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Proper tire pressure somehow related to rider weight?

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Proper tire pressure somehow related to rider weight?

Old 09-16-10, 09:53 AM
  #26  
Wanderer
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OK, I did a little calculating using Schwalbe's formula and diagram. That 2" wide tire is about 52mm. That's one BIG tire. Schwalbe considers the 28X2 a 50mm wide tire. Schwalbes diagram is fairly close to Berto's diagram.

Assuming a 40/60 split in load, and the loaded weight - a 45# base pressure for a 160# rider - it works out to approximately this at 290#.

60# for the rear, and 55# for the front.

This should be considered your base pressure at that load, with those tires. You should not exceed the max pressure printed on the side of the tire, nor go lower than the minimum. If I were you, I wouldn't go below that number! This would be for optimum ride quality, on very smooth roads.

I would also increase the base pressure by 5# if you plan on riding on anything other than VERY SMOOTH surfaces.

You will have to experiment from there, to find out what you like the best. I would start out by increasing the pressure in 5# increments, and giving them a trial ride to determine if there is any benefit. I would go all the way to 70#, the tires maximum, as part of the test. You, very well, may find that you like the easier roll at higher pressures - that's OK. If you are satisfied with the roll, softer tires are also OK, as long as they don't squat when laden.

Don't forget to look at the tire - if it squats too much, like in Schwalbe's photo, it NEEDS higher pressure. Only exceed the maximum at your own risk, not recommended. (Yes, I know I do it!) My Supremes (very similar to the Dureems) roll much more nicely at higher pressures, and are not uncomfortable to ride on.

Also, during your pressure trials, a 1# change can make a big difference, or no difference. Just things to keep in mind.

Also, it's easier to let a little burp of 1# out of a tire, than it is to increase..... You just might want to start high, rather than low, after your initial test at Schwalbes recommended pressure, of 60R and 55F, from their charts.

I hope this helps

Last edited by Wanderer; 09-16-10 at 11:00 AM.
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Old 09-16-10, 10:25 AM
  #27  
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Hey Timberline - I'm confused! Which tire, and size, did you order - you mentioned two different sizes 700X40mm (622X42,) and 28X2" (622X50mm)

Last edited by Wanderer; 09-16-10 at 10:58 AM.
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Old 06-04-13, 07:35 PM
  #28  
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Originally Posted by khutch View Post
You can do a curve fit to the data in a spreadsheet like Excel by adding what Excel calls trend lines to the graphs for M and B.
This old thread is just about the only place I have seen where somebody proposes a formula to fit Berto's tire pressure chart.

I have a different formula that is partly from fitting that chart but also based on some simple physics and geometry.

P = 120 * L * W^(-1.5)

P is tire pressure, in psi
L is single wheel load, in pounds
W is tire width, in mm

* is multiplication
^ is exponentiation
you can pretty much plug that formula into excel.

One interesting data point is the 102 mm Pugsley tire. My formula gives a pressure of about 11 psi while khutch gives 26 psi. khutch was just fitting the chart, which makes extrapolation difficult. Because I have a bit of physical analysis in there, extrapolation is more likely to work. 11 psi is about right for a Pugsley.
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Old 06-04-13, 07:46 PM
  #29  
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Originally Posted by timberline12k View Post
I changed my tire order to Marathon Dureme 700 X 40C tires.

http://www.schwalbetires.com/marathon_dureme

http://www.schwalbetires.com/marathon_dureme_home

They are half the weight of the Marathon Plus 700 X 45 tires and provide better grip. It should suit my riding situation better. The are 1/4" more narrow, but the same width as the standard tire provided on the Salsa Vaya.

Marathon Dureme 700 X 40C tires have a manufacturer's suggested range of 50-80 PSI. Using the ratio provided in the above posting would suggest 50-60 PSI in the front tire and 70-80 PSI in the rear. I will probably start out at 55 front and 75 rear. I am assuming that is close to the optimum pressure for a 250 lb. load.
I've been riding a set of 40mm Duremes and always ran the at the max listed pressure of 85 psi. For an experiment I ran them at 75 and they felt pretty good so I dropped down to 70 rear and 65 front, all at a current weight of 380 lbs. At those pressures the tires provide a very smooth and comfortable ride and I have had no pinch flats or any indication that one is iminent. There is a rolling penalty for these tires but the cushion is worth it.

Just two days ago I swapped out the Duremes for my Vittoria Randonneur Hypers (32mm) which by most accounts, are a comfortable tire. I aired them up to 85 psi (recommended max is 95) and although my bike now feels like a racer, it's like riding in a wagon....very rough. I don't think going much lower on this narrow a tires is a good idea so I'll probably put the Duremes back on. I just wish my bike would fit the 50mm Duremes or even the 60mm Big Apples.
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Old 06-04-13, 08:45 PM
  #30  
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Originally Posted by Wanderer View Post
Hey Timberline - I'm confused! Which tire, and size, did you order - you mentioned two different sizes 700X40mm (622X42,) and 28X2" (622X50mm)
I ended up putting the 28X2 Duremes on my Salsa Fargo. I run around 40 PSI. Still running great. I have the larger 28X2.3 Big Apples on a Kona Unit with Nuvinci 360 and run those around 30 PSI.
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Old 06-04-13, 08:57 PM
  #31  
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Playing around a bit to fit the Berto chart better, this seems to work well:

P = 110 * L * W^(-1.5)

I could superimpose my formula against the Berto chart:


32 mm tires at 190 pound load, my formula gives 115 psi
40 mm tires at 190 pound load, 83 psi.
50 mm tires at 110 pound load, 34 psi
60 mm tires at 110 pound load, 26 psi

One cool thing: the formula gives about 10 psi for the 102 mm Pugsley tires - just about the right answer!

This stuff is mysterious!
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Old 06-04-13, 09:49 PM
  #32  
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Originally Posted by khutch View Post
You can see a good discussion of tire pressure and weight in this article. More pressure does mean less rolling resistance because it reduces the tire sidewall flex where most of the rolling resistance comes from. You will get a harsher ride at a pressure too high for your weight and the handling/grip of the tire will be worse when it carries more pressure than your weight requires. The amount of change in resistance and handling from the "correct" pressure is small, the claim is that the gain in comfort is large when the pressure is correct. A correctly inflated tire may even have less rolling resistance on rough roads. The bottom line is that you are doing nothing wrong, a whole lot of people run their tires at the max sidewall pressure regardless of their weight and more than a few run overpressure. If you are happy running your tires where you are then there is no great need to change. You might find that pressures more in line with what that article suggests will be better, but if not what you are doing is fine.

Ken


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Old 06-05-13, 01:25 PM
  #33  
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This version of Berto's work - http://www.bccclub.org/documents/Tireinflation.pdf -
includes a chart for wider tires. My formula doesn't fit as well... but hard to say why. Sure seems like
pressure should go to zero as wheel load goes to zero, but that's not what Berto's lines do!


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Old 06-06-13, 06:37 AM
  #34  
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I have a Trek 7.3fx and i run at 80-90psi, i weigh in at 150lbs and have 1700 miles on the tires and going strong. The tires are the stock bontragers that came with the bike.
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Old 10-02-16, 02:52 PM
  #35  
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Originally Posted by khutch View Post
You can see a good discussion of tire pressure and weight in this article. More pressure does mean less rolling resistance because it reduces the tire sidewall flex where most of the rolling resistance comes from. You will get a harsher ride at a pressure too high for your weight and the handling/grip of the tire will be worse when it carries more pressure than your weight requires. The amount of change in resistance and handling from the "correct" pressure is small, the claim is that the gain in comfort is large when the pressure is correct. A correctly inflated tire may even have less rolling resistance on rough roads. The bottom line is that you are doing nothing wrong, a whole lot of people run their tires at the max sidewall pressure regardless of their weight and more than a few run overpressure. If you are happy running your tires where you are then there is no great need to change. You might find that pressures more in line with what that article suggests will be better, but if not what you are doing is fine.

Ken
Old thread but I want to revisit this, having just looked at that article and having just bought a floor pump today. I bought my bike 2 months ago and haven't touched the tires since the shop sent me out the door. I have a 700 x 30 tire. My sidewalls list a pressure range of 85-95 psi (FWIW it looks like they were around 45 psi today after two months of pressure loss, lol - no idea what the shop set it to when I bought it, forgot to ask).

I tested the front/rear wheel weights as best I could, and came up with 68.4 lbs front, 113.2 lbs rear.

Based on that, according to this article I should be right around 40 psi front, and around 75 psi rear, both well below the range on the sidewall. I set both front and rear to 85 based on the sidewall, and a quick trip down the street and back and it seems comfortable enough.

But if that article is accurate, I should be operating well outside the specified sidewall range, and I'm not sure that's ok; my gut feeling is that the pressures determined in the article should supersede what your sidewall says since it doesn't account for ride loads, etc, but I'm not sure I want to go that far out of the range.

