Hi, I am going to try some spacers when I change my fork seals shortly, however I have read mixed messages about material type and size, from solid aluminium, pvc pipe, nylon rod, old sockets and 20c pieces.
Can someone point me in the direction of the preferred option and pros and cons of each.
I can only find 30mm nylon rod locally, just wondering if it can be a solid spacer or more similar to a washer? I measured the 20c piece and its about 28mm.
Thanks for your help :D
Any of the materials you mention will work. One thing to consider is how much air the spacer displaces, because that will affect the effective spring rate when compressing.
The air space in the top of the fork acts like an air spring and adds spring rate progressively as the fork compresses. Some people tune this by setting oil height so there is more or less air. The less air, the higher and more progressive the spring rate will be.
If you use a solid nylon spacer then you will reduce the air volume much more than if you use aluminum tubing. PVC tubing is somewhere between because the walls are thicker. If you use a solid material like nylon then the spacer will not only add preload but it will also have the effect of reducing the oil height by the same amount. This needs to be taken into account when you set the oil height so you don't set it too low and blow the seals.
Thanks mr72, excellent info, I forgot to think about the air gap and oil height change, now I am reconsidering.
Anyone have advice on sizing of spacer?
Also I have noted on here the air gap of 91mm does that sound right and should I consider reducing / increasing that first? Alternatively how do I calculate oil volume / air gap with the use of a spacer.
Sorry for al the noob questions....
Thanks,
Rather than start a new topic, I am jumping on this one--it seems relevant. I changed the fork seals on my GS500f today--what a pain. The broomstick method worked, but it took a lot of time and many tries for each side. I used a 1/2" drive air impact, a 3/8" butterfly air impact wrench, and a hand impact driver. Anyway, I put in inner valve springs from a Yamaha SR500 on top of the standard spacers. The springs fit right into the fork nuts and stuck out about 1 3/4 inch. I put in 380cc of (gasp) synthetic ATF. Anyone who knows me knows that I do not like to spend money on parts if I can avoid it. I have not had a chance to go out to the mountains and gas it up, but around the neighborhood and over some railroad tracks the front end felt much better. The front end does not dive nearly as rapidly and as much under heavy front braking. The dampening seem about right. Overall, with my R6 shock conversion and the front fork work, I am quite pleased so far. I'll see if I can get to the mountains--southern Appalachians--(less than one hour away) within the next few days. I always appreciate your feedback, whether positive or negative.
Yea ATF is a close enough swap - in fact old school bikes in the 70's may have had ATF from the factory - specifically those KZ400 kawi's if I recall.
Or did they have 10wt motor oil ??? something we would now think as "redneck".
Anyway valve spring as a spacer ??? why not just use a PVC sch 40 pipe ? Yea, you gotta make a lowes run and that's no fun on a bike with a split apart Front end.
Cool.
Buddha.
The valve spring is not all that stiff, so my thought is that i'll get a little more spring action than a solid spacer.
A couple of things:
If you put another spring on top of the existing spring you LOWER the spring rate ( Hooke's law). You may increase the preload ( NOT a good thing overall, but sometimes necessary) but you will lower the rate.
ATF is the equivalent of 25 to 40wt depending on which type. It worked a bit in the pre 80's because the forks were garbage and had internal leaks. It is considerably less advised nowdays ( although the GS500 stock forks are pretty aweful).
Hi density nylon is a fine preloader but Al is better. Hard plastic guttering/plumbing pipe is good and easy to work with. Big washers from building supplies work well and you can stack them until you have the right size then solder them together especially if they are zinc coated.
Getting the correct spring rate is the best way. That way you have the least preload and better spring control. Small bumps will be less harsh and you won't just blow through you travel on big hits.
Thanks for the feedback. You are giving me exactly the kind of information I desire.
I am much more in tune with the empirical rather than the theoretical. I need to be able to make sense of something by observing it. Therefore, I sometimes am in error over things. For example, my empirical observation is that the more I compress a valve spring, the harder it is to compress. However, I read in several sources that the spring rate is constant, unless, of course, one has a progressive spring. Since the Yamaha valve spring is a different wire diameter and coils per inch, wouldn't it act as a kind of progressive spring?
