Nicole, I've had this happen on a Woodrup touring frame with a rack trunk. I really didn't resolve it, but I did stop using a trunk in favor of a saddle bag that snugged under the saddle and did not attach weight to the rack. The bike had medium-low trail, so it was decent with a front bag. I added in the front bag with a rack for a long SAGged tour, and that worked quite well. It does make me want a bike with a low trail.
The fact that you don't have lateral veering when riding no-handed indicates you don't have significant fork/frame/wheel alignment issues, though few bikes are perfect. Jan Heine has recommended going to a roller bearing headset like a Stronglight or Miche, and several members of iBob have stated that it works. The principle is the same as tightening a headset, but I would not suggest tightening a ball-bearing headset. The risk is accelerated wear of the bearing surfaces (races and cups), which are not restorable. Could be an expensive proposition. I'd only suggest it if I knew that you or your mechanic are very well-skilled. But knowing the risk you can think about proceeding. The overall principle is to add drag to the steered part to damp an oscillation (the feeling of shimmy is oscillation). There are a few ways of adding damping - head bearing drag (tighten or go to a roller), a damping cylinder (not uncommon for motorcycles), or perhaps tires. This last is a little hypothetical, but suppose your 33's are inflated to full pressure so you're prepared for your 50# load. When you later ride with only 15# load (purse et cetera plus MacBook), the bike feels lighter because the tires have more pressure than is needed for that weight, and the front tire contact patch is smaller and not as long. This smaller contact patch has less resistance to force applied at the handlebar, and to the inertia of the front wheel as it rotates around the steering axis. Going to the Berto paper on tire pressure (google "Frank Berto tire pressure," www.bikequarterly.com/images/TireDrop.pdf), suppose your total bike weight is 135 +30+15 = 180#, and the front weight is 45% (81#), rear weight is 55% (99#). For your tires the chart gives target pressures (I use these as minimums) of 48 psi and 59 psi, front/rear respectively. If you reduce your tire pressures to 50/60 front/rear, you'd get as much tire damping as possible. I'm not sure if it will be enough to solve the problem you're experiencing, but take it as a suggestion. If you become worried that the rear is too soft, just up it another 5 to 50/65. You can use the chart to see what your target pressures are for the heavy load case - you should see bigger numbers. Another way to look at tire damping (it's a term I've coined) is "self-restoring force," a concept from mechanical engineering and vehicle dynamics. You can Wiki it, but the math gets beyond me, and I'm an experienced engineer. Good luck! There are probably some people here who have tried this as a shimmy correction. I and others have used it to determine target tire pressure values. On Mon, Sep 5, 2011 at 7:03 AM, NME <[email protected]> wrote: > Hi All, > > Sorry to bring up a well-discussed topic, but after reading through > the history of threads, I'm still not sure what to try. I've had my > Hilsen for about five months now, and I've always ridden with > Sackville TourSacks (the rear panniers) mounted on a Nitto Big Back > Rack. I've got less to carry on my commute now, so I took off the > panniers and decided just to strap my bag to the rack. The problem is > that I get a consistent low-speed shimmy when trying to ride no- > handed, which had never happened with the panniers. > > > I'm not sure what info is relevant, so I'll give you what I know, just > in case: > > -- Me (135 lbs with shoes) > -- 55 cm. Hilsen, 33mm Jack Browns, SKS fenders, Nitto Campee Big Back > Rack. > -- U-Lock > -- Messenger Bag: 10 lbs of macbook, power cord, wallet and phone. > Smallish canvas bag, but extends 2" over each side of the rack. > -- Two bungees to strap down the bag. > > What I've tried: > > -- strapping the bag every which way, moving it forward and backward, > turning it sideways, multiple bungees to lock it down. > -- tightening the rack attachments. > -- changing the tire pressure > -- a lighter load. A 5lb bag (leather purse/wallet/phone/u-lock) > *doesn't* shimmy. > > > I'm glad the bike handles so well with the occasional 50 pounds of > books and groceries in the panniers, but it would be a real shame if > it couldn't handle 10 pounds on the rack. > > Has anyone actually had luck tightening the headset? > Any other suggestions? > > Thanks for your advice! > Nicole > > -- > You received this message because you are subscribed to the Google Groups > "RBW Owners Bunch" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/rbw-owners-bunch?hl=en. > > -- Ken Freeman Ann Arbor, MI USA -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
