Difference between Stiffness and RestLenghtFactor

Replies to This Discussion

Permalink Reply by Weiyi Tan on February 1, 2014 at 6:09pm

do you know what is the difference between a regular string and a rubber band? 

Permalink Reply by Cy Wa on February 3, 2014 at 2:58am

Hey,

you mean, that a regular string isn't flexible but a rubber band is?! With flexible I mean the ability of extending and compressing.

Permalink Reply by Daniel Piker on February 4, 2014 at 4:55am

Hi.

Good question.

Simply put, rest length is the length the spring tries to reach, while stiffness is how strongly it tries to get there.

It is true that in some situations you can achieve the same effect by altering either of them, but it is good to understand the difference.

For instance, if we take a spring which has rest length 1 metre and stiffness 1000, and move the ends apart until it is 2 metres long, the spring will be in tension and will apply a force pulling its end points towards each other.

If we then double the stiffness to 2000, the force pulling the ends together will double.

Alternatively, if we kept the stiffness at 1000 and reduced the rest length of the spring to 0m, the force acting on the ends would also double.

This is because the force is calculated according to Hooke's law, which tells us that the force is proportional to the stiffness multiplied by the displacement - the difference between the rest length and current length.

However, when we release the ends, if there are no other forces acting on it, the spring of rest length 1m will bounce around until damping causes it to settle at 1m long, whereas the 0m spring will shrink down to a point.

The RestLengthFactor input on the SpringsFromMesh component multiplies the starting length of a line and uses that as the rest length.

A RestLengthFactor of 1 means the spring starts out in a state of no stress, while a factor of less than 1 corresponds to pre-tension.

What is called the stiffness in Kangaroo can also be referred to as the spring constant, and it can be calculated in terms of material properties using the formula:

EA/L

Where E is the Young's modulus, A is the cross-section area, and L is the length.

Permalink Reply by Cy Wa on February 11, 2014 at 2:08am

Thanks Daniel Piker, you helped me a lot! So if the stiffness is referred to the spring constant, it is measured in N/m?

Thank you again!

Permalink Reply by Daniel Piker on February 11, 2014 at 7:36am

Yes, that is correct - you can measure the spring constant in terms of N/m.

If you want to relate this to known material properties using the EA/L formula, then E is in Pa, A is in m², and L is in m. More information on the units used can be found here:

http://www.grasshopper3d.com/xn/detail/2985220:Comment:717587

Note that as most common units have Young's modulus of many gigapascals, you may need to increase the particle masses by several orders of magnitude to keep things stable (remembering that these can be fictitious masses, not affecting the final equilibrium position).

Permalink Reply by Cy Wa on February 11, 2014 at 1:43pm

Thanks a lot!