Trek 520 wobble

[James]

I have a 2008 Trek 520 with a unicrown fork.  When I tour I use front and rear panniers and handlebar bag.  The front rack is a lowrider.  I love the bike with one exception, when it is loaded it seems to have a slight wobble that feels like it is located in the front.  It only occurs when I pedal,  no wobble on fast down hills, or unloaded.  The problem is minor, but still irritating.  Am I justified in thinking another fork would help, if so any recommendations?

[John Nelson]

I have a 2009 Trek 520, with a unicrown fork, run with full panniers, a low-rider front rack and a handlebar bag. I have never detected a wobble. Check all the usual suspects (loose bolts, lose headset, etc), and make sure your handlebar bag isn't overloaded. These things are very tough to diagnose--you may never figure it out.

[Pat]

I have had the same problem with my Trucker.  I suspect it's the handlebar bag.  I can see it wobble when I pedal.  It is on the larger side and I think I've overloaded it (maybe 3-5 pounds).  I'm going to do some testing - maybe take it off and see, and put a little weight in it, or lash it to the top of my front rack (it wouldn't really make that much difference for accessibility).

Good luck,

Pat

[driftlessregion]

You've had the bike almost 4 years; what has changed so that now there is an issue? Old hubs? Headset need adjusting? Does the bag have bungie cords attached below to keep it from swaying? Really weird that it only does it while pedaling. Usually wobble/shimmy is when not pedaling. Good luck. Let us know the solution.

[waynemyer]

To add on to the preceding diagnoses: If it occurs at low speeds, a binding headset can be at issue. The headset should turn freely without any fretting ("indexed" headset), stiction, or clunking (too loose). The fretting and clunking should not cause you any speed wobble issues, but the stiction will, especially at low speeds.

Another thing to check is your upper body while pedaling. Is your upper body "quiet" while pedaling? You may be introducing a slight wobble that is compounded by the "resonance" of the loaded bicycle. Wider handlebars can help quiet all that down; the wider bars require a greater input to disrupt the handling of the bicycle.

[John Grossbohlin]

Another issue that I did not see specifically mentioned is how smoothly you pedal. I'm sure someone could add a lot more technical information here but in layman's terms, if you have a nice round pedaling stroke the bike and load are generally more stable. The smooth pedaling combined with good core body strength go a long ways towards preventing the development of dynamic sway, i.e., wobbling while pedaling. Clipless pedals, or toe clips and straps, let you pull and push through the rotation. Some slight pull at all times helps smooth things out and a lot of pull helps you power up hills or accelerate.

It took quite a while for my mid-teen son to develop a smooth pedaling stroke as both technique and strength are involved in doing so. To help him understand what I was talking about I told him to watch how well his rear wheel tracked behind the front (relatively dry roads with puddles or cinder rail trails help here). When he saw how much his front wheel was constantly going from side to side as compared his rear (and compared to my tracks) he understood. After a couple thousand miles of loaded touring over two summers, growing in height and gaining a lot of core body strength during that time, he pedals much more smoothly now and it shows in his tracks.

I noticed that it also takes a bit of time at the beginning of a tour for my body to automatically and seamlessly make the core body muscular adjustments needed to keep the bike steady. I'm talking about a few hours at most... By the second day the muscle memory is restored and there are no more wobbles.

[Pat Lamb]

Unless you've wrecked the bike lately, I doubt the fork is the problem.  My favorite things to blame include a loose headset, loose wheel bearings, and out of true wheel.

Also see the article at http://www.sheldonbrown.com/brandt/shimmy.html.

[DaveB]

....when it is loaded it seems to have a slight wobble that feels like it is located in the front.  It only occurs when I pedal,  no wobble on fast down hills, or unloaded.

The tells me it's caused by the load you have on the handlebars and/or front panniers and amplified by your pedaling style.  If the bike rides smoothly with no wobble while unloaded, particularly on fast downhills, and only when loaded and while you pedal then there is not likely to be any problem with the bike.  Pack the front panniers tighter and fasten all the lashing points tight.