Suzuki GSX-R Motorcycle Forums Gixxer.com banner

Installed and adjusted preload in mm

4K views 21 replies 4 participants last post by  Anthony D 
#1 ·
I'm hoping some of the racers and builders among you, might be able to shed some light on this. Have you ever measured the installed preload (that is, the difference between installed and free spring length with the adjuster all the way out) on relatively "modern" GSXR forks? Have you measured the preload with the adjuster fully in?

I'm wondering what the typical preload range is (actual preload in mm, not clicks or lines) on recent models, assuming there is a "typical range".

If you've got measured data on shocks, I'd be interested in that as well.
 
#2 ·
You're asking about free spring length vs. minimum preloaded spring length? You're correct that there's quite a bit of load on the spring just installed. It has to be sufficient to hold the weight of the bike. Just a guess, but on the stock K5 600 fork, it looks like there's about 30mm of adjustment in the preload adjuster. The measurements really aren't much on their own, you have to pair it with the spring weight.

For instance, stock springs are .850kg/mm. Let's compare that with a 1kg/mm spring.

Assume installed preload (just a guess)100mm, 30mm of adjustment, and 120mm of travel.

Spring Minimum Maximum Compressed
0.850 85kg 110.5kg 187kg
1.000 100kg 130kg 220kg

The magic is finding the spring rate that can give you both the correct free sag and rider sag numbers at the same preload. I know it's not really an answer to your question, but I'm trying to figure out why you're asking the question.
 
#4 ·
The magic is finding the spring rate that can give you both the correct free sag and rider sag numbers at the same preload. I know it's not really an answer to your question, but I'm trying to figure out why you're asking the question.
The basic reason for my question, apart from curiosity, is that I can't see why this process has to be magic. In theory one should be able to estimate, both a) what sort of stiffness and preload are currently installed, based on easily performed measurements of sag, as well as b), what sort of springs to install so as to be able to achieve either the desired free and race sag, or some other parameter, such as natural frequency.

Besides easily accessible numbers, such as machine mass, rider mass and caster angle, only few other parameters would be needed, such as shock leverage ratio, horizontal center of mass location and total unsprung mass. These shouldn't be too hard to estimate with sufficient precision for useful results.

I had calculated my preload (based on my race sag of about 40mm and known spring stiffness of 10 N/mm with fully wound in preload adjuster) to be about 33mm and I wanted to see whether I was in the right ballpark, or whether I was missing something. For instance, this video got me a bit worried, as a 636 suspension is measured and the results are 12mm installed preload and an additional 8mm of adjustment. It doesn't say which 636 they're from, but all models seem to have comparable stock spring stiffnesses (based on RaceTech) and similar era Showa forks as our machines.

Furthermore the video seems to indicate that the amount the adjuster moves in and out, doesn't translate directly to spring preload. This puzzled me a bit, but then again, I'm not familiar with the details of the construction of cartridge forks. In the video, the fork cap seems to move upwards, as the adjuster moves downwards. This might explain why the maximum additional preload is less than the maximum exposed height of the adjuster.

You have me curious now. Looking into some details, it looks like my assumed numbers need some tweaking. Free spring length is closer to 250mm. So initial preload is probably much less than 100mm. The adjuster marks look to be about 1.5mm, so that would make total travel more like 20-25mm.
Actually, for the K4/5 model, according to K-Tech, free length is 240mm and I just measured the preload adjuster and it's actually 15mm (I was surprised too). Your method of estimating installed preload is basically sound, but you forget that there are two fork springs, so actual preload should about half.
 
#3 ·
You have me curious now. Looking into some details, it looks like my assumed numbers need some tweaking. Free spring length is closer to 250mm. So initial preload is probably much less than 100mm. The adjuster marks look to be about 1.5mm, so that would make total travel more like 20-25mm.
 
#5 ·
The service manual says about 240 is the service limit. A new unstressed spring is a bit longer. And you are correct, I was only using one side for my example.

You're absolutely right. With a linear spring rate, you should be able to find right spring rate where the line crosses 5mm free sag and 40mm sag with the rider. The delta is in the weight of the rider as perceived by the fork (rake angle adjusted). I get the curiosity, but you're talking some pretty complex formula to capture it all. You'd need to know the weight of the entire sprung system right at the point of the lower tripple. Most of the spring slingers already have calculators for getting the right spring weight. Unless you're going for extra credit.....

