Thread: Regarding the front and the rear ride frequency

1. i have read those things now what i mean to say is :
i need to know the stiffness in the rear and the front. let me assume a "X N/mm" front stiffness spring. This I assumed arbitrarily. According to the post i should fix more stiffness spring in the rear so that the frequency would match. So how to select the stiffness in the rear can i just take the "X N/mm" as the reference and take the rear one as just higher value. Is this so is there anything that i should know while selecting the spring..?

2. Look at this

Would you look at that....even gives the equations and the whole deal! From the master himself...there's even a link a couple down thats good too!

Even a spark-E / engine guy like myself can make some sense out of that.

you owe me a beer now

3. This thread, its links, and the other thread by the OP on "Pitch point", are typical examples of automotive cottage industry codswallop.

As usual, NO DEFINITIONS of terms used.
==================================
What is "front ride frequency"?
What is "ride"?
How does "ride" differ from "heave" or "pitch"?
Is there such a thing as a distinct "front-sprung-mass" that is somehow separable from the "whole-car-sprung-mass"?

Having made no effort with the first step, there is no point applying any rigour to the rest of it.
================================================== =====================
Is the discussion about a 2-D side-view of the car with the whole-car-sprung-mass conveniently (and unrealistically!) reduced to two point masses located above front and rear axle lines?
Why so, given that when Rowell and Guest did more realistic 2-D side-view analyses in the 1920s they found that, in general, the two fundamental modes of this 2 DoF system give nothing like the "front" or "rear" frequencies calculated in the links?

With regard to the "Pitch point" thread, is that whole discussion about kinematic anti-pitch properties of the suspension linkages (eg. anti-dive/squat/etc.), or about the side-view oscillation centres (as per R&G above) as determined by F + R spring stiffnesses, and CG position + pitch MoI?

Is the real reason that automotive engineers talk about "front and rear ride frequencies" because it sounds oh-so much more sophisticated than saying "its got stiff springs"?

Z

4. Originally posted by Z:
Is the real reason that automotive engineers talk about "front and rear ride frequencies" because it sounds oh-so much more sophisticated than saying "its got stiff springs"?
You if anyone should know that spring stiffness is not only parameter affecting the natural frequency of a system... Using frequency is convenient as it combines two values into one and allows for easy comparison between designs. But you are probably right, automotive engineers don't do it for the convenience but to sound sophisticated!

5. Originally posted by HenningO:
But you are probably right, automotive engineers don't do it for the convenience but to sound sophisticated!
Well, I think we've at least defined "hypocrisy".

6. It's ALWAYS entertaining when you read the comments that those outside of the Gates of Industry make up only to sound like they are inebriated with the exhuberance of their own verbosity.

Perhaps one day they can hitch a ride on a tour bus that takes them into the labs, corridors, conference rooms and cubicals to catch a fleeting glimpse (out of the corner of their eye) of how sophisticated, technical, creative and secretive the industry actually is. Once you have been there, you'll never have to pretend you know what engineering is.

Mechanical engineers design weapons, civil engineers design targets. Un-civil enineers wind up serving nourishment for others to dispute.

7. The thing is, there is no such thing as "front or rear ride frequency". It is fiction.
~~~~~o0o~~~~~

If it was real, then surely you could measure it.

How would you do this? Push the nose of the car down then measure time for, say, ten oscillations?

Well, with dampers you barely get one oscillation, and they change the frequency. So remove all dampers and fit ultra-low friction bearings everywhere. Now push the nose down again.

Except this time you notice that pushing the nose down also lifts the rear of the car. Leverage, aaargh! So, push the car down directly above the front wheels.

So, the \$64,000 question: Will the front of the car now oscillate at some fixed frequency?

Answer: Of course not! As explained (independently) by Rowell and Guest in the 1920s. And repeated in a lot of Vehicle Dynamics texts since then. (Hint: the "front and rear" motions of the car are coupled, because the car's mass is "joined up" in the middle! There are indeed two fundamental harmonics, but in general these are not "front and rear".)
~~~~~o0o~~~~~

But thanks to progress over the last ninety odd years this rather simple 2-D problem has been dumbed-down to 2 x 1-D problems (ie. two totally independent, and unrealistic, 1 DoF spring-mass systems).

And could you even start to use this "independent front and rear" gibberish to describe the side-view behaviour of an interconnected suspension, such as used on the Citroen 2CV? Well, the answer is also the reason why there are so few interesting cars these days. Or good suspensions...

Z

8. Originally posted by Z:
The thing is, there is no such thing as "front or rear ride frequency". It is fiction.
~~~~~o0o~~~~~

If it was real, then surely you could measure it.
Following that logic, "stress" is fiction too since we can't measure it... we measure strain and then calculate stress.

All things engineering are fiction... we just come up with equations to easily characterize object. Some represent reality more closely than others, but how much fidelity is "close enough"?

9. Originally posted by Z:
This thread, its links, and the other thread by the OP on "Pitch point", are typical examples of automotive cottage industry codswallop.
They are typical examples of terminology and methods used by people who actually build cars for a living mate...

10. So, the \$64,000 question: Will the front of the car now oscillate at some fixed frequency?
It doesnt matter one iota. Nobody cares.

We use the ride frequency to get a baseline value for the spring rate and then we move on. The final spring value is then refined after looking at load tranfser calcs, full vehicle simulations and test data.