shimmy 71.5 head angle

[Sam Perez]

why does my front loaded bike shimmy at speeds when I let go of the handle bar head angle is 71.5. any thoughts?

[Brian Campbell]

Headset installed correctly? Fork bent? Too much weight in the front? Weight too high on the bike? Lightweight tubing that flexes too much? Saddle height too high which generates a rocking motion and destabilizes the bikes center of gravity?

[James Valiensi]

Your down tube isn’t stiff enough.

On Aug 31, 2020, at 10:54 AM, Sam Perez <sam.per...@gmail.com> wrote:

why does my front loaded bike shimmy at speeds when I let go of the handle bar head angle is 71.5. any thoughts?

--
You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group.
To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owners-bun...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/rbw-owners-bunch/175f8396-b5d8-4463-aa4d-e951b7946158n%40googlegroups.com.

[Sam Perez]

interesting , I is light weight tubing and a small front rando bag in the front.

[Patrick Moore]

Shimmy can be caused by any number of things; it seems to be common with low trail designs built from very light tubing. 

Eliminating those 2 things leaves an indefinite number of other things. My 2003 stout-tubed custom shimmied after I changed tires; with other tires, it disappeared.

Start by eliminating single things, like adding a couple of lbs or removing a couple of lbs from your load. Change tire pressure. Change tires. 

Without more details about your bike and its setup, one can only conjecture in a void.

On Mon, Aug 31, 2020 at 11:54 AM Sam Perez <sam.per...@gmail.com> wrote:

why does my front loaded bike shimmy at speeds when I let go of the handle bar head angle is 71.5. any thoughts?

--

You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group.
To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owners-bun...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/rbw-owners-bunch/175f8396-b5d8-4463-aa4d-e951b7946158n%40googlegroups.com.

--

-----------------------------------------------------------------------

Patrick Moore

Alburquerque, Nuevo Mexico, Etats Unis d'Amerique, Orbis Terrarum

[Sam Perez]

interesting, ok ill get more details and see if that will help narrow down the issue.

[Bill Lindsay]

Patrick Moore stated:  "Shimmy can be caused by any number of things"

That's true and correct.  Shimmy is resonance.  Resonance in a mechanical structure means everything important is just right.  

Patrick Moore is also spot on in the way to fix it: Change something and see if it fixed it.  If the resonance goes away, good, now you know.  If the resonance doesn't go away, change something else.  It's natural to want to change something that you can change for free, like tire pressure.  Try those things first.  Another thing you can change for free is the way you sit up and ride no handed.  See if your own weight distribution changes the shimmy.  It might, and if it does, maybe you will decide to simply fix the shimmy with your own riding technique.  

Nobody on earth will take any number of details from your machine and say "The reason your bike shimmies is because of X".  Nobody understands it completely, and nobody has sufficient understanding to look over a bike and say "this bike shimmies".  It is entirely an empirical exercise.  Theoretical generalizations can be applied after the fact, but none are universal.  

Bill Lindsay

El Cerrito, CA

On Monday, August 31, 2020 at 12:43:47 PM UTC-7 Patrick Moore wrote:

[ascpgh]

Trail for sure makes sense, lower has less stabilizing effect at rolling speeds but the other variables do have profundity beyond their list relegated value. My Rambouillet scared the heck out of me on a long steep downhill when it broke out in a full-on shimmy oscillation. I had no load other than my small seat pack with spare tube, spartan tools and a folding spare outside under the cinch. That was on 28c tires and was on a county asphalt road. Happened another time following a nominal brush of the handlebars going downhill in a local cemetery in a group.  

Never happened again after moving to 32 tires, regardless of load. 

The violence of that oscillation is sure memorable. Knees to the top tube! If you figure out the right formula of initiating a speed wobble, learn it and never repeat it!

Andy Cheatham

Pittsburgh

On Monday, August 31, 2020 at 3:43:47 PM UTC-4, Patrick Moore wrote:

Shimmy can be caused by any number of things; it seems to be common with low trail designs built from very light tubing. 

Eliminating those 2 things leaves an indefinite number of other things. My 2003 stout-tubed custom shimmied after I changed tires; with other tires, it disappeared.

Start by eliminating single things, like adding a couple of lbs or removing a couple of lbs from your load. Change tire pressure. Change tires. 

Without more details about your bike and its setup, one can only conjecture in a void.

On Mon, Aug 31, 2020 at 11:54 AM Sam Perez <sam.pe...@gmail.com> wrote:

why does my front loaded bike shimmy at speeds when I let go of the handle bar head angle is 71.5. any thoughts?

--
You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group.

To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owne...@googlegroups.com.

To view this discussion on the web visit https://groups.google.com/d/msgid/rbw-owners-bunch/175f8396-b5d8-4463-aa4d-e951b7946158n%40googlegroups.com.