Al1943

I think I would experiment along these lines instead of arbitrarily increasing the pressure.
Interesting problem.

AnkleWork

Is it possible the rim is failing?

Alias530

I'll check it closely in the morning but it's an almost brand new rim, 250 miles on it.

Jiggle

Their 29er rim at 32mm inner width, that is 97 square inches of rim bed. Inflate the tire to 40psi and you now have 3880lbs of force compressing the rim.

Divide by 32 spokes gives 121.5lbf/spoke

Your average DS tension loss is about 340lbf-185lbf = 155lbf/spoke

The 30lbf discrepancy could be the tire bead tension. It all adds together quite well.

Solution: Use a narrower rim. I know those LB rims are stiff and light, but I don't think these wide designs are the best for tubeless mtn tires.

FBinNY 

Your analysis is flawed and overly simplistic. It fails to take into account the rim's own resistance to compression.

Spoke tension loss is a function of dimensional change, namely getting shorter. That's dictated by Hooke's Law. So let us for a moment imagine a rim with a cross section of 3 square inches of high strength steel. All the tire pressure in the world wouldn't shrink the rim, and if the rim doesn't shrink, the spokes don't change length, and therefore don't lose tension.

OTOH- let us imagine a rim with little resistance to compression. First of all, if that were true we'd see material size change when we laced it up, but let's let that slide for the moment. If the rim were a soft spring, the added compression from the air pressure (or tire bead) would further shrink the rim and we'd see significant spoke tension change.

In reality the rim is someplace between the two examples above. It does shrink slightly (very slightly) from both spoke tension and tire pressure. But how that small change in length affects the spoke tension is a function of the spokes cross section and length in accordance to Young's Modulus. A thinner spoke changes length more than a thicker one in response to the same tension. Conversely it changes tension less in response to a change in length.

So, spokes that are relatively stiff compared to the rim, will undergo greater swings in tension with rim deflections or shrinkage, than thin spokes.

So, Yes, tire pressure will drop spoke tension, but how much depends on 1- the surface area of the rim (as you pointed out), 2- the rim's weight (since length is the same, all weight implies greater cross section), and 3- the original spoke tension and cross section area, or % elongation if you prefer.

Jiggle

Yet the spokes are losing tension, which means they are undergoing a dimensional change, which means the rim is compressing, and they're doing it in a way that is in line with my simplistic calculations. So it must not be flawed or there's something else going on. What else is going on?

I agree a thinner spoke is a good solution.

FBinNY 

Yes, the rim is shrinking ---- slightly, which is why spokes are losing tension, especially the slacker left side ones which were barely stretched in the first place. It's just that you made a quick flawed calculation, that coincidentally gave you an answer close to what you wanted. In essence you went once around the monopoly board without bothering to pass Go.

Read Post 2 and you'll see that there is agreement on the effect, --- inflate tires ---> spokes lose some tension. The disagreement is in the amount and what factors are in play.

Alias530

Is it possible that I'm losing tension from hitting something HARD?

This is a cyclocross bike that I use for road biking or light trail duty but I've hit a few pot holes or gaps in the trail that I hit so hard that my hands instantly went numb. I'm wondering if maybe this whole time the tension isn't gradually going low, but going low all at once because I hit something hard. My other bikes (road and MTB) either never see impacts (road) or I have a big cushy tire and over 5" suspension to soak up the bump. This is a rigid frame with a 35mm tire. Although the bike isn't seeing drops bigger than a curb or jumps at all, some of the stuff it has hit might pass more force to the actual wheel than a 3ft high jump on a mountain bike with huge tires and suspension.

FBinNY 

Anything is possible, and without being there, it's impossible to do a thorough analysis. But rough use as a cause isn't consistent with your original post, where in the 2nd paragraph you clearly describe a radical pressure drop simply from mounting and inflating the tire. (ie. without riding).

IMO- it's equally likely that you're somehow misreading the tension gauge, but that too is only a possibility.

My suggestion is to build the wheel, mount the tire, then measure tension before and after inflation, so air pressure is the ONLY variable. You might bring the wheel to a knowledgeable builder and demonstrate the same issue in front of him, and solicit his opinion.

Alias530

Sorry if I wasn't clear, thought you were in the original thread.

I had someone else build the wheels for me and I was losing tension every couple rides. He told me it was normal in the first few rides (which I later found out was false) but when it kept happening he said I should raise the tension. So I raised the tension and couldn't get rid of a vertical hop I created so I rebuilt from scratch.

