Jump to content
YellowDuck

The Suspension Thread

Recommended Posts

Found this on the Ohlins website.

 

The spring pre-load affects the ride height, it does not affect the spring stiffness. Therefore, on models with a linkage to the shock absorber, the suspension may actually feel harder when you reduce the pre-load and the shock absorber gets into the harder range of the link system.

 

Also found this http://fixbroke.tumblr.com/post/39188918110/spring-preload-and-sag-what-does-it-really-do

 

Decent explanation of preload

Share this post


Link to post
Share on other sites

Thanks for bringing up the air gap. Though i have an academic knowledge of it's working, i wonder if any of you have experimented with it and found what difference it makes?

 

I have actually replaced the springs on my last 5 bikes, because the standard stiffness is too low to support my weight (I'm currently 200lbs, looking to drop 10-20lbs) and the forks would simply bottom out during hard braking.

 

The first bike I tried this on was my old YZF600R ("ThunderCat") that I got in 1999. I found that when braking hard, the front fork would go "klonk" (bottom out) and then start to rise up on the front tyre in a rolling stoppie. No matter of preload and compression damping would stop the fork from bottoming out.

I then bought a set of second-hand Ohlins fork springs from a YZF600R friend, who had sent the forks to Traxxion(sp?) in the UK for a full revalve job.

 

I installed them the day before going on a Norwegian motorcycle course, which BTW was heavily inspired by CSS (Wobble MC, the precursor to "Full Control"). There we were asked to practice braking, and it was only when I let go of the brake in the end and the back wheel came back down, that I noticed that I had done a rolling stoppie (and a rather large one too).

 

On my current YZF-R1 road bike, I had a shop install the harder springs and replace the oil. Unfortunately, they put in too much oil, so the spring rate became excessively progressive and rather harsh. When I got the oil level fixed, the effective spring rate became much more suitable for my weight and allowed the fork to pass through more of the stroke, which made the front end much smoother.

 

Before someone says "Wait, spring rate isn't (compression) damping", let me say: Yes, correct, spring rate and compression are too different things. However, the spring rate both determines the static position of the fork (given a specific load) and how much travel the fork will go through, while the compression & rebound damping are only controlling the dynamic position.

 

Let me explain with an example: Imagine that you add 50kg on top of the bike. What's going to happen?

- From a static point of view, it's easy: the spring rate will determine how much more the fork will be compressed. The damping of the compression and rebound valves are irrelevant (we have infinite time to wait for the system to settle).

- From a dynamic point of view, it's a bit more complicated: The speed that the fork will move with, will be determined by the compression valve, but how much the total stroke will be, will be determined by the (effective) spring rate. So with a very high spring rate, you're not going to get much fork movement, and that feels harsh to the rider. That's why you want the spring rate AND the preload to be sorted out first.

Share this post


Link to post
Share on other sites

I agree with khp that getting the spring rate "correct" is about the most fundamental thing you can do to alter the feel and performance of your suspension. I say "correct" in quotes because there is some element of personal preference in this as well. One rider may prefer to have a more rigid suspension under all conditions and so might like a higher spring rate. That setup promotes stability without having to use excessive rebound damping, and helps ensure that travel is always adequate. The compromise is that the suspension may not be sufficiently compliant over positive deflections (bumps) so that traction might suffer in certain scenarios such as accelerating over rough pavement exiting a corner.

 

Another rider might like a lot of compliance and so uses a lower spring rate but increases the preload to prevent bottoming; the tradeoff there is that the suspension will more often be topped out, under acceleration for the forks and under braking for the rear, which can create harshness and chatter under those conditions.

 

Spring rate and compression damping work together to determine the suspension position at any given moment as cornering forces are applied or positive deflections occur, so some will say that they prefer a soft spring but "hold the bike up with compression damping". There is some truth in this but for sustained (more than a second or two) cornering or braking loads it is pretty much all spring rate and preload that determine the maximum suspension compression; as khp explained, under a steady-state (static) condition that is long enough, the spring is eventually going to compress to the point where it supports the load; compression damping just determines how long it takes to get there.

 

Factor in separate high-speed and low-speed compression damping adjustments as found on many modern sportbikes, and you can get pretty lost in all of this... In my experience, compression damping is the most difficult thing to get right because it is pretty hard to make the adjustment and then clearly see the effect of the change on a stationary bike. You can feel it by bouncing the suspension, but not to the same extent that you can see the effect of a rebound damping change on the rate of suspension extension, or measure how a preload change affects sag.

 

Fortunately, the compression damping setting also tends to be the least critical. Setting the sag (preload) is easy - do it once and forget it, at least until your braking intensity or cornering speeds increase to the point that you need more travel. Set the preload, then set the compression damping to some kind of middling best-guess setting, and then by only tuning rebound damping you can get most of the way to a setup that is sufficiently optimized for all but the highest rider skill levels. This is why when a suspension component only offers one damping adjustment, it will pretty much always be rebound damping - it just has a much more profound effect on how the bike handles.

