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  1. #1
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    Tire Pressure and Spoke Tension

    In case you want to know what happens when you put a 700x28 commuter tire on your Crest 29er rim and pump it to 85 psi for use on the road and a trainer...
    The figure shows drive-side tension on a rear wheel in six different scenarios: at four different tire pressures declining from 85 psi to 0 psi, one with the bead off, and after readjustment.
    The "readjusted" tension line is just about how the wheel left my hands the first time... not that long ago, and right on the 100 kgf mark. Now it has a little more variation. I asked Stan's what they thought and they said increase tension. I'm not sure this isn't going to happen again fairly quickly.
    The most intersting thing to me was to see that the tension stayed about the same (the middle shape is actually four lines), even dropping from 85 psi to zero, until the bead came off.
    The tire is a Specialized All Conditions Armadillo 700x28.
    Attached Thumbnails Attached Thumbnails Tire Pressure and Spoke Tension-crest-29er-pressurization.jpg  

    Last edited by meltingfeather; 03-27-2012 at 12:19 PM.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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    Quote Originally Posted by meltingfeather View Post
    In case you want to know what happens when you put a 700x28 commuter tire on your Crest 29er rim and pump it to 85 psi for use on the road and a trainer...
    The figure shows drive-side tension on a rear wheel in six different scenarios: at four different tire pressures declining from 85 psi to 0 psi, one with the bead off, and after readjustment.
    The "readjusted" tension line is just about how the wheel left my hands the first time... not that long ago, and right on the 100 kgf mark. Now it has a little more variation. I asked Stan's what they thought and they said increase tension. I'm not sure this isn't going to happen again fairly quickly.
    It was intersting to me to see that the tension stayed about the same until the bead came off.
    The tire is a Specialized All Conditions Armadillo 700x28.
    What's different about road wheels? Shouldn't this happen with those as well, considering they are run at over 100psi?
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  3. #3
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    Quote Originally Posted by Buzzaro View Post
    What's different about road wheels? Shouldn't this happen with those as well, considering they are run at over 100psi?
    The interesting thing to me is that the bead seating, not the tire pressure, seemed to be the cause of the lion's share of the detensioning.
    This fits with anecdotal evidence of some folks having issues with Stan's tubeless road rims dropping tension, assuming they have a similar bead design.
    So... a typical road clincher with a 622mm BSD would not experience the same detensioning because the tire bead doesn't fit as tightly on it... meaning the rim doesn't get compressed, or not nearly as much.
    This wheel had been ridden wobbly for a bit (not sure how long), which is why I think the tension was no longer uniform.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
    29er Tire Weight Database

  4. #4
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    I have a set of Stans road rims that eventually got some Maxxis tubeless tires on them. Shortly after the rear wheel wouldn't stay true. I am running a set of Crests on another road bike with 28mm tires and they have been solid so far.

    I understand the data but I don't know what to conclude. Should such a wheelset built to run road tires be retensioned? How much of load can a wheel carry if its spokes are detensioned by half as appears here?

    Also, would the type of spoke used effect these results?

  5. #5
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    Quote Originally Posted by craigsj View Post
    I have a set of Stans road rims that eventually got some Maxxis tubeless tires on them. Shortly after the rear wheel wouldn't stay true. I am running a set of Crests on another road bike with 28mm tires and they have been solid so far.
    I've heard some similar reports about road tubeless, to which Stan's has recommended raising tension. I think that variability in tire beads may play a role here, as it does in mounting tubeless or tubeless ready mountain tires on Stan's rims.
    Quote Originally Posted by craigsj View Post
    I understand the data but I don't know what to conclude. Should such a wheelset built to run road tires be retensioned?
    I'm facing that quandry myself. I don't want it to happen again, and my options seem to be: 1) tell the customer he shouldn't do what he's doing, and 2) jack the tension up on the Crest, neither of which I really want to do. I did bump the tension up a bit, but I wonder if it's pissing in the wind by looking at scale of what happens.
    After I retensioned the wheel to an average of 106kgf I re-inflated the tire to 65 psi and the average tension dropped to 80 kgf, which is better than the 58-and-all-over-the-place it came in at. I didn't go through total stress relief because I knew I was going to try this out, but after I remounted the tire, inflated to 65 psi, and pulled the tire back off, the tension was again all over the place. I've attached that figure. Based on my inability to dial the tension uniformity to what I typically do with a new wheel, I think the rim is warped from being ridden wobbly. Two NDS spokes had zero tension when it came in.
    Quote Originally Posted by craigsj View Post
    How much of load can a wheel carry if its spokes are detensioned by half as appears here?
    Not sure... I'd assume about half, which is apparently less than what the wheel is subject to. I've built quite a few Crest 29er wheelsets, even for guys significantly heavier than this rider, and never had this issue.
    Quote Originally Posted by craigsj View Post
    Also, would the type of spoke used effect these results?
    It's built with Revos. I'd think the phenomenon would be even more pronounced with a less elastic spoke.
    Attached Thumbnails Attached Thumbnails Tire Pressure and Spoke Tension-pressurizing-crest-after-readjustment.jpg  

