I just found that 5W was too thin and 10W too thick, but mixing the 2 together gave me an oil whose weight was fine.......Don't sweat the numbers
Weight is how much it takes to move and rate is how much it takes to move an inch, for instance. What needs saying depends on who you are talking to. I have found that when it comes to preloading forks (for some strange reason not so much with shocks), you often have to start way back at the beginning.
For those in the know, does this look to be the OEM fork spring for a /5? This is what's in my bike. I don't know if it's original, or a replacement by the previous owner. It has a white paint mark on it. Not sure if this indicates top/bottom/if it even matters.
I know nothing about \5's but later BMW springs were dual rate with the closer coils at both ends like that one. For this type of spring which way up doesn't matter. Some aftermarket springs are dual rate but with the close coils only at one end in which case the close coils go to the top. You might be able to identify your springs here http://www.bmwscotter.org/topics/suspension-resources/john-chay.htm If you want to identify the rate of your spring it's possible to calculate it by accurately measuring the wire diameter, number of coils and coil diameter. The formula is: Rate =(wire Dia to power 4 * 1470000)/(coil dia to power 3*no of coils) For the higher spring rate subtract the number of closely wound coils and run the calc again. This is not an easy calc to get right unless you know roughly what the answer is. I have it on an Excel spread sheet if you need it. Looking carefully at the picture and running the calc backwards, if you have something like 74 coils with 10 - 12 close wound at each end and the wire diameter is 4.0mm (this dimension is critical) I'd say there is a good chance that's an early soft \5 spring with 18.48\26.88 spring rate. <table border="0" cellpadding="0" cellspacing="0" width="64"><colgroup><col width="64"></colgroup><tbody><tr height="17"> <td style="height:12.75pt;width:48pt" height="17" width="64"> </td></tr></tbody></table>
The only way that the coil will have a dual rate is if the closer wound coils actually become coilbound, in the range that it runs when fitted and working on the bike. And if you actually count them, you will be lucky if any more than half a coil each end becomes coilbound - all the gaps close by the around the same percentage, and very few seem to want to go to zero. I dont know why. In theory it should be possible to make a progressive spring, but the technology simply does not exist to make the gaps accurately enough - remember that if the manufacturers sites are to believed be they are heat treated twice , scragged and powdercoated after they are coiled.
Sorry for delay, I was out riding on the weekend, copped a Laos monsoon in the middle of the dry season getting home yesterday- no wet weather gear of course:eek1 SS, Thanks for defining your own understanding of this, It is clear to me how you could use 'weight' and 'rate' talking about suspension travel, but is not clear to me how you are using 'weight' and 'rate' to talk about a spring... especially through how you then go on below to talk about them... however, working with your definition: This makes perfect sense to me and is exactly how I understood it. If I put a certain weight on my handlebars that put 10Kg extra weight down through the forks, it will move my forks through their travel a given amount, for my linear wound .65Kg/mm springs in my WP50 forks, lets say this is around 8mm. If I wanted to reset my sag to where it was before the addition of the weight, I'd need to put an extra 8mm preloading spacer in. Add another weight and another 10Kg force down through the forks (making an extra 20Kg force on the forks) I'd get another of the exact same extra 8mm suspension travel and need another 8mm spacer to reseat the sag. Every 10Kg extra weight through the forks, regardless of where it is in the range of the travel moves the suspension 8mm through this travel. The rate of the spring is not changing with preload, instead at a given weight on the front suspension, the sag or the position of travel at rest through the potential travel is changed. back to what you said on page 2 This is incorrect to me. 'Rate' for starters seems the wrong word for the shock. To me, preload doesn't change the rate of the shock from the beginning of its travel to the end, it changes the range that this same rate operates within. Putting in an extra 16mm preload spacer in my WP50 front end moves the range of the potential suspension travel 20Kg heavier (ie with 16mm more preload it will take 20KG more weight/force for the suspension to bottom out). It is the exact same spring, and as far as the fork is concerned it is exactly the same rate spring- not higher like you've said. So long as the spring doesn't bind, 10Kg weight still moves the suspension 8mm regardless of 10mm or a 100mm preload.