I remember doing an oil change on my fork and it seemed like I had to compress the spacer a mile to get the nut off. So maybe that's why my initial 100mm assumption was off. Divide that by two and you've got 50mm on each leg. On a 1kg/mm spring, that's about 220 pounds. I don't recall exactly, but aren't the bikes weight biased towards the front a bit? On a 430 pound bike, 220 seems to be reasonable.
 
#6 ·
You're absolutely right. With a linear spring rate, you should be able to find right spring rate where the line crosses 5mm free sag and 40mm sag with the rider. The delta is in the weight of the rider as perceived by the fork (rake angle adjusted).
I assume 40mm is the "one third of the total travel rule", correct? How did you arrive at 5mm though?

I get the curiosity, but you're talking some pretty complex formula to capture it all. You'd need to know the weight of the entire sprung system right at the point of the lower tripple.
It shouldn't be that bad. You only need to know the unsprung mass and the horizontal location of the center of mass. The total wet mass of the machine is available from here for instance, and to estimate the sprung mass, you need to subtract the weight of the wheels and perhaps half the swingarm and part of the fork. These weights are actually available from measurements on a 1000K1 here. (This paper is a treasure trove of information in its own right.)

After that, estimating the weight on the front spring, is a matter of simple interpolation, and taking into account the rake angle is only a matter of a few cosines.

Most of the spring slingers already have calculators for getting the right spring weight. Unless you're going for extra credit.....
They do? The only one I've found, is the one from Race Tech. The problem with that, is that I don't know how it works and, although I have to assume that they know their stuff better than I do, I still find a few things suspicious.

For instance: For our K4/5 and my weight, they suggest springs that are 21% stiffer front, but only 11% stiffer at the back. If the aim is to adjust the suspension for your weight, I'd expect the same relative change for both suspensions.

Ok, so perhaps the people at RaceTech have arrived at better suspension values, compared to the OEM. But then I'd expect the values suggested for the K3/4 1000 to be similar, as it seems to share most of the rear suspension components. The linkage part numbers are the same, the shock dimensions are the same and comparing wheel stroke to shock stroke indicates that the leverage ratio should be similar if not the same. The only difference, is the (relatively slightly) larger machine mass. Still if I change the weight of the machine to that of the K4/5 the suggested rear suspension is still significantly stiffer, compared to the suggested value for my 750.

Anyway, it is such considerations that have incited me to study this a bit and see how one could arrive at reasonable suspension values, at least values that somehow directly map to suspension performance, so that two different people, perhaps on different machines, could meaningfully compare them.

I don't recall exactly, but aren't the bikes weight biased towards the front a bit? On a 430 pound bike, 220 seems to be reasonable.
Both the paper I referenced above as well as another article I can't find right now, contain measurements of the weight bias. It is a bit forward biased, but with the rider sitting mostly behind the center of mass of the machine it probably gets closer to 50/50. In any case very accurate measurements shouldn't be necessary for meaningful results.
 
#7 ·
There is some great information in this thread, thanks for sharing guys...
 
#8 ·
Nearly every suspension setup bible out there calls for free sag to be somewhere around 5mm. 40mm was your number from post #4.

That paper is pretty impressive. For such a detailed analysis, I found it very odd that they did not include the sprung components of the fork for their chassis mass calculations. The inclusion of the crown of the tire and its effect on steering torque during acceleration was very interesting. Especially the diagrams regarding the differences in the front and rear. I need to read it again, but it looks like they missed that the front contact point moves laterally with steering input because of trail.
 
#13 ·
Nearly every suspension setup bible out there calls for free sag to be somewhere around 5mm. 40mm was your number from post #4.
I think race and static sag should only be considered together. It's easy to show that the sum of these two sag numbers directly relates to the mass-stiffness ratio, which mostly characterizes the dynamic response of the suspension.

