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Old 05-10-13, 06:57 AM   #1
TandemGeek
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Playing With Tire Pressure

For sh*ts and giggles, when I put our conventional wheels back on the Calfee I only went to 100psi vs. 135psi on our 700x25 Vredesteins; lets call it the lower-limit for those tires and our 275lb combined team weight.

Handling: Significant reduction in stability and tracking through the corners.
Comfort: Quite plush up front, even over the broken pavement sections. Debbie said nothing and was unware that I'd changed the pressure.
Rolling Resistance: Definitely more drag / less responsive when climbing above 5% grades, but otherwise not much difference and no real change to average speed over our normal loop.
Out of the Saddle: Much more "springy" and heavy-feeling.
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Old 05-10-13, 10:41 AM   #2
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Matches my experience that with good tires the lower pressure makes the ride feel much slower before actually effecting speed very much.

Observation - 25mm at 100 psi is pretty low - you do have smooth roads.
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Old 05-10-13, 11:35 AM   #3
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Observation - 25mm at 100 psi is pretty low - you do have smooth roads.
It's a beautiful thing....
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Old 05-10-13, 04:27 PM   #4
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For some time, I've been playing with the idea that, given load and tire width, it should be possible to calculate the appropriate tire pressure. If we go:
((all-up weight)*8)/(nominal tire width) = minimum pressure
that's somewhere close. So for us, in summer trim, that's 350*8/25=112. We use 120 and never pinch flat. Ride is a little stiff, but it goes where I point it.

This falls apart at light rider weights, though I don't see quite why, f.i: 150*8/23=52. Which might be correct in a way. A 125 lb. rider probably wouldn't normally pinch flat at 52 lbs., but the bike would handle terribly.

At heavy weights, it seems decent, f.i: 450*8/32=112.5, though it might be hard to find a 32c with that high a maximum pressure. And about there, one starts to run into rim problems.

Thoughts?
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Old 05-10-13, 05:13 PM   #5
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We run 120 PSI on Maxxis Re-Fuse.
On Pasela TGs on rear of tandem cut down from 120 to 110PSI at stoker'srequest.
We are a sub-250 lb duo.
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Old 05-11-13, 07:09 AM   #6
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Interesting calculation and differs considerably from the chart that has been posted here on occasion.

PK
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Old 05-11-13, 08:44 AM   #7
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Interesting formula CF, we are relegated to 26" wheels, run narrow as we can get, 26 to 28mm, formula says 85 for us, a 260 lb team. We tend to run 90-95 psi. Started out three years ago, more at 105-115 psi , and interestingly enough, we had more flats. Sure seems to perform better at higher psi, but our roads are terrible, so we both enjoy the softer ride.
R&J
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Old 05-11-13, 09:30 AM   #8
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Quote:
Originally Posted by Carbonfiberboy View Post
For some time, I've been playing with the idea that, given load and tire width, it should be possible to calculate the appropriate tire pressure. If we go:
((all-up weight)*8)/(nominal tire width) = minimum pressure
that's somewhere close. So for us, in summer trim, that's 350*8/25=112. We use 120 and never pinch flat. Ride is a little stiff, but it goes where I point it.

This falls apart at light rider weights, though I don't see quite why, f.i: 150*8/23=52. Which might be correct in a way. A 125 lb. rider probably wouldn't normally pinch flat at 52 lbs., but the bike would handle terribly.

At heavy weights, it seems decent, f.i: 450*8/32=112.5, though it might be hard to find a 32c with that high a maximum pressure. And about there, one starts to run into rim problems.

Thoughts?
How did you decide on a factor of 8?
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Old 05-11-13, 10:44 AM   #9
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I suspect it is not a linear function.
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Old 05-11-13, 03:29 PM   #10
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Given that the area of a circle's relationship to its diameter I agree.

Last edited by waynesulak; 05-11-13 at 03:35 PM.
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Old 05-11-13, 05:40 PM   #11
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How did you decide on a factor of 8?
Empirical.
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Old 05-11-13, 05:42 PM   #12
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I suspect it is not a linear function.
Correct. We have a wide range of empirical values, from this forum and the 41. Given the number of engineers and maths on this forum, it seems to me someone should be able to fit a curve through those points. Rim width might also be a factor.
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Old 05-11-13, 06:39 PM   #13
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Below is the chart posted by twocicle in the 19mm rim thread:




I have not measured the interior width of a Dyad but Velocity lists the outside measurement at 24mm. If we subtract 3mm (and 2mm might be more accurate) on each side then the interior rim width is 18mm. Using the chart above that would make a 25mm tire marginal. 25mm tires may work well on Dyads but they may also be subject to some additional pinch flats and sidewall damage. On the other hand carefully keeping the tires fully inflated may avoid problems.

It will be interesting to read TandemGeek's results.
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