Leaf spring vs coils and 3/4 link 4

[347stroker]

Hello all, I am building a leaf spring model Falcon for track days. The rear suspension is my big problem. Can I make the leaf spring rear into something acceptable with a watts or mumford link, or am I best off changing to the later 4 link coil spring rear end with the watts link. If I can use the lower control arms from the coil spring setup for my lower links and build a long 3rd link similar to the RRS conversions it would be a great setup I think, but am hoping for simpler answer to the problem. There is not enough room for diagonal top links attached to the diff. Would simply adding a mumford link give me a large gain in handling, close to what I would achieve with coils, or are leaf springs a lost cause?

 

[BrianPetersen]

You can use leaf springs with a Panhard rod, just make sure it is as long as possible, horizontal at nominal ride height, and the attachment points at both ends are close to the same height as where the leaf springs attach to the axle (at nominal ride height) to minimize bind-up. You will still need to do some fabricating, because the attachment point at the chassis end will be in the middle of nowhere. Do you have enough clearance to the fuel tank behind the axle?

Some (all?) chrysler minivans use leaf springs with a panhard rod. Maybe it's not a stunning example, but it shows that it can work.

It will never work as well as a properly engineered link type suspension with coil-overs; for one thing, it won't solve axle wrap-up, but the compromises necessary in fitting something like this into a chassis that was never meant for it might make that option not much better unless you feel like cutting up the whole rear end of the chassis, which it sounds like you don't.

 

[NormPeterson]

For a track day car, I don't think you want to combine triangulated upper links with any other method of laterally locating the axle. Triangulated uppers by themselves, unless rod-ended, tend to be either too compliant (permitting wheel hop under hard acceleration) or too stiff (resulting in additional rear lateral load transfer). Even when you replace all eight bushings in a triangulated 4-link with rod ends, you still have a roll center that's several inches higher than you'd like.

I'd build either something similar to the RRS kit (which is actually a torque arm much like GM's 3rd and 4th generation Camaro/Firebird) or a real 3-link (like the current Mustang). An upper 3rd link need not encroach extensively into the passenger compartment, though its additional attachment structure needs to be thought through carefully to avoid fatigue issues. The kinematics and the stress analysis of a torque arm differs from that for a third link, so in spite of what the advertising copy says it's best not to consider a "torque arm" and a "3-link" to be the same thing.

Should you retain the leaf springs and add any other sort of lateral locating device, avoid replacing the spring eye bushings with stiffer stuff, as that will increase whatever roll binding results. I'd investigate all three (so far) additional lateral locating devices for their roll center migration over the anticipated range of suspension movement and choose the one that can be kept closest to the RC determined by the leaves.


Norm

 

[GregLocock]

Leaf+watts is pretty easy (it nearly got into production).

Compared with the coil sprung setup, well, it might be OK but you'll never really sort the shocks out, and the weight is not going to be your friend.

ED and EL live axle setup was pretty good, if you've got less than 300 hp. If you've got more that you might want to copy the Supercars setup and go to a long parallel UCA rather than short inward pointing ones.

I have no opinion about 3 link setups for a Falcon, the floor is designed around 4 links.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[347stroker]

I've been trying to find diagrams of the supercar stup on the web, but haven't been able to find anything yet. Thank you for your replies so far. I'm really hoping not to just settle for the old panhard rod, I'm prepared to do a fair bit of fabricating and strengthening ot the chassis where needed. I just don't know where to start yet.

 

[patprimmer]

As Greg suggests, use a Watts link, similar to the ones on Aussie Falcons from about the mid eighties until they introduced IRS.

To prevent spring wind up, use a hefty link that is parallel to and equal length the front half of the spring leaves.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[MikeHalloran]

I'm on my sixth Camaro now, and have become rather fond of the torque arm (+ long Panhard rod + 2 LCA), mostly because it doesn't misbehave under power or braking. (All it does is react the crownwheel torque to the transmission crossmember; the forward end slides in a rubber lined socket.)





Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

I doubt you'll get much off the web for the Supercar geometry, but the demo version of Susprog3d contains some likely looking hardpoints-basically make the arms long, straight, equal length and parallel.

If you look at the back of an RTV you'll see the arms Pat is talking about. You might want to think about lifting them a bit, if you don't need all the wheel travel.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[347stroker]

I appreciate all your feedback and ideas, looks like I'm going to go th rrs type setup, anything I do (other than a simple panhard rod) is going to take a fair bit of work, so I may as well go the whole hog. I hope RRS doesn't mind me copying their idea.I think their system with the mumford link can't be be surpassed, in my humble opinion