jmX

It's been implied a couple times now on this forum that I'm running too high of a PSI on my Trek 7.3FX because I'm not 200+ lbs. The tires are 700x32 race lite hard case tires, and I run them at 100-105psi. On the sidewall it recommends 110psi.

I weigh 165lbs, and I ride on fairly smooth road surfaces only.

Am I doing it wrong, or what? My presumption was higher psi = less rolling resistance. Also I should mention that comfort isn't much of an issue as 60 mile rides on the bike have been fine.

Thanks for the help.

AaronJohnTurner

If you're enjoying them on that pressure keep 'em there! You can always experiment too. I keep my 700x32's at 110psi on my Trek Valencia, no exceptions, due to my weight of 245pounds, and also because I'm not a fan of unnecessary rolling resistance and rarely ride offroad.

At your weight you have some play in where you can set your pressure without a huge risk of pinch flats.

khutch

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

jmX

Wow, the article implies 50psi front/60psi rear for my setup would be optimal. That is pretty surprising.

I'll ride a day or two at 90 (and then maybe 80) and see how it goes. Other than loss of comfort, does high PSI cause any other side affect such as increased chance of flats or blowouts?

timberline12k

I ordered a Salsa Vaya and asked the LBS to swap out the standard tires for Marathon Plus 700 x 45 tires. The suggested PSI is 45-70.

If I weigh 245 lbs., what is the proper front and rear tire pressure for optimum performance based on the article referenced above and what is the calculation?

How does that change for paved vs. unpaved roads?

Wanderer

At 245, with those tires, I'd run 'em at max.

But, experiment a little, as those tires aren't known for their ability to spin up quickly - hard tires won't hurt.

At my 190 + 45 for the bike, with Sch Marathon Supremes size 35 and 40, I run mine at 92 (rated for 85) because that's where they roll the best.

timberline12k

Sounds reasonable. My road bike tires are rated up to 120 lbs and I always make sure they are at least 110 lbs.

I could not figure out the formula since 45 wide tires and my weight of 245 lbs. were not on the chart.

khutch

I've been running pressures based on that article all summer and have not detected any loss of speed. The shock absorbing of the tires is noticeably better as the article claims. High psi does increase the chances of a blowout but as long as you don't exceed the sidewall pressure of the tire by a large amount the risk of blowout is quite small until the tire wears out or is damaged so it is nothing to worry about. People argue all the time that high psi does or does not reduce the chances of a puncture and none of them have any science to back up their opinions so I class the topic as "unknown". The late, great Sheldon Brown claimed that high psi reduces the risk of punctures, for what it is worth.

Ken

khutch

That is a failing of the article, it does not consider tires as wide as many folks use these days. I tried modeling the data in the chart and extrapolating it to wider tires. If I assume that your bike weighs 25 pounds and you have 40% on the front, 60% on the rear my extrapolation recommends 33 psi front, 46 psi rear. The formula or the chart will often recommend pressures on the front that seem like pinch flat bait to many folks so use your own judgment. I have been running according to the article without any problems all summer but YMMV. Some tire manufacturers have detailed and weight based pressure recommendations buried on their web sites if you look hard enough so that is another source of this information.

Ken

meanwhile

Yes, your chance of flats will increase with a higher pressure. You'll also possibly reduce turning and braking ability. The best thing to do, as you've guessed, is to try several different pressure.

Sheldon Brown's website is the best place to research most cycling questions. Here's the tyre section: https://www.sheldonbrown.com/tires.html

..And this is the widely used Rivendell tyre width chart:

https://www.rivbike.com/assets/full/0...ick_a_tire.pdf

meanwhile

https://www.rivbike.com/assets/full/0...ick_a_tire.pdf

Drop the pressure and run wider tyres for unpaved roads. Or if you want to have only set of tyres, run the wider ones unless you like rim damage. Again: https://www.rivbike.com/assets/full/0...ick_a_tire.pdf

Btw the Marathon Plus is considered possibly the toughest tyre you can buy, but it's something of a boat anchor.

timberline12k

I am still not quite sure how you came up with those figures, but if I were to keep the same ratio and adjust each to stay within the manufacturers recommended range (45-70 PSI), I should be running around 45-50 PSI in the front and 60-70 PSI in the rear. When my bike shows up in a couple weeks, I will probably start with that pressure and adjust as needed.

