Paravia

I have a front wheel professionally custom built with Archetype rim, Hope hub and Sapim spokes. It runs true and it is absolutely perfect.
I wanted to switch to dynamo lights so I thought to build a new wheel (rather than replacing the hub on my 'perfect' wheel) with the same components (except for the hub of course which is an SP dynohub).
So I've built an *identical* wheel, which also runs 100% true (vertical, radial and dishing). This was my first wheel built.
However, I noticed that if I manually spin the wheel WITHOUT THE SKEWERS (or with the skewers loose) it tends to 'dance' in the dropouts a little (unlikely the professionally built wheel which is steady as a rock even with skewers untightened).
I thought no big deal, since cycling with the new wheel shows no apparent issues. Except, no-hands cycling starts a terrible wobble (front wheel, handlebar etc.).
Please avoid replies such as 'just ride with your hands on', since I am very fussy with my bikes and this bothers me. I also like to ride no-hands frequently to stretch my back etcetera.
I'd like to know how to stop my new wheel moving in the dropouts with skewer off, which I believe is not right and almost surely the cause of the no-hands wobble.

FBinNY 

I don't understand the problem.

Are you spinning the wheel with the bike upside down so gravity is keeping the wheel in place?

If you're seeing tiny rapid movements, It might be that the generator has magnets which give small tugs on the axle as they fly by the coils.

Otherwise, if the wheel is spinning fast, there may be some unbalance causing it to hop up and down with each revolution. Bike wheels aren't precisely balanced as anyone who's ever spun a wheel in his hands can tell you. Some (but not all) rims are unbalanced, being slightly heavier opposite the valve in the hope that the valve will help equal things out.

Of course, none of this matters, since gravity and the QR keeps axles home when you ride.

Paravia

Yes sorry I didn't specify that I am in fact spinning the wheel with the bike upside down (skewer loose). It moves in the dropouts, sort of 'dancing'. Since everything is new and basically a duplicate of the other wheel, to me it shouldn't move. I see your point about the magnets and it makes sense, thanks. However, I'd like to fix the no-hands wobble, because there is no wobble at all with the other wheel.

kingston 

Sounds like the OLD on the SP hub is a little narrower than the Hope hub. A spacer, thin washer or a fatter locknut will fix the spacing on the SP hub. You could try that, but my guess is that the weight of the hub not the OLD is causing the shimmy. There are a few things you can try listed in the link below.

An inconclusive discussion of speed-wiggle

Paravia

Thanks I'll check with a caliper! I've been reading about shimmies for two days but everyone is discussing it in 'absolute' terms i.e. bike load, rear wheel alignment, fork, rider position etcetera but in my case it shows only when switching the wheel, so all the other (relatively common) causes are excluded. Since I am new at wheel building I thought it was my fault somehow (even though it runs positively true).

FBinNY 

No hands wobble when riding (with QR tight???) is totally unrelated to what happens when the wheel is loose. Odds are that even if the wheel is true (at the rim) the mounted tire isn't, or the tread line/contact patch isn't. That's fairly common with raised center ridges which aren't perfectly centered on the crown of the tire.

The other difference may also be tire related, and about width and inflation pressure, which combine to determine the friction that works with caster to provide steering stability at speed.

FBinNY 

One other possibility is that the wheel isn't dished correctly. This is rare on front wheels, but bears mentioning because a rim slightly off center from the bike's central plane wreaks havoc with steering forces.

The quick and easy test is to mount the wheel, measure from one side of the fork to the rim. Remove, flip and remount and remeasure to see if the rim moved. If so, you need to dish the wheel to center.

For a more precise check, you can rent my patented table top dishing gauge.

You'll need 3 soup cans, cups, or similar and a bunch of quarters. Spread the cans in a triangle so the rim rests on them as on a tripod. (tire should be removed for this). Stack quarters up to where they touch the axle face that rests on your fork (not the end). Flip the wheel, and if the stack is now the wrong height, so is your wheel.

BTW- I get a $0.50 licensing fee every time anyone uses this method, let me know and I'll send you payment details.

Paravia

FB, i am glad you mentioned dishing, because as odd as it may sound, it is the professionaly built wheel ( the one that doesn't wobble) that is slightly uncentered. In fact, its disc needs a couple of shims otherwise it is not centered in the brake pads. I was pleased that my wheel was dished better than the professionaly built one, yet that does not wobble and mine does! I am also using the very same tyre and inner tube...

Andrew R Stewart 

A few comments- I'd like to have built just one (of my hundreds) of my wheels "perfectly" Really no wheel is perfect but great ones are close. Unless you ride with the wheel not secured by it's QR how it behaves spinning loosely in the fork and how it does when properly tightened are two unrelated things, FB is correct here. The way to prevent the wheel from dancing about is to spin it with the QR tight. The disk rotor's location has nothing to do with wheel dish. However both are adjustable and therefore correctable.


As to shimmy while riding- The Op is at the start of the learning curve surrounding speed wobble, or whatever name one wishes to call it. One interesting test might be to shift the dynamo wheel off dish the same amount the "pro built" wheel is and see what happens. Are the tires the same? Are the spoke tensions the same?


BTW in the OP's perfect wheel how even are the spoke tensions? I ask this because I know of at least one shimmy experience when some spokes were quite loose and after retensioning the shimmy "went away". Andy

FBinNY 

I was going to suggest the same thing. It's entirely possible the fork is off a bit, and the "incorrect" wheel is right in that fork. I wouldn't change the "correct" wheel, but would simulate the effect with a spacer pushing the hub across a bit.

