Fork Springs

[Igofar]

The OEM ST1300 factory springs are tapered, with the tight coils smaller in diameter than the more open coils….however, several aftermarket companies (sonic) comes to mind, use a spring that is the same diameter on both ends, so for those types of springs it would not matter.
Some aftermarket springs also provide you with large special washers so they will sit properly on the cartridge etc.

 

[MidLife]

........the only difference would be in the air volume and the spring effect of compressing said two air volumes (close coils up or down).

Re-read @TerryS very simple equation.

Air Volume is what is left after you pour oil and insert the spring, which both have constant volumes.

The oil heights being different does not mean the air volumes are different. More coils in oil means less coil taking space in air.

 

[ST1100Y]

It seems to me that the close-wound ends should be up, so the bike's mass has greater control over the movement of the front wheel. With the close-wound coils down, there is more mass down low that the front suspension has to maintain control over.

The debates about the "heavy end" of fork springs can get 'religious'...
Masses, oil displacement, etc...

I always stick the instruction given in the OEM manuals; ST1100 and NT700 show close wound end down... both not tapered...

 

[SMSW]

Re-read @TerryS very simple equation.

Air Volume is what is left after you pour oil and insert the spring, which both have constant volumes.

The oil heights being different does not mean the air volumes are different. More coils in oil means less coil taking space in air.

I did, my comment was based on not knowing if the spring is fully immersed in oil or if part of it is above the oil line. If the spring is fully immersed, then there is no difference in air volume above the oil line regardless of its orientation. If part of the spring is above the oil, then the volume of steel coils immersed will have an impact on the oil level and thus the volume of air.

 

[dwalby]

I did, my comment was based on not knowing if the spring is fully immersed in oil or if part of it is above the oil line. If the spring is fully immersed, then there is no difference in air volume above the oil line regardless of its orientation. If part of the spring is above the oil, then the volume of steel coils immersed will have an impact on the oil level and thus the volume of air.

there is a column of air above the oil level, if there weren't then the fork wouldn't compress.

but if the oil level becomes lower as the spring is inverted, because there's less coils in the oil, then the additional air space above the oil has its air content reduced because there's more coils in the air section. So more air is displaced above the oil level than would be with the spring the other way around and the oil/air ratio remains the same in either orientation.

 

[SMSW]

there is a column of air above the oil level, if there weren't then the fork wouldn't compress.

Of course there is air above the oil level.

but if the oil level becomes lower as the spring is inverted, because there's less coils in the oil, then the additional air space above the oil has its air content reduced because there's more coils in the air section. So more air is displaced above the oil level than would be with the spring the other way around and the oil/air ratio remains the same in either orientation.

Interesting thought. I'm not sure if there is a significantly larger volume of steel in the straight coils than the progressive coils or the other way 'round. It would depend how much of the spring was exposed above the oil level. If the spring is fully immersed, this is moot.

 

[MidLife]

I did, my comment was based on not knowing if the spring is fully immersed in oil or if part of it is above the oil line. If the spring is fully immersed, then there is no difference in air volume above the oil line regardless of its orientation. If part of the spring is above the oil, then the volume of steel coils immersed will have an impact on the oil level and thus the volume of air.

Look again at @TerryS equation.

It doesn't matter if the spring is fully or partly immersed or upside down. The air volume will always be the same.

Equal to total fork enclosed volume minus oil volume minus spring volume. It is really as simple as that.

 

[dwalby]

It would depend how much of the spring was exposed above the oil level. If the spring is fully immersed, this is moot.

I guess that would depend on the size of the spacers in the forks. For the 1100, the conventional fork spring has a 130mm spacer between the spring and the fork cap, and the cartridge fork has a 50mm spacer that is retained by a clip that sits just below the fork cap. Given the length of the forks, and the amount of oil inside, when at rest there's no way either of them could ever be fully immersed in oil. I think the spacers in the 1300 forks are longer than the 1100, so it may be a bit different, IDK.

Back to the topic of displacement, let's take a slightly different approach, and add some numbers to help sort it out. Let's say you have a fork inner cylinder with a volume of 500cc, that is empty. You then put the spring in, which has a volume of 100cc, now there's only 400cc of air left in the cylinder because the spring displaced 100cc. Now pour 200cc of oil in, and the air volume is now 200cc because the oil displaced another 200cc. The math will be the same regardless of which way you put the spring in, there will be 200cc of oil and 200cc of air.

Now, one other thing that occurred to me while thinking about this is the fork response to compression. Let's use the example above, and add that the fork slider has 150mm of potential travel.
Let's also state that with the spring oriented with the tightly wound coils down, the oil column is 200mm long and the air column is 200mm long. I suspect the spring in the air might complicate the equation, but let's say by using pv=nrT that if we compress the slider by 100mm, the air pressure in the tube doubles because the air column went from 200mm to 100mm. Now reverse the spring so it displaces less oil and let's say now the oil column is 180mm and the air column is 220mm. I'm not 100% sure, but I think in this spring orientation it would take 110mm of slider travel to double the air pressure inside the tube. In both cases you'd have 200cc of air volume, but its distributed over a different length of tube.

