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what makes the steering twitchy?

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what makes the steering twitchy?

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Old 03-17-09, 06:04 AM
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erbfarm
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what makes the steering twitchy?

Was out on my old Sequoia for the first ride of spring the other day. I've got 650B wheels on the frame, moustache bars, and a 75mm stem (bike originally had an 80mm I think). The steering is really twitchy. My question: is the twitchiness from A) the smaller wheel size (bike is from mid 80's so probably had 700c wheels) B) the smaller stem size C) both of those things or something else entirely? I've never had this issue w/ any other bike. Thanks!
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Old 03-17-09, 06:16 AM
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Soil_Sampler
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smaller wheels and tire = less trail+short stem=twitchy
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Old 03-17-09, 06:48 AM
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That combo seems like it would be a little twitchy, but how bad is it? Is it enough to annoy you, or enough to scare you?

Since you mention it's the first time you've ridden the bike this year, I'd check the headset. It's possible the grease has seized a bit. The most scared I've probably ever been on a bike was when my headset got tight during a ride (I think as the grease cooled down), and the bike got just ridiculously twitchy.
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Old 03-17-09, 07:15 AM
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If there is only a small wheel on the front it will make the steering angle steeper which will contribute to twitchiness
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Old 03-17-09, 07:25 AM
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Rake is the perpendicular distance between two parallel lines, one through the center of the hub, and one through the center of the steering tube. Trail is the horizontal distance between the tire contact point and a line through the steering axis. The more trail, the more stable the bike (slower steering). The less trail, the quicker the steering. Tire radius, rake and head tube angle affect the amount of trail. Steepening the head tube angle, increasing rake or reducing tire diameter will decrease trail, reducing stability and quickening the steering. The formula for trail is as follows, where R is the tire radius, and H is the head tube angle. Trail = (R/ tan H) (rake/sin H).

The stem length issue is debatable, since the shorter length reduces the steering arm length, requiring less movement of the bars, but the shorter length also makes it harder to turn. The change to the steering arm length is not nearly as large as many people think. Changing from a 120mm stem to an 80mm only reduces the steering arm length by about 15%, not 33%.

You also have to be specific about steering at high/normal speeds or low speeds. At low speeds, the wheel is turned to the right to make a very slow right turn, but at high speeds, you have to push on the right side (countersteering) to make the bike lean to the right and turn right. The two processes are quite different. A lot of people probably don't even realize that they do this, unless the are also motorcycle riders. Any motorcycle rider who makes a sharp right hand turn knows that you have to push hard on the right side of the bars to make the bike lean to the right and turn right. Quit pushing and the bike will go straight.
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Old 03-17-09, 07:40 AM
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dave is correct, but just in short: smaller tires can be twitchier than larger, all other things being equal.

Was it this twitchy last season?
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Old 03-17-09, 07:52 AM
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[QUOTE=Was it this twitchy last season?[/QUOTE]

I had a different set up on it last season so this is the first time I've ridden w/ the smaller stem and mustache bars. What I mean by twitchy is any small movement of my shoulders or arms seems to be immediately magnified. I was just tooling around the neighborhood the other day, not going fast, but it just seemed any small movement of the bars resulted in a very large movement of the front wheel which I'm not used to. My other bikes are kind of tanks and take a lot of movement up top to produce movement on the bottom. Would the mustache bars be doing anything? I'm used to drops.
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Old 03-17-09, 08:00 AM
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I've got 650B wheels on the frame
is the twitchiness from A) the smaller wheel size (bike is from mid 80's so probably had 700c wheels)
you switched from 700c to 650b, correct?
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Old 03-17-09, 08:14 AM
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Moustache bars move your hand positions radically compared to drop bars. If your hand positions are more widely spaced that adds to the leverage, and more response from the bike for a given amount of hand push. The distance of your hands in front of the steer axis could have a similar effect.

This effect isn't related to trail, just leverage.

I don't think the trail difference is very large, and it might or might not be perceptible. If I have a bike with a 72 degree head angle and a fork with a 45 mm offset, and 700x28C wheels, I'll get a trail of around 63 mm. If I change the wheel to a 650x28B, the trail will be around 57 mm. Both values are in the "racy bike, pretty stable" category. It's not like you'd be changing the bike into the 40 mm "low trail" category.

