Mini Chopper flop

[telling]

Hi folks
Maybe some of you can help me with a flop prob with the mini chopper I'm building. I used an old Mini Baja and stripped it. Cut the neck and raked it 40 degrees. Made the offset for the forks 2.5" and made the forks 30". The flop was terrible. No control slow or fast. Did a little research and raked the triple trees 6 degrees. This helped for high speed and slightly improved slow, but still not good enuf. I was able to get the trail to around 2-3" by moving the front tire in or out. By the way, the rear tire is a fat boy and the front is a 20" off a bike. Thing looks good but is useless if ya can't ride. Any help would be appreciated. Thanks.

 

[landuse]

Hi folks
Maybe some of you can help me with a flop prob with the mini chopper I'm building. I used an old Mini Baja and stripped it. Cut the neck and raked it 40 degrees. Made the offset for the forks 2.5" and made the forks 30". The flop was terrible. No control slow or fast. Did a little research and raked the triple trees 6 degrees. This helped for high speed and slightly improved slow, but still not good enuf. I was able to get the trail to around 2-3" by moving the front tire in or out. By the way, the rear tire is a fat boy and the front is a 20" off a bike. Thing looks good but is useless if ya can't ride. Any help would be appreciated. Thanks.

Welcome!! How about some pics? This would help us with your problems. Are you saying that your speed is being influenced by the position of your forks etc? I am going to go out on a limb here and say that your gearing or engine might actually be the problem. Pics and more engine and gearing info would help

 

[itsid]

well first:
you raked it TO 40°
or you raked it BY 40° ?

the latter will be impossible to control with nearly every wheelsize.

What's your caster set at?
imagine three lines..
straight line (a) down the fork, another parallel straight line(b) through the head
and one vertical straight line through the center of the front wheel(c)

the wheel contact area should not only cover the point where c crosses the road plane,
but also where b crosses the road plane.
ideally b is at the front edge of that contact area (c is always the center of it)
if it's too far in, your caster is too small and the bike tends to wobble.
if it's too far out, your stability will get better but the handling will suffer.
it must be well under the front tyre!

the straight line a though (or where it crosses the road plane) should always be outside and in front of that contact area; but still 'under' the tyre!
otherwise again you risk a very bad steering behaviour.

'sid

 

[telling]

mini chopper flop

Hi,
Here are the pics of my build. The first one is a wide shot to get an idea of where I'm going. I've got the tire on top of the forks. acceptable? I have also switched to below and moved the tire along the notch. Closer seems to be better. #2 is the raked triple trees. They are at 6 degrees to the neck. #3 is the trail. #4 and 5 are the angle finder at the neck. If I turn the finder to the left I get 48 degrees, go the other way and get 43. Which is correct? So anyways, there it is. I would love to get this thing on the road and be fairly certain I won't kill myself. Please bump this post to the top. I need all the help I can get! Thanks everyone.

 

[itsid]

well I see what you did there,
and I think I understand WHY you did that.

the bad news now is, I don't think that's a good idea...
Although your caster looks alright'ish to me;
maybe you need to move it even further out with a steering angle like that
-I need to point out, that I seriously HATE raked forks,
the two I rode handled like ; and since then even the look turns me off -
So what I said about where your caster should be placed at is based on motorcycles with 'ordinary' steering geometry.

What I want to say is, maybe I'm totally wrong here and a fork that long on a lowered bike like yours needs to have a MUCH larger caster than a road bike, racer or bobber.
*shrugs*
I simply don't know
If you have, check with a larger front wheel.
(that way you increase the caster without changing the geometry)
if that helps, maybe you can adjust it a little.

My guess that raking the triple trees that much doesn't help with the geometry, since you tilt the wheel offset to it's center.
What that does is,
going straight your front and rear wheel are in line (well at least I hope so )
as soon as you steer you shift the frontwheel sideways a bit.
and a tiny bit actually helps leaning the bike,
but with a fork that long you might be out of line with the rear wheel already;
and that surely makes things worse.

but to be honest, I know next to nothing about making forks.
surely not enough to fix your issue I'm afraid.

I'm sorry, maybe someone else can kick in.

'sid

 

[OzFab]

If I understand correctly, the term "flop" you refer to is the wheel flopping from side to side in turns, twisting the forks?

How do you prevent that? By welding a bridge between the forks...

Now for the angle finder: When it reads 43°, what number corresponds to the centre of the fork tube? Likewise, when it reads 48°, what number corresponds to the centre of the fork tube?

 

[landuse]

I cannot help you out, but just thought I would say that your chopper looks great!

 

[FL350R]

i need just a little more info

in the picture where you have the blue straight edge and the square

is that blue straight edge running the same angle as the forks or is it running the same angle as the pivot point on your tree ?

 

[itsid]

i need just a little more info

in the picture where you have the blue straight edge and the square

is that blue straight edge running the same angle as the forks or is it running the same angle as the pivot point on your tree ?

Oh dear.. I haven't even noticed that earlier...
that indeed looks parallel to the fork (as I asked)
but it must be straight through the headset( is that the correct term...??)
with raked triple trees that's NOT parallel!
totally my fault. (missed the raked triple trees in your first post completely)

So yeah, run it straight through the pivot point and check again,
with an angle offset like yours I'm afraid you might even ended up with a negative caster...