Cheers, yeah, that is a good analogy and it fits the way I understand it. I have on-the-fly adjustment of rebound and compression damping (on my WP50's) but have to pull the cap off the forks to change my preload.
I do it once on my bikes, write down what I did, then premix next time, or buy the right weight. You are right though, for most use, "in the ballpark is good enough" - problem is that working that out is a serious PITA and expensive if you are going to work through a dealers supply of fork oil one bottle at a time. Starting with something that's probably too heavy (say ATF), confirming that with a test ride, pulling halt out and replacing with 2.5 --- repeat going heavier or lighter came up with something vanishingly close to 7.5 wt. Next time, I just brought 7.5. Not so much going for the perfect mix like 7.6543 - but finding out which of the stock weights is closest it works and is a lot cheaper. Pete
Consider two flat springs. Flat bar of spring steel laying n the table top. Same cross section, alloy and hardening. But one is twice as long as the other. Bolt one end of each to the table. Now when you lift the other end exactly 2", which is stiffer? The shorter one. Ok, now make those flat bars into round bars. Again, same section, alloy and hardening. One twice as long. One end of each restrained. Again lift the free end 2". Which is stiffer? The short one. If it isn't intuitive yet, imagine two strips of ordinary mild steel. Each is 1" wide and 1/4" thick. But one is 6' long and the other only 3". Fasten one end of each to the bench and lift the other end 2". You can do the long strip with a finger tip. The short one takes some hydraulics. inch for inch, the longer strip has less metal displacement and thus requires less force. Now coil those round bars (still spring steel) around a 3/4" shaft. Remove the shaft. Same cross section, alloy, hardening and distance between coils. One was twice as long as the other and coiled is still twice as long as the other. Compress each 2". Which is stiffer? The short one. Now connect the short and long coils end to end and compress the stack with some force. What happens? The force on the stack is transmitted to both coils so BOTH experience some displacement. The long softer coil compresses more (inch per inch) than the short stiffer coil. But both compress. The long softer coil doesn't have to bottom out before the short stiffer coil compresses. This is a variable rate spring and it has a complex spring rate. As the long spring compresses more and more it gets up on it rate curve and more force is transmitted to the stiffer spring which then compresses more and more. Formally, the spring rate is the slope of the curve of displacement vs. force on the spring. Go back to the flat spring bar on the table. If you measure the force required to lift one end 1", then 2" then 3" etc, and graph this against the force required, you don't get a strait line. The rate is not a constant. This is due to the properties of metals in deflection. But in practice you can get it pretty constant by making the bar long enough. Make the bar round and coil it up and the effect holds. Enough coils and you can get a "good enough" constant rate. Engineering is applied physics. "good enough" counts. Vary the spacing of the coils so there is more metal here than there and you can accentuate a variable rate. How you vary the coils (different wire diameter, coil spacing or coil diameter) affects the final "rate" you get. "rate" in this last instance is "rate" at some given compression of the spring. The real "rate" is a curve and you can only evaluate it at points, so the real rate become the derivative of the curve at some point. You hardly care. You want to know what is happening in the first couple of inches of spring travel and the last couple. For the motorcycle you want the fist couple soft to soak up road bumps and the last couple real stiff to prevent bottoming out and to support heavy braking. Preload on a spring means you start out with it squashed some amount to get you up on the rate curve some known amount starting off. You might preload a fork spring to give you a good initial stiffness for the weight of the bike and rider and then things get stiffer from there depending on the shape of the rate curve. For the road bike. you screw around with preload and variable rate springs (including multiple springs) to give you a nice ride and anti-dive performance. Your fork valve "jetting" and fork oil viscosity also effect the fork movement in compression. You change the speed at which the fork can move. Fat jets and a thin oil means the fork can move fast and all the compression work is on the springs. The converse is true. Different jets control the rebound (in some forks ) but you only get to use one fork oil. Then you can add some compressed air acting as a spring... ---------------------- Disclaimer: I working from old memory and I'm almost too drunk to type so I might have missed (or screwed up) some things.