Also, I have read that ATF averages 10w (from 7.5 to 15) although it is really measured by a kind of viscosity index that is new to me. My empirical observation is that it pours much more rapidly than 10w30 oil.
Quote from: ajensen on August 13, 2017, 10:43:38 PM
Thanks for the feedback. You are giving me exactly the kind of information I desire.
I am much more in tune with the empirical rather than the theoretical. I need to be able to make sense of something by observing it. Therefore, I sometimes am in error over things. For example, my empirical observation is that the more I compress a valve spring, the harder it is to compress. However, I read in several sources that the spring rate is constant, unless, of course, one has a progressive spring. Since the Yamaha valve spring is a different wire diameter and coils per inch, wouldn't it act as a kind of progressive spring?
You're confusing total force and spring rate. In a constant rate spring, the amount of
additional force it takes to compress a further increment of distance stays the same.
For example, consider a 5.0kg/mm shock spring. If you set that on the floor and place a 50kg weight on it, it will compress 10mm. If you want to compress it a further 10mm you need to add another 50kg weight. If you're pushing on a spring by hand, you'll feel that it takes more and more total force to compress the spring further.
Quote
Also, I have read that ATF averages 10w (from 7.5 to 15) although it is really measured by a kind of viscosity index that is new to me. My empirical observation is that it pours much more rapidly than 10w30 oil.
This is about right, although different ATF types vary quite a bit in viscosity. If memory serves, the 1978 Honda Hawk I had waaaaay back when spec'd ATF in the forks.
On adding spacers, they do nothing to change the spring rate. They will make the bike ride higher, because sag is reduced. You'll still get as much brake dive, although bottoming is reduced since you start higher in the stroke. That's offset by an increase in harshness at the top of the stroke.
Thanks again. When I get a few bucks ahead, I'll try some progressive springs. As it is now, the front end feels much better, but small bumps do get through. By the way, the oil I took out of the forks--I suppose it was the original--was the nastiest carbon color I have ever seen fork oil. Was all the factory oil like that, or has something discolored it?
That's all the wear metals from your fork bushings. I change fork oil every time I change the front tire. The most important change is the first one, lots of crap in the oil from the sliding parts of the forks wearing in to each other. After that you could probably go every other tire change.
Thanks--good to know. I never had Yamahas that made the oil that dirty, but the oil was quite easy to change on them.
Quote from: ajensen on August 13, 2017, 10:43:38 PM
Since the Yamaha valve spring is a different wire diameter and coils per inch, wouldn't it act as a kind of progressive spring?
The way a progressive spring works is as the spring compresses, due to coil spacing some coils bottom out (bind) and this results in fewer active coils left to compress. Fewer coils == stiffer spring (higher spring rate) if wire diameter (and material) remains the same.
For your new setup to perform like a progressive spring, assuming the valve spring actually has larger wire diameter, it would require the main spring to completely bottom out, all coils in bind. When this happens, then all that remains to compress is whatever travel is left in the valve spring before it binds. The only way for this to occur would be for you to put in a solid spacer big enough to force the main spring to bottom out, and the only way to make that fit in the fork along with your new valve spring would be to cut coils off of the original main spring to make space for the spacer. And a fair amount of math. Now, while technically this would be like a progressive spring, the reality is your valve spring in there would exist mostly to soften the bottoming out, kind of like a bump stop. Ordinarily the air trapped in the top of the damper assy performs this function because air as a spring is progressive and it actually gets stiffer the more it is compressed unlike a coil spring, so it is very good at providing bottoming resistance. Reduce the volume of air in the top of the fork and you will get stiffer response when the springs bottom. Careful though because if you reduce the air space too much, you will increase the air pressure enough to blow the fork seals. And of course there's a way to reduce it so much that there's not enough room for any air plus the oil, springs, spacers, etc. when the fork is compressed so it'll basically blow the seals the first bump you hit. This is why you need to follow the manufacturer's guidelines carefully when setting "oil height", which really is "air volume".
Whether your "empirical" results validate it or not, by adding a spring (of any rate), you have softened the forks, which is generally the opposite of what's recommended for these bikes. I put "empirical" in quotes because it is a word you used, but in fact for it to actually be empirical you would need to do a measurement, which would validate that the fork is indeed softer than before. However, you have increased the preload (which is also generally not recommended) which may have the effect of making your
subjective perception be that it is stiffer, but that's because it takes more force to begin moving the fork at all. In other words, it's more topped-out more often.
