# Bicycle Mechanics - What is rolling resistance ?

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View Full Version : What is rolling resistance ?

slouw
12-09-00, 03:25 AM
Could anyone point me to an academic explaination of this ?

Buddy Hayden
12-13-00, 04:43 AM
Think of it like this :a solid steel ball bearing rolling on a steel surface has an extremely minute contact point no matter how big the ball ie:a very low amount of rolling resistance ! , then on the other hand a ball of soft clay would have a high rolling resistance...so a 700cX23-110 psi tire low resistance, and a 26"X2.10 35psi = a high rolling resistance comparatively !!

Jean Beetham Smith
12-13-00, 07:08 PM
Slouw
You may want to look at these sites:
Sheldon Brown's recent article on tires
Analytical Cycling
San Francisco Exploratorium's "The Science of Cycling"
You may also want to search for back issue articles in Bicycling. Hope these give you the answers you are looking for. Happy surfing.

pat5319
12-13-00, 10:01 PM
Friction

Overcoming inertia

ramius
09-13-05, 09:15 PM
I know this question may be impossible to answer, but I figure if anyone can tell me, you folks can.

I'm trying to 'keep track' of calories burned, etc. with Cyclestats program on my PC. One of the required fields for figuring this is 'rolling resistance.' It has several radio buttons for road tires, Mtn bike tires, etc., and also a window to enter the factor manually. I ride a Trek 7000 with 26 x 1.95 tires, so I hit that button, fine, it shows 0.012 rolling resistance factor. Great. I ride a rail trail 50+ miles long, with a lot of different surfaces depending where I ride. On good days, hard pack. Fair days, loose gravel and cinders. But yesterday I hit the mother load of crappy trail- it felt like beach sand, and in places I sank up nearly to the rims. About half of my 46 mile ride was in this stuff, and when I got done I felt like I'd ridden 70 miles.

So here is the question, Physics gurus:

Is there any good, simple way to 'estimate' the rolling resistance, or increase of same, to make my calculations a bit more accurate? I roughly doubled the number for yesterday, just to see what effect it had on my stats, and it was huge. I don't want to artificially inflate my numbers, but I know the standard number would be way too low for yesterday.

Any thoughts anyone has will be greatly appreciated.

:D

MudPie
09-13-05, 09:44 PM
There's a book called "Bicycling Science" that offered scientific and technical approaches to bicycles. I bought a copy back in the late 80's, and there may've been updates. Sorry, I forgot the author, but somehow, M.I.T. figured into the book somehow (like the author was associated with MIT, the research came from MIT labs.

DannoXYZ
09-13-05, 10:52 PM
I So here is the question, Physics gurus:

Is there any good, simple way to 'estimate' the rolling resistance, or increase of same, to make my calculations a bit more accurate? I roughly doubled the number for yesterday, just to see what effect it had on my stats, and it was huge. I don't want to artificially inflate my numbers, but I know the standard number would be way too low for yesterday.

Forget it. The energy required to overcome rolling resistance in infinitesimally small. The amounts would be bearing-friction, chain-friction and tyre's rolling-resistance on the road. Combine them all at 20mph and they'd make up less than 5% of the energy you're expending. At 40mph, they'd make up less than 2% of the energy-output.

By far and large, the most significant single factor is air-drag (wind resistance). Whereas rolling-resistance goes up fairly linearly with speed (^1), and bearing friction actually goes down somewhat with speed, depending upon type of bearing and lub used, wind-resistance goes up by the square function of speed (^2). And the power-required to over come that drag goes up by the cube power of speed (^3). So in order to go from 20mph to 40mph, you have to put out 8x the power!

If you want to count calories, it's easiest by datalogging speed vs. time. Record terrain data with GPS and topo maps to cancel out elevation changes. Then integrate velocity vs. time to find total drag. Plug into power equation and you'd get power required. Work through gearing used at each part of the course to calculate pedal-force on the pedal for each revolution and you'd get a total amount of work done....

Bicycle & Rider Drag Coefficients (http://www.users.globalnet.co.uk/~hadland/lafford2.htm)

bostontrevor
09-14-05, 03:37 PM
There is an updated 3rd edition of Bicycling Science, published by the MIT Press, a publisher of academic books and journals (which currently employs my dear wife as an assistant editor).

There's not a lot known about what causes rolling resistance. It's now commonly believed to be largely a function of sidewall deformation, but the impact of various environmental influences on that resistance is very little studied. So we know what it is, but we can't predict it.

genericbikedude
09-14-05, 03:50 PM
Your not wrong, but if the poster was riding in beach sand, then rolling resistance (the amount of the tire contacting solid matter) would be quite high. Also, given that very few people can go very fast in this type of environment, the wind resistance would be quite a smaller factor.