[EDIT]
I made a quick sketch..

that should be close to your real setup,
not too much about the size of the triple trees but the rest (incl the angles) are as close as possible;
I took 45.5° for the fork.
because of two reasons:
1) you intended to have 46° and 45.5 is close
2) you said you have 43 and 48 and 45.5 is the average of the two.
So as you can see the caster is more or less neutral;

the point is, if you do not want to redo the whole geometry, shortening the fork will get you closer to where you want the caster to be:


see?
the same can be achieved with a larger front wheel;
try to find a 24" front wheel for testing for example.


'sid

 

[FL350R]

it looks to me like the "trail' is either negative or close to it

on a street bike the trail is usualy somewhere between + 2.0 to 4.5 inches. below the short end the steering feels touchy...above the longer end it would feel like your turning a semi. but those numbers are just a starting point they depend on the make up of the bike and what the rider likes for feel
a street bike is more forgiving you have a suspension and a wide tire thats made to run on the sidewall. but even with that, with a short trail if you lose it your chances of getting it back are slim.

raked tree's are awful you should only use one if your unwilling or are not capable of modding the frame they usualy come in 3, 5 and 7 degress
as we can see from the pictures that doesnt seem to be any problem

your set up looks good its just not going to be forgiving with a short trail

my opinion for what its worth

change the tree make the forks and the neck parallel.
if you want more rake change the angle where the neck attaches to the frame.

if you have to keep the tree raked
keep the neck and forks close in your tree...if the center point distance of your forks is closer to the center point distance of the neck. the set up will be more forgiving of a raked tree. { dump the raked tree }

last thing if can move the connection point of your handle bars from where it is in your pictures.
to in between the center point of the forks and tree it wont change anything but it will have a better feel

i hope i explained it well enough
i think it looks cool with a slight mod you should be all set

 

[telling]

Mini chopper flop

Thanks to all for the input. Some very good ideas. Guess I will try to answer your questions, here goes

sid: I raked it to 40°. But I guess I'm a little closer to 43° By caster, do you mean trail? Depending on the tire I get 2" to 3". I will get rid of the rake trees and test with a bigger tire. Maybe shorten or lengthen the forks. Heck, pipe is cheap. Love the sketch. Very helpful.

Fabroman. Yes a bridge is in the works. Need something to attach the dinky front fender. On the angle finder and the blue straight edge, both are on the steering neck, (head).

landuse. Yup. Once I powder coat this thing and some patriotic decals, It is gonna be sharp. That is if it ever get's to that point!!!

FL350. All your suggestions make perfect sense. I'm thinking that will help immensely. I will move the forks up to original place and the handle bars back down. Should look great.

To all, thank you.

 

[itsid]

well, yes trail (due to caster angles..) sorry not my language

Anyways. it looks like the headset has a 40° angle alright, the thing is your additional 6° rake in the triple trees, since your anglefinder obviously isn't calibrated (anymore)
you measure 43 holding it one and 48 holding it the other way around.
that makes me believe your angle is right down the midlle of those two numbers (45.5°)
which is not exactly 6 but 5.5 more...

Anyways..
2"-3" is not enough... according to customchoppers.info a good trail is exactly twice of that amount (4"-6")

So maybe you could loose one degree on your triple trees or something.

'sid

 

[telling]

Sid

You are right. I'm going to loose the tree rake and move the neck and forks closer. Right now I have 3" spacing from the center of the neck to the center of the forks, (if you measured it as a T).

Think I'll take it down to about an inch

 

[Poboy kartman]

Sid

You are right. I'm going to loose the tree rake and move the neck and forks closer. Right now I have 3" spacing from the center of the neck to the center of the forks, (if you measured it as a T).

Think I'll take it down to about an inch

.....You mean 'sid is actually right? ???I'm comic relief....'sid is the "go to" guy......this forum wouldn't be the same without either of us. .... but it would still be the best forum in the world. ...

 

[OzFab]

I'm comic relief....'sid is the "go to" guy......this forum wouldn't be the same without either of us.

You're absolutely right about that...

.... but it would still be the best forum in the world. ...

...& that...

 

[telling]

Sorry guys, but Sid nailed it. Got rid if the tree rake. Moved the offset a little closer. Shorten the forks and VIOLA!!!!!!!!!!! Now we have a fun chopper. Very good handling at slow and fast. No more worries about castration. Will send pics upon completion. Now all I gotta do is weld it up, grind it down, take it apart, powder coat, bake, reassemble. Easy, huh?

 

[landuse]

Awesome

 

[itsid]

Sorry guys, but Sid nailed it. Got rid if the tree rake. Moved the offset a little closer. Shorten the forks and VIOLA!!!!!!!!!!! Now we have a fun chopper. Very good handling at slow and fast. No more worries about castration. Will send pics upon completion. Now all I gotta do is weld it up, grind it down, take it apart, powder coat, bake, reassemble. Easy, huh?

Glad I could help

'sid

PS if you don't mind.. there are those "Thanks" buttons on the bottom right of all post..
click the ones that you think did help...