tire pressure and rolling resistance

[handlebar]

My father used to say the roughest riding vehicle on the road was an unloaded truck. Radpower says  that with a Radrunner, you should use the pressure on the sidewalls, which is 30 psi. I used that pressure because there was less rolling resistance than at 25 or lower. It rode like an unloaded truck.

I bought a Radmission partly for the 27.5 x 2" tires, which would give a smoother ride and lower rolling resistance. The sidewalls specified 40-65 psi. i tried 60.

The Radrunner is geared to be equivalent to a 76" wheel (before bicycles had chains). The Radmission has 86 gear inches, but I found it easier than the Radrunner to pedal without motor assistance. 

To compare, I coasted down a hill that drops about 16 feet in 100 yards. The Radrunner was faster. I recall reading that road cyclists often pump their tires too hard. That would minimize resistance on a perfectly smooth surface, but pavement has bumps. On a freshly paved and rolled surface, bumps will be small. Around here, where pavement has sat in the sun and frost more than 50 years, bumps are likely to be higher than an inch. Hitting the upside of a bump slows you down.

Dunlop devised pneumatic tires for his mother's wheelchair. He discovered that they would roll faster down a slope. Tire rubber isn't bouncy, but air is. By pressing a bump on the downside, air pressure can return some of the energy you lost pressing against the upside. Raising it can make the tire too stiff to apply much pressure on the downside of a bump, so you regain less energy.

On fairly smooth pavement at 30 psi, the Radrunner has a fairly narrow contact patch, somewhat like the Radmission. With half the pressure, the Radrunner was obviously regaining more energy on bumps. If I'd found the Radmission easier to pedal, perhaps it's because I was pedaling without assistance on smoother surfaces and at lower speeds, where there was less energy to regain from bumps and resistance was mostly from flexing rubber.

Radmission tires recommend 40 to 65 psi. When I lowered the pressure to 50, it was as fast as the Radrunner on the hill, and I didn't notice an increase in resistance where I pedal without assistance. The most efficient pressure depends on how bumpy the surface is. I may try 45 or 40, but first I need to refine my coasting test.

[Altema]

Good observations, and I usually try to find a balance between comfort and efficiency.

[Eric7]

I agree with you all.  I think since some Rad bikes are 60 to 70 pounds, and since we have a battery, we might as well adjust for some comfort.  If we lived in a situation where we need to extract every ounce of energy out of the battery, or we are constantly caught with a dead battery on a ride, we would have gotten some other bike or an extra battery.  With a 60/70 pound bike, we might as well be comfortable vs efficient. The option has been decided when we got the bike.

[rjp]

Comfort and ease of operation is why I got an ebike. If riding efficiently, to be able to get to your destination is necessary. I'd make sure I have a big enough battery to get there easily to make the ride more enjoyable.  I hate range anxiety!  I also like a smooth ride, so front and seat suspension is key. Tire pressure and rolling resistance goes back to the bigger battery again. But sometimes you need to lower tire pressure to float, like in soft sand and then you use battery a lot faster. It all depends where your riding and if it's up or down hill and with or into the wind. Ride on everyone!!!        

[handlebar]

With a 60/70 pound bike, we might as well be comfortable vs efficient. The option has been decided when we got the bike.

That's what I had in mind when I bought a Radrunner. In fact it was terribly uncomfortable. The worst part was not the bumps but the instability of the riding position with the saddle high for pedaling. I got tennis elbow maintaining control with my right hand when I lifted my left hand to signal a turn. I got a layback seat to make my relationship to the bars more stable. I didn't realize that changing my relationship to the pedals would do even more for stability.

With a better position, I discovered that I loved pedaling, but I still wanted the electric motor. The weight difference between an ebike and an equally durable unpowered bike adds about 10% to my gross. Real-world experiments have shown that bicycle weight makes little difference in travel time. However, a motor will help me maintain speed on hills and in headwinds.  That's convenient. It will help me accelerate to 20 on a road with traffic.

At 65 pounds, the usual Schwinns of the 1950s, marketed as gifts for grade-school children, weighed as much as an ebike these days. They were popular because they looked comfortable. In England, the Raleigh Sport was considered a utility bike, but Americans called it a racer because it was so much faster than a Schwinn. With narrower, higher-pressure tires and a forward-leaning riding position, it looked uncomfortable, but riders found that it was far more comfortable than a Schwinn.

The Raleigh Roadster looked like the Sport but weighed nearly twice as much. For practical purposes, it was as fast, and it was more comfortable. A heavy bike doesn't have to be inefficient for pedaling.