I've never seen loose spokes cause shimmy, but all that means is that I've never been in the right place and time.

However, the common cause of shimmy is tracking error, so that's where the OP needs to look. As I hinted, lower front tire pressure has a dampening effect, and increases caster forces, so before going nuts the OP might experiment to see if it helps. He might also experiment to see if flipping the "incorrect" wheel induces shimmy. If not, all questions relating to dish are off the table.

I wish I could help more, but this kind of stuff is near impossible to diagnose by remote control. All we can do here is toss stuff against the wall until something sticks.

Gresp15C

Could the added mass of the dyno hub be a factor?

Paravia

Spokes tension is the second variable (besides using a different hub) since i could only check it by feel and eye ( guitar tuning apps etcetera did not work for me) but i believe it's pretty similar to the 'pro wheel' (and anyway the wobble is really severe, could just a slightly different spokes tension cause it?!). I appreciate all your comments lots of good ideas thanks!

FBinNY 

No. First of all it's not a lot, and most of it is static. Secondly, added rotational mass would only increase stability.

70sSanO

I'd use a different tire wider/heavier to see what happens. I've heard a wives tale that adding a computer magnet opposite the valve stem out close to the rim might help.

John

WizardOfBoz

As FB asked, is the wheel balanced? If you put the valve stem at different angles, does it stay where it is? If not, does it end up at the low point? If the the wheel is not touching anything, I have to think that its unbalanced to get a dynamic movement.

FBinNY 

FWIW - while I suggested wheel imbalance as a possible cause of hub movement when the QR was open. I most definitely do dot suspect imbalance a cause of wobble when riding.

Paravia

Eh unfortunately with the new dynohub the wheel does not spin as freely as a normal wheel, because of the magnets, so i can't check balance properly. I am getting lots of good thoughts in this thread, still convinced if i figure out why the hub moves when the QR is open i will sort the wobble too.

WizardOfBoz

Agree. The OP was asking about the wheel jumping around when loose in the dropout with the bike upside down. I can't think of much else if the wheel runs perfectly true.

FBinNY 

Please trust me on this, the problems are not related (if the wheel is true).

Wobble when riding is a harmonic issue whereby small steering inputs get amplified to a wobble through resonance.

One thing you can try is to take a crayon and hold it against the center of the fork just barely touching the center of the tire and spin the wheel to mark the running track on the tire. Odds are the tire doesn't run true, so you'll get some skips, but maybe you can see it move a bit side to side at the same time.

We'll wait for a report, but as I said before, the easiest test is to drop tire pressure considerably, say down to 50-60psi and see if it makes a difference.

AnkleWork

Maybe the field coils are storing and releasing energy causing a periodic reaction force at the axle. What is the approximate frequency of the "dance" in the dropouts relative to the wheel revolution? One to one, or more frequent? Try shorting the generator output when spinning the wheel to see if it lessens.

Andrew R Stewart 

OK here's a rough experiment. Take the dynamo wheel in hand. Grab the axle ends with fingers and holding the wheel out away from you spin it, keeping the axle stationary. Feel the axle's stutter from the hub's internals forcing past each other. Next hold the same wheel by only the QR ends. Again spin it. Now the axle won't be turning but the rest still will be. Feel the difference. Smooth like the "pro built" wheel?


Are we wrong to suggest that the hub's internal forces (from the dynamo function) are not the cause of on the road shimmy? Maybe but I doubt it. The inertia of the rim/tire rotation is likely a far higher factor.


A new thought is this- Is there a light on this bike yet? Was it there with the "pro built" wheel? If not then this added mass likely hanging off the steering axis a bit is a change that can effect shimmy.


I do wonder what else not noted was different before and after shimmy was first felt. All the understandings I have reviewed over the years (starting way back in 1979) about steering and shimmy come back to a harmonic flex/resonance which starts with a seemingly tiny seed and grows to what we call shimmy. Body positioning, body stiffness, rotational inertia, steering geometry, frame and fork flex, weight placement, tracking alignment, wind pressure, handle bar leverage, wheel stiffness and more factors are at play. Change any one (or three), whether you know you have, and the game is different. The biggest variable is the rider/body. I have had more then a few bikes that shimmied for me yet when others ride then on the same roads, the same day, their results are stable. I suggest that one can't develop a real double blind study which will result is a true understanding as to how to completely avoid any chance of shimmy, or completely stop it happening on a bike where it has happened before.


So we toss a lot against the wall and see what sticks. (As said recently already). Andy

Paravia

Thanks guys I will do some further tests and possibly a short video and post back.

Paravia

Uhm okay I think I figured out why my new dyno-wheel is dancing with the QR open (and it doesn't with the Hope Pro hub). As per photos, the SP dynamo hub is not only thinner but also (for unknown reasons...?) it's got a big cut-out on one side. So that when spinning and the skewer is loose, it bounces on the flat part (which I didn't notice before). I have now wrapped both dynohub ends with some tin so they're now round and snug, I will try no-hands tomorrow. Although at this point I believe what most of you said i.e. the "open QR dancing" has nothing to do with the no-hands wobble, so the only thing left to check would be spokes tension (sadly the ParkTools tension gauge is over $80 here and I cannot afford it).

wschruba

Someone who has taken one apart may be able to comment, but that's probably where the wires head into the hubshell from the connector. Some (most?) manufacturers run them in in a groove, but a flat would work just as well, and probably needs much less careful machining.

FBinNY 

I don't know about those particular hubs, but doubt the flats are related to anything electrical.

Generators create bit of reaction torque on the stators, so the flat is probably an element of a keyway system to ensure that nothing works loose over time.