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Thought about it, but the PITA problem of having to make up a special tool to adjust the normal preload put me off.

If you do it, be sure to let us know how it goes ;)

Pete
 

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Discussion Starter #3
Thought about it, but the PITA problem of having to make up a special tool to adjust the normal preload put me off.

If you do it, be sure to let us know how it goes ;)

Pete
Bicycle shock pump like this

I suppose regular pump would work but it would be harder to get pressure right. And it would be one extra thing to carry.

The beauty of air preload that along with sag you will be also adjusting spring rate.. which makes sense as more sag usually means heavier rider.
 

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Using air in the forks was done by a lot of manufacturers a while back. I don't know any do it now?
I had a tiny hand pump that was used back then.
A search shows air suspensions for Harleys. Pricey stuff!
 

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Adding air to damper rod forks was popular back in the late 1970's and early 1980's. It was abandoned because all it did was make the forks harsher as pressure was added. Modern air forks are very different, and even the best new air forks are only just now approaching the performance levels of the best spring forks. Purchasing the correct spring rate will always be much better than simply adding air on top of a weak spring.


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I have a rear air shock (Cloud 9) on my Kona mountain bike and it works fabulously.
 
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I had several Honda's with air forks and shocks. If all the air was removed the fork was very soft. As soon as you put a little (1 or 2 lb.) in them, they started firming up. I think they were designed to have a very large range of stiffness and load capacity. They gave a very comfortable ride and I liked them a lot. That said, I'm not a professional motorcyclist like some folks and wouldn't know the performance levels of the best spring forks from a 2x4.
 

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Discussion Starter #8
I decided to take a dive and got a set of oem 43mm caps off fazer.

If my back of the napkin calculations are correct, adding 15psi would firm up stock Glee springs to 0.98kg/mm and 30psi to 1.12. Insistently per RaceTech calculator rates correspond to 220 and 300lbs rider.

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Discussion Starter #9
Adding air to damper rod forks was popular back in the late 1970's and early 1980's. It was abandoned because all it did was make the forks harsher... Purchasing the correct spring rate will always be much better than simply adding air on top of a weak spring.
I think the major advantage of air is adjustability. Old Concours had rear air shock and you could adjust it from solo to 2up loaded by adding a few PSI.

The issue I am having with Glee forks that the rate is right for my weight, but not for full load. Yes most of additional weight is riding over rear and Intiminators helping with brake dive, but still being able to stiffen up front would be of help.

And no I don't wanna stiffer front springs as I only spend small ℅ of the time 2up loaded.
 

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I think the major advantage of air is adjustability. Old Concours had rear air shock and you could adjust it from solo to 2up loaded by adding a few PSI.

The issue I am having with Glee forks that the rate is right for my weight, but not for full load. Yes most of additional weight is riding over rear and Intiminators helping with brake dive, but still being able to stiffen up front would be of help.

And no I don't wanna stiffer front springs as I only spend small ℅ of the time 2up loaded.
That old Concours had adjustable damping too, so you could tailor it to the air pressure. Hard to do that with damper rod forks.

The other downside to air in motorcycle forks is that it makes the rate more progressive, which is a bad thing.

MTB suspension needs and priorities are very different from motorcycles, very little theory or practice transfers between the two.
 

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Discussion Starter #11 (Edited)
That old Concours had adjustable damping too, so you could tailor it to the air pressure. Hard to do that with damper rod forks.

The other downside to air in motorcycle forks is that it makes the rate more progressive, which is a bad thing.

MTB suspension needs and priorities are very different from motorcycles, very little theory or practice transfers between the two.
At 15 psi and stock oil level we are talking about 45lbs additional on unloaded and roughly 90lbs at the top. This is compared to 570lbs from stock springs, 45lbs extra is just a drop in the bucket.

I don't think there will be much issue with damping stiffer spring = less damping needed, but more rebound. I just fitted HyperPro rear spring which goes from 8.7 to 11.4 and was half expecting problems with rebound. Surprisingly no issues whatsoever and compression damping didn't needed any adjustment.

With air preload rate change will not be as drastic, this shouldn't be any issue at all. And it isn't on bikes where this convention popular like old GW or was supplied from the factory.

