OldBike876

Hello,

I am chasing a shudder/wobble in the front of my Norco Indie aluminum commuter. I loosened the stem and tightened the bolt in the cap. Two things:

1. I turned the bolt maybe 270° and the bolt bottomed out, and
2. That had no effect on the steering or the shudder.

I have read conflicting accounts of lowering a starnut. Is it true that a compression plug is preferable? If it's a starnut, should I have a specific tool to accomplish this? Should I have a new one on hand, or is the old one usually reusable? Am I right that this solves the bottomed out bolt?

Any thoughts?

Many thanks.

FBinNY 

Start by properly diagnosing your problem. A steering wobble can be caused by a loose headset, but it's much more commonly caused by other stuff.

Your question implies that you cannot tighten the headset. You should confirm that before going farther.

The definitive test for a loose headset is to lock the front brake and rock the bike forward and back, while looking and/feeling for play at the lower cup.

Assuming the headset is loose, and you're correct that the bolt bottomed out, its time to determin why. Leave everything tight, remove the.bolt and top cap and verify that the steerer is 4mm or so short of the top of the stem. If not, the top cap may be what's bottoming. You can confirm by threading the bolt as far as it goes, and seeing if the head drops well below the top of the stem.

So if it's the top cap, you need a spacer below the stem. If it's the bolt, reassemble everything with the bolt raised about an 1/8" or so. Then tap it down so it pushes the star nut deeper.

Finish by trying anew to adjust the headset.

Note the importance of working sequentially to identify the issue, so you don't try to fix the wrong thing. Also note that tightening the headset may not solve the wobble.

OldBike876

Thanks for your reply.

Kontact 

You can lower a starnut. It is simply tapped in further. If you don't have a tool use a donor bolt with a nut that you can tighten against the starnut, then tap slowly to avoid twisting the starnut off center.

This is better than removing the starnut. Starnuts are superior to expanders, but expanders are necessary for carbon steerers.

OldBike876

Thanks for the replies so far.

I will say, the bike has done this for ages - sometimes are worse than others. I have gone through several rotor/pad combos, and very rarely - when new - it shakes less. My winter project was to build a bike wheel for the first time and fit a larger rotor, so the tire, rim, spokes, hub, rotor, and pads are new.

I do most maintenance myself, but had a local shop look at the bike for a few issues, and they changed the front pads of their own accord, and the shaking remained. I asked them, and they said they didn't feel it.

I concluded it wad the headset from everything else having been changed. There's no knocking on the "push with a locked brake" test, so I had always focused on the problem being at the wheel.

maddog34

something i noticed.. the steering angle of the frame is 70'... this is fairly steep..road spec.... the Offset specified for stock fork is 50mm.. that's kinda high... the Offset reduces TRAIL in steering geometry... more offset equals less TRAIL.. Less TRAIL equals less STABILITY, especially during braking...... a lack of stability can create a situation that manifests itself as HEADSHAKE..

try a fork with less Offset (40mm), and a bit more distance from the dropouts to the crown race seat (10-20mm), and see what happens.... call it "tuning of the bike's handling"

jp911

You can test your loose headset theory as described and it if does seem to be moving you can add a spacer above the stem and try to add tension to the headset (you will need to loosen the stem bolts when you do this). If that resolves things you can then drive the star nut deeper in the steerer tube. Using an old bolt works OK, but if you can bar a star nut setting tool it will be much easier.

Kontact 

Low trail is not less stable. Low trail is more stable at low speeds. High trail is less stable at low speeds.

FBinNY 

Here we go again.

Let's not chase this rabbit down the hole until/unless the OP confirms that he's resolved the headset issue, and STILL has a head shake problem.

OldBike876

I had a look this weekend. I tried the headset test with the brake locked, with the wheel against the wall, with the wheel turned 90°, and felt around with the bike on the stand. I can't feel anything knocking during those tests.

So, I cleaned the rotor (it's only a week old), put in new pads, tightened the brake hardware, checked the hub, and then bedded in the brakes. Like I said, this wheel is fairly new and shake predates it, but I couldn't figure what else to do.

The shake is still there, but subdued a bit. it happens after I've been on the brake more. This problem has ebbed and flowed over time, so while i am not surprised that fidgeting with the brake changed things, i don't really know whay to make of it.

Let me say: this bike appears to have Shimano's cheapest hydraulic calipercaliper, and it has to be 5 or 6 years old now (I bought the bike used 3.5 years ago). I don't know how that would fit in, but it IS a part of the system that would make one wonder. I am also a very, very heavy rider, if that is relevant.

FBinNY 

We need to separate wheat from chafe here. Head shake (aka high speed shimmy or wobble) is a complex issue and threads about it are like those on wheel building, or chain lube (as examples) with plenty of lively debate.

HOWEVER there's general agreement that brakes are NOT involved, unless it happens when the brakes are used, in which it's a different, brake related problem. So, please clarify:

Describe the nature of the head shake, ie. high speed only, while braking, or whatever other patterns you've noticed so people here can have enough info to help intelligently, rather than throw spaghetti against the wall to see if it sticks.

OldBike876

Ah, ok: it shakes when decelerating down a hill using the front brake. It rarely does it on a flat stretch. The bike has done it through various rotors, pads, tires, hubs, but does ebb and flow. I had a shop change the pads once and they claimed they didn't notice the shake (which was there when I rode it home).

FBinNY 

OK it seems that you MIGHT have a classic head shake issue, apparently triggered by front braking.

