Ducati Monster Forum

The rear spring on my new Penske is too light.  I have it preloaded 21.4mm to give 30mm rider sag and about 3mm free sag.  I understand that ideal preload is more like 15mm and free sag should be 5-10mm.  That's what I'm shooting for.  The current spring is rated at 500lbs and the application is an M695.

How much more spring do I need?  In my quest to answer that question I came across an interesting post with some seemingly sensible math behind it.  I want to avoid the trial and error approach to spring selection, and I don't trust online charts.

http://www.ducati.ms/forums/showthread.php?t=48737 (http://www.ducati.ms/forums/showthread.php?t=48737)

I collected the following data on my M695:

- Bike wt rear = 194lbs (measured by me)
- Rider wt = 180lbs
- % rider wt rear = 55% (measured by me)
- Unsprung wt rear = 40lbs (guess from linked post)
- Force from shock = 55lbs (measured in linked post)
- Linkage ratio = 2.71 (delta sag/delta shock travel, measured by me)

I ran some scenarios according to the calculations in the post, and it looks like I'd be good with about a 575lb or 600lb spring.  That's consistent with a few reality checks from folks of similar weight who also know their actual sag numbers.

Anyone used this approach to help pick a spring?

BK

Are you unhappy with the springs performance?

21.4mm is well within a good range, unless you are trying to change things for a specific effect I think you might be chasing a theoretical number.

If you increase your spring rate 1 rate change you should be at the 15mm range but do not take it that the number is a magic bullet.

You will see the bike sit higher in travel when in cruse mode and it will squat a little bit less. The big bumps will be better absorbed with the mid bumps more pronounced. Small bumps will be noticed less, some prefer this others prefer more feedback.

Your measurements are in the good to go range you are in the "preference" area, meaning now it is more about what type of set-up you prefer (loose of firm)

Thanks, ducvet.  Good to hear that amount of preload is not atypical.  With moderately aggressive riding I'm contacting the external bumpstop and the rear feels a bit soft and out of balance with the front.  I know there is no single magic spring, but I definitely want to be in the right zone, especially since the shock is new.  The available online charts are all over the map!  My former Ohlins shock used a 542lb spring.

Just for giggles I used the data from my bike and plotted out some sag curves for three different spring rates.  I think the calculations might be a bit too simplistic and there are definitely tolerances in terms of accurately measuring the travel ratio, etc. and making certain assumptions about the unsprung weight contribution.  Anyway the charts are interesting.  With a target spring preload of 16mm (indicated by the vertical bar) I would get a predicted total sag of 33mm with a 600lb spring.  About 22mm preload on a 500lb spring gives about 30mm sag, which is what I'm seeing.  The free sag values are off, which leads me to suspect some of the input numbers.

BK

(http://i42.tinypic.com/29oingy.jpg)

BK, thanks for the link to that discussion.

Let me look this stuff over and I'll be back.

Bk
FYI the bump stop is considered part of the travel of the shock, Yes 100% of it. Realisticly if you crushed it that far it would explode. You are supposed to contact the bumper if using close to full travel. Think elastomer like on mountain bikes.

As for the rear being soft I am assuming you have adjusted the damping to firm things up?
If the sag is close (and it is) and the free sag is close (and it is) the spring rate is in the ball park so you might get where you want to be with a couple of clicks . your damping settings should be somewhere in the middle of the range. if you are at the edges of the range something will need changing.

I have a different perspective on all this suspension voodoo.

I ask ducvet what to do...

and the bike feels great.

It still doesn't make me go faster.   :'(

Good point on the bump-stop, ducvet - Penske even lists lb/inch ratings for their various bumper material types.

Clicker settings are pretty close to right at the halfway point on the low-speed.   I haven't tried to tune the compression yet, but I got the rebound in a happy place.

I'm a little skeptical of my 3mm free sag reading because it seemed to hold steady with additional preload.  I did my best to account for stiction.  I think it might actually be less than the 3mm that I measured.

I mentioned above that I dont trust the online charts.  Traxxion lists 650# for an '04 M620 and Penske lists 500# for the same bike.  A few rows down Penske lists 650# for the S2R (same rocker configuration, very similar sprung weight).  On the TOB Ash tried multiple springs starting with the 525# his Penske shipped with, and worked up to a 650# in order to get the theoretically ideal free/rider sag values ('04 M800 and 180lb rider).

But like you said, if the sag values are reasonable and things feel right, it's "good to go" territory.  I still think I want to try a heavier spring for educational purposes  [moto]

BK

Penske sent me three heavier springs to test:  575, 600 and 625lb.

I installed the 625lb spring tonight, preloaded it 15mm, and measured 7mm free sag and 31mm rider sag.

If my road tests are favorable, I will probably go with the 625lb and not bother experimenting with the 575lb.

BK

I actually threw on the 600lb spring and I've been running that for a bit with good results.

Here's some video of the shock in action on a nice smooth road:

84E penske 2 (http://www.youtube.com/watch?v=F9SYp2lG7pY#lq-lq2-hq-vhq)

And on roads with more irregularities:

pmr penske 2 (http://www.youtube.com/watch?v=rBjjPhP7xBs#lq-lq2-hq-vhq)

BK