Fork subtanks... are they for you?

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

  1. mars

    mars Starbucks anyone?

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    Yep, the rear is a 71 260 and I have the preload cranked up so high its not working right. I do have a 72 91 progressive but thought the 76 would give me a better sag setting, I took this spring off my 05 250 exc. I weigh in about 205. A friend of mine does suspension for a living, he rode it for a bit and suggested the 52 springs with a 145mm oil level and 7.5 weight, its near perfect in my opinion and more than sufficient for my riding skills. I will probably not go with the remote tanks since it so good right now. This has been the most successful upgrade I have made to my 640.
    #61
  2. Jan from Finland

    Jan from Finland Been here awhile

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    I agree. Stiffer forks really make the difference. I went from 4.4 N/mm to 4.8 N/mm. One adverse effect (besides a bit harsher ride) is that my corner entry speeds have increased dangerously in dirt.[​IMG] The front tire also wears a lot faster than earlier.
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  3. mars

    mars Starbucks anyone?

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    I put the 76 spring on the rear tonight, the manual said a 70 260 spring is stock with 27mm of spring preload. I cranked the new spring up to 27mm and I now have a tiny bit of static sag, maybe 5 mm. I am going to give it a try tomorrow, who knows it may work.
    #63
  4. bmwktmbill

    bmwktmbill Traveler

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    Wow!!
    I am liking this thread.
    This is a case where everyone is right the way I see it.

    The subtanks allow you to increase the oil volume and not pay a penalty because you can lower it in effect by opening the tanks and increasing the air volume or vice versa.
    On top of that the rate of changeover can be controlled by the magic air valve which can be varied to find a middle ground if that is desired.

    Now I am going out on a limb by saying the subtanks will allow light springs and high oil volumes for a more progressive spring effect or stiff springs and low oil volumes for a more linear effect(straight wound spring), but either condition can be reversed by adjusting the tanks.
    This is just what is needed for dual sport riding IMHO.
    It will be possible to adjust for sand vs. hard pack, rocks vs. gravel, or road vs. trail.

    Depending on the majority of your riding you can chose hard or soft springing as a biased place to start and then adjust for the opposite when necessary using the tanks.

    Would someone please tell me that I am wrong and that I don't understand how these darn tanks work.
    Bill in tomahawk, WI.
    #64
  5. creeper

    creeper Still alive...

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    I'd have to consider the light spring/high oil volume at greater length to estimate it's value Bill.
    There are limits that would cut an experiment like that short... hydraulic lock for example.
    As the stock springs on a 640A are too light for most people, if you still have them, you would be a good test subject for your theory.

    The idea, at least in the generic sense I suppose, is to start off with a firm or at least borderline firm fork set-up... say for MX track days or high speed rough terrain use, then by adding the tanks and fine tuning the oil level, one can have a softer set-up by opening the tank valves... for use in such areas as tight, low speed, rocky or otherwise ugly terrain.

    The same thing could be said of a set-up for a SM track days bike, that would also be used on the street, where a bit more compliance would be advantageous.

    In Washington, due to the wide range of terrain we can encounter on a given day, I like the ability to go from a firm fork that can take a high speed logging road, big surprise pothole hit without upsetting my intended direction much, to a fork that is better suited to tracking over bowling ball size rocks at 2 MPH on a tight single track.
    The number one reason I considered the subtanks was to try and get back some of the low speed compliance I lost when I went to firmer springs and damping.

    Subtanks do not produce the perfect-do-all-fork by any stretch of the imagination, but they certainly do allow a range of on-the-ride tuning options that would not available without them.

    C
    #65
  6. meat popsicle

    meat popsicle Ignostic

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    That last bit sounded familiar to me, so I took a peek back in the thread and copied Zerodog's replies to my very similar question. Not sure if this is 100% applicable, but it all sounds fairly familiar.

    #66
  7. creeper

    creeper Still alive...

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    Well, how many ways can you say subtanks make your forks softer, so putting them on already soft forks is not such a good idea... before it all kinda starts to sound the same? :dunno
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  8. meat popsicle

    meat popsicle Ignostic

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    ya'but...
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  9. Zerodog

    Zerodog Long timer

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    Yep, subtanks on a bike that is sprung too soft will only make problems worse. When in the on position they would make the forks ride even lower in the travel and just make the fork bottom easier. And the lower the forks ride in the travel the less sensitive they become to small bumps. This is due to the air volume being much smaller in the fork.

    Jan if your forks lost sensitivity to small bumps with the stiffer springs try a lower oil level and see how it works for you. Go with 140mm or 150mm to start with and see how it effects it. You will be surprised with how it effects your fork. Try it it is free. Yes you will lose bottoming control but you will also gain small bump sensitivity. With subtanks you can have both. Especially when using my usual MX bike setup with control valves. The ball valve has a similar effect but it is more to the side of drastically dropping oil levels because of the higher air flow.

