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Soft forks

Discussion in 'Technical and Troubleshooting Torque' started by duff_boy, Mar 21, 2008.

  1. Hey guys, im loving my GSF250v bandit, but the front forks are a little on the soft side! So i was thinking perhaps ill try replacing the oil first, see if that helps. What i need to know is what grade of oil(heavier) would be good, and how much i need to add.

    Im a diesel mechanic so doing it myself is no worries, just have little experience with bikes. Can it be done with the forks on the bike? Im assuming theres a drain plug on the bottom to drain it, and then just fill it through the top, in the correct quantity?

    Also, is it possible to get aftermarket springs that are stiffer? if so, who sells them?
  2. If noone else chimes in with the official information, here is some general advice:

    Most non-USD forks are designed around 5W or 8W oil. Try 10W for a stiffness improvement.

    You can usually put an inch of tube or broad rod on top of the spring underneath the fork cap, in order to pre-load the springs with extra pressure. This will feel firmer on the road.

    Voila, cheap front fork improvement!

    BEFORE YOU Drain the forks, take the cap off the fork, gently slide the spring out (allow the oil on the spring to drain back into the fork) and measure the air gap from the top of the fork to the surface of the oil. Accurately.

    Most bikes these days don't have a fork oil drain screw anymore. You could take the fork out and pour the oil out (work the fork up and down a few times to get the oil out of the circuit .

    When you re-fill the fork with new oil, top it up to the same air gap (or fork oil level) as it had before, or even better, the correct one as per the specifications (if you can find out).
    Make sure you check the specs to find out whether this is measured with the springs in, springs out, fork compressed, or fork extended.
  3. Factory recommended fork oil is 10w, next step up is 15w, can't actually remember which one I used.
    Some bikes do have a drain for the oil, the Bandit doesn't. So the forks do need to come off.
    Since you're the 2nd person to ask about changing the fork oil on a Bandit I've uploaded the relevant chapter from the workshop manual which can be found here:
    Should hopefully make things a lot easier.
  4. Stiffness and oil weight shouldn't really be in the same sentence. It's more like a ghetto valving adjustment, and can be handy if you can't quite get perfect with dampening adjustment (or if the bandit forks aren't adjustable).
  5. The 250V has drain screws in the base. (Two phillips head screws in the side) maxxing out my stock settings on the bandit (for a 120kg solo + venturapack), I run 15W oil in the front, and max preload on the rear shocker (for ground clearance, mainly, and I find it wallows less), The next step up for me would be upping the coefficients/retempering the front and rear springs. try maxxing the stock settings and see if you can live with it, personally i'm not much for throwing $100s after the suspension when I'm not riding at 9/10ths all the time, the only thing that annoys me is the excessive dive on the front due to the chinsy springs under braking (anything over a slight bump bottoms the front out)

    (Also, being a 250-market bike, with small aussie sales, not only are the shocks virtually unadjustable, playing with the valving is going to cost big time, hence the need for underhanded tactics) (And there is no spares market like for the CBR250RRRRRrrrr....)
  6. If you plan on keeping it, change the oil to 10W and get some new springs for your weight for about $110 AUD landed.

  7. Okay, had a bit of a play this arvo. Rigged up some spacers that ive put into the front forks to preload the springs a little. They're a little over an inch long, definitely make the front a little firmer, perhaps not as much as id like, but it still feels like its lacking a bit of damping. So ill change the fork oils(15w sounds like the go) and i think then it should be pretty good, both for average commuting and my favourite, the twisty bits :twisted: So if anyone has any idea on the correct oil ammounts, that would be great!
  8. You have just added preload to the springs, which changes the ride height. It does not change the rate of the springs or make them stiffer. While it is true that you now have to apply more weight to get them to the same distance of movement, they will still be as soft as before. If it took 10 kg to move them 1 cm before, it will still only take 10 kg to move them 1 cm. If a spring is too soft it needs to be changed for a firmer spring.

    You also need to check the spring's mechanical characteristics before you leave that spacer in - it is quite possibly too long and will coil bind the springs before they reach the end of their travel.

    Changing the viscosity of the oil will do little to "stiffen" the fork's action. The damping oil is most effective on rebound, and there is normally not a lot of compression damping.

    The damping oil viscosity is established by checking the way the forks return when you compress them. Butt the front wheel against a gutter or solid object, or hold the front brake on. Then push down on the forks as hard as you can, but allow them to return without your full weight on them. As they reach the top of their travel maintain a slight downwards pressure, usually just a small proportion of your weight.

