More unsprung Weight, Wheel frequency etc.

[Morls]

I bought the book, "Colin Chapman Lotus Engineering" at Stoneleigh and found it a fascinating insight into how CC developed his cars (and planes and Boats).

 

As an Engineer me'sen I'd like to think I understood most of the theories discussed, but one interested me that was jogged by the "Unsprung weight" thread.

 

Apparently CC used a "Suspension Frequency" calculation to work out the frequencies and magnitude of forces that the springs and dampers would need to absorb.

 

So...

 

Does anyone understand, and can explain, the theory, the formulea and the practicequestion.gif

 

 

 

Mark

[ian crocker]

Suspension frequency = 187.8 * sqrt(wheel rate/sprung weight) This gives cycles per minute and around 120 cpm is reckoned to be a reasonable value for a race car by some people. However there are many different theories on the ideal value, and whether there even is an ideal value.

 

Wheel rate = spring rate / motion ratio^2

 

Motion ratio is the ratio between how far the wheel moves to how far the spring mounts move.

 

Ian

[Morls]

Ian,

 

Cheersthumbsup.gif

 

This doesn't appear to take any account of the ratio of sprung to unsprung mass which seems odd confused.gif.

 

My understanding (not much), is that spring's force (for a given length of deflection) controls the force of a vertically accelerating mass (the wheel /brake/hub), so surely this (mass x rate of deflection) must have an impact on spring rate choicequestion.gif

 

Oh well, I'll best go back to my previous state of...

 

 

Where ignorance is bliss.....smile.gif

 

Mark

[ian crocker]

Yes, I suppose it should have an effect. However it seems that most worked examples for cars with varying sprung/unsprung weights still come out in a similar ballpark. There are even worked examples for the McLaren F1 road car in a book about the car and they started off at 120cpm in racing configuration and that has a dire sprung/unsprung weight ratio - actually quite similar to a se7en at 5.5:1 and 5.8:1 F&R.

 

At the end of the day it is only a guide and many more practical considerations enter the picture such as limiting bodyroll to a sensible value, making sure you don't use the bumpstops on any track etc etc. Also, the experts often disagree on fundamnetals such as whether the rear frequency should be higher than the front.

 

Damping is even more complex than getting the spring rates right!

 

You can run all sorts of calculations on a car but you are likely to make plenty of changes to those settings once you drive it, and different drivers will obviously prefer different setups.

 

Regards,

 

Ian

 

Edited by - ian crocker on 25 May 2002 13:30:31

[Peter Carmichael]

The frequency that Ian quotes is only for *heave* suspension displacements. You really need to know all about the distribution of masses and roll axis/pitch axis before you can work out what is going on for the other modes of suspension movement. Modes are heave, pitch, roll and warp.

 

Simply put, the suspension frequencies are for the car body's movement. The same forces that accelerate the car also act (in reverse) on the wheels and the other unsprng components. The unsprung weight is less, so that same force accelerates the unsprung wheels and tyres at a higher rate, enabling them to track the road surface accurately.

 

The unsprung mass can't go anywhere because the ground is in the way. The unsprung mass is subject to the springing and damping inherent in the car's tyres which operates at much higher frequencies than the suspension.

[Morls]

Cheers chapsthumbsup.gif

 

I think the number of variables are an ideal candidate for computer modelling, as my brain has run out of memory already.confused.gif

 

But then, of course, there would only be one "right" answer and Blatchat would be a little quiet.smile.gif

 

Mark

[Doug Jackson]

My cats called Mittens!