    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
    29er Tire Weight Database

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    Thanks, MF. Very interesting.

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    Quote Originally Posted by meltingfeather View Post
    The interesting thing to me is that the bead seating, not the tire pressure, seemed to be the cause of the lion's share of the detensioning.
    This fits with anecdotal evidence of some folks having issues with Stan's tubeless road rims dropping tension, assuming they have a similar bead design.
    So... a typical road clincher with a 622mm BSD would not experience the same detensioning because the tire bead doesn't fit as tightly on it... meaning the rim doesn't get compressed, or not nearly as much.
    This wheel had been ridden wobbly for a bit (not sure how long), which is why I think the tension was no longer uniform.
    Did this detensioning affect the radial or lateral true of the wheel? Have you tried any other tires on the wheel? Not insinuating your data is erroneous, just trying to isolate the cause.
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    Quote Originally Posted by Buzzaro View Post
    Did this detensioning affect the radial or lateral true of the wheel? Have you tried any other tires on the wheel?
    Yes... that's why the rider brought it back to me. It was out of true pretty badly until I fixed it.
    I don't recall specifically, but I don't think it went significantly out of true when I re-inflated the tire to 65 psi after fixing the wheel, even though it skewed the tension. Don't quote me on that, though. I went over the wheel again thoroughly after doing that.
    Quote Originally Posted by Buzzaro View Post
    Not insinuating your data is erroneous, just trying to isolate the cause.
    Not taken that way... I'd be stoked if you're able to see or think of something I missed. That's why I posted.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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    For what it is worth, I just built a set of Pacenti tl28's (a lightweight 26" rim about 390 grams each) with Wheelsmith DB14 spokes and DT Swiss 240s hubs, and these wheels have done the same thing. Tension was right at 100kgf (recommended) in the truing stand. After a ride the rear rim dish was off center and had a couple of loose spokes (NDS). Took the tire off (Continental Trail King 2.2), re-tensioned everything, re-installed the tire (32 psi)...then I noticed that dish was off-again. Checked tension and the DS had fallen about 15-20% and some of the NDS spokes were so loose that my park tension meter wouldn't even register a tension (off the 'chart'...the TM would show a number like '4'). It happened after the tire was installed and inflated, and the effect was more pronounced on the rear than the front. I didn't take notes/data points as MF, but I was so stumped on what to do that I have just been riding a different set of wheels and bouncing the same ideas as above around in my head (ride it with loose spokes is asking for trouble, so more tension, tension with a tire on the rim, maybe split the difference). The guy who taught me to build wheels suggested bumping the tension up on the rear to close to the recommended tension with the tire on the rim and inflated. But I haven't decided what to do. I want to see what is the effect of using a different tire. I wonder if the tires make the difference? Maybe a different manufacturer? Or a different bead design (UST vs 'tube' bead)? I'm a relative novice compared to many of the highly experienced wheel builders here, so I don't have much to add, other than my observations at this moment. I'll try to report back after trying some other tires.
    Last edited by jason300b; 03-27-2012 at 10:27 PM.

  10. #10
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    bump for a calculation of bead tension due to mounting. It's hoops stress from spoke de-tension plus rim circumferential compression. Come on, you know you want to do it.

    It's no surprise that psi doesn't affect spoke tension. Think of the force of air pressure on a rim. One part pushing radially inward against the spoke bed. An equal cross section pulls radially outwards via the bead hooks.