Forget it. The energy required to overcome rolling resistance in infinitesimally small. The amounts would be bearing-friction, chain-friction and tyre's rolling-resistance on the road. Combine them all at 20mph and they'd make up less than 5% of the energy you're expending. At 40mph, they'd make up less than 2% of the energy-output.

By far and large, the most significant single factor is air-drag (wind resistance). Whereas rolling-resistance goes up fairly linearly with speed (^1), and bearing friction actually goes down somewhat with speed, depending upon type of bearing and lub used, wind-resistance goes up by the square function of speed (^2). And the power-required to over come that drag goes up by the cube power of speed (^3). So in order to go from 20mph to 40mph, you have to put out 8x the power!

If you want to count calories, it's easiest by datalogging speed vs. time. Record terrain data with GPS and topo maps to cancel out elevation changes. Then integrate velocity vs. time to find total drag. Plug into power equation and you'd get power required. Work through gearing used at each part of the course to calculate pedal-force on the pedal for each revolution and you'd get a total amount of work done....

Bicycle & Rider Drag Coefficients (http://www.users.globalnet.co.uk/~hadland/lafford2.htm)

ramius
09-14-05, 03:56 PM
Thanks for the information, which is obviously well thought out and good, but perhaps I did not state clearly enough what the problem was: the difference between a smooth surface and a really horrendous one. I can easily see that wind drag would be by far the biggest factor riding on pavement, hard pack, or a similar roadway. But I think it just as obvious that riding, say, on a sandy beach with tires sinking up in the sand or through deep mud would require a great deal more energy even at a speed so low as to make wind drag minimal. I was just wondering if anyone had a way of quantifying it easily- which I know would be difficult, if not impossible, to do.

Was thinking of it more as an interesting mind exercise than expecting to be able to come up with perfect numbers, although it would be nice to get those as close as possible, also.

Checking out the links offered, thanks to all for the replies.

R.

genericbikedude
09-14-05, 04:05 PM
Thanks for the information, which is obviously well thought out and good, but perhaps I did not state clearly enough what the problem was: the difference between a smooth surface and a really horrendous one. I can easily see that wind drag would be by far the biggest factor riding on pavement, hard pack, or a similar roadway. But I think it just as obvious that riding, say, on a sandy beach with tires sinking up in the sand or through deep mud would require a great deal more energy even at a speed so low as to make wind drag minimal. I was just wondering if anyone had a way of quantifying it easily- which I know would be difficult, if not impossible, to do.

Was thinking of it more as an interesting mind exercise than expecting to be able to come up with perfect numbers, although it would be nice to get those as close as possible, also.

Checking out the links offered, thanks to all for the replies.

R.

I dunno--but you'd have to calculte the surface area beneath your tires, as well as the force required to lift your rims out of the gook if they are buried (the weight of the material above your tires).

The rolling resistance function (whatever that may be) given the area beneath your tires + the force required to "un-bury" your tires as you go. Since we are dealing with tubular sections of circular bodies, there are going to be lots of sines and cosines and sh*t. Who needs that? Just go and ride your bike and eat total in the morning.

ramius
09-14-05, 09:12 PM
Lmao... yep, well put. I think I'll do just that.... :D

DannoXYZ
09-14-05, 09:48 PM
I dunno--but you'd have to calculte the surface area beneath your tires, as well as the force required to lift your rims out of the gook if they are buried (the weight of the material above your tires).

The rolling resistance function (whatever that may be) given the area beneath your tires + the force required to "un-bury" your tires as you go. Since we are dealing with tubular sections of circular bodies, there are going to be lots of sines and cosines and sh*t. Who needs that? Just go and ride your bike and eat total in the morning.

You can figure out rolling resistance through sand and dirt the same way as on normal roads. You figure out momentum first, MV at the time you first hit the sand/dirt (coasting). Then time how long it takes to stop the bike and measure how far you went. With those three pieces of data, you can come up with rolling resistance of the surface.

ramius
09-15-05, 08:45 AM
DannoXYZ-

That is exactly what I was looking for. Just enough to jog my brain into the right direction. I may just do a little more research and try an experiment in the coming weeks....

Thanks for an informative and precise reply to my question.

Ramius.

Little Darwin
09-15-05, 09:08 AM
IMHO - I can't see a reason to record calories expended to any significant level of accuracy beyond minutes spent at a specific level of effort... Perhaps augmented for some additional accuracy by a HRM. There are too many variables to get any real accuracy by computing the physics involved.

If accuracy was important to me, I would buy a PowerTap and do my computations based on power expended. Of course the cost would roughly equal what I have spent on bicycles and components for the three years I have been involved in cycling, but I think it would provide the most accuracy available. I would also expect that with comparative readings, this could be the best way to accurately determine rolling resistance of a particular bicycle configuration over a specific surface as well.

I am not a scientist, but I did sleep at a Holiday Inn Express last night... No, wait! That was my apartment... never mind! ;)