Fork Springs

[SMSW]

Progressive fork springs are usually wound with more turns per inch at one end than the other end, but the ID and OD of the spring coil is uniform over its entire length*. These springs are unconstrained over their length in the fork tube, i.e they are dropped into the fork and sit on the valving at the bottom while the other end (slider) is capped with a preload spacer. Inverted forks will have these parts reversed.

So why do manuals usually tell one to insert the springs with the closely wound coils down (or up)? It should make no difference at all which end of the spring is down. Force applied to either end will flex that part of the spring where the resistance to bending is least, regardless of position. What am I not considering?

*Even if the spring were wound with differing diameters, as long as the spring is constrained only at the top and bottom it should not matter which end goes into the fork first.

 

[dwalby]

I've always considered that to be related to the fork oil spec. If they spec a certain volume of oil, and expect it to be displaced by the tighter wound end, that determines the height of the oil. Reversing the spring would change the oil level, so you'd have more air in the chamber than with the spring in the other orientation.

However, there's usually an oil level spec as well as oil volume, so if you flip it over and adjust the oil level accordingly, then its the same. But, as the forks compress and the tightly wound end compresses, now that's at the top, out of the oil, instead of at the bottom, in the oil so that might be a bit different while riding.

 

[ST1100Y]

And/or: unsprung weight...

- tight windings up: the whole/most of the spring must be moved along...
- tight windings down: only the few/soft windings add to the unsprung partition...

 

[Igofar]

There are at least two misprinted instructions in the Honda service manuals.
Even showing pictures of the wrong way to insert them.
Then more pictures of which end THEY are calling the tapered end.
Inserted incorrectly, as you probably figured out, the large end slips over the shelf and will collapse the fork.

 

[Willsmotorcycle]

Progressive fork springs are usually wound with more turns per inch at one end than the other end,

I may be out of my depth here. As the name implies, the spring will compress from a given rate to a different rate as it is compressed more. Surely the softer to firmer is preferable over harder to softer.

Looking forward to more information on this.

 

[Igofar]

The correct instructions state to insert the smaller end into the fork tube first.
This smaller diameter sits correctly on the shelf of the cartridge.
The larger “tapered” end is wider than the shelf and can slip over the ledge and jam stuff up!
The OEM springs have different diameters on each end.

 

[SMSW]

The correct instructions state to insert the smaller end into the fork tube first.
This smaller diameter sits correctly on the shelf of the cartridge.
The larger “tapered” end is wider than the shelf and can slip over the ledge and jam stuff up!
The OEM springs have different diameters on each end.

The key here is different diameters land on different diameter washers/shelves/valves, etc. In this case there is a mechanical reason for one end to be down or up - because it mates with a fitting of the same diameter. Or, in my case, on my NC750xD, the OEM preload spacer has a shoulder that fits into only one end of the spring (the larger diameter, 'unprogressive' end). But not all springs have different diameters at opposing ends, and that's where my question originated. @dwalby's air/oil space makes the most sense to me. I'd thought of @ST1100Y's sprung/unsprung weight reasoning but dismissed it as not being significant.

 

[Igofar]

Here is a picture (instructions) from another Genuine Honda Service Manual

 

[TerryS]

I've always considered that to be related to the fork oil spec. If they spec a certain volume of oil, and expect it to be displaced by the tighter wound end, that determines the height of the oil. Reversing the spring would change the oil level, so you'd have more air in the chamber than with the spring in the other orientation.

However, there's usually an oil level spec as well as oil volume, so if you flip it over and adjust the oil level accordingly, then its the same. But, as the forks compress and the tightly wound end compresses, now that's at the top, out of the oil, instead of at the bottom, in the oil so that might be a bit different while riding.

This is a great thought exercise (so I'm out of my depth already).

Within the fork there are two fixed volumes, the oil and the spring. These are incompressible. Then there is the air gap which is compressible. As you compress the fork, the volume of the oil and the spring don't change but the air gap does. The orientation of the spring makes no difference to the rate of the air compression.

The spring itself should be though of as a torsion bar, just wrapped into a cylinder. When the spring is compressed, the wire is twisted evenly along the full length of the spring (unless the spring material itself is tapered). The "progressive" bit of the spring comes from the tight coils becoming coil-bound i.e. in contact with each other after a small amount of compression. At this point they no longer twist as the spring is compressed further so the remaining looser coils twist more per mm of compression which increases the apparent spring rate.

Having looked through a fair few Honda service manuals there is no obvious pattern around orientation of the spring, and I'm sure Igofar is perfectly correct that the diameter of the spring bearing on the top of the damper cartridge is the key.

 

[Larry Fine]

And/or: unsprung weight...

- tight windings up: the whole/most of the spring must be moved along...
- tight windings down: only the few/soft windings add to the unsprung partition...

If one end of the spring is tapered, there is no choice. However, this is my understanding:

Sprung- vs unsprung-weight is the ratio of the weight of the moving (relative to the vehicle) suspension parts (unsprung weight: tires, wheels, hubs, brakes, half of the shocks and springs, etc.) vs the weight of the rest of the vehicle (sprung weight: the vehicle, load, the other half of the shocks and springs, etc.)

The heavier the vehicle and/or the lighter the suspension parts, the greater control the vehicle has in moving the suspension parts around to keep the tires in intimate contact with the road, rather than the wheels moving the vehicle around. That's why heavier vehicles can be designed to handle the road well.

When the closer-wound coils compress to touching first, that end effectively becoming a solid cylinder. If that end of the spring is up, it becomes sprung weight. If the closer coiled end is down, that end becomes unsprung weight. Which is better or handling, a heavier front wheel or a lighter front wheel?