Simpler to put a spring in a jig and measure what happens. And if you do you will find that the rate only changes when some of the spring becomes coilbound, and for some reason the coils are reluctant to do that. So the progressiveness in a spring is next to nothing, which is why Ohlins, Traxxion Dynamics, Maxton and most others who know something about suspension dont use them. And Ontic , you are overthinking this thing, again the theory just doesnt match reality. As you increase preaload the spring reacts less to a given load, and this applies not only when the bike is static, but also when the suspension is moving , both ways, as you move along the road. So it fells stiffer and reacts less to and imposed load, like a bump in the road. It is because of this two way movement that suspension needs some preload in order to work at its best. Most people have found that somewhere between 16 and 20 mm preload works well , and if you get the can get the suspension working to your liking with the preload in this range you have the correct spring. But the balance , front to rear, is the critical setting - you can set up a bike soft, hard or anywhere else that suits and it will still handle reasonably well, if you get the front/rear spring and damper balance correct. Which is why quite a few bikes only have fine tuning provision at one end. Around 10% stiffer at the rear is a good starting point, but the final percentage is dependent on factors you aint ever going to read about in a set up guide, or a suspension thread. And of which I know little, apart from their existence.
No, the progressiveness of springs can be anything you want it to be. Depends on the engineering of the spring. I've never cooked up any spring myself. I pick ones out of catalogs. In the book, the constant rate springs have a displacement and a force value. The force is constant within some tolerance over that displacement. The variable rate springs have a graph of the rate. You eyeball up the one you want. On something like a shock, you might use a constant rate. It isn't truly a constant rate, that is a physical impossibility. But it is so close over a listed displacement that it does the job and is considered constant rate. The reason the coils on a variable rate are resistant to becoming "coilbound" is because some force is always being transmitted to the stiffer section and that stiffer section is compressing even without the softer section bottoming out. Put that sucker in a vice and yeah, you can compress it solid. But it's not like these coils work and then those coils work. They ALL work at the same time. The soft coils just move a whole lot further than the stiff coils. So the first 3" of fork travel are soft while the soft coils compress a lot and the stiff coils compress a little, then the the stiff coils really kick in and the next 3" of travel are a lot stiffer. Tricky to visualize. The force applied to the spring is distributed to the whole thing and it compresses the whole thing...just more here than there. Go back to the flat spring lying on the bench. Suppose the first 12" of spring steel are 1/4" thick...pretty stiff...but then there is a 12" section welded on that is only 1/8" thick..more flexible. Fasten the thick end to the bench and lift up on the thin end. What happens? The thin end lifts up plenty but the thick material lifts up some. Force and hence displacement is distributed over the whole thing. This visualization comes from another ones used to explain the force exerted by screws. A screw is simply a wedge wrapped around a cylinder. Turning a screw and pushing a wedge in a strait line are the same thing. The pitch of the screw and the angle of the wedge are analogous. It can be easier to visualize things in their simpler form. lifting one end of a flat chunk of spring steel off a bench can be simpler to visualize than compressing a coil. But they are the same thing. look under cars and you will see both flat bar springs and coil springs. Exactly the same action, different geometry.
Bringing it back to fork oil...if you keep everything most very clean dearly beloved, and keep track of your math, you can drain the fork oil, reblend it, and just put it in the forks again. No need to blow a lot of cash. Cook up something that looks likely, load it, ride it for an afternoon and if it doesn't suit drain into your antiseptically clean pan after sterilizing the drain area on the bottom of your legs, reformulate it and try again. I actually go through this BS with wax formulations for candles. When I am looking for a particular formula that works well with a particular wick at a particular diameter I blend again and again and again, keeping track of the ratio of each of 3 types of wax in the current batch. Do a test burn, melt it down and adjust the ratios, recast, test burn, melt it down again, etc. Pain in the wahzoo but when I finally get my ratios I'm done forever. You might start out with something that you know is too thick and then do a couple rounds of thinning it until it is just right. When you get done you have a nice jug of just the right stuff and a formula to make more. Just don't store it at cold temperature. Keeps forever..or long enough anyway. A side benefit is when the fork oil threads start (or rally campfire convos) you mention that you use a custom tuned blend. Only the proletariat use off-the-shelf stuff.../sniff You can determine the viscosity of your base blend. Paint shops (like auto paint) have the little viscosity measuring cups you need. Dunno if the range is appropriate however. I image they should be, I've cooked some very thin lacquers with them.
