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  • Bicycle Tech Thread --->

    Two quick questions for any bike geek so inclined:

    First, what causes "speed wobble?" My mountain bike starts to shimmy at high speed if I take my hands off the bars (the bars weave back and forth). I have lots of road bike experience at high speeds with very light touch on the bars and no shimmy... so, is it the slack head tube angle? I was considering tightening up the headset a little, but considering this bike just ate a headset seal I don't know if that's a great idea. For what it's worth this is a FS 29'er with 120 mm of travel.

    Second, what's the pulsing break feel in the front brake of my cruiser? It has cheap side pulls on it. Once again, only at pretty high speed and can be minimized by changing the proportion of breaking between front and rear.

    Neither of these are huge issues as I don't really have to go "no hands" at 35 on my mtn. bike although I do enjoy it sometimes

    The cruiser is certainly not a performance ride, so dialing it back on that thing is always a good idea, but I don't want to eat through pads either or be losing breaking performance if it's just an adjustment issue. I assembled the bike and aligned the pads and toed them in properly.

    Times like these I miss Pinnah and Robrox.

  • #2
    Is your front wheel trued?
    Coastal Crest Snow Patrol
    https://brentheffner.smugmug.com/
    http://www.youtube.com/user/MrJibmstr
    https://www.strava.com/athletes/1816044

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    • #3
      Originally posted by jibmaster View Post
      Is your front wheel trued?
      Good question. I don't feel like this is the likely culprit, but I can certainly check. Thanks.

      Comment


      • #4
        On wobble: http://sheldonbrown.com/brandt/shimmy.html

        In addition to making sure your rim is true, check for a bent or dented section where the two sides aren't parallel. Look at the rim joint and make sure it's still smooth. Also check the brake pad alignment and toe-in. Changing brake pads might help, old pads can become glazed like a pencil eraser and will magnify any irregularities in the wheel.

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        • #5
          I'll take a crack Matt.

          I used to have a racing bike with a steep head angle - like 72 degrees. If I took my hands off the bars when going fast it shimmied like crazy. I think it may just be a side effect of the head angle and the other bike geometry that makes it unstable under those conditions. I just learned not to ride it that way. A slacker (lower) angle actually makes a bike more stable at the expense of maneuverability.

          Assuming your rim is true, the brake pulsing may be either an old, hard brake pad which doesn't grab the rim as it should, or a loose brake pad mounting or if the pads are not adjusted correctly. They need to be toe-in, that is, the front of the pad (forward when looking down on them from the cockpit) need to contact the rim first when braking. If the pads neutral or toe-out they can grab the rim and chatter when you brake.

          Here's a good description of pad alignment
          http://www.parktool.com/blog/repair-...-brake-service

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          • #6
            Originally posted by dangerDan View Post
            ...They need to be toe-in, that is, the front of the pad (forward when looking down on them from the cockpit) need to contact the rim first when braking. If the pads neutral or toe-out they can grab the rim and chatter when you brake.

            Here's a good description of pad alignment
            http://www.parktool.com/blog/repair-...-brake-service
            I was just about to mention that
            Coastal Crest Snow Patrol
            https://brentheffner.smugmug.com/
            http://www.youtube.com/user/MrJibmstr
            https://www.strava.com/athletes/1816044

            Comment


            • #7
              Tires may contribute to speed wobble. I think. Even if your wheel is round and true, the tire may not be balanced. Might try a different tire. Also, higher or lower profile tires will change the effective trail of the bike in ways that I am not sure about, but which Pinnah could no doubt expound upon at length (that is a compliment by the way, not sarcasm).

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              • #8
                Slackening head angle reduces shimmy, but slackening head angle also slows steering response, so you can't have it both ways.

                Some folks have tried dampeners, but they just add friction to your steerer, you can alternatively use a longer bar or longer stem, that may help quiet things down.

                A slacker head angle requires a new frame or alternatively you can raise your head tube (more fork extension or fork spacers), or if you have the right sized heat tube you can try an adjustable angle head set.

