Suspension spring rate calculation question

[austin_ob]

i am having problems with a question i have to do for an assignment and i was wondering does anyone have a solution for it the question is: A suspension coil spring has a diameter of 10mm and a mean diameter of 50mm. the lenght of the spring is 140mm and the youngs modulus for steel is 20600N/mmsquared. calculate the spring rate if there are 8,9 or 10 coils. how much force must be applied to the springs to compress them by 40mm?

[phoenix]

(modulus x wd^4) / (coils * pi() x cd) x (180 / pi()) = spring rate

where wd is the wire diameter (10 mm) and cd is the mean coil diameter (50 mm)

divide spring rate by displacement (40 mm) to get the load required to compress the spring.

The length of the spring given - 140 mm - is irrelevent to the calculation. You should be able to answer why that is......

[greenamex2]

Are they trying to catch you out with the 10 coils? Would the spring be coil bound?

[phoenix]

Originally Posted by Denis Bassom

Are they trying to catch you out with the 10 coils? Would the spring be coil bound?

Good point Denis - but by my calcs 10 coils of 10 mm diameter wire take up 100 millimetres. At 40mm of deflection the coil would just be solid, though.

[greenamex2]

If the free length is 140mm, the wire diameter is 10mm and there are 10 coils, aren't only 9 'gaps' of 4mm and hence a spring travel of 36mm?

Or am I missing something (probably, I spent over a year driving with coil bound rear springs!).

[phoenix]

Originally Posted by Denis Bassom

If the free length is 140mm, the wire diameter is 10mm and there are 10 coils, aren't only 9 'gaps' of 4mm and hence a spring travel of 36mm?

Or am I missing something (probably, I spent over a year driving with coil bound rear springs!).

Maybe I am missing something! If there are 10 x 10mm of wire, then that is 100mm. Yes, there maybe only 9 'gaps' - but they must total 140-100 = 40 mm if the spring is 140mm long, so the gaps must be 4.444mm each - or am I doing something wrong?

[greenamex2]

Originally Posted by phoenix

Maybe I am missing something! If there are 10 x 10mm of wire, then that is 100mm. Yes, there maybe only 9 'gaps' - but they must total 140-100 = 40 mm if the spring is 140mm long, so the gaps must be 4.444mm each - or am I doing something wrong?

Yeah, you're right.

It's been a long day!

[austin_ob]

thanks guys i was struggling there