Bumpstop length math

Anyone handy with shock/bumpstop math? Getting some new shocks and want to make sure I’m protecting them from bottoming out.

Shocks are set at ~45° angle and will have ~4” of exposed shaft. I’ve got ~7.5” of space between the bump perch and upper contact point. The bump itself is ~1.5” fully compressed.

Without consideration of leaf arch, how to I select bump mount length to protect the shocks? Not sure if it’s right but I’m playing with: shock travel = vertical travel * cos(shock angle), using different values for vertical travel based on different bump mount lengths. Is this the right way to do it?? I come up with needing to limit vertical travel to less than 5.75” (1.75” bump + mount) to match the 4” shock travel.

 

4.0/cos(45) = 5.65 in. vertical travel

It's an approximation because the shock angle changes as the axle moves up.

 

Thank you! Glad I was on the right track.

 

Could always just cycle the suspension and go from there.

 

Kind of what I was thinking.
Get a tape measure and start measuring.

 

Yep, and the geometry is slightly different between two-wheel bump (e.g. landing after a jump) and one-wheel bump (e.g. flexed out on a rock).

 

Thanks guys, I appreciate the feedback! I don’t have the shocks quite yet but wanted to build the bump mounts now so I can hopefully get everything installed for a trip this weekend, so ballpark is all I need for now. Will dial it in with shims or spacers (or rebuild the mounts) once the shocks are in and I have more time to play with it.

In the meantime @time623 built me a model to pull some measurements from because he’s a badass! Our math got pretty close!