If I were running a MechEng department... [Some of this stuff is really practical!]

...I'd want a class for "Practical Application of Engineering Tools." 3000 or 4000 level, based heavily in Matlab. Probably 4000 level, since MCEN3030 - Computational Methods, is a junior class and pretty much murders you. Back then I eventually got decent at Matlab from having it thrown at me mercilessly, but when I got to work I couldn't do anything PRACTICAL with it. Like.. how to load a damn Excel file. Matlab has so much built in stuff it's unbelievable and I didn't know about any of it.

On the other hand, a bunch of crap that was just theory without application, as far as I was concerned, is really practical! Bode plots for example, who knew!! I sure didn't. I think I either skipped that lecture to work on the racecar, or it was when I had sliced my finger open on a lathe. (Freak accident, but don't EVER think you're the master of the machine. It can, and will get you).

Anyway. Don't let anyone try to fool you into thinking ride quality isn't important on a racecar. It's critical. There's a reason why professional race teams put a lot of time in 7-post ("shaker") testing.

What is a 7-post test? You put your vehicle on a special contraption, and shake the hell out of it. More precisely you recreate the disturbances and undulations the car is going to experience on a racetrack.


Why? We know that tire load variation is bad for grip. If you didn't know that, it's easy to conceptualize. If your suspension is stiff as a brick, if you had rigid links instead of springs, the car would chatter, skip and bounce over the race track. The whole reason for having a suspension is so it can be compliant and ride easily over bumps without upsetting the tires and chassis. The ratio of input from the ground, to force passed up into the vehicle, is transmissibility.

Where do Bode plots come in? It's a way of plotting signal amplification (transmissibility) and phase lag versus input frequency.


Changing masses, spring rates, and damping rates will change the shape of your curves. If I know what the major frequency content is of my suspension in a critical part of the track (big braking zone, corner exit, whatever) I can adjust my suspension rates to deaden that vibration out and increase my mechanical grip.

Wheel rate, tire rate, and damper rate are easy enough to get, as are the sprung and unsprung mass at a corner. If you knew the tire damping rate (hint hint) you could in theory fire up Matlab, play with some of your rates, and do a form of virtual 7-post testing. The Bode plot is a good way of seeing at a glance if your suspension is doing what you want or if you're moving in the right direction. Up until now I just thought it was some silly thing without a practical use.

As an aside, since at each corner of the car you have a sprung and unsprung mass, you'd think one would be acting as a mass damper relative to the other. For a long time there's been the phrase that you want to "always reduce unsprung mass." Generally I think this is true. There may be the case though that you could tune your unsprung mass to go along with everything else and really zero in on the frequency response you want.

Aw snap.