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Old 10-20-2014, 07:08 AM   #1
mjc506 OP
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Suspension set up - rate and preload for sag

Not sure if this in the right area, but I ride a DL650, so suck it

I'm trying to set up the suspension on my bike, but need some advice.

Apparently, the 'optimum' sag settings on any bike are 14% static, 25% race sag on the front, and 11% static, 34% race on the back. That's fine, it makes sense to me (but how were these numbers determined?), but I'm not sure its correct for every bike? (A half-ton Harley with a 50kg rider will surely have different sag settings than a 250kg rider on a TW200?)

Anyway, assuming the above is true, I've calculated I'd need 0.775kg/mm front springs, with 46.75mm preload, and 9.98kg/mm rear spring with 8.33mm preload.

All the online calculators I can find recommend stiffer springs than this (ie, Racetech recommend something around 12kg/mm on the rear).

So which way is 'best'? Low spring rate, and high preload, or higher spring rate with low/0 preload?


mjc506 screwed with this post 10-20-2014 at 07:13 AM Reason: Can't fkucnig slpel
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Old 10-20-2014, 11:34 AM   #2
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I didn't do any of that "calculating" stuff. There is such a huge following of the DLs, I just researched as many threads as I could & came away with a starting point for best spring weight. You'll have to get correct total sag (sack) spring weight first.
I'm about 200 lbs. geared up, & found .90 kg fork springs to be about right. For the rear I'll start with 10.7 kg.
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Old 10-21-2014, 06:44 AM   #3
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25 to 30% of total travel...doesn't matter if its mini 50 or HD road king. Paul Thede of Race Tech offers a good How to article (google it) but Race Tech spring caluculator is often wrong (too stiff). Get Race Sag correct and you are 80% there for suspension setup. On DS and offroad I shoot more towards 30%. My track bikes are set at 25%.
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Old 10-21-2014, 07:10 AM   #4
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Alright, but if we assume that this isn't a DL650, or is sufficiently different to stock (for example, +40mm travel and height on both ends), what then?

I get the ~30% race sag, that works well on pretty much everything, and I understand wanting perhaps a little less sag on the front, as the bike will then ride a little nose-up - more stable at higher speeds, and the front wheel hits stuff first so perhaps needs more travel haha

but to get that 30% sag, you can run a stiff spring with no preload, or a soft spring with lots of preload. A soft spring will 'float' in the travel better, but could be prone to harsh top-out and will not resist bottoming (but that should be the job of the bottom out cones in the damping circuit), and a stiffer spring with no preload shouldn't top out hard and bottoming will be resisted more, but it will be harsher over a series of bumps than the softer spring.

So I suppose the 'happy medium' does depend on rider preferences, but should be between those limits. Based on my calcs, for 11% static and 25% race sag, with the DL650 at 175mm free travel (I'm 85kg geared up, so front wheel weight is 110.4kg static, 140.3kg race) I'd want a set of 0.775kg/mm springs, with 47mm preload. This seems like a lot of preload to me?!
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Old 10-22-2014, 08:27 AM   #5
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The standard that I've always uses is 20 % of total suspension travel. So for example a rear with 6 " of travel needs about 1.2 " of sag to keep the suspension working in its best range. Same applies to the front. If you find that you need to crank up all the preload of the stock set up to keep the suspension either the front or rear in this range then you need to replace the springs. Various good sites on how to measure this so I won't get into it here.

On my vstrom I put a 12kg/mm on the rear and on occasion riding 2 up and loaded I've had to use all the preload available to prevent bottoming out. This is with the bike about 100 lbs over its total GVW. I wouldn't go to a stronger rear spring as when you ride solo it would get too rough.

