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Discussion Starter · #1 ·


So my wheel completely taco'ed itself in a berm last week, and over the bars I went. Ouch. The crummy part was that I don't recall hitting anything, it just seemed to let go as I loaded it up in the turn. I have a few questions before getting a new wheelset built.

Damaged Wheel (built by Mike at oddsandendos) - wheels about 3yr old, I bought them used but true from an XC racer my size.
- Velocity Synergy Asymmetric 32hole (IMO the rim was the failure point, but who knows) - rims were slip fit, not welded.
- Alloy Nipples
- Wheelsmith 15ga Titanium Spokes
- WTB Laserdisc Lite Hubs

Rider (me) - 205lb with pack, moderate XC - no jumps over 24"

1) Would the ZTR Arch rims be a stronger rim and okay for my use, and can I use them with tubes? (not quite ready for tubeless yet)

2) Is there any chance the alloy nips could have been at fault? My understanding is they just can be a pain to true, but not a failure point.

3) Could the ti spokes or rims create a handling issue? I've had a horrible time this year feeling stable in corners vs my old setup. I'm guessing this has a lot to do with going from a heavy coil spring to a much lighter REBA fork and the front being lighter, but maybe these light wheels were flexing??

Other advice?
 

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i'd like to hear what the wheel guru's think about this...

i'd guess that the 'slip fit' area failed...railing a berm will put a lot of tension/compression in the spokes...if some of the spokes give way more than others, then you might have what you call (in seismic events) a periodic/rhythmic loading....add in a little gyroscopic action from the spinning of the wheels, and poof, catastrophic failure.

doesn't look like the alloy nips had anything to do with it...but i would stick to brass next time...i like sun singletracks...they take all sorts of abuse and keep rollin....

ZTR rims don't have eyelets, do they? i prefer having them...peace of mind...

maybe i'm rationalizing what happened though...

it's fun to see busted parts :thumbsup:
 

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here's some info that may be interesting...

even though your Ti spokes are alloyed, looking at the elastic values for unalloyed titanium vs steel, it appears that ti has a lower 'E'(modulous of elasticity), meaning that titanium will irreversibly deform more quickly than steel, if subjected to equal stresses.

this may have made the ti spokes stretch to the point where it helped create the crack in the rim, or perhaps vice versa....

if anyone knows i'm totally off, please let me know....i just took a test on structures and if i'm wrong, i've got bigger problems...:mad:

if what i say is true, better stay away from ti spokes!
 

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Discussion Starter · #4 ·
I read highly mixed reviews for Ti spokes, ranging from them being the best and problem free to being blamed for every problem in the world. Problem is, I can never hear good reasn why ti spokes are bad.

I did send this all to a good friend who is a metallurgical engineer for Rolls Royce jet engines here in town. He felt the rim being slip joint was the failure point. He was surprised the ti spokes were 99% pure ti and rolled not cut threads. He did find an odd stress riser in every single thread at 500x magnification.
 

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BQuicksilver said:
I read highly mixed reviews for Ti spokes, ranging from them being the best and problem free to being blamed for every problem in the world. Problem is, I can never hear good reasn why ti spokes are bad.

I did send this all to a good friend who is a metallurgical engineer for Rolls Royce jet engines here in town. He felt the rim being slip joint was the failure point. He was surprised the ti spokes were 99% pure ti and rolled not cut threads. He did find an odd stress riser in every single thread at 500x magnification.
Do you mean to say that the spoke threads had stress risers? There's one big big reason to not use ti spokes. I'm also surprised to hear that the spokes are not alloyed..

I'd like to hearmore about what he says about using ti in this application.
 

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Discussion Starter · #6 ·
The stress risers were VERY!!! small. More of an interesting observation than anything. I'm pretty certain the rim was the failure point. Could a spoke tension issue have preceeded the rim failure? Possibly, but all I can say is that the rim failed and we couldn't find a failure in the ti spokes or alloy nips.

This is one single thread from the spoke (you know how fine these threads are) at 100x. The defect is VERY small as you can see.

 

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cool pic.

even if they didn't show any wear or damage, since ti can stretch more easily than steel, it may have contributed to premature failing of the rim.

with that said, keeping a close eye on the spoke tension would be a good idea. remember, the spokes not only undergo lateral loads from turning, but the torque from the disc brake will wreak more havoc onto the spokes. if done enough times, ti will eventually become deformed and tension would be lost.
 

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BQuicksilver said:
I read highly mixed reviews for Ti spokes, ranging from them being the best and problem free to being blamed for every problem in the world. Problem is, I can never hear good reasn why ti spokes are bad.

I did send this all to a good friend who is a metallurgical engineer for Rolls Royce jet engines here in town. He felt the rim being slip joint was the failure point. He was surprised the ti spokes were 99% pure ti and rolled not cut threads. He did find an odd stress riser in every single thread at 500x magnification.
The rims are not "slip fit." They are sleeved. Properly made the seam is as strong as a welded rim. The force required to break the sleeve would also break a welded rim. Your rim is also cracked away from the seam. The rim did fail at the seam but I doubt that is where the collapse started.

Rolled threads are considered to be stronger than cut threads for the same reason forging is stronger than machined. You can see how the metal grain structure follows the thread shape in your pic.

