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Discussion Starter · #1 ·
I recently built up my old 29er hardtail frame into a street bike for riding around the neighborhood with the kids and making Starbucks runs. I'm getting a weird "gyroscopic" effect though when turning at low speed with the handlebars cranked at a big angle, that I am having a hard time diagnosing. It feels like a low frequency wobble that yanks the handlebars left and then right in an alternating fashion as long as I am turning enough. Straight line stability and lower input turns do not exhibit any odd behavior.

I am running a rigid fork with tubed Maxxis Grifters 2.1 on a Sun Inferno 23/XT hub wheelset from BWW and Avid BB7 mechanicals. Not sure if my headset is contributing to the problem as I am having trouble getting the preload correct and wondering if the cups seated correctly on the powdercoated frame.

Any ideas on what to look at? Tire issue? Hub preload? Headset? Too much traction causing a fat bike type situation?

Thanks!
 

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since 4/10/2009
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odd. almost sounds like speed wobble, which I've experienced on my commuter bike before. It's a weird harmonic thing that sounds like a higher frequency than what you've got going on.

Major difference is that the speed wobble happens when riding in a straight line more than any other situation. As such, I've thought about getting a Cane Creek Viscoset, but since it's something that's only come up for me once, I haven't been extremely motivated to make the purchase.

https://www.canecreek.com/product/viscoset/

I don't think you're dealing with gyroscopic issues, either. THAT shouldn't really be a big deal at low speeds. And that should feel different, besides. Gyroscopic effects should more or less feel like the bike wants to keep going straight at speed and that you'd be fighting the bars whenever you try to turn them. It should feel pretty consistent, not any sort of alternating, wobbly sensation.
 

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Life's a Garden, dig it!
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You may want to check your effective head tube angle. Frame head tub angle plus fork offset could be causing an issue. Kind of like an extreme caster angle on a car. If it's too extreme, you may be dealing with the front wheel's propensity to "flop" off center.
 

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Discussion Starter · #6 ·
You may want to check your effective head tube angle. Frame head tub angle plus fork offset could be causing an issue. Kind of like an extreme caster angle on a car. If it's too extreme, you may be dealing with the front wheel's propensity to "flop" off center.
sounds like you put the wrong type of fork on and messed up the geometry
Just to clarify, I'm experiencing an alternating tugging sensation during low speed turning not a resistance to turning like a fat bike. Apologies for my confusing nomenclature. Do you still think it could be a geometry issue?
 

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U sayin' Bolt ?
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Which fork did you put on which frame? Older 29ers didn't have much head angle to spare on going too low up front. Not sure if that would produce a wobble though, more like catching an edge ime.

Might be worth it to take a couple spanner wrenches to the hubs and make sure everything is tight.
 

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Life's a Garden, dig it!
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Just to clarify, I'm experiencing an alternating tugging sensation during low speed turning not a resistance to turning like a fat bike. Apologies for my confusing nomenclature. Do you still think it could be a geometry issue?
Yes, the "flop" that I mentioned would feel like a bit of a tug as it tries to over turn in the direction you want to go.
 

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You may want to check your effective head tube angle. Frame head tub angle plus fork offset could be causing an issue. Kind of like an extreme caster angle on a car. If it's too extreme, you may be dealing with the front wheel's propensity to "flop" off center.
sounds like you put the wrong type of fork on and messed up the geometry
+1

It all works until you go too far one way or the other.

-F
 

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May seem like an odd question but, are you leaning or just turning via the handlebar?
 

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Is it once per wheel revolution? Put a chalk mark on the tire and see of the wobble is in sync with the mark.
 

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Discussion Starter · #13 ·
sounds like you put the wrong type of fork on and messed up the geometry
You may want to check your effective head tube angle. Frame head tub angle plus fork offset could be causing an issue.
Which fork did you put on which frame? Older 29ers didn't have much head angle to spare on going too low up front ...
Might be worth it to take a couple spanner wrenches to the hubs and make sure everything is tight.
I put a Soma Straight Blade fork (485mm, 47mm) on a 2010 Motobecane Fantom 29PRO SL frame with a 71.5* HTA. The stock fork was a Reba SL 100 that was 490mm sagged with a 46mm offset.

I will have to get some spanners and figure out how to preload Shimano hubs. My only other set are some XTs from 97 that I've never touched.

Giddy, circle back here and let us know what you found.
I'll be sure to do that once I get it sorted. Between vacation, work travel, and taking delivery of my Ripley :cool:, this issue hasn't been getting any attention.

May seem like an odd question but, are you leaning or just turning via the handlebar?
Leaning ... well, leaning as much as you can riding slowly in tight circles.

Is it once per wheel revolution? Put a chalk mark on the tire and see of the wobble is in sync with the mark.
Feels like it oscillates back and forth a few times but I'll give that a try too when I get time.
 
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