fork sub tanks leaving unbalanced suspension

[jaguar]

I've been reading about the sub tanks for the forks which gives you a "softer" front end until you land from a jump or hit some nasty bumps, during which it changes to be "harder" (more resistive to movement).
But the question that comes up for me is how to change the rear end to match? Maybe if the shock has a high and a low speed compression adjustment then you could lessen the low speed damping and increase the high speed damping to compensate.
But in my case the shock only has one compression adjustment.
I thought that maybe reducing the gas volume of the bladder (by compressing the shock a bit, letting out the gas, and then refilling the shock with oil while the shock is at full extension, and then repressurizing the bladder with gas) would cause it to be more progressively harder which may approximate the action of sub tanks once the compression is also lessened. I doubt that would completely match but at least it would lessen the difference between the front (with sub tanks) and rear.
any comments?
new thought: since the pressure on the rear shock is a multiplication of what force is moving the rear wheel up, then utilizing pressure to affect the action would be minimalized. Its not like the front where there is no linkage system to multiply the wheel force to the fork. So I doubt my idea would be of much use. So then, what could be done? I doubt anything since nothing else aproximates the action of a sub tank system. It makes the forks stiffer with hard hits, whereas valving can only make it softer with hard hits.
On a MX course if the front is softer than the rear then low angled jumps would leave you nose diving if you didn't gas it, and woopdies would leave you swapping ends. :(

 

[marcusgunby]

I havnt heard of people saying they make the bike feel unbalanced, they should do something other than just soften the front-we can do that with valving easily.They make the front airspring speed sensitive not just position sensitive.This is how they improve the feel.I wouldnt worry about your concerns as most who have them like them alot.

 

[mxneagle]

Be sure to increase the oil level in the forks and you shouldn't notice that the forks fell much softer. They should just feel stiffer on the big hard hits (like comming up short or over jumping)

 

[jaguar]

In browsing posts on various sites I have seen where the rider thought the bike was left unbalanced which does make sense.
I think it would be possible to make a sub tank for the rear also just by connecting to the valve. Anyone know of an online supply store that could provide the right connectors?

 

[Jeremy Wilkey]

Hey guys!

Yeah you can do a sub for the shock and you could see it last year on Ezra Lusks KX.. Ross Maeda worked on for the KTM as well.

Here is the deal.. I worked extensively on this with Ross and MACE in writing some fantastic software that allows the effect of rod force relative to gas volume and a speed through a restrictive barrier to be modeled in a way that gives a real number.. Mace worked his butt off on it and it was slick, unfortunately the project never got very far.. However it was a fun thing to at least get your head around.. After much thought the best way to do it is with 3 tanks.

In general the rod charge is flat. In other words on a normal shock the gas charge impacts the rod like adding weight, it is not a multiplying force as the gas volume is so large relative to rod displacement. So for a visual example you can take the spring off a shock and place onside on a scale. You start to push and nothing other than the numbers on the scale dial going up. After you continue to add weight you'll notice the shock will start to move, at that point the scale will stay put until you've completely compressed it.. So gas pressure relative to volume can be exponential it is not in high volume to displacement ratios.. So gas is not a spring, although it can act like or a spring can act like gas they are very different as they deal with force and displacement. This is why the ICS spring in a twin chamber fork acts not like gas or preload, but rather adds force in even incoromens through the stroke..

So the best way to accomplish a speed sensitive gas spring is to do it with three tanks a short one with 2 exits.. One exit goes to piston with a movable stop the other with a restive barrier.. When the displaced gas has 2 exits the division results no net force gain, when the tank 2 piston hits the stops the third tank now has the volume which the restrictive barrier now controls.. When you take the time to model it out the effect can be massive and the volumes are very small. I wish it had gone somewhere, but o-well..

As for the practical side of it.. Don't worry it’s not a problem to have them on the forks and not the shock.. The major advantage in the forks is the whole lack of a motion ratio, and that’s why the tanks work so simply and so well.

BR,
Jer

 

[jaguar]

thanks Jeremy for your thoughtful answer. a couple more questions...
If the system works so well on the forks then why don't more pro riders use it? Do you think its just a matter of time before they do?
Is there any type of revalving that approximates what a sub tank can do? Or was I right to say that nothing else can do it?
My bike has 42mm forks and the current oil level (on these conventional forks) is 130mm from the top (std is 140). Just guesstimating, what size sub tank should be used for each fork?
thanks man!

 

[Jeremy Wilkey]

Jaguar,
I think if you walked through the pits you'd see that many do... But here is something to think about. In my experience Pro’s are not good choices to look at. The factory guys yes, because they generally run what works best.. Not what they say they run but what they actually use.. Once you’re below full factory level then unfortunately riders are just "ho's" running whatever they can get for free, or more likely today what they get paid to use.. It’s a joke really that has reached academic proportions. Start paying attention to rider interviews it’s a joke. So in many ways it’s all about money.

I think many of the factory systems have them integrated into the fork... BTW..

For an example KTM's come with straight rate springs, when I was building the factory supermoto suspension guess which one worked best between straight or progressive? Well it was a progressive by the stopwatch and rider feel. When you buy the factory replica suspension guess which spring you get? If it comes from KTM it is a straight.. See the point?

Anyway as for your forks, I'm not convinced they would be the most bang for the buck.. I would try to figure out some speed sensitive valve system like a RT emulator before I worried about subs... I would do some swag between volumes, but you'll need to make them small..

BR,
Jer