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Uneven dirtbike front suspension?

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  • Uneven dirtbike front suspension?

    Hi. Iv been rebuilding a 1980~ Yamaha MX80 dirt bike (80cc). After draining the uh, 10mL of oil out of the front forks, I noticed the oil hole that regulates dampening rate in the shock tubes was very different on the left vs right. On one it was two 1/8" holes or so (one is maybe +1/64"), and on the other is a 1/8" hole and 3/16" hole. (On both tubes, the holes are one above another and just get covered up by the tube guide when the suspension travel is at its max)

    Is it normal for the front shocks to be that different in dampening? or is that just wrong and/or dangerous?

    Is a 1/4" + 1/8" hole really 'soft' for a 80cc bike suspension? Its got 4" of travel if that is any help.

    Should I drill out the smaller hole on the other tube to match the larger hole? (And use thicker oil if I need more dampening?)
    Manual specs 10 weight oil, its a small (130lb) 80CC bike with a 210~lb rider, likely rather larger rider then was intended.

    The springs are also a little different (one is an inch longer and hence has force when the suspension is topped out while the other does not) but when compressed more then an inch or so they feel about even.

    I don't really have money to buy new tubes and I would rather not weld/braze them as the tube guides go over the holes and id need to sand/file it down.
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

  • #2
    separate the two forks from the triple clamp and conduct your own tests with two hands forcing each shock down against the ground separately --- go by "feel" on the compressional -------- go by time frame on the rebound, if both shocks are air filled make sure your using identical pressures...

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    • #3
      Someone has replaced one of the tubes/forks . They are supposed to be exactly the same. I have no solution except to find the specs for the year an vin # of the bike and match them. There is no reason for them to be different.

      But I hear you on spending money on a bike like that. I would go ahead and match the any holes that you can but I really would try to find a set of matching springs. That's a before they got really techno on those bikes and usually just cleaning everything up and matching as much as you can will give a kid a good ride.
      Last edited by topct; 08-23-2014, 08:44 PM.
      Gene

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      • #4
        I remember on my old Yamaha Enduro that both forks had even pressure. Unless you have a bad / broken spring or bad seal.

        JL.............

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        • #5
          Originally posted by topct View Post
          Someone has replaced one of the tubes/forks . They are supposed to be exactly the same. I have no solution except to find the specs for the year an vin # of the bike and match them. There is no reason for them to be different.

          But I hear you on spending money on a bike like that. I would go ahead and match the any holes that you can but I really would try to find a set of matching springs. That's a before they got really techno on those bikes and usually just cleaning everything up and matching as much as you can will give a kid a good ride.
          That is what I expected. the internals are a little different too. The springs are close enough I think, I just cringed to think of what would happen landing off a jump or something with greatly uneven dampening, assuming it had not been done for some reason I was not aware of or was just some 'standard practice'. I guess I will drill out the fork with the smaller hole to match the other fork as best I can.

          I have another set of forks for it, but they are so rusted its not worth even replacing the seal on them, let alone filling them with oil.
          Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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          • #6
            The idea is to have both fork sliders move together and be secure at the axle. Watch out for any wheel spacers so that there is no pinching of the fork sliders together. If you can do that you will be alright. If the other forks have springs that match you might be able to use those. Oil weight will make a difference in their reaction both up and down. Pick a number in the middle.
            Gene

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            • #7
              It is normal for them to be different. Gold Wings have the anti dive on only one side. My GS 1100z had the adjustment for the anti dive on the right side.

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              • #8
                It should not be a problem. The new mx bikes have the spring in one leg and the dampeners in the other. Much bigger difference than you are describing. The main thing is how well it is doing the job of dampening for your weight and style of riding. Adding oil will help to keep the forks from bottoming on big bumps. The less air in the tube will act like a spring when it is compressing. So adding some oil will feel like a stronger spring. Upping the weight will help with the compression dampening. What weight oil does it call for?

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                • #9
                  Originally posted by outlawspeeder View Post
                  It is normal for them to be different. Gold Wings have the anti dive on only one side. My GS 1100z had the adjustment for the anti dive on the right side.
                  The OP's bike isn't a Gold Wing. Most of the old Yamaha's forks were tied together with the fender bracket so they had to move evenly.

                  JL............

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                  • #10
                    Originally posted by JoeLee View Post
                    The OP's bike isn't a Gold Wing. Most of the old Yamaha's forks were tied together with the fender bracket so they had to move evenly.

                    JL............
                    Ummm-the Gold Wing's forks are tied together with the axle, so they, too, need to move evenly.

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                    • #11
                      Interesting that some bikes use the spring on one side and shock on the other. Sorta seems like it could try and cock and jam but I guess the tubes are too long for that.

                      I hope I can just add oil then, it only had about 10ml of the strongest smelling oil I have ever smelt in it. Manual calls for 10W.
                      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                      • #12
                        I think you may have sucessfully found a fault that doesnt exist.
                        My neighbours diary says I have boundary issues

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                        • #13
                          Many bikes have compression damping in one fork and rebound in the other. I wouldn't sweat it.
                          Peter - novice home machinist, modern motorcycle enthusiast.

                          Denford Viceroy 280 Synchro (11 x 24)
                          Herbert 0V adapted to R8 by 'Sir John'.
                          Monarch 10EE 1942

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                          • #14
                            Could be accidental (odd tubes) could be by design (slightly different damping rates or stages for each leg). Either way, I wouldn't Futz with it until you've actually ridden it. You're best off dialing in the rough damping range by playing with the oil weight first anyway and you can mix different weights to get there too (there's some funky calculator you can use to work out exact volumes to get the right right). You can also vary oil height to get the progression how you want it. Might be worth checking out what the recommended spring weight is for you weight.

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                            • #15
                              Be sure and get all the old oil out by inverting the fork and pumping it till empty. I usually flush mine with diesel or paint thinner before refilling.

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