That paper is pretty impressive. For such a detailed analysis, I found it very odd that they did not include the sprung components of the fork for their chassis mass calculations.
I think they did. The front frame (that is, the forks) are modeled as two masses of which Mff_str is the steering head and is part of the sprung mass and Mff_sus is the unsprung part. It wasn't directly incorporated in the chassis, beacause it's important with respect to its effects on steering response and oscillations.

Incidentally, the model works surprisingly well. I implemented it (with some modifications) in a real-time simulator a couple of years ago. The result felt pretty close to riding the real machine (considering it was a mouse-operated simulation). Here are a couple of videos onboard and showing the model.

You seem to have this idea that there is "a" proper setup, and you seem to think that achieving the proper sag is the goal. It isn't, like i said, we haven't checked the sag on any of my bikes in years, and i get around the track pretty good.
I can assure you, I am under no such misconception :). If anything, I'd be hard pressed to cite any single parameter, that had a "proper" value in engineering, regardless of the conditions of operation. I therefore have no intention of devising a magic formula that will calculate optimal parameters for a man-machine combination and then insist whoever doesn't like the them is plainly wrong. I also never implied that the "proper" rate, if it existed, would be related to body weight; I mentioned considerations of (geometric and mass) parameters of the machine as well.

In fact I appreciate (to the extent my experience allows me) and agree with everything you've said. Nevertheless you're someone with very particular requirements and who's in the position to experiment with many different setups in order to satisfy them. As a result you've amassed great experience. Although enviable, that doesn't apply to everyone (or "mere mortals" as Geek puts it). So besides my academic interest in understanding the basic concepts of how suspensions work and how they're designed, I think a process whereby one could determine values that would at least, say, prevent bottoming out under near-stoppie braking, or ensure that the sag won't be "too bad" given the available preload adjustability, wouldn't be entirely useless. I'm not saying that's necessarily possible, just that it would have some use if it were.

Then there's also the question, of whether one can calculate some parameter of a suspension setup that would characterize it, so that it can be used to compare it to other setups, or to estimate other setups. To clarify, consider this: you've arrived, after much expererimentation at some sitffness and preload values, that you like for a particular track. Could one somehow, translate this setup to another machine, given their differences in mass and geometry, so as to estimate initial parameters, to be further refined through experimentation. Again, I don't say these questions have positive answers; they probably don't and even if they had, I'd not be in a position to answer them. But since you brough the matter of utility up, I just say they're interesting and would be useful if they did.

Now, getting back to my original question, you do seem like someone in a position to provide some concrete data. :D
 
#10 ·
Where to start...

There is no "proper" spring rate.

Contrary to popular belief, bodyweight has very little, if anything to do with spring rate selection. For example, a buddy of mine weighs 135lbs and at some tracks uses springs "designed" for someone who weighs 220lbs. If someone has absolutely NO idea what springs to use, then sure, get some for your bodyweight as a very basic starting point. But I can honestly say that we haven't checked the sag on any of my race bikes in 8 years. What matters is how it acts on track. I like a certain feel and stability from the front end, I don't like to use all of the fork travel (some people do), etc. Those things matter.

Bodyweight is a factor in how the bike acts on the brakes, because the bike is trying to slow more weight, but bodyweight isn't necessarily the reason for selecting certain springs.

But even then, the necessary spring rate will change depending on the track. Bike setup changes for every track, including spring rate. That is why we have a suspension guy we work with all the time, they are constantly making changes.

dpapavas said:
I can't see why this process has to be magic. In theory one should be able to estimate, both a) what sort of stiffness and preload are currently installed, based on easily performed measurements of sag, as well as b), what sort of springs to install so as to be able to achieve either the desired free and race sag, or some other parameter, such as natural frequency.
You seem to have this idea that there is "a" proper setup, and you seem to think that achieving the proper sag is the goal. It isn't, like i said, we haven't checked the sag on any of my bikes in years, and i get around the track pretty good.

Getting the bike to handle properly and have good tire wear is the goal. I think that is where the confusion is coming in. Bike setup is a living thing that changes track to track, day to day, maybe even morning to afternoon depending on the weather.

