I've run similar loading experiments on my Atlantis. The unloaded set- up is a Nitto small front rack with Acorn Box Rando bag which I often load up to 6-8 lbs without issue. I have a Nitto Big Rear Rack (2 lbs of real steel) for anything I may pick up along the way. Unless I've got a box of books or similar casually strapped to the back, the bike is stable although the steering feels a bit wandering if I need to take both hands off the bars. Not a big deal.
The Atlantis will shimmy with extra weight only on the rear (2 panniers, total weight less than 20 lbs), such as a lodging tour load. Move that same load to front low riders, no shimmy. Add another pair of panniers for camping (15 lbs total), and the most stable arrangement is with the lighter pair on the rear. 4 bags, with the heavier on the rear, and the bike has that "tail wagging the dog" feel, and has developed a nasty shimmy more than once. The above is all with the same 35 mm touring tires and a 175 lb rider. I've fiddled with the headset adjustment, etc., and will change to a needle bearing HS when the present one dies. My conclusion, for me & what I do with the bike, is to put the bulk of any load on the front, with whatever's left over on the rear. William may have reached the same conlusion in his post where he reports eliminating the shimmy when he removed the rear saddlebag (12 lbs). In any case, these experiments are great fun to perform and we learn a lot about our bikes, preferences, and what works for us. Rene (above) is the only other Atlantis owner I can recall posting about shimmy. dougP On Dec 22, 11:09 am, William <tapebu...@gmail.com> wrote: > The drinard video is very good. That's exactly the thing I'm looking > at when running my experiments. > > On Dec 21, 9:34 pm, Tim McNamara <tim...@bitstream.net> wrote: > > > > > Try the simple things first: move the saddle slightly forward or > > back, maybe 1/2", to change the weight distribution of you on the > > bike. Change the distribution of stuff you carry on the bike (or > > consider carrying less stuff). Raise or lower the bars to change > > your position. Make sure the headset is adjusted. Make sure the > > wheels are true and properly aligned in the frame. Swap the front > > tire to the back and vice versa; look for casing defects in the > > tires. Try less inflation pressure (or more) in the tires. Try a > > different size tire. If your bike starts to shimmy with your hands > > on the bars, stand up slightly and ease your weight off the saddle- > > this will usually stop a shimmy instantly. > > > The cause of shimmy is not exactly known. Since it tends to be speed- > > dependent, it is likely that vertical and gyroscopic forces play a > > part. There appear to be at least two fulcrum points, the rear tire > > contact patch and the saddle. The bike frame appears to act as a > > spring, whipping back and forth; nutation can be observed with the > > steerer turning slightly in the head bearings as the front part of > > the frame travels laterally. Interestingly the front tire contact > > patch usually continues to track in a straight line but, as the > > shimmy worsens, the contact patch can begin to arc back and forth on > > the road. This is what is often meant when someone talks about a > > death wobble, because the vehicle becomes unstable and will often > > crash unless the wobble can be stopped. Wobbles can be started by > > road surface irregularities > > > *Great* video by the much-missed-in-rec.bikes.tech Damon Rinard of a > > deliberately induced no-handed shimmy: > > >http://www.youtube.com/watch?v=xODNzyUbIHo > > > I wish that was in slo-mo. > > > This is Rob English crashing at the Battle Mountain IHPVA event on > > level terrain at about 70 mph (I bet he appreciated his fairing very > > much). It appears to me that his pedaling effort caused the problem > > and that as the bike started to wobble, his pedaling amplified it. > > >http://www.youtube.com/watch?v=i5Dapy1xUq0 > > > My observations with my bikes (a 1996 Riv A/R [26 x 1.25], a 1995 > > Ritchey Road custom [700 x 25], a 1998 Gunnar Crosshairs [700 x 28] > > and a home-built road bike [700 x 25] all with Panaracer Pasela tires > > at the rated maximum pressure) is that: > > > 1. none of them currently shimmy with my hands on the bars. > > > 2. the Gunnar shimmied when I mounted a Nitto mini-rack on the front > > fork, even with no load on the rack, and stopped as soon as I removed > > the rack- I could feel the pulsation in my hands when they were on > > the bars. The Gunnar sometimes shimmies if I ride no handed and lean > > back. Reynolds 853 with Waterford fork upgrade. Oddly it did not > > shimmy with a really old Eclipse bar bag (one of the ones from the > > late 70s with a rack that slipped over the bars and under the stem > > and with elastic cords going down to the dropouts). > > > 3. the Riv doesn't, with or without the same Nitto mini rack, with > > or without a load. Reynolds 753 frame with 531 fork. No shimmy that > > I can ever recall having on this bike. > > > 4. the Ritchey shimmies at certain speeds no-handed (19-20 mph and > > up) if coasting but not if pedaling; worse if I lean back. Tange > > Prestige Ritchey Logic tubes and fork tubes. > > > 5. the home built bike shimmies occasionally no-handed. Reynolds > > 501 frame and fork tubes. > > > 5. the scariest shimmy I ever had was descending on a Bianchi > > Reparto Corsa-built bike when I tried the aero trick of sliding off > > the back of the saddle. It felt like the rear wheel was shimmying, > > very disconcerting. The Ritchey replaced that bike and handles so > > much better than the Italian job.- Hide quoted text - > > - Show quoted text - -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To post to this group, send email to rbw-owners-bu...@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