Share this post


Link to post
Share on other sites
Okay, here is one more thing about springing and preload – this is getting into more advanced stuff. It relates to topout springs and how the forks behave near full extension.
In the simple model described by the figure on the previous page, at full extension we have 15 mm installed preload and 10 mm adjuster preload, for 25 mm total. With our 1.0 mm / kg springs in each fork leg, that means we have to apply 2 x 25 kg = 50 kg force before the suspension starts to move at all.
Think about that…under hard acceleration with the forks extended fully, even if compression damping had no effect, the forks wouldn’t move at all unless the force applied to them exceeded 50 kg. So, if the wheel is skimming the surface, any bump that supplies anything less than a 50 kg force is going to be transmitted directly through the suspension to the chassis, as if there was no suspension at all! That is going to feel harsh. Put another way, below forces of 50 kg, the suspension behaves as if the spring rate were *infinite*.
In practice it doesn’t work like this in modern suspension, because the phenomenon of “hard topout” has been eliminated by the incorporation of topout springs. These are (usually short, stiff) springs that are engaged as the forks approach full extension. Here is a diagram from Ohlins that shows the typical configuration, with the topout spring installed inside the cartridge.
So, when the forks are at full extension, the force exerted by the main spring is countered by the topout spring. Neglecting the weight of the wheel and lower suspension (just to keep things simple), at full extension the force from compression in the topout spring will equal the total preload force in the main spring – the forces are in balance and nothing moves.
But because the spring forces are just exactly in balance, *any* additional force will cause movement. If you hit a bump, the main spring will compress a little more and the topout spring will extend the same amount - and you will get some suspension movement rather than complete rigidity.
But here’s the weird thing…when both springs are engaged in this configuration, their rates *add*. Not obvious why that should be, is it? I can explain that later if anyone likes, but maybe for now just take my word for it. This means that in the range where the topout springs are engaged, the suspension will still move in response to an external force like a bump, but it will have a much higher effective spring rate and so the stiffness will be higher than normal – but at least the effective spring rate is not infinite! This diagram from Andrew Trevitt shows how the effective spring rate changes with suspension position, with and without topout springs.
How the suspension behaves near full extension depends significantly on the length and rate of the topout springs. With short, stiff springs (most common in the past) the suspension will be very rigid near full extension, but once it compresses even a little the topout springs are disengaged and the rate is determined just by the main spring.
By contrast, incorporating long topout springs allows the engineers to use topout springs with a lower spring rate, so the suspension will not be so terribly stiff near full extension – it might be just, say, 3x as stiff as normal. But, the range over which that rate applies is much larger because the springs are so long. So, you could be under mild acceleration with the forks just barely extended past the static sag point, and the topout springs are already engaged, making the suspension feel very rigid.

Share this post


Link to post
Share on other sites

The other weird thing about topout springs is that they can mess with your sag measurements.

 

Look again at the diagram from Ohlins that I linked above. If the topout spring were not there (i.e., you had "hard topout"), then the fork would have the same fully-extended length whether or not the main spring was installed. Once it is fully extended it is fully extended - it doesn't matter how stiff a spring your squish in there.

 

But, with a topout spring, when you install the main spring (with preload - there is always installed preload in a fork), the topout spring compresses enough to balance the preload tension in the main spring. This allows the fork to extend beyond what previously seemed to be the fully extended position. This extra extension, equal to the distance the topout spring compressed, is marked as "growth" in the top left of the diagram. The upper part of the fork is higher in the right figure than it was in the left figure.

 

Get it? Okay. So say now you are setting sag. Rider is aboard and the topout springs are disengaged, so only the main springs are supporting the weight. You measure 35 mm total sag. You crank in 5 mm additional preload. Sag will now be 30 mm from the *original* fully extended measurement. That's normal - add 5 mm preload, lose 5 mm sag. But if you re-do the fully extended measurement, you will find that it is greater than before. For example, if the topout spring rate is 2x the main spring rate, then in the fully extended position the fork will have "grown" by 2.5 mm when you added 5 mm preload (since the topout spring had to compress by 2.5 mm to build enough additional tension to counter the additional 5 mm preload on the main spring). So, your actual sag from the *new* fully extended position is going to be 32.5 mm, not 30 mm.

 

It gets even stranger on certain modern suspensions where there are long, soft topout springs that are normally engaged even with the rider aboard. Now, even your static sag position will not change in 1:1 correspondence with the preload you add. Instead, the reduction in sag will be equal to the added preload x (1 - main spring rate / topout spring rate). I think. :)

 

And that is pretty much all I know about springs and preload. Shall we go on to linkage rates? Or am I just talking to myself at this point?