Before reading this thread and linked article, I would have set both tires near 70 PSI. The lower pressure should deliver a softer ride. One of the main drivers to purchase a second bike was to find one that could travel where my road bike can’t and transport my DSLR. Anything that reduces shake will be better for my D700 which cost more than both bikes.

I have read the Marathon Plus can be slow, but I plan to ride on nearby Rail Trails. I don’t want to hassle with flats and am not looking for the speed of my road bike. The Marathon Plus is about the widest tire (1.77") I could find that fits on the Salsa Vaya. It provides maximum durability and protection while still offering moderate grip and speed.


That being said, is the Marathon Plus overkill for Rail Trails and should I look at the regular Marathon instead? The suggested PSI is the same for both.

khutch

Well I did not explain it, I only gave you an example because I had no time to do more.

If you look at the "curves" on the chart in that article you see that they are all straight lines. A straight line is described by the equation:

P = M * L + B

where P is the inflation pressure for the tire in psi, M is the slope of the line for that tire width, L is the load on the tire, and B is the intercept of the line. If you calculate the M and B values for each tire width from the data on the chart and then plot how they vary with W, the tire width in mm you get nice smooth curves. 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. The trend lines can be extrapolated to show you the values for wider and narrower tires than are found in the original data and Excel also allows you to display the equations of the trend lines which allow you to directly calculate the M and B values. So, having done all that my results were as follows:

If you want to calculate the tire pressure for tire widths not given in the article you first need to know the load on each tire. The article itself tells you how to come up with that. You can weigh each wheel of the bike with you and your normal gear compliment on the bike or you can take the total weight of you, the bike, and your gear and multiply it by the percentages given in the article for the front/back distributions for typical bikes. You should come up with the weight on each wheel in lbs or kg.

Next you need to calculate the m and b values for your tire width w, in mm. The formulas for this come from the spreadsheet analysis, not the article. My results were:

M = 11.941 * EXP(-0.068749 * W) / 2.2046 if your weights are in lbs

M = 11.941 * EXP(-0.068749 * W) if your weights are in kg

B = 23.552 * LN(W) - 83.219 B is the same whether you use lbs or kg

Anyone with much math training can apply those formulas and they can be pasted directly into Excel if you have that program. W is the width of the tire in mm, EXP() is the natural exponential function, LN() is the natural logarithm function. Both should be found on the keypad somewhere on any scientific calculator.

Once you have M and B for your tire width you can calculate the inflation pressure from

Pf = M * Lf + B
Pr = M * Lr + B

where Pf, Pr, Lf, Lr are the front and rear pressures and the front and rear tire loads respectively.

I'd attach a spreadsheet set up to do this but I don't believe the forum permits that. If you are too math adverse to deal with the EXP() and LN() functions I will include a table below with precalculated values for M and B with tire W. It probably won't come out formatted correctly but the data will be there.

For very large tires the results are an extrapolation and are not based on data so in addition to any mistakes I may have made in this analysis there is some question about how accurate an extrapolation can be. The intercept values for narrow tires are negative, which seems odd but the original curves do cross zero for small positive weights so this is a feature of the original data. For any likely tire load from real life the pressures will be positive except possibly for children's bicycles and it would really be painting the lily to apply this work to children's bikes. Supposedly the tire diameter does not matter but I do not know how rigorously this has been tested. Inflation pressures that are below the manufacturer's recommendations are common from this analysis so use your own judgment. Low pressures can lead to pinch flats in rough terrain and the manufacturer's minimum value may be a better choice than the recommendation from this work.