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Old 03-17-09, 09:15 AM
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Before you do anything else check the headset adjustment. A headset that is a tiny bit too tight will steer goofy.
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Old 03-17-09, 01:40 PM
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I thought that the effective diameter of a 650b was very similar to 700c. My vote here is with roadfan, the increased leverage could make it feel twitchier.
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Old 03-17-09, 06:29 PM
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If you're able to ride hands free, that would settle the debate between hand leverage and headset condition. Personally, I think it's the headset
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Old 03-17-09, 06:51 PM
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If the headset and other mechanical stuff check out OK, just try riding it for a while and see if your mind and body get used to it. When we've been riding our tandem for long rides all weekend, it can feel really weird going back to my French Jeunet for commuting Monday morning, but only for a hundred meteres or so, then it becomes natural again. Ditto with my long-trail Mariposa randonnee bike. It's amazing how kinesiologically (is that a word?) adaptable we are.
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Old 03-17-09, 07:24 PM
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Originally Posted by FlatMaster View Post
If you're able to ride hands free, that would settle the debate between hand leverage and headset condition. Personally, I think it's the headset

I would not say it isn't the headset. If it were mine I would make sure the headset is proper as part of the analysis. But I really haven't seen headsets causing what I think of as twitchiness. It's just I hate to go taking things apart and putting them back together, just to have to do it all again when the headset appears to be the only possible cause (I didn't discover this based on a twitchiness or handling problem) or some new condition is traced to teh headset.

That mindset mainly comes from my days as a self-taught air-cooled VW mechanic, being too cheap and lazy to replace clutch plates when the engine was out of the car.

I am so glad I don't ahve to deal with car mechanics any more ...
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Old 03-17-09, 07:36 PM
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Originally Posted by conspiratemus View Post
If the headset and other mechanical stuff check out OK, just try riding it for a while and see if your mind and body get used to it. When we've been riding our tandem for long rides all weekend, it can feel really weird going back to my French Jeunet for commuting Monday morning, but only for a hundred meteres or so, then it becomes natural again. Ditto with my long-trail Mariposa randonnee bike. It's amazing how kinesiologically (is that a word?) adaptable we are.
I definitely agree with seeing if the OP acclimates to the bike. There might be nothing wrong at all.

Just a question, and please don't take this as argumentative: Does "long trail" refer to a lot of trail, or a lot of rake? How much trail does it have? I usually think of a randonneuse as having a lot of rake, and hence not a lot of trail. At least that's what the math says. That characteristic helps the bike handle well with a large 'bar bag. It's just I find that term confusing.

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Old 03-18-09, 05:21 AM
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I'll check on the headset (will probably get to it tomorrow) and report in after that, thanks!
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Old 03-18-09, 06:34 AM
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Originally Posted by Road Fan View Post
But I really haven't seen headsets causing what I think of as twitchiness.
It does. It keeps the front wheel from self-centering the way that it's designed to do.
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Old 03-18-09, 06:36 AM
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Originally Posted by Road Fan View Post
But I really haven't seen headsets causing what I think of as twitchiness.
I've had a tight headset make the bike almost uncontrollable. Any movement at all would cause a pretty steep turn. Scared the living crap out of me.

I'm guessing the winter layoff for the bike let the grease seize up.
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Old 03-18-09, 06:44 AM
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Originally Posted by Mr. Underbridge View Post
I've had a tight headset make the bike almost uncontrollable. Any movement at all would cause a pretty steep turn. Scared the living crap out of me.

I'm guessing the winter layoff for the bike let the grease seize up.
I have had that stiction problem.
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Old 03-18-09, 10:41 AM
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Originally Posted by Soil_Sampler View Post
smaller wheels and tire = less trail+short stem=twitchy
+1 The trail basically means how twitchy your frame/fork's going to be. The short stem / handle bar differences can usually be modified.

For the stem/handle bar, it's basically geometry. The longer the stem, and the wider your hands are on the bar, the less twitch the steering is going to be. I kinda learn that in a jarring way when I tried out the aero bars on Monday, even though I know the steering was going to be different. With my hands basically next to the stem, any slight movement translates to steering.
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Old 03-18-09, 01:45 PM
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When you test riding no hands, you should see the front-wheel waggle back & forth with each pedal stroke. If it veers off to one side back & forth slowly while causing you to lean over, the headset may be too tight and won't allow the trail to re-center the wheel automatically.l
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Old 03-18-09, 02:52 PM
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Originally Posted by calyth View Post
+1 The trail basically means how twitchy your frame/fork's going to be. The short stem / handle bar differences can usually be modified.

For the stem/handle bar, it's basically geometry. The longer the stem, and the wider your hands are on the bar, the less twitch the steering is going to be. I kinda learn that in a jarring way when I tried out the aero bars on Monday, even though I know the steering was going to be different. With my hands basically next to the stem, any slight movement translates to steering.
But he said 650B not 650C. Given that 650B bikes generally have large tires and 700c bikes have skinny ones, this may be a non issue. I converted a bike from 650B to 700C last summer and while the brake track moved a fair amount, the final loaded tire diameter was within 1 or 2 mm. I think something else is the issue, but I'm not sure what just yet. Have you checked the hubs to make sure that they are not loose?
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