So it seems to me that it might be possible that the fork response to compression does change based on spring orientation, the question is by how much and does it make any difference to the average sport-touring rider on a 700lb bike. This might explain why Honda specifies to put the tight coils down, but I'm just guessing. Curious what others may think about this aspect of the question.

 

[Igofar]

The only reason Honda specifies to put the "smaller" or "tighter coils" downward, is the shelf that the spring sits on is about the size of a quarter (same diameter as the smaller end of the spring) and if the spring was put in (ST1300) with the larger, or looser end downward, it would slip OVER the ledge and jam things up.
It's as simple as that, as far as how the spring needs to be installed, again, on ST1300's.

 

[RaYzerman]

Larry's right, that's the only reason. IF you were to upgrade to say RaceTech or someone else's straight rate springs, you'd likely find the springs the same diameter for the entire length... in which case you use one of the supplied washers to drop in first to act as a spring seat.

One usually uses fluid volume as a rough reference to get started.... one sets the oil level per the spring manufacturer's recommendation.... usually lower than OEM spec, since you want more air cushion with stiffer springs.

 

[dwalby]

The only reason Honda specifies to put the "smaller" or "tighter coils" downward, is the shelf that the spring sits on is about the size of a quarter (same diameter as the smaller end of the spring) and if the spring was put in (ST1300) with the larger, or looser end downward, it would slip OVER the ledge and jam things up.
It's as simple as that, as far as how the spring needs to be installed, again, on ST1300's.

you keep saying this, and its true, but it ONLY applies to the 1300, not the 1100. The 1100 manual says tight coils down even though there's no reason at all you can't invert them, they're the same diameter at both ends.

 

[Igofar]

That’s why I specified ST1300.

 

[MidLife]

CASE 1 the spring oriented with the tightly wound coils down, the oil column is 200mm long and the air column is 200mm long. I suspect the spring in the air might complicate the equation, but let's say by using pv=nrT that if we compress the slider by 100mm, the air pressure in the tube doubles because the air column went from 200mm to 100mm

CASE 2 Now reverse the spring so it displaces less oil and let's say now the oil column is 180mm and the air column is 220mm. I'm not 100% sure, but I think in this spring orientation it would take 110mm of slider travel to double the air pressure inside the tube. In both cases you'd have 200cc of air volume, but its distributed over a different length of tube.

So it seems to me that it might be possible that the fork response to compression does change based on spring orientation,

In both cases, the pressure is to be doubled, which means that in both cases, we have to end up with a compressed air volume that is exactly the same, half of the original air volume

According to the equation from @TerryS , the resulting fork enclosed volume will be exactly the same in both cases, it will be equal to

oil volume + spring volume (that doesn't change under compression) + half of original air volume.

And same resulting fork enclosed volume also means same resulting fork travel in both cases.

 

[dwalby]

In both cases, the pressure is to be doubled, which means that in both cases, we have to end up with a compressed air volume that is exactly the same, half of the original air volume

According to the equation from @TerryS , the resulting fork enclosed volume will be exactly the same in both cases, it will be equal to

oil volume + spring volume (that doesn't change under compression) + half of original air volume.

And same resulting fork enclosed volume also means same resulting fork travel in both cases.

I agree both cases have the same volume of air, I'm not disputing that.

But because of the two different oil levels from the different spring orientations the depth of the oil is not the same dimension in both cases. If the oil depth changes, then the air "depth" above it also changes.

let's use my example figures, in one case the air/spring section is 220mm high, the oil/spring section is 180mm high. In the other case the air/spring section is 200mm high, the oil/spring section is 200mm high. Both cases have the same 200cc of air, but the oil level is different because the tighter wound section displaces more oil, raising the oil level inside the tube relative to the other orientation. Do we agree so far?

Now this is where I can't say for sure what would be the result of this, but I think that even though the air volume is the same in both cases, the way its distributed in the tube would cause it to respond differently to compression.

Consider the case where the spring wasn't there and we had the exact same volume of air in two different cylinders, but one cylinder was slightly longer than the other. The longer cylinder would have to be compressed over a greater distance than the shorter cylinder in order to double the pressure. I think we agree on that.

That's what I think might happen with the two different spring orientations, but the fact that the spring is in the mix does complicate things a bit. It maintains the same air volume in either case, but the lengths of the air and oil portions of the fork tube will be different in the two different spring orientations. I think the fork compression pressure rise would be based on how far the tube compresses relative to the original length of the air column, not just the air volume. So what I'm suggesting is that the spring causes the air to respond like the longer cylinder in the paragraph above where there's no spring in one orientation, and as the shorter cylinder in the other orientation.

it is a bit esoteric to discuss, and I can't claim to be an expert, so throwing it out as food for thought.

 

[MidLife]

I agree both cases have the same volume of air, I'm not disputing that.

