Push rod suspension/ Anybody help me out?

First of all I'm too dumb to even know what it's called when you use a set up similar to a rocker arm to suport your suspension. Also, does anyone know where I can get some good, understandable, information on this set up? How you do you figure out things like load when you are using this set up? How do you figure transfer of suspension load when it's pivoting on the rocker arm? I need everything so please give me a site, a book, or something else to look at. I tried to search here, but since I don't even know what this set up is called, it's kind of tough. Thanks

 

Engineering Pro Tips has an automotive section

www.eng-tips.com

But be carefull they are cranky bastards overthere. The only thing they seem to care about is SAE forumula cars. Old car stuff will apparantly get you kicked out, at least that is my experience.

 

I'm too dumb for that web site. However, I did read where someone gave some pretty great advise. They said to draw out full scale plans with the suspension at each end of travel and measure to see how much travel you get on the inboard shock. Pretty straight forwar I guess. Now my only question is Since you are putting the shock in a true vertical position do you still get 100% eficiency even though the push rod is at an angle?

 

Scratchbuilt,
the shock ratio is relative to the angle of the shock compared with the linkage which operates it. The only way to get 100% is with a live rear end going straight up and the shock dead vertical. In every other case the ratio will change as the suspension articulates. THe best way to do a pushrod deal is to set the geometry to give a rising rate. That is, the shock gets closer to 90 degrees to the actuating arm as the travel increases.
I have built several cars with either pushrods or rising rate bell crank linkages but I don't think that I have any digital images to post.
I will have a look at the shop tomorrow to see what I have that I can scan and post.
The easiest way is to simply use a rocker arm as the upper control arm and then the geometry is straightforward. The pushrod deal takes a little more calculation to get all the ratios right.

Roo Man

 

I built my first Push Rod suspension,back in the mid '80s,
on a One Off single seater Formula Car.The Carrera coilover
shocks ran front to back,above the drivers legs,with an inboard sway bar.Designed and built ALL the suspension,a-arms,hubs,
spindles,tie rods,push rods,rockers,welding jigs for all the parts,
billet Aluminum brake calipers,Push Pull shifter,8" leather wrapped steering wheel,cockpit adjustable brake bias,etc.

At the time Formula 1 teams were switching back and forth
between Push Rods and Pull Rods,searching for an advantage.
Most people we consulted favored Pull rods.
Now EVERYONE uses Push Rods.

You can get just about anything you want,or need,
as far as wheel rates,by changing the design of the rockers.
The proportions,ratios,angles,etc will affect wheel rates.

A lot of people take the "safe way out" and make their rockers 1:1.

Take a look at current F! and Indy cars for ideas.
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I'd suggest you make a scale model from carboard and push pins,and see what happens.Move the wheel up one inch,and see
how much the shock moves.If it moves less than one inch,
you will need stiffer springs than if the shocks were mounted outboard where the push rods are.

This is a good exercise for a CAD program.
There is a lot of stress on the rockers,and the pivot.
Having a decent Mechaanical Engineer to consult would be very helpfull.

Sounds like you've got a LOT of research ahead of you.

 

Thanks for the info and please keep it coming as much as you can. I thought about keeping my rockers 1:1 as well just so I would know everything was just matching up so to speak. My real reason for doing this is because I want to make an "A" arm that has little room for a shock, but I also want the support arm to be out as close as possible to the lower ball joint. I hope this design can help me. I would be interested in hearing more about the rocker as the upper arm. Pictures would be great do help me visualize during my research. Also, what type of bearings did you use as the pivots in the rocker arms?

 

Our buddy Boyd Coddington has written a book on custom chassis- it should cover that.....

 

Kenny, don't you mean someone ghost wrote a book that Boyd gets to take credit for?

Frank

 

Te he he he h heee-I suppose!

 

John Hotchkis designed the suspension on the latest Chipster

 

there was a neat build of a rear engine mini tube chassis with acura motor www.kimini.com that the guy makes a cantalever front suspension that the upper a arm is also the push rod.

 

Is there an advantage to having the pushrod instead of the upper arm just acting as the rocker like in the above pictures? Is the only reason for the pushrod due to space contraints or is there another reason?

 

The guys on the Locost forums have done this a number of ways. I've seen the shocks laid down horizontally, laid down front to rear, vertical, angled.....many shocks can operate at any angle.

Here's a link to one build using a pair of Yamaha R6 motorcycle shocks. Cheap, neat rockers, and effective. http://www.locostbuilders.co.uk/viewthread.php?tid=47098



This is a homebrewed IFS, but the same could be done for a straight axle or rear end.

 

Bump

Still wondering about the upper arm as the rocker. Is this set up just as good as the pushrod that hooks to the lower arm? Thanks

 

In my personal opinion, no. Whenever chassis load is transmitted to the suspension through the upper control arm, caster angles need to be near negative for proper trail.


Example, Early Falcon/Mustang and Chevy II suspensions mounted the coil spring to the upper control arm. These cars handling suffered quite a bit for it too. So much in fact aftermarket guys are offering coil over kits to pass throught the upper arm and attach to the lower.

It is accepted practice to suspend from the lower control arm. This allows greater caster angle and a more favorable weight jacking durring cornering and a tendency for more neutral handling. Negative caster or near zero would have the opposite effect with a tendency to understeer.

