GravelMN

The roads are getting icy around here. After mounting the winter tires on my all-rounder, I noticed that there was one spot that was slightly out of lateral true on the rear wheel. I put the wheel in the truing stand and as part of my wheel inspection, spot checked the spoke tension. I built these wheels last year to a tension of 105 kgF on the drive side but now they are at 117+ kgF. The tensions are still pretty even (about a one mark variance on the Park Tension Meter) but the overall tension has increased. I'm the only one who works on my wheels and this wheel has not been touched since I built it last summer. The only difference I can think of to account for the increased tension is that my shop area is only semi-heated and went from about 80F when I built the wheels to about 32F this morning.

Could the contraction of the steel spokes in the cold explain the increase in tension? My bikes get ridden in temps from below zero F to about 100F.

Bill Kapaun

The aluminum rim has a greater coefficient of expansion, so it would "shrink" more.

Thermal expansion - Wikipedia, the free encyclopedia

Wilfred Laurier

Greater by how much? The rim will shrink circumferentially but the spokes will shrink radially.

Bill Kapaun

Kind of hard for it to shrink "circumferentially" without it shrinking radially.

Wilfred Laurier

Correct, but it shrinks 3.1415 times more circumferentially than it does radially.

Bill Kapaun

So?

rpenmanparker 

So what? The radial shrinkage is what is important. And the coefficient for Al is greater than steel, but the length of steel that is shrinking is much greater than the depth of the aluminum rim. The coefficient of linear thermal expansion for stainless steel is very dependent upon type, but is always about 10 (X blah, blah, blah) or greater. Aluminum's coefficient is 22. Suppose the rim is 25 mm deep and the spokes are 275 mm long. So the ratio of the shrinkage of the steel to the aluminum is at least (10/22) X 11 or at least 5. It would appear that the tendency for shrinkage of the steel spokes could indeed explain the increase in spoke tension. Maybe something else, but that is not a bad guess. BTW the higher tension is well within normal limits and should cause OP no concern.

Of course, this is an oversimplification because the tensile moduli of the steel and aluminum also increase with decreasing temperature. I think that suggests even greater tension to stretch a cold steel spoke than a warm one. So the temperature explanation is looking pretty good.

Andrew R Stewart 

I am suspect of any significant shrinkage happening across the aprox. 50*F stated. If this was the case so many other devices in our lives would do so also. We don't hear of auto engines, transmissions, body panels getting loose or tight from a mild Summer to a mild Winter. Or am I ignorant and don't travel in the right circles to hear of such issues? I do know of machines which need a warm up to achieve correct operating conditions (the SR-71 blackbird comes to mind) but these go through many times the temp difference then the OP 's case. Andy.

Andrew R Stewart 

This question of spoke tension changes due to temp changes has a simple and quick solution. The Op should cool off his wheels to whatever temp he decides is a minimum one, measure the spoke tension and record. Then bring the wheels inside and allow them to warm up fully. Then re measure and compare the two findings. I wonder if the spoke tension meter's temp comes into play, maybe keeping it the same temp is a good idea. Andy.

rpenmanparker 

Assuming a mid-range coefficient among the various stainless steels and based on the "blah, blah, blah" part of the coefficient units I come up with about 0.004 in or 0.1 mm shrinkage for the spoke over a 50 degree F range not counting the modulus change or the accommodation of the rim. Is that significant?

Andrew R Stewart 

I don't know if .1mm over 290mm is significant WRT how much the rim also changes. I'll wait for the experimental data Andy.

rpenmanparker 

Good idea and good point about the meter. Just using inside and outside the house should provide a good basis for the experiment.

Bill Kapaun

Possibly this. One would have to know the temperatures when the rim was checked. I keep my truing stand & tension meter INSIDE.
OR the winter tires are lower pressure and don't "compress" the rim as much.

Andrew R Stewart 

Bill has a very good point Re tire pressure. So the experiment needs to be with bare wheels, no rubber. Andy.

rpenmanparker 

I just remembered that 2.0 mm diameter spokes stretch about 1 mm at 120 kgF. Voila! 1/10 of that is 12 kgF the exact difference OP observed. Verrrry interesting! Of course we have no idea what spokes OP used. But still verrrry interesting.