Now my tension is going way down when the tire is mounted. My latest post was referring to whether I was losing tension to begin with because of hitting something hard rather than a poor wheel build.

Another variable--after letting the tire sit mounted/inflated overnight, tension comes back up a bit (bead stretching?)

FBinNY 

As I said, we have possibilities, but not enough specific information to form a theory.

My original theory, based on the limited early info was that you have too much spoke for the rim, and rough use, or maybe even simply filling the tires is allowing slacker spokes to go full slack, which allows nipples to unscrew. Once that starts it's all down hill from there. As a rule it starts on the left, but as the wheel gets slack it spreads across the entire wheel like kudzu.

So, yes, rough use may be a contributing factor, but the wheel should have some tolerance for rough use. My commuter sees very rough use, and light wheels stay tight and true, and they weren't crazy tight to begin with. My road mikes run on 300 gram rims and tubular tires, and I treat it like an SUV, never being afraid to ride wherever I feel like, including singletrack "goat" trails.

There's no reason that you should be having these problems, but you are. Unfortunately, I can't diagnose the issue over the net.

BTW- I'm still having a problem with the amount of tension drop you;re getting from tire pressure. That alone tells me that something is fundamentally wrong, but not what that something is.

FWIW and as a basis for comparison. My current road bike (actually Debby's) has 15mm inside width rims, weighing roughly 390 grams, built 1.8 DS and 1.55 NDS (thin part of butted spokes) at tensions of about 75 and 40 respectively. I find zero measurable change in tension when inflating the tire to 100psi. So far the wheels are only a few thousand miles old, and I might touch them up and add a bit of tension (to 90/50 if I keep riding it since I weigh about 60% more than Debby does.

hueyhoolihan

probably just shows how pointless all the measuring of spoke tension and calibration of measuring tools is.

Alias530

I only have 1 ride since the rebuild but I'm not losing tension this time around (so far)... however, the NDS spokes just FEEL loose compared to the other wheels I have.

I have another wheelset with 24 spokes in the rear and the same spokes as this one and they feel a lot stiffer on the NDS. With less spokes I would imagine they'd have to be.

And my MTB wheelset has 32 spokes of 2.0mm straight gauge spokes and these feel stiffer too. I would expect this, but by quite a bit.

I'm at about 70kgf with the tire on with the wheelset in question so I'm shocked you're running 40kgf on the NDS and they don't feel like boiled noodles.

FBinNY 

I've been building wheels for close to 50 years. I started long before it became A "science" and people got into the notion that more tension is always better. Prior to 1970 or so, 2mm spokes were rarely used for anything except utility bikes. 15mm and 15mm DB spokes were SOP for performance. I still build wheels with much lower tensions than is fashionable today, using comparatively light rims and spokes, and my underbuilt, under tensioned wheels with spokes that feel like overcooked spaghetti (no quite) often outlast, heavier, tighter wheels.

I wish I could give you practical advice, but all I can say based on the info you've provided, is that something is seriously wrong. Without knowing things that I could only know with the wheel in my hands, I can't give you any specific answers.

BTW- my theory of wheel building focuses more on spoke elongation (strain) and rim deflection, than on tension per se. By way of comparisson, a 1.5mm spoke at 40kgf is under the same strain as a 2mm spoke at 65kgf, so that puts these wheels squarely on target (my target that is) based on my wife's weight. If they were built for me, they'd be somewhat but not much tighter.

One other note-- if you follow wheel threads, you'll see posts by various people with solid wheelbuilding knowledge and experience. They (we?) all weigh different amounts and have different styles of riding, and do things differently. This is cause for much debate here, but the simple truth is that there's a very wide band of specs and methods that produce quality wheels. There are light and heavy well built wheels, more or fewer spokes, higher or lower tensions, various patterns, etc. All is good, if done well and consistent within itself.

Alias530

I'm wondering then if that my mountain bike with 2mm spokes is at the same strain as this wheel build with 2.0-1.8-2.0 spokes? The spoke count is the same but it's on aluminum rims.

My road bike has 24 2.0-1.8-2.0 spokes on it and an aluminum rim but it's not tubeless and fewer spokes might necessitate a higher tension?

I'll look around the forums for a reputable builder in the area and see if I can take the wheels to them and see what they think.

FBinNY 

Let's start by defining terms. Stress is the tension (force) divided by the cross section area (comparable to pressure). Strain is the elongation divided by the original length (often expressed as a percentage. Stress and strain are proportional, and the ratio is defined as Young's Modulus.