Ken


Width M B
20 3.01922 -12.6635
21 2.81862 -11.5144
22 2.63136 -10.4188
23 2.45653 -9.3718
24 2.29332 -8.3695
25 2.14096 -7.4080
26 1.99871 -6.4843
27 1.86592 -5.5955
28 1.74195 -4.7389
29 1.62622 -3.9124
30 1.51817 -3.1140
31 1.41731 -2.3417
32 1.32314 -1.5940
33 1.23523 -0.8693
34 1.15317 -0.1662
35 1.07655 0.5166
36 1.00503 1.1800
37 0.93825 1.8253
38 0.87592 2.4534
39 0.81772 3.0652
40 0.76339 3.6615
41 0.71267 4.2430
42 0.66532 4.8106
43 0.62112 5.3648
44 0.57985 5.9062
45 0.54133 6.4355
46 0.50536 6.9532
47 0.47179 7.4597
48 0.44044 7.9555
49 0.41118 8.4412
50 0.38386 8.9170
51 0.35836 9.3834
52 0.33455 9.8407
53 0.31232 10.2893
54 0.29157 10.7296
55 0.27220 11.1617
56 0.25412 11.5861
57 0.23723 12.0029
58 0.22147 12.4126
59 0.20676 12.8152
60 0.19302 13.2110
61 0.18020 13.6003
62 0.16822 13.9833
63 0.15705 14.3601
64 0.14661 14.7310
65 0.13687 15.0962

On Edit: As I feared the formatting came out awful, I have tried to clean it up and I am adding the data below again in a comma separated format which can be pasted into a spreadsheet or cleaned up with a find/replace in a text editor or word processor.

Width,M,B
20,3.01922,-12.6635
21,2.81862,-11.5144
22,2.63136,-10.4188
23,2.45653,-9.3718
24,2.29332,-8.3695
25,2.14096,-7.4080
26,1.99871,-6.4843
27,1.86592,-5.5955
28,1.74195,-4.7389
29,1.62622,-3.9124
30,1.51817,-3.1140
31,1.41731,-2.3417
32,1.32314,-1.5940
33,1.23523,-0.8693
34,1.15317,-0.1662
35,1.07655,0.5166
36,1.00503,1.1800
37,0.93825,1.8253
38,0.87592,2.4534
39,0.81772,3.0652
40,0.76339,3.6615
41,0.71267,4.2430
42,0.66532,4.8106
43,0.62112,5.3648
44,0.57985,5.9062
45,0.54133,6.4355
46,0.50536,6.9532
47,0.47179,7.4597
48,0.44044,7.9555
49,0.41118,8.4412
50,0.38386,8.9170
51,0.35836,9.3834
52,0.33455,9.8407
53,0.31232,10.2893
54,0.29157,10.7296
55,0.27220,11.1617
56,0.25412,11.5861
57,0.23723,12.0029
58,0.22147,12.4126
59,0.20676,12.8152
60,0.19302,13.2110
61,0.18020,13.6003
62,0.16822,13.9833
63,0.15705,14.3601
64,0.14661,14.7310
65,0.13687,15.0962

khutch

By the way you can hear an interview with Jan Heine here. He discusses the article among other things. It is currently the third podcast on that page, titled It's Simple: Comfort = Speed.

Ken

Whiteknight

I ride a Trek Multitrack. Two weeks ago I pulled the Conti Touring Plus tires off the bike. Wore them out in 1900 miles. Bought a Bontrager Race Lite Hardcase in 700x32c and a Bontrager Race Lite Hard Case 700x30c. Local LBS did not have either as a pair.
Mounted them. I am running them at 80 psi. Lower rolling resistance compared to the conti's. Not a hard riding tire at 80 psi.

The Multitrack comes with budget price rims. The LBS suggested that I not run the new tires at the max 110 psi as I might see distortion of the rims as a result. I weight 180 pounds. The bike has a rack on the back. Medical kit, repair stuff and a few odds and ends. In this hot weather I am carrying 4 frozen 20 ounce drink bottles and one in the rack on the frame. So on most rides the weight is roughly 200 pounds.

So far, I really like the way the new tires ride and handle.

acidrock23

Thanks for the interesting discussion. I have always used the manufacturer's maximum pressure recommendation on my bikes figuring that it would make it go faster and faster= better? After reading this, I figured that I am too lazy to bust out a scale and actually measure it but I tried letting some air out of the front tire and it did feel quite a bit comfortable? I'd already gone 26 miles earlier today so I just spun around the block a couple of times and then let a bit more out and it the ride felt noticably smoother or I had a big dose of placebo? I'll probably hit another longer ride tomorrow and see how that goes to get more non-data but thanks again for the information!

timberline12k

I changed my tire order to Marathon Dureme 700 X 40C tires.

https://www.schwalbetires.com/marathon_dureme

https://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.

timberline12k

I put my Salsa Vaya order on hold. I realized the Salsa Fargo may be a better option.

meanwhile

The Dureme is an excellent choice. People have been getting spectacular results with these tyres.

Wanderer

I believe the Dureem is modelled after their very sucessful Marathon Supreme - which I absolutely love. I am riding Marathon Supremes, right now. Folding bead.

Currently, I'm running a 35 on the front, and a 40 on the rear, because I get the best handling and ride like this. They are rated at 85PSI, but I run both of them at 92, because that's where they roll the best, with my (me and bike) combined weight of about 240#.

Why the mix of sizes? Because I get noticeably improved handling, without sacrificing ride comfort with the 35 on the front. Also, noticeably improved comfort with the 40 on the rear, without sacrificing better handling. My bike came OEM with 45s - and these are better in every respect. Traction, under all conditions, has been SUPERB. For wide tires, these things are very fast, and spin up very easily.

I absolutely love them, and ride on pavement, aglime, packed gravel, packed dirt, and regular road gravel. I try to stay out of mud or soft sand.

I predict you will fall in love with the Dureems - so make sure you check back in with your findings.

Schwalbe makes really nice tires.

So far, they have been flawless.

timberline12k

Have you ever tried running your Supremes at air pressure levels suggested by the article above?

I am changing my bike order from the Vaya to the Fargo. The Fargo is available in an XXL frame and the Vaya is not. The Fargo will also allow me to run with 29" X 2" tires which I originally wanted anyway. I plan to try running lower air pressure; 55 PSI rear and 40 PSI front.

My two top choices are the Marathon Supreme and Marathon Dureme.

Do you think the Supreme is a faster tire than the Dureme?

Wanderer

I think you will find very little difference in the Supreme/Dureem.

Yes, I have tried many different pressures, and pressure combinations - and find that the 92# works the best for me - you may be different. I find the comfort comes from te sizing, as much as the pressure; but, fine tuning with pressure can improve even that.

If the Dureem was available when I bought my Supremes, I might have bought them - but--------- right now, I have 5 new ones in the basement(as I found a really good sale price a few times)

Also, consider putting a 35 on the front, with the 40 on the rear - you get better handling, and still get a good ride.

timberline12k

Phil,

Thanks for the information. I am in a holding pattern at the moment. I called my local bike shop to order a Salsa Fargo, and they were sold out for this model year. I have to wait until the 2011 models are released next month to order. Looks like Marathon Duremes are also on back order.

At least the weather has been good for my road bike.

timberline12k

I am not sure how to calculate proper air pressure. Could you provide a sample formula using the following variables?

29” X 2” Dureme Tires
Manufacturers suggested PSI: 30-70
Rider, Gear, and bike weight: 290 lbs.

I would like to stay within the manufacturer’s recommended pressure. Should I have 30 PSI in the front tire and 42 PSI in the rear tire?

My rims are on back order until the first week of October. The rest of the 2010 XXL Salsa Fargo build kit will be in the bike ship by the end of this month.

Wanderer

https://www.schwalbetires.com/tech_in...ation_pressure

Go here for a good reference - it even gives a formula for conversions to different weights - if this doesn't help, get back to me and I will see if I can help