Pullrod vs. Pushrods

[KU_Racing]

In this argument, I choose pullrods. Pullrods have quite a few advantages over pushrods, in my opinion.

As far as packaging goes, if the chassis is designed with one system or the other in mind (I would say it should be....) Then it is relatively easy to package a pullrod at an angle very close to the angle a pushrod would see in action.

I like pullrods for their strength- Much better to load a member in tension than in compression. You can make the pullrods either lighter for the same strength level or stronger for the same weight level as a pushrod.

Also, the CG gains from placing the shocks under or close to under the chassis are obvious.

One small side benefit is that if a pullrod fails, there are no members that are going to go into the ground (which has caused FSAE cars to roll in the past) or through the shielding and into the driver's compartment.

[flavorPacket]

I don't understand why you guys are saying you can't get a good motion ratio from a pullrod system. I was able to get a nice linear .93 damper/wheel with minimal headache.

And I don't think the cg benefits are that compelling. Instead of having dampers (which weigh all of 2 lbs) under the driver, why not have the 60+ lbs of driver's legs under the dampers?

[Gareth]

Historically UW has run pullrod suspensions for the CG benefits and the buckling requirements. We tried a few different configurations, each with its own pros and cons. Last year we switched to pushrods for a variety of reasons. The first was simply that we were tired of lying on the ground to work on the springs/dampers. As silly as that sounds, our philosophy was that we could go faster by making more setup changes and tuning the car than by putting the mass a little lower. After the initial concept we discovered massive hub-to-hub stiffness gains, but that's very dependent on your chassis concept. The other benefit was that it moved around the forces in the control arms, making them more evenly distributed. Ultimately we came to a more efficient structure that also gave us more room to package a kinematically better anti-roll system.

I don't think there is a clear winner between the two, but it's definitely good to have the various compromises in mind when making your decision. Of course you can achieve good kinematics with both, provided your other parts don't get in the way.

[C.Zinke]

Overall, i think this is a thread with a lot of good arguments. Furthermore i agree that the decision is depending on your kinimatc, structure and package.

I like to add that a pushrod and a pullrod produce reaction forces. But normally your lower A-arm is loaded higher. So it stands this reaction forces anyway. If you use a pullrod, then you also have to use big tubes for the upper A-arm.

Furthermore you reduce the risk of buckling (rear tube of front lower A-Arm) during braking with a pushrod

We use Pushrod. So its possible to build the upper A-arms this way:

http://www.lionsracing.de/component/option,com_ponygall...func,detail/id,1171/


With a pullrod and small upper A-arms this can happen:

http://evilengineering.com/gallery/v...32679.jpg.html

[D J Yates]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by C.Zinke:
I like to add that a pushrod and a pullrod produce reaction forces. </div></BLOCKQUOTE>

I was getting worried there that everyone had forgoton newton's third law. Further more, since the angle between pullrod/pushrod is rarely greater than 45 degrees to the horizontal, the reaction in the wishbones is greater than the normal load on the tyre.

Another thing to consider is the cyclic nature of the tyre normal load. Since only cycling tensile loads reduce fatigue life, ask yourself would you rather have to replace a pullrod or the lower wishbone of a pushrod assembly.

With a carefully designed pullrod you could garauntee the failure mode of your suspension and the subsequent damage it could do. Say that your inexperience/rubbish driver hit a large curb head on, if your suspension could withstand the shock loading chances are your wheel couldn't. Would you rather replace a simple pullrod or an expensive ultra light weight wheel. If the pushrod was a thin walled tube with a tether running down thinside, the pullrod could fail in tension and allow the wheel to ride up over the curb and save the wheel, but the tether would stop the car bottoming out when it landed.

Personally i think that pullrod or pushrod choice should be determined by the location of the steering joint. If your pullrod/pushrod and steering link connect to the upright at the same end (steering from top with pullrod, steering from bottom with pushrod) you are free to move the ball joint of the other wishbone, giving yourself easyilly adjustable caster and kpi without any effect on steering geometry or motion ratios.

[Iloper]

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

</div></BLOCKQUOTE>

You miss the point here, a pullrod never pushes the car, hence the name. The only time a pullrod gets buckeling forces is when the wheel is in the air and gravity is forcing it down.

Whe had quite som problems with this, but only when the car is in the pit, when the vehicle is of the ground and an exhausted needs to rest, he sometimes wants to sit on a tire. Or one other time when our brake discs melted and the car took a halt on the track. When it was time to lift it up, the tyres were glued to the ground and Bamm! both front pullrods looked like a banana.

We have been using 6mm steel rods, we're changing that to tubes now.

Per
CFS

[jowen]

Per, how could your brake disc melt?! Shouldn't the brakes be fading a long time before that happends?

[Iloper]

I didn't drive, so I don't know if they faded. The discs didn't realy melt, just deformed heavily. The problem was dirt in the brake fluid, the pistons didn't retract enough, together with cermic coated alu (6082) discs. The coating is still there though.

Per
CFS

[js10coastr]

Chariots of Fire by Steve Matchett has a pretty good section arguing the points of each, and why F1 uses pushrods now.

[TG]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by js10coastr:
Chariots of Fire by Steve Matchett has a pretty good section arguing the points of each, and why F1 uses pushrods now. </div></BLOCKQUOTE>

I'm pretty sure that was mostly for aero reasons. F1 is heavily dependant on the aerodynamics of their vehicles. This is plainly evident in the fact that they are now compromising on sub-optimal suspension geometry in the zero-keel designs to improve aerodynamic efficiency.