Spring Rate Driver: Rates or Bump Travel?

[LLOGAN]

It seems to me like this thinking sets a floor on possible suspension spring rates, assuming you don't want your shock to bottom out.

Max Expected Push Rod Force / Motion Ratio = Spring Force
* motion ratio as proxy for ratio of rocker arms

Spring Force = k * available bump travel

minimum k [lbf/in] = Push Rod Force / (Motion Ratio * available bump travel)

ex: k = 300 / (.75 * 0.95) = 421 lbf/in

From the rates perspective, Spring Rate is a function of tire rate, target ride frequency, motion ratio and unsprung mass. The only strong lever available to us is Target Ride Frequency. Motion ratio is the immediate candidate since moving it closer to 1.0 reduces the required shock reaction to prevent bottom out - unfortunately MR has an inverse effect on Spring Rate. From this perspective, I have to jack up my target ride frequency to 4.0+ Hz in order to meet minimum spring rate previously established. This seems like a pretty immovable floor since I'd like to avoid bottoming out suspension at all costs.

This is very nebulous but if anyone could give this a read and (hopefully) point out where my thinking is incorrect that would be much appreciated.

http://s7.postimage.org/m0tfz4p23/Capture.jpg

[LLOGAN]

Push Rod force might be on the aggressive end of the spectrum so that could play into the divergence.

Regardless, is this a valid line of reasoning?

[LLOGAN]

Anyone have thoughts on this?

[BillCobb]

Thoughts:

1) The roll gradient is unrealistic. Try 2 - 2.5 for starters. Tires alone might give you 1.5 deg/g

2) Your rear is too low for the magnitude of the front. It will 'hitch' (azz drag). Instead, set up a balance rear to front bounce frequency RATIO of about 1.2 : 1. That tends to give you a 'flat ride' (rear has to catch up when you hit a bump). Then choose some frquencies that fit into your travel box.

3) make up a dynamic model (In Simulink) of coupled pitch and bounce with ride unsprung and sprung masses and pitch DOFs. Set some damping levels and run the play with a small havesine jounce input and plot the results vs ground speeds.
Your driver and your undercarriage will appreciate your efforts.

Just sayin'...

[GSpeedR]

LLOGAN, check your equations, the rear ride spring (K_RR) seems off, like you used the front mass instead of the rear.

[SteveHarasym]

To answer the question in the title; I prefer to have the motion ratios determine total travel, then spring rates are set so that at max load case the damper will almost bottom out or have a desirable roll grad/frequency.

The primary goal of a suspension is to make sure nothing hits the ground that shouldn't. So if your damper bottoms out first, it can never happen (unless there is significant deflection or something breaks). After that, you don't want your damper to bottom out, so set the spring so that it won't unless there is an unpredicted load case. If you want it stiffer for a certain ride freq or tweaking the roll grad, then change it.

Also, your calc is planning for 0.95" travel in bump, pretty sure there is a lower limit to the travel you can have in either direction from ride http://fsae.com/groupee_common/emoticons/icon_wink.gif