So there's a relationship between spoke gauge, count and tension. IMO- tension (elongation) must exceed the maximum expected rim deflection, and anything above that and a safety margin is unnecessary dead load on the components. So (for me) the process begins with estimating the expected rim deflections, at least qualitatively, if not precisely quantitatively. There are those who believe in higher tensions than I do, and they also build good wheels, so there's no magic specific "best" tension.

If you want help, start by including your city of residence (or a close lie if you're concerned about privacy) in your profile. Then possibly somone might see it and give you the name of a good local resource.

fietsbob

Analogy : steel music wire strings . thinner string reaches the proper 'concert' pitch at a lower tension .
but a Thicker string with its higher tension can Be wailed on with more force ..

SRV was said to have brutal, thick strings on his electric guitar..

Alias530

Sacramento, CA

I just want a wheelset that lasts and doesn't give me issues... I'm starting to wish I had just bought a set of factory wheels from ENVE or something and paid the premium. I've definitely put enough hours into this at this point that it would have been worth it.

The needs of this build were too specific though... I'm running low pressures so road tubeless rims that depend on high pressure don't work, I'm running discs so I don't need a brake track, but it's a CX frame so it has QR's so I couldn't just go with regular MTB stuff for thru axles.

chriskmurray

I vote for a much simpler approach. I have built a bunch of rims that came from light bicycle and never had issues with any spokes ever coming loose or wheels not holding true. I did notice in stress relieving that it takes a few times around the wheel squeezing pairs pretty hard before they quit to loose tension so you can finish the wheel but this is not terribly uncommon in carbon rims. Some of these wheels were built with DT Comps but most were built with DT Revs or Comps on the DS and Revs on the NDS.

I would simply email light bicycle and explain your issue, carbon fiber work is very hands on and there is a very fair chance something simply was not laid properly. Ask them if they have had an issue and if they are a good company they will be up front and replace your rims. I have never had to deal with a warranty issue with them so I have no personal experience how they handle those issues but all it takes is a quick email and you may have your questions answered.

Alias530

I emailed them about recommended spoke tension and explained that I was losing major tension with the tire on and they just responded with recommended tension and completely ignored the part about the major tension drop

chriskmurray

Keep in mind, English is probably their second language and there is a chance they got confused by what you were asking.

You can try skype or send another email back being more direct. I have learned many Chinese manufacturers like very to the point emails and tend to forget to address things when the conversation gets a bit long winded. It may not be the case for you, there is still the chance they are having a problem with selective hearing but I try to give others a fair chance to make things right.

An email that is extremely straight forward like:

"I bought X rim and built it to a tension of 150kgf and after installing a tire and inflating to 40psi the spoke tension drops to 70kgf, is this normal for X rim?"

With that being said, those tensions while not ideal are workable and should not loosen further unless something else is wrong like excessive spoke wind up, not stress relieved, etc. The HED Big Deal carbon fat rim is not to be built above 90kgf drive side, with a tire mounted it drops to about 70kgf with a spoke that has a center section of 1.6mm and this is with a "by the book" build. Many of these wheels have into the thousands of miles and still going strong.

Alias530

Alright thanks I'll try them again. I found a recommendation for a builder in my area too so I'll contact him.

FBinNY 

One of the bits of advice I give new wheel builders is the same as given to new bakers ---- "don't overwork the dough". I'll often refuse to do anything more than touchup, or basic alignment on wheels that have been heavily worked by others. Reworking someone else's wheel is often harder than building fresh, and not knowing what happened before makes me uncomfortable taking responsibility the way I would on a wheel I built fresh.

BTW- I've had people bring me situations like these and sometimes refer to them in terms that are a bit too crude to post here -- think "messy leftovers".

I tell you this so you won't be surprised if the builder isn't comfortable or willing to make you any promises. Of course he can and should offer advice and explanation, and may be willing to give it a go for you, but don't be surprised by a disclaimer.

Alias530

Yeah I couldn't fix the original builders wheel so I started fresh. I just emailed the builder so we will see what he says.

chriskmurray

There is some truth here for sure but I will say the most valuable skill I have learned when it comes to turning wrenches is expectation management, a skill almost more important than the actual work you put out. If both parties are on the same page, especially if it is in writing if the case justifies it, there is rarely an issue if things are less than perfect when finished. If customers are being unreasonable I will simply refuse the work but that is a very rare occurrence. Working in a shop that was known for being able to make nearly anything semi-rideable again even if you do not have much cash to work with was a good crash course in this skill and made for some very happy customers who were glad they could at least continue to get to work when other shops refused the work flat out.