Pullrod vs. Pushrods

[Bandit_06]

Does anyone have any strong feelings as to whether a pull-rod or push-rod actuated suspension is better suited to an FSAE car?

I was going to start out by listing the reasons I like pullrods better, but I realized, that I'm actually pretty torn...

On one hand, using pullrods accomplishes one major goal: locating the spring/damper assembly very low on the vehicle. While pushrods on the other hand are a heck of alot more reliable, and probably can be constructed with less weight.

Has anyone managed to construct a reliable pull-rod suspension?

[Bandit_06]

Does anyone have any strong feelings as to whether a pull-rod or push-rod actuated suspension is better suited to an FSAE car?

I was going to start out by listing the reasons I like pullrods better, but I realized, that I'm actually pretty torn...

On one hand, using pullrods accomplishes one major goal: locating the spring/damper assembly very low on the vehicle. While pushrods on the other hand are a heck of alot more reliable, and probably can be constructed with less weight.

Has anyone managed to construct a reliable pull-rod suspension?

[Matt Gignac]

We have four main design criteria for determining pushrods vs pullrods: packaging, motion ratios, load pathes and CG height.

Pullrods are easier to package for us, as at the front the dampers end up below the driver's knees, which would otherwise be empty space, and at the rear it makes sense with the way we have other stuff set up.

Since you'll have a shallower angle between the upper a-arm and the pullrod than between a pushrod and the lower a-arm, you'll have less flexibility in terms of motion ratio (namely, you won't get it as high as you'd like).

For load paths, either can be good, depends on your chassis concept, and CG height is a no brainer, except think if there's something heavier you can put in the dampers' place.

Oh, also, with pullrods, no worries about buckling, and if you have carbon fiber pushrods with Al. inserts, if your glue delaminates, you can still have a flange to keep your chassis off the ground.

That's my take on it at least.

Matt Gignac
McGill Racing Team

[Boston]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Matt Gignac:
if you have carbon fiber pushrods with Al. inserts, if your glue delaminates, you can still have a flange to keep your chassis off the ground.
</div></BLOCKQUOTE>

I agree with everything else you said, but can you explain this in more detail?

[Matt Gignac]

The insert was a cylinder with a flange, ID threaded for a rod-end. The CF tube was bonded to the OD of the insert, and the end rested against the flange. In this case, the tube is loaded in compression via the flange, with the glue bond there to keep it all together.

Basically:

(edit: my little ASCII image didn't work)

With pullrods, the glue is in shear, and once that fails, you're effectively driving a sled. We managed to get the pullrods to work this way by going with a larger bond area (thus longer insert), but on our way to testing one day with the car tied down, one of the bonded inserts came out, so that killed our confidence in them, and we decided to analyze the situation a little further before trying that again.

Matt Gignac
McGill Racing Team

[John Stimpson]

I'm not a super chassis/suspension guy, so maybe I'm crazy...

As an engineer, it seems to me that pullrods are superior. The biggest loads on your push/pull rod will be when you hit a bump, in which case the pullrod gets loaded in tension. A pushrods could buckle.

Of course, it is the push/pull rod's job to lift the chassis back up from a jounce condition, meaning a pullrod has to "push" the car back up. However, there are much lighter loads required to "slowly" push the car back up to ride height.

I'm not understanding how you can't achieve any reasonable motion ratio curve with either push or pullrods.

I've read a couple places that you should try to package your damper vertically, so as to avoid cavitation and air bubbles. Again, I'm no expert when it comes to shocks, so I could be off base.

[B Hise]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I've read a couple places that you should try to package your damper vertically, so as to avoid cavitation and air bubbles. Again, I'm no expert when it comes to shocks, so I could be off base. </div></BLOCKQUOTE>

Most shocks seen in this competition are gas pressurized with a divider piston. This means you can orient the shocks any way you want and if you stay above the mfg. reccomended minimum gas pressure the shock will not cavitate in normal conditions.

Bryan

[jowen]

The shallower angle on a pullrod will also cause a higher force in the pullrod, rocker and rocker mount compared to a more steeper pushrod.

You may also consider the direction of the loads in the a-arms. A pushrod connected to the lower a-arm will lower the compressive force in it and a pullrod will lower the tensile force in the upper.

Anyhow, I think the most important criteria to consider is packaging.

[fade]

the upper a arm could also buckle from the additional loading from the pull rod. Also if you use a rod end for ride height adjustment in you pull rod it could fail from the tensile load. The motion ratio problem is due to vertically mounting the springs and then trying to shorten your lever arm but having to go under the shock. Either method poorly designed can be unreliable. Come up with a goal for your design and rank the importance of the variables, that should get you started developing your own strong feelings. If your still not sure flip a coin.

[Matt Gignac]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by John Stimpson:

I'm not understanding how you can't achieve any reasonable motion ratio curve with either push or pullrods.

</div></BLOCKQUOTE>

Our problem with pullrods was that the angle between the upper a-arm and the pullrod in the front view wasn't very big. Think of the extreme case where the pullrod is in the same plane as the upper a-arm (for instance, a pullrod setup with very small VSAL), your motion ratio is effectively zero. Whereas with pushrods, you have more flexibility as you can vary the angle between the pushrod and the lower a-arm.

Matt Gignac
McGill Racing Team