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Utility Cycling Want to haul groceries, beer, maybe even your kids? You don't have to live car free to put your bike to use as a workhorse. Here's the place to share and learn about the bicycle as a utility vehicle.

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Old 05-08-08, 12:25 PM   #1
JeanCoutu
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xtracycle death wobble

K, so I built this a few days ago.

The bike I started with was good all round and had nothing evil about it. Now it'll get a death wobble going at maybe 20-25 km/h with no hands. Going faster is ok, and it's kinda hard to ride no hands slower. The rear of the red bike is slightly higher then it was before, I've clamped down the bb on the mtb rear triangle so hard the bearings must be ruined, so there's no slack there. Both wheels are reasonable true and the bearings are allright, only thing is the headset's a little loose. Front tire is mounted slight off since it was standing up when I inflated it, so there's a slight lump on the part that was on the ground. I'm just happy it happens at a low speed rather then say going downhill at 70km/h, but I'd rather get rid of this behaviour. Suggestions?

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Old 05-08-08, 12:36 PM   #2
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It could be frame flex, and it could be just the wobbly headset and the lumpy front tire.

I'd deal with the tire first, then move to the headset if you have to, and if those two don't fix it start investigating where it may be flexing.

That said, loaded, my longtail gets twitchy as hell.
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Old 05-08-08, 02:31 PM   #3
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Look up wheel shimmy and see if that is the problem that you are describing.
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Old 05-09-08, 02:40 AM   #4
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If the bike is unsafe to ride at 20-25kph with no hands, what's the problem?

None of the (non long tail) utility bikes I've had were any good for no-hands either.
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Old 05-09-08, 08:12 AM   #5
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You should not be riding a bike with a loose headset or a "lumpy" tire. Fix both.
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Old 05-09-08, 08:22 AM   #6
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Why are you riding a long bike no-handed?...... or at 70km/h!!!
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Old 05-09-08, 11:32 AM   #7
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I had a "blowout" on my unicycle the other day. The tire didn't actually blow out, but the tube did. I think the tire managed to work around to where it could slip off the rim in one place, and did, and blam! flat tire. So beware.
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Old 05-09-08, 12:45 PM   #8
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Why are you riding a long bike no-handed?...... or at 70km/h!!!
Oh, heh, I don't think it's ever reached anywhere near that fast... yet!

No handed, well I was coming back from a ~5km trip and with flatbars you only really have one hand position, tho it rides superbly with one hand. I was going down this slope and the bike was coasting nicely, so I got up and let go the handlebar. It kept tracking where I wanted it to go though it needed a bit more body language, but then I realized it was shaking, I look down at the bars and it was pretty bad. Putting hands on the bars made it alright.


Ok, so I've taken off the wheel and checked it out. Somehow the rim was ~5mm off center, took one that needed less work from my junk pile, made it true and round ~1/2mm, installed the tire partly inflated going around pushing pulling making sure it's as evenly seated as I could possibly get it. Then I added two reflectors on opposite sides but closer to the valve stem and added some fishing leads near the valve stem. So, at this point I can't really get it much better then this.

Still not touched the headset but I'm off to get tandem cables for the rear end shortly, we'll see what happens downhill on the return trip. I suspect the tire's lump compounded with ~5mm offset was what caused it.
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Old 05-09-08, 12:53 PM   #9
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K, so I built this a few days ago.

The bike I started with was good all round and had nothing evil about it. Now it'll get a death wobble going at maybe 20-25 km/h with no hands. ...
This is the problem I expect with these add-on things; even the one Xtracycle themselves makes but especially the home-builts that people take so much pride in building out of junk metal.

I've not ever seen one in real life so I don't know how well it attaches, but I'd think it would be a lot safer to just get a steel frame and cut off and re-weld the rear end longer.
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Old 05-09-08, 01:57 PM   #10
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Welp that didn't fix it, now it does it much more smoothly. I would guesstimate 30-40km/h. Also I've noticed while turning the bike seems to want to turn more then I tell it to rather then just riding like an extension of my body.

