Stiffer spring and oil level

[clw]

Is there a standard to apply when installing stiffer fork springs? Just went from .40 to .42 in the Gasser, softened up the compression two clicks and slowed down the rebound 3. It's close (and better) but I had raised the oil level before to offset the softer spring and a bottoming condition. Now it's difficult to bottom so I'd like to open up the air gap. Suggestions as to a good first guess?

 

[marcusgunby]

I normally go in 10mm increments.

 

[clw]

Thanks Marcus, seems I remember reading a KTM article where they put in stiffer springs and lower the oil a lot, like 20mm or so. That sounds too much.
Thanks again!

 

[KTM-Lew]

I suppose it would also depend on where you started from and how much you previously added.

I prefer to start low and add through the bleed hole when nec. :thumb:

 

[clw]

GG comes at 150mm, I raised to 140 (same as KTM MX bikes). I'll just go back to 150 and re-ride.

 

[marcusgunby]

It all depends on the fork diameter-a 48mm fork will have a lesser pressure rise for 10mm of oil than a 43mm.

 

[DEANSFASTWAY]

clw, Sometimes you can buy fork springs that are wound different or made of a somewhat different wire and the spring actually takes up less volume in the fork than the old spring and the spring could be even firmer. Youre level could be lower already .Ever look at street bike heavy low tension springs as compared to the same rate high quality racing spring?Example Sometimes Eibach 43 spring looks a bit different than the stock 43 spring ,It may be wound different or use a different quality wire than stock maybe less or less closely wound coils .The spring may take up less/more volume than the ones youre replacing. There may be a difference in weight usually a heavier spring of the same material as replaced will take up more volume in the fork altering the oil height A thinner wire thicknessor looser wound spring may take up less volume in the fork and have a lower oil height .Im sure somewhere there is a mathematical weight/volume equation that can be applied somewhere/sometime, not right now though . Actually I learned this when swapping out 42s for 43s and the 43s were Eibachs , The bike rode abit higher and was more stable but still bottomed with the same oil level . We added just abit more oil .Sound like you should suck out just abit .Your forks will probably work real well thenprobably nice & smooth.GOOD LUCK

 

[clw]

Thanks Dean:
The 0.42 springs were original eq. in a 400EXC. The 0.40 were the GG original eq. springs, both White Powers but as you suggested they were a bit different. When I had both springs on the bench I found it odd that the stiffer spring had more coils (3 more), the opposite of what I expected! So you're right , I hadn't thought about it, the 0.42 does displace more oil once assembled compared to the GG spring. I measured both springs and ran the numbers through a spring rate calculator and they calc'd out as labeled.

What a difference 5% in spring rate makes! or maybe it just put the front in balance with the rear, but a very positive improvement.

 

[clw]

Originally posted by marcusgunby
It all depends on the fork diameter-a 48mm fork will have a lesser pressure rise for 10mm of oil than a 43mm.

Marcus: can you explain that one to me? The bigger fork would need more oil (cc) to raise it 10mm but are the internal pressures given the same travel going to be different?

 

[marcusgunby]

Hmm i start o get confused when we get one variable making another the opposite.
I think im right in saying a 48mm fork has a larger volume due to the diameter-if you raise the level 10mm in a 43mm fork you, as a percentage decrease the space left more than in a 48mm fork.I wouldnt be surprised if im wrong on this.

 

[James]

If you have two tubes 300 mm in length but one is 43 and the other 48. The 43 capacity is 435ccs and the 48 543ccs. If you set the oil level at 150 mm in each, then that is 50 % full in both. If you raise the oil level by 50mms in each, they will BOTH be 66% full. In my opinion, the effect on stiffness would be identical (or so nearly so as to be impossible to notice a difference.)

Now if we start both tubes 50% full, the 43 would have 218ccs and the 48 would have 272ccs. Add 200 ccs to each, the 43 becomes 96% full and the 48 becomes 87% full. I think the 43 would be noticeably stiffer than the 48 in this case.

But notice we had to add 200ccs to create less than a 10% volume differential….leading me to believe that there would have to be much larger differences in tube diameters to create a concern about adding ccs of fluid equally to fork tubes.

 

[marcusgunby]

isnt it the amount of how much airspace is left that matters, a small airspace will give a very big airspring-so a 48mm fork will have a bigger airspace left after adding 10mm height of oil.You are right in the fact that any difference will be small and compared to most forks whose levels are often miles out we would be splitting hairs.Its still fun to debate it however;)

 

[clw]

What we're hashing about is Boyle's Law-for a given volume of gas if you decrease the volume in half the pressure will double. So I believe that the rate of change as measured in pressure will be the same in either fork (43 or 48) as it relates to volume. eg, 150mm air gap compressed in half, to 75mm, will be 2atm in both. But, and this is the part I don't remember from Physics class, I believe that for the same weight (force) the 43 will go through a longer stroke because of the less internal surface area. That's why high pressure hand pumps (like the ones that pump up MTB shocks to 300psi) have a very small pistons yet are easy to operate. Anybody fresh on their gas property from physics class? It's been a long time for me!

 

[James]

Now I see EXACTLY what you are saying Marcus (well I think I do anyway). I dunno, I think I am agreeing with clw on this one, well the first half of his post anyway about compressing air gap in half doubling the pressure. I'll have to wait and see where the hand pump thing goes.

Can we assume that forks are a perfectly sealed system? My assumption is that they are..BUT..things aren't always perfect and if air does escape as the forks are compressing, and that escape was related somehow to the size of the tubes (seals) then I could see where your point would be valid Marcus. Can air escape without fluid escaping also??

