# Fork subtanks... are they for you?

Discussion in 'Thumpers' started by creeper, Aug 27, 2006.

1. ### creeperStill alive...

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Installing Infinity Machine & Design subtanks

Air, Huh! What is it good for? Absolutely somethin'!

To preface this little article a bit about air as a spring.
As it relates to its confinement in a front fork suspension, air functions as a spring a pure, perfect spring.

At sea level, in our fork air chamber we have 14.7 PSI, or atmospheric pressure equivalent to outside pressure.
If we reduce the air chamber in the fork by half, the pressure doubles to 29.4 PSI, or 14.7 PSI above. Reduce that by half again, and our pressure quadruples to 58.8 PSI.
What our initial air chamber size is to begin with determines how quickly the pressure will increase as the chamber size is reduced from suspension compression.
If we have a 150mm long air chamber, the pressure will increase at a rate slower than a 100mm long air chamber for a given amount of suspension travel, with a lower final pressure as well.

An example:

Suspension A has a 100mm air chamber with atmospheric pressure. We compress the suspension 75mm, reducing the air chamber to 25mm. The pressure of the air is now 58.8 PSI

Suspension B has a 150mm air chamber with atmospheric pressure. We compress the suspension 75mm, reducing the air chamber to 75mm. The pressure of the air is now 29.4 PSI

For a given amount of travel, suspension A pressure numbers demonstrate that, although it starts out with the same pressure as suspension B, the rate that it increases will be faster and the end pressure higher due to the smaller starting volume.

The idea of subtanks:

What does this have to do with Infinity Machine subtanks or any brand of subtank for that matter?
Simple a subtank is the bolt-on, external equivalent of having both A or B air chambers at your disposal.
By adding a secondary air chamber, with a variable size orifice for transfer of air, you effectively increase your total air volume and can regulate how fast or slow that air moves from one chamber to the other adding another layer of tuneability.

Riding fast on rough ground and need a firmer suspension? Turn the subtank control valves to closed or near closed eliminating or reducing the combined air chamber size and slowing the transfer of pressure between chambers.
Want to tune in a little more anti-dive control for a spirited ride in the twisties? Same adjustments.

Riding on slow, tight, technical ground where a softer, more compliant suspension would be beneficial? Open the subtank control valves more or all the way, increasing the combined air chamber size and increasing the transfer of pressure between chambers.
Riding around town with ugly, pothole infested, frost heave checkered roads? Again same adjustments.

Two forks in one at the turn of a valve.

Tuning subtanks is a matter of adjusting the fork air chamber by adding or removing oil, and adjusting the control valve to suit the terrain and speed.
On your fork, with subtanks added and the control valves turned off, you have not changed a thing; you have the same fork as before.
Open the valves and the fork instantly becomes more compliant.

My experience so far:

My fork is fitted with a set of .52Kg/mm springs vs. the original .44Kg/mm springs. I did this to firm up what I felt was a far too soft suspension for my riding style and weight.
The fork works great when speeds are high and the ground is rough, but I lost much of the sweet, low speed compliance so handy for tight, slow technical terrain. Instead of tracking over things, I was being bounced off them.
My option, while still retaining the .52 springs, was increasing the air chamber volume within the limits of fluid requirements this helped, but not quite enough.

I was wondering what I would try next (cant leave shit alone for long) when I read Zerodogs post in the vendor forum.
Id considered subtanks but only in passing, as what I saw available was kinda pricey at \$275 and up... well over \$300. Infinity subtanks are quite a bit less, and even cheeper with a KTMtalk member discount.

Zerodog (Rob in the world) owns Infinity Machine & Design, and his subtanks are his current and only product.
Hes an experimental kinda guy, and his post was about the use of ball valves on DS bikes, in particular his KTM 640 Adventure, as opposed to the more tunable control valves.
The idea being that a typical DS guy would like two basic suspensions at their disposal one for street and one for dirt.

A simple ball valve would allow an open or closed position and even some un-indexed middle positions if you want to play with it.
With proper placement, the valves would be accessible from the saddle for a quick on or off.
I liked Rob's ball valve idea quite a bit enough to comment, contact and finally order a subtank kit.
I guess I got the jump on him, as he was still in the process of making a pair of delrin mounting blocks for attaching the tanks to the KTM 640 Adventure headlight subframe.

What do I think? I think it was money well spent. I have not had a chance to ride off-road as I've re-injured my wrists and they need a little break. I have ridden on-road, experimenting over differing ground with the valves open and closed.
Compliance over stutter bumps and irregularities in the road have been softened greatly with the valves open, and with the .52 springs, brake dive is minimal and easily regulated. With the valves closed, the fork is as it was... firm.
I intend to experiment with oil level a bit the first experiment will be reducing the level 5-10mm to see if the compliance improves a little more with out negatively affecting the valve closed position.

Installation:

Installation on a KTM/WP 4860 MXMA fork (640 Adventure) requires removing the fork caps, drilling out the existing 4mm bleed screw holes with a 5/16 or R letter drill, and tapping those holes with a 1/8 - 27 NPT tap.

