Lift Geometry: Rear Spring Axis vs Rear Shock Axis

SSTJ

SSTJ

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Overthinking the basics of lift geometry:

1) Given the difference between rear shock axis and rear spring axis, is there any notable difference between how much of lift one gets along the shock axis for a given amount of lift along the spring axis?

2) Forgetting shocks for a moment: given the angle of the rear shock axis (vs vertical), how much vertical lift does a TJ get for every inch of lift of lift along the spring axis?

I can go measure the angles and figure out a theoretical answer, but I bet someone already knows the answer from experience. @mrblaine, perhaps?
 
There is a decrease in the rate of lift height the closer the arms approach perpendicularity to the frame. There would also be a related decrease in the rate of vertical spring lift as the springs are pulled away from vertical by the control arms.

This feels like a calculus problem more than geometry.

This also feels like an argument for a long arm kit! 😉
 
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SSTJ said:
Overthinking the basics of lift geometry:

1) Given the difference between rear shock axis and rear spring axis, is there any notable difference between how much of lift one gets along the shock axis for a given amount of lift along the spring axis?

2) Forgetting shocks for a moment: given the angle of the rear shock axis (vs vertical), how much vertical lift does a TJ get for every inch of lift of lift along the spring axis?

I can go measure the angles and figure out a theoretical answer, but I bet someone already knows the answer from experience. @mrblaine, perhaps?
Click to expand...

What are you trying to figure out in the hardest way possible?
 
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jjvw said:
There is a decrease in the rate of lift height the closer the arms approach perpendicularity to the frame. There would also be a related decrease in the rate of vertical spring lift as the springs are pulled away from vertical by the control arms.

This feels like a calculus problem more than geometry.

This also feels like an argument for a long arm kit! 😉
Click to expand...

And tuned springs!
 
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SSTJ said:
Overthinking the basics of lift geometry:

1) Given the difference between rear shock axis and rear spring axis, is there any notable difference between how much of lift one gets along the shock axis for a given amount of lift along the spring axis?
Click to expand...

It's not perfectly 1:1 or linear, but it's close enough within small distances, like you're probably going to expost about 3/4" more shaft if you install a 3/4" spring spacer. I would not suggest that 4" of lift can go with 4" longer shock or that a 4" longer shock calls for a 4" bump stop extension.

SSTJ said:
2) Forgetting shocks for a moment: given the angle of the rear shock axis (vs vertical), how much vertical lift does a TJ get for every inch of lift of lift along the spring axis?
Click to expand...

The coil sits on the axle so close enough to consider it the same. If we had big long travel suspension with a coilover attached at some point along the length of the radius arm, then that would be different.
 
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