Can G force be converted to Newton?

[Mechaman]

I'm sizing a shock absorber at the moment for a project and I have accelerometer readings of a road going over bumps (3 gs). I have already applied gravity to the load to get my 5.5kN

If the weight of the static load at that point is 5.5kN would it be safe to say the spring needs to be sized to take:

F = 5.5 x 3 = 16.5kN

 

[FactChecker]

Yes. For a given mass, acceleration of 3 g will require 3 times as much force as 1 g.

 

[Randy Beikmann]

I'm sizing a shock absorber at the moment for a project and I have accelerometer readings of a road going over bumps (3 gs). I have already applied gravity to the load to get my 5.5kN

If the weight of the static load at that point is 5.5kN would it be safe to say the spring needs to be sized to take:

F = 5.5 x 3 = 16.5kN

I've got a question about your measurement. Where did you measure the 3 g's at? Was it on the ground itself? On a suspension knuckle? On the vehicle body?

I have a hunch the answer to finding your loads is more involved than it sounds, because the vehicle and suspension are a dynamic system with compliances.

Also, are you sizing the shock, or the spring? They both actually react part of the force from the tire/wheel, and the proportion reacted by each would vary by condition.

 

[Mechaman]

I've got a question about your measurement. Where did you measure the 3 g's at? Was it on the ground itself? On a suspension knuckle? On the vehicle body?

I have a hunch the answer to finding your loads is more involved than it sounds, because the vehicle and suspension are a dynamic system with compliances.

Also, are you sizing the shock, or the spring? They both actually react part of the force from the tire/wheel, and the proportion reacted by each would vary by condition.

Hi Randy, I took the 3 g's from an accelerometer measurement from a study just to avoid any serious dynamic equations.

I'm sizing the spring for now but sizing it for an impact load (16.5kN). Am I right in saying the spring should absorb all of it and dissipate that energy through the damper? Normal riding conditions will be a lot less than that though.

When you say they vary by condition, do you mean a sudden impact load as opposed to a wavy road surface?

 

[Randy Beikmann]

Hi Randy, I took the 3 g's from an accelerometer measurement from a study just to avoid any serious dynamic equations.

I'm sizing the spring for now but sizing it for an impact load (16.5kN). Am I right in saying the spring should absorb all of it and dissipate that energy through the damper? Normal riding conditions will be a lot less than that though.

When you say they vary by condition, do you mean a sudden impact load as opposed to a wavy road surface?

Where the accelerometer was located will make a big difference. The suspension knuckle is probably the most useful, because it will be about the same as the tire and wheel assembly.

Thinking it through, wouldn't the force in the spring just be equal to k*x, so the maximum force is equal to the spring rate k times the maximum deflection x_max?

But the damper force depends on the velocity of its motion. The worst case would be at its maximum velocity, which could be estimated by integrating and filtering the accelerometer signal.

So my take is that you size the spring for the maximum allowable deflection (travel), and consider the maximum stroke velocity in the damper design. In both cases, the appropriate safety factor must be used.