    As to whether you should increase the spoke tension - that depends on what the limit of spoke tension is. Usually, as Jobst Brandt points out, it's when you stress relieve a wheel and it goes loopy. Same as when 4slomo finds that the rim gets increasingly sensitive to spoke adjustments. It's when the rim is starting to get near instability. In the case of Stan's rims, I though the tension limit was due to lack of spoke eyelets?

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    I had the same thing with my Podium MMX rear. Built the wheel to 100kgf with less than 5% variance. Fitted a Race King Supersonic 2.2 and within half an hour of riding had to stop and redish/retension the wheel. Spokes were loose everywhere. The wheel was so sloppy that it felt like I had a flat tyre.

    Redid the wheel with the tyre off, nice even 100kgf all round, fitted tyre and within half an hour had the same thing happen. WTF? Posted on mtbr and kdiddy mentioned the Alpha 240s problem.

    Redid the wheel again, then mounted the tyre and inflated it. DS tension dropped to between 70kgf and 80kgf. NDS tension went down too (can't remember by how much). The wheel did not go out of true.

    I then brought the wheel up to final tension (100kgf) with the tyre on and inflated to 28psi. Since then, the wheel has been bombproof. No loose spokes and runs true, I've done around 1500km on it.

    Perhaps there are just some tyre/rim combos or even batches of a particular tyre that have extremely tight (or perhaps strong) beads.

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    I had another idea. Maybe on the rims this is happening to, the joint is loose. Perhaps their diameter is changing slightly when the tire is mounted. I mean a diameter change is the likely cause either way, but this could be the culprit. A 1mm excess at the joint could be overlooked, especially with rim strips or tape. This would decrease the finish diameter by about .3mm or so. That might be enough to throw the tensions all off.
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    Quote Originally Posted by TigWorld View Post
    I then brought the wheel up to final tension (100kgf) with the tyre on and inflated to 28psi. Since then, the wheel has been bombproof. No loose spokes and runs true, I've done around 1500km on it.
    Do you change tires very often? Any issues? I'm assuming your Podiums are now at ~120kgf w/out the tire mounted, right?

    Quote Originally Posted by TigWorld View Post
    Perhaps there are just some tyre/rim combos or even batches of a particular tyre that have extremely tight (or perhaps strong) beads.
    This is what I'm thinking... probably a combination of the two factors.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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    Quote Originally Posted by beanbag View Post
    bump for a calculation of bead tension due to mounting. It's hoops stress from spoke de-tension plus rim circumferential compression. Come on, you know you want to do it.
    I'm pretty sure spoke tension and circumferential compression are directly related, not additive.
    Quote Originally Posted by beanbag View Post
    It's no surprise that psi doesn't affect spoke tension. Think of the force of air pressure on a rim. One part pushing radially inward against the spoke bed. An equal cross section pulls radially outwards via the bead hooks.
    It remains surprising to me, and I don't think this argument is plausible for two reasons: 1) the bead is supporting itself in large part (i.e., the bead hook is not the only thing holding it on), and 2) I don't believe the "bead hooks," if you can even call them that, on a Crest would be capable of providing much support.
    Quote Originally Posted by beanbag View Post
    As to whether you should increase the spoke tension - that depends on what the limit of spoke tension is. Usually, as Jobst Brandt points out, it's when you stress relieve a wheel and it goes loopy. Same as when 4slomo finds that the rim gets increasingly sensitive to spoke adjustments. It's when the rim is starting to get near instability.
    Probably going this route, or the-tension-with-tire-on route.
    Quote Originally Posted by beanbag View Post
    In the case of Stan's rims, I though the tension limit was due to lack of spoke eyelets?
    I'm thinking it's more due to the light weight.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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    Quote Originally Posted by Buzzaro View Post
    I had another idea. Maybe on the rims this is happening to, the joint is loose. Perhaps their diameter is changing slightly when the tire is mounted. I mean a diameter change is the likely cause either way, but this could be the culprit. A 1mm excess at the joint could be overlooked, especially with rim strips or tape. This would decrease the finish diameter by about .3mm or so. That might be enough to throw the tensions all off.
    Spitballing....
    Not a bad thought, though I'm pretty sure the joint in this case is tight.
    After I broke the bead and was adjusting the tension, I spent a few minutes chasing a "click" in the rim, thinking that the joint pin had dislodged, only to find out the presta valve in the tube was flopping around.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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    Quote Originally Posted by meltingfeather View Post
    It remains surprising to me, and I don't think this argument is plausible for two reasons: 1) the bead is supporting itself in large part (i.e., the bead hook is not the only thing holding it on), and 2) I don't believe the "bead hooks," if you can even call them that, on a Crest would be capable of providing much support.
    I wasn't sure of his perspective either. When you consider that all the elements in the structure are elastic to some extent, I don't think the result is so easily predictable.