                Tire pressure and suspension set up could be part of the problem, so maybe start there, try various changes, one variable at a time, then see how it goes. Generally speaking, a stiffer from end will be more responsive, so more potential for shimmy with a hard tire and stiff fork.

                As for the cruiser, it could be a bent rim, but more than likely it's "sticky" spot. Clean the rim with brake cleaner and or use light sandpaper to "resurface" the braking sidewall, pull your pads and sand them down with emory cloth to remove oxidized rubber, then reset and go ride.

                Oh, and real cruisers are fixie
                Last edited by Nurse Ben; 29 October 2013, 03:22 PM.

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                • #9
                  Thanks for the ideas. The MTB wobbling is more an issue of curiosity. It could certainly be attributed to the off road tires or geometry.

                  The cruiser brakes are pretty nice and powerful for a super inexpensive bike as it was spec'd with side pulls. I think the pad material is real hard and that may be contributing to the pulsing. I'll probably run these pads for awhile longer and then upgrade the pads to see if that helps. Once again the performance is not outside of what I would consider "normal" or "reasonable" for the ride. No one designs a three speed cruiser to be doing mach 3. I'll take another look at the pad adjustment, thanks all for taking the time to respond.

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                  • #10
                    Originally posted by dangerDan View Post
                    .I used to have a racing bike with a steep head angle - like 72 degrees.
                    Assuming you mean "road" bike, 72deg is not steep. That's touring bike angle that is. Just sayin'..........

                    Originally posted by teledad
                    Look at the rim joint and make sure it's still smooth.
                    Or it could be a depression where the welded join was ground smooth.
                    “Taking away someone’s opinion is no different than sewing a man’s butthole shut.”

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                    • #11
                      -you're a telemarker! fix the heal, fix the problem

                      -clearly you need to replace your trail bike and give it to me.

                      go faster

                      drag your feet to slow down

                      glad i could help

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                      • #12
                        Anyone running tubeless with any recommendations?

                        Is it expected to run very low pressure? I just installed the Stan's valve cores and rim tape and am using their sealant. My LBS told me that sometimes there are compatibility issues with the cores and some rim profiles... anyone aware of this?

                        I'm struggling with getting a good seal and certainly can't hold pressure over about 30 lbs. I personally like higher pressure. It just feels faster to me and don't typically have traction problems on climbs. Any help appreciated!

                        Comment


                        • #13
                          Originally posted by Matt J View Post
                          Anyone running tubeless with any recommendations?

                          Is it expected to run very low pressure? I just installed the Stan's valve cores and rim tape and am using their sealant. My LBS told me that sometimes there are compatibility issues with the cores and some rim profiles... anyone aware of this?

                          I'm struggling with getting a good seal and certainly can't hold pressure over about 30 lbs. I personally like higher pressure. It just feels faster to me and don't typically have traction problems on climbs. Any help appreciated!
                          Couple of questions, Are you using dedicated tubless wheels and TR tires? If not it is always a crap shoot on wither the tires will seal up as well as be reliable. Anyway, in the mean time, make sure the valve core is tightly secure and use 4-6oz of Stan's in each tire. There is another good sealant I just heard about maybe called Orange something and heard it is better than Stan's. Once dialed in you should have no problems running whatever pressure you like. I like running bigger tires on my XC 4x4 2.35 dedicated TR tires and about 27 #. I run more pressure on smaller 2.1 tires.
                          "Just say no to groomed snow"

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                          • #14
                            Be cautious Matt. If it won't hold pressure above 30 it won't handle a decent hit either. A role off at any speed aint no fun.
                            Lift served and proud of it.

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                            • #15
                              Thanks fellas.

                              The rims and tires are both tubeless ready. I can't really figure out where they're losing air. I've been putting more pressure in and spinning the wheels each time I walk through the garage. At first they were definitely leaking through the valves then the mechanic that installed the valves and rim strips adjusted them and put more sealant in and they held air for a little while. It's kind of a saga now. From slow leak to complete loss and back. I miss tubes, but I had a ding in the rim that was pinching the tube which was part of the reason I gave this a whirl. I bought sealant so I guess I can keep on adding more.

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