 

I was about to suggest that you take a peek at the rear suspensions of some bikes. Depending on the amount of weight you're going to be carrying, some of them might be good inspiration. Take a look at some of the latest BMW's, but if you really want to go "hmmmm....", check out the latest Kawasaki Concours....four-bar linkage and interesting castings. See the December, 2006 issue of Cycle World. Kinda hi-tech, tho.....

 

Please take a look at this and tell me what you think. I'm trying to get rid of as much exterior linkage as possible.

 

Overly complex. Am I to assume this is for a lakester or something? a normal pushrod doesn't get in the way too much. Most Forumula cars run them.

You could cantilever the lower arm much simpler. It was done for a while but the arm is usually pretty heavy and bulky. But it does work.

 

Thank you. That is pretty cool, and simple. Does that have the same response as the normal pushrod set up?

 

My next questions would be

Is there any way to eliminate the need for an anti roll bar?

What is a good ratio for lengths of the upper and lower control arms?

 

Do you mean responce as in ability to control the wheel? Sort of, depends on how you design it and the materials you use. The killer on this type of suspension wether it be upper or lower control arm cantiliver is the arm itself. A bulkier arm is required to carry the load of the entire vehicle from the fulcrum point to the balljoint. Where as a pushrod system would only have to resist braking and cornering loads because the pushrod mounted nearer the ball joint would take the weight. Hence the concern for making your pushrod rocker stout.

Just as the other designs, lower cantilever arms can be set up to keep the shock at near right angles for the best mechanical advantage against the shock. Also you could incorporate mulitple mounting points to change the shock angle to get different results in comounding.

It really depends on what your intent is with this system.

 

it's becoming clear that you have a lot more knowledge about this than I'll ever have. You've been very nice about not telling me to just use pushrod and stop being an idiot. That's what I'll do. When I go to design the system I'll use pushrods mounted out as close to the ball joint as possible. One more slim rod out there won't make any difference. Thank you for steering me in the right direction.

 

The location of the attachment point of the suspending medium should have absolutely no effect on the trail, castor, etc. It is simply a mechanical connection for the shock/spring. Castor is a function of the relativity of the upper and lower ball joints and that should not be affected by the location of the shock unless there are packaging problems. The location of the inner mounting points for the control arms can also affect castor as the suspension moves through its travel due to built in anti squat/anti dive geometry. Trail is also a function of positive castor, that is the steering axis is ahead of the tire contact patch. Again the location of the shock mounting point has absolutely no effect unless the suspension bushes are extremely soft to the point of letting the control arms deflect.
The reason that the early Falcons/Mustangs don't handle all that well is not so much related to the spring location but rather the original geometry. As I understand it the design came about as a result of the need to tie the upper spring/shock mount into the car's structure. Remember that prior to these cars Fords were all full frame vehicles. The shock tower tied into the fender mounting rail and the cars also had a brace to the firewall area in most cases, spreading the load into the unibody structure.
Some good friends of mine in Australia built a GTP/FIA Group C sports car in the mid 80's. That car had rocker arm front suspension and as far as I know it still holds the sports car lap record at every track in Australia that it raced on. It also held the outright lap record at several of those tracks so I don't thing that having the shock connected to the upper control arm is a problem, plus it is a simple deal to engineer and mount.

Roo Man

 

My guess is that you'd want an anti-roll bar if:
1.the car has a high roll center,
2.you plan on doing gymkana or high speed cornering work,
3.the roads where you live offer the opportunity to throw it into a corner! it's always tempting.

 

Roo, yes keeping the spring load on the top control arm can be made to handle well. IE motorcycle fork geometry comes to mind. But for the ability to self center a top control arm mounted spring load is an inferior design. It can be masked by increasing the srcub radius or increasing the KPI to inhibit more weight jacking durring turning. More than likely your friends GTP car did this along with engineering the car to work with this type of suspension. To my point though most all current designs use conventional mounting of shocks and springs or pushrods. Why? Solid reasons, lower scrub radius and lower KPI needed for better tracking and less weight jacking under cornering loads.

Scratchbuilt, you may not need the anti-roll bar if you pick your spring, damper and roll centers correctly. Otherwise there are some real compact ways of adding one in if needed.

 

I saw how you can add an antiroll bar by hooking the rocker arms to a bar that pivots in the middle. It just seems that for street driving I wouldn't get much travel out of this.

 

Again,it really depends on what the purpose of the car is. A car can be made to ride/handle well with a limited ammount of suspension travel with a high quality shock and sophisticated valveing.

My contact at RCD Suspenison has been able to build some trick Bilstien shocks to tame some really low cars. It's really amazing stuff to deal with once you start tinkering.

So you going to tell us what all this is for?

 

Just about everyone has gotten away from rocker arm
suspension,for a few reasons.

To save weight.Rocker arms end up being heavy
to handle the stresses.

Ease of adjustability.Changing pushrods and bellcranks
is much easier than making new rocker arms.

Crash damage.Rocker arms don't want to bend in a crash,
which will tear up the chassis instead.

Aerodynamics.Bulky rocker arms are less aerodynamic
than streamlined pushrods.

There is more to it,but you get the point.
------
As a side note,my friend Joe Sulpy Sr. has coilover shocks
actuated by billet Aluminum rocker arms on the front of his
Hemi powered Model A show car,that he built in the early '70s.
The shocks mount horizontally,and run on the outside of the frame,beside the motor.