Ducati Monster Forum

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

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.

Are they Showa forks?

So they're much like most OEM Ducati Monster fork springs.  :-\

 [laugh]

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.

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.

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

Symbols

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

(https://www.acxesspring.com/media/coil-spring-mean-diameter-calculation-from-inner-diameter.jpg)

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".

(https://www.acxesspring.com/media/compression-coil-spring-design-basics-end-types.jpg)

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.

https://youtu.be/OG5n1RpVrrQ

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.)

https://youtu.be/vjmzIDvkPuA

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.

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.

Öhlins has springs for Sachs, l had a 95 Nm (me 92kg) on mine.

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...

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.

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.

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.

Improving the suspension was half of the project - I needed to lower the bike as well, and could only raise the stock forks 5/8".  I put on the Showas from the 848 sbk which lowered the front about 1 1/4".  Rear sag was in spec with the stock Sachs shock, so I took the spring from that and put it on the 1100 Sachs shock, along with the adjustable link rod, which I also shortened a bit, so the rear is lower, but I forgot to measure how much.  I've yet to ride it as the exhaust is off and in the bin, waiting for some new pipes, but it looks right.  If I ever find out what rate the original spring is, I may replace it with a single rate spring, but we'll see...

You stated being 5'5", 135lb.

I am 5'5", 125lb with gear on. (Have had the bikes with crazy tall seat.)

So, I somewhat understand your situation.

Maybe you already know, but let me quickly explain about short / light weight rider and altering the ride height.

#1
when you get the rear shock that's tailored to your weight, it effectively lowers the seat.
(When you sit down on the seat, the rear suspension strokes more.)
Not a huge amount, but enough to make difference when you stop.

#2
The adjustable link rod you have there, you know
It is NOT the seat height adjuster, right?

The primary purpose of the adjuster is to alter the swingarm droop angle.


When you do major tampering to the suspension, it is very important to "change one thing at a time".
In your case, I'd say, leave the front end the way it is now, and test ride the bike on your favorite piece of road.

Basically, you want to figure out the "Best swingarm droop angle setting FOR YOU"., first.
(You probably can't lower front end anymore than you already have. So, leave the front end.)

The signs of "Not enough swingarm droop angle"  are

1)front forks tend to top out, thus, front end feels unstable, flighty at exiting corner (especially on rough pavement)

2)In general, the bike tends to run wide under power when coming out the corner (understeer)

The opposite , "too much swingarm droop angle", then ...

1)too steep swingarm droop angle = too much anti-squat effect = not enough weight transfer

2)as a result, poor traction under power.

3)more importantly, it makes the bike more prone to sideway slide (rear wheel stepping outside. Combined with too much rebound damping in the rear shock, it is almost guaranteed to cause lowside.)



In your situation, you are not increasing swingarm droop angle (which means you are making the seat higher).
So, it is a little bit safer direction. (At least, you are not making your bike prone to "sudden rear tire grip loss then lowside".

Thanks for that on swingarm angle. I'd read enough going in that I didn't just get one of those "ride height adjusters" for the rear and just twist it all the way down.  First I removed 1 1/2" of foam from the seat, and when that didn't get me there, and lacking any useful input, I decided to just go for it when I found a set of 848 forks on Ebay for $300.  Low miles, perfect condition.  Same with the 1100 shock, $80.  I've lowered the front of every bike I've owned up to 3/4" and it's always been good.  That said, I'm not looking for track level performance, not with road conditions and traffic as they are these days.  Just want to be able to toss it around a bit and have it feel good, like my Monster!