Then you'd also have to stick with the pV = cst law you were invoking. It's all about volumes.

The fork inside volume is a closed chamber; if the compressed air volume is the same in both cases, the total closed chamber volume HAS to be the same in both cases as well (spring and oil volumes are not changing). And same closed chamber volume means same fork travel in both cases. Basic physics. No esoterism here.

Spring and oil are just taking away volume in this chamber, regardless of how they are distributed. Volume is volume. Same fork travel will result in same fork internal volume and same compressed air pressure, regardless of spring orientation.

And then, if you want to get deeper into your example, you'd have to put more work into it, rather than generating random numbers that are misleading you.

For instance, thinking that you'd have to slide 100 mm out of the 200 mm to double the pressure is incorrect, as the spring is also being compressed in the process and its volume is not divided by two as you slide down. So, if you were to slide 100 mm down, you'd end up with an air volume much smaller than half the initial volume once you have taken out the correct spring volume remaining in this space, and then you'd have more than double the pressure you were looking for.

Then you'd also have to account for the rise in oil level as you compress the fork.

But try to wrap your head around the simple physics of it first.

 

[FastST]

The only reason Honda specifies to put the "smaller" or "tighter coils" downward, is the shelf that the spring sits on is about the size of a quarter (same diameter as the smaller end of the spring) and if the spring was put in (ST1300) with the larger, or looser end downward, it would slip OVER the ledge and jam things up.
It's as simple as that, as far as how the spring needs to be installed, again, on ST1300's.

I think that the whole debate is would it be any difference in riding if the spring were reversed (providing the shelves were reversed too or added adopters).
Would we or a bike feel a difference?
I.E - the lower part compressed more than the upper, or v/s.

With the normal spring, the entire spring comprises equally either you hit a bump or a dip.
A progressive one transfers more compression on the weaker part be it closer to the wheel or to the bars.
But the end result (pls correct me if lm wrong) must be the same.

 

[dwalby]

For instance, thinking that you'd have to slide 100 mm out of the 200 mm to double the pressure is incorrect, as the spring is also being compressed in the process and its volume is not divided by two as you slide down. So, if you were to slide 100 mm down, you'd end up with an air volume much smaller than half the initial volume once you have taken out the correct spring volume remaining in this space, and then you'd have more than double the pressure you were looking for.

Then you'd also have to account for the rise in oil level as you compress the fork.

But try to wrap your head around the simple physics of it first.

I do understand the simple physics of it, but I thought that maybe I'd stumbled onto something else, which might explain why Honda recommends a particular spring orientation for the 1100. We agree the 1100 spring could be inserted in either orientation, so if it made no difference at all, then why would Honda bother specifying an orientation?

If the tight coils are up, the (air volume+spring volume) above the oil is greater than if the tight coils are down. The sum of those volumes being greater causes the air to be distributed over a longer distance inside the fork, which we know is true because we agree the oil level will be lower if the tight coils are up.

So I see this as a case of the same volume of air being present in two different length cylinders. For that to be possible, the diameter of one cylinder has to be smaller than the other, but we know that isn't the case. But I think the difference in spring orientation causes the "effective diameter" of the air cylinder to be different. The fact that there's more spring displacement in one case forces the air to occupy a longer distance inside the fork, which would be the same effect as if we reduced the diameter of the tube. This seems to be the part that you disagree with. I think the spring can be ignored because its volume doesn't compress, as you said, so maybe I'm oversimplifying, IDK.

 

[Larry Fine]

With the normal spring, the entire spring comprises equally either you hit a bump or a dip.
A progressive one transfers more compression on the weaker part be it closer to the wheel or to the bars.
But the end result (pls correct me if lm wrong) must be the same.

I agree. The only difference would be whether the coils that are touching are at the top or bottom, and the only difference then would be whether they become sprung or unsprung weight..

 

[Andrew Shadow]

...... so if it made no difference at all, then why would Honda bother specifying an orientation?

I have no idea why. I offer an opinion for a possible reason that has nothing to do with a technical reason, and is from my perspective of having done technical writing.

If it really makes no difference in the case of the ST1100, and if there is a difference in a part and it can be assembled in two possible ways, this can create issues for the technical writers who write the instructions for the manual. The problem that it creates is if you do not specify which way to assemble it, regardless of the fact that it doesn't matter, you will be bombarded with requests from people who use those instructions asking which way is the correct way to assemble it. To stop that you will have to issue a revision to the manual to specify how it is installed. This usually requires engineering intervention to review and sign-off on the change, so it isn't necessarily a simple matter to correct. It is possible that the technical writers, after confirming with engineering that it doesn't matter, simply flipped a coin and wrote that it must be installed XX way to preclude the time wasting process of having to deal with countless requests and from having to revise the instructions later.

It really might be this simple. I know from experience that once you have had to deal with this because of something that you wrote, believe me you try to avoid doing it again.

 

[FastST]

And that's why on st13 they insisted for it to be "head" down.
But if the result doesn't change - an elephant in the room question: Why progressive?