Rear Suspensions

J

jeffreylyon

New member
Has anyone played with a 3-link rear suspension with triangulated lower links to avoid the need of a panhard bar or Watts linkage? Sort of a Satchel 4-link on the bottom and a traditional 3-link on the top?

Thanks in advance.
 
I have it on my Lotus 7.

Broken it three times. will be changing to a different set-up soon.

Problem is all lateral forces have to go through one joint with this set up.

Peter
 
Ted: I can't tell from the photograph, but I think that's a traditional 3-link with a wishbone replacing the panhard rod. I was thinking more along the lines of triangulating trailing links to the outside of the axel and a single upper link on top of the pumpkin.

Peter: What are the angle of your lower links? At 45 degrees they are supporting 50% of the lateral force, right? Are you breaking the top link?
 
Peter and I have had the same issues with it on our Lotuses.

Problem is with one rod end (like what you see on McMahon's car), that piece takes a lot of drivetrain shock. Don't care how big it is, you're pulling on threads, particularly if you have a dog box and are downshifting without it. I found if I chunked a downshift, the rear of the car would move side to side because the threads stretched. When I ran a 1/2" rod end, it would eventually break. I upped the size to a 5/8" rod end and it didn't break, but would still stretch the threads.

My fix: I kept the same concept but used two radius rods coming back from in front of the axle at a 45 and connected them next to each other on the bottom of the diff. Got the rods from Speedway Motors. I think I spent about $100 total on the parts and then did some fabricating. Never had another issue.
 
My GTL car has the twin rod setup with 3/4 hiems. Bottom of diff has a plate with threaded bosses and rods go forward at about 45 degrees and mount below front mount for upper arms. Upper arms go straight back and mount (guessing) 4-6" above axle tube.
 
Jeff, what you describe sounds a little bit like a variation on a truck-arm rear. The exception to this is that you are talking about using a third link vs. the "U" bolts over the rear end. Using the lower links to converge in the center of the car (under the drive shaft) and using a single upper link centered on the rear end housing. I haven't seen it done this way exactly but I would think that as long as you can get the anti-squat that you are looking for there shouldn't be a reason that this wouldn't work. The kinematic roll center will still be at the point the trailing arms cross the c/l of the axle in the "z" dimension. The only question I would have is the introduction of the load in the center of the chassis rather than the perimeter. I have seen this before and if you pm me I will explain.
 
Don't forget about the location of the rear rollcenter. You can very easily tranform the handling of your racer into an ill manered pig if the front and rear roll couples aren't properly matched.

Also, rod ends should never (we all do it though) be loaded in bending. They are designed for tension and compression only in the shank. Take a look at the tension vs bending load ratings on any rod end and you will quickly concur.
 
Jay,

Point well taken - my front suspension design will allow me to easily change kingpin heights, giving me the ability to change the upper wishbone angle, thus changing the roll center to match that of the rear.
 
Jeff,

Conventional wisdom is not that you want the front and rear roll centers to be the same. What you really want is an upward climbing rear to front roll axis.....since the rear CG is lower than the front in a front engine car.

It's all about the roll couple and trying to minimize changes in the roll couple front to rear as the car pitches up and down front to rear and rolls right to left.
 
Jay,

So I guess that means that I should shoot for a very low rear roll center to allow the declining slope from front to aft. I believe that would push me even further towards the lower wishbone design due to its very low roll center.

Thanks,

J
 
No, just the opposite....sorry I confused you. Front RC should be lower than rear. Actually the rear mass centroid is higher than the front.

Typical "good" RC locations for small prod cars are about 1.5" in front and maybe 8" in rear. Your results may vary!
 
Sorry..........sometimes my pathetic little brain plays funny tricks on me. I can get so CONFUSED!!
 
"Roll center" adjustments, I hate that name, it does a terrible job of describing all that's going on there, are my favorite tuning and/or chassis design tool. There is very little outside of aero that can change the car's characteristics as dramatically. Love playing with that geometry.
 
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