-------- Original Message --------

Subject: Spring Rate / Spring Length Calculation

Date: Tue, 26 Feb 2002 05:23:28 -0800

From: Mike Craig

To: sv650-l@LISTSERV.HAWKGT.COM

The formula for spring deflection is:

d = (64)(P)(r*3)(N)/(G)(D*4)

Where d = deflection

D = diameter of the spring material

P = downward force

r = radius of the spring coil

G = modulus

N = the number of active coils

(The active coils equal the total number of coils

minus 1.75 coils.)

r*3 = r raised to the third power

D*4 = D raised to the fourth power

This is an approximation but a very good one.

Knowing that a stock SV spring has a spring rate P/d

of 0.706 kg/mm (from Race Tech); a spring length of

315 mm and 26 coils (12.1 mm per coil) we can reduce

the equation above to something useful.

Let K be a constant equal to (64)(r*3)/(G)(d*4);

substituting into the formula we get

d = (K)(P)/N or P/d = 1/[(K)(N)] or N = 1/[(K)(P/d)]

Remember spring rate is P/d, force per unit distance

deflection. Let's call it SPR. P/d = SPR

So now N = 1/[(K)(SPR)] or K = 1/[(N)(SPR)]

We know spring rate for the stock spring is 0.0706

kg/mm and N is 26-1.75 = 24.25. Therefore

K = 1/[(24.25)(0.706)] = 0.584

>From the above N = 1/[(SPR)(0.0584)]

Total spring length is (N+1.75)(12.1). Let's call it

TL

Therefore for a spring rate of 0.80 the spring length

must be:

N = 1/[(0.80)(0.0584)] = 21.4 active coils

TL = (21.4 +1.75)(12.1) = 280 mm total length

For a spring Rate of 0.85

N = 1/(0.85)(0.0584) = 20.15

TL = (20.15 + 1.75)(12.1) = 265 mm

I have not measured the free space between coils but

keep in mind you will need to maintain enough free

space to equal the fork travel. Don't shorten the

spring too much. My guess is the stock spring is

limited to around 0.95 spring rate. Of course that's

enough for a heavy rider.

Visit Jim Stewart's web site for an excellent method

of cutting the spring (It's not easy) and an equally

good discussion of suspension tuning:

http://www.strappe.com/suspension.html#rate

Please note the constant calculated for the SV(S)650

spring (0.0584) is good only for the stock spring.

Other springs will have a different constant. The

method is valid for other springs but you will have to

know the initial spring rate, length and number of

coils. Then calculate a new constant.

Cheers

to Suspension Tuning

Subject: Spring Rate / Spring Length Calculation

Date: Tue, 26 Feb 2002 05:23:28 -0800

From: Mike Craig

To: sv650-l@LISTSERV.HAWKGT.COM

The formula for spring deflection is:

d = (64)(P)(r*3)(N)/(G)(D*4)

Where d = deflection

D = diameter of the spring material

P = downward force

r = radius of the spring coil

G = modulus

N = the number of active coils

(The active coils equal the total number of coils

minus 1.75 coils.)

r*3 = r raised to the third power

D*4 = D raised to the fourth power

This is an approximation but a very good one.

Knowing that a stock SV spring has a spring rate P/d

of 0.706 kg/mm (from Race Tech); a spring length of

315 mm and 26 coils (12.1 mm per coil) we can reduce

the equation above to something useful.

Let K be a constant equal to (64)(r*3)/(G)(d*4);

substituting into the formula we get

d = (K)(P)/N or P/d = 1/[(K)(N)] or N = 1/[(K)(P/d)]

Remember spring rate is P/d, force per unit distance

deflection. Let's call it SPR. P/d = SPR

So now N = 1/[(K)(SPR)] or K = 1/[(N)(SPR)]

We know spring rate for the stock spring is 0.0706

kg/mm and N is 26-1.75 = 24.25. Therefore

K = 1/[(24.25)(0.706)] = 0.584

>From the above N = 1/[(SPR)(0.0584)]

Total spring length is (N+1.75)(12.1). Let's call it

TL

Therefore for a spring rate of 0.80 the spring length

must be:

N = 1/[(0.80)(0.0584)] = 21.4 active coils

TL = (21.4 +1.75)(12.1) = 280 mm total length

For a spring Rate of 0.85

N = 1/(0.85)(0.0584) = 20.15

TL = (20.15 + 1.75)(12.1) = 265 mm

I have not measured the free space between coils but

keep in mind you will need to maintain enough free

space to equal the fork travel. Don't shorten the

spring too much. My guess is the stock spring is

limited to around 0.95 spring rate. Of course that's

enough for a heavy rider.

Visit Jim Stewart's web site for an excellent method

of cutting the spring (It's not easy) and an equally

good discussion of suspension tuning:

http://www.strappe.com/suspension.html#rate

Please note the constant calculated for the SV(S)650

spring (0.0584) is good only for the stock spring.

Other springs will have a different constant. The

method is valid for other springs but you will have to

know the initial spring rate, length and number of

coils. Then calculate a new constant.

Cheers

to Suspension Tuning