Calculating installation ratio

[Sathersc]

Hey guys,

We've run into a problem with calculating the wheel rate and motion ratio of our suspension. We haven't been able to find or figure out how to relate our calculations of wheel travel to shock travel (the installation ratio).

We've determined our desired travel, percent sag, and we know how much travel we have in the shock to play with. We're using a rocker arm that pivots on an axis perpendicular to the plane the pushrod and shock create which complicates things, but is necessary for packaging. Can anyone help point us in the right direction for determining the motion ratios we would get using different rocker arm geometries short of using a 3D modeling program and hoping I can pull measurements for each? Thanks.

[Sathersc]

Hey guys,

We've run into a problem with calculating the wheel rate and motion ratio of our suspension. We haven't been able to find or figure out how to relate our calculations of wheel travel to shock travel (the installation ratio).

We've determined our desired travel, percent sag, and we know how much travel we have in the shock to play with. We're using a rocker arm that pivots on an axis perpendicular to the plane the pushrod and shock create which complicates things, but is necessary for packaging. Can anyone help point us in the right direction for determining the motion ratios we would get using different rocker arm geometries short of using a 3D modeling program and hoping I can pull measurements for each? Thanks.

[kwancho]

Wingeo works really well for this. The rocker arm SHOULD pivot on an axis normal to that plane. For Wingeo at least, you need to give it two points to make the axis that the rocker pivots around, which I had to use a 3D modeling program.
I've also seen CSUN's COSMOSMotion seminar where they quickly outlined how they did one set up in Motion, and got everything, motion ratio, forces, MR curves, etc. Very slick.

[J. Vinella]

Also make sure you test your ratio when you are finished. It is a quick double check and if you have gotten it wrong with errors in the jigs, design... etc then you will know something needs to be fixed.

We all hope the components we design work in the way we designed them, but physically testing them is a must.

[Sathersc]

So it's really just, in my case, Solid Edge to Wingeo, back to Solid Edge, back to WinGeo?

Atrocious.

Is there anyone who's looked at figuring out the geometric relationship and created Matlab scripts to determine this? It's something I'd like to do, but I haven't figured out how to set the relationship between the arc created by the lower mount on the pushrod to the arc of the bellcrank between two planes. I really just haven't had the time to sit down and figure out the math.

Also, is there any word on if OptimumG is going to offer OptimumK to formula teams at a discount or sponsorship level?

[HenningO]

We've done a MATLAB approach to it, although our system is a bit different. It's just a matter of defining circles (using the unit circle) and points in space. Calculate lengths of the "push/pullrod", move a point and solve for where the point on the circle moves.

You should be able to figure it pretty quick if you sit down and look at it.

Another option could be using the Excel/Solid Edge connection.

[J. Vinella]

We use a Excel Spreadsheet linked to Soildworks.

I know I found your exact solution in a suspension dynamics book. I returned it some time ago so I can't be sure of the title but it think it is:

Vehicle handling dynamics / by J.R. Ellis

Not a very big book, but it has a whole section on rockers/bell cranks.

[Samo Simonian]

Hey People,

What we did when we started designing was do a 2D sketch of the inner and outer suspension
points, with inner fixed and fixed arm/upright lengths. To that there would be a fixed length
push/pull rod linked to a rocker/bellcrank with pivot and it's own geometry. Extremely easy
in Catia. The IR is indeed the movement of the whole system so be sure take the arms and
upright into account.
This sketch with movable lines and readable dimensions also gave us first glimpses of camber
gains and such.

I hope I understood the question correctly?


Samo Simonian,

Chief Suspension
DUT Racing 2006/2007
www.dutracing.nl (http://www.dutracing.nl)

[Sathersc]

Sorry to drudge up one of my old topics, but I'm having a little trouble wraping my head around what's going on with motion ratios. With our design, we've got what I think is an overal 5:3 wheel:shock travel ratio. Plotting our shock length against our wheel travel at quarter inch intervals, we get this:

http://www.users.muohio.edu/sathersc/length.JPG

To what little I know, this says our motion ratio is perfectly linear, which I have trouble believing. So I went ahead and plotted the amount of change in the shock travel for every quarter inch of wheel travel and came up with this:

http://www.users.muohio.edu/sathersc/increment.JPG

To me it looks like this is a fairly linear with a slight rising rate since the shock is moving less as the travel increases.

I feel like I'm missing something fairly profound here. Help please?

[JHarshbarger]

Those graphs look fine. That's probably just about as good as you're going to get. A slight rising rate is common/acceptable/preferred.