 magazines each month and are, of course, visiting 4x4Wire.com on a regular basis. They are also being exposed to other seriously built rigs, and hear the tales of tougher trails like the legends of the Rubicon, Moab, Tellico, and others.

A great driver can take a mildly built rig through some pretty hard core trails, but it is just human nature to want a bit more - just in case…

Now our recent enthusiasts see that what they really need to need conquer the world (or at least those trails) was not just a mild lift, 32" tires and a winch - but a bigger lift, 33" or 35" tires, better gearing, and maybe a locker or two. Yep - that will be the ticket!

SUA vs SOA
Bolt on kits Fabrication required
Often has u-bolt ends & mounting plates protruding below the axle Clean under axle profile
Higher spring arch reduces lateral stability and up-travel Less arch in spring means more lateral stability and often better up-travel
More moderate lift heights and available in different ranges Higher lift heights, usually all or nothing with no way to make it shorter
Moderate lifts require fewer concurrent modifications Higher lift almost always requires changes in shock mounting, steering & braking
Springs help protect the tie rod Often increases exposure of tie rod
Better control of axle wrap Increased incidence of axle wrap
Better spring life negative flex decreases spring lifetime

For drivers with conventional spring under axle (SUA) leaf spring suspensions, this is generally were you will find a divergence in the evolution of the modified 4x4. Some people stay with a standard spring under configuration, going to a 4" or so lift kit; others try the spring over axle conversion, gaining 5" to 6" of lift. Both lifts can also be combined with body lifts and other tricks to allow larger tires to fit or reduce interference with articulation. A family of "standard" 4-inch lift kits has long been available from a variety of manufacturers while the spring over axle conversion has long been a matter of custom fabrication. Unfortunately no manufacturer has marketed a workable kit for a dramatic increase in driving skill.

What I have described as evolutionary growth also has coined another descriptive term - "incrementalism".

I have the very good fortune to work with some real 4 wheeling gurus, among them Jeff "Jefe" Reynolds. Now Jefe has said that anybody who is really serious about rock crawling will eventually run a spring over axle (SOA) suspension configuration. There are some advantages to the SOA, but like everything in life, there can be some drawbacks. Jefe has also advocated a move away from incrementalism. It's not that the incremental steps are bad, it is just that over time, building a rig in small steps costs much more than doing it right the first time. Jefe is a wise man, and he is usually right, but I occasionally like playing the devil's advocate...

I may be the current designated poster child for the spring under axle configuration, with my express goal being to develop my spring under Jeep (YJ2K) so it is as capable as most SOA rigs or the lifted coil spring TJs. I am proud to say that I think I have gotten pretty close to this goal. I have done this in an incremental fashion, not because of any philosophical reason, but because I went through a version of that evolutionary growth and, to be honest, I only had the money to make small changes at a time. If I had to start all over again I may have chosen to take Jefe's advice and go SOA from the git-go, but I am very pleased with YJ2K's setup and handling at this point. Of course, I always aspire to more, and will continue to upgrade and modify YJ2K with the best I can find (and afford), but I don't know what path this may take.

You have been very patient reading this far without a single mention of how YJ2K's suspension is now configured. Doing a customized suspension correctly is actually a complex process, so I am going to present this for you in three parts: Part I (this month) covers the latest suspension modifications on YJ2K, next month Part II will side track a bit to take a look at some of the basic factors to address when your want to build a flexible suspension as well as some things you might want to know when shopping for lift kits, and later Part III will cover other often overlooked yet critically-important modifications, such as steering and braking systems and show you some of the best shots of this suspension in action over the past year.

Now - let's take a look at the suspension on Page 2

| OutdoorWire | 4x4Wire | JeepWire | Jeep Tech |
SUA Part I: | Page 1 | Page 2 | Page 3 | RE YJ 4.5" Lift Kit |


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