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Fork Air preload?

2K views 21 replies 8 participants last post by  cyclopathic 
#1 ·
Anyone had done this on Wee?
 
#3 ·
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.
 
#5 ·
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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#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.
 
#7 ·
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.
 
#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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#17 · (Edited)
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.
 
#16 · (Edited)
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.
 
#20 ·
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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