    To see the effect the airspring has on total fork travel try taking your air bleed screws out of your fork caps and see how easy it is to push your forks down without them in. Another example is if you never bleed the air in your forks they do build pressure and lose sensitivity to small bumps and become pretty harsh. That is why on mx bikes a lot of guys use push button airbleeds to do this all of the time without tools. It is also why I have incorporated airbleeds into my subtank system.

    As far as graphs go I don't have one. Shock/ fork dynos aren't cheap to buy. But I do have a ton of experience with offroad motorcycle suspension and the effect the system has on it. It isn't snake oil and there are a lot of subtank systems on the market. They just cost around $100-$200 more than my system for pretty much the same thing. I have added the ball valve option for dual sport guys as a compromise. They are not ideal for all around riding with them on all the time but they give the broadest effect for the very different terrain you encounter with a DS bike. They are also really easy and convenient to use.
    #69
  10. meat popsicle

    meat popsicle Ignostic

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    Fixed. :lol3

    No seriously now - ya'but (just facilitating here - last nail in the coffin I hope, unless I really am not following this discussion):

    What would be the effect of using your subtanks on an undersprung bike with a higher oil level?

    I am guessing that question (which is not mine) assumes that oil level and air volume are a tradeoff in suspension behavior, although there was an additional hypothesis mentioned that undersprung/high oil level might emulate a progressive spring, and oversprung/low oil level might emulate a linear spring - it all sounds very technical but it is beyond me.
    #70
  11. autolycos

    autolycos True vulgarian

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    You kids and your language are too crazy for me.

    So, nu, which is better, the cat's ass or the dog's nuts?
    #71
  12. Zerodog

    Zerodog Long timer

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    Adding extra air volume with subtanks to a fork will not help it's airspring effect to overcome a weak spring. But if you didn't have subtanks on and you had an very high oil level you could hydraulicly lock the fork when you are close to bottoming. I had a guy last year that was racing areana cross blow the fork seals out of his bike because he turned his tanks all the way off when running a very high oil level to see how it felt.

    You could if you wanted an progressive airspring effect on an undersprung fork add some shrader valves on your fork caps and put a few lbs of air in your fork and get the effect you might be after. But I would think it would be best if the oil level was lower not higher. But then again excess pressure in the forks causes the fork seals to try to seal harder causing more stiction........
    Schraders are cheap. Tap your caps for the fittings and try it. It might work or it might suck. See what happens! In the end getting the correct springs for your weight is the best upgrade to suspension you can get for an offroad bike.
    #72
  13. meat popsicle

    meat popsicle Ignostic

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    Thank you Zerodog,

    I see now that Creeper had answered those points... and that I have much to learn about the physics of suspension. Thank you both for putting the time into this.

    I am thinking that if you did not put Schrader valves on your subtanks I don't need to try them on my forks. :D Besides, my plan is to have my suspension reworked with your subtanks included in the mix. Now where did I put that extra C-note? :scratch
    #73
  14. bmwktmbill

    bmwktmbill Traveler

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    Boy, I really like this thread. It really is forcing me to think about my suspension.

    This question for Zerodog.
    What is the proper spring for a KTM Adventure with a WP USD fork?
    And what is hydraulic locking?

    Meanwhile, let's say my fork is a little undersprung but not bottoming excessively, I just have lots of movement in the fork especially braking on the street and handling is mushy off road. occasionally I bottom there too.
    Sometimes I load up my camping gear, fill the gastank, etc increasing the ride weight by 100 lbs.

    So to compensate I increase the preload to get some of the static height back(adjustable preload caps) and add say 25mm of oil to the fork and I add the subtanks. This is basically what WP recommends to do in the manual. But not subtanks and maybe not that much oil at once.

    With the valves closed, this is my road, gravel road setting- now not so much movement or mush, for trail riding I open the valves either half or whole.
    For fast trails with potholes and ruts I run the valves half open.

    Is this the best solution to the most common problem with the Adventure bike.
    Zerodog, please tell me this will work.
    Bill in Tomahawk, WI.
    #74
  15. meat popsicle

    meat popsicle Ignostic

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  16. Zerodog

    Zerodog Long timer

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    Ok I will try to answer this the best I can. Suspension stuff is pretty overlooked on Dual Sport bikes. To get the most out of them they need to be tuned for the rider and the terrain. If you ride roads all the time it doesn't matter a whole lot. But as soon as the speed starts increasing offroad it becomes more and more important. Basic set up like springs and sag are the most important parts of suspension set up. The right springs for your KTM are the right ones for your weight and what you are hauling on your bike. For the rear spring you need to be able to set your rider sag to around 4 inches without preloading the hell out of the spring.

