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Death wobble on the Schwinn Circuit today!

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Death wobble on the Schwinn Circuit today!

Old 09-05-16, 07:54 AM
  #51  
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Originally Posted by jamesdak
It amazes me how many people seemed freaked by the speed.
I don't know if "freaked" is the right word, but risk does go up with speed. It's fine for each of us to have his/her own limits, but the concept of limits isn't strange or novel. 55 mph is an unusual speed for bicycles, and it doesn't seem strange to me that it is beyond some people's willingness.

I'd bet no bike maker tests their bikes at speeds like that.
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Old 09-05-16, 07:59 AM
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Originally Posted by jamesdak
It amazes me how many people seemed freaked by the speed. It's really a product of climbing in our mountains out here and no big deal. Plenty of bikers run these mountains at speed.
Me too. It's odd.

It's quite routine for racers to go this fast. There were some fast long straight descents near wear I grew up that allowed higher speeds than this. We would routinely play peg the speedo on them (65). It was just doable by sprinting into them and using full tuck.

That said I suppose if you are not confident at that sort of speed, it would be unwise to go that fast.
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Old 09-05-16, 08:05 AM
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Originally Posted by Salamandrine
Is the alignment correct and has this been checked? Did you respace the frame yourself? Was it in fact respaced or was the 11 speed wheel just crammed in there? The point I'm getting to is that frame alignment should be done correctly. It really bugs me when I hear people recommend cramming a 135 or whatever wheel in the wrongly spaced frame, without properly realigning the frame. It is wrong to do this.

It's possible it is an issue of geometry: short wheelbase, steep angles and low rake. I can't measure it with my eyes but it looks kind of like that bike has those qualities. If the bike has that and/or some unfortunate combination of geometry and mass that leads to oscillation, there's not a whole lot you can do about it. Proper set up is controllable however. Checking the alignment would eliminate that possibility.

55 or ever faster should be no big deal.
Well the frame was checked and respaced for the build. I still need to check it again to see if something was missed. But I'll also say that with all my various bikes over the years It's pretty obvious that all of them are a bit different on that rear spacing. I doubt that many actually measure a specific 130mm and doubt a mm or two either way really matters but alignment certainly does.

The wheelbase is actually one of the longest when I just compared it against all my road bikes. Yet it's measures as a 54 cm where as some of the others are 56cm. It is steeper than my Giordanas with a seattube angle of 74 and headtube of 73 going by catalog specs.
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Old 09-05-16, 08:11 AM
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Originally Posted by Salamandrine
Me too. It's odd.

It's quite routine for racers to go this fast. There were some fast long straight descents near wear I grew up that allowed higher speeds than this. We would routinely play peg the speedo on them (65). It was just doable by sprinting into them and using full tuck.

That said I suppose if you are not confident at that sort of speed, it would be unwise to go that fast.
Being a solo rider the idea of riding in a paceline or gasp, a peleton scares me.

I remember the one time I saw the Tour de France. It was '86 I think, the time they came through Karlsruhe Germany. You had this sudden rush of a mass of riders, going really fast, on a narrow german street, and through a curve. I was like, "how do they not crash into each other"???
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Old 09-05-16, 10:56 AM
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Originally Posted by jamesdak
Being a solo rider the idea of riding in a paceline or gasp, a peleton scares me.
Me too. No, make that me three.
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Old 09-05-16, 11:07 AM
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Originally Posted by SquidPuppet
Glad you saved it.

I very much doubt you caused it though. In the motorcycle world, "Tank Slappers", are caused by unstable geometry. Certain extremely aggressive Rake/trail/short wheelbase combinations make for an excellent handling racing motorcycle for cornering and quick change of direction. But they result in nasty straight line instability, which is exaggerated at higher speeds and by bumpy surfaces. It's actually the accepted norm and that's why you'll see serious dampers fitted to all race bikes.

As you know, it doesn't really matter what initiates the wobble. Once started, they take on a life and energy all their own. Even if everything else, bearing adjustments, spoke tension and true, frame alignment, etc, are perfect, some bikes are just very likely to do this once they reach a certain speed. Of course I don't know if that's the case with your bike. It may have been a one-off incident caused by a group of factors all occurring at just the right time. I hope it was just a freak occurrence.

The "Great" thing about a really violent slapper on a motorcycle is that head shake will drive the brake pads and pistons ALL the way back into the calipers. So if you survive the wobble, right when you need the brakes, you have NONE. And it usually takes quite a few pumps to restore the hydraulics to normal operation.
Not in all cases. From personal experience, had an agressive 'tank slapper' on a brand new GSXR-1100 in 1986. First year import to the US. Factory equipped with a damper. To properly break these tight motors in, one would rev to redline in most gears and under load. Mine might have been one of the first releases as Suzuki never advised of the potential disaster awaiting.

