lotek 

Ok all you wheel builders here's the question
of the day.
I finished the rear wheel, true tension, true tension, true
prestress tension and I'm pretty satisfied with the result.
My front wheel has a smallish (1/4 in?) hop opposite
the valve hole. What is really baffling is that its
small, maybe 1 spoke hole arc, it isn't gradual.
No amount of tensioning or untensioning opposite
side seems to move it.
So, Bad rim? or am I just not doing something right?
What if any method is there for correcting this type
of wheel hop? Anything I've read online doesn't seem
to address this.
The rims are Mavic GP4's, new when I bought them.
Appreciate the assist,

Marty

WoodyUpstate

Try adding a little tension right under the hop. Use two spokes from opposite sides to try to maintain true the best you can.

mechBgon

Let's say spoke C is the one closest to the hop, which I assume refers to an outward bulge, not an inward bulge:

a b c d e (so spokes a, c and e come from one hub flange, and b and d come from the other)

if you tighten C and countertighten B and D as Woody suggests, the wheel can maintain trueness. I'd like to add that if C is much looser than A and E, then lower tension on A and E and add it to C also. You can determine the relative tension levels by plucking the spokes and comparing tone.

Overall, however, if it really is 1/4" out-of-round, that's quite a lot. That's ~6mm, and usually one would want to get the roundness down to the inherent limitations of the rim, typically less than 1mm. If the rim is showning a 6mm bulge concentrated into the area of one spoke, I'd certainly be thinking "bad rim" unless this was due to a radical imbalance in spoke tension in that area. Hope it works out for you!

lotek 

Mech,

Using your example (above) the outward bulge is at C but as
I said it doesn't seem to gradually bulge (say from B upwards
and down to D). That's why I'm thinking bad rim.
I'll try the suggestions from above and let ya'll know what
the results are.
Thanks again
Marty

bikerider

At this point, consider taking it to a shop to allow an experienced wheelbuilder check it out. It sounds like it could be a poorly aligned rim seam. If it is a defective rim and you are lucky, the shop you got it from may replace it out of their pocket. Don't hold your breath expecting a warranty replacement from Mavic.

lotek 

Well, I tried retruing for the wheel hop as
mentioned above, no dice, still have the hop.
I'm taking the rim to LBS to see what their
number one wheelbuilder says.
I'll let ya'll know.

Marty

lotek 

well that was interesting.
I went to the LBS, they brought out their premier
wheelbuilder who is going for the Sheldon truely Brown
look alike contest.
Anyhow he takes one look at the wheel and states:
"wheels no good, spokes are too short" huh?
He asked the length, then went to the online calculator
ran the hubs and rims and came up with the same
302. Told me sometimes its wrong, and said I'll exchange
em if you remove em.
He then prep'd the new spokes, gave me 4 extra and
said when I finished to bring em in he'll eyeball them
and tweak em for free.
He also said that no matter how much I tried I would have
never gotten them right and have well tensioned wheels.
So, I'm gonna go relace them now.
I'll keep ya posted.

Marty

threadend

You didn't happen to sneak a peek and see where the on-line calculator is located did you?

mechBgon

Odd. It's the tension of the spokes, not their length, that influences the wheel's trueness and roundness.

If anyone ever needs a second opinion on spoke length, I can help in the majority of cases, particularly once I get my hands on a Quality Bicycle Products catalog with the ERD data for the newer rims that aren't in my 9th-edition Wheelsmith book.

AndrewP

The tension of the spokes will only determine the wheels roundness if they are all the same length.

mechBgon

I disagree. The spoke could extend a meter past the nipple, but the force exerted on the rim by the nipple would still have the same effect on trueness and roundness. The tension of a spoke is generated by the amount it is stretched along the span between the nipple and the hub.

Now, if by "tension" you mean how many threads the nipple was screwed onto the spoke, then unequal spoke lengths would produce uneven tensions, but they're not hard to discern by tone, or by using a tensiometer.

cogito

I'm a wheelbuilder. You mentioned the hop was opposite the valve hole. This is an assembly point for most rim manufacturers. The rim in question has a pinned joint, not welded and machined like currently produced rims. Pinned joints are prone to giving a hop because the aluminum was bent into a circle. The ends don't want to stay exactly where they were bent, so you get a little hop. Often, when building with older Mavic rims, a very calculated smack with a rubber mallet was what it took to take care of this.

IMO, spoke length in this case is irrelevant.

lotek 

Ok, to clarify a bit (hey it was late when I posted).
The thing the wheelbuilder at the LBS noted
was a number of the spokes had threads showing
even tho tension was just about correct for the wheel.
His thought was that the amount of tension I would need
to add to pull in the hop was enough to produce an
unsafe wheel.
I relaced the wheel last night and things seem
much better using spokes 2mm longer.

Marty

mechBgon

Cool!

lotek 

It seems there were 2 different types
of Mavic GP4 rims, older ones with 615.0 Effective Rim
diameter and newer rims with 612.0 ERD.
that would explain the inaccuracy of the spoke calculator.
I believe that the wrong rim was entered and that
messed things up from the getgo. I've also replaced
spokes on rear (although I had it trued etc.) and generally
feel a whole lot better about the entire wheelbuilding
adventure.
Threadend,
Sheldon Brown has links to spoke calculators. I have Damon
Rinard's excel spreadsheet which is really good.
Check Here caculator links
Marty