There are two ways to remove the caps. One is to remove the forks from the bike and clamp the legs into vice. The other is to leave the legs in place and lower the bike in a controlled fashion to expose the damper rod "nuts".

I either instance, you must be able to compress the fork to extend the cap/damper rod/spring assembly up and out of the fork leg.
I choose to use the bike as a vice, left the forks in the trees and used a motorcycle scissor lift to lower the bike, exposing the parts I needed to access.

You will need a 22mm wrench to hold the damper rod and unscrew the cap assembly. The cap nut is 24mm.

Once you have removed the caps, they can be drilled and tapped, then reinstalled.
You must tap the holes all the way to the end of the tap threads for the diameter to be sufficient to fit the air hose ends.

Once the forks are reassembled, you can install the push fit or bayonet style air hose ends included in the kit.

That was the hard part the rest is easy. Locate the position for where you want the subtanks to be attached on the headlight subframe. Mount the tanks so the bleed valves (thats right, bleed valves are included) are facing up. This is necessary so that any oil that collects in the tanks will be drawn back into the forks.
I havent received my delrin mounting blocks yet (told you I got the jump on Rob) so my subtanks are, for the moment, mounted with large, high tensile zip ties and polyethylene sleeves between the frame and subtanks to act as a cushion.
The mounting system is still evolving for this application, so I cant say for sure what you would get today, or a week from now... ask Rob.

Finally, route your included hoses so that they are not pinched when you turn the forks lock to lock and fit them to the subtank valves.

Thats about it. Go for a ride and see what its like to have two different forks at the turn of a lever. It's pretty handy if you ask me.

Caio,
C
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2. ### dagwoodBanned

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Cool shit creep.
I recal back in the day we had something like that on our street bikes but with the advent of upside down forks they seemed to fade away as uneccesary.
We also linked the forks together and had/ran positive air pressure of like 3 to 8-12? psi. been along time, don't recal exatly.
(different app I guess) but this is cool for the kattom.
3. ### creeperStill alive...

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You're gona laugh at this one.

Back in the late 70s early 80s, I built a rigid framed Panhead... kind of a little, light weight, open belt drive, magneto ignition "Bob job". About 400 lbs.

I did some kinda odd stuff with brakes and forks. First, I ran a 750 Honda rear drum brake... with a cush hub.
The front fork was a Fox Factory 44mm RSU MX air fork for a Honda CR250R... brand new in the box, the latest in "dirt tech", and a complete front wheel assembly with cool conical hub for the same bike.
I had to modify the trees to accept a 1" fork stem from a Paughco springer fork... the last thing was to slap a 21" Avon Speedmaster tire on it, and I was good to go.

Anyway... the Fox fork was cooler than shit. I had the cross-over with schrader valve you refer to, and could with a squirt of air, raise and lower the ride height (12.5" of travel in those babies) to, in effect have a extended fork chopper, or an in the dirt bobber.

Wish I could find pics of that bike... it was all white ya' know.
4. ### Two DotBeen here awhile

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We REALLY need pictures of this bike... what if I volunteer to come up there and help you look... I could bring my seat...
5. ### creeperStill alive...

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You mean you haven't sent that seat yet? :huh
If you want to see your future cover... look here.

I think my wife has all the "good old bad old days" pics in storage.
The bike was in the... October '81 issue (I think ) of SuperCycle magazine. White bike, big boob'd Israeli chick in white leather g-sting thing... and a long blond wig.
Oh... and in an Easyriders spread on the '82 LA. Toy Run at the Coliseum.

(I never did save either magazine... what a dumbass )
6. ### creeperStill alive...

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OK... I dug around and found an old pic in a really old wallet. It enlarged pretty good.

This was taken in '81 or '82 after arriving in Ruidoso, NM. from Fontana CA. I got that raccoon look 'cause a few days of riding in to the sun in August with just shades will do that to ya'.

I know, I know... I look just like an extra from an old 'B' HA movie.
7. ### 2whlrcrgooligan

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I knew you looked familiar.

8. ### ZerodogLong timer

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Thanks for the big review / install guide creeper.

Here is a shot of the new delrin blocks for mounting. They should give you a little more room to move stuff around.

If there is enough interest in the system for 640s I will develop a cool mounting system like I have for MX bikes made of billet aluminum. But instead of clamping on the fork legs they would clamp on the headlight frame rails.

Creeper that was a cool bike you had there! It must have been the cat's ass back then too. Do you know what ever happened to the bike?
9. ### creeperStill alive...

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Well, you have a well-made product at a fair price Rob. Might as well make mention of it should anyone else be considering same.

The delrin blocks are the cat's ass. Looking forward to getting mine... the current set-up is secure, but kind of unfinished looking.

Scattered to the four winds no doubt. Might have a few bits still stuck in some hole someplace.
10. ### motomikedhBeen here awhile

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I recently installed subtanks on my KX250, man did it smooth out the small chattery bumpy stuff. When I go trail riding I open up the subtanks and it's smooth as glass on the small bumpy trails, then out on the track, keep em closed when you go to the track. I plan on putting some on my '94 Elefant to help out on the small rocky dirt roads we ride on. Get some, they are worth it, and are top quality!!!
11. ### creeperStill alive...

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Recently got the 640 Adventure specific delrin mounting blocks in the mail... very nice machine work on these!

They are a bit on the slippery side (Delrin slippery? What a shock!) so I put some double sided foam tape on the grooves to hold things in place.

The spacing is just about perfect and allows you plenty of latitude for positioning the subtanks on the fairing frame.
I was considering cutting the blocks in half, so that they could be positioned at the ends of the subtanks rather than the center... much like I had in the initial install, but I got lazy.
The silly and not so silly reasons behind doing that would be better, more stable support... and 'cause there would be no strapping over the spiffy decals.

Anywhoo... here is the current mounting. Two industrial strength zip ties per subtank. There is a tiny bit of movement if you "lean" on them, but still plenty tight not to go anywhere.

Still not able to ride in the dirt just yet... which bums me out, but I'll try and keep up with reports on the tuning stage as it goes. At the very least, a full report of performance after Loadedpalooza v4.0.

That's it,
C
12. ### Ricardo Kuhna.k.a. Mr Rico Suave

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Wow Don ZeroDog good Job on the Subtanks and about you Creeper I do't even say nothing since you work is flawless everytime..

anyway I do have a little suggestion, after many years of routing bicycle wires and hydrolic lines, looks to me like will be better for the hoses on your sistem if you have a little better of a radius for them (less sharp, more gradual) so my suggestion is to run the Subtanks on the opposite side (left leg hose going to the right side and viseversa) with the hoses routed forward into the fairing, that way all the slack (grow when your are turning) of the hose will be asimilated in small increments and under the fairing as oppose to be crush by the radiators and the fork legs (like at least it look on the pictures) the only other compromise is that you will need to run the subtank aircaps at the bottom of the cilinders (hopefully that is not a big deal) or get a longer air hose for them..

just a suggestion that I'm sure you even consider already since you are a master mind when it comes to mecanical things..

Oh ZeroDog even the stickers look good on your product
13. ### creeperStill alive...

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Hi Ricky

The radius' are not as tight as they appear in the pics... and the hose is a very flexible, thick-wall product rather than the harder stuff we're used to.
I don't know where Rob gets it, but it's nice stuff for this application.

I had the hoses running "cross-over" initially (left leg, right subtank etc) but then realized that there was only a little less than two inches separating them, so I "un-crossed" them.
I'll be keeping an eye on the routing for awhile... as they are QD fittings, it will be an easy thing to shift them around should the need arise.

Combined, even at full left or right lock, there is only one large, less than 180 degree bend and 1 shallow radius 90 degree bend in each hose.
As the fork goes from lock to lock, it's more of a "back and forth" angular change rather than a radius change for each hose.

I'm not sure if I understand you on the subtank mounting... but the ball valves must be at the "bottom" of the mounting so that any oil collecting in the tanks will be drawn back into the forks... under the vacuum of full extension.
The bleeder valves as well, must be at the "top"... unless you want a possibility of them squirting oil at every use. :eek1

Good to hear from you Ricky... taker easy,
Chris
14. ### buffallodanLong timer

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Could these be made to work with the Husky 610? It seems the mounting points would need to be some type of fabricated bracket as there is no solid forward attatchement points on the Husky's...

Dan
15. ### creeperStill alive...

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Actually, the Adventure mounts are what's out of the ordinary. With more conventional bikes with a number plate/headlight arrangement you would mount them either behind the numberplate (if there's room), behind the forks, or to the side and offset to the front or rear... but attached to the fork legs themselves.

On a Husky SM (as seen below), there seems to be room directly behind the number plate... just behind and inboard of the outer edges.

The subtanks themselves are about 7 inches long, including the valves, and 1 1/16" in diameter.

C
16. ### buffallodanLong timer

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Nice, thanks Creeper,

Dan
17. ### creeperStill alive...

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ZeroDog is the expert on all this; he may have already sold a set to someone with a Husky SM and might know the particulars... or at least he could offer some insight into the possibility of fitting them.

C
18. ### Ricardo Kuhna.k.a. Mr Rico Suave

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as ussual you are right on target Chris, Some how i was sure you explore every possible routing and problem since after all your are Creeper the master blaster.

yeap that was my idea and yes it make perfect sense for the check vlaves to be were they are...

thanks for the explanation as ussual, very imformative and educational
19. ### creeperStill alive...

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Thanks Ricky.

So how you been doin'? Makin' any nifty parts lately?
20. ### Ricardo Kuhna.k.a. Mr Rico Suave

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Oh man don't ask, is like I have a dark cloud over my head with thunder lightning and everything...
actually Yes, just for my own pleasure and to keep my head above water, soon when i get a new camara I post the progress.