Learn me: pushrods with live axle

Looking at my build inspiration (an X-prepared AE86) I noticed pushrod shocks on the rear end. Does anyone have some good articles or tips on designing a setup?

On a closed wheel car there are two justifications for pushrod shocks on any system, live, DiDion, or independent.  They are reducing unsprung weight and increasing shock displacement. That said, I have seen, and experienced driving, a set up where the rocker geometry reduced the shock displacement, done I think for packaging, but it did not work well. My recollection of your Toyota is a little vague, but I think the original shocks were inclined about 45* and parallel to the axle. That gives 50% shock displacement on bump, but close to 100% on roll. That is a pretty good compromise for a street car live axle, and its cheap to build. If they tip for or aft it reduces displacement in any motion. How ever you choose to do it, you will do best if you can maximise shock displacement. Matching shock travel to suspension travel (100% displacement) is the starting point for design unless you are targeting very rough terrain with limited overall travel. Then I would try hard to exceed 100%. Sturdy flex free anchors for the rocker are super critical. The down side is increasing the number of places for wear to introduce slop (delay) into the shock's reaction time.

In reply to TurnerX19 :

lots of good info there. Gives me a starting  point for the design.

Pushrod on a live axle immediately made me think of Mike Maier's '66 Mustang. I believe he said he did it so he could use a shock with a longer stroke.

He did it because it looks cool.  Anybody that says it's for performance is lying to themselves.

I'd build it myself, but purely to show off.

In reply to Streetwiseguy :

I remember something about packaging, but yeah, I'm sure you're right. Definitely looks cool.

In reply to Daylan C :

Longer stroke is what I have described as displacement, and he certainly has with that design, but as pretty as it is, the best thing I see here is the ability to change them without going under car. The pip pins he uses to retain them are too sketchy though! Most of the folks on this board can do something better than this, if not quite as pretty.

 What’s wrong with the four link set up? I loaned a few AE86s and a couple of Celicas they all perform very well for a live axle set up on track 

In reply to _ :

The four link can remain exactly as it is, the OP question related to the shocks, not the suspension geometry per se. Reducing unsprung weight will always help on bumps, and increasing shock travel will always help transitional behavior. 

In reply to Streetwiseguy :

You clearly have never driven a car that had inadaquite shock travel relative to suspension travel. Really scary at Bridgehampton. Also the OP's Toyota is light enough to make the unsprung weight enough reason by itself. 

I've seen offroad guys do something like that with the shocks parallel to the floor under the car. They call it cantilever though, not pushrod or bellcrank. The reasoning there is to get huge travel without putting the shocks into the passenger compartment/bed. 

In reply to TurnerX19 :

I have, actually, driven a car with inadequate travel.  There are easier ways to get more shock travel than filling the trunk with billet aluminum, and to be honest, if we are having a conversation about unsprung weight in a car with a stick axle, well, I'm not gonna bother.

I'm completely onside with doing it because you want to, or because its cool, or you want to make use of your new welder.  However, I have had my ass handed to me on a very rough track by an AE86 using shocks in the stock location.  And by handed to me, I mean taking my ass, braising it in a red wine reduction and serving it to me on a fine silver platter.  He was GT3, I was ITa. I had a bigger motor, and was the fastest car in my class at the time, and I should have been able to at least keep him in view for more than 30 seconds...

My main reasons are: better shock choice and gets around the issues of trying to move the stock spring perches to the new axle.

In reply to Streetwiseguy :

I did not say inadequate travel, in the absolute, I said it relative to suspension travel. Very different. The part about the live axle and unsprung weight is another animal, but the corolla piece is surprisingly light. Well located too. Relocating the shocks would probably move 10%  of that unsprung weight to sprung. Probably worth doing, if there was another car in class that was equal it could be important. As for filling the Mustang's trunk with pretty aluminum, it could have been done both lighter and stronger if it was not so pretty. Also the Mustang is not all that short of travel to start, I think your conclusion about did it 'cause he could is about right.

In reply to TurnerX19 :

Relocating unsprung weight is also a plus. If I wanna play in XP I’ll need all the help I can get.

Fitzauto said:

In reply to TurnerX19 :

Relocating unsprung weight is also a plus. If I wanna play in XP I’ll need all the help I can get.

There you hit another of the wrongs in the Mustang photo above. All mass should be kept as low as possible. Mount the shocks vertically with the rocker at the top so the mass is kept low.

I had a completely different idea of what this was going to be about. 

Gah! Not that again.

Inboard shocks do not automatically reduce unsprung mass. In fact, they often increase it like in that Mustang setup above. When you it a bump, the suspension will accelerate that push-rod, that huge rocker and that shock. It's all still connected to that wheel movement. Thus it's still all unsprung mass.

You can reduce unsprung mass if you take advantage of the rocker ratio to use a much smaller shock like the FSAE teams do, but that comes with it's own huge sets of problems (over-heating, too little movement of the shock piston for proper dampening)

There are many good reasons to use inboard shocks :

- Mount the shocks in a better location (central and low)

- Mount the shocks so that it's reaction better controls the suspension (no leaning shocks)

- Feed the forces of the suspension into the structure at you preferred location (important on a CF formula car)

- Get the huge shock out of the air stream (again important on a formula car)

- Run a mono-shock setup or a really complex ARB setup that is much lighter

- Make your drift car look baller

But reducing unsprung mass is rarely one of them.

Sorry this is a pet peeve of mine.

So to the OP, what's your goal?

In reply to fanfoy :

Only 1/2 the mass of the rocker is unsprung. None of the shock mass is unsprung. All of the pushrod mass is unsprung. Inertial loads and static sprung loads are not quite the same thing.

I think the big things about it are the ability to run whatever shocks you want and have near complete control over the motion ratios, both of which you can't really do with other setups, especially with the overall suspension geometry fixed. It's very possible to reduce unsprung weight, but everyone likes to get caught up in that and not consider that putting the dampers up in the trunk and adding structure to mount them adds a bunch of sprung weight, mounted very high in the chassis which I have trouble believing doesn't at least offset the gains from reduced unsprung mass. Of course you can do it properly and get the dampers mounted super low but that's usually pretty difficult to package within an existing chassis structure.