 

[marcusgunby]

The fork will not let air escape as then the airspring would not be working and also on rebound it would create a vacuum.

 

[marcusgunby]

The best way to think of this is to take it to a extreme-if we had a 15mm fork and we had only 100mm of airgap would it have a bigger air spring than a 150mm fork with a 100mm airgap?

hmm ive not convinced myself of this theory so im open to others opinions.

 

[svi]

Clw,
150 and 75mm are lengths not volumes. you also need to take acount of the cross sectional area.

 

[clw]

svi:
True, but it the case of a suspension fork we are dealing with a cylinder. A cylinders volume is directly proportional to its length.

I think what we'll find out, if someone can confirm, is that Marcus is right: a 10mm rise in fork oil level in a 48 will feel the same as a 12.5mm rise in a 43 because of the 25% larger volume of the trapped gas in the 48. It deals with volume, not pressure.

Maybe, waiting on the sweep crew!

 

[DEANSFASTWAY]

Marcus is correct about the pressure rise assumption . The 43s fork has a smaller internal volume than a 48 fork so the smaller fork is more affected by a 10 mm rise . The closed fork leg has a mathjematical compression ratio that you could figure out if you really want to . Think about all the air space when the fork is extended and the smaller air space when it is compressed . There is a big difference even without factoring in the springswhich take up volume(constant). If you think about it in more liquid terms its like adding a shot of Jack Daniels in a small glass as opposed to adding a shot to a larger one, the smaller glass has more affect right ? Sort of like mixing your gas but not exactly.Please dont scrutinize me on this but if you think about it it makes sense . Unless of course, you throw in bladders or enzo reservior type stuff . Oh yeah and at the Delaware Enduro there was a very light turnout due to the rains during the week .??

 

[James]

clw, if you are agreeing with Marcus, then wouldn't it be that 10mm in a 43 would feel like 12.5 in a 48??

Dean, Marcus, for your theories to be correct, are you saying that air is more compressable in larger volumes even though the amount of compression is the same? If air is compressed to half it's original volume, in either a large or small container, isn't is still under the same pressure and thereby offering the same resistance. As in 10lbs of pressure in a tire is 10lbs regardless of the tire size. If anything, I would think that the larger fork would offer a greater air spring as it would be 10lbs of resistance across a larger surface area of fluid/fork. Is this the angle you were coming from clw?? I was tempted to make a bicycle/auto tire comparison but I think tire construction has a lot to do with that one.

Also, all the spring does is reduce the internal capacity. If we fill both forks internal oil capacity to 50% (adjusted for the coil spring) do we still not end up with comparable air springs?

I am willing to agree if it is presented properly...so I am not arguing just to argue. I have thought through the shot glass comparison and it makes me want to ask more questions. Is the shot you are adding to both glasses volumetrically the same or are you raising the level of the liguid in the glasses from one level to another by adding 10mm of JD? If it is the latter, then your alcohol/coke (assumption) ratio would be the same..unless of course, the glasses are not concentric.

Is there a way we can simulate cavitation with carbonation bubbles ?? ;)

 

[clw]

James:

I think I stated it correctly the first time. Note I said I think, don't really know for sure!

The problem here is because of the word pressure. Pressure is a factor but not the only one in this model. The larger volume of the 48 will have a larger amount of molecular activity based on the larger cross sectional area of the fork, in this case 25%. 43mm has a surface area of 1452mm2 and a 48 is 1810mm2, an increase of 25%. It's a 25% larger engine. Same reason a CR500 is a lot harder to kick over than a CR125 even though the 125 has a higher compression ratio (read higher pressure). That's even with a lot shorter kickstarter and more revolutions per kick (disadvantages) on the 125. Another analogy is in bicycle tires as I think you started to make. If I sit on my Razorback (MTB) with 35 psi in the tires they're fine, but if there's the same 35 psi in a racing road bike with their narrow (19mm) and short aspect height tires they're on the rims. Same pressure but with very different results.
I agreed with what Marcus had to say not because of the termnology he used but because I believe what he means perceptually will be true.

 

[James]

Sorry if that came across wrong clw, I was just trying to clarify what you were saying so that I could be sure I was understanding you correctly as it looked like you were saying the larger air volume would be stiffer where Marcus was saying that the smaller air volume would be stiffer? I'm still trying to master the art of inquisitve questioning via chat boards ;)

I am doing some research on some other suspension stuff so I'll keep my eyes open for anything that might apply in this topic.

Thanks for the conversation guys! :thumb:

 

[marcusgunby]

I spent ages today with a bunch of different syringes to represent forks of different sizes-i got a colleague involved who is IMO very clever and we concluded its very hard to prove much-A 5kg weight moves a 10ml syringe down 15mm and it moves a 50ml syringe down 15mm but the small syringe has a much smaller piston and so it has a higher force in psi-i got all confused by my brighter collegue and went home;)

 

[Glitch]

I'm thinking that if the 43mm and 48mm both had the same level, height, of oil, then the 48mm would have more airspring because of its AREA. This goes along with the CR500 and CR125 concept. So then the 43mm would need a higher level of oil in order to have the same airspring as the 48mm.

 

[clw]

James--Was I curt? Didn't mean to be! Sorry if I offended you.

Marcus-- Really!! Throws my 125 and 500 theory out the window. But I know that a 125 is easy to kick over, but why?

BTW, did I mention how neat the Gasser is now with the stiffer springs!