Bicycle Mechanics - wheel building question

I recently built a new rear wheel for my road bike. I used the following components -- Speedcific sealed rear hub (Campy 9 speed cassette compatible), Velocity Aerohead 36 hole rim. and 2.0/1.7 mm double butted Wheelsmith steeel spokes. I have a truing stand, dishing tool and a Park tensionmeter. I have read Brandt's book and Sheldon Brown's article on wheelbuilding.

When I finished building the wheel, I thought the tension measurements looked pretty good. On the right hand side the average tension was 133 kg, with all measurements within +/- 15%. On the left hand side the average tension was 82 kg with a +/- 20% tolerance.

I put the wheel on my bicycle and went for a single 20 mile ride. There was nothing special about the ride, but afterwards the wheel was badly out of true. I measured the tension as soon as I removed the rear wheel. The spoke tension on the right hand side seemed to have decreased fairly uniformly by about 15%. The spoke tension on the left hand also seemed to decrease by about 15%, but there were three spokes that lost a lot of tension. I adjusted the tension and re-trued the wheel fairly easily.

My question is why did the tension in these spokes change so much in one ride? My guess is that I did not adequately stress relieve the spokes (although, I tried) during the build process. I don't have a lot of experience building wheels so I would appreciate any comments.

Thanks,

Yeah, sounds like not enough stress relief, definitely. The main thing that stress relieving does is it forces the elbow of the spoke to fit tightly to the shape of the hub flange.

You have to put *a lot* of force on brand new spokes to stress-relieve them! Grab neighboring spokes and pull them towards each other with pretty much all your might. Do that all the way around the wheel. Do it again :) After that go back and touch up the truing. I've even seen people who WALK on top of their newly-built wheels to stress-relieve them.

Another great way to pre-stress the wheel is to use the palm of your hand and apply as much pressure as possible, pushing in on both the right and left sides of the wheel, moving through each spoke intersection until you've done all of the intersections 4 or 5 times. You can add tremendous lateral pressure to the wheel this way without having to step on it.:D

Did you measure the tension after the ride with air pressure in the tire, because that reduces spoke tension.

I never found grabbing pairs of spokes to be very efficient. I put the hub axle against the floor, and push on the edges of the rims. Work the wheel around until you get it all. Do it with both sides, you will eventually stop hearing that pop. Just keep truing it after this.

Did you measure the tension after the ride with air pressure in the tire, because that reduces spoke tension.
How can air pressure affect spoke tension???

How can air pressure affect spoke tension???
I had a hard time believing that one too...
The only thing I could think of to justify this would be the idea that the tire does not really flex outward when you pump up that tube. So what else must? The rim must flex inward.
A metal rim is stronger tahn the tire, it's not going to flex anywhere :).

I had a hard time believing that one too...
The only thing I could think of to justify this would be the idea that the tire does not really flex outward when you pump up that tube. So what else must? The rim must flex inward.
A metal rim is stronger tahn the tire, it's not going to flex anywhere :).
Agreed. It's ESPECIALLY not going to flex anywhere when pressure is evenly applied all around its circumference. In the same way, an eggshell will not break if you squeeze it very hard but very evenly.

Andrew is correct. The spoke tension can decrease after tire inflation, but probably not 15%. I've experimented with this a couple of times and definitely had decreases of 5% to 7% prior to riding the wheel. This was with DT RR 1.1 rims and Revolution spokes.
I would also suspect not enough stress relieving.

Al

Andrew is correct. The spoke tension can decrease after tire inflation, but probably not 15%. I've experimented with this a couple of times and definitely had decreases of 5% to 7% prior to riding the wheel. This was with DT RR 1.1 rims and Revolution spokes.
I would also suspect not enough stress relieving.

Al
Surprising, but good to know! I've not had this problem, so I've never tested spoke tension after inflating the tire. Any idea how this actually works? I still can't figure out how it would happen.

Surprising, but good to know! I've not had this problem, so I've never tested spoke tension after inflating the tire. Any idea how this actually works? I still can't figure out how it would happen.

I was surprised too. I assume that with really light weight rims and spokes the wheel can be compressed that much. I noticed that the wheel's dish changed slightly too, by a fraction of a mm. I supose that's due to the asymetrical tension.

Al

Surprising, but good to know! I've not had this problem, so I've never tested spoke tension after inflating the tire. Any idea how this actually works? I still can't figure out how it would happen.

A slight loss of spoke tension is caused by radial compression imparted onto the rim. It's a little more pronounced on a clincher rim due to the fact that the crochet hook acts as a fulcrum. As the tire's tube is filled with air, the increasing air pressure, with the resulting deformation of the tube and tire that follows, becomes the applied force as the hook bead on the tire becomes the lever. The crochet hook on the rim gives a solid support for the increasing pressure towards the rim. Simple(*) mechanics at work.

*[edit] Perhaps not quite so simple. There are many dynamics at work here. Should we start another thread related to this issue? We can call it 'Brownsfan's Wheel Thread Continuance' :)

Andrew is correct. The spoke tension can decrease after tire inflation, but probably not 15%. I've experimented with this a couple of times and definitely had decreases of 5% to 7% prior to riding the wheel. This was with DT RR 1.1 rims and Revolution spokes.
I would also suspect not enough stress relieving.

Al

Check out this thread (http://www.bikeforums.net/showthread.php?t=125934). There were some practical measurements done. I thought there was a thread that had a website of someone who did a more thorough study of the affect to spoke tension from tire inflation.

My question is why did the tension in these spokes change so much in one ride? My guess is that I did not adequately stress relieve the spokes (although, I tried) during the build process. I don't have a lot of experience building wheels so I would appreciate any comments.

Thanks,

The responses to this question are all spot on. Stress relieving in one method or another is required during the build process. If you research the different ways to stress relieve a wheel,and find a technique that works for you personally, I'll bet your next attempt at building a wheel will be a success.