Damping Ratio Calculation 1

[JakubMech]

Just after a comfirmation and any correction if need be on the terms to use in the calculation of compression and rebound damping ratio.

For compression, use the chassis spring and tyre spring rate combined, the combined damper and tyre damping, and the mass of the unsprung system.

For rebound use only the chassis spring, only damper damping, and mass of the sprung system.

Does this seem reasonable? or do I add the tyre rate in rebound and/or drop it in the compression calculation?

Also regarding tyre damping, does anyone have some real world numbers, ive been playing between 800 and 1500 N.m/s,
the higher values relevent for race type tyres.

Jakub

 

[GregLocock]

Why are you leaving the tirecontribution out of rebound?

The tire is still part of the system

I haven't seen numbers for tyre damping, but you could get a great estimate by dropping a wheel and tyre on the floor. That would give you the stiffness as well. I'd /guess/ about 10% of critical damping for that system - you'd get 3 or 4 good bounces.

Cheers

Greg Locock

 

[JakubMech]

Thanks Greg,

I left out the tyre rate from the rebound caclulation as the norm (? well in milliken it is) to calculate chassis frequency is to only use chassis spring and unsprung mass.

In other text they do combine the two in series.

Thanks.

Jakub

 

[GregLocock]

Whoa thar, Trigger.

I am 99% sure that you can calculate NOTHING meaningful from the unsprung mass and the referred roadspring rate, ignoring the tire wall contribution.

I think you may have either (a) got confused or (b) made a transcription error.



Cheers

Greg Locock

 

[Tmoose]

Does Milliken refer to damping ratio as anything but a shock absorber supplied phenomenon?

http://www.tokicogasshocks.com/car/performance.html

From the shock perspective, This chart is typical of what I recall usually seeing published before in "CHEVY power", "Race car engineering", and motorcycle magazines of the 70s and 80s.
Small and fairly constant compression damping.
Higher and velocity sensitive rebound.

 

[RegisAlstom]

I still don't get how to calculate the Damping Ratio for the data supplied by Bilstein.

They say that the shock valving is 180/75, what means 1800N for rebound and 750N for compression at 0.52m/s

Maybe I misunderstood Milliken, but it looks like

F=C(dx/dt) and dx/dt is the velocity (0.52)

Ccrit = 2 SQRT(Km)

Damping Ratio is C/Ccrit

Are these assumptions right?

Can I say that this Bilstein with 180/75 valving has 1800/0.52 = 3462 N.sec/m as C in rebound, and 750/0.52= 1442 N.sec/m in bump?

Thanks

Regis

 

[JakubMech]

That all seems correct Regis, forumale and your damping coefficients. For damping ratio use the unsprung mass for the bump cooefficient, and sprung for rebound coefficient.

Im still a little hazy about the inclusion of tyre stiffness in the damping ratio calcs. Im only after a simple comparison that will get me with in 5 or 10% of actual. I had a good look at milliken the chassis spring and unsprung mass are only used to define chassis natural frequency and have nothing to do with damping.