View Full Version : Solid axle two wheel bump vs single wheel bump ride frequency

funcars

05-08-2012, 09:05 PM

I've calculated a solid rear axle ride frequency both ways I've seen it done - one from an old Steve Smith book calculates ride frequency based on single wheel bump which includes the motion ratio from the track width and spring mounting points. The other method from Milliken treats ride frequency more like a two wheel bump situation and for the leaf spring example uses the leaf rate directly. Both of these approaches give significantly different ride frequencies and wheel rates. Both approach roll rate calculations basically the same. With an independent suspension there would not be a difference between the two wheel bump and single wheel bump ride frequencies.

I'm in the process of changing front and rear spring rates along with installing a different rear suspension and want to get my ride frequency starting points close. I tend to lean more towards the Milliken method which results in a higher frequency and then calculate the roll rates separately. Is this the most realistic approach?

Thanks

MIKE67

05-09-2012, 09:16 AM

I think this is a great question and shows that you have a handle on the problem. I like the quote at the front of chapter 16, Milliken and Milliken.

"Figuring the suspension of a car is almost entirely a matter of making useful approximations. It is not an exact science. But neither is it a blind application of rule-of-thumb principles."

Maurice Olley

In the example on page 593, they decouple the rates when calculating ride and roll spring rates. i.e. They use the verticle spring rate for ride. They increase the roll spring rate by 40% due to leaf springs higher roll rate due to lateral and torsional deflection of the leaves. Then they apply the motion ratio because the springs are mounted inboard.

I think your approach is good. Usually ride calculations are looking at two wheel bumps. Post up more of your calculations and spring selections!

exwestracer

05-10-2012, 04:52 AM

It seems to me that suspension leverage (and frequency) in roll should be figured using a leverage ratio from the roll center of the system. Now, when you get to a leaf spring setup where there are 2 lateral anchor points (each leaf pack) what do you do? I've always used the idea of connecting the anchor points of the springs to determine height and placing the roll center at the axle width centerline. BUT, the only truly fixed points in the system are the front spring eyes... We assume no deflection of the shackles, but this isn't always the case. And, since the spring is running front to rear and is also the locating link for the axle housing, the leverage against that mounting point may change significantly as the suspension compresses in roll.

The usual fix is to err on the stiff side, and this seems to work. I wonder if anyone has ever compared real world wheel travel tests to a computer model to see if leafs behave the way we think they should?

Mateospeed

02-11-2013, 08:35 AM

while this is not a technical answer, I tend to lean towards Milliken, as the Steve Smith book is really just a handbook, assuming you already know all the background behind the calculations.

Bryce

02-11-2013, 08:54 AM

I wonder if anyone has ever compared real world wheel travel tests to a computer model to see if leafs behave the way we think they should?

There is a SCCA porsche guy that teachs a modal analysis class using his car as the test article. Measuring frequency response and input loads. I will see if I can find some notes.

exwestracer

02-11-2013, 01:09 PM

There is a SCCA porsche guy that teachs a modal analysis class using his car as the test article. Measuring frequency response and input loads. I will see if I can find some notes.

Porsche...and leaf springs...WHAT!? :screwy:

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