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Thread: fork spring question

  1. #16
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    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.
    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!

  2. #17
    Registered User toooldtocare's Avatar
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    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.

  3. #18
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    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 -- 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. #19
    Registered User toooldtocare's Avatar
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    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

  5. #20
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    Quote Originally Posted by toooldtocare View Post
    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.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
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  6. #21
    Registered User toooldtocare's Avatar
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    One inch spacer, 50 pounds, 2 inch spacer, 100 pounds. I was think of two inches to show a larger value.

    Wayne

  7. #22
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    OK, I think I have it...this (hopefully) will illustrate what's going on.

    We've got our spring with a one inch spacer which results in 50 pounds of spring tension, pushing back. Now, visualize a way in which you could remove the spacer, but put your fingers (say) in place of the spacer in such a way that the spring didn't move. You'd be pushing on the spring with 50 pounds, right? That is necessary to balance the tension in the spring. If you begin to increase the force from your fingers by even just one pound, the spring will start to move. Keep pushing to 100 pounds. The spring should now be at 2 inches of travel.

    How's 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!

  8. #23
    Registered User toooldtocare's Avatar
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    well to a point. The spring would move two inches, but the fork only one. Remember, the spring in the fork is compressed at one inch, not the fork itself. If you agree, than

  9. #24
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    I just consider that the "far" end of the spring is fixed and we're pressing on the other end of the spring...it doesn't matter if there's a fork tube or slider or whatever. I guess I'm going to have to find some kind of setup to run a siple test. Not sure how to deal with preload.
    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!

  10. #25
    Registered User toooldtocare's Avatar
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    Look at your rear shocks. Set them for soft and push down. Notice how far they go down. Then set them for hard and push again. It is the same thing, you add more preload, thus the force needed to compress them without changing the shock length, just the spring length.

    Have a nice evening.

    Wayne

  11. #26
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    Quote Originally Posted by toooldtocare View Post
    Look at your rear shocks. Set them for soft and push down. Notice how far they go down. Then set them for hard and push again. It is the same thing, you add more preload, thus the force needed to compress them without changing the shock length, just the spring length.
    I wonder if the "feeling" of how easy it is to push at least initially is masking the idea of what's really going on. With preload, you have to apply the force equivalent to the preload just to get the spring to start moving. That feels like it takes more force but in reality it takes the same force overall, once you exceed the effect of the preload.

    I think we have to look at the equation that's been given. For a constant rate spring for which we know the spring constant (either by a test or from the specs), if you measure the distance, then you can figure out what the force was to compress the spring that much. So, in the case of adding a spacer, it has no bearing on the spring constant and thus, knowing the distance moved gives you the force. A 50 lb/in spring without a 1-inch spacer compressed 6 inches or a spring setup with a 1-inch spacer also compressed 6 inches will all take the same amount of force...300 pounds.
    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. #27
    Registered User toooldtocare's Avatar
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    Hi,

    I think you are missing one thing. The spring without a spacer is only compressed 6 inches, thus the 300 pounds. A spring with a 1 inch spacer is already compressed 1 inch inside the fork. Pressing it down another 6 inches is actually compressing the spring a total of 7 inches, thus the 350 pounds, but the forks only moved 6 inches. You are not increasing the spring rate, but you are increasing the force to compress the forks. The key is, with the spacer you are already starting with 50 pounds, you cannot ignore that, it is part of the equation.

    Wayne

  13. #28
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    An oil thread indeed. Setting aside the theological discussions here, I would find springs matched for my weight and then set the rider sag to 35-40mm for a compliant ride. In general, a softer ride will entail more brake dive. In my experience, progressively wound springs give a soft ride, a lot of brake dive, and then stiffen up more than I want as the travel is used up. Race Tech sells single rate springs that you can match to your weight. The ride will be firmer than the stock springs and have less dive. That said, I was happy with the Progressive springs I had on my '84 R100. I fiddled with the oil weight to get the feel I wanted. IIRC, 7.5 worked best.
    61 Gold Star, 76 R90S, 03 CBR600RR, '13 690 Duke, '14 Street Triple R

  14. #29
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    Quote Originally Posted by toooldtocare View Post
    I think you are missing one thing. The spring without a spacer is only compressed 6 inches, thus the 300 pounds. A spring with a 1 inch spacer is already compressed 1 inch inside the fork. Pressing it down another 6 inches is actually compressing the spring a total of 7 inches, thus the 350 pounds, but the forks only moved 6 inches. You are not increasing the spring rate, but you are increasing the force to compress the forks. The key is, with the spacer you are already starting with 50 pounds, you cannot ignore that, it is part of the equation.
    I think I can agree with this but we have been talking about a "different 6 inches". And I'm not really considering a true fork setup...I'm just thinking about a spring being compressed...and how preload effects the forces. My example with a spacer is 1 inch for the spacer and another 5 inches...6 inches total. You're saying 6 inches for one situation and 7 inches for another. So, I agree with 300 pounds versus 350 pounds.

    I think we're basically talking the same thing, but using different references. I'm not sure why I even try to think about these things...I still have the original springs (no spacer) in my 100K /7 with a Lufty fairing, using 7.5wt oil, and I like the way the bike handles. I suppose someone else would think it is a death trap. As always, YMMV!
    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!

  15. #30
    Registered User toooldtocare's Avatar
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    Hi Kurt,

    We probably are saying the same thing all along, just thinking different. Now, time for a nap, love being retired.

    Have a nice afternoon.

    Wayne

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