On the newer vstrom ( can't comment on the previous generation ) mine
is a 2013 there is a small amount of nitrogen in the shock that you will loose when you replace the spring. There is no way to put this nitrogen back in the shock without modifying it. The person who worked on my shock ( a raceteck dealer ) figured that this was put in to prevent cavitation and that I'd likely not notice it gone. He was right I've never noticed any difference with or without the small amount of nitrogen.
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Old 10-22-2014, 08:47 AM   #6
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Quote:
Originally Posted by cycleman2 View Post
If you find that you need to crank up all the preload of the stock set up to keep the suspension either the front or rear in this range then you need to replace the springs.
I agree that this is standard wisdom. But I can't determine any reason why - a softer spring will be plusher over rough surfaces, but will require more preload to reach a certain sag setting. But so long as the spring will not coil-bind before bottom out, and sufficient adjustment is still available on any adjusters (remember, new or longer/shorter spring spacers can be fitted to change preload without affecting the adjusters), why not run this softer spring?

One example where it does make a lot of sense is the rear shock on the Wee - as only so much preload adjustment is available, a stiffer spring will be better able to be adjusted for different loads (pillion, luggage).

Another small note - changing the spring should in no way mean tearing into the shock itself, there should certainly be no loss of oil or nitrogen. The preload adjuster, on the other hand, may ingest a little air if fiddled with when removed from the shock, but this will soon be compressed into nothingness when loaded back up again!
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Old 10-22-2014, 05:32 PM   #7
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Once you take the tower off a modern shock any pressure inside the shock is released, ie: nitrogen, if you keep the shock upright you won't loose any oil. Older style shocks where a sealed unit and the rod wasn't hollow like the newer shocks are to allow for adjusting damping. It really just depends on what type of shock you are talking about.

You have to take the tower off the shock to remove the spring, there is no other way to remove the spring. Sometimes the tower ( with the rebound adjuster ) is on the top or the bottom of the shock, as it sits on the bike, but this is the end that always has to be removed to replace the spring.

What you are trying to do with an initial 20% sag rate, is to keep the suspension within its operating limits and without bottoming out under load. This initial setting is a guide and the rider may want the preload slightly stiffer or softer.

I've seen stock bikes rear suspensions use up just about all the preload to get the 20 % sag with nothing left for a passenger or a load. If you want a good handling bike, this is not a good idea.
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Old 10-23-2014, 03:47 AM   #8
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I don't want this to turn into a 'how to change a shock spring' thread, but I would recommend you do some research yourself - it sounds like your suspension guy has done you a disservice, I just hope that he's caused no damage, but would recommend you avoid him for any future work.

I understand and agree with the reason for sag, no questions there. What I want to understand is:

  • Is there an advantage to having a stiff spring with zero preload, over a softer spring with lots of preload?
  • Is there a limit to the amount of preload than should be applied, and why?
Or... is the DL just too heavy to worry about static sag (and if so, why?)
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Old 10-24-2014, 07:08 AM   #9
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Starting to answer my own questions now, which I suppose is good

The reason I was tempted by soft springs with lots of preload, is that I imagined it would give a plusher ride. This is technically true, but I have now roughly calculated that the force due to damping (when hitting a 1" square bump at 15mph) is approx. 5 times the change in spring force due to the same dump. ie, the spring makes very little difference, and any change will be swamped due to the damping force.

So, while I'd still like to determine the reason for recommending a particular static sag as well as race sag, instead of just specifying race sag and aiming for 0mm preload, on the heavy DL I suspect it makes little difference (apart from making springs hard to find - for zero preload, and 25%/34% race sag front/rear, I get spring rates of 1.6kg/mm front, 20.2kg/mm rear, and I'm 85kg geared up)
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Old 10-24-2014, 09:14 AM   #10
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Spring preload is an often misunderstood concept. Without delving too far (actually I'll just skim the surface) the spring preload does not in any way change the spring rate. The spring has the same rate regardless of how much or how little preload is applied. You need to learn understand differences of preload force vs preload length. Consequence of too much spring preload that results in the suspension being too extended is that there will not be enogh travel available for the suspension to extend into dips/holes etc. Too little preload results in wasted ground clearance and cause bottoming.

This leads into suspension sag/rider sag and another topic entirely. Bottomline, the correct sag depends on quite a number of things but in general (as I previously stated) should be approx 30% of total travel. Yes, proper sag numbers can be arrived with dramatically different spring rates. With either a too soft spring with much preload or too stiff spring with little to no preload the quality of the ride can suffer. Too soft and you will get suspension that dives/easily bottoms. Too stiff and ride will feel harsh as force is building too quickly and not move enough. You want a suspension that uses all of its travel with matched valving to spring rate.
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Old 10-24-2014, 10:13 AM   #11
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I know the difference between preload and spring rate. And preload force - spring rate * preload (mm), nothing too complicated there. Please assume I know what I'm talking about, and can revalve as needed.

This is what I'm getting at. What is 'too much' preload, and what is 'too little' (I'm excluding mechanical concerns such as coil bind, I'm just gonna go ahead and assume that I'm not going to need that much preload...)

The dirt guys seem to have it figured with measuring static and race sag, and adjusting spring rate and preload to get those figures. But either my DL is too heavy for these values (fine, but then what values should I use?) or I do actually need to run 47mm of preload in my forks, with .775kg/mm springs.

Please feel free to play with this:
https://docs.google.com/spreadsheets...it?usp=sharing (I think that'll allow you to download and edit?)
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Old 10-24-2014, 10:55 AM   #12
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Your asking specifically about 'free sag'. On a DL type bike I'd be shooting for 8mm +/- 3mm. Race Sag must be correctly set prior. Top out springs change every thing and many rear springs have max preload parameters from the manufacturer.

Its been awhile since I've done suspension work on a DL so use my numbers as reference only. DL rear has approx 160mm travel. If you set race sag at 48mm but free sag measures out at 3mm what do you do? If target 8mm would you want a stiffer or softer spring? I'd be switching to a stiffer spring with less preload. With the current spring you dialed in too much preload to arrive at proper race sag. The excessive preload is not allowing the bike to sag enough under its own weight.

Nothing is written in stone and understand some experimenting/playing with different spring rates is required for dialing in Race AND Free sag.

You should note that I'm not a professional suspension person. My experience comes from 20+yrs racing both road and offroad and spending countless hours taking notes during practice events, figuring out why I crashed or bike acted in some way and modifying suspension components into the wee hours of the night. Often I proved I have much to learn but feedback from others who I helped tune/adjust since I retired shows I may know more than even I'm aware of. Good luck with your suspension!

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Old 10-24-2014, 11:21 AM   #13
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Ok, so on the front, i have 28mm static/free sag, and 41mm race sag. With 175mm total travel (modified, and measured) which would suggest my current spring is too stiff, and i should go to a softer spring with more preload, going by your 8mm free sag. It does have top out springs in the forks, but I'm well clear of them with my measured sags.

On the rear, which is still stock travel, i have 15mm free and 50mm race, which are both a little low, going off 11% and 34% sag respectively. Again, if working on 8mm free sag, i should get a softer spring with more preload. (i am assuming you're measuring sag at the wheel/axel and not the shock?)

I understand there may be an extra limit on the rear shock for max preload, limiting top out force, as most have no top out protection aside from damping, and of course to prevent coil bind. I like to think that setting the correct rebound damping will prevent damage there.

Thanks for your help by the way. I've had these sag percentages shown to me as if they're gospel (and indeed, they work very well for lots of riders), but I've been unable to determine if they should work for all bikes, or otherwise.
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Old 10-24-2014, 11:34 AM   #14
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Will this help?

http://www.peterverdone.com/archive/springs.htm
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Old 10-24-2014, 11:43 AM   #15
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My prev post was reference to rear only. Front free sag I typically shoot for double vs rear as starting/reference point.
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