PS and I respect your opinion but disagree that progressive is a bad thing. Actually in my limited experience it had always been a good thing.

MC makers also widely use progressive setup most of rear linkages set up as progressive, and many dirt forks use progressive springs.
 

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The problem with air is that the progression is linear. Rear linkage suspension offers engineered progression rates.


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...MC makers also widely use progressive setup most of rear linkages set up as progressive, and many dirt forks use progressive springs.
The load on the rear varies much more widely (passenger, luggage) than the load on the front does. That Connie shock was actually a good idea in theory, but they screwed up the execution. If they had put a stiffer spring in it, so that 0psi worked well for a lighter rider, and had the damper adjuster done more, then it would been nice. As it was even at 160lbs I need 30psi and it was always underdamped. With wife and luggage it was undersprung even at max pressure (50psi)

Dirt bikes have 12"+ of travel, and have to deal with very hard landings off botched jumps. It makes sense to stiffen up the last few inches to handle that, there's still plenty of travel available for "normal" situations.

Even with a straight rate spring, a fork has a somewhat progressive overall rate due to the trapped air volume. Depending on the setup, the final rate is going to be ~15-20% stiffer than the initial rate. That amount of progressivity is ok. (You need a little bit because of the inherently pro-dive nature of telescopic forks) Add a progressive spring to the mix though and things get out of whack, with the final rate being ~35-50% greater than initial. Adding air preload jumps that up even higher, maybe a lot higher depending on how much pressure and what the trapped air volume is.
All the Japanese manufacturers tried air caps in the early '80s, and they all abandoned them. Like the various anti-dive braking schemes they fitted around the same time, it just didn't work.
 

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Discussion Starter #14
The load on the rear varies much more widely (passenger, luggage) than the load on the front does. That Connie shock was actually a good idea in theory, but they screwed up the execution. If they had put a stiffer spring in it, so that 0psi worked well for a lighter rider, and had the damper adjuster done more, then it would been nice. As it was even at 160lbs I need 30psi and it was always underdamped. With wife and luggage it was undersprung even at max pressure (50psi)

Dirt bikes have 12"+ of travel, and have to deal with very hard landings off botched jumps. It makes sense to stiffen up the last few inches to handle that, there's still plenty of travel available for "normal" situations.

Even with a straight rate spring, a fork has a somewhat progressive overall rate due to the trapped air volume. Depending on the setup, the final rate is going to be ~15-20% stiffer than the initial rate. That amount of progressivity is ok. (You need a little bit because of the inherently pro-dive nature of telescopic forks) Add a progressive spring to the mix though and things get out of whack, with the final rate being ~35-50% greater than initial. Adding air preload jumps that up even higher, maybe a lot higher depending on how much pressure and what the trapped air volume is.
All the Japanese manufacturers tried air caps in the early '80s, and they all abandoned them. Like the various anti-dive braking schemes they fitted around the same time, it just didn't work.
It is my understanding that Kwak used the same shock on Connie and ZX600. So no surprise it was too light on C10. I had used zx600 shock on much lighter ex250 commuter and it worked very well. Only needed a few PSI.

Air shocks are very popular on mountain bikes, and they work very well there. Partially because MTB do not use progressive linkage for space and weight concerns. Forks too there were forks designed with air springs.

The progressive feel of fork due to trapped air isn't 15-20℅, with 150mm level and roughly 150mm of travel the internal volume reduced by x2, so pressure goes from barometric 15psi to 30, and additional 15psi = 45lbs of force.

Springs themselves generate 0.85*2*150=255kg or 560lbs, and additional 45lbs is only 8℅. Pumping additional 15psi will add another 8℅, 16℅ total. That is minor comparing to how progressive say Progressive or HyperPro are, and still better than just adding 1/2" of preload on straight rate. And you can adjust progressiveness by changing oil level.

If you think air preload is bad idea, read reviews of those who did it on bikes like GW or Honda CRF.
 

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Your math is significantly off.
 

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Discussion Starter #16 (Edited)
Your math is significantly off.
I don't think it is. The internal area of fork tube ~1.5sq.in, 1.5 x 2 tubes x 15psi = 45lbs. The ID of the fork is only slightly larger than OD of 1" PVC pipe we used to cut preload from, so 1.5 is about right.

