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fork spring question

dpmonk

P Monk
Since I bought my /6 in 08, the only thing I have done is change fork oil once.
I have never been real happy with the ride and am going to disassemble and do a rebuild on them.

Supposedly the bike has progressive springs according to the previous owner. I am considering the heavier fork springs as recommended by Max.

I am looking for just a little softer ride than I have now, but without creating a whole lot of fork dive.

suggestions anyone?
 
I do not understand the heaver spring to give a softer ride? Anyway, best to know what is in there first. Some springs came with spacers to increase their compression force. It may be enough just to remove it, if one is in there, or shorten it. Good luck.

Wayne
 
Some springs came with spacers to increase their compression force.

Spacers don't change the rate of the spring...they just change the amount of sag that will be there once loaded. Unless the spring is a dual-rate spring, but even then you have to get the spring very much compressed so that part of the coils become coil bound...which then effectively changes the spring rate.
 
Thanks Kurt. What I meant is, spacers increase the preload of the spring, thus increasing the force to compress it, same as on the rear shocks. This is only true if you have to force the top caps in place while compressing the springs, which I have had to do most of the time. I have tuned the rate and amount of dive on various bikes by changing the length of the spacers to suit me instead of always going with a heaver or lighter spring.

Wayne
 
Wayne -

I'm having trouble wrapping my head around that. If we're talking about a constant rate spring, that means it takes X pounds to compress the spring Y inches. Each inch requires the same force of X pounds...the total force is cumulative, so 2Y inches is 2X pounds. Is that the "increasing force" you're talking about?

If we didn't have a spacer and the spring was not compressed a bit even with the rider sitting on it (unrealistic, I know), hitting a certain bump will cause the spring to deflect a given amount...I'll say the spring deflects 2 inches. Now say we have a spacer which compresses the spring just one inch...again, the spring wouldn't compress any more with the rider sitting on it. Go back down the road at the same speed, hit the same bump, the spring is going to deflect the same 2 inches. No additional force is required to deflect the spring in the latter situation with the spacer.

Am I not understanding things correctly in this simple situation?
 
Example: First, assume we are only working with a single fork bike. If you are talking two forks, multiply everything by 2. Also, we are talking constant wound springs, not progressive, they are slightly different.

Steps

1. A spring not under load just sits there. Apply force, say 50 pounds in our example, and it compresses 1 inch. To hold it there it there would take a constant 50 pounds, so you cannot let go.

2. To compress it another 1 inch (2 inches total) let's say it takes an additional of 50 pounds. So now you are holding the 50 pounds you first applied to hold it down 1 inch, plus a second 50 pounds for the second inch to drop it 2 inches total, or 100 pounds to keep it dropped two inches.

4. Now, put that spring in the fork without the spacer and assume that it does not compress when you put the cap on. Apply 50 pounds and it would drop 1 inch. Apply any force and it will start to drop, even an amount as little as 1 or 2 pounds and it will start to drop. This is the same as step 1. Add an additional 50 pounds of force (100 total) and it would drop 2 inches. The same as step 2.

5. Now, remove the top cap and put a 1 inch spacer in, compressing the spring 1 inch inside, but not changing the length of the fork. The cap is providing that first 50 pounds. Same as step 1 again, but the bike stays all the way up.

6. Now start to apply force. To even start to move it would take more than 50 pounds, because it is already preloaded to 50 pounds. Since you put 50 pounds on it to start it moving, you would have to put another 50 pounds (100 total) to drop it that inch. Remember, the fork only dropped 1 inch, but the spring inside 2 inches. Same as step 2 again.

This is what you do when you turn the preload on your rear shocks, you partially compress the spring. This causes the bike to take more force to move, thus you can carry a heavier load.

Does this make sense? Am I thinking right, or getting confused because I am comparing it to the rear shock?

Wayne
 
I think I'd have to run an experiment. But if I understood #6 right, the spring will move 2 inches under 150 pounds of load...50 from the preload and another 100 per what you say. Doesn't seem right for a constant rate spring. :dunno
 
Kurt, my head is starting to hurt, I am not sure that I am right with my engineering. What we need is a mechanical engineer, I am a EE, retired.

But, I do know that when I added spacers my forks they dove less on braking, just like when I add preload to my rear shocks. When I ride single I have my rear shocks set on soft and they do not bottom out. When my wife rides with me then bottom out badly unless I move the adjuster to the middle position, which shortens the springs, not the shocks. I do get more bounce, but the preload is overcoming the extra weight.

On the experiment, I was thinking the same thing, maybe on a smaller scale. Take a spring out of a ball point pin and work with that. It will give me something to do tomorrow after a doctor's appointment. All I need to do is devise a way to hold it compressed and a scale to push it down. Let me know how your experiment goes too.

