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fork spring question

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? :dunno
 
:clap 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 :clap
 
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. :scratch
 
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
 
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.
 
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
 
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.
 
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!
 
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
 
Kurt,
I think you would be surprised how much you can improve your handling by setting the proper amount of sag.
 
photorider -

Maybe, but as I said, I'm pretty comfortable with what I have. Seems like it would be difficult to really tell the differences because, as I see it, you ride a given setup, then come back home. You take a couple of hours to change the setup somehow...sag, oil weight, oil volume, etc....then back out. I don't think my physical memory would really be able to tell the difference. :dunno I have been noticing that I don't quite look over the fairing screen like I used to...I have to sit up higher. Hmmm...methinks. I wonder about the sag in the rear shocks. Konis installed long time ago on the softest setting. I'm beginning to think maybe I should try the next preload level...that makes even more sense as I pack for the trip to Salem.

Granted changing the rear sag would be easy, but the front seems time consuming to a point where I woudn't remember the previous "setting" responses. I'm not an agressive rider by any means, but the bike seems to suit me.
 
"I'm pretty comfortable with what I have"

Fair enough.

If you ever want to experiment though, here is an easy way...

1. Undo the bottom clamp of your fork boot
2. Get a sharpie
3. Sit on the bike
4. Reach down and draw a line where the slider meets the tube (you might need a friend's help here with your fairing)
5. Put bike on center stand and pull down on the fork tubes to make sure they are completely extended. Measure the difference from your line to the slider. That is you sag.

Takes five minutes and at least you'll know where you stand.

If you want to reduce the sag, just pop the tops of the fork tubes and insert a PVC spacer roughly the same size as the amount you want to reduce the sag by (yeah, I know this is not exact science but it will get you pretty close).

Go for a ride.

I'll bet you a beer you WILL be able to tell a difference!


BTW,
The sharpie trick works for the rear as well.
 
have you looked at the Race Tech website? I recently rebuilt my forks ( 1990 R 100 GSPD, 40 mm marzocchi forks ) with RT springs matched to my weight , and RT gold valve emulators . I have been thrilled with the results, I have mine set to the stiffer end of the range. there so many small adjustments to make, it makes your head hurt. the preload affects how stiff it rides, more preload, stiffer ride, oil weight also affects ride, the higher the weight the stiffer the ride. then if you get a gold valve there are 3 different springs each with 28 different recommended settings. I am running 17 mm preload, 10 wt oil, and GV with yellow spring ( the stiffest ) set to 2 turns, RT presets them to 4 turns, I will try that when I change oil.
 

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Race Tech

If the 74 was my main ride or money was no option I would be interested in these. As it is I have 3 times or more invested in the old bike than it is worth. I am looking for some improvement in ride without investing too much money (I know, cheapest thing on a motorcycle is me)

But the Race Tech emulators are really impressive.
 
in that case start with preload, and oil weight, and maybe new springs, have fun!
 

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Agree only with progressive springs

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, ...

Kurt,
I absolutely agree. I wasn't clear. The spring rate changes with compression only with a progressive spring. Thanks for adding the clarification here.

DaveM
1975 R90S
2000 R1100RT
 
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