Suspension Examination:

[n_green]

So whilst I measured the spring length I never measured the gap between the bottom of the fork cap and top of the ICS piston. Some pistons were 107mm length, some were 100mm length. How much preload would be on each spring?

[Jakobi]

Originally Posted by n_green

So whilst I measured the spring length I never measured the gap between the bottom of the fork cap and top of the ICS piston. Some pistons were 107mm length, some were 100mm length. How much preload would be on each spring?

I don't think there is any difference in pistons. Just the spring lengths. I didn't measure the length from the cap down either.

The piston purges the oil through the center and when bled will sit in the same position regardless of the spring. If the spring is too short, as in the case of Natos (-3mm) then the rod won't extend all the way back on its own. Pulling the leg out will still pull the piston down into the lowest position.

[n_green]

Ah sorry I meant springs not pistons. What I was trying to say is even through the ics piston always sits at the same height, obviously the longer 107mm spring will have more (potentially 7mm more) preload on it. Or does the 107mm have 0mm preload and the shorter 100mm spring have negative preload??

[nato]

Jake in my case does this mean I am more than likely going to end up turning mine in more tgan others to get the same results?

[Jakobi]

The preload varies by spring length and PFP setting. I don't know if they have any other variances in terms of the actual shaft length either. Minor variances can occur depending on how the loc nut for the comp clicker is positioned, however even between several clicks its not a lot of difference in length.

Basically, when the rod is bottomed into the cart it displaces a set volume of oil. This oil then displaces the floating piston (IFP). When the cart is over filled for bleeing the volume pushes the piston up high enough that it slides over the cutway/taper in the rod and purges air and oil through the center of the IFP past the clearance of the seal.

So once bled, with the rod fully compressed, the piston will sit with the seal at the edge of the taper on the ICS shaft. This should be the same for all, with some variation for poor bleeds as the air is compressable. At this stage the ICS spring is pushing back against the IFP on the oil. It forces the IFP back down, extending the rod. If the spring has negative preload it won't extend all the way.

You could mathematically calculate the position of the IFP at full extension quite easily by working backwards.

So in short, yes the length of the spring has a profound change on preload. The PFP runs over a range of about 10mm (say 1mm a turn). So in theory, with all things the same, If my spring is 107mm and yours 100mm, you running your PFP at 7 turns in would be similar to mine at 0. A bloody big difference.

Glenn did the calcs at some point (which I also need to do), but he advised that with a spring that holds the piston against the stop, that the ICS would compress 7mm just through assembley. My piston floats around 6-7mm from the stop, so I would have a little preload once assembled. Working backwards I think Natos worked out to be -3mm (and I wound on a few turns of PFP).

All of this number crunching has me questioning the length of my spring again though and wanting to do more testing.

Green, with your ICS assembly standing piston side down, slide the IFP and ICS down towards the valve stack. Where is the spring in relation to the cap? Mine will sit with the edge of the spring just below where I can slightly preload it and push it aside. Natos floated around in there I think. I also notice some variance from one side to the other. My guess is that even stock, the springs are all over the shop.

[Jakobi]

Really got me wondering what the length of my springs are now

I checked the notes on Nathans and it definitely is 3-4mm difference in ours when measuring from the stop back to where the piston floats against the spring (when laid out on the bench) Exposed part of shaft below the piston.



I'm thinking I need to pull mine down again, but what would make sense is that Glenns were 107mm which would put them against the stop. Natos I wrote down the spring dimensions and am confident it was 100mm. So working off that mine would have to be 103mm. I also found some notes from the good man, Dave at STM Suspension who said he's measured a set around 102mm.

Like I said earlier, there seems to be some variance across the springs, which no doubt makes for some very interesting results across the board.

[n_green]

Hmm mine are back together now and I didn't pay particular attention to how much free play the spring had when the ifp was held down against the BV. But I estimate it would be 5-10mm max. Probably 5mm.

But my shaft does extend fully on its own so obviously there is some preload on the spring, even if it's 0mm. Dunno next time I have them apart I'll pay closer attention.

[nato]

Ok interesting. Last weekend a 2 day ride i found them to be soft and felt low in the stroke just sitting there? I wound a full turn on the pfp after the first run and felt instantly better so I'm thinking i will go another 2 turns just for experimental reasons and see if that helps with the "soft settled" new springs. 2 turns is excessive i know but they are lighter springs than standard so shabby right Thanks for all the info in this thread guys its great.

[Jakobi]

You'll notice 2 turns for sure, but in comparison mine at 0 turns is equivalent to yours at 3 and that without taking the lighter rate into account. I find around 1/4 turn has been working for me. Your base valve has also been setup firmer than mine. How many hours you racked up since you got them back? I serviced mine after yours, and have done mine again since. Found quite a bit of contamination in one of the legs. Didn't find any signs of damage/wear and had thorougly inspected, so put it down to the springs breaking in to a certain extent. Will be pulling them down in another 20hrs to inspect again. On another topic, I think my trail tech has been on the blink. It eats batterys, and I think stops reading at times (possibly over volting). As such the time I thought I put on them could be wrong. Ride before last which was a fat loop of around 80kms of singles only showed as 40kms on the taco. These things are shit and nothing but trouble!

The other thing to note with this, is that the purpose of the ICS spring is to keep pressure on the fluid to fight off cavitation. By having negative preload the initial action will be very soft, but also defeats the purpose of a closed chamber fork.

[nato]

Hence the reason it felt better with the extra turn. See how i go if i don't find it good enough i have the original springs still for pfp.I have only put 2 tanks of juice through unfortunately.
I agree with the trail tech mine went to the restart screen last ride so will throw a battery in to see if thats all it is.