AFAIK Wheelsmith never made ti spokes. DT and Marwi have and they were straight 14g. A 15g ti spoke would be very elastic and make for a less stable wheel. As it is ti spokes need to be tensioned higher than most steel spokes. This can lead to rim fatigue and failure.

You mention this was a used wheel when you got it. Even though it was true, the spoke tension may not have been balanced if it had been damaged. Uneven spoke tension can let a wheel fold under high side loads.

The aluminum nipples were not a factor. I rarely build with anything else.
 

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Discussion Starter · #9 ·
Sorry, I used the terms slip-fit and sleeved interchangably...maybe you can spell out the difference. Either way I know I'm not using them again. Yes, I'm aware the rolled threads are superior. The rims from oddsandendos had a "built with wheelsmith spokes" decal on them and the original owner said they had not been rebuilt.

I can also buy into the wheel being too elastic with these spokes, as I've had a terrible time feeling 'right' in corners since I started running them. Maybe I just forgot how to ride, but I'd rather blame the new wheels vs my old 2300g luggers. :)

Here's what I'm looking at for a new build:

- Stans ZTR Arch - 420g
- Reused WTB Laserdisc Lite hubs - 144g,260g
- Alloy Nipples - 22g
- DT Comp Double Butted Spokes 14/15ga - 382g

About 1650g and a lot more stable IMO (assuming they're built well)
 

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BQuicksilver said:
Sorry, I used the terms slip-fit and sleeved interchangably...maybe you can spell out the difference. Either way I know I'm not using them again. Yes, I'm aware the rolled threads are superior. The rims from oddsandendos had a "built with wheelsmith spokes" decal on them and the original owner said they had not been rebuilt.

I can also buy into the wheel being too elastic with these spokes, as I've had a terrible time feeling 'right' in corners since I started running them. Maybe I just forgot how to ride, but I'd rather blame the new wheels vs my old 2300g luggers. :)

Here's what I'm looking at for a new build:

- Stans ZTR Arch - 420g
- Reused WTB Laserdisc Lite hubs - 144g,260g
- Alloy Nipples - 22g
- DT Comp Double Butted Spokes 14/15ga - 382g

About 1650g and a lot more stable IMO (assuming they're built well)
Sounds much more sensible than going ti again. Also I've heard that its wise to use brass nips instead of alloy with ti spokes, which would negate the weight savings?
 

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BQuicksilver said:
After a bit more reading it sounds like I'll need to use brass nips since the ZTR Arches don't have eyelets.
i did some quick calculations on elongation of a spoke with the following assumptions:

14 gage spoke, 10.5" long
(has a cross sectional area of 0.064084")

Titanium modulus of elasticity @ 70degrees F: 15,000,000 psi
Steel modulus of elasticity @ 70 degrees F: 29,500,000 psi

applied force is 5000 lb*ft.

using the formula:

total deformation = (stress*length) / (area*modulus of elasticity)

A titanium spoke subjected to 5000 lb*ft at 70degF will elongate 0.0546"
A steel spoke subjected to same stress @ 70degF will elongate .00277"

Conclusion, under equal conditions, the titanium will elongate twice as much as steel.

i have no idea on the applied force, but i'm guessing that the force would be quite high, in a worst case scenario, such as landing on flats or railing a berm while braking. if anyone has an idea about what the actual applied force would be in this situation, it would make thecalculations more relevant..

just geeking out on a slow sunday morning....
 

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Just to be clear about modulus of elasticity, we're talking about non-permanent, or ELASTIC deformation here. To permanently (plasticly) deform, the stress has to exceed the yield strength of the material, which may or may not have happened in this case.

Ti can work as a spoke material, but per volume it is 1/2 as stiff as steel. So a 14G Ti spoke will be roughly the same stiffness as a 14/17/14 DT Revolution steel spoke, in resisting tensile (stretching) force. A 15G Ti spoke will be scary flexy....not the thing to be railing berms with or jumping. You finally flexed the wheel enough fold the rim.....

A couple more tips on lateral stiffness and strength...a stiff structure made with strong materials and no weak points.....will be strong structure:

- More spokes are stiffer/stronger
- Thicker spokes are stiffer/stronger
- Less crosses are stiffer/stronger laterally (including radial, the stiffest).
- Wider hub flanges build a stiffer/stronger wheel
- Taller hub flanges build a stiffer/stronger wheel

Almost all high quality Ti hardware for bikes has cold rolled threads, not cut. Makes for stronger threads due to cold working of the material, preventing them from stripping out. I doubt the stress risers are an issue - how often do you see a spoke snap in half at the threads while inside the nipple?
 

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understood...i guess my point is that since ti will 'stretch' more easily than steel, spoke tension is extremely important. chances are that the spokes have not 'plasticly' deformed...i guess i used the wrong terminology....

props on the clarification however!
 

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misctwo said:
understood...i guess my point is that since ti will 'stretch' more easily than steel, spoke tension is extremely important. chances are that the spokes have not 'plasticly' deformed...i guess i used the wrong terminology....

props on the clarification however!
Even with high tension, the wheel will flex more than one with thicker/stiffer spokes......that's going to push most rims beyond their limits.

However, it will probably flex much further before allowing slack in the spokes.
 
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