Page 1 of 3 1 2 3 LastLast
Results 1 to 15 of 37

Thread: fork spring question

  1. #1
    P Monk
    Join Date
    May 2005
    Location
    Port Neches, Texas
    Posts
    636

    fork spring question

    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?
    P. Monk
    My prized possessions. 74 R90/6 Mine! (also know by bride as the Black Hole). 09 R1200 GS. My wife, 1953 model who has survived aplastic anemia and a bone marrow transplant. My most prized possession is my relationship with Jesus!

  2. #2
    Registered User toooldtocare's Avatar
    Join Date
    Mar 2008
    Location
    Lake Saint Louis, MO
    Posts
    1,698
    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

  3. #3
    Administrator 20774's Avatar
    Join Date
    Sep 2005
    Location
    San Antonio, TX
    Posts
    12,896
    Quote Originally Posted by toooldtocare View Post
    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.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  4. #4
    Registered User toooldtocare's Avatar
    Join Date
    Mar 2008
    Location
    Lake Saint Louis, MO
    Posts
    1,698
    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

  5. #5
    Administrator 20774's Avatar
    Join Date
    Sep 2005
    Location
    San Antonio, TX
    Posts
    12,896
    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?
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  6. #6
    Registered User toooldtocare's Avatar
    Join Date
    Mar 2008
    Location
    Lake Saint Louis, MO
    Posts
    1,698
    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

  7. #7
    Administrator 20774's Avatar
    Join Date
    Sep 2005
    Location
    San Antonio, TX
    Posts
    12,896
    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.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  8. #8
    Registered User toooldtocare's Avatar
    Join Date
    Mar 2008
    Location
    Lake Saint Louis, MO
    Posts
    1,698
    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

  9. #9
    Monza Blue 1974 R90/6
    Join Date
    Dec 2006
    Location
    Delaware
    Posts
    427
    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

  10. #10
    P Monk
    Join Date
    May 2005
    Location
    Port Neches, Texas
    Posts
    636

    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.
    P. Monk
    My prized possessions. 74 R90/6 Mine! (also know by bride as the Black Hole). 09 R1200 GS. My wife, 1953 model who has survived aplastic anemia and a bone marrow transplant. My most prized possession is my relationship with Jesus!

  11. #11
    Administrator 20774's Avatar
    Join Date
    Sep 2005
    Location
    San Antonio, TX
    Posts
    12,896
    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."
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  12. #12
    Rally Rat
    Join Date
    Sep 2004
    Location
    Arlington, TX
    Posts
    151

    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.
    Boxerbruce

  13. #13
    Registered User toooldtocare's Avatar
    Join Date
    Mar 2008
    Location
    Lake Saint Louis, MO
    Posts
    1,698
    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

  14. #14
    Saltwater
    Join Date
    Feb 2013
    Location
    Washington State
    Posts
    3

    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.

  15. #15
    DaveM Dave Backmarker's Avatar
    Join Date
    Jun 2007
    Posts
    48

    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

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •