How low can I go on tire pressure on my tires?
 

I use Schwalbe Marathon HS420 Green Guard tires - 44-584.

They measure 650b x 40.5mm wide in real life on the rim and inflated to 55psi.
Lowest recommended on sidewall is 50psi.

But can I go lower, and if so, how much is still safe?

Going lower is possible if:

1. You are a light rider. A 130 pound rider can get away with lower pressure than a 230 pound rider
2. You ride "gently", i.e. you don't go over bumps and potholes too fast and don't jump curbs, etc.

The real lower pressure limit is the point where you start getting pinch flats or you go so low the tire shifts on the rim and tears out the valve stem.

just as a reference...

at 150lbs, good roads, 80PSI and 20mm 700c tires i have no problems with pinch flats.

and possibly a little more esoteric...

i use a dual wheeled pneumatic 11inch tube and tire wheelbarrow to haul a two days worth or horse manure about 200 yds per day. the tires have been flat for about a year now. it's only a problem during the winter here in California. it rains and causes the manure to weigh two to three times a much. then it's a beetch...:)

You can go as low as you want, until you start getting pinch flats.

FWIW I'll run my 34mm cross tires at 20psi for racing, but those are tubulars so not a valid comparison. I also weigh 130lbs. For a 40mm clincher I'd guess I could easily go down to 35psi.

As said before, it depends on your weight and your riding style, but if you're not too heavy and ride on good paved roads, probably about 30-35 psi.

I'm 145, ride on fair to mediocre roads and would have pinch flats all the time at that pressure, even with 700-23 tires. You must be VERY gentle on your bike.

With this calculator it's simple to figure it out, go to the second heading that says bike + rider then enter your total ready to ride clothed weight and your bikes ready to ride weight, then select the F/R weight distrubution (I like the 45/55 choice), then select your tire size for both front and rear (37 is the largest size), once that's done it automatically calculates and puts the recommended PSI in the boxe; see: http://www.dorkypantsr.us/bike-tire-...alculator.html Once you get that figure I would substract about 5 to 8 pounds in both tires since your tire size is larger then the largest provided.

i hear ya.

i might have an advantage in that i ride solo and don't have to "hold a line" and take it on the nose with potholes and such...

for really low pressure high volume tires use sewup glue to glue one tire bead to the rim.

low pressure tires creep around the rim, and shear off valve stems..

I can run 45 PSI in my 26X2.1 mountain bike tires (closest size I have to yours). If I am going to ride where there are plenty of tree roots or stones, or if the terrain is unknown to me, I'll inflate to 65 PSI.

Brad

Here's a guide to optimal pressure, based on weight and tire width, though there's room lower if you're riding pothole free roads, or packed dirt, or mud free of large rocks.

Your speed and riding style are also factors. At lower pressures a 2" step, like driveway entrance can be climbs slowly, but at higher speed the rim will dent before the tire lifts it clear. Low pressure tires are also very flexy in corners, so watch yourself on twisted descents.

Th real question is how low do you want to go, and why.

Low tire pressure open you up to pinch flats, and IMO shorten the tire lifetime due to excessive flex.

I find this odd because I weigh 164 and have ridden at 65 psi on a 23c rear tire due to a crappy pump I bought and rode the bike about 8 miles home on city streets without pinch flatting.

Too-low pressures can lead to increased pinch flats, but pressures used to achieve 15% tire drop should be high enough to avoid those flats.

Correct, and that is what that Michelin tire pressure calculator is doing, achieving the 15% drop.

I've done the same thing when I had to change a flat on the road and didn't want to spend 20 minutes with the mini-pump. I got away with it too but I rode VERY carefully and took great pains to avoid as many bumps and potholes as I could. I would not trust anywhere near that pressure for routine riding.

I am interested in lowering the pressure to achieve the 15% drop talked about in that Bicycle Quarterly article. But I have two problems:

1. There is no drop provided for a 40 or 42mm tire on the chart.
2. If I go below recommended low tire pressure listed on my tire, will it damage the tire to ride it that way? The low end is 50psii on my tires, and looks like I will be in the 40's from what I see on the chart.

1. this isn't rocket science, so you can interpolate and get a good enough approximation. If you look at the chart you'll see that changing from 32-37mm drops the pressure by about 20psi at the right edge (70kg). The rate pf pressure change declines as tires get larger, so taking a crapshoot guess, figure that from 37-42mm, the pressure will drop by 17-18psi. Plot that on the graph, and draw the line to see other values.

2- the minimum pressure value is only a marketing guideline with no basis in reality. As long as your riding with pressure drops close to 15-20% your tires won't be affected.

Just use some common sense, see how your tires ride, and be attuned to bottoming (rim shock) on potholes, and you'll be fine.

My commute bike presently has a slow leak in the rear tube which I am too lazy/busy to attend to, so I pump it every day. 700C. 25 mm. If I don't pump for 4 days, I find myself riding around 60 psi, where the rear gets squirrelly on curves. I can run as low as 80 psi without alarm, but normally prefer 100-120 psi. 185 lb rider. I really gotta fix that leak.

If you feel the need to lower tire pressure below the minimum to enhance comfort, you have the wrong size for your weight, riding style, and riding conditions.

You should consider a tubeless setup. With roughly the same width tires you mention, running tubeless, I run about 28 psi rear, 26 psi front.....especially for areas where there are lots of roots and rocks, those are the situations where you need low pressure the most. :)

I have considered tubeless rims when these are due for replacement. :)

Brad

Rim width?

I've mentioned this before about these graphs and calculators-in my experience and opinion, the suggested pressures are too low.
example- for 28mm tires, at my weight 140, on a 30lb bike, lets say 35lb with bottles, repair stuff (lets round up to 180lbs) it suggests 79psi rear and 50psi front.
Maybe in a perfect environment (marble road, not a bump in sight) this would work, but as mentioned, you bring in potholes or whatever and this suggestion is just too low.

More importantly, I really feel these graphs do not take into account going around corners fast, there is no way I want to be going around a fast corner with 50psi in the front, the tire will move around and it if the front is loaded braking into a downhill corner, and then you hit a bump, you are going to smack the rim.
And this doesnt even consider the "feel" of a tire this low. Again, from my experience, this low a pressure is going to feel very squirrely in corners and not a confident feeling thats for sure.

last point, if you ride a bike with a rack and panniers, from one day to another you can have 0lbs on the rack, or 10lbs the next, or 20, so running a psi that is (arguably even too low imo) right on the limit of these calculators, you add some weight, or lose some air over days, and you risk damaging a rim and/or having to deal with a flat--that again, imo is not worth the small increase in comfort.

re increase in comfort, my 28s can be pumped to 110 or more, but there is a marked increase in comfort in running them at lets say 90 front and 100 rear, but 50 front? thats a huge decrease.