PDA

View Full Version : "Beyond" steering on my new cargo bike



RodM
07-21-2014, 11:33 AM
What causes what I am calling "beyond steerage"? When turning to the right (the steering arm prevents this from happening when turning left) on a slow roll, if turned too far, the wheel continues "beyond" the turn and pivots all the way around. Can this be corrected by changing the point of attachment for the steering arm? or do I have to weld in stops (and spoil my new paint job :dunce2: )?

http://forum.atomiczombie.com/gallery/showimage.php?i=4268&c=15
http://forum.atomiczombie.com/gallery/showimage.php?i=4269&c=15
http://forum.atomiczombie.com/gallery/showimage.php?i=4270&c=15

bambuko
07-21-2014, 12:48 PM
Looking at your pictures (and it is very nice looking bike indeed!) I can see two possible issues:

- one is the steering pushrod, by virtue of it being bent to clear the obstructions it could be possibly be flexing/buckling under the load?
- steering geometry, it would appear that you have very little rake?

On my LWB when not loaded, the front wheel has a tendency to swing round "beyond" but it doesn't happen when I seat on the bike and control it. This btw could be a hint of possible another issue i.e. weight balance front/rear (does your problem happen when the bike is loaded with cargo?)

bambuko
07-22-2014, 06:44 AM
Here is your problem:

http://forum.atomiczombie.com/gallery/files/2/9/3/8/img_0577.jpg

This bend makes the pushrod sloppy as as you turn - I will try to sketch something to explain.

Twinkle
07-22-2014, 07:08 AM
nice looking bike ,

that why most of us run the unpainted toy for 100 miles to eliminate these little opportunities .
But bolting the stable door after the horse has bolted wont do any of us any good .
A limit stop on the lower steering arm under the load bay might be an answer , using a bolted on limit stop without damaging that nice looking paintwork .

regards emma

bambuko
07-22-2014, 07:15 AM
Sorry Emma, limit stop will not make blind bit of difference :evilgrin:
and here is why:

https://lh4.googleusercontent.com/-oPqkMGTsY3s/U85GfrNnltI/AAAAAAAAPDY/HBvL5bdYwVg/s800/Clipboard01.jpg

Compare the geometry of both pushrods when turning.
As soon as angle "A" is greater then 0 you lost the control of wheel angle.

https://lh6.googleusercontent.com/-5_vQYk7i2gE/U85GgPlTemI/AAAAAAAAPDc/xc7yxYJGzN0/s800/Clipboard02.jpg

Indeed as soon as you start approaching, just getting close to it your control will be very poor :mad:

bambuko
07-22-2014, 07:53 AM
You should be able to solve your problem by redesigning the pushrod.
Here is how it is done in commercial cargo bikes (viewed from above with front wheel at the top):

https://lh5.googleusercontent.com/-4Zf3iRJcBCo/U85OnDZqTII/AAAAAAAAPD0/qAopLNNEslw/s800/framekit3.jpg

Notice no curve and the angle at front end.
This is not the only solution, but it is easiest to do and will not damage the paint (apart from the paint on the pushrod).
There will still be an angle of front wheel deflection beyond which the wheel will flop beyond and to the other side, but it will not be as bad as with your current pushrod.

If I were doing it I would prefer this type of pushrod:

https://lh6.googleusercontent.com/-rmFMGCAedks/U85P1Zwy1VI/AAAAAAAAPEM/HLZlhf1Tjdc/s800/WorkCycles_Kr8_RainCover_800x600.jpg

Here (when viewed from above) the pushrod is straight line (all the bends done in side view) but this would be more complicated and involve bit more paint work :)

RodM
07-22-2014, 10:09 AM
Good morning, and thank you both for your very helpful suggestions. Yesterday, while waiting for the Forum to respond, I was able to add stops (without welding - bonus!!) and hence solve the problem. Yes, Emma, I know about the unpainted 100 miles but I usually have so much confidence in my work that I forego that prerequisite, and then live to regret my impatience!! Chris, great diagrams - I understand what you are saying about the pushrod, and although the first one I made does not seem to be flexing, I am leaning towards changing it over to one similar to the shape you suggest. What I am interested in learning is this - could the problem be prevented at the design stage? I expect to be building another one in the future and it would be nice to be able to avoid the problem all together. By the way, I checked out both of your builds - nice work!! And thanks again for your interest and good advice. This Forum is a wonderful asset for those of us who mostly work in isolation from other hackers.

bambuko
07-22-2014, 11:02 AM
Good morning, and thank you both for your very helpful suggestions. Yesterday, while waiting for the Forum to respond, I was able to add stops (without welding - bonus!!) and hence solve the problem...
That will work (kind off) as long as the stops prevent you from rotating the wheel beyond the critical point where pushrod stops working as intended (as explained hopefully in my sketches).


...Chris, great diagrams - I understand what you are saying about the pushrod, and although the first one I made does not seem to be flexing, I am leaning towards changing it over to one similar to the shape you suggest...
It is not flexing that is an issue, but simple geometry - once the front over the pushrod goes over the "top dead centre" (to use piston analogy) you will loose it. At least that's what I was trying to explain :evilgrin:


...What I am interested in learning is this - could the problem be prevented at the design stage?...
Yes, of course it can be easily avoided using simple geometry and either of the two recommended layouts.

If you want (next time) seek advice before painting :surprised: and I am sure you will have no problems.
Anyway... if I were you I wouldn't accept that "you have solved the problem by adding stops" and use this bike to make better pushrod before moving onto another bike?