Have a good evening.

Wayne
 
I would offer that fork springs are not quite in the same league as, "which oil is best?", but are about the same as tires. That is, there really isn't one "right" answer. If your /6 forks have original (or old) springs and after 30+ years, haven't had the benefit of a good cleaning, I'd say you would do just fine with the BMW heavy duty or the Progressives, or maybe Hyperpro (http://epmperf.com/hyperpro-springs.htm). With a new set of springs, nice clean insides, 7.5 weight oil, and some fresh rubber bumpers in the bottom, you should see an improvement regardless of which springs you choose.

Once you get it back together, the key seems to be adding the required thickness of spacer to get between 1-2" of "sag" as you sit on the bike. Here is a detailed discussion from Robert Fleischer about the front end. The sag part is in the middle. Note the comment about not replacing the wiper rings while you have the forks apart for cleaning.

http://bmwmotorcycletech.info/frontforks.htm

In short, my $0.02, don't worry too much about which springs are best. If yours are worn out, get a new pair, clean up the forks, add some new oil, set the sag at 2" and you'll be in good shape.

Barron
 
HD versus Progressive springs

Max recommends 0631421232017 reinforced springs rather than the standard 0631421231358 which came in the bike. Guess my question is which would give a little softer ride, the progressive or the heavier springs from Max.

However i probably am way premature asking at this point since like lots of other things on this bike, I really don't know what I have. I was looking to buy a rebuild kit and considering the ---017 springs. Could be I need to measure the sag and see where I am before I really get into this project.
 
Wayne -

I found something that makes sense...you're right on what preload does. This is from:

http://www.ntnoa.org/suspension_preload.htm

One key area states:

"Increasing preload does not result in a stiffer spring. It only changes the amount of load it takes to begin to compress the spring. Spring stiffness is determined by the manufacturing process, and is usually expressed in lbs/in or kg/cm. As an example, a 100-lb/in. spring preloaded 2 inches will not compress further with any weight less than 200 pounds, but will compress 1 inch per 100 pounds above that."
 
Springs

I'll have to go with the MAX suggestion. The BMW sport springs with the part number ending in 017 are my choice, and a bit softer than the Progressives. The Progressives are a bit too stiff for me. They come with spacers that, if installed cause the springs to bind and are way too stiff, even with a big fairing like a Windjammer installed.
The difference between the 017 springs and the stock springs is that they are a heavier guage wire and are actually shorter that the soft stock springs. The 017s are recognized by a splotch of white paint on one end. The stock springs usually have yellow paint.
 
Thanks Kurt, glad my memory is not that bad. On my second new bike (74 Moto Guzzi 850T) I replaced the springs because I added a Wind Jammer III fairing.

The spring kit came with spacers. One about 1 inch long for normal riding, one about 1.5 inches long for riding double-or single with a Wind Jammer, and one about 2 inches long for riding double with the Wind Jammer. It was hard to get the cap on because I had to push it down a lot to preload the springs. This is the method I have been using to determine the amount of fork dive under braking since then, I cut my own spacers from PVC tubing and force them in instead of installing stiffer springs.

This is why I suggested finding out what is in his bike before he starts replacing parts, maybe the pervious owner did the same thing. But, as he suggested, best to start over with a clean sheet of paper and do it right.

Wayne
 
Been there, done that recently

I refurbished a neglected, non running 1978 R80/7 recently - really enjoying it. I dismantled the forks, cleaned out the disintegrated rubber parts, and installed all new everything. The stock springs measured the correct length, but the sag was low, like half way down. I weigh about 165 pounds fully dressed, probably 175 with full riding gear. The bike is all stock, no fairing. I then purchased the HD springs from MAX. They are shorter than the original springs, so you will have to install a spacer. Near as I can tell, the HD springs are actually the OEM part on another model. I finally settled on 5/8? spacer cut from 1.5? schedule 80 water pipe (the white plastic type). I am pleased with the result, and the sag is approximately what Snow-bum recommends. I can?t tell the difference in the ride between stock and HD; it just sags less.
 
Spacers do affect loading on progressive springs

Maybe my Bachelors of Science in Mechanical engineering may help. For springs, force is proportional to the distance the spring is displaced. The force times a spring constant (K) equals the displacement. The formula is F = Kx where F = Force and x = displacement. The definition of a progressive spring is that the spring constant changes (increases) over the compression or displacement of the spring. From that, a progressive spring is soft as small compression (displacement) and stiffer near the limit or big compression. Inserting a spacer is a great way to tune or make the front end stiffer for little compressions.

I hope this helps. Don't mean to be too much of a geek.

