3

The recommended tire pressure is when the tire's height drops by 15% when it's loaded because it makes the ride comfortable without increasing the tire's rolling resistance too much. Less bumpy rides mean the tires may even roll faster. Unfortunately, the recommended pressure may exceed the tire's minimum rating. Should we still follow the chart to get a 15% drop? Would handling be poor?

I use the 700c x 38c Schwalbe Marathon Greenguard tire which is rated at 50 to 85 PSI. Even though I keep its pressure between 50 and 60 PSI, the ride is still bumpy on neighbourhood streets, the edges of potholes, and expansion joints.

My weight is 140 lbs which means the front wheel's load should be less than 40 kg including the weight of my bike. Based on the chart, I should inflate my front wheel to less than 40 PSI.

Tire pressure for 15% tire drop for different tire widths

Chart from article in Bicycle Quarterly (pdf)

Móż
  • 23,532
  • 3
  • 71
  • 120
Brian
  • 1,043
  • 1
  • 14
  • 21
  • 1
    The short answer is "yes". Try it, see what happens. You might get snakebite punctures, but I suspect not. – Móż May 28 '16 at 22:25
  • IMO that chart is a bit on the low side – BSO rider May 28 '16 at 23:24
  • "Recommended pressure" ranges for tires are just that. You can go maybe 20% over or under the limits in most cases. The chief danger of underinflation is snakebite, so if you do not experience that problem you're probably OK. (But note that there is no way to eliminate the bumps entirely.) – Daniel R Hicks May 29 '16 at 02:48
  • When I inflated my front wheel to 40 PSI, it did drop by around 15 percent. I measured the change of height of my handle bar with and without loading my bike and compared it to the tire's height measured from the tire's edges to the rim's edge. I guess I will pump a bit higher than the chart's recommendations. – Brian May 29 '16 at 05:27
  • I can understand the upper limit but not the lower limit. Overinflation can burst the tire. How much the tire drops depend on its load and pressure. If a 700x38c tire has 10 lbs of load and is inflated to 10 PSI, I don't think it will pinch flat if it attempts to jump the curb.

    I weighed my front and rear wheels. Their loads were 80 lbs for the front and 125 lbs for the rear which means their recommended pressures were calculated to be 37 PSI and 55 PSI respectively.

     – Brian Jun 05 '16 at 04:33
  • 1
    I imagine upgrading to a suspension seat post would give you a better (and safer / less potentially damaging) result in ride comfort than would under-inflating your tires. – Jeremy Jameson Jun 20 '16 at 06:21
  • The 15% drop chart is just a reference, different tire will give slightly different drop results (some walls and bead are thicker ) , you should check it out yourself than assume the figures. The psi range from the tired is another indicator. – mootmoot Jul 06 '16 at 12:08
  • @CameraCyclist - Trust me. If you hit a curb with 37 psi in your 38mm front tire you definitely can get a snakebite. – Daniel R Hicks Jul 20 '16 at 11:33
  • You have to play around with it, but Schwalbe will recommend 55+ psi. – Batman Sep 08 '16 at 17:45

1 Answers1

2

You can try to lower the pressure further, but 2.8bar (40PSI) is already quite low for a 38mm trekking tire. Expect snakebites (pinch flats), increased rolling resistance and cracked sidewalls. There is no hard limit and if your rims are wide enough, handling shouldn’t suffer too much.

You could upgrade to wider tires if your rims, brakes and frame allow for it.

More flexible tires (e.g. cyclocross tires) would be a bit more comfortable and better at handling low pressure. However, they are generally less puncture resistant.

On a sidenote: A more relaxed and active riding style helps a lot. Try to avoid potholes or at least get out of the saddle!

Michael
  • 27,311
  • 1
  • 24
  • 86
  • 1
    With even 80 lbs on the front wheel (~60% of the total), 40psi means 2 square inches of contact area, or a deformation of about 1/4" on a 38mm tyre. I'm not doubting your experience, but I'm very curious to see photos and hear more about what happened when you did this. If it's just speculation, I'd like references and some evidence that you might be correct. Maths would be fine. – Móż Jun 20 '16 at 07:35
  • @Móż: Sharp edges (e.g. potholes, curbstones etc.) or braking “into” them will put much more pressure on the front tire. My rule-of-thumb test is pressing down the front wheel on a curbstone. If you can push through to the rim it’s too little pressure. – Michael Jun 20 '16 at 10:20
  • @Móż - I agree with your assessment, I ran my fully loaded touring bike with 700cx38 tires at 45 psi down rough dirt roads with pot holes with no pinch flat issues. I did however put my front wheel out of true due to a couple hard hits (50:50 split of about 225 lbs total load). – Rider_X Sep 08 '16 at 18:32