cause of wobbles

gothicsurf

10 mW
Joined
Jul 9, 2017
Messages
29
Location
Pacifica, CA
With a forum search on 'wobble,' there seemed to be little I saw relating to rear racks specifically. The bike I am riding has a rear-mounted battery, and it did not wobble before the conversion. I have noticed this problem before by simply loading a rear rack either unbalanced or over its design capacity. In this instance, it seems to be the mass of the rack + battery not being inline or parallel with the direction of the bike. To minimize the effect, moving toward parallel and balanced decreases the wobble produced. Maybe someone can explain the physics behind this?

Described as the 'death wobble' surely because the shaking, if undampened, increases with resonance until remaining vertical becomes more difficult. Any advice on mounting a rack methodically in the plane of the bike?
 
This is a link to a list of posts of mine during previous times this has come up, that may include info on the basic problem and potential fixes (depends on the design of your rack, design of your bike, compromises you may be willing to make, and your DIY skills (or budget). Not all results are relevant, but many are, and some have pics or drawings.

https://endless-sphere.com/forums/search.php?keywords=rack+triang*&terms=all&author=amberwolf&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=-1&t=0&submit=Search

I know there are other posts as well, if you search my posts for "rack*" you'll find more of them (but also more non-relevant results). One of hhem is a bike I did up a custom heavy-duty rack for my friend Bill; it's only real weak points were the bolt-on points to the frame itself since there was a requirement was that it be removable (not welded to the actual frame), IIRC.

If you post good clear well-lit pics of top, sides, rear, etc of your bike/rack setup, with and without the battery / cargo on it, we can suggest specific fixes.

FWIW, "tail wag" is probably a more common description of the problem; sometimes "sway" is used.

"Death wobble" usually has more to do with overall frame stiffness and front-end geometry.
 
Improper weight balance can be a source of wobbles.

https://www.youtube.com/watch?v=4jk9H5AB4lM&t
 
Besides carrying 70 (52v) 18650 cells in my trunk bag it also has tools, wet/cold riding gear, spare tube and at times groceries. I used to have an issue with the "wobbles" or shakes with the back rack and struggled to keep it firmly attached under the seat. It seemed to always be loosening. I added two stainless hose clamps to firmly attach it to the seat stay on the crossbar where the fender attaches. No more issue with the wobbles even though my rack would be considered extremely light weight.
 

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In my experience and observation, rack stiffness is much more influential than side-to-side weight balance in determining tendency to wobble and its severity. Forks and wheel stiffness play a significant part too.

For what it's worth, I use batteries up to 26 lbs on nothing-special commodity rear racks with no related wobble at all. The only resonant phenomenon I've had was mild headshake due to inadequate fork offset and resulting excessive trail. Adding half an inch of fork offset cured it.
 
In terms of physics, high and to the rear is the worst place to mount something heavy.

Go to a triangle mounted battery and you'll never look back. :)
 
This ...

neptronix said:
In terms of physics, high and to the rear is the worst place to mount something heavy.

Go to a triangle mounted battery and you'll never look back. :)
 
neptronix said:
In terms of physics, high and to the rear is the worst place to mount something heavy.

Go to a triangle mounted battery and you'll never look back. :)

In terms of actually riding a bike, the worst place to put a bulky mass is between your knees.

Cycle tourists have been carrying heavier gear than our batteries on the back rack since, well, cycle touring. That's where I usually put the battery, and it always works just fine.

Here's the last one I turned out into the wild. Battery (48V/15.6Ah) is in the rack trunk, surrounded by closed cell foam:
IMG_20221104_183702925~2.jpg
 
Quality racks mounted solidly are important ... of course.

An experienced touring cyclist knows the art of balancing the load for best handling conditions.

Top level cycle designers are known to use scales under the wheels (with rider) to measure and understand the front to rear loads.

Thorn.nz.JPG
 
neptronix said:
Go to a triangle mounted battery and you'll never look back. :)

Challenges arise with step-through frames which is when I started putting the battery on the rear rack.
 
Weight on top of a rear rack = bad
weight behind rear axle = very bad
flexible rack and/or mount = very bad
Weight low, centered and rigidly mounted = all good

You can make a full sized, 1000 lb motorcycle "death wobble" by mounting as little as 25 lbs too high and rearward.
 
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