DaveM
1975 R90S
2000 R1100RT
 
Dave -

I think this is where things get murky...you say inserting a spacer makes things stiffer. That might be the case for a progressive spring, but what I was describing above was for a constant rate spring. According to what I copied, a spacer just adds preload and it affects sag...does not make for a stiffer spring. If you have an effective 200 poungs of preload, you must load it higher than 200 to make it begin to deflect again.

In these situations, isn't the spring (with spacer) pushing back against any further force and that you must overcome this pushback before the spring moves anymore? Using Hooke's law for a constant rate spring, with a spring rate of 100 lb/in, the 200 pounds (due to the spacer) will deform the spring 2 inches. If we put 200 pounds onto the spring/fork setup, the restorative force will just balance the 200 pounds of added weight and the spring will not move. If we increase the 200 to 300 pounds, the spring will deflect another 1 inch for 3 inches overall. But this still satisfies the original equation for a spring rate of 100 lb/in...300 pounds deflected the spring 3 inches.

Progressive springs get more complicated, but the situation is still the same...the preload creates the restorative force...you must over come that with more force created during preload in order to deflect the spring any more. That's why you increase spring preload, say with the knobs to turn on the outside, when riding with a passenger or when traveling with all your gear.
 
Hi,

My 2 cents again. A constant wound spring with compression rate of 1 inch equals 50 pounds. Use a fork with 6 inches of travel and no spacer. To push it down all the way to the bottom would take 50 pounds per inch times 6 inches, or 300 pounds.

Same spring, add 1 inch spacer preloading it 50 pounds. Fork still has 6 inches travel, now push it down all the way. To start it moving it would take 50 pounds, to take it all the way down would take an additional 300 pounds, so the total force is the 50 plus the 300, total 350 pounds.

Same spring, 2 inch spacer with 100 pounds preload. Now it would take 400 pounds to push it all the way down.

The rate of the spring did not change, still takes 50 pounds per inch of movement, but with spacers it is harder to push down because it is harder to start moving (must over come the preload), but it is still 50 pounds per inch once it does starts moving.

Before shooting holes in my example above, read the reasons below, it has to do with sag. At my age I have plenty of sag, but that is not what I am talking about.

SAG: This method of tuning I have used to eliminate bottoming since it takes more force to push it all the way down, can give a harder ride IF THERE IS NO SAG when I sit on the bike, not because it changes the rate of the spring, but adds the preload that must be over come.

IF THERE IS SAG with the spacers, than nothing changes except the ride height. That was not the case with my bikes, I do not like sag because I ride aggressively and do not like the fork bottoming out, so the spacers take it out which results in the preload. I found out that every airhead that I have owned with stock springs is much too soft, so I add spacers. Sometimes this does not work well, but this is where I start before I start buying springs.

And the reverse, removing the spacers will often soften the ride without problems as long as too much sag is not introduced.

Make sense?

Wayne

PS, dropped my R1200R today in the drive. My sag prevented me from picking it up, so my neighbor had to help. Rather embarrassing. :blush
 
Wayne -

Doesn't make sense to me. I think we're talking constant rate springs here. The spring rate is 50 pounds per inch and you have the one inch spacer. So there is already one inch of travel which takes 50 pounds to get that. If you now begin to compress the setup (I'm not sure where we're pressing), but if you put 50 pounds onto the setup, you haven't added another 50 pounds, you've only just balanced the pushback of the spring. 50 pounds, still at one inch. Add 50 pounds for a total of 100 pounds, by theory, you will have moved 2 inches...still 50 pounds per inch. By the time you get to 6 inches you will have put a total of 300 pounds....50 pounds per inch. That can be the only answer with a constant rate spring.

You're suggesting a stiffer spring due to the preload, but it was stated on the link I provided that adding preload doesn't change the stiffness of the spring. The rate of the spring is always the same...50 pounds per inch. That is the "K" in the equation...it is constant...for a constant rate spring.

Sorry...about the "moment"...I hate when that happens!!
 
Kurt, with the spacers the forks are holding the spring under tension. But, once you push down enough to start movement you are providing the force. So you are starting with say 100 pounds of force already applied before you have moved more than a fraction of an inch.

Since you are already pushing down 100 pounds this must be your starting point, you cannot let go or the forks will return that fraction of an inch to full extension. To move the extra 6 inches takes 300 more pounds. You are now pushing down 400 pounds. Without the spacer, only 300 pounds.

Ever use a bicycle pump to inflate a bicycle tire to say 120 pounds. When the tire is flat it takes very little force, but much more force when it is almost inflated. That is because the air acts like a spring under compression, another word for preload.

Wayne
 
So you are starting with say 100 pounds of force already applied before you have moved more than a fraction of an inch.

I'd agree with you if you said 50 pounds instead of 100. The spring is under tension and pushing back with 50 pounds. Once you push with 51 pounds you begin to overcome the tension and the spring begins to move.
 
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