First of all, try to isolate things by not braking on.some high speed descents, and/or using the rear brake alone to moderate speed.

If that works to avoid the problem, you're done. Otherwise, try managing by shifting weight back and stiffening your arms before using the front brake. (this is.always good and should be SOP)

Other things that help stabilize a shimmy including bracing a knee against the top tube on descents. This changes the harmonics and can prevent shimmy, or delay it until higher speeds.

Also, make a note whether the front brake is smooth or if there's a pulsing action.

Keep in mind that it's often best to manage this kind of issue because actually fixing it can be either expensive or sometimes impossible.

OldBike876

Interesting, thanks. I will try putting a knee on the top tube. I can (and have) not use the front brake on slow descents, but kind of need it at speed.

FBinNY 

One of the things that separate experience cyclists from those less so, is managing descents effectively.

The first choice is to use the air brakes. Aerodynamic drag, normally our enemy, is our best friend here. Every bike on every descent has a terminal speed based on grade and air drag. You can significantly slow (or speed up) descents by changing your aero profile.

Rather than relying on the brake, descend with the pedals at 6 and 12 and stiffen or brace the extended leg. Without going nuts, raise your head and torso to maximize your drag. Also, note that very often your terminal speed is only a bit higher than what you're shooting for.

Lastly, rather than dragging the brakes, allow the bike to accelerate to something above your preference, then use brakes to scrub some off to below target. Repeat this sequence as necessary. At the same time, learn to be comfortable with higher speeds and not slowing until conditions, ie. an approaching intersection or turn, sharp curve, pavement change, require slowing.

Also,and especially important with your problem is to learn that the front brake is best for stopping, the rear is preferred for slowing.

Trakhak

Piggybacking on FBinNY's excellent analysis: does the shake feel as though the oscillation is side to side, or is it front to back? (Sorry if you already reported the answer and I missed it.)

Side to side would likely be a frame issue, i.e., a high-speed shimmy unrelated to the mechanical parts. Front to back would likely be a headset adjustment issue (or, in rare cases, deformation, a.k.a. "ovalization," of the bottom of the head tube).

Kontact 

A shudder that only happens when braking is unlikely to relate to anything but the way the brake and fork interact.

FBinNY 

Is this a suspension fork?

If so, and given that the front brake is implicated, then consider play in the struts as the cause.

This is not repairable, so go back to work on techniques to avoid front brake on high speed descents.

OldBike876

Some very interesting points, thanks. The shake feels up/down, not side/side.

It's not a suspension fork.

mitchmellow62

CAUTION: I never escaped the 9 speed era. If the following is nonsense, please ignore.

Could this symptom be brake shudder? It's relatively common in other disc brake systems. It is often a warped rotor but can also be caused by uneven pad wear. I have no idea if this is common in bike disc brake systems. I'm getting the impression that the OP tends to "ride the brakes" a lot. Perhaps quite hilly terrain? I don't understand how new pads and/or rotors would improve shimmy/wobble/head shake even briefly. OP: do you use your front brake a lot? Were the pads excessively or unevenly worn on replacement? Shimmy/wobble/head shake is a oscillatory movement around the steering axis inherent in all caster wheels. Shudder should be almost purely linear fore and aft. I also think it's probable a loose headset would have been found by the LBS or OP. But I've been wrong more times than I like to admit ;-)

OldBike876

I don't know that I ride the brakes more than usual; I squeeze them to stop. FB's comment about using the rear to slow and the font to stop is news to me, but I doubt I use the brakes more than needed. The only component that remains unchanged on the fork is the caliper (and it's line, I guess). The wheel, tire, skewer, and brake parts have all changed - sometimes multiple times - since this started. If I use expensive or cheap pads, I tend to go through them quickly, but evenly.

mitchmellow62

From the inexperienced 9 speed guy:

A web search for bicycle disc brake shudder produces a number of hits including:
https://bicycles.stackexchange.com/q...-cause-and-fix
https://www.mtbr.com/threads/shudder...brake.1087134/
https://www.cyclingtips.com/2018/03/...brating-noise/
https://forum.bikeradar.com/discussi...ng-disk-brakes

Apparently shudder isn't that uncommon. Perhaps the more experienced could comment. I don't mean to impugn OldBike876 in any way but I am curious as to your comment "I tend to go through them quickly". Does that mean the brake pads wear out quickly? I'm simply trying to establish how much the bike is ridden and under what conditions.
Thanks for tolerating me. I'll do my best to quit after this.

grumpus

To lower the star nut, unscrew the stem cap bolt a few turns and tap it back down with a hammer.
Expanders are for carbon forks, they offer no advantage for metal forks unless you want to run a cable through the steerer.
You don't actually need a star nut/stem plug - in a pinch just loosen the stem clamp bolts slightly and tap on the top of the stem with your copper hammer (or a bit of wood you found on the trail) to add a little pre-load to the bearings, and re-tighten.

OldBike876

I ride the bike every day at least 4km, usually more than that though (probably 75km a week as an average). I commute on it, use it for long city rides, and one bike trip per year. I will use st least 3 sets of front pads per year, and at least one set of rears.

grumpus

Beware the mechanic who doesn't test ride his work. There was a steep hill just a block away from one of the shops I worked at which was perfect for checking gears on the way up and brakes on the way down - testing isn't as easy on the flat but it can still be done a lot better than just spinning the wheels in a work stand, which is I suspect all that a lot of shops do.