Even tho while I'm using mostly garbage salvaged parts, but I don't expect it would behave significantly differently were it made out of top end bikes.
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Old 05-09-08, 04:40 PM   #11
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Headset.
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Old 05-09-08, 06:04 PM   #12
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K, I'm a try that next.
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Old 05-09-08, 07:28 PM   #13
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Also I've noticed while turning the bike seems to want to turn more then I tell it to rather then just riding like an extension of my body.
Since you said you put the rear of the original frame a bit higher than it was pre-modification... that changes the headset angle (relative to the ground) and might cause this problem.

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Even tho while I'm using mostly garbage salvaged parts, but I don't expect it would behave significantly differently were it made out of top end bikes.
In my opinion, if it were skillfully welded together from the minimum number of parts (metal tubes), instead of bolted together from a slightly larger number of parts, there would be less room for flex where stuff fits together. You can't beat plain ol' triangles for stiffness, and lack of stiffness is a likely cause of or contributor to a long-bike wobbling.
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Old 05-10-08, 11:00 AM   #14
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Headset didn't fix it. Made it happen smoother, more sharply, at what seemed like a higher speed. same as attention to the wheel really. I noticed the rear part of the rack can mode sideways a little tho, gonna try clamping it down better and see what happens.

And yeah I think I'm a have to read up on bicycle/motorcycle geometry stuff to dial this thing in properly. It's gonna be great.

Nothing to stop me from hauling my buddies around town pretty much everyday since the day I built it tho. Thing is hauling a passenger around town makes for a slow ride on the flats and somehow doesn't make you wanna go fast down hills.
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Old 05-10-08, 11:36 AM   #15
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Did you bolt it together the "fordfasterr" way or the way I did with my first one? I am guessing you did it "my way" since you talk about the bearrings. Remember when you use the triangels axle as a bolt to connect the bikes dropouts to the triangle you must remember that the two axel pieces is not the same lenght. that way the rear wheel is not right behind the front wheel. Could that be the problem?

Also the added bar (steel flatbar?) need to be properly in place. I like to use a "square tube" from a car`s roof rack. Easyer to do a good job with that one in my opinion.
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Old 05-10-08, 12:50 PM   #16
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Uh, well I had a bunch of parts and bolted the pedal things in the drops of the red bike with a handlebar used as brace (still has a brake lever).


So far:
- Respace rear triangle of front bike so it's centered (sheldon brown string method I guess)
- play with angle of front bike
- Use a square, tight fitting tube for brace, eliminate play as much as possible
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Old 06-06-08, 05:20 PM   #17
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BTW, I've not fixed the issue or given up or lost interest or anything, reason I've not updated is that about a month ago I fell off a cliff and broke both my legs and a wrist. I'm typing this from a computer in a physical rehabilitation center and it's likely going to take at least 3 months before I can ride a bike again, so that's that. Also the buddy I was with when I fell off the cliff kept using said bike and somehow managed to get a slow leak on the front tire (well worn 26X2.0 Big Apple), so the fine balencing will have to be re-done I guess.
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Old 06-06-08, 06:24 PM   #18
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about a month ago I fell off a cliff and broke both my legs and a wrist. I'm typing this from a computer in a physical rehabilitation center and it's likely going to take at least 3 months before I can ride a bike again, so that's that.
Ouch. Hope everything heals quickly and well!
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Old 06-06-08, 06:31 PM   #19
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Sounds like you like to live with danger, excuse me for saying so..

Did think about you some few days ago. Comming from the shop, two bags of groceryes, my 26 kg dog and myself on the bike ging down a small hill. Suddenly the donorbikes bb axel dropped out of the dropouts of the main bike (remember I told you not to do it the way I did?). If it had happened in the speed you used to ride yours I do not know what would have happened. We skidded on the senter kickstand for many meters until I realised what had happened, that I was still balancing the thing (dog still sitting in his place) and decided to try to breake. It worked. Just a small scratch on one knee.

This has happend once before, but in a much more safe way, just noticed the rig was a bit "soft" when riding it and not quite straight. Think I was too fast and too lazy when repairing it. I am thinking I should change the setup a little, and also maybe change to a hub gear. The thing is I like this bike so much I do not want to start working on it in fear of that I get into problems so it is not ridable for days (also want to change the rear triangle to a alu one I just found for weight reasons). 80% of my riding is done on this bike since I almost always ride with my dog, and most of the time much longer than he can run.

Did approx 40 km today. He was riding on the bike for approx half of this.
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