I am trying to work on the front end on my friend's scrambler. We've got the rear end done with a 4-link with uppers and lowers triangulated. Rough sketches show the roll center to be just above the axle tube and the roll center axis to be relatively flat. It exhibits minimal rear steer (maybe an inch or two over the range of motion) so it seems that the data backs up the real world handling.

Right now the front end is SOA on 4" springs and isn't as flexible as the rear. He wants me to show him a couple of setups and I think a 3-link with traction bar would probably be the easiest and possibly the hot ticket. Plan is to use coils, likely stock TJ coils since there is a set laying around. The front end is 70s J20 D44 with pass side pumpkin. Steering is already high-steer so the drag link is relatively flat, it's not hydro steer.

Here's the basic setup I plan to use, but the upper link (traction bar) will be setup differently to minimize binding. I like it on the driver's side because of less junk in my way (track bar, drag link, pumpkin).

http://mywebpages.comcast.net/jjacobs149917/suspension/three_link.jpg

The lower links will be parallel or almost parallel. Likely running from just inside the knuckles to the outside of the frame. This will cause my roll axis to be parallel to the rise of the lower control arms, correct?

This should mean that the roll center will be greatly affected by the height of the front trackbar, meaning that the lower we mount it the lower it will make the roll center. A big factor in setting it up will be to match the length and angle of the drag link.

How will the front vs rear roll center height affect the handling of the vehicle? Should I try to keep them the same, or make one lower than the other?

How will a near flat roll axis in the rear and a sloped roll axis in the front affect the vehicle?

Thanks.

Is he gonna be road racing, or wheeling at high speeds?? If so then the questions you asked are valid. If not, then just build it to flex. And like you mentioned, make the panhard bar equal length/angle as the drag link.

EDIT: nice diagram, btw

Nope, just your good old east coast rockcrawler / trail rig. I'd like it to be able to handle on the street decent enough to be able to leave the trailer at the campground and drive it to the trailhead or run to the store to get some beers.

I stole the diagram from an SAE book. Here's the other ones from the book:
Four Link (http://mywebpages.comcast.net/jjacobs149917/suspension/four_link.jpg)
Four Link Parallel Lowers (http://mywebpages.comcast.net/jjacobs149917/suspension/four_link_parallel.jpg)
Three Link (http://mywebpages.comcast.net/jjacobs149917/suspension/three_link.jpg)
Leaf Springs (http://mywebpages.comcast.net/jjacobs149917/suspension/leaf_spring.jpg)

well, in that case, you need to be alot more worried about wheel hop and squat/anti-squat than roll axis. Just build it how you described, and try not overthink it. I'm sure it will work fine.

General rule of thumb: Keep the front roll centre lower than the rear. Of course roll stiffness comes into play but this little rule keeps the seat of the pants feel better.

Ken

I have the same setup on my ride, it works well and I am happy with it, the only thing I dislike is the inherent problems from using a panhard bar, the axle moves laterally through it's travel. Yeah, generally speaking, keep the front RC lower than the rear, and make sure you have a long SVSA, I would try to make the lower links converge to the frame to give you a more level roll axis so you have less roll stear. The lateral placement of your upper link affects which direction the front dives when you brake. I never did the calcs, I put I where I could which is on top of the diff on the pass side and I have no problem with pulling when braking, however I didn't put much anti-dive in the front suspension.