    MF, do you have an idea how much compression of the rim is required to see the net reduction in tension you are measuring? I wish I could be of more help here, but I find this really interesting.

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    I too experienced this phenomenon when I first mounted tubeless road tires onto my Alpha rims. There was a considerable loss of spoke tension (I'm not sure of the amount- I don't own a spoke tensiometer.) I then tried a regular, kevlar beaded clincher road tire at the same pressure and the result was nowhere near the same amount of spoke tension loss. It was evident to me that the nearly inelastic carbon fiber bead of the tubeless tire was compressing the rim; more so than just the air pressure alone. I simply retensioned the rims with the tires mounted and inflated on the rims. That was almost a year ago, and I'm still running the Alpha wheelset with no problems and no need for further adjustments. (Although I might lower the spoke tension if I ever switch back to regular clinchers. That's probably not likely, though.)

    So it stands to reason that any tight fitting beads can reduce the spoke tension of a rim; particularly lightweight aluminum or scandium alloy rims. BTW, I don't believe there is any 'upward' force exerted on the bead hook of a rim by a bead that's also exerting a compressive force on the rim.

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    Interesting

    Quote Originally Posted by meltingfeather View Post
    Probably going this route, or the-tension-with-tire-on route.

    I'm thinking it's more due to the light weight.
    Guessing it has to be the light design/open profile. . would love to see your chart comparing these profiles

  19. #19
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    Quote Originally Posted by meltingfeather View Post
    I'm pretty sure spoke tension and circumferential compression are directly related, not additive.

    Yes, they are directly related, AND they are additive for hoop stress calculation.

    It remains surprising to me, and I don't think this argument is plausible for two reasons: 1) the bead is supporting itself in large part (i.e., the bead hook is not the only thing holding it on), and 2) I don't believe the "bead hooks," if you can even call them that, on a Crest would be capable of providing much support.

    In the same vein, the air pressure pulls up on the bead, causing it to compress on the spoke bed by less.


    I'm thinking it's more due to the light weight.

    That sounds like engineer-speak to me. "The part wasn't too weak - it was too light."
    ...

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    Quote Originally Posted by beanbag View Post
    Yes, they are directly related, AND they are additive for hoop stress calculation.
    OK... maybe I misunderstood... as I see it, bead tension in the tire is going to come from tire pressure and rim compression, the latter being directly related to the change in tension.

    Quote Originally Posted by beanbag View Post
    In the same vein, the air pressure pulls up on the bead, causing it to compress on the spoke bed by less.
    I see now what you're getting at... I thought you were saying the bead pulls on the rim, which I don't think happens, but it's materially the same affect as compressing less.

    Quote Originally Posted by beanbag View Post
    I'm thinking it's more due to the light weight.