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 total weight does not change, of course.

 

[dwalby]

Within the fork there are two fixed volumes, the oil and the spring. These are incompressible. Then there is the air gap which is compressible. As you compress the fork, the volume of the oil and the spring don't change but the air gap does. The orientation of the spring makes no difference to the rate of the air compression.

Thanks for the kick in the butt, I hadn't considered the effect of the spring displacing air as well as oil. So although the oil level drops if you flip the spring to tight coils up, the volume of air does stay the same. There's more potential air space above the lower oil level, but more air is displaced by the tighter wound end of the spring, so it ends up about the same volume of air being trapped after you put the cap back on, regardless of spring orientation.

However, this applies only if you measure the fork oil by volume, not by height. If you measure by height, then you'll use a different volume of oil depending on which way the spring is oriented, and the equation will not balance. The potential air space will remain constant, but the amount of air displaced by the spring will be different depending on which way the spring is oriented.

 

[Igofar]

If one end of the spring is tapered, there is no choice. However, this is my understanding:

Sprung- vs unsprung-weight is the ratio of the weight of the moving (relative to the vehicle) suspension parts (unsprung weight: tires, wheels, hubs, brakes, half of the shocks and springs, etc.) vs the weight of the rest of the vehicle (sprung weight: the vehicle, load, the other half of the shocks and springs, etc.)

The heavier the vehicle and/or the lighter the suspension parts, the greater control the vehicle has in moving the suspension parts around to keep the tires in intimate contact with the road, rather than the wheels moving the vehicle around. That's why heavier vehicles can be designed to handle the road well.

When the closer-wound coils compress to touching first, that end effectively becoming a solid cylinder. If that end of the spring is up, it becomes sprung weight. If the closer coiled end is down, that end becomes unsprung weight. Which is better or handling, a heavier front wheel or a lighter front wheel?

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 total weight does not change, of course.
[/QUOTE
You’re over thinking this.
Regardless of what you think about sprung or insprung weight etc.
The larger (wider) end when inserted “upside down” will slip over the cartridge seat and not work.

 

[TerryS]

Thanks for the kick in the butt, I hadn't considered the effect of the spring displacing air as well as oil. So although the oil level drops if you flip the spring to tight coils up, the volume of air does stay the same. There's more potential air space above the lower oil level, but more air is displaced by the tighter wound end of the spring, so it ends up about the same volume of air being trapped after you put the cap back on, regardless of spring orientation.

However, this applies only if you measure the fork oil by volume, not by height. If you measure by height, then you'll use a different volume of oil depending on which way the spring is oriented, and the equation will not balance. The potential air space will remain constant, but the amount of air displaced by the spring will be different depending on which way the spring is oriented.

No butt-kicks intended mate; I scratched my head over this one for a while, it is only my 2 cents worth.

I will take exception to your comments about measuring by height or volume. The oil level should always be set by height, with the springs removed (at least that is what Mr Honda says). I'm not sure that you could realistically do it accurately with the springs in place.

 

[Igofar]

Is nobody paying attention?
The spring CANNOT be inserted with the small end upwards and work properly.

 

[SMSW]

Thanks for the kick in the butt,

However, this applies only if you measure the fork oil by volume, not by height. If you measure by height, then you'll use a different volume of oil depending on which way the spring is oriented, and the equation will not balance. The potential air space will remain constant, but the amount of air displaced by the spring will be different depending on which way the spring is oriented.

Not clear why you like a kick in the butt. Honda wants the oil measured by height from the top of the fork without the spring in the tube. Putting the spring in one way will result in a smaller air volume than the other way.

I wonder how much difference in oil height the closely spaced spring coils would displace vs. the regularly spaced coil and if a few mm difference in oil height would be felt by the rider.

 

[Igofar]

The oil is measured without the springs in place, with the forks fully collapsed.
Filling the forks with oil, then measuring the oil height from the top of the tube down.
By drawing it out from the top with the proper tool, both levels will be even.
Then you must push/pull the damper rod 20 or so times to get the air out, then push up/down on the fork lower to remove air etc.

 

[Igofar]

By putting the large end down, would be like placing a coffee can over a soup can and expecting the soup can rim to support the coffee can.

 

[SMSW]

Is nobody paying attention?
The spring CANNOT be inserted with the small end upwards and work properly.

Of course not, Larry. I posed a hypothetical question about a spring constrained by two planes, top and bottom and wondered if dropping the spring in one way or the other would make a difference. The discussion has been helpful, even if it has gone astray. My take is that for my original question, the only difference would be in the air volume and the spring effect of compressing said two air volumes (close coils up or down).

Of course, if there is a mechanical reason (there is in my NC750 and also in STs) for one orientation, then the dang spring will only fit one way and the rest of the q is moot.

 

[Larry Fine]

My first sentence was: "If one end of the spring is tapered, there is no choice."

The rest of my post was generalization, which is why I said "vehicle" instead of "bike."

 

[dwalby]

Honda wants the oil measured by height from the top of the fork without the spring in the tube.

The oil level should always be set by height, with the springs removed (at least that is what Mr Honda says).

thanks for pointing that out, I've always done it by volume so I never read the details on how to do the level measurement.

regarding the comment that it won't work at all to invert the spring, I haven't looked at it much over the years but in my weak memory the springs are the same diameter on both ends, there's no taper to them. The illustration in the service manual also looks like a cylindrical spring with no taper, but I don't know that the illustration really matches reality. Anyway, if there is no taper, then why wouldn't it be possible to invert the spring?

Or, maybe the 1300 springs are tapered and the 1100 springs aren't?