RodM
07-23-2014, 08:51 AM
Morning, Chris, and thanks again for your interest. You have made your point clearly regarding the push rod - the stops are working but I could see that they might not be adequate if I were to hit a pot hole or some other sudden obstacle. I have a commitment today but I assure you that I will make a new one, using your diagram and photo when I get back into my shop tomorrow.
In your original response you also mentioned the question also of rake. That was an issue that I spent considerable time on before building, and this is something that will be different in the next build. The rake on this bike is 72 degrees - similar to the rake on the Kyoto Cruiser I built a few years ago, as I recall. It is a bit "twitchy" - no "hands off" riding for sure and would be better if more relaxed. How much more relaxed? What are your thoughts?

bambuko
07-23-2014, 12:40 PM
Terminology with regard to front geometry is confusing and conflicting (not to mention the differences between the motorcycle and bicycle world).
When I was referring to rake I was using the following definition: "...distance between the front wheel axle and the line extending through the head tube..." (see more at AZ tutorials (http://atomiczombie.com/Tutorial%20-%20Rake%20And%20Trail%20-%20Page%201.aspx))

or, as shown in one of my pictures:

https://lh3.googleusercontent.com/-gsfxRZ_-cLw/UW_y_QO2DCI/AAAAAAAANms/2gJ_QKNYdiA/s800/rake_and_trail.jpg

What you are referring to as rake is "rake angle" (mostly motorcycle term), which in cycling is more often called steering head angle.
BTW in motorcycling they measure it from vertical line, whereas in cycling it is measured from horizontal line (just to confuse :evilgrin:)
The angle that you have (ie 72 deg is perfectly fine, mid range, safe value).
I was more concerned about your forks being straight, with minimal amount of rake as per photo above, or as below (typical bicycle terminology)

https://lh6.googleusercontent.com/-DW5u3Yvq6Ww/U8_jCh7ApxI/AAAAAAAAPF8/opZDp1cO5xY/s800/800px-Bicycle_dimensions.svg.png

Bottom line is that whichever one you change ie head angle, rake or trail will affect the remaining two.

Having said all this... I am reasonably convinced that your main problem is the pushrod.
Rake, trail and head angle less likely to cause you problems that you are experiencing - your bike is effectively "normal" bike so you can't go far wrong by copying your regular DF bike.

Ticktock
07-23-2014, 06:21 PM
Hi Rod,
I,ve been hibernating for a few days, so just picked up on this thread.
All the answers you have are good and correct.
You could improve this at the design stage by avoiding the little things that contribute to the problem.
One thing not mentioned is that , because your steering rod acts at axle level, any flex in the forks, or even the slightest looseness in the steering head bearings is going to add to steering loosness, and this is exaggerated once you get near 45 degrees, and out of control past that!
It is far better to have the steering rod act at the top of the forks if possible, even though this means a bent rod.
The actual steering forces are quite light, but the whole thing is very sensitive to any loosness anywhere in the system. Using longer steering arms helps considerably, but is not always possible on our normal builds. You are lucky here--you can use longer arms without clashing with peddles , chains and legs!
Steering stops are almost mandatory on the layout you have, as any impact on the front wheel when at 45 degrees right will cause over swing and lock up the steering, no matter what layout you have.
Nice bike, good paint job, and happy you have a solution without having to re-do the paint.
Steve G

RodM
07-26-2014, 09:54 AM
http://forum.atomiczombie.com/gallery/showimage.php?i=4288&c=15

Morning, All, and greetings, Steve. Running a bit behind here but just to let you all know that I have been taking notes. All contributions gratefully received. The pic shows the new pushrod fab-ed yesterday afternoon with Chris's suggestion. Emma, just so you know I was listening, :o),I will try it out on the bike BEFORE painting. I would love to have made the curved version used on the Workcycles, but my home made pipe bender and my pipe bending skills are not quite up to that standard. Again, thanks to the Forum (and Forum moderators) for this valuable resource.

bambuko
07-26-2014, 10:34 AM
...The pic shows the new pushrod fab-ed yesterday afternoon with Chris's suggestion...

http://forum.atomiczombie.com/gallery/files/2/9/3/8/new_22improved_22_steering_pushrod.jpg

This is the picture you are talking about? :)
I am assuming that the front, shorter straight part, is no longer than it's necessary to clear front wheel?

I await results of the test with interest :rolleyes4:

Twinkle
07-26-2014, 11:48 AM
I'm watching

regards emma

Frippegubben
06-23-2015, 04:24 AM
I don't know how this ended up, I just happened to stumble upon this thread when browsing.
But the design of the push rod has absolutely nothing to do with it (assuming it's not caused/worsened by it flexing). It could be designed as a snake or even a spiral and still act exactly the same way. You will easily (?) realize this if you imagine welding a (big) flat sheet of metal around the push rod, adding this will not change the way the push rod acts. Are you with me so far?
Now you could remove the original push rod, just leave the two attaching points on the sheet metal. Still no difference, right?
Now you can cut the sheet metal to any design you like, just leave the attaching points intact and this will still act in the same way, assuming it's not flexing.
Another way of realizing this is to imagine putting the newly designed push rod on top of the old one, still attached to the bike. This will not change things a bit.

No the issue here is the geometry, where you'll get to a point where you 'go over the top' of the steering circle. It's like when the pedals are at the top most/vertical position, they can either go forward or backwards. You have really no control of the way they move if you apply a force straight down. You need to construct your steering so it's acting in the most favorable section, corresponding to where the pedal arms are in a horizontal position. Working in a +-45 degree (or maybe +-30?) around this position will give you good control of the steering. The pedal arms in this case are both the arm from the steering head and the arm from the fork, but they should be of equal length and parallel.