[of course it could actually be stiffer if you set the "oil height" much lower than it was before...]
BTW damping doesn't affect "dive" as you describe at all. What affected "dive" is addition of preload via your extra spring, or potentially less air (oil height) in the fork due to a fluid change. Damping is only really meaningful under high-speed/high-force things like going over a hard-edged bump. Braking is a very slow thing in suspension terms, the oil flows freely through the damper at that rate. You feel less dive because the fork is operating (statically) in the early (high) part of the travel more often due to the extra preload. But in fact the total spring rate is less, so it will dive more than before once you have overcome the preload.
You have to think about preload like this. Let's say your new spring rate is 0.6 kg/mm. That means for each 0.6 kg, the spring will compress 1mm. Since there are two springs in parallel, they share the load, meaning it would take 1.2 kg to move the fork 1mm. Bike + rider is maybe 240 kg and let's say 1/2 of that is on the forks (it's less, but I'm just trying to make an example. I chose round numbers so it will make sense). With no preload, the fork would compress 100mm under bike+rider weight in this circumstance... 120kg (half of the 240kg total) divided by 1.2 kg/mm = 100 mm. Now, let's say you put a total of 60 kg preload on the fork (30 kg per side). In that case, under bike+rider load the fork will only compress 50mm. But with each additional 1.2 kg it will compress another 1mm, whether preloaded or not. With the preload, it just sets the ride height 50mm higher.
If you were to put, say, 150 kg of preload on the fork, then with bike + rider weight the fork would be topped out and would not compress at all. It would begin to compress once you added more than 30 kg of additional force.
GS500 fork has about 120mm of total travel stock. So as you might imagine, you need some preload otherwise the fork will have consumed 100mm out of its 120mm of travel just sitting still before it hits a bump or you brake. Adding some preload raises the ride height. Until you get the suspension completely topped out under static conditions (which would be pretty insane, and also likely ruin the fork), preload really is more like ride height. Adding a 1" spacer to increase preload will simply raise the ride height by 1". If you did that with something like PVC tubing, then this will mostly affect the geometry of the bike, but it will also add bottoming resistance at the expense of topped-out operation. Adding like 20-30mm of extra preload spacer is pretty safe, but much more than that and you restrict overall travel (making it bottom sooner). Again you'd have to do a lot of measurements and math to determine the real upper bound. But in any case you won't ever be able to install enough spacer to cause the bike to truly operate topped-out under normal riding. It'll still have the fork compressed at least some even with no rider on it until you have added enough spacers to reduce travel, with stock springs.
The goal, most likely, is to get the fork, under static conditions, to operate about 1/3 of the way down in the travel, or on this bike, about 40mm. The poor man's way to do that is with preload spacer adjustment. The real way to do it is with spring rate adjustments. You do this by measuring sag with an assistant to keep the bike from falling over when you get on and put your feet on the pegs. Google it, there are tons of instructions and opinions about what's best. BTW when you change the rear shock or adjust the rear preload, it shifts weight on or off of the fork, so you would have to redo the adjustments to the fork preload in order to keep the geometry constant. Adding a stiffer rear spring (say, a kat600 shock) will result in shifting weight to the front of the bike, steepening the head angle and cause more of a tendency to dive etc. You would need to then adjust the fork spring rate to suit. Once you change something it potentially throws off the entire suspension and chassis balance, or at least changes it so that the bike behavior might change.
If you really want to increase bottoming resistance, increase the spring rate and not sacrifice suspension travel on the cheap then you can adjust the spring rate by cutting the coils. It's very simple, you just cut the coils and replace the space taken by the coils with a length of PVC tubing. I did all the math and measurements on mine and cut some off, and if you are really interested either search the forum here (I wrote it up) or hit me up and I'll go out and count the number of coils on the chopped-off coils. BTW I did this to try to get the fork to match my Kat600 shock. I would have (and should have) replaced the springs but I couldn't talk myself into spending $80+ on springs for a $900 motorcycle.
In the end you probably
don't want progressive springs. Nothing at all wrong with straight-rate springs on a GS500.