4 replies to this topic

[Damped]

Hi,

Very simply my question is when generating damper coefficiants for basline damper curves from first principles for ride frequencies applicable to the body, it seems that the unsprung mass is overlooked in many of the claculations i have seen. It seems to me that when you calclulate the ride frequency of the vehicle you are in effect isolating the unsprung mass and and neglecting its effects.
Whilst it is obvious that variations in the unsprung mass have a significant affect on the damping requirements I am not entirely sure how you could quantify it as accurately as you can for the sprung mass.

Any ideas?

[Fat Boy]

While according to people who are much more mathematically adept than me this is wrong, I have found some general trends that seem to work.

Use the unsprung mass to generate your damping coefficient in compression and the sprung mass to generate your damping coefficient in rebound. For bigger cars you can run the same coefficient on bump and rebound (although the force values will be different) and have something that is reasonable. You'll always have to tune it to the track, but this is fine for a first cut.

For cars with really light unsprung masses, it might not work. I haven't really worked with that type of stuff for a while.

[Greg Locock]

Yup. That's about as mathematical as it is worth getting, so far as I can tell. We do have people who work away at trying to generate first pass curves for shocks, if you follow what they are doing it is 9/10 same as last time, 1/10 from their models fudged to fit the previous model, ratio-ed for mass. The most successful of these models is no fancier than an excel spreadsheet.


I'd guess it is 'easier' to predict shocks for handling rather than ride, but the allowable error would be much smaller.

[murpia]

http://www.optimumg....ps_techtips.htm
The 3rd and 4th articles deal with this issue. I wonder how their 2/3rds 3/2nds rule of thumb compares to the sprung / unsprung method mentioned above for some typical racecar values...

Regards, Ian

[Greg Locock]

Very good. Time to get some splinters in my fingers.