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Discussion Starter · #1 ·
I sometimes experience speed wobbles on steep descents. My frame is a large, custom titanium to fit me (6'-3"). The frame is beefy, but the bike has a lot of factors that are known to increase speed wobble tendencies: Tall steerer tube,
Tall seat tube,
Lightweight carbon forks with 50 rake (ENVE 2.0),
Higher bottom bracket for 180 cranks,
Deep-ish lightweight rims (Velocity A23),
Flexy handlebars (Salsa).

I'm looking to replace the wheels, handlebars, and forks. Suggestions?
 

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Do you have another set of wheels? If not, borrow a set to eliminate that as a cause. Don't start replacing things until you're certain of the cause. Speed wobble is very difficult to pin-point. There are many theories but the one that makes the most sense to me is the weight distribution is off. Could be the rider's weight distribution on the bike, not enough weight on the front wheel? Good luck, this is a tough one to solve.
 

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Discussion Starter · #3 ·
The rear triangle is standard, while the height and overall length of the frame is greater; this would undoubtedly put less relative weight on the front.

My remedy while riding is to try to relax and put weight on the bottom bracket and forward on the hoods.

Without adding length to the rear triangle, I'm looking for other contributors that can more easily be changed.

I balanced and trued the wheels, and got a lot more stability. But when the bike gets a hard wobble, it continues until the bike is almost brought to a stop--with the front wheel appearing from my point of view almost as if it is warped--but it isn't.
 

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Discussion Starter · #5 ·
What I'm looking for are specific fork, wheel, tire, and handlebar suggestions from riders/mechanics who have successfully eliminated speed wobble.

In the last two years, I've probably read all the anecdotes about what causes speed wobble and how to adjust your body when it happens, and also have read quite a bit of engineering analysis of how the system contributes. Riders have reported that changing wheels or forks have made their bike more prone to wobble. . . But what about changes that have helped?

Here's what I've done so far: I have eliminated speed wobble up to about 50mph on straight, smooth pavement by stiffening the top tube and handlebars with foam inside and by correcting wheel truing and balancing the wheels with weights. I have also learned to relax and get a feel for the resonance differences I can control with posture and knee pressure on the bar. However, the bike will still wobble on hard downhill hairpin turns, especially when I'm braking hard. The speeds that induce wobble are too fast to really play with, and the conditions too dangerous. So I have just changed one thing at a time and then seen if it helped.
 

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The thing that cures wobbles better than anything else is to get your butt off the saddle, and use your knees as suspension. When you stay seated, you clamp the top tube with your knees. Gotta ride loose, man!
 

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How do you get a wobbel in the middle of a turn?
If your braking and the front wheel starts feeling choppy, ease off on the front brake a little, your about ready to lose the front end!
 

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Discussion Starter · #9 · (Edited)
Thanks for the suggestions about riding.

About wobbles on curves: This happens on steep and rough descents, with or without braking. It starts with the front end getting knocked around.

I'm mostly interested in how riders/mechanics/builders have solved wobbles with different parts/hacks.
 

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Get new forks with more trail, remove all that other baloney.
Duriel has it right.

While I don't know the geometry of your custom frame, I will assume the head angle is typical for a frame your size. That would be 73.5-74 degrees.

At those head angles, 50mm of rake results in too little trail and great potential for shimmy. If I'm correct about your head angle, then you should have 40mm maximum fork rake on that bike.

If that doesn't work, or better yet-try this cheap experiment, first.

If your bike uses a spoke mounted front wheel magnet, remove it and see if the shimmy goes away. I don't care if you balanced the wheel with the magnet on it, just try it. It's free.
 

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Discussion Starter · #13 · (Edited)
Duriel has it right.

While I don't know the geometry of your custom frame, I will assume the head angle is typical for a frame your size. That would be 73.5-74 degrees.

At those head angles, 50mm of rake results in too little trail and great potential for shimmy. If I'm correct about your head angle, then you should have 40mm maximum fork rake on that bike.
Yes, the head tube angle is 73 degrees, and the current fork has 50 mm of rake.

So, reducing the fork rake increases trail, right? Right now, I measure 52 mm of trail. A 40 mm rake fork would give me about 10 mm more of trail--so 60 mm.

If that doesn't work, or better yet-try this cheap experiment, first.

If your bike uses a spoke mounted front wheel magnet, remove it and see if the shimmy goes away. I don't care if you balanced the wheel with the magnet on it, just try it. It's free.
The magnet is on the back.
 

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Yes, the head tube angle is 73 degrees, and the current fork has 50 mm of rake.

So, reducing the fork rake increases trail, right? Right now, I measure 52 mm of trail. A 40 mm rake fork would give me about 10 mm more of trail--so 60 mm.
Even 52mm of trail is too little. 55-57mm is considered "neutral" or not too hot, not too cold. Even up to 60mm wouldn't be bad; much like a mountain bike and I like the way my mountain bike handles.

I had a custom frame once, and I was struck by a car. I had the frame rebuilt and I spec'd the front end at 73/50 just like your bike. In short it was a poor handling bike. A framebuilder of great reputation merely re-raked the steel fork to 45mm and vastly improved the handling. Perhaps you'll enjoy the same improvement.
 

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Discussion Starter · #15 ·
So now I'm looking for a stiff fork with a 40 mm rake and a long steerer. I'm currently using 362 mm of steerer tube. Could take out spacers to a 350.

I have been looking for a carbon fork with a metal steerer or a much thicker wall if it's all carbon. Any suggestions?
 

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I sometimes experience speed wobbles on steep descents. My frame is a large, custom titanium to fit me (6'-3"). The frame is beefy, but the bike has a lot of factors that are known to increase speed wobble tendencies: Tall steerer tube,
Tall seat tube,
Lightweight carbon forks with 50 rake (ENVE 2.0),
Higher bottom bracket for 180 cranks,
Deep-ish lightweight rims (Velocity A23),
Flexy handlebars (Salsa).

I'm looking to replace the wheels, handlebars, and forks. Suggestions?
If the bike shimmies in turns, that could be the titanium frame in 60-62 cm size is too large for the tubing and flexes from lateral forces. I had a steel bike that did that. Putting the foot down hard on the outside pedal helped a lot, but the bike still didn't corner very confidently. It was a badly machine brazed lugged frame, 56 cm. Yours must be custom, but titanium used to have a reputation for being a bit flexy in larger sizes. Leonard Zinn would know all about that.
 

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I don't know why everyone is talking changing the rake, you can't change that without modifing the frame. From my research the rake is the head tube angle. Now if you had a triple tree on it, you could change the rake there.
The only thing one can do is change the trail, the rake is fixed once the frame is built.
 

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I don't know why everyone is talking changing the rake, you can't change that without modifing the frame. From my research the rake is the head tube angle. Now if you had a triple tree on it, you could change the rake there.
The only thing one can do is change the trail, the rake is fixed once the frame is built.
You have a misunderstanding of terminology here.

Head tube angle is fixed

Rake is the off set of the fork

Trail is the distance the tires contact patch is behind an imaginary line where the head tube angle intersects the ground.

Dave Moulton's Blog - Dave Moulton's Bike Blog - Trail, fork rake, and a little bit ofÂ*history

Line Diagram Circle Drawing Line art
 
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