Need help with spring rates and how to compare them

[bicketybam]

I am replacing the shocks in my Granite 4x4 3S BLX with shocks from a V3 Typhon. I was able to get the spring rates for each and I was wondering how to interpret them. Here is what I got:

Granite front spring rate: 1N/mm
Granite rear spring rate: 0.85N/mm

Typhon front spring rate: 0.75N/mm
Typhon rear spring rate: 0.56N/mm

Granite uses 500 cst shock oil and the Typhon uses 550 cst

How do these specs compare?

 

[Yonic]

I don't think you can compare the shock oils, as the valves and shock body size are part of the equation there as well.

 

[bicketybam]

Some more info:

Granite front springs: https://www.arrma-rc.com/part/AR330538
Length is 2.76" Stiffness is 5.71lb/in

Typhon front springs: https://www.arrma-rc.com/part/AR330235
Length is 2.76" Stiffness is 4.28lb/in

Granite rear springs: https://www.arrma-rc.com/part/AR330539
Length is 3.15" Stiffness is 4.8lb/in

Typhon rear springs: https://www.arrma-rc.com/part/AR330239
Length is 3.31" Stiffness is 3.2lb/in

So are the stock granite springs STIFFER than the Typhon's? That doesn't seem right....

 

[Yonic]

It can be, since just like in the shock oil, the mounting position and the geometry of the suspension is part of the equation as well. So for example, springs which are half as stiff, but mounted further from the wheel - will cancel each effect out, so the result could be the same

 

[woodtoolguy]

bicketybam: It's been a while since I attended any Physics classes, but I'll do what I can to assist.

YES, the Granite springs are stiffer. Let's examine the Granite front spring:
Simply put (and using SAE units) with a spring rate of 5.71lb/in it requires 5.71lbs of force to compress the spring 1 inch.
If we measure the spring length when it is fully compressed and it is 0.76 inches, the spring travel (distance compressed) =
2.76" (open) - 0.76" (compressed) = 2" of spring travel
5.71lb/in X 2in = 11.42lb. That means it will take 11.42lbs of force to fully compress the spring.
We can assume a constant spring rate if the spring wire has the same diameter throughout it's length and the coils are spaced evenly.

Other factors (shock design & engineering)? The piston and the oil inside.

The piston moves up & down inside the shock body. It has holes in it to allow the oil to flow through the piston as it moves.
Small holes = more resistance to oil flow = stiffer shock
Large holes = easier oil flow = softer shock.

The oil "dampens" the spring. That means it resists the springs movement. The oil is what differentiates a shock absorber from a pogo stick.
Thicker oil = stiffer shock
Thinner oil = softer shock

Yonic is correct about suspension geometry. Where the shock lives in the mount and control arm determines it's travel distance and relative forces placed upon it. If we go down that road we include levers, fulcrums and distances into the equation and the physics complicates accordingly.

To answer your original question, YES, the Granite springs are stiffer, but the oil in the Typhon shocks is thicker.
Maybe the Granite shocks are engineered for jumping while the Typhon shocks are engineered for speed running across uneven ground.


Needless to say, shock & strut makers for the motor sports industry spend a lot of time engineering their products.