Straight front axle 2WD ???

[michael]

OK, I may regret this, but here goes. On a primarily street driven (Chevy) 2WD prerunner, what would be wrong with scraping
the stock A-arms and converting to a straight axle...similar to the race Cherokees. It looks like good
travel would be fairly easy and inexpensive. No camber changes, no scrub, steering issues maybe....any thoughts?
Thanks!

Michael <A target="_blank" HREF=http://jmartin.net/parker/goose.html>http://jmartin.net/parker/goose.html</A>

[jeff]

I'm not an expert by any means...

And you've probably got a reason... but why not a set of I-beams?

Highspeed handling is better on anything IFS, be it a-arm, beams, anything that isn't just one big solid system.

I too am curious to see what people think about a mono-beam front end. I've ridden in a few old Fords that have the monobeams and they sucked on-road and off.

[vwguy]

well thats what good aftermarket springpacks are for they are made thin so the have flexibility
but i agree with jeff that i beams would probably be better, cheaper to adapt and get good wheel travel where a arms suck thought cuz stock pivot points leave no room fo improvement without being way wide and thats ugly

[Bob_Sheaves]

Hi michael,

What you propose is completely do-able, from a technical standpoint. The old Class 7 MJ's (Commanche's) and Class 4 (I think?) XJ's (Cherokee's) (circa 1987-1991) used a 4 link system effectively, BUT there was several issues initially of unsprung weight and harmonics. I am assuming you are not concerned with the NVH of your conversion (NVH=Noise, Vibration, and Harshness). To see a practical application of a "dead" Dana 60 front on a 5 link coil suspension, go to your nearest Dodge dealer and look under a HD 1 ton 4x2 cab and chassis.

Even simpler would be a Hotchkiss suspension with leaf springs instead of coils. In this instance, you would use an old Scout II (1972-1980) as a "go-by" for your design for suspension geometry. I'd start off using measurements on the 4x4 Hotchkiss Chevy to start for spring attachment, then attach like a Scout.One caveat. however. You will still have a scrub radius, depending on the configuration of kingpin inclination, brake package, and wheel offset. The greater the radius, the more effort and loading you are going to have on the steering linkage and gear box. Too much and you will just tear up wheel bearings and possibly wreck the truck-Be Careful.!

Best regards,

Bob Sheaves

[michael]

Hey Bob, thanks for the comments. NVH would be a big concern since the truck is a daily driver. I don't see where unsprung weight
be be allot worse than stock A-arms. I would think a "mono tube' front could be made fairly light weight. I'm totally unfamiliar with the
"harmonics" issue. I was thinking of a setup similar to a 2WD Cherokee.....4 link with/without(?) a trac bar. They seem to drive just fine.
Some locating links and coilovers......seems pretty straight forward to me and much easier than I beams....besides that's been done.
Of course I could've just bought a 4WD! Talk to me.......

Michael <A target="_blank" HREF=http://jmartin.net/parker/goose.html>http://jmartin.net/parker/goose.html</A>

[Bob_Sheaves]

Hi Michael.

No problem (you do this stuff long enough and it tends to get a bit old..LOL)

The first issue is one of unsprung weight AND harmonics. What was happening was relatively simple (the same thing can happen in a live rear axle). At some vibration point, the thickness of the tube used for the dead axle will resonate, creating a standing wave. This is a function of the mass placed on each end of the tube (brake package and wheel/tire assembly). The key is to design the axle such that the tube never reaches this natural frequency point. In the MJ's the front tube was a specially made Spicer lightweight tube (very low mass) but the 33" tires, disc brakes and aluminum wheels set up a wave that bent the tube in the middle and caused it to collapse. The remedy was to eventually go to a 1/2" wall tube of DOM 4130, adding about 1/3rd the weight to the assembly, but moving the natural frequency above the racing speeds obtained.

The OEM design for the XJ (and the Dodge BR- full-size - 4x4's and HD 4x2's) is a 5 link, over-constrained link- coil system. There are several potential problem areas for the unsuspecting builder of a street truck with this design. First and foremost is brake steer due to the location and angle of the track bar (or if you prefer, Panhard rod). AMC and Chrysler had a serious problem on Europe's high crowned roads with brake instability on the "XJ series"- they would pull seriously to the right as I remember. Dana/Spicer (the mfgr. of the axle ended up building a series of special axles for warranty replacement with asymmetrical geometry to correct this.

