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jawnn
10-22-2012, 02:17 PM
Why does this bike shimmy? http://james.architectureburger.com/cycle/cargo.html (http://james.architectureburger.com/cycle/cargo.html) I think it is the round tube they used. I think it can be avoided by using flat bars or a square tube.
The size will still matter 1.5” should be enough if flat bar is used. But I can’t find thin (.09”?) flat bar in 4130 airplane steel.

trikeman
10-22-2012, 06:45 PM
I think the answer is in the article - not enough trail. Bikes with almost no trail tend to have quite sensitive steering inputs (even if that input is from bumps in the road). I always like to shoot for about 2" of trail, which gives me a nice smooth hands off ride, but if you load down the front fork with 30 pounds of stuff it does make steering more difficult at low speed. You can't have everything I guess. Since the shimmy seems to be caused by steering geometry, larger frame tubes probably won't tame the beast. As you say, cantilevering that much load out in front or back can't be helping either. In the back you could add a brace from the back of the rack to near the axle. On the front, there is really no place to attach a brace. Perhaps that is why many cargo bikes put the load in the middle of the bike.

Strangely, this is the third thread on trail and its effects I have participated in this week. Two were on BROL, and now one here. One was a homebuilt with too much trail and lots of wheel flop. Two were factory builts with almost no trail and twitchy or shimmying steering. It must be bike geometry week.

trikeman
10-22-2012, 07:42 PM
When I build a bike and it has bad geometry, I just cut it up and make it better. If I bought a factory bike and it had bad geometry I would probably be a bit pissed.

graucho
10-22-2012, 11:10 PM
Sweet looking work bike jawnn. I agree with trikeman on the importance of good front end trail.

I wonder if your wheels are true and balanced. I wonder if your inner tube has a twist. Any spokes a little wobbly. Spoke tension, rim alignment or tire straightness. Maybe the front wheel bearings are loose on the axle, causing the axle to wobble. Remove the front wheel. Does the axle wiggle slightly?

Shimmy will also happen because a frame lacks lateral stiffness. Oscillation occurs between the two gyros (the wheels), most often at speed. If it is a frame issue. clamping your knees against the top tube with your crankarms horizontal will usually stop it. Try slightly tightening the headset bearings. Slight, not overtight!

Looks like you carry weight from the stack of papers you haul. There could possibly be flex in the rear wheel if you consider that the upper spokes are under tension and the lower spokes are not; making the wheel likely to flex at the bottom. Also a dished wheel has unequal tension on the drive and non-drive sides. So if you have a bike that is prone to shimmy maybe think about switching to a stronger more tightly built wheels.

Nitty
10-23-2012, 07:12 AM
The weight (load) is too central to it's two anchor points [Wheel & Handlebar] & [Wheel & Seat]. This is the least point of resistance & most elastic. You never see an archer place his arrow at the top or bottom of his bow.

The Head tube / forks and the seat tube will bow side to side (looking from front or back) this will cause twist in the down tubes and as these are both the same angle, they cannot counteract or resist each other. The rider will try to counteract these forces, (remember, he / she is an anchor point) and the whole load / bow / twist problem reverses until the resonant frequency of the seat tube & head tube is reached.

Unsecured loads do not actually 'provide' damping, they merely do not 'input' as much kinetic energy.

....... I think

Ken

graucho
10-23-2012, 08:56 AM
(Shortened thread) The weight (load) is too central to it's two anchor points [Wheel & Handlebar] & [Wheel & Seat].
The Head tube / forks and the seat tube will bow side to side (looking from front or back) this will cause twist in the down tubes
Ken

Also great points Nitty. I wonder if jawnn would shift his weight to way back on the seat... then way forward, in front of the seat to see if the shimmy stops. Maybe it's body weight positioning. Maybe as simple as changing seat location forward or back 3"?

Also I don't like the rake on the bike. It looks to vertical. You think the manufacturer would have had better geometric engineering applied. Which could be fixed by trikeman's orig first post with fixing the trail. LOL! Staring at an enlarged shot of the bike... I think if the manufacturer should have given it 5 deg more rake, 1 inch more trail, and a 24" front wheel it would have straightened up the bike as it should have been.

Radical Brad
10-23-2012, 09:43 AM
I think there will be an oscillation effect between the steering and the load. If you think about, it - this bike is partially rear steered, and we all know that a rear steering cycle is almost impossible to maneuver safely.

Rear steered you say?!?!.... yup.