A better riding position made the Radrunner comfortable except for my hands. At 60 psi, the Radmission is easier on my hands than the Radrunner at 20 psi.

[handlebar]

Comfort and ease of operation is why I got an ebike. If riding efficiently, to be able to get to your destination is necessary. I'd make sure I have a big enough battery to get there easily to make the ride more enjoyable.  I hate range anxiety!  I also like a smooth ride, so front and seat suspension is key. Tire pressure and rolling resistance goes back to the bigger battery again. But sometimes you need to lower tire pressure to float, like in soft sand and then you use battery a lot faster. It all depends where your riding and if it's up or down hill and with or into the wind. Ride on everyone!!!        

The EBR video review, hosted by the Radpower founder, sold me. It was fraudulent. The editor said immediately that he had reduced the tire pressure for a more comfortable ride, but he said what the correct pressure was or what pressure he was using. That should have been a red flag. He said the battery location made the bike balanced and the fat tires made it stable. He seemed to be talking through his hat. He was tall, and I discovered that the bike was terribly unstable with the seat raised as he had it.

The editor pointed to a sign warning that there were 13-degree hills ahead. (That's 22%.) He said he would use the throttle only because when he was younger, sports like swimming had ruined his knees. Judging by the seat position of the Radrunner, I thought it was pedaling bikes like that, that had ruined his knees. Lance Armstrong has said that exercises that damage knees can strengthen knees if you don't flex them so far under strain.

With throttle alone, he sailed past two Rad employees using pedal assist. On similarly steep hills, the CEO gave him a ride on the back of a Radrunner with the seat down, where he didn't pedal. Way overloaded and on steep hills, the bike was brisk. He said he had 18 psi. In other words, the bike had plenty of power and the tires could be run soft for comfort.

I bought one, and the manual said it was unsafe to run less than the pressure on the sidewalls. It was deceptive for the editor and the CEO to reduce pressure without saying it was unsafe. For months, I couldn't find that pressure on the sidewalls because white markings obscured it.

I found that it would barely climb a 10% grade. Instead of the advertised 80 newton meters, I was getting 53. With a stopwatch, I found that the terminal speed on a 6.1% upgrade was 11.5 mph. The Radrunner was advertised at 750 watts, but I was getting 503 on the pavement, in addition to some for rolling resistance and air drag.

I revisited the video . When the editor passed the pedalers, their cadence showed they were going 13 mph. He must have been going at least 15. He was passing a house whose front yard was in the same plane as the road. He must have been putting 1000 watts on the pavement in addition to something for air and tire resistance. The CEO was getting similar performance with twice the gross weight, calling for 2000 watts on the pavement. The bikes they were advertising were not the bikes they were selling.

When I asked Customer Service about my slow climb, the reply was that I have to pedal. Others have complained, and Radpower has come out with a disclaimer that you may see less than advertised power because of safety laws. Still talking through their hat. In fact, 500 watts output is reasonable. Like many ebike brands, Radpower advetises input power, and 67% efficiency is pretty good.

My Bolton controller indicates 1500 watts' input on that hill and will accelerate the bike to 20. Even then, pedaling makes a difference in performance, increasing my speed on the hill to 24. Tire pressure also affects performance even with 1500 watts on tap.

I hate the noise and handling of those tires at 15 psi. 30 is much better of both counts. The controller makes the motor noisy, which is a reason I prefer to keep the motor off as much as possible.

With the standard seating position, I, too, had a suspension seat post. The ride was still so rough that I ran only 20 psi. I'm much more comfortable at 30 psi with the new seating position than I was at 20 psi with the suspension post.

Two youtube cyclists tested a Radmission on a 9% grade 600 yards long. Throttle alone got the 170-pound rider to the top, but not the 220-pound one. With pedal  assist, both made it at about 10 mph. The fact that the motor didn't make the lighter rider faster, impresses me. If I want to be sure to get somewhere on an ebike, pedaling ability may matter.

[handlebar]

Comfort and ease of operation is why I got an ebike. 

My discomfort in riding my Radmission at 50 or 60 psi was the vibrating mirror. I use the same old-school kind I've used more than 60 years because I need a clear view behind to be comfortable.

I dropped the pressure to 45 and the image is clear. Bumps can start vertical oscillations, and at 45 psi the rubber flexes enough to damp them quickly. I may be losing energy, but it's worth it to be comfortable. Besides, the easiest way to operate a bicycle is not to get hit by a car.