Range vs. Tire Pressure

[EbikeTestLab]

Good Morning EBR folks! I wanted to test the accuracy and repeatability of my road load testing and have some fun with tire pressures. Therefore, I setup my Magnum Metro+ just like the last test, but I aired up the tires to 90 psi and ran the 20 mile per hour road load measurement test. I then repeated the same 20 mph road load test at 70 psi, 50 psi, and 30 psi. The results were interesting. From my previous testing, the power needed to maintain 20 mph at the wheels was 323 watts with the tires at 80 psi. At 90 psi, the power needed came out to 315 watts (that's more repeatable than I thought it would be). Here's the full chart of power vs. tire pressure:

The power needed to maintain 20 mph increases dramatically from 70 psi to 50 psi, and even more power is needed at 30 psi. How does this affect range?

A drop from 90 psi to 70 psi equals a 5% penalty in range. From 90 to 50 psi yields a 12.7% penalty, and going down to 30 psi increases that penalty to 18%.

I like to keep my tires at 80 psi (the sidewall lists a range of 50 to 85 psi) but that does make for a firmer ride than say 50 psi. However, my commute is 35 miles and I need all the range I can get to not run the battery too low. Find what works for you and your ride!

 

[drewberz]

do you have any data on weight, road surface, tire print? would be interesting to see some more factors integrated. Paris Roubaix comes to mind (eg tire deflection on uneven surfaces)

 

[Deleted member 4210]

The crazy thing about all of this, is if you simply ride a few mph slower, to keep wind resistance down,since the resistance is non linear, it would make a larger difference in range than 10 psi of tire pressure change alone will. I.e., If you ride at 12 mph, instead of 16 mph, you'll see more than a 7 % difference in range. Try it sometime.

 

[EbikeTestLab]

do you have any data on weight, road surface, tire print? would be interesting to see some more factors integrated. Paris Roubaix comes to mind (eg tire deflection on uneven surfaces)

Good points! The test rider plus instrumentation weigh 200 lbs, I adjust this weight with by adding water bottles to a pack pack so it’s right at 200 lbs every time. The bike weighs 59 lbs. The test surface is smooth asphalt. The tires are Schwalbe Marathons 700x38c. The bike and tires have roughly 2000 miles on them.

Different tire types and sizes will certainly give different results. However, the Marathons are pretty common among city commuter ebikes.

I’ll try to repeat the testing with my daughter’s Faraday, which has 26” x 1.75” Continental Contacts. It will be interesting to see the difference. A fat tire ebike would show the most drastic changes, I think...

 

[EbikeTestLab]

The crazy thing about all of this, is if you simply ride a few mph slower, to keep wind resistance down,since the resistance is non linear, it would make a larger difference in range than 10 psi of tire pressure change alone will. I.e., If you ride at 12 mph, instead of 16 mph, you'll see more than a 7 % difference in range. Try it sometime.

Absolutely! I’m usually in a hurry though! The chart attached is from my full test of the Magnum Metro+:



It shows the non linear increase in power needed as speed increases. Just like with cars, the aerodynamic drag increases with the square of speed.

 

[jim6b]

What "rolling resistance" are you measuring?

The resistance of a rough surface?

The resistance of tire sidewall flexing?

 

[EbikeTestLab]

What "rolling resistance" are you measuring?

The resistance of a rough surface?

The resistance of tire sidewall flexing?

I’m not directly measuring rolling resistance. The road load test measures how much power is necessary to keep the ebike and rider at a steady speed, in this case it was 20 mph. However, by varying the tire pressure, the rolling resistance should be the only change. In essence, I have determined how increasing the rolling resistance affects road load and range. A perfect test would generate a straight line of road load vs tire pressure since the response should be linear.

 

[GregGritton]

A perfect test would generate a straight line of road load vs tire pressure since the response should be linear.

The response can't be linear or with enough pressure the rolling resistance would go negative.

 

[EbikeTestLab]

The response can't be linear or with enough pressure the rolling resistance would go negative.

I sort of agree but it can only get to zero. If changing the tire pressure only affects rolling resistance, then only the “B” term of the three term equation that defines road load would be changing (A+B time speed + C times speed squares). By changing only the b term at a given speed, the response should be linear until the b term becomes zero, which physically isn’t possible. There will always be some base load (A), some rolling resistance (B time speed), and some aerodynamic resistance (C times speed squared).

In reality, road load is more complex but the automotive companies and the government (EPA) simplify it to three terms. Changing the tire pressure actually affects both the B and C terms but most of the change is to the B term.