built a low rider, but bike is unstable. anyone know why ?

M

motorbike57

New Member
I built a 20" low rider bicycle that I had intended to give to my daughter for christmas, however, once completed, I took it out for a ride and it's pretty wobbly / unstable. Can anyone tell me why? is it just the geometry of the bike ? Even just pedaling the bike, it's sort of unstable. if you dont hold the handlebars dead straight, it's wobbly. when you try to turn, it's like "whoa"...feels very unstable. Everything on the bike is tight. maybe its just that the 20" tires and the springer front end make it that way ??? I've probably built 20 or so motorized bikes so far, including a micargi puma stretch that has a springer front end, and I've never had this problem. please let me know if you know what is going on here, before I have to completely abandon the project !

cheers
 
I've built a few recumbents in the past. Had that type of problem on the first one, very wobbly feeling to it. Come to find out, it was the rake and trail of the front end. A couple of changes made it a very useful bike, so maybe that's it?

Good luck.
 
Smaller bikes are usually "twitchy," and if the front is raked out with those bent springer forks, it's going to want to "lay over" when turning or going slower, as the rake angle naturally pushes it sideways.

Not a good setup for motors, and why you don't see many lowrider bikes out for a long cruise.
 
dgm099 said:
I've built a few recumbents in the past. Had that type of problem on the first one, very wobbly feeling to it. Come to find out, it was the rake and trail of the front end. A couple of changes made it a very useful bike, so maybe that's it?

Good luck.
Click to expand...

What exactly did you change that fixed the problem ?
 
I read a bunch about rake and trail. I then went to a standard bike that was stock and measured out the rake and trail of it and compared it to what I had read about so I knew that I was figuring things out correctly and I had the measurements of a bike that worked well. I ended up changing the front forks out to one that wasn't quite as curved as the ones I started with. Made all the difference in the world.

https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry

This is a good starting point. Good Luck
 
That answer isn't really very good is it?

Once you know a bit about rake and trail, there are a number of things you can change that will change the rake and trail. The angle of the head tube, the curve of the forks, adding a spacer to move the tire forward or backward (kinda like a springer does) to change the geometry, the angle that the forks exit out of the head tube. That's just a few things you can do and you can combine them together to help save the overall look of your bike and to get the actual geometry to work for ya.
 
dgm099 said:
That answer isn't really very good is it?

Once you know a bit about rake and trail, there are a number of things you can change that will change the rake and trail. The angle of the head tube, the curve of the forks, adding a spacer to move the tire forward or backward (kinda like a springer does) to change the geometry, the angle that the forks exit out of the head tube. That's just a few things you can do and you can combine them together to help save the overall look of your bike and to get the actual geometry to work for ya.
Click to expand...

Good info.. thanks. I ordered a more 'straight' springer fork and hopefully that will help a bit with stability.

cheers
 
built a low rider but bike is unstable anyone know why

Id love to see some pictures of bike racks on your TD. Any preference to the front or back of the TD?
I did some searching in the forum, but no luck find any pictures.

Thanks
 
If 20" bikes were wobbly and hard to ride there would be nothing but lawsuits out there....most kids learn on one so I'd guess you changed something drastically.
 
Bent springer forks introduce more than a mere trail issue, they actually cause the the front wheel to "sweep" instead of "twist."

Imagine standing astride a bicycle with normal forks looking down at the front wheel.
Twist the handle bars, and the wheel twists along directly underneath.

The same experiment with the bent forks will show the wheel to be some distance out in front of the bike so twisting the handlebars and head set results in the wheel "sweeping" in a concentric circle (arc) around the headset. The longer the distance, the greater the effect.

(straight chopper forks introduce another effect as the wheel stops turning, and simply lays or "flops" over instead of turning the bike )

The upshot is; bicycles have evolved to look the way they do for a reason, but it sure is fun to change 'em - you can really feel the geometry!
 
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