Spokes coming loose on Magic Pie

[Who42]

Spokes coming loose on Magic Pie???

Spokes don't come loose on this wheel see below pic
Maybe we need to straighten up the high angle of the spokes on the Magic Pie

[Who42]

(Quote) Bikemad

Jerry,

I fully agree with the spoke angle problem, which cannnot help to maintain an even tension on the spokes.

Due to the large angle between the spoke and the rim (See attachments below for more details), the nipples cannot seat correctly and as a result of this, the tensile force of each spoke is concentrated on one very small point of contact, instead of being spread evenly around the head of the nipple.

As the nipple tries to pull itself down to seat onto the rim, this single contact point will act as a pivot for the nipple, causing it to bend the spoke whilst trying to achieve the impossible (a fully supported and correctly seated position in the rim).

Unfortunately, the nipple will never be correctly seated in the rim, so it has no option but to spend the rest of its life under unnecessary strain waiting for something to eventually fail.

I suspect the loosening of the spokes is probably due to this:

A common reason for spokes to fail on hub motors isn't because the motor puts extra strain on the spokes, or because the spokes aren't a thick enough gauge, it's because of fatigue failure from spokes that aren't held snug against the flange. If the spoke bend radius is too large or too far from the head, then it can flex up and down at the bend with each wheel rotation, eventually causing it to crack and fail.

This problem has been legendary with overseas built hub motors, and we had some Crystalyte shipments where about half the customers would experience spoke breakage on a recurring basis. Ideally the distance between the head and the bend in your spoke will match the thickness of the hub flange, and you won't have problems. But if not, there are two ways to address the situation. One is to insert a washer under the spoke head. The second way is to lace the wheel in an over/under pattern, such that the spoke tension compresses the bend part of the spoke into the flange.

Please note:
Ideal hub hole Ø equals spoke thread Ø + 0.1 mm (eg. spoke thread on 14G/2 mm measures thread 2.25 mm + 0.1 = 2.35 mm: max. hole Ø 2.50 mm).
- flanges which are too thin and / or hub holes which are too large in which case the spoke angle of 95° could increase to 120°

Correctly mounted SAPIM spokes never elongate. However, an incorrect hub can cause the bending angle to enlarge (elongation of the bend up to +1.5 mm per spoke) and buckle wheels.

- Oversized flanges cause extra pressure on the spoke heads. In consequence, the heads pop off.
- Consider spoke washers

Does anyone else have any thoughts or useful suggestions regarding this subject?

Alan
 

[GM Brazil]

Maybe use some type of sphere shaped contact face nipples to improve the contact area?

[Bikemad]

If you take a look at the following diagrams, you can see my proposed solution to the problem:
(The current spoke pattern on the Magic Pie is shown in dotted lines.)

View enlarged image

The following close up shows the proposed modifications in more detail:
(Spoke thickness and hole sizes are not to scale.)

View enlarged image

By spacing the pairs of holes 20mm apart, it would produce an angle on the spoke of 9° from perpendicular, which is about the maximum angle that spoke nipples are able to correctly accommodate.
A small conventional hub is more than capable of transmitting enough torque to pull wheelies, even with thin spokes angled at something like 3 or 4°, so an angle of 9° should easily be able to handle more than twice as much torque without any problem whatsoever.

Fitting the spokes at a more acceptable angle should completely cure the curved spoke and misaligned nipples, which are very obvious, and it should also help the loose spoke problem.
But the poor spoke fitting at the flange end also needs to addressed at the same time, as the current spoke holes are too large, the flange is too thin and the countersinking is too deep for the actual spokes currently being used.

If I had the choice, I would prefer to use standard gauge spokes instead of the thick ones currently used, as a thinner gauge spoke will have more "give" and should help to absorb more of the shocks being transmitted from uneven road surfaces etc.
If these wheels were to be used for BMX or stunt riding etc, It might make more sense, but the majority of these conversions will be for general road use.  It's like putting heavy duty springs and dampers onto a car, it may well handle better when pushed to extremes, but unfortunately it gives a very uncomfortable ride when just driven normally.

Bigger is not always better. 

Alan
 

[GoldenMotor]

Thanks for the feedback. I will send this information to the factory ASAP.

[e-lmer]

I think the large angle between the spokes and the
perpendicular line is needed because of the short
spoke length.


The closer to perpendicular the spokes are, the more
force is applied to the hole in the rim when you apply
torque.

The torque parallel to the tangent to the wheel
is the applied force times the sine of the radial
angle.  The smaller the angle, the smaller the sine.
*till it's 0 at 90 degrees.

I have been wondering about putting the hooked
end of the spokes closer to the hub (making them
longer (and yes, heavier, but so?)

It means

[electrorocket]

I just finished my pie build on a Mavic A719 29"rim. http://www.mavic.com/mtb/products/tn-719-disc.995921.2.aspx I am using the stock gauge spokes, in a radial lacing pattern.
The radial lacing was the best solution I could come up as well, given the resources I had. I have dished the wheel 2mm to allow for the 8s cog set. This is obviously not ideal... So far it has proven to be reliable. I will advise if I run into a problem.

I might add that I am a small rider, and never ride off road. I do not recommend doing this yourself, unless you are confident it will work!

I am now building mini motors onto Mavic 317's. I have yet to receive the hubs, so I am undecided as to how I will lace them, and with which spokes.

[ezbikin]

Interesting to read about wheel/spoke problems as I just ordered a Pie. Have been using Bionx for years now, and had some problems with truing/broken spokes in the beginning. My spoke problem was the same as you are experiencing with the Pie, and it was due to the sharp angle at which the nipple and spoke join. I solved the problem by rebuilding using Sapim Polyax nipples and Sapim 2.3/2.0 single butted spokes. I purchased the nipples originally from Biketoolsetc.com, and the spokes from Wheelbuilder.com (I think they also have the Polyax nipples). The Polyax nipples have a rounded shoulder where they pass through the rim, rather than the traditional almost flat shoulder, allowing them to take a more radical angle out of the rim. The only concern I have with this approach is that I only know of their availability in 14g, but that works with the single butted spokes, as the thicker end of the spoke (the elbow) is 2.3 and the nipple end is 2.0 (14g). I've not broken a spoke is two years on three different units since taking this approach, and one of the wheels is mounted on a RANS Screamer recumbent tandem, with a weight of bike, cargo, and riders sometimes approaching 500 pounds! Also, a mistake newer wheelbuilders make is in overtightening the spokes. This is what often causes the rim to crack around the nipples. There should be some give in the wheel. Most rim manufacturers as well as most shops can give you the optimum tension for their rims. Good luck.
Jim

[GM Brazil]

The Polyax nipples have a rounded shoulder where they pass through the rim, rather than the traditional almost flat shoulder, allowing them to take a more radical angle out of the rim.

Nice! Yao, see if you can find these rounded shoulder nipples for the GM motors, I think I already said that here somewhere!

[Bikemad]

I don't know how well they would fit into the deep V rim that GM are currently using, but the seem to be available as 13 and 14 gauge in three different lengths.

Polyax Nipples The rounded design of the Patented Polyax nipple head reduces the stresses produced by axial and lateral forces (inherent in wheels with low spoke counts and crossings). Polyax nipples re-align themselves within a rim hole up to 9 degrees in a radial direction and 6 deg in an axial direction, making for a much superior fit.

That's still nowhere near the 23° on the current 26" Magic Pies. But 9° would be perfect for my proposed modification.


View enlarged image

Alan
 

[jvr]

How can I calculate the spoke length needed with this new schema?

Rim has an ERD of 539.

Joe