Long vs short shock spring?

[stopintime]

With the great new fork, I need to follow up with a heavier shock spring. If my notes are correct... the first WP shock I had was 150N. It was extremely long. My current unknown rate Bitubo has a short spring and I've been advised to MAYBE get a 120N for it.
Higher numbers for a longer spring to work as a shorter?

[ducpainter]

I could be wrong, but I think the spring rate should be higher than the 150 you already have if you want stiffer.

Maybe Eric can confirm.

[stopintime]

I could be wrong, but I think the spring rate should be higher than the 150 you already have if you want stiffer.

Maybe Eric can confirm.

The 150N (?) WP was replaced a while ago. My current Bitubo has a shorter spring. I can have it measured before I order the new, but it will delay the project. AFAIK Bitubo doesn't offer more than 120... That's confusing if 150 was correct for the WP

[ducpainter]

For a given wire diameter, a shorter spring is stiffer.

Did you replace the original WP spring with a lighter or heavier one?

My limited understanding of suspension tells me that if you required an x rate spring on a bike, you'll need the same rate spring on that bike regardless of shock manufacturer. The spring rate being determined by your weight and the linkage rate of the bike.

For a given rider, the ST framed bikes require stiffer springs than the 851 framed bikes due to linkage rate. The SBK's require lighter springs for the same reason.

You may have to look to another manufacturer for a spring.

[MonsterHPD]

If you still have the "150" spring I'd  suggest you try and have it measured for spring rate to establish a base value. In my experience, 150 N/mm would be a very stiff spring indeed;  can' remember I ever saw one, really.
Spring rate is independant of shock brand, but of course the linkage makes a big difference. Don't remember, what frame do you have?

[ducpainter]

He has an S2R

[MonsterHPD]

Well,
S2R has the same frame as my now-sold M900-2002. The Öhlins shock for that came with a 100 N/mm spring; the St4S with basically the same frame came with a 95 N/mm spring. These springs were on the stiff side for me (60 kg without gear).  I tried an 85N/mm spring, which was definitely too soft, so I now run the 95N/mm spring on my M800 (same frame, same linkage again), which is just fine, really.

The linkage on these bikes is very progressive (35% - 40%, if memory serves) so in a sense a wide variety of springs will give some kind of OK spring rate somewhere in the suspension movement, but for a track day bike you'd want to stay in the early part of the stroke. With a heavy rider, I suppose 120 N/mm would be a good starting point .... but knowing what springs were already tried, and how they worked, would help. Lacking that, my own experience as related above is all I have as reference, but I am quite a bit lighter than Stopintime ....

[stopintime]

It's too bad I don't know what I have (shop closed). The sag numbers and ride issues are clear though. "Too soft" isn't such a problem when both ends are similar, IME.

I'm guessing my current spring is +/- 95 and if 150 was real on the WP..... some of the difference might be due to length...

[Speeddog]

Springs can be made any length at any rate, within the allowable material properties of the wire.
So the length of the spring isn't important from a spring rate view.
It is important for installation, as the positions of the collars varies by manufacturer and shock.

Lars had a 150N/mm spring on his WP shock according to the paperwork, oddly enough I was able to find that in our PMs.
At the time, I thought that was VERY stiff, but I'm not riding where he does.

The Bitubo spring should be marked as to it's rate, somewhere.
Ohlins and some others are printed on the outside of one coil, not always visible depending on how it's positioned.
Some are marked or engraved on one end.
Dunno about Bitubo, not seen one here.

I'd set him up with a 120N/mm spring.

Ohlins would be a -44/120 spring.

Length codes for Ohlins springs are:

1093 -> 150 mm - 5.9"
1091 -> 160 mm - 6.3"
1092 -> 170 mm - 6.7"

The american springs I fit to Ducs are 7" length, 2.25" inner diameter.
Similar american spring to the above would be an Eibach 07002250650 which would be softer, 650 lb/in (114N/mm) or the 07002250700 which would be stiffer, 700 lb/in (123 N/mm).

I've run 100N/mm and 105N/mm springs, and I'm ~180lb/82kg without gear.
That is a slightly soft setup, for general street riding on pretty smooth roads.

[stopintime]

The shop I used for my last service, in Germany, one of few with any experience, said there might have been a sticker on it, but that's usually it. I'd like to know, by testing, so I don't buy and install a new spring that I only hope/guess is heavier.

The ride and numbers don't make sense to me.... For my own amusement/confusement: at max shock compression/suspension travel the long WP spring didn't compress more than, let's say, half it's total range. Shouldn't that mean something?

[Speeddog]

Yes, best to actually find out what you have right now.

~~~SNIP~~~

Shouldn't that mean something?

Nothing relevant to the spring rate.

The equation for calculating the rate (k) of a spring is :

k= (G*d^4)/(8*n*D^3)

G is torsional modulus, a property of the spring material; steel, titanium, plastic, whatever...
d is the wire diameter
n is the number of active coils
D is the mean coil diameter

So a spring designer can play with d, n, and D to get any rate of spring at any size.

[manwithgun]

Some springs that have been individually tested will have their actual rate written or engraved into the flat ground section on top or bottom.  Not a practical location for a mounted shock but is a possibility. 

[Moronic]

As was noted, Ohlins specced a 100N/mm rear spring for the S*R bikes. The S4Rs came with a 105.

I have used both of those, and a 115, and a 120 on my S4Rs at various times.

I am about 170 lbs without gear. Both the 115 and the 120 were unnecessarily firm for me solo. They were still okay, however, and especially with some touring luggage on board.

The 100 I have on now is nice and soft but with an Ohlins shock just about right for a comfortable ride that makes the most of the S*R Monster's long axle travel (148mm). I've not had any problems with bottoming. Seems to me that Ohlins got it about right for average sized people. The 105 gives a bit more stability at the expense of compliance.

Both the 115 and the 120 gave plenty of support for me and a 120 lb pillion.

I find it hard to believe the 150 did not feel like a solid steel rod for a solo rider of just about any weight, if it really was a 150 in the same units.

As others have said, a spring of a specified rate should compress at just that rate, no matter how long it is. A 100N/mm spring of any length should compress 1mm under a force of 100N - or roughly 10kg weight.

[stopintime]

....

Both the 115 and the 120 gave plenty of support for me and a 120 lb pillion.

I'm just 20% less than that.... 

.....

I find it hard to believe the 150 did not feel like a solid steel rod for a solo rider of just about any weight, if it really was a 150 in the same units.
..........

IIRC the sag numbers were ok (~30 something mm), but I can't verify that the spring was 150N. It's just what I was told by the suspension guy. It wasn't very compliant - I remember that. Maybe I had little preload to get ~30mm rider sag and the rest was on the (too) firm side.

[manwithgun]

If your sag numbers are spot on then you might hav a more aggressive compression stack in the new shock.   Or if you're feeling the harshness mid-corner, rebound could be too slow.  I do know that it's futile to troubleshoot suspension on the internet but try opening up the  compression circuit, that's a free mod.    Document where you are now, then open up a circuit, test, and repeat.     

And any discussion of suspension deserves mention of Ducati Up North and their page for reference.

EDIT: This link contains a Ducati spring rate chart, covering many years and models, including what comes stock and what you should have based on rider weight.  It also deciphers the Ohlins spring code.

http://www.ducati-upnorth.com/tech/suspensionspring.php