    If they return to a higher position than when you first pushed them down, the damping is too little and the oil should be changed for a higher viscosity. If they return slowly and don't reach the original position (remember you are maintaining a little extra weight on them) the damping is too great, and you need lighter (lower viscosity) oil.

    A second way to check for excessive damping (too slow a return) is to push down as rapidly, repeatedly as you can. If you can make the forks go further down the more often or longer you push them, the damping is too great. This results in the suspension packing down over bumps.

    Another sign of too-little rebound damping is when you are riding and hit a bump or a ripple in the road which makes the bike rise up as it passes over it.


    Trevor G
  9. Aren't the standard springs progressive rate? if they are, i would have thought preloading them would stiffen them? As it seems to have done. Its really just a temporary measure until i buy some new ones anyhow.
  10. Trevor G is absolutely right, it's not as good as a new spring set
    and valving job, but Trevor you have left out one important thing...

    this has cost $0.00 so far :grin: :grin: :grin: and new oil is likely
    to cost $10-$15 so might as well try it before buying new springs
    and paying for valving ($$$$).

    I think that this is value-for-money. I've used this method to make quite
    a few bikes rideable and safe, where the cost of valving and springs
    would have been more than the bike cost me to buy. And on most bikes
    it works reasonably well for average sensible riding styles, as long
    as the rider doesn't get too over-enthusiastic. And it's nearly free!

    Disclaimer: This method will not make your bike steer like an R1. It will
    not make you ride like a God. It is not guaranteed to make
    anything better, it might make some things worse, depending on your
    bike and how you apply it. It worked for me on a few bikes but that's
    not a guarantee. If in any doubt, consult a professional whom you can
    sue if it goes wrong.
  11. See the link I posted earlier.
  12. A set of comp and rebound valves for alot of bikes run at about $310 for both sets, + $120 for springs, would see you with a much improved front end for sub $500, and in most cases it isnt hard to do yourself.
  13. Oh yeh i see now. So thats a chapter out of the GSF250 manual? If i read it correctly, the oil level, with the spring removed and the inner tube fully compressed should be 105mm from the top of the inner tube? thats my interpretation anyhow.
  14. Chucking in some preload can do wonders.

    It's good for your riding to have shit suspension for a while anyway. Nothing smooths out your throttle to brake and brake to throttle transitions more than horrible suspension.
  15. I agree, devotard. This generation of kids these days that grew up on RGV250s and Aprilia RS wouldn't know a chassis twist or a pogo if you
    threw it at them. Why, when I was learning, we had steel-spaghetti frames
    and forks filled with blancmange and if you didn't gently stroke the
    handlebars it'd buck you into the middle of next week!

    But seriously, my first road bike was an XV250 and if you're not smooth
    on one of those, you're into the scenery.
  16. Yep that's from the workshop manual for the GSF250V. And you're spot on, Fork oil level is 105mm from the top of the inner tube (or 495ml of oil).
    The E-03/E-28/E-33 models referred to are all US/Canada, I've no idea why they're that little bit different (Australian model is E-24).
  17. Except that it might not be the correct solution if the springs are too soft...


  18. I can't see them, so I cannot say.

    You can tell if they are progressive wound springs by looking at them. If the coils are the same spacing all the way along then they are straight wound.

    If they are progressive and you have added preload then yes, you have probably "stiffened" the springs, because they could be running on the stiffer section.

    However, if the springs have weakened (which they probably have, it's a problem for many bikes) then adding preload will lead to coil binding if you go too far. There is only one way to find out - do the tests I mentioned.

    Another solution is to fit the fork caps with air valves and add just a psi or two. Air is a wonderful spring, especially when used in conjunction with steel springs.

    This is a simple mod and is made perfect by joining the two forks with a T piece and having just one valve. I have modded several bikes in this way, including our current VTR250. By adding just one stroke of a foot pump I raise the forks 10mm and overcomne the bottoming shock when crossing over a gutter such as into a servo or driveway, at 20 kmh or so.


    Trevor G
  19. I doubt that revalving a 250 is worth it - you are unlikely to benefit from the experience. ;-)

    I repeat that changing to oil which is heavier than the springs require will make the fork rebound too slow and the forks will end up locking up over ripples or repeat bumps because they cannot rebound quickly enough to return to their original position and absorb the next bump.

    Heavier oil, because the compression damping is fairly light anyway, will hardly show up in the compression area before it slows the rebound too much.

    Remember it is the springs which absorb the bumps - the oil is to stop the springs bouncing.


    Trevor G
  20. Dont spose you've got the rest of the manual available? Particularly interested in the carb adjustments and valve clearances etc. :grin: