How to Calculate Shock Compression and Rebound rates

[regiscastro]

I'm not sure if this is the right board to discuss this, but as I am a Newbie I'm puting this question in this board, and if it is better to put in the general discussion it can be moved to or posted in the General 4x4 discussion it with an advice of someone with more experience than me ;)

Ok, I've searched a lot for the shock valving calculation and everything I've got was Spring rate calculations... but no shock valving, so my question is:

Since I know the spring rate I will use, how can I determine the best compression and rebound rates for the shocks? :confused:

Thanks

Regis

[KC_JoNeS]

Well, since you know the spring rate that you want to use, then this should be simple. All you must do is decide what degree of damping rate you want to accomplish. This however may require some extensive field testing, depending on the sprung weight of the vehicle.

Damping factor = R-shock / R-spring

The higher the damping factor, the faster the oscillations will die closer and closer to nothing.

But like I said, a damping factor must be established which may require field testing.

KC

[regiscastro]

Thanks KC,

But as higher is your dampening rate is, more time the spring takes to go to the "standard" position, so if you are passing thru a place with lots of holes, like in a competition.. hi-speed (BITD for example) the spring will be more and more compressed, until you have no spring (just bumpstops) ...

I agree that the best way to do this is to test it in the track and find the better valving, but I don't know even something to start with.

Isn't there any calculation about this? something like linked to frequency or speed?

Thanks

Regis

[KC_JoNeS]

That is not true. I understand your point, but that is not how a shock works. The best way to describe how a shock works is by simulating 2 sine waves being put on a spectrum at the same time.

Here read this site...

http://www2.eng.cam.ac.uk/~vyby2/shock_absorbers.html

Good info about real world apps of a shock absorber.

KC

[KC_JoNeS]

another good read. This is a bit more mathematical in nature.
And keep in mind this factors in other forms of action against the main body.

http://writing.eng.vt.edu/me2024/memo.pdf


KC

[The Joker]

What use are you trying to fit a shock to? Daily Driving, Desert Racing, Rock Crawling?

Wouldnt an adjustable shock let you figure it out as you drive? Dont like the ride make an adjustment drive some more.

[regiscastro]

Thanks again KC... very clarifying...

I was thinking that the dampening forces of a shock absorber were constant... so I will have to review my concepts...

The problem is that I have a total travel of 24" and a sprung weight of 400 lbs and 100 lbs unsprung.. considering the geometry of the suspension, a recomended spring rate should be 66 lb/in... what gives me the spring compressed 6" on rest.

Now I need to define the start point for the valving in the shocks to go to the field and test...

If the shock has 16" travel and I consider the starting point a dampening value of 1 for compression on 11" (6" of rest position plus half of the travel that I still have after the rest position = 6+5), my shock should have a bump force of 726 lbs and if I use the same rationale, the rebound rate should be something like 200 lbs (3" in the spring).

Is this correct?

Thanks again

Regis

[regiscastro]

Originally posted by The Joker
What use are you trying to fit a shock to? Daily Driving, Desert Racing, Rock Crawling?

Wouldnt an adjustable shock let you figure it out as you drive? Dont like the ride make an adjustment drive some more.

It is a desert racer.... I am starting the design for the suspension and the dampening rates are making my brain burns :D

[KC_JoNeS]

Originally posted by regiscastro


It is a desert racer.... I am starting the design for the suspension and the dampening rates are making my brain burns :D

What I can tell you is this:

The goal of a desert racer is to keep the tires on the ground as much as possible. In reviewing this diagram of two sinusoids, you will see a small signal incorporated into a larger one. The smaller signal will happen always and is a simulation of the different bumps on the ground that one will encounter...

The larger signal gives an idea of the terrain, jumps, hills, and larger scale bumps along the trail.

The goal will be to minimize the larger signal through proper damping. If too much damping is attained, then a hop will occur which will cause less traction and less speed. If not enough damping occurs, there will be big oscillations of the bigger signal. The ideal case would be one large oscillation and then exponentially smaller ones occuring after that.