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Thursday, June 2, 2011

Optimal Tire Pressure For Cycling

THE QUICK & EASY METHOD:  TIRE PRESSURE CALCULATOR & CHART
If you don't want to weigh and calculate each tire pressure, check out the tire pressure calculator below.  It was created around Frank Berto's work that experimentally determined the tire pressure that produced the best grip.  Compared to the tire pressures I calculated, the calculator was 2-5 psi off from my calculated front and rear tire pressure.  That's a fairly small error considering that the longer method is still not exact- you still need to experiment with the pressures.  I recommend adjusting by 1 psi increments to fine tune your tire pressure.

http://www.dorkypantsr.us/bike-tire-pressure-calculator.html

Michelin also made their own chart.  Notice how tire pressure depends on both weight and tire size.


THE LONGER METHOD:
After reading an article called "All About Tire Inflation" by Frank Berto, I stopped habitually inflating my front and rear tire to 120 psi.  Based on a discussion with other bicycle tire manufacturers, they determined that a ~15% tire drop produces optimal tire contact for cornering.  Due to different tire construction and materials, this percentage can vary by +/- 2%.

Note: The article also claims that a 15% drop produces optimal comfort and puncture resistance, but since the tests they performed never measured these variables, I will report my experience based on real riding conditions.  I would also like to know what others think so that we can validate this claim.

A WORD ABOUT TIRE PRESSURE GAUGES:
Based on personal experience with seven different motorized pumps and two floor pumps, I learned that the gauge that comes built-in is never accurate.  I have seen pumps give numbers that were 20 psi off of what the pressure actually was.  Floor pumps were more accurate (+/- 5 to 10 psi), but still not good enough.  To get the most out of the recommended tire pressures, use a dedicated tire pressure gauge to calibrate tire pressures.  Believe it or not, but a 5 psi difference can make a big difference on cornering.

For convenience and reliability purposes, I recommend using a dial gauge instead of a digital gauge.  I use a gauge that is designed for presta valves, but there are other options out there.  If you already have an automotive tire gauge, there are $1 valve adapters which can convert presta valves into schrader valves.  This is a better option for those who already have a tire pressure gauge.

CALCULATE TIRE PRESSURE HOW-TO:
If you hate measuring and calculating percentages, try this bicycle tire pressure calculator.  The calculator tends to give slightly lower numbers than what you'll get from manually calculating it.  When I used the single wheel calculator, it was 2 psi lower than my calculated number.  The total body weight calculator was 5 psi lower than my calculated value.  If you want to calculate your tire pressures the long way, try Frank Berto's method below:

  1. Lift bike up, stand still on weighing scale and record total weight (bike + rider).
  2. Measure weight distribution (rider on bike) with weighing scale.
    • Sit on the bike with both hands on the flats, hoods or drops.
    • Level the bike- raise the rear wheel when measuring the weight on the front wheel.
    • Record front wheel weight
    • Record rear wheel weight
  3. Check error in measuring the front and rear wheel.
    • Add front wheel weight and rear wheel weight to see if they match the total weight measured in step one.
  4. Calculate percentage of weight distribution
    • Divide front wheel weight by total weight
    • Divide rear wheel weight by total weight
  5. Find difference between weight percentages.
    • Subtract big percentage by smaller percentage.  55% - 45% = 10%
  6. Use chart to determine optimal pressure
    1. Subtract 10% for front wheel.
    2. Add 10% for rear wheel


EXAMPLE (MY NUMBERS):
I measured my total weight (bike + rider) which was 169.4 pounds.  Based on my numbers on the drops, I held 44.6% of my weight on the front tire and 55.4% of my weight on the rear tire- a 10% difference. Based on my total weight, Frank Berto's chart says that my optimal pressure was ~89 psi.  I subtracted 10% to get the front tire pressure (~80 psi) and added 10% to get the rear tire pressure ~98 psi.

MY EXPERIENCE (ROLLING RESISTANCE + COMFORT):
Compared to running 120 psi in the front and rear tire, I surprisingly didn't notice any increase or decrease in rolling resistance.  I was able to maintain my usual average pace at the same effort, and during climbs, it felt similarly easy and possibly easier to get up the hills.  The new, lower pressures felt exactly the same with the exception that bumps and small potholes didn't feel nearly as harsh.

I thought it was very weird to feel the same low rolling resistance as the high pressures since I was running 40 psi lower in the front tire and 22 psi lower in the rear tire.  My guess is that after exceeding a threshold pressure, rolling resistance reduces insignificantly as tire pressure increases.  For example:

  • 10 psi would feel extremely grippy compared to 30 psi
  • 30 psi would feel a lot more grippy compared to 50 psi
  • 50 psi would feel more grippy compared to 70 psi
  • 70 psi would feel similar to 90 psi
  • 90 psi would feel similar to 110 psi

This is just a rough example, I want to see a real study which can test my hypothesis.  The threshold in this example would be around 70 psi.  Anything above 70 psi would feel about the same with regards to rolling resistance.  This could explain why the lower pressures felt just as easy as higher pressures, but more comfortable at the same time.  It seems that above 70 psi, comfort is the only variable which is greatly affected by increased pressure.

MY EXPERIENCE (PUNCTURE RESISTANCE):
I've been riding on these new pressures for several months now and never had a flat despite running over glass, small potholes and very bumpy roads.  Although the pressures were lower, it didn't feel like it was so low that any bump or pot hole could cause a pinch flat- the ride was still fairly harsh.

Compared to 120 psi, I actually experienced more flats with the higher pressures than the new lower pressures.  I used to get flats from riding on very bumpy roads and I think it was due to the large spikes in pressure within the tube.  Since the tube was already inflated to a very high pressure, the impacts from rough roads continuously spiked the pressure beyond the tube's elastic limits.  By keeping the pressure lower, I think the tube was not spiking as high above its capabilities which might explain why I haven't experienced as many flats.

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