2016 Plan: Amended

I think I got something wrong in the last post.

I said “However, the sway bar, unlike the springs, acts across the car to create an increase in the total weight transferred from the inside to the outside, which tends to decrease total lateral G capability.”

Did you agree or disagree?

Some people have definitely thought this in the past. They (and I) may have been misinterpreting such statements as this, from Carroll Smith, in Tune To Win, page 38: “…the stiffness of the anti-roll bar  will both decrease roll angle and increase lateral load transfer.”

We’ve got to be careful with that statement.

The sprung mass rolls. If there is no sway bar, the inside spring extends, reducing weight on the inside tire. The outside spring compresses, increasing weight on that tire. If you could put a scale under each wheel you will measure what looks like a “weight transfer”, but it’s not, really. It’s a differential in forces at the tires and it will disappear once the car stops cornering. The amount of force differential is controlled by the mass, the lateral acceleration, the track distance and the moment arm which is the distance from the CG to the roll center. The amount of roll doesn’t matter. The stiffness of the spring doesn’t matter. The existence of a sway bar doesn’t matter.

If there is a sway bar, some of the roll energy goes into it instead of the springs. The roll is reduced, but that energy creates another force differential at the tires. The sum of the spring differential forces and the roll bar differential forces exactly equals the previous   amount with springs alone.

So, there is no negative effect on maximum cornering force due to using a stiff roll bar. There is no increase in the total amount of weight transfer, or what I call force differential.

But, during the transient, that is, during turn-in while the lateral-Gs are rising, roll stiffness from springs and bars is a good thing because it makes things happen faster. Energy absorbed by the shocks is a good thing as the effect is to (temporarily) increase roll stiffness and make things happen even faster. The force differential at the tires is going to do what it’s going to do, which is to decrease the maximum lateral-G capability due to the shape of the tire load sensitivity curve. Not much I can do about that in Street class other than set up the car as low as possible. Or, go on a diet and get a lighter helmet!

2 thoughts on “2016 Plan: Amended

  1. I enjoy reading your blog. Seems like you’re getting a little lost in the woods with this one though. Try this: imagine the springs not as springs but as indicators or gauges. Imagine looking at the car from the outside and watching the outside wheels compress and the inside wheels extend. Now imagine the spring by itself sitting in your garage on a scale that’s been zeroed. If you added weight to the spring it would compress a specific amount relative to that weight and the entirety of that weight would display on the scale. Now imagine the car on course again. The springs position and the subsequent body roll is a direct reflection of the amount of weight on those tires. It’s that simple.

    So softer springs allow MORE weight transfer to the outside because they allow more body roll. Body roll is simply evidence that the CG is higher than the Roll Center and that a lateral force is causing a tipping. The more roll, the more the CG gets closer to being above the outside contact patch. Ever see the underside of an E36 DSP BMW? 🙂 Doesn’t take much to remove ALL the weight from the inside tires.

    Shocks only influence the transitional phase of weight transfer. Turns out, the transitional phase makes up the majority of time in an auto-x run… so that’s kind of important. 🙂 That’s why shocks are useful in Street classes.

    Also, get this book and read it. it’s pretty simple but does a great job of explaining some really important fundamentals. https://books.google.com/books/about/Chassis_Engineering.html?id=rY2ujnNrhf0C&source=kp_cover&hl=en

    See you in March.
    Matt

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    • Matt: Thanks very much for the thoughtful comment and the book recommendation. I made an update to point 6 in the original post where I may have been unclear.

      However, I must disagree with: “softer springs allow MORE weight transfer to the outside because they allow more body roll.” Changing the stiffness of the bars/springs changes how much roll occurs at the final state, but does not change the maximum amount of weight transfer. See the formula for lateral weight transfer on page 8 of Adams’ book. While this formula is only exactly correct for the weight transfer contribution from the unsprung mass, it is close enough for all the mass for most practical purposes. Roll stiffness is not a component in that formula (or other more correct formulas) for weight transfer.

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