The violent wobble only came from straight line and rolling off throttle at well over triple digit speeds. When first experiencing it, scared the jeebeez out of me and the effect was so wicked it transmitted thru the chassis to the rear wheel. Thats how bad it was. To save it, had to roll back 'on' the throttle, then drag some rear brake and slowly roll off the throttle, all while praying I have enough time and distance ahead of me.

My bike sat in the service department only a few days when a recall notice came in. The problem - improper headset preload. After that and even some time later with highly modded motor and gearing, the bike was bulletproof.
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Old 09-05-16, 11:09 AM
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I'm neither an engineer nor a bike designer.

My experience with racing motorcycles (that have adjustable front and rear geometry, ride height, and wheelbase) led me to believe that the same things that casue wobble on motorcycles cause them on bikes.

Jobst Brandt describes it much better than I can.

Shimmy is not related to frame alignment or loose bearings, as is often claimed. Shimmy results from dynamics of front wheel rotation, mass of the handlebars, elasticity of the frame, and where the rider contacts the bicycle. Both perfectly aligned bicycles and ones with wheels out of plane to one another shimmy nearly equally well. It is as likely with properly adjusted bearings as loose ones. The idea that shimmy is caused by loose head bearings or frame misalignment seems to have established currency by repetition, although there is no evidence to link these defects with shimmy.


Bicycle shimmy is the lateral oscillation of the head tube about the road contact point of the front wheel and depends largely on frame geometry and the elasticity of the top and down tubes. It is driven by gyroscopic forces of the front wheel, making it largely speed dependent. It cannot be fixed by adjustments because it is inherent to the geometry and elasticity of the bicycle frame. The longer the frame and the higher the saddle, the greater the tendency to shimmy, other things being equal. Weight distribution also has no effect on shimmy although where that weight contacts the frame does. Bicycle shimmy is unchanged when riding no-hands, whether leaning forward or backward.
My bolding.

The second bolding is why the only "successful" (it helps, but doesn't save them all) method of killing a death wobble on a motorcycle is to raise your butt an inch off the saddle and put as much weight on the pegs as possible.
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Old 09-05-16, 11:32 AM
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Originally Posted by crank_addict
To save it, had to roll back 'on' the throttle,
That's pretty unusual. I'm glad it worked. Those Gixxers got a lot of press for that issue in their day.
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Old 09-05-16, 03:38 PM
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Well, I weighed my options this morning and decided to head back up the same mountain. But I wound up not taking the Circuit. I've been wanting to do this same climb with the Giordana since putting on the Shamal wheels and decided that was the one to ride today. Great climb up as this bike has always felt good climbing and getting a pound off the wheels made it feel even better. Probably not any faster though with my slow old legs. LOL!

Just thought I'd mention todays ride as a way to explain how the speed experience can be on this mountain. Today the wind was coming up the mountain as I did the main descent and really messing things up. I was getting buffeted around a lot and not able to get anywhere near the speeds I did the other day. Today's max was right under 48 mph and in the area where I hit 55 mph and had the shimmy I was only getting 37 mph and decelerating due to the strong winds. That's just how it goes riding in the mountains out here. Conditions constantly change and it's not always going to be a full on "death defying, warp drive" run back down. It's just really cool when it is!

The view was spectacular today and that's one of the reasons I love riding this route.

This is just a quick snapshot with the cellphone about 1/2 up the mountain.

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Old 10-01-16, 06:18 PM
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After 30-ish years on lugged-steel Trek, I added a seatpost-style water bottle rack (2 bottles) to my handlebar stem. When the bottles are full,or nearly so, I have experienced a wobble at high speeds that was not present before. Recently it occurred on a fast descent in a group and I thought I was going to buy the farm. Finally slowed enough to get control. I have found that clamping knees against the top tube will greatly diminish it, a sort of damping effect. From an engineering perspective, this is a complex dynamics problem worthy of a dissertation by a sharp grad student. Lots of factors at play.
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Old 10-02-16, 08:25 AM
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Good point about the grad student, @SherpaBob. Is there a good index of bike research? It seems that there has been a nice increase in bike physics research in the last 10 or 20 years.
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Old 10-02-16, 09:15 AM
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Have you ever figured out if this was a fluke or if there was a mechanical cause to this wobble?
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Old 10-02-16, 09:49 AM
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Originally Posted by Steve Whitlatch
Have you ever figured out if this was a fluke or if there was a mechanical cause to this wobble?
I haven't yet. Been back up that mountain just not on that bike. Then I tweaked the left calf and while I've kept riding I've avoided the climbs so it will hopefully heal. I did 40 miles on it yesterday without any pain afterwards so I think it's about there. Hopefully I can get back up there sometime this week. It'll need to be soon as once the snow hits the roads and they start dropping the chemicals down I keep the good bikes off of them.
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