Basically the issue can be described as the lateral "pull" of the track bar on the axle (side to side movement) and the suspension moves from jounce to rebound due to the arc of movement and angularity of placement at a static, or starting, position. This, combined with the low "anti-dive" of the Cherokee (18.5% or so- where the Dodge BR was designed to 38 percent) - this allows the front end to compress more under braking- allowed the axle to move laterally under the vehicle almost an inch towards the left. The steering linkage, being a non-compressable attachment, forced the tires to the right a greater amount that the vertical travel, leading to the "pull" the driver felt to the right. A secondary issue of a lateral track bar is one of "head toss". The shorter the bar, combined with the greater the vertical travel, will lead to a form of "seasickness" in some people from the lateral loads imparted onto the middle ear.

To minimize these issues, I'd recommend using a 3 or 4 link only- 2 parallel lower links and 2 "splayed" upper arms or a single 3 point upper arm to control the axle lateral displacement. The steering linkage in such a system would need to be radically different to minimize loading and steer effect based on vertical travel. Basically, this means that you would need to position the linkage such that it did NOT travel side to side, as on an XJ, but rather fore and aft, like a Chevy 4x4 C/K from '73 to '87. Point is, you want to have the linkage follow the suspension arc of travel front to rear, so you have precise control over where the tires are pointed at any given time.

Frame rail starting point will affect the total amount of travel available for you to playwith. Chevy rails are notorious for their "flatness" in the 2WD configuration. For this reason, I'd start out with a 4x4 frame (you'd gain about 4-5" extra jounce travel).

I realize this sounds like a bunch of "hogwash", but this is the same process that I go through in my job every day. Failure to account for this "stuff" in a street driven truck can possibly make you liable in an accident.

Best as always,

Bob Sheaves

[FABRICATOR]

Michael,
It really is a good question. This may be an area which is not as fully explored as most would assume. If it was'nt for Bob Sheaves, the Jeep TT's may have never left the dream stage. The worst thing about a solid front end is trying to go fast. If the course were bumpy and the car was going straight ahead all the time it might get to be fast To put it simply, a really pliable front end goes over the bumps without upsetting the rest of the car, then its up to the rear to just be able to follow over the same obstacle without bottoming out or loosing too much traction. It is nearly impossible for a solid front axle to do this because of the constant reaction of entire axle, not just piece of it. The section which has the unneeded movement is the part that the rest of the car is sitting upon. This literally causes a single or a one-two upward punch to the chassis just when it needs to be as calm and level as possible. This also sets up the rear end for uncontrolled reactions. About the only proven advantage is toughness and even that is minimal. I would not attempt to get alot out of this system.

[John_Bitting]

Fourwheeler magazine did this conversion a couple of years ago with an IFS 4x4 it turned out really bitchin and they loved it until the hit a whoop/ditch at speed and pushed the whole front end over a couple of inches. I looked for the article but could not find it. I would love to read it again now that I have had 3 years of learning more since the article was published..

[michael]

Hey thanks Bob and Chuck! As with most things these days....nothing is as simple as it first appears!! Isn't the "net" great! Bob, the description was really good....I've been studying up on frontend geometry so it all made since. (I liked the "scrub" question answer too). So, are the 3" wider A-arms the real deal or not? Can I still get a good caster/camber set up out of them. Refering to the type from California Super trucks or Fabtech. Thanks!!

Michael <A target="_blank" HREF=http://jmartin.net/parker/goose.html>http://jmartin.net/parker/goose.html</A>

[Stan]

"Basically, this means that you would need to position the linkage such that it did NOT travel side to side, as on an XJ, but rather fore and aft, like a Chevy 4x4 C/K from '73 to '87. "

Question Bob,
Keeping the old, stock steering setup is better than converting to side to side (crossover)? I've read up on this conversion (I have an '87 K5) and the drawbacks of the stock steering and a lot of guys are going over to crossover. Helps in the rocks, but since I want mine for the dez, I would want to keep the stock setup?

Stan

"I love donuts. My arse is bigger because of them; but I'm not a Five-O."