On progressive springs? I think both camps are right at the same time. I think it works both ways to some degree but I think the coil binding side of the story holds the most weight. 16-20mm preload means you have the right spring? You can have have way more than that and have a spring that works just as well as one with less.
Thanks for the explanation, (and sorry for the thread drift from oil- I'll stop after this) I'm just killing a little time, when I have it (in spades for two days at least now), and don't consider thinking and conversing in an attempt towards clarification of opinion and experience as overthinking. I don't post much on forums but occasionally I do like to hash things out a bit, apologies for anyone I've annoyed. Again, I have not and am not arguing for using nor aiming to use a spring with no preload. Of course not. All I am emphasising is to first use a spring that is the right rate- then use preload. It seems many consider the stock springs too light, including myself, and consider 'progressives' too heavy (including myself again, though I find them preferable)- thus it was a point I thought deserved making. As far as 'theory' vs 'reality', I understand that there is probably a grey area that is being talked about here- where the riders reality (say adding more preload giving a 'feel' of a higher spring rate) is the most important factor (even if it contradicts the physical reality). However in appreciation of actual theory and laws of science and so on, I also understand that even though the rider may feel like the spring is operating at a higher/heavier rate, it is not. If there is something else at play here I be happily corrected. Within the recommended range (itself a very interesting debate), if extra preload gets the job done, then all well and good, and the spring is probably within that grey band of what is suitable for that rider/bike/application. Could a slightly heavier spring without the 'extra' preload (but still with preload) have also got the job done? IMO probably. Which one would be better? I wouldn't have a clue, and no doubt in side by side use it would come down to the rider preference- at least both these examples are talking about being in the range of what is OK. Personally I do not like the feel of a spring that is too light- not at all- and I do feel that I can differentiate between the rate and preload. I've felt it obviously on the first springs that came in my WP50 forks, they were way too light for an airhead and significantly heavier springs feel excellent- I'd love to test side by side slight differences in rate, but that will have to wait for a bit. Anyway, thanks for the conversation all. Back to oil, I'm using Pentrite full synthetic ATF fluid in the WP50's and it feels pretty good. Rated as equivalent to 10W fork oil (by them). Cheaper than good fork oil, is available at any oz auto junk supermarket (repco et al)- as is commonly a problem for me when I need it. Penrite used to have a page on their site last year with a lot of details of various fluids of theirs (with other names such as ATF or powersteering fluid) that are usable for fork oil, with proper product specs and equivalent weights. I started a brief thread on it here but they have since changed all the links I made (and I failed to note the actual products they listed) and have now bottled and marketed actual fork oil. I highly suspect these oils are actually just the previous equivalent product they used to recommend, rebottled and labled, and I highly suspect that the price advantage has now disappeared. I am going to try to talk to a Penrite rep when back in oz to figure it out, as eventually I want to play with different weight oils in my WP50 fork legs with their independent adjustable rebound and compression legs. Cheers, lunch time
The point of the theorizing is to hopefully indicate a particular direction to go in if you want a particular feel. Swapping springs around can get costly. Experimenting with the preload and the oil is super cheap. You might find a preload that makes it work the way you want except you had to give up too much travel. So then you would go looking for a spring that had the same stiffnes/progression but didn't cost you so much travel.
I didnt say preload outside 16- 20 mm didnt work acceptably well , just that 16- 20 mm seemed to be the optimum. This is based on every spec sheet I have ever seen for a new shock - all have been supplied with 18 mm preload, and the instruction not to change it by any more than 2mm either way. All have worked very well this way, and even better when I dialed in the front to match. I once bought an almost new Ohlins GS shock for $200-, which the owner couldnt get to work , and he replaced it with a cheap and nasty WP which worked much better, for him. Problem with the Ohlins was the spring - although marked as a 450 lb it actually measured almost 475, and he had reduced the preload to 6mm in an attempt to make it work. It didnt. But with a actual measured 450 lb spring and 16 mm preload it was just about perfect, with stock HPN insert front springs. Good result, and I was left with the feeling Ohlins just might know what they were talking about. Even if they couldnt make a spring to withing 25 lbs, or wind one to close enough tolerances to make it progressive ----.
Sure they say that. It sells springs. The fact of the matter is that if the different springs have the same rate it makes no difference.
