tire size vs effort?

[parajared]

If you were forced go on a long distance tour and you had to choose between a bike with cartoonishly small tires and a bike with cartoonishly large tires, which bike would you choose assuming these are magic bicycles that both weigh the same and have gearboxes that let you put it in any gear you want and you are traveling down a magical road that doesn’t have potholes and bumps and stuff for the small wheeled bike to catch in.

Which bike would take less effort to peddle and why?

 

[dogman dan]

My unscientific guess would be that a lighter tire would take less effort to keep spinning. Ever thrown pots on a kickwheel? That cement wheel spins a long time, but takes a shitload of effort to keep spinning all day.

I

 

[cal3thousand]

It would take less effort to get rolling on the smaller tires, but you will also roll longer on the larger tires once up to speed. It boils down to moment of inertia on the larger tire being larger.

 

[teklektik]

This is an interesting piece on rolling resistance and tire width by Schwalbe.

This isn't exactly what you asked, but supports some remarks here and puts other issues such as inflation pressure on the table.

 

[spinningmagnets]

I suspect that when you put the same load on a larger footprint of tire, each square inch of tread that is in contact with the pavement has less pressure on it. I would guess that doing so would reduce the chance of a nail/screw being pushed through the tread and producing a flat?

Two tires on 50-lb E-bike and 200-lb rider, so 250-lb, divided by two tires is 125-lbs on each tire. If you have a small diameter tire at high pressure, maybe one square inch is in contact, so 125 lbs is pushing it down...4-inch fattie at 20-PSI? maybe two or three times as many square inches? (who knows precisely, but it is more) so maybe 40 to 80 pounds of force per square inch trying to give you a flat.

Of course fat tires have been proven on E-fatbikes to suck more amps in order to do the same job, so...

 

[Drunkskunk]

Of course fat tires have been proven on E-fatbikes to suck more amps in order to do the same job, so...

I agree. Though air pressure comes into that equation as well. Your tire's pressure on the ground can't exceed the air pressure in the tire. if you have 10 pounds per square inch in the tire, then the tire can only push on the ground with a force of 10 pounds per square inch maximum.
If you over inflate a fat tire to 50 pounds, then roll over a big sharp rock, the tire will only be able deform to a maximum of 50psi pressing on that sharp rock, (Localized pressure on the point is higher) but that might be enough to get a puncture.

As for that last statement though, I've found that not to be true. My monster fat bike is on the extreme end of size and weight, and even with it's current 4.8" tires, I've noticed no real difference in power consumption compared to any of my other skinny tire bikes over time.
But in the short term, I've noticed in grass, mud, and sand, the fat tires roll much better and use significantly less power than one of my thinner tired bikes.

 

[gogo]

There are many factors. Inertia favors lighter/smaller in proportion to frequency of start and stop. Heat produced by carcass deformation favors wide vs. narrow and bigger diameter vs. smaller. Wind resistance favors narrow vs. wide and short vs. tall.

 

[friendly1uk]

I freewheel downhill past lycra's pedaling. The difference is huge. Like 50% more energy to keep a standard road bike moving at 20kmh that my 2.15" Apples. As shown in the graph posted above.

There is no way I would cruise about on a racer. They are for sudden sprints and have aerodynamic advantages that you won't see below ~20mph. Apart from that they are uncomfortable and weak.

Interesting thoughts on tyre pressure and punctures. That wider tyre with half the pressure covers twice the catchment area. Which appears to null the situation.


I would take the bigger tyres and run them softer to create the same drag but offer more comfort. Less shock loading for my legs would aid me approach my limits with greater control. I would be more efficient. The losses seen in getting a big wheel moving are all returned if you don't use the brakes to scrub it away. Sometimes you must. You can't always coast to a steady stop. It's the stop/start frequency that would really define my choice, but touring sounds like 14mph, all day long.

 

[Punx0r]

Notice how cars designed to achieve high fuel efficiency have skinny tyres?

 

[dogman dan]

The more I think about the question asked here, the more I get confused. There isn't going to be a magic wheel that's fat and weighs the same as a race bike wheel. Trying to think about that premise is making my head hurt.

What works best just depends on the conditions. A quick test with two similar CA equipped bikes shows how a wide tire rolls better in soft dirt, sand, mud. The deeper the rut the more power it took to dig that deeper rut. Floating over the top uses lots less energy. But if it's bottomless sugar, all tires short of ATV tires may sink in and suck amps.

But to look at rolling resistance, go back to horse power. Back in the day, a wagon wheel was 3-4 feet tall. Two reasons, one being rolling resistance. But it was rolling resistance over rocks, logs, stumps. Not smooth road. Also, the tire was steel, so narrow was cheaper, and weighed less. It would cut to the bottom of a deep mud hole, and leave the axle above the ground. So it needed to be tall.

It was still horse and buggy days when the safety bike came out, and roads sucked. Wheels were big for good reason, as they still are for off road bicycles. But add enough power, and the need for that big wheel diminishes. Motorcycles and quads don't require a 29er wheel to handle rough stuff.

 

[parajared]

In my mind a rollerblade wheel bicycle would take the same effort as a penny-farthing wheel bike although I'm not sure why.

 

[dogman dan]

Think of the curve of a wheel as like the slope of a hill. The smaller the wheel the steeper the slope you climb to roll it.

Crew of guys moving landscape rock all day. 4 wheelbarrows. Big fight over who gets the barrow with the biggest tire. Every time. The difference is instantly felt with 200 pounds in that barrow. By the end of the day pushing about 10 tons of rock, you are really glad you fought for that better barrow.