Bill Kapaun

Even then, if would be very difficult to check the tension with the wheels at 2 different temperatures while keeping the tension meter at the same temperature.

It'd probably be easier to leave the wheel at one temperature and check the tension meter at 2 different temperatures, checking the SAME spoke after quickly taking the tension meter out of the freezer with a gloved hand.

Andrew R Stewart 

More food for thought.

A 0.1mm spoke length change is about equal to 1/4 turn of a nipple. (56TPI spoke threads). To me this is a minor tension change and NOT significant. But some one needs to check my "I have to go to work soon" math.

Bill- It would be east to standardize the tension meter's temp. keep it inside till just before the outside wheel is measured. Then return it to the inside, let it's temp stabilize and check the inside wheel.

I suspect that the lack of really precise methods and tools will provide too much scatter/noise to this experiment to have a good result. But some one should go through the effort and report back. Andy (who's riding to work in shorts today)

Looigi

This reminds me of the hold basic engineering quiz question: If you heat up a metal washer does the hole get smaller, bigger or stay the same?

FBinNY 

If aluminum shrinks, the rim diameter would get smaller so spokes would slacken not go up in tension.

I suspect that the issue has little to do with the wheel or temp, and probably is more related to calibration error in the tool.

However, if the OP (or anyone) is curious about the effect of temperature on spoke tension (not that it matters) they should put a tension meter on a spoke and take a reading. Then, without touching anything, gingerly carry the wheel out into the cold, let the wheel come to temp and take a new reading. To draw a meaningful conclusion, the experiment should be repeated a number of time to confirm what is really happening.

I'll take the opportunity here to crank up the broken record. People are expecting too much from their tension meters and are getting OCD about tension. The "correct" tension is a wide band, not a single precise value. The tools themselves have working tolerances of calibration, and repeatability. The identical tool will give various results depending on where along the spoke the measurement is taken, the orientation of the wheel and tool, and how quickly the tool is released to take a reading.

SquidPuppet

My car (Camry) is 100% rattle free and solid during the warm months. It develops a number of quirks as the temperature drops below ~35f. When it drops into the low teens and single digits those quirks disappear and two new and distinct audible rattles appear in the left rear door panel. It sounds like a large wiring harness had broken free from it's mounting location. When the temps rise, the rattle vanishes. This cycle repeats itself like clockwork every year, for the past eight years.

Electric auto windows function far more slowly.

Garage doors get louder and the electric openers sometimes need a manual assist. I am convinced that is because of the effect that the cold has on the springs.

Padlocks, even when not exposed to moisture, become a challenge to operate.

The cold play tricks on many other mechanical devices here...

bigfred 

Your meter's calibration has changed.

/thread/

79pmooney

2 observations here. First, internal combustion engines go through temperature changes of hundreds of degrees, so the 80 or so we are talking of here isn't much. Second, ever driven old cars, ones from the 1940s or earlier? Winter or summer made a huge difference! Auto engineers have done a lot of work to come up with a package that works seamlessly through the seasons. The fact that we don't see it is testimony to good engineering, not that temperature expansion/contraction is not a major issue.

Ben

Andrew R Stewart 

I think to a degree (bad pun) what is happening with cars are the rubbers/plastics and the atomization of fuels/combustion changes that occur with large temp swings. Until I see some well controlled experimental data with bike wheels I'll be skeptical. Heck, I get different tension meter readings depending on how I attach the tool to the spoke. Interesting that no one has mentioned pluck tone as a gage of relative tension yet. Andy (who has blown far past his usual 3 replies to a thread guideline long ago).

cderalow

the thing that amazes most people is that the hole gets larger.

hence why when interference fitting a bolt or other ring shaped object onto a rod, you heat it.

hot object = expanded hole. cooled object = good friction bond.

FBinNY 

Actually this is too simple. The question about whether a hole gets bigger or smaller, depends on the size relationship between the hole and what it's in. In blocks of metal, the metal expands into the hole making it smaller. In rings the ring expands and the hole gets bigger. So the question boils down into whether you're looking at a hole or a ring.