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MountainBikeReview.com's Forum Archives - - Passion -
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Biking Viking, wanna do some math for me? (12 posts)
|  Biking Viking, wanna do some math for me? | lidarman
Sep 6, 2002 2:54 PM | | Anybody for that matter. And it's bike related.
Heck, maybe math isn't required, it's just that I'm trying to convince myself. I've gotten trapped in MathCad and need another opinion.
I'm putting my Marz x-fly 100 on my Schwinn hardtail and as it is, it has way too much travel for the geometry--the head angle must be 69 degrees. I don't want that so I'm reducing the travel.
My first notion was to just pressurize it less (allowing more sag) but this should cause it to bottom out easily.
Next, I decided to add a spacer on the negative spring to basically compress it some and reduce the travel. However, it seems that first order, when the shock is pressurized to have say 20% sag, the result is the same as leaving the full travel and having the same pressure in the shock.
It seems the key is the amount of oil. It determines minimum volume of air that can be in the shock when fully compressed. And when fully compressed, this volume determines the pressue (via ideal gas law) and restorative force on the shock.
So the questions are:
1. Will sag alone do the job? (I think not)
2. Will a negative spring spacer alone work? (some reason this doesn't make sense).
3. Or can the oil height be changed and the pressure reduced to make a lot of sag witout bottoming out.
4. Or the combination of a negative spring spacer with a different oil height? |
| You're a little unclear here. Either that or I'm slow today. | Biking Viking
Sep 6, 2002 3:13 PM | | It is Friday afternoon after all.
"Next, I decided to add a spacer on the negative spring to basically compress it some and reduce the travel. However, it seems that first order, when the shock is pressurized to have say 20% sag, the result is the same as leaving the full travel and having the same pressure in the shock. "
I read that as spacer/no spacer makes no difference. That I don't understand.
BV |
| Clarity.. | lidarman
Sep 6, 2002 5:20 PM | | Basically that's what I mean.
Normally I run at 32 psi. Say that gives me 20 mm of sag.
If I try to solve the problem by having more sag (say 25 psi) giving me say 40 mm of sag, the shock bottoms out.
Now, If I put a spacer (at 32 psi), there is no sag. So as I decrease the pressure to get sag, the situation where it bottoms out would appear again. All speculation--Gedanken!
The key is that the spacer is in series with the negative spring and I think it effectively compresses the shock that amount. It's designed normally to keep the shock from rebounding with a bang. I think with the set oil level, there is about 25 mm of air left when the shock is fully compressed. One can easily calculate a pressure based on this and therefore an equilvalent restorative force.
If the shock had no air volume when fully compressed, the force would be infinity so I think this remaining volume is the key. |
| Got it. However... | Biking Viking
Sep 6, 2002 5:24 PM | | ...the oil in the fork also lubricates the interior. If you put less oil in there you may risk parts of it running dry.
BV |
| I would imagine | lidarman
Sep 6, 2002 8:38 PM | | having to put more oil. if I run lower pressure to get sag at 80 mm, I need a smaller volume of air when the thing fully compresses to produce the required restorative force. |
| spacer won't work | tweezle
Sep 6, 2002 3:31 PM | | the space may reduce the travel of the fork, but your concern is ride height rather than actual travel. the spacer would retain the same ride height.
seems to me that could use a heavier weight oil. the heavier oil should increase compression damping, thus counteracting the potential for decreased pressure to allow bottoming out. i think increasing the level of the oil has the same general effect, but a heavier weight oil (at the normal level) would allow the fork to function more normally. |
| The spacer does | lidarman
Sep 6, 2002 5:03 PM | | decrease the travel and ride height. I put a small one in and it does work. It goes on the negative spring side and basically compresses the fork a little. If I were to take it out, I could increas my travel at the cost of a harsh knock when the fork rebounds.
Thanks for the oil viscosity tip. I haven't even thought of that. I don't understand how the forks compression damping works that well but it seems that could help. |
| Thought you knew this. Compression damping is a dynamic force... | Biking Viking
Sep 6, 2002 5:22 PM | | ...counteracting the compression of the fork. It decides how fast the fork will compress under a certain load. No matter the damping, the fork will always compress the same amount given time to stabilize. The electrical equivalent is charging a capacitor with a resistor in series. The value of the resistor determines how fast the cap will charge for a given voltage source.
Compression damping is achieved by designing the interior of the fork so that when compressed, the oil in the fork is displaced. When that happens, it flows through a narrow opening somewhere to get from one place to another. The force of pressing the oil through this opening generates damping. The thicker the oil, the harder it is to press it through. But if you load the fork with 100lbs, it's going to sag to the same spot no matter the weight of the oil - eventually.
Sorry if you already knew this. It seemed from your post as if you didn't.
BV |
| It wasn't clear to me | lidarman
Sep 6, 2002 8:43 PM | | that the fork has any compression damping. I figured since it only has rebound damping, there is virtually no resistance to the oil flow as the fork compresses. However, after it was brought up, I do remember a hole in the left side rod that may act as a compression damper--only adjustable by oil viscosity changes.
Yes, I do understand compression damping principles.
I think the real problem is I don't understand marzocchi's design. |
| Just ride it with 100 | steakcheese
Sep 6, 2002 4:10 PM | | Just ride with a 100 of travel. After a week you wont even notice. I put a 100 Fox Vanilla on my OCLV hardtail and at first I hated it but now I can't imagine riding without it. |
| Better solution | Fast Eddy
Sep 6, 2002 4:45 PM | | Sell it on ebay, and get a real fork. I sold my POS, flexy, sticktiony, x-fly for $179, and I got a brand-ass-new, buttery-smooth Z2 for $200.
The 100mm really screws up a hartail made for 80mm, IMHO. The problem with depending on sag to fix the geometry is that when you're climbing anything steep, the sag comes out, and you're back to 100mm at the time when it counts most. |
| Guess why it's going on the hardtail... | lidarman
Sep 6, 2002 5:08 PM | | because I got a sweet fox forx for my truth.
And that's exactly why I'm wanting to reduce the travel. I rode a trail the other day with the fork and I couldn't make a lot of the tight switchbacks on the climb.
The fork on it was a 1999 z2 but it's 70 mm with 5 mm neg so it's really on 65 mm. I'd like at least 80mm. Plus, If I figure this problem out, I can make it anything (83.4541232 mm. plus Iahve a bike with a manitou three that I want to put the z2 on. |
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