Fork sub tanks

[brent j]

After reading several magazine articles on fork sub tanks I thought I’d have a play with making some.

They are a small air tank connected to the top of the fork leg via a small passage.
These are supposed to allow you to run a high fork oil level to resist bottoming while still giving the effect of a low oil level to give a supple ride on small bumps.
The magazine explanations said how they worked and why so I looked for a quick and easy way of making some that could be altered easily for testing.

I used 25mm PVC pressure pipe and caps. The pipe is just long enough to allow the caps to touch and I made another set with about 25mm of pipe in between the caps.
One end is threaded to accept a 1/8” nipple and the fork cap is tapped to accept the same. I did use a 1/8” ball valve to try adjustment between the fork and the tank but it wasn’t any better. I do need to fit a schrader valve to the top end of the tank to bleed off accumulated pressure. I do this on my forks anyway.

At present they sit above and inline with the fork tubes, they are about 50mm long. In this position they self drain of any oil that makes its way in. I understand that the commercially available ones need to be removed and drained periodically.
I will make a neater fitting version now I know they do work.

The results were quite surprising. I was able to raise the oil level to minimise fork bottoming then fit the tanks and regain the suppleness to small bumps I had at a lower oil level. With the tanks fitted I have better bottoming resistance than before and overall, a much better ride.

I see these as another tool or option in suspension tuning. I already have my spring rates sorted and I’m using PD valves and I’ve worked out my oil weight and levels.

I’m about to start work on the suspension of my wife’s CRF230 and it will definitely be getting sub tanks.

The down side is that my previously good rear suspension now feels like complete rubbish.

Brent

[jimson]

You got some pic's Brent sounds interesting.jimson

[DR]

ironically Brent I spotted these the other night..kinda similar..no other details excepting it's an RN80 they're fitted to

[evo550]

Doc,
Marzochii manufactured forks with a similar cartridge attached to the bottom of each fork leg, I remember them fitted to the can-am sonics, not sure what they did though.

Brent,
 Does the tank increase the volume of air in each fork leg? If so how does this increase resistance to bottoming? I was always under the impression that increasing oil height in the fork leg, increased resistance to bottoming by decreasing the volume of compressable air in each leg.
Not questioning your judgement, just trying to understand how it works.

[LWC82PE]

in my opinion The Too Tech tank is one of the best for the reason being they are self draining http://www.tootechracing.com/Home%202nd.htm

all the twin tanks ive seen hang down below the top cap and after each race the tanks have to be removed to allow the oil to drain back in. There are many, many brands making these now eg Tech Care Suspension, Enzo Racing, Race Tech just to name a few.

I was reading the other night that the Can Am Marzocchis have a gas chamber in the canisters at the bottom of the forks, not sure if its bladder or piston though, but they said that it was to cushion fork travel at full compression.

evo550 have  a read here
http://www.tootechracing.com/Kayaba%20Air%20Tanks.htm

[evo550]

Aaahhh!! The airflow valve is the secret.

[brent j]

Jimson,

I'll try and get some piccies tonight, I warn you tho, they are not flash.

Doc,

I've seen those in the past but I'm not sure just what they do.

555,

The tanks add to the fork volume and the small 1/8” nipple allows air to transfer between the two with minimal resistance under slow movement hence the smoothness on small and slow bumps. The same nipple creates a massive resistance to the air flow under high speed fork movement and effectively takes the sub tank out of the equation. There is a bit of air transfer but not much.
When I tried the ball valve to create a variable orifice I found no difference between having the valve open and closed on high speed movement.

I’ve thought about trying different sized tanks (they cost about $4.50ea to make!) but I need to upgrade the rear suspension now to be able to assess the front any further.

I’d love to put a fast rider on this thing to see what it will do and how they find it.

[brent j]

It's taken a while but here's my take on sub tanks.

I never said they were pretty, just that they work. I do intend to make a version that mounts horizontaly accross the top of the fork tubes.
I've seen them set up to use the inside of the handle bars but I prefer to keep the two tanks separate so that the oil does not drain from one side to the other if you drop the bike.

Cheers

Brent

[mx250]

Interesting work Brent. I would be interested in more details (close up pixies ) of your dooverlackie. Many moons ago I did a similar thing on my enduro'ised DT360. It made a big improvement, mainly to responsiveness but also softened off the suspension (like you I also ended up with a mismatch front to rear). I had a problem of oil build up in the 'tanks' and the lines blew off on big impacts (my dodgy workmanship . But it was worthwhile. I stopped mucking with the idea because it was in the period that suspensions were improving week by week and it was easier and faster to just let the factories do the development, jump on the latest and greatest and ride the wheels off the thing (as best I could ).

I dreamed of turning the 'bars into a 'tank', I though that was the neatest solution. Maybe I'll revisit the dreams and experiment of my youth .

[LWC82PE]

you could have one tank mounted horizintally but have it split in 2 with a internal divider so its actually looks like 1 tank but is separated in 2 in the middle.

[mx250]

I've had these for a while, I think they are off an RM. I often wondered what the idea of them was. They are the same as most fork caps but have added length to them. Either they allow greater range in lifting the forks to get steering geometry changes and/or they give extra air capacity to give advantages of the air tank idea.

[mx250]

Maybe use something like this .

[LWC82PE]

The tall caps just allow you to slide forks out further and run a higher oil level and change the air spring curve. The Yamaha ones are different and have a dual pressure set up.

These have a floating piston inside and would be same as the Yamaha ones. Someone told me they were made by 'Ottos Works' or something
They are a period itrem though and NOS. just gotta get some lines for them to see what they do to fork action. I wonder if you can still have fork springs though?


Here we go, I thought I had the info somewhere. This is what I was told.

I would be willing to bet these are ``Ottows Werks`` Accumulators. I
used to know the guy that made those. His name was Bob Ottow. At the
time he was in Downey, California, USA. I have long since lost contact
with him. But I am pretty sure that is what those are. I had them on my
1974 Honda CR125. You had to have an air tank to fill up the bottom
side with a certain amount of air, and then fill the top side with a
little less. As the forks compressed the air pressure in the top side
would equal, then excede, the bottom. This would then slide the piston
down in the accumulator.

[brent j]

Leith that's exactly my next plan, a horizontal tank internaly separated. I'll join it to the forks with 6mm nylon air tube and push-in fittings. That will allow me to put the crossbar pad back on without cutting it up.

MX250, The accumulator set up you posted replaces the springs, FROM MEMORY!
The key to getting the subtanks to work is in the bleed between the fork tube and the tank. The tank size will be relavant as well.
I'm not going to play with these again until I get the rear suspension to match the front. The rear is masking what is happening at the front too much.

Brent