With the load being positioned ahead of the front wheel, each tiny steering adjustment sends the load moving in the opposite direction of the steering motion. This inertia is then instinctively counter-acted by the driver in an attempt to move in a straight line. The load is then forced into the other direction, and the vicious cycle continues into a a nasty shimmy. A rear steered cycle suffers this effect.

Being an upright cycle makes it even worse since the load ahead of the wheel is tightly affixed to the flatbed whereas the rider is loosely affixed on top of the seat. So the front load is really the pilot, and the poor sap trying to counter act this rear steered machine is the stoker.

If the load was simply placed behind the front wheel, this bike would then perform admirably.

Brad

graucho
10-23-2012, 10:07 AM
Brad you have a lot of brains stuffed into that skull. LOL
jawnn it looks like your model is different than the link you provided. If your bike was built like the one
in your link, I think your bike has manufacturing issues.
One of my qustions is... I wonder why the manufacturer didn't even build the bike to their own specifications?
In their plans looks like they had the correct rake and trail. Even the bottom tube isn't the same degree?
The rake was 5 deg more. the trail was longer. I thought the rake looked too vertical.

http://i284.photobucket.com/albums/ll13/grauchosbikes/cargocombine2.jpg
http://i284.photobucket.com/albums/ll13/grauchosbikes/cargocombine.jpg

jawnn
10-23-2012, 01:48 PM
OK, maybe I didn’t make it clear that the bike I was talking about is not mine and I do not know them. But I made my bike before I found that article, and the steering axis is 74 degrees (for the same reason they did) with no shimmy at all. I used 1.5”x 0.125” flat bar mild steel for the cantilevered front rack. I know it is over engineered because I sat on it with my 210lbs with no ill effects other than the rear popped up. I agree that long delta trikes need long trail.

But I would like to build one of the kind in the article, and still think it should be made stiffer. They were trying to make it light weight, something I would not do; Except to use thin .090” 4130 flat bar.

I also think it should have braces on the rear of the rear rack section, I have chain stays for that. In fact there is really no reason to have it cantilevered. Too bad we can’t communicate with that bike’s builder.

This is my bike and it has about 1 inch trail with a 74 degree axis.
24" rear wheel.
http://forum.atomiczombie.com/gallery/showimage.php?i=2405&catid=newimages

http://commutercycling.blogspot.com/2012/04/construction.html

This is what I want to do next but maybe too simple.

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darnthedog
10-23-2012, 03:08 PM
jawnn
I think Brad let you know what the issue is. Weight distribution over the front wheel. Have you considered a different style such as the Dutchman. I can attest to the weight over the front wheel being an issue. Was I assisting a friend in delivering papers with a bicycle. Talk about shimmy. Didn't think I was going to make the whole route. That was many years ago. However I never knew until now what I had issues.

Radical Brad
10-23-2012, 04:07 PM
Or more specifically, weight "ahead" of the front axle.
At least, that's my guess as to what I would say sounds like inertia induced oscillations amplified by operator feedback (IIOABOF).

.... had to give it a proper title.

Brad



jawnn
I think Brad let you know what the issue is. Weight distribution over the front wheel. Have you considered a different style such as the Dutchman. I can attest to the weight over the front wheel being an issue. Was I assisting a friend in delivering papers with a bicycle. Talk about shimmy. Didn't think I was going to make the whole route. That was many years ago. However I never knew until now what I had issues.

savarin
10-24-2012, 09:59 AM
Its like the old butchers delivery bikes that had the huge basket on front with a 20" wheel.
A real pig to get used to.

jawnn
10-24-2012, 02:44 PM
"weight ahead of the front axle" sounds resasonable, bouncing.
maybe they can just add verticle suport?

Mistake:
I measured my BMX fork last night and it has 1.25" rake so it has 1.7" trail, that must be why I do not get any shimmy.

Good thing I didn't l did not weld it at 70 degrees, it would have had over 2.5" trail and I would not be able to ride it, because of of the wheel flop.

Short trail is good for SLOW speed hill climbing. I had too much wheel flop on my recumbent and fixed it by shortening the trail.

I have another bike with only .5" trail and it works well, but it is a recumbent bike. No photos yet.

See this page for info about low trail on recumbent bicycles, and there is a motorcycle that has zero trail at the bottom:
http://commutercycling.blogspot.com/2007/10/ultimate-touring-bike.html

jawnn
10-25-2012, 01:19 PM
My bent with 79 degree steering axis and .5" trail, note that it has counter ballance in the handle bars (my arms)
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