Share this post


Link to post
Share on other sites

I was wondering if there was going to be a follow up post on the topout spring since that is what I ran into on my 636. I was having my son take measurements and they just didn't make any sense after making adjustments. I was almost convinced he was reading something wrong. That led me to research this same topic.

 

BTW, keep going, some may not comment but I am sure plenty are taking it in.

Share this post


Link to post
Share on other sites

Kind of inline with your talk about the top out spring and the preload~

 

I recently helped install springs in a 2009 CBR1000RR and ordered new fork springs while replacing seals and oil and lower bushings. According to the online calculators it was calling for me to get .90kg/mm springs replacing the oe (approx. 1.02kg/mm progressive springs that had tons of built in preload ~ something like 60mm)

 

However no mention to how much preload to cut the new spacer to.

No mention of how much the top out spring will actually compress

So it was really a big guessing game on cutting a spacer and installing for me.

 

I ended up first cutting what measured to be 20mm built in preload, but once actually installed it felt super soft and I could actually pull up on the cap and fairly easily have zero preload on the new springs. So I assume the top out spring compressed quite easily and perhaps it even compressed 15mm? before I one handed pulled on the cap and showed zero preload~ I am not even sure how one would measure the true "as installed" preload on these new springs seeing that the top out spring is hidden and also cannot be measured as to how much it is compressing

I did end up cutting new spacers that would appear to have 35mm preload- knowing it really didn't, but not knowing what it really did have

 

Perhaps this is too many problems all mixed together as one?

 

Maybe the spring rate is actually wrong?- how does one really know what is the correct spring rate?

Maybe the built in preload is wrong?- how does one figure this out, besides trial and error?

Maybe it is some combination of both?

 

As set up above, after doing some preload cap adjustments it ended up with something like 23mm free sag and only 25mm rider sag

 

I assume~ and could easily be incorrect~ but the springs are too stiff with not enough preload? (140lb rider with all the gear on)

Shouldn't the numbers be more like? 15-20mm free sag and 25-30mm rider sag

Share this post


Link to post
Share on other sites

I'd need to sit and think a bit longer about this to give you the best answer, but a few things off the top of my head.

 

1. The CBR1000RR is often mentioned as one of those models that have long topout springs that mess with preload / sag adjustments. So no surprises there.

 

2. Yes, having a really small difference between free sag and rider sag is normally diagnostic of a spring rate that is too high. But remember that having topout springs engaged creates exactly that impression, since the effective rate is higher in the top part of the travel.

 

3. In the absence of "normal" behaviour, a good alternative way to set preload is to ride the bike as you normally would with a zip tie on the forks and check how much of the total travel you use. If you are not getting to within 30 mm of the bottom even under super hard sustained braking then the setup is likely too stiff.

 

All that said, yes, 2 mm difference between free sag and rider sag is whacked. If that is indicative of the effective rate in use it is going to feel REALLY stiff. Are you 100% sure you are measuring right (accounting for stiction)?

Share this post


Link to post
Share on other sites

Yes measurements are correct.

 

The suspension felt tons better than before (compared to the entirely stock set up)but possibly it is too plush now? (when leaving the compression and rebound damper settings the same as previous which was nearly dead center of total available)

 

The zip tie would end up about 5mm from the fork bottom now (+3mm zip tie width-so really using about all but 8mm of available travel under hardest braking)This was after a couple sessions of fidgitting with stuff, in the end it was left at 1 3/4 turn out from full on the compression damping and 1 1/2 turns out from full on the rebound damping with 2 more rounds of spring preload wound in. This did not feel harsch it actually felt soft for the bulk of use except when hitting the ripple bumps under hard braking, then it seemed to jolt the bike substantially. Oil level was set at 130mm (service manual spec)

 

As it was in oe form there would be about 40mm of the suspension unused under the same conditions

 

The rear of the bike is still on an oe shock with the spring collar spun to the top to get any rider sag, so that is still on the hit list too, but for now the front was seemingly far more important than the rear, although I think he is quite quickly realizing he needs plenty more addressed.

Share this post


Link to post
Share on other sites

Nice writeup!

Some links of mine:

http://www.risingsuncycles.com/bikespecific/suspension.htm > HRC stuff!!

http://www.gostar-racing.com/club/motorcycle_suspension_set-up.htm> not sure if its 100% correct in there

and preload as a % of your total fork travel (I'll just keep it on the front side as the rear... too many varieties and I dont have the luxury of a bike with rear linkage suspension so to speak)

"A general rule of thumb is that the front sag should be about 30-35% of travel, while the back should be at about 25%. That works out to be 30-40mm at the front and 25-35mm at the back, for most bikes."

taken from here:

http://www.promecha.com.au/sag_preload.htm

 

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
×
  • Create New...