If you were me, would you go with 85 psi or the pressures determined by the article?
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Old 10-02-16, 03:23 PM
  #36  
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Originally Posted by puma1552 View Post

I tested the front/rear wheel weights as best I could, and came up with 68.4 lbs front, 113.2 lbs rear.

Based on that, according to this article I should be right around 40 psi front, and around 75 psi rear,

If you were me, would you go with 85 psi or the pressures determined by the article?
It's a funny thing tire pressures, almost a religious issue in many quarters. Aside from optimizing your contact parches and optimizing the rolling resistance there is the comfort factor to deal with. A harsher ride (higher pressures) equates to more control on smooth road as far as I'm concerned. At lower pressures I feel like the bike is wallowing under my weight and sapping my strength so I run higher pressures and are careful not to lean the bike too hard.

I never run the big differences you have come up with, I always have more in the rear of course but the front is not far off this. It will be interesting to hear your thoughts once you have ridden at those pressures a bit.
Frank.
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Old 10-02-16, 08:05 PM
  #37  
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I have never seen any discussion about how tire makers set their minimum pressures. Some folks claim that the maximum pressures are one third to one half the pressure it takes to blow the tire off a rim but I can't guarantee that is correct either! When I expect to be running on smooth pavement I typically run both tires at the same pressure. I typically set them at max pressure and then check them again in a few weeks! If I know that I will be running on rough pavements or unpaved roads then I will set them to the Berto pressure. Usually that gives me a large pressure difference front to rear and the front or both will be below the minimum on the sidewalls. I've never had any issues with handling or rolling resistance or pinch flats or anything when I do this. But I can't tell you that you cannot have issues if you run below the minimum sidewall pressure. Certainly you will improve your rough road comfort and rolling resistance by running at the min sidewall pressure rather than the max and if it makes you feel better to comply with the manufacturers recommendations then by all means do so. And likewise there really is no advantage to running below the max on smooth roads so do that too, as I do.

Lower pressures do improve the ride considerably on rough roads. RAGBRAI has included an intentional and optional gravel loop the past two years. Last summer I did a training ride on a gravel road to boost my confidence before RAGBRAI and I started out at the max pressures front and rear. After a mile of being pounded by the rather large stones on this particular road I had had enough so I stopped and dropped the pressures to the Berto suggestions. HUGE, HUGE difference in the ride!! So last year on RAGBRAI I stopped at the gravel loop entrance to adjust the pressures and then again at the next town after the loop to boost them back up to max. That worked fine. This year I was quite a bit more casual. I had pumped the tires up to max "recently" so I headed out on gravel loop day which was the first day this year without even looking at the pressures. I rode the loop without any trouble and then decided to top the tires off the next morning for the road running the rest of the week. Well, they were both at about 35 psi which is pretty much the Berto pressure for the front but only half that for the rear! Yikes!! I'm running Vittoria Voyager Hypers in the 700x38 size so I got away with that although I had more rolling resistance than I should have. Obviously if I had tried that on the 23s that most everyone on RAGBRAI runs I would have had a couple of pinch flats within the first mile of the gravel loop. Assuming I made it that far....
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Old 10-02-16, 08:23 PM
  #38  
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The ultimate tire pressure blog: https://silca.cc/blogs/journal
Skip over part #5 (aero stuff) down to part 4a to get into the pressure aspects.
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Old 10-03-16, 11:08 AM
  #39  
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Interesting, the more they study tire pressure the more complex it becomes. One would think that all this would have been known decades ago but I guess not. It will be interesting to see if they can ever reach any guidelines that would allow us to optimize pressure for tire size, surface conditions, and load.
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Old 10-05-16, 06:54 PM
  #40  
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Originally Posted by Shimagnolo View Post
The ultimate tire pressure blog: https://silca.cc/blogs/journal
Skip over part #5 (aero stuff) down to part 4a to get into the pressure aspects.
Thanks for the link.

Warrants more careful consideration than a quick scan, so I have bookmarked the site for further study.

The more I read about tires, the more important they seem to be to overall performance / comfort, but the more complex it becomes with so many inter-related factors.
Certainly seems to be indications that certain traditional 'rules' repeated as a matter of 'truth' may well be much more debatable than their proponents realize.

Perhaps as power meter get more popular, more people will start seeing with their own eyes some of the evidence as to the reality of what happens as you change tire pressures / tire quality / rim widths
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