3 link rear end

[11thhourfabrication]

Just a quick question. I have read alot of the links in the hardcore thread and the books that I have but I still have questions, so here goes. I am making a triangulated A arm upper link for my for my ride that has a single upper link and a panhard rod. I was under the impression that if I replaced the single upper with a A arm I could do away with the lateral locating devices (panhard, watts, etc..). My lower links are angled in about 20 degrees and angle up about 5 degrees. Also is there a a ratio for the lengths of the rear upper and lower arms. My stock lower is 24" and the single upper is 13 5/8" making the upper arm 56% of the lower in length. The layout in the car would make it difficult but not impossible to make the arm longer. Any help would be appreciated.

[MrQuick]

We did a 68 chevelle a few years back and we ended up with a 17" upper arm with 75% anti-squat. but you are correct about the lateral locating device but it does aide in putting your roll center in a more optimum location.

question? upper A arm like a Jeep Grand Cherrokee style?

https://static1.pt-content.com/images/pt/2010/03/rearsuspen-1.jpg
Vince

[Norm Peterson]

What you're building is a wishbone or T bar sort of rear suspension.

Yes, it eliminates the need for a separate method of laterally locating the axle. If I'm picturing your intention correctly with two chassis side pivots and a single one at the axle, the RC height will be about at the 3rd "link's" axle side pivot height, and the roll steer effects will be set from that by the LCA plan- and side-view skew angles.

Note, however, that you are somewhat restricted in the choice of rear roll center heights. Given that the single pivot is going to be on the axle, you have the choice of placing it up toward the top of the pumpkin, or down underneath it (with the outboard trailing arms moving to the top sides of the axle tubes), but not easily anywhere in between as the driveshaft would tend to get in the way of suspension movement. Alfa-Romeo used the higher RC arrangement, Lotus and a few one-off race cars have used the lower.

56% is a bit on the short side, but the current Mustang's "true" upper link (car uses a separate PHB) is shorter than that, so it can be done.


Norm

[ArtosDracon]

I had the exact opposite impression as norm upon first reading it, probably stemming from my truck research.

A lot of "Mini-truckers" use a wishbone upper link for their airbag suspensions, and typically with the dual mounting points on the axle.

As best I can tell, as I'm no expert, not even an internet one, this is going to present one of two problems. In the case of a solid mount like poly or delrin at the single mount point, you will be incredibly limited in your axle deflection, but you should retain good lateral support. In the case of a super pivot or johnny joint or the likes at the single mount point you will retain your deflection, but lose some of your lateral stability. The second example would certainly be the better of the two evils, but still not be ideal in terms of both lateral stability and roll center.

[ITLBTU]

I think you would want the "wish-bone" to be able to swivel along the centerline on the single part of the link, but the point where it attaches to the chasis must be kept from lateral movement via some sort of solid type of bushing, delrin... etc

[terryr]

Aerostar van also used an upper L link, with lots of rubber.

[Norm Peterson]

I think you would want the "wish-bone" to be able to swivel along the centerline on the single part of the link
Not really necessary as long as there is sufficient rotational freedom of movement at the single pivot itself. Rod ends or similar spherical pivots would be the better kinematic solution, but if you're totally committed to the use of cylindrical bushings - yes, you would want to engineer in some sort of torsional moment release. That's one approach, and I think it is already being used in somebody's rear control arms (at the moment I can't recall whose they are - sorry).



but the point where it attaches to the chasis must be kept from lateral movement via some sort of solid type of bushing, delrin... etc
All you need is something like the flanges that are molded into most control arm bushings. Other than manufacturing convenience, I can't think of much reason you couldn't make these "flanges" from stiffer material than the rest of the "bushing" if you didn't want to make the whole thing relatively rigid.


Norm

[11thhourfabrication]

Thanks for all the responses. This is for a 1960 Chevy Sedan Delivery (don't laugh, pigs can fly). The arm is U shaped like the 1958 Impala but it will be alot more substantial. I have fabricated adjustable frame brackets and will use 2 inch poly bushings on the frame side and a Currie johnny joint on the housing end. My first question was answered but the upper to lower arm ratio is still a mystery.

thanks