Here is an example. The season opener was the first time I ever rode the 1299S. They set it up with a good baseline based upon how they did my other bikes. We struggled with setup all weekend. At first the tire wear was great...as in TOO great for the abrasive track we were at. It was because I was sitting at the top of the stroke and just spinning. So he changed the rear shock spring to a softer one (ended up changing it again later, so we ended the weekend at 2 rates lower than what we started). Then it had better drive grip, but it was trying to wheelie too much coming off apex (it makes 202hp and 105ft/lbs of torque on race fuel). So we had to play with the wheelie control. That helped the wheelies, but then it was trying to push me wide and wouldn't finish corners on the gas. Not to mention i was bottoming out the forks under hard braking. By the end of the weekend we ended being 2 spring rates lower on the rear, 2 spring rates higher in the front, added a total of 20cc's of oil to the forks, and had altered the geometry several mm on both ends. Not to mention all of the playing he did with rebound, compression, and preload.

The next weekend was at Road Atlanta. I was hitting the rev limiter in 6th gear on the back straightaway and running out of top end, so we had to drop 2 teeth on the rear sprocket. That wheelbase change sent us back to the drawing board and we spent all weekend chasing a "Road Atlanta setup". We made more spring rate changes, preload and geometry changes.

For another example, I run completely different geometry and springs at Jennings than I do Road Atlanta. Obviously my bodyweight hasn't changed, but the track has changed. Different tracks require different setups.

I run different spring rates on the R6, RSV4, 675R, and 1299S. Even at the same track, the spring rates will vary for each bike.

Something else that is interesting. A few years ago me and a buddy were riding the exact same bike, with the exact same motor build, on the exact same tires, with the exact same parts, with the exact same suspension components, and we were running damn near the exact same lap times........but we had DRASTICALLY different setups. That is something else you aren't considering...riding style and/or rider preferences. Setup isn't all mechanical and it isn't all about numbers and equations. Like I said, it is a living thing that changes all the time.

Every time I come off the track, especially since we are chasing setup on the Panigale, Livengood will ask how it felt, if there were times I couldn't get on the gas like I wanted to, how stable the front end is, he will look at tire wear, check the zip tie on the forks, etc. And he will make changes. You cant think of spring rates or setup as one entity that you just plug and go. It just doesn't work like that.


How much riding/racing experience do you have? To be honest, it seems to me like you are WAY over-thinking this. You are asking questions that either don't need to be asked, or are completely irrelevant.
 
  • Like
Reactions: Giorgio_Rp
#11 ·
I think it's more of a curiosity thing really. But TJ, you're correct. A track with good grip will require a stiffer suspension than one with less grip. In a turn, centrifugal force is acting on the compression of the suspension. How much of that force is possible is directly tied to grip in the corner. The article was less about what a good setup is, but moreso what the mechanics behind it all are. I also agree that proper sag numbers are just part of the solution. Let's say you want perfect free and rider sag numbers. There's obviously one perfect spring rate for it. But go back to that gripy track and you may be bottoming the suspension in every turn. Sag numbers just get it close for most of us non-racing mortals. Racing is a different world all together. You don't want the bike topping out all the time, but you also don't want to hit bottom on the suspension mid corner. The individual use case becomes the deciding factor. It's about finding the right range, not the right value. You want a range that ideally will let you get good sag, but also keep you close to, but not touching the bottom at maximum demand.

What really get's fun is when you start playing with the air spring at the top of the fork. That air volume is compressing with every stroke. Changing your oil height can have big effects. I don't see much of this in road racing, but in off road long stroke forks, it's a big component of a good setup.
 
#12 ·
Ah ok, if this is all being discussed out of general curiosity, and not necessarily for riding performance....then my bad.

Carry on. :D
 
#16 ·
I honestly don't to be honest. I am at work and don't have any notes with me. I would have to ask Livengood, he keeps up with all of that stuff. I just know from where each bike started, we have went up 2 rates on the R6 and down 2 on the Panigale.

I do know Ohlins told him the spring on that TTX on the R6 is the stiffest spring they make for that shock. If I wanted/needed more, they would have to design/make one from scratch.
 
#18 ·
I do know Ohlins told him the spring on that TTX on the R6 is the stiffest spring they make for that shock. If I wanted/needed more, they would have to design/make one from scratch.
And when are you having custom "Broome Edition" springs made?
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top