    That sounds like engineer-speak to me. "The part wasn't too weak - it was too light."
    Could be... I have been know to do that on occasion... I just don't think it's due to the lack of eyelets.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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  21. #21
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    Quote Originally Posted by bholwell View Post
    I too experienced this phenomenon when I first mounted tubeless road tires onto my Alpha rims. There was a considerable loss of spoke tension (I'm not sure of the amount- I don't own a spoke tensiometer.) I then tried a regular, kevlar beaded clincher road tire at the same pressure and the result was nowhere near the same amount of spoke tension loss. It was evident to me that the nearly inelastic carbon fiber bead of the tubeless tire was compressing the rim; more so than just the air pressure alone. I simply retensioned the rims with the tires mounted and inflated on the rims. That was almost a year ago, and I'm still running the Alpha wheelset with no problems and no need for further adjustments. (Although I might lower the spoke tension if I ever switch back to regular clinchers. That's probably not likely, though.)
    Your point about dropping the tension back down is particularly interesting. Raising tension would be a much easier decision if this wheel did not see use both with the road tire for training and with tubetype mountain tires for trail riding. He changes pretty frequently between the two.
    Quote Originally Posted by bholwell View Post
    So it stands to reason that any tight fitting beads can reduce the spoke tension of a rim; particularly lightweight aluminum or scandium alloy rims. BTW, I don't believe there is any 'upward' force exerted on the bead hook of a rim by a bead that's also exerting a compressive force on the rim.
    I agree.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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  22. #22
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    Quote Originally Posted by meltingfeather View Post
    Do you change tires very often? Any issues? I'm assuming your Podiums are now at ~120kgf w/out the tire mounted, right?
    I have changed rear tyres - now using a Race King 2.2 Racesport setup tubeless which has slightly thicker side walls than the supersonic version (used with a tube) which originally gave me the problems. I just measured the spoke tension and DS is right around 100kgf with the Racesport version as well. I assume the tension would be ~120kgf without a tyre on but I haven't measured it.

    Interestingly, at the valve hole I've got the overlap of the Stans tape. It's 3 layers thick for the spokes either side of the valve hole and two layers thick everywhere else. This should create extra deflection by the bead for those two spokes, but those spokes do not seem to have lost any tension. 1 extra layer of tape seems inconsequential.

    It would be interesting to know whether the extra tension (sans tyres) will damage the spoke holes.

    It seems for these weight weenie rims that spoke tension really needs to be dialled in with the intended tyre in place.

    Meltingfeather - your stroke of genius was to measure with bead seated and no pressure. The fact that inflation pressure makes only a small difference to spoke tension makes me a little more comfortable on setting the spoke tension with the tyre seated esp. tubeless. In the event of a flat, the bead will still be in place containing the spoke tension.

    120kgf without a tyre on may not be a problem as you can't ride on it without a tyre.

  23. #23
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    Quote Originally Posted by craigsj View Post
    MF, do you have an idea how much compression of the rim is required to see the net reduction in tension you are measuring?
    By my estimate a ~1.0mm reduction in circumference (a 0.30mm reduction in diameter) would give the ~20kgf tension drop I'm seeing.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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    I recently ran in to the same issue building a set of Alpha 340s for a friend. I initially built the wheels to 120 kgf, and once we got on to building up the rest of the bike, I noticed the spokes being very soggy, and measured them at 70 kgf with the tire (and tube, not tubeless) at about 110psi. The front, radially laced wheel dropped from about 100 kgf to something below what the Park tension chart would give me a deflection conversion for (I believe 54 kgf was the lowest listed conversion)

    I had him ride the bike home and back a couple times to get out any possible kinks and then had him bring me the bike to retension the wheels. Though I did not put a tensiometer on the spokes while doing so, we slowly let out air pressure while plucking the spokes, and there was a distinct change in pitch as the tire deflated.

    I jacked the spoke tension up to 135 kgf in the rear this time around, making sure to keep an eye on any weird occurances in the rim due to the high tension, but it seemed fine. When I inflated the tire again this time around, the tensioned dropped only to 110 kgf, half the difference that occured after the initial build.

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    Thanks for all the above info and ideas.

    I too have just build up a set of Crest 29ers and got a nice even tension at 95 +/- 7%. On fitting a super tight Racing ralph this had dropped to 50!

    I tried another tire (Ignitor) that i could mount by hand and found the tesion had dropped but by a slightly less amount.
    I then re tensioned the wheel with tire on. Yet to ride.

    Last year i had a new Crest/ Hope Hoop that after a crash in it's 1st race had an unrepairable taco, it was also running a RR that was not easy to get on. I now wonder if loss of spoke tension could have contributed to the wrecked wheel??

    I also see in a lot of reviews of light wheelsets that the reviewer complains of loose spokes after a short ride. Perhaps the spokes were loose after fitting tire before the bike was ridden (or got worse because of loss of spoke tension).

    I am not an exierienced wheel builder but my 1st thought was a wheel should not need to be tensioned to every tire but perhaps overtensioning spokes before fitting tires is what needs to be done with light rims?

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