    Rider sag is the measured by first measuring the axle to fender or some static point directly above the axle with the bike on a stand with the bike unweighted. I put a marker mark on the fender and axle. Then you need to sit on the bike with your feet on the pegs and have someone measure this again. The difference is your rider sag. This needs to be attained without sacrificing static sag. This is how far the bike sags without you on it. To much preload = too little static sag. This will actually make the bike have harsh ride and the rear will tend to kick. Too little or too much rider sag causes geometry problems with your bike leading to all kinds of handling problems. If you have too much rider sag your bike will tend to wash out the front wheel. If you have too little rider sag high speed riding is unstable and the bike tends to knife in on turns. You also miss out on small bump performance and some of your suspension travel.

    This stuff is the same for the front too. But the front is hard to measure accurately with sag. The front it is more of a matter of feel, recommendations from a suspension/ spring place and from other guys. If your spring is too light your fork rides low in the stroke. The lower your fork is in the travel the more it is trying to "ramp up" because of the airspring in the fork. This will cause a fork to be harsh on small bumps even though it is undersprung. It also bottoms easy. To heavy of a spring leads to a harsh ride and a fork that will not settle into a turn and cause it to wash out or not hold a line. You want the front to dive a turn. This makes the head angle of the bike steeper driving the front wheel into the ground for increased traction and makes the bike turn faster. The first time you hop on a BMW with the Telelever fork you will see how weird it feels when turning. It doesn't dive much at all. Without this geometry change caused by diving the bike takes more effort to turn.

    My set up is this. I weigh 190lbs. Rear spring is 8.0 and the front is 5.0 in each leg. Rider sag is 4.25inches on the rear spring. The oil is set to 100mm on the fork. On the road I ride with the tanks in the off position. This makes the forks less divey and active under braking and helps keep the front up in the travel and aid high speed cruising on freeways. With the valves on I get head shake starting at 75mph with knobby tires. With them off I can run up to over 100 with no head shake.
    Offroad I switch the tanks to fully on. This allows the forks to eat up small rocks and washboard on dirt roads even at very high speed. But it also allow the fork to absorb big hits and not bottom because of the variable airspring effect of the subtanks.On slow speed rocks this also allows the fork to use more travel and be more plush because they ramp up much later in the stroke. I don't really use the 1/2 way position on the valves. Not to say you can't I just don't with my set up.

    Hydraulic locking is this. When the oil can't move through the valves fast enough and causes the fork to stop. This is a big part of bottoming control and is effected by the fixed orifices in your suspension valves and how your valve stacks are arranged. Oil viscosity is a factor too. Another form of this is if the air chamber is too small/ fork oil really high there isn't enough air in the fork to compress when the fork moves through its full stroke. This can cause you not to use all of the travel and even worse it can cause your fork seals to leak or in an extreme case blow them right out of the legs. :eek1
    #76
  17. PackMule

    PackMule love what you do

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    :clap Zerodog!!

    This is the finest lay-person's explanation of suspension I've read! Thanks for putting it into words that I (and hopefully his Meatness) can understand! :freaky
    #77
  18. meat popsicle

    meat popsicle Ignostic

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    bmwktmbill is NOT my Sock Monkey.
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  19. bmwktmbill

    bmwktmbill Traveler

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    Meat, in northern Wisconsin it's "my mitten monkey", Zero, thanks for the explanation.

    I would add that everyone might benefit by checking the static drop on their front forks and adjusting the preload. The WP recommendation is 25-40mm. Check the WP manual for fork oil height handling recommendations.
    I searched for an official ride sag(rider+mc+gear) for the front fork but haven't found one . Most agree it is around 75mm with the rider seated on the machine with one foot on the ground.

    If you add a hundred pounds of travel gear good luck. I ended up making peace with numbers on the low end of the scale when I am packed up and on the high side unpacked and bags off, ready to trail ride. Adjustable preload caps come in handy for this transition.

    I spent a couple of days reading on the KTMtalk suspension forum. The actual 640 Adventure fork is rarely discussed.
    This is not simple stuff, our models are not all the same and how we ride and where will affect our final setups.

    I hope this is just the beginning of a longer discussion based on actual riding experience.
    Heaven would be a chart that shows the stock spring weights and shim stacks for every year.

    The ride weight on a KTM Adventure bike can vary easily by 150 lbs when gear and gas are considered.
    We ride from the race track to the rock gardens.
    We ask more of our suspension than any racer except maybe the Dakar guys and we rarely rebuild.
    Oil changes should be on the 10K list but are they?

    If someone has it all figured out, I'd like to know about it.
    I'm going to try the sub tanks.
    Bill in Tomahawk.
    #79
  20. Zerodog

    Zerodog Long timer

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    The wide range of dual sport bike use is what the tanks are all about. More adjustment for very different conditions. If you ride your dual sport offroad a lot you should treat your suspension like you would with a real dirtbike. Oil changes at least once to twice a year are the norm.
    #80