Jimmy Johnson's rear suspension design

[Payton King]

Ok,

I am not claiming to be any sort of expert on suspension design. I am just explaining this as it was explained to me. Apparently in the 60's when Chrysler was into factory sponsored drag racing they were running a class, I think Super Stock, and the rules stated that if the car was produced with a rear leaf spring suspension it must be run. They were having a hard time getting the leafs to hook so they came up with this idea to get around the rules.

Basically the leafs act as the track bar, spring (duh) and lower link. The rear is not mounted to the spring by a conventional perch but instead by a swivel link(my term). The upper link is just like normal.

This type of suspension was also used on an Austin Healy 3000 in the 60's. Jim Pettigrew use to have one and dropped a small block in it. He stated that rear worked great and it would really hook and drive great.

So with all of that said, lets hear some opinions. Pros and cons.

Jim would be more that happy to make these for anyone as a low cost alternative to what is out there now.

[evilzee28]

Had a similar set up on my '55 Chevy with a "floating" axle like that. The car ran arrow straight a 10.57 @135mph, with no lateral movement at all. Nice easy to make set up. :thumbsup:

[XcYZ]

Cool, thanks for sharing, Payton. :thumbsup:

[Payton King]

and the dude at AME thought that there was going to be roll bind due to the short upper link. I am no susupension guru, but I thought the problem with 4 link and tri-bar set up was the same roll bind as this set up. What I saw from the times at the run through the hills...all of them performed about the same, including the leaf cars.

Is this a therory thing on a computer model at the absolute limit of suspension travel or is this a problem on the street and track days?

[black2002ls]

that is a very interesting set up.

[astroracer]

I have done a bit of sketching and thinking about this and have come to the conclusion that the "Leaf Link" rear suspension will have much less roll bind then your typical 3 or 4 link, coil over suspension.
I loaded a few Power Point drawings into my Fotki site here:
http://public.fotki.com/astroracer/suspension_stuff/articulation_study/
I didn't do a lot of labeling but I think you will get the picture. Remember the diagrams are hand drawn so take them as they are...
The typical 4 link has a ton of freedom in a straight up and down articulation. Roll bind comes into play when you induce jounce (bump) or rebound (droop) into the suspension in a non-planer direction (say, RH wheel up 2" and LH wheel down 2"). The suspension will want to rotate around the lower links. A perfect "rotation" would have ALL of the 4 bar pick-up points on the chassis and axle in the same rotational axis. This can't happen of course so the links end up working against themselves. This is multiplied when you throw in the panhard bar which is on ANOTHER rotational axis, actually controlling the lateral movement of the housing and inducing even more bind...
The "Leaf Link" (for lack of a better term?) will control roll with the springs, it is inherent to the design. You can see in the last diagram there is much less induced roll with the springs controlling both lateral movement and rotational deviations. The springs also "work together" because they are tied together through the housing. The spring/housing mounts create a hard line of reaction (through the housing) between the two springs that a 4 link does not see.This forces the housing to rotate around the front and rear spring bushings of BOTH springs. This requirement also centers the rotation "point" somewhere under the middle of the housing. The amount of roll can be controlled with the type of bushings being used. Controlling body roll with good shocks and sway bars will also help reduce roll bind with this suspension.
On a race course, with controlled variables, the induced roll bind will probably be negligible with minimum suspension movement.
I am just thinking out loud here so jump in with ANY comments or thoughts you may have.
Mark

[Payton King]

Just looking at it in the shop and bouncing the car on jack stands it looked like it will work. Obviously, that is no real test.

Thanks again for your time Mark

[David Pozzi]

I think it will work OK but the concern would be if the upper link were too short compared to the lower link, there might be some binding near the limit of suspension travel due to the links moving out of parallel as they move. If the links were not parallel to start with at normal ride height, then binding is more likely. If the links converge (in side view) towards the front of the car, there will be more anti-squat and more traction, but the binding issue begins to turn the axle housing into a big anti-roll bar.

Since he allready has the four links (if you count the leaf spring) on the car, all he needs to do is add a "real" lower link, coilovers and a panhard bar, and no leafs to save a lot of unsprung weight. I think the car would ride better.
David

[Payton King]

Never a 4 link on the car

[Marcus SC&C]

FWIW that`s *exactly* the rear suspension configuration designed by Gene Hardig for use in the Studebaker Avanti in 1961. They used it up until around 1987. They actually handle pretty nice considering they`re on a modified Lark chassis designed in the late 50s! So it works,it`s just a question of how well it`ll work in a given application. Mark SC&C

[ccracin]

Hi Guys,
I am fairly new to the site, but I was hoping to throw in a few comments. A very similar configuration to this has been used in circle track racing for years. More specifically short tracks, dirt and asphalt. In the circle track configuration there is normaly only 1 leaf on each side that is cut off just behind the rearend housing perch. Then the upper links are mounted as shown above, only either a panhard bar or watts link is added. The main goal was to try and eliminate the shock to the tires when the throttle is hit coming out of the corners. This is accomplished by the axle wrap that the leaf spring allows. As the system of spring and rubber loaded third bars in a three link were developed these configurations started to disappear. On several asphalt tracks lately with rules that do not allow spring or sliding links this setup has been brought back. It works well although it takes some trial and error to get the correct spring rate. Oh and 1 last point most guys use spherical bushings in the front mount of the leaf to reduce roll bind. It works well considering dirt cars will sometimes have +- 5" of travel from side to side. Anyway just something I thought you all would be interested in.

Chad

[Payton King]

Car is gong to paint today and should be driveable by Jan. I guess we will find out how well it works