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Author Topic: Shock Spring Rates  (Read 3632 times)
cencalal
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« on: August 10, 2020, 09:48:09 AM »

Been searching but not finding - can anyone link me a chart of Ducati OEM shock spring rates?  I'm working on my Hyper 796 and the forum for that bike is, well - let's just say not as good as this one.  Race Tech doesn't list it and seems to be on voicemail...
Thanks
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stopintime
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« Reply #1 on: August 10, 2020, 10:50:31 AM »

I don't think Ducati publish these numbers. Not to worry - all you need to know is that they're rubbish - mostly because they're not really progressive - more like sooooft and then suddenly quite hard.

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« Reply #2 on: August 10, 2020, 12:11:49 PM »

Are they Showa forks?
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« Reply #3 on: August 10, 2020, 01:45:36 PM »

I don't think Ducati publish these numbers. Not to worry - all you need to know is that they're rubbish - mostly because they're not really progressive - more like sooooft and then suddenly quite hard.

So they're much like most OEM Ducati Monster fork springs.  Undecided
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« Reply #4 on: August 10, 2020, 01:54:39 PM »

So they're much like most OEM Ducati Monster fork springs.  Undecided

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MonsterHPD
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« Reply #5 on: August 10, 2020, 02:01:00 PM »

Shock spring rates, or fork spring rates?
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« Reply #6 on: August 10, 2020, 02:16:29 PM »

Shock spring rates, or fork spring rates?

I was ranting about fork springs, but now I see I was wrong. However, shitty front suspension will affect the rear, so both ends should be adressed.
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cencalal
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« Reply #7 on: August 10, 2020, 06:08:15 PM »

Yes it's the shock spring rate i'm looking for.  I found a link to a formula to figure the rate by measurements of the spring, number of coils,etc, but the link was dead. I won't give up.
Thanks for the replies regardless.  Over at the Hyper forum at Ducati.ms, ninety six people have read this question and none have replied, which has been typical for this project.  
It's coming along well, though.  I'm trying to both lower the bike (I'm 5'5'', 135 lbs) and upgrade the suspension, using oem parts from Ebay - on a budget.  I'm trying to figure the shock spring rate to use the Hyper 1100 shock. Both are Sachs, the 796 has rebound adj. the 1100 rebound and compression - same length, external reservoir on the 1100.  The 1100 is a heavier bike by 20 lbs or so and I've lightened this 796 about 15 lbs.  The softest spring Race Tech has for the 1100 is 8.5 kg/mm, nothing listed for the 796.

« Last Edit: August 10, 2020, 07:28:00 PM by cencalal » Logged
S21FOLGORE
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« Reply #8 on: August 10, 2020, 11:16:49 PM »

Yes it's the shock spring rate i'm looking for.  I found a link to a formula to figure the rate by measurements of the spring, number of coils,etc, but the link was dead. I won't give up.
<SNIP>


The formula to calculate coil spring's rate is as follows

Symbols

d = wire diameter
Do = coil's outer diameter
D = mean diameter



D = Do -d


E
= Young's Modulus of Material

v = Poisson's ratio of Material

G = Shear Modulus of Material


G = E ÷ 2 ( 1 + V)


k = spring rate

Na = Number of Active coils


k = Gd^4 ÷ (8D^3 na)




Most suspension springs are "closed and ground".



With these springs, Na (Number of active coils) is

Na = number of total coils - 2


Now, here's the catch.

You really can't accurately calculate spring rate UNLESS you know exactly what kind of material is used.

Young's modulus is essentially modulus of elasticity. You can think it as the ratio of tensile stress to tensile strain.

Poisson's ratio is essentially a measurement of deformation (either expansion or contraction) of a material, in a direction perpendicular to the direction of applied force.

Shear modulus of material is essentially the ratio of shear stress to shear strain.
G = stress / strain

Young's modulus and Shear modulus (AKA Modulus of rigidity) can vary with both cold work and heat treating.

The material itself, you can guess, in some cases.
For example, Penske says their springs are made out of "lightweight chrome silicon spring steel",
so, you can assume they are  ASTM A401, then shear modulus can be found in the steel manufacturer's catalogue.

Any calculation that skips Young's modulus, Share modulus, etc is useless.

For example, take a look at this video.



His formula is

k = 9913 x d^4 ÷ (D^3 na)

He says in the video "9913, it's just a shear strength of the steel ..."
well, he clearly doesn't understand the physics.
And his calculation is wrong.

This video's explanation is excellent. (It may not be entertaining. But, if you don't like physics and math, it's not going to be entertaining anyway.)



More practical way to figure out spring rate

If you have access to hydraulic press, you can use it, then calculate spring rate from Hook's law.

Hook's law
k = F ÷ x

 k = spring rate
 F = force applied
 x = distance traveled (how much spring is compressed)

Apply force to spring, measure it, then measure how much the spring gets compressed.

This is actually far better way than the first one.













« Last Edit: August 11, 2020, 12:14:36 AM by S21FOLGORE » Logged
MonsterHPD
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« Reply #9 on: August 11, 2020, 12:05:05 AM »

Agree with Folgore, check the spring you have and go from there.
I have not checked the spring dimensions of the Sachs spring, but there´s a good chance you could find an Ohlins spring that would fit.
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greenmonster
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« Reply #10 on: August 11, 2020, 06:49:53 AM »

Öhlins has springs for Sachs, l had a 95 Nm (me 92kg) on mine.
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cencalal
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« Reply #11 on: August 11, 2020, 07:37:16 AM »

Thanks for the spring theory.  The last little bit about Hook's law tells me I should just send the spring to Race Tech and have it tested.
Their softest spring for this application is 8.5 kg - the stock Hyper 1100 spring is 9.25 kg.  I'll put the 796 spring on the 1100 shock and ride it to see how it feels, and proceed from there.  Got an adjustable link rod to play with too...
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« Reply #12 on: August 11, 2020, 07:41:51 AM »

Try what springs you've got, that's free outside of the labor.

If you're going to be swapping them yourself, be aware they can be dangerous if suddenly released.

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« Reply #13 on: August 11, 2020, 07:55:02 AM »

Most of the Duc stuff I see (that's not Ohlins) are progressive springs.

At best they can be functional for a wide range of rider weights.
But they're not optimal for anyone either.

You should be able to find a local suspension tuner who can test the springs.
Car suspension guys should be able to as well, it's same sort of size as small single-seat race cars.
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« Reply #14 on: August 13, 2020, 06:41:21 AM »

However, shitty front suspension will affect the rear, so both ends should be adressed.

I replaced the Sachs rear with an Ohlins 737 on my 796 Monster... unfortunately, as good as it is, it really shows the limits of the front forks, now.  Adrianni now makes a drop in cartridge set for the 796, I just don't know if I can pay the price of admission.

OP, I've still got the old Sachs rear shock and spring off the Monster.  If you are interested, I can get the numbers off the spring to compare to what you have.
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