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On Bushings and deflection...
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Posted by 2Slow (more from 2Slow) on Tue, 13 Nov 2001 14:43:14 Share Post by Email
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The longer I monitor this board, the more posts I see where people replace various suspension bushings with other (stiffer) materials. (I saw one on the 9-3 board just recently and it sparked this post) People complain of the stock bushings beeing too soft and allowing parts to deflect under load. They mention the fact that some bushings have _voids_ (AKA holes) in them...

I have resisted posting in the past but just wanted to speak from the suspension engineer's point of view. We are not stupid. We know the bushings are soft and this allows parts to deflect under load. In fact we often design in three rates in the two planar and axial directions.

Not only do we know that the suspension bits to move, we count on them to do so. We do this _not only_ for ride requirements, but also handling. Lateral force compliance steer is a big tuning parameter on many vehicles and when people start putting very rigid bushings in all directions, you can easily screw up something that was tunned meticulously.

I will admitt that for most vehicles we suspension engineers must err on the side of ride quality and safety rather than handling and fun factor so some many vehicles could benifit from different tuning. The problem is that the performance bushuings I see marketed are pretty much equally stiff in all directions, and quite stiff at that. That is not really intelligent tuning... This removes an entire degree of freedom from the suspension design. The compliant motions.

Usually we start with a fairly good kinematic model (where we assume that the bushings are revolute joints with infinite stiffness) from there we make up for the suspension's shortcomings with compliance. We tune bushing rates in three directions per bushing to get the user the best ride, and safest handling we can. When you go to super stiff bushings, you basically only get the kinematic motion of the suspension, not the compliant one. We try tog et lateral force compliance steer, and minimize steer effects under load or braking. (We also like to assure that the steer effects are understeer effects under braking. This is one of the reasons that 9000's are so great in the snow. It takes a lot to upset that rear suspension... 9-5's can go oversteer in a corner when applying the brakes because of the suspension geometry and compliances.)

An example of this is good ol lateral force compliance steer. Since you really only want steer effects from the suspension under lateral force, not ride events, it seems logical to me to leave the suspensions steer up to the compliant elements. If you put it in the kinematic ones, the suspension can't tell the difference between a bump and a corner, so you will get the same steer effects for either. (for those who play with toe curves, the person wo is designing only for kinematic motions forces himslef into haveing some slope on the toe curve to achieve roll under steer. If you play the compliance game you can have little to no roll steer, but lots of compliance understeer and better achieve your handling targets) Since you only want them in a corner it is better to tune the compliance rather than the kinematics for understeer...

So please be careful when changing suspension bushings and make sure that you understand what you are doing. You could very easily screw up both handling and ride if you are not careful.

This brings me to another point. We were working on a particular vehicle and pretty much maxed out the chassis' handling capabilities. The ride still felt "too soft" according to a brand manager so we had to stiffen it up. There was no gain in performance, just a degredation in ride. Many people percieved the stiffer tuned vehicle as having sportier handling, but it was no better. We sold that. That seems sad to me, but I fear that is the type of error that people can make when blindly swapping out heavily tuned bushings in place for simple stiff ones... Ride will defineately get worse, but perfomace bennifits will be marginal to negative since the kinematics of the suspension were designed with the compliant motions in mind.

Sorry for the long post. I realize as long as it is, it does not fully explain the hows and whys of bushings. Feel free to ask any questions, but I warn the performance minded driver to be very careful when messing with bushings. Those voids are there for a reason...

-Joe



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