Rain-groove causes crazy wobbling? Or is it me?

Eric, which size Roadrider are you running on the front? I've got a theory about about "over tire-ing" a given rim size. The 90/90-19 has a recommended rim size of 2.15", WM3. The min-max range is 1.85"-2.50" On the 100/90-19 the recommended rim is 2.5, WM4; with a range of 2.15"-2.75". I think that if one squeaks in at the minimum rim size, or undershoots it, then that may distort the tire by squeezing it at the bead and making the tire taller with less support. Modern tires seem to have thinner sidewalls. I don't think they are a stiff as the old style tires. I think they are happier at their recommended rim size or maybe even at the max within the given range. So, the WM2, (1.85"), original rims might be pushing it on a 90/90 and asking for it with a 100/90 tire. Cosslter mentioned in his paper that wobble is a function of sidewall stiffness and fork flex. Raingrooves are a perfect way to kick off a wobble every few seconds or so.

I think it's true that the death-grip on the bars only makes things worse. The front tire may make little excursions but they are self-correcting. The D-G only exacerbates things because you can't react fast enough till you begin to amplify the excursions.

http://www.avon-tyres.co.uk/sites/defau ... 1%2010.pdf
http://www.vf750fd.com/blurbs/vibmode.htm

I ain't experimented enough on tire size-rim combos but do have to diddle total air pressure and ff/rr difference to compensate some for the side wall stiffness in old to new tires, to reduce the fork wondering that temps me to try to dampen it too much, which then interferes with its self correcting tendency. My 2 Combats came with 110s on front WM2 rim, but Wes's '71 has 100 front. Can't tell the difference so going with 100 on Peel next time just to see if I can living on it. Questioning forum gave feedback 100 just felt better than 110, so good enough for me to try to upset it. Fascinating to frustrating all the things that can feel the same bother in Commando's

Bob- this is very interesting, thanks for the tip. I have the original stock rims, and I am indeed running the 100-width tires. As I wrote, beautiful on the asphalt or non-grooved concrete, but the grooves really get it shaking. I think my grip is pretty relaxed in general, and I did experiment with even more relaxed vs. tighter grip to see what would happen; at times just barely holding my palms against the grips with open hands, but still shake and bake.

From the sound of it, lots of guys in Southern Cal are having this issue, so I am pretty sure it's not my setup. Narrow wheels, grooves, light bike, equals shimmy I guess.

I know a few people have said "Just relax- it may shimmy but nothing will happen." Try telling that to your panicking brain at 70mph :? Also, I tried higher speed/lower speed, and the shimmy continues well up to 85, and who the heck wants to go that fast on a Commando anyway? Oh, Hobot. So I have kind of resigned myself to staying off the grooves and traveling surface roads. Takes longer, but hey, that means more time on the bike, right?

The narrow wheel/wide tire combo might be making it worse for sure- I'd be curious as to what experience others have had with this?

Thanks again- ejd

I ride my '73 750 in LA too.

The rain grooves on LA's freeways DO aggravate my Commando's high speed weave. On ungrooved asphalt, the onset is at 80 MPH, and usually more. On the rain grooves, it comes on at 70 MPH or more. In all cases, I can attenuate it by moving my butt toward the rear, shifting the center of gravity or perhaps lowering my chest/center of air pressure?

There is never any weave when cornering - that fact seems important but I don't know what to make of it.

I installed a Taylor head steady to get better handling and that it did but I also installed it, in part, to see if it would eliminate my Commando's weave - it did not.

Now, I will have to return to experimenting with relative front to rear tire air pressure in this regard. I would rather optimize relative tire pressures with cornering in mind, than do so with the focus on the weave.

I'm running WM-3 19 front and WM-3 18 rear with 100 width up front and 110 on the rear, AM18s.

Well guy's I live behind the Orange Curtain but ride all over all of our freeways & my bike is smooth as silk at all speeds as far as the rain grooves go. It had problems before but with new Avon's, new rims and spokes from Buchanon's all is well. I did add a new headsteady from CNW/ Jim Comstock so who knows but the rain grooves do almost nothing to this bike. This bloody wind however IS kicking my ass!!!

MPH, and usually more. On the rain grooves, it comes on at 70 MPH or more. In all cases, I can attenuate it by moving my butt toward the rear, shifting the center of gravity or perhaps lowering my chest/center of air pressure?

There is never any weave when cornering - that fact seems important but I don't know what to make of it.

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UNLESS you are hard on brakes, both the above condition UN-loads the front tire, lifts-extends the forks. This how I used to have to ride THE Gravel, my butt on very back of seat and least amount of weight of me on bars, or it began the Cdo Cha Cha. Less front pressure does similar and improve my Cha Cha's but not completely solved it on old K81's. Its unnerving and does move ya that much closer to out of control. Peel's corrected Roadholders, tri-links and new tires removed any hint of Cha Cha even hands off. Make ya a wish list check list and see what it takes in your case. As Noted one can not expect much form a head link alone but its a step towards settled calmness. 85 is my after dark deer protection speed and 70 mph in many sections of THE Grit commuting speeds now. I do not go fast into blinds so pretty handy on braking on loose stuff now w/o crashing.

Something that may be related to grooved Cha Cha is coating down from 30 mph about hands off to see if wobbles onset to scare ya. Factory Trixie does that but not Peel no more, she wiggles forks some but does not continue to magnify out of control.