DrSpoke
11-05-2015, 11:03 AM
Tire Pressure - Have you ever asked anyone what tire pressure to use? Has anyone ever asked you what tire pressures to use? For me, the answer to both questions is yes. A good articles on the subject is by Jan Heine (editor of Bicycle Quarterly and owner of Compass Bicycles), here: https://janheine.wordpress.com/2010/...-and-pressure/ The article includes a chart originally produced by Frank Berto using the concept of 15% tire drop. The problem with the chart is that it doesn't include all tire sizes and, since it's a chart, not so accurate. With a little internet research, I found a spreadsheet with a formula. I didn't like the design of the spreadsheet so created my own, attached. The inputs are the various weight categories, tires size and weight distribution percentages. From that, it calculates weight distribution in lbs and recommended tire pressures. Note that fairly small changes in weight have a larger than expected impact on tire pressure. The 45/55 weight distribution is an approximation but should be pretty close for road bikes and probably mountain bikes too. But you can actually calculate this using a bathroom scale and refine the percentages. I intend to do this for myself soon. If any questions, contact me. I tried it on Friday on my mountain bike and ended up dropping about 7 psi in front and 5 psi in back and it had a very noticeable, positive impact on the ride and handling. On Saturday, I used it on my road bike and dropped about 5 psi in front but added about 10 psi in back - again a noticeable, though subtle, improvement.
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This is from an email blast I sent out to some riding friends on Monday as well as a reply to a post here on The Paceline yesterday.
Since the spreadsheet is an Excel format, I can only attach a .pdf version of it here.
The first three sections are inputs for weight, tire size and weight distribution/%.
The weight distribution/% shown is, for me, an estimate as I have not measured it yet.
The last two sections are output.
The formula for weight distribution/lbs is a simple calculation of total weight * weight distribution/%.
The formula for tires pressure is as follows: =ROUND((153.6*C20)/(C12^1.5785)-7.1685,0) where C20 = weight/lbs (on that tire) and C12 = tire size (in mm).
A simplified method would be to measure your weight over both the front and rear tires while sitting on the bike in normal riding position w/all gear, water, etc. using a scale.
Then use this formula to calculate tire pressure for each tire: Pressure (psi) = (153.6*weight in lbs)/(tire size in mm^1.5785)-7.1685
As stated above, one of the things I have noticed is that a relatively small change in weight, and likely weight distribution, can have a significant impact on recommended tire pressure. The other thing I noticed is a much larger delta in tire pressures between the front and rear than I had expected. I would use these calculated pressures as a starting point and adjust from there depending on intended use, riding style, road/trail conditions, rim width, tire construction, tubed/tubeless, etc.
Hope this helps someone.
PS - TP won't let me upload the spreadsheet again as it's included on another thread - #15 here: http://forums.thepaceline.net/showthread.php?t=176892
===============================
This is from an email blast I sent out to some riding friends on Monday as well as a reply to a post here on The Paceline yesterday.
Since the spreadsheet is an Excel format, I can only attach a .pdf version of it here.
The first three sections are inputs for weight, tire size and weight distribution/%.
The weight distribution/% shown is, for me, an estimate as I have not measured it yet.
The last two sections are output.
The formula for weight distribution/lbs is a simple calculation of total weight * weight distribution/%.
The formula for tires pressure is as follows: =ROUND((153.6*C20)/(C12^1.5785)-7.1685,0) where C20 = weight/lbs (on that tire) and C12 = tire size (in mm).
A simplified method would be to measure your weight over both the front and rear tires while sitting on the bike in normal riding position w/all gear, water, etc. using a scale.
Then use this formula to calculate tire pressure for each tire: Pressure (psi) = (153.6*weight in lbs)/(tire size in mm^1.5785)-7.1685
As stated above, one of the things I have noticed is that a relatively small change in weight, and likely weight distribution, can have a significant impact on recommended tire pressure. The other thing I noticed is a much larger delta in tire pressures between the front and rear than I had expected. I would use these calculated pressures as a starting point and adjust from there depending on intended use, riding style, road/trail conditions, rim width, tire construction, tubed/tubeless, etc.
Hope this helps someone.
PS - TP won't let me upload the spreadsheet again as it's included on another thread - #15 here: http://forums.thepaceline.net/showthread.php?t=176892