stencilman

hi, i built this recently;



its nothing special, 50 x 50 1.6 mild steel box section, the welding is sound and the geometry what i think it should be at 80mm of trail.

for reference the bullet is around 60mm and the messenger v2 compact is around 80mm (measured from a drawing)

when i get it up to 20km/h i get the wobble - not fun!

i tried fliping the forks which was a non starter at 200mm trail.

the frame seems reasonably stiff so what is the best way to go from here?

1 - 0 offset forks to increase the trail
2 - stiffen frame further - i reckon there are way flimsier frames out there that don't exhibit this
3 - steering damper
4 - other??

i would really like to be riding this, the motor is 3kw and it will sit on 60km/h (last bike did) but i need to resolve this or it will kill me.

thanks

unterhausen

the articles about shimmy in the scientific literature show that alignment is the primary cause. That's my guess, particularly if you are sure that the bike is stiff enough.

dsaul

Aside from alignment, I would make sure there is no play in whatever steering linkage you are using.

fietsbob

Triangulate ..

Only 1 tube . bears the load , Note some other designs Larry vs Harry - Bullitt and CETMA cargo bikes for carrying kids, made in the USA ? CETMA Cargo

unterhausen

with a bike that long, I think you can tell if it's a problem related to stiffness. It's a good point to look at the steering setup to make sure it's not the issue. This type of bike is not known for shimmy.

stencilman

thanks for your thoughts guys;

Aside from alignment, I would make sure there is no play in whatever steering linkage you are using - i spent my weekend machining little parts to take the play out but there is still the 20mm tube with the bend linking the steering.

the articles about shimmy in the scientific literature show that alignment is the primary cause. That's my guess, particularly if you are sure that the bike is stiff enough. - this will be my next investigation - i have tweaked welded structures before with some success (my workbench top beam supports are (according to my digital level) 0.1 deg out of parallel after tweaking.


Triangulate ..

Only 1 tube . bears the load , Note some other designs Larry vs Harry - Bullitt and CETMA cargo bikes for carrying kids, made in the USA ? CETMA Cargo

i don't disagree but there are many home built cargo bikes that are not triangulated without this problem - why mine?

tuz

What is the tubing section for the steering linkage? It needs to be stiff. On my cargo bike by it's 1x0.035. I tried 5/8x0.35 at first... awful shimmy.

The front wheel naturally wobbles like a caster wheel when moving, but that wobble is usually dampened by simply holding the handlebars. Now if the steering linkage in between flexes you can't prevent the wobble, and any load exacerbates the effect.

stencilman

Tuz - i pretty much made the realization and came in here to write this down and you beat me to it. currently it is 18mm and 1.2mm wt.

here is a little video of the setup and problem;

https://www.youtube.com/watch?v=TMFn...ature=youtu.be

so tomorrow 25mm and 1.6wt is getting fabricated - i wish i had a tube bender... it will be kerf bent.

thanks all, i will let you know how it goes.

stencilman

the correct link;
https://youtu.be/TMFnfHMdCtk

sorry for the crap quality and lighting