EDIT: ok adding additional 15psi at the bottom will result in additional 90lbs at 150mm compression, which will be 15℅ more over stock setup. However out of those 90lbs 45lbs will be at the bottom, so increase is only by 45lbs, or 7-8℅.

Overall increase over spring only is 24℅ total.
 

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Discussion Starter #17 (Edited)
The problem with air is that the progression is linear. Rear linkage suspension offers engineered progression rates.


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No it is not linear it is hyperbolic. P*V stays constant, so reducing volume by 1.5 times increases pressure by 1.5 time. It actually rises slower at first but then it rumps up pretty quickly. And you can change progressiveness by changing level adding extra 50mm of oil will give you 250mm to 100mm - 2.5 ratio. Another 25mm and it will be 225 to 75 or 1:3.

Edit: read this https://en.m.wikipedia.org/wiki/Ideal_gas_law

For the purpose of this we can ignore rise in temperature which increase pressure. Forks are hearing up but not by significant amount.
 

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I don't think it is. The internal area of fork tube ~1.5sq.in, 1.5 x 2 tubes x 15psi = 45lbs. The ID of the fork is only slightly larger than OD of 1" PVC pipe we used to cut preload from, so 1.5 is about right.

EDIT: ok adding additional 15psi at the bottom will result in additional 90lbs at 150mm compression, which will be 15℅ more over stock setup. However out of those 90lbs 45lbs will be at the bottom, so increase is only by 45lbs, or 7-8℅.

Overall increase over spring only is 24℅ total.
Your arithmetic is good, but the input numbers (and assumptions they are based on) are wrong.

First, you use the outer diameter of the fork tube, 43mm. Consult any reference on hydraulic system design for the reasons there. Converting that to in^2 we get 2.251. 2.251 x 15psi x 2 tubes = 67.53lbs.

More crucially though, you're assuming that the oil height rise in the tube is equal to the amount of compression of the fork assembly. It's not, the oil height rise is significantly greater. Exactly how much is difficult to calculate, you need to know the volume, on a per unit length basis, of the spring, spacer and inner fork tube.
If you do get air caps on the forks you can test it empirically though, compress the fork an inch at a time and measure the pressure each time.
 

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Discussion Starter #19
Your arithmetic is good, but the input numbers (and assumptions they are based on) are wrong.

First, you use the outer diameter of the fork tube, 43mm. Consult any reference on hydraulic system design for the reasons there. Converting that to in^2 we get 2.251. 2.251 x 15psi x 2 tubes = 67.53lbs.

More crucially though, you're assuming that the oil height rise in the tube is equal to the amount of compression of the fork assembly. It's not, the oil height rise is significantly greater. Exactly how much is difficult to calculate, you need to know the volume, on a per unit length basis, of the spring, spacer and inner fork tube.
If you do get air caps on the forks you can test it empirically though, compress the fork an inch at a time and measure the pressure each time.
The OD of the fork 43mm but ID is less. I think I have used 38mm to calculate 1.5sqin area (it was slightly less)

Point taken on level the lower has greater displacement than tubes. I had measured oil level on fully extended when we had forks out installing Intiminators, but can't find the number. You also need to take into account pressure at static sag (should be ~2psi) and volume of spring and adjuster to be precise. Dropping level to say 330mm on fully extended only amounts to 1:2.2, 10℅ error.

But the calculations above are rough estimate, so 20℅ more or less would only mean that you need 12 or 18 psi instead of 15. Close enough you can fine tune it at that point.

The same way as fork rate calculator is a rough estimate it doesn't take into account many things, weight of furkles, tank bag, bike load, riding position, etc. And then you limited to existing rates if you are in-between you have to choose one.
 

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I experimented with adding schraeder valves to fork caps back in the late 70's and early 80's. Yes, the air increases spring rate. I don't remember needing anywhere near 15 psi to do the job, but that was a long time ago. As mentioned above, air springs are progressive and the damping is NOT and I found my front tires didn't follow bumps very well and that was felt in turns in a very negative way.
The problem I never found a solution to was equal air pressure. I suppose one could fabricate a tee valve to do that. Unequal pressure causes a LOT of stiction in the forks! These were not designed to seal air, so they didn't maintain air pressure very well over time.
 
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