Skip to content

Rolling resistance – theory and practice

15/01/2013

In theory, the rolling resistance of wheels decreases as the diameter of the wheel increases. This is on the assumption that all other factors are equal: the tyres are of identical cross-section and carcase construction, with equal internal air pressures and equal external applied loads, rolling at low speeds in still air where no significant aerodynamic effects apply, on smooth hard road surfaces, with the wheels on hubs with insignificant bearing friction.

Yet it is clear from observation and testing that, under some circumstances, some smaller diameter bicycle wheels can roll as easily, or even more easily, some larger diameter wheels. This does not mean that the theory is wrong – merely that one or more of the “other factors” is not equal. The easiest factor for the average rider to control is tyre pressure. It’s a fair assumption, confirmed by everyday observation, that most cyclists ride on tyres that are at sub-optimal pressures. So, pumping up the tyres of a small-wheeler to the maximum recommended by the tyre manufacturer may well be enough to allow it to roll more easily than many other cycles with larger wheels. Choosing a small diameter tyre with a supple carcase will also help. At racing speeds, wheel aerodynamics and unsprung mass of the whole bicycle and rider ensemble can also enter into the equation and may, for example, favour a well-designed small-wheeler with suspension.

Between 1998 and 2002, British engineer John Lafford carried out rolling resistance tests on various tyres, ranging in tyre bead seat diameter from 305 mm to 622 mm (i.e. nominal wheel diameters of 16-inch to 28-inch). The manufacturers and product types, cross-sections, tread patterns, state of wear and tyre pressures all varied quite considerably. His full data may be found here: http://www.legslarry.beerdrinkers.co.uk/tech/JL.htm

Below is a chart generated directly from John Lafford’s data using Microsoft Excel. The vertical axis shows the various tyres tested, ranked by bead seat diameter – biggest at the top and smallest at the bottom. The horizontal axis shows rolling resistance – the less the better. The straight, backward-sloping, black line is a computer-generated trend line which reflects the general truth of the theory that rolling resistance decreases with tyre diameter. But it is immediately apparent that the rolling resistance of any particular tyre diameter may vary considerably, confirming the variance due to those “other factors” that may not be equal in reality. Hence we find some of the smaller tyres under certain conditions have actual rolling resistances lower than some of the larger tyres.

Rolling resistance of bicycle tyres of differing diameters using data from John Lafford's 1998-2002 tests.

Rolling resistance of bicycle tyres of differing diameters using data from John Lafford’s 1998-2002 tests.

About these ads

From → Small-wheelers

4 Comments
  1. Do you have a version of this graph which lists the make, model and construction of each tire (OK, tyre — you’re British). And the pressure? These would make for much more useful information. Also, I note the work done at Bicycle Quarterly magaine, with the surprising result that wider tires roll better and increasing pressure above an optimum value is of little advantage — see http://janheine.wordpress.com/2012/06/13/bicycle-quarterly-performance-of-tires/

    • Hi John,
      Thanks for your comments. The precise data is all in the Excel table to which a link is provided in my article. To cram that onto the chart would be beyond my skills but maybe you’d like to have a go. The main point of the graph is to show that, whilst the general theory may be correct, the variations between tyres of the same size can be considerable.
      The points about contact patch and ultimate pressure highlighted by Jan’s recent work have been made in the past. Wide section tyres with supple walls were advocated by VĂ©locio a century or so ago, for example, and I remember reading Frank Whitt on the subject a third of a century ago.
      By the way, Mike Burrows tells me that, in his recent tests, he’s finding the “Wellington boot” Schwalbe Marathon, officially their heaviest, highest drag tyre, actually rolls better than their supposedly faster tyres.
      Tony

  2. This is a subject which has fascinated me for a long time. The bike I use for triathlons is a Litespeed Tachyon with Vredestein Fortenza tires, which have a psi limit of 160. I’ve done several races with the pressure at 160 and others at 120. Tests have shown that lower pressure is better because the tire doesn’t “bounce” on tiny bumps in the road. I’ve done roll down tests on a hill near my home and I always seem to hit the same speed when I reach the mailbox on the bottom (31.4 mph) regardless of the tire pressure. Is tire pressure levels pretty meaningless?

    • Hi Jack,
      Generally speaking, road bike tyre pressures above about 100 psi are unlikely to reduce rolling resistance in real world situations. As you have found, increasing the pressure further just gives a harder ride and you are likely to get more energy loss due to the inability of the tyre to absorb irregularities in the road surface. (The bike behaves as if it has solid tyres and some energy that should be driving you forward is lost in vertical motion of the whole unsprung mass of bike and rider.) But below about 100 psi, tire pressure can be more critical, and especially so with smaller wheel diameters. On a smooth road, assuming good tyre design, at 100 psi there may be little difference in rolling resistance between good 20-inch and a 26-inch tyres. But drop the pressure in both tyres to 50 psi and the fall-off in performance of the smaller diameter tyre will be much worse.
      By the way, coast-down tests should not exceed about 9 to 12 mph, otherwise aerodynamic variations will mask the effects.
      For more on rolling resistance, see the third edition of David Gordon Wilson’s classic book “Bicycling Science” (not in any way to be confused with Max Glaskin’s new book “Cycling Science”).
      And it’s always worth bearing in mind that, because of constructional differences between brands/models of tyre, rolling resistance can vary considerably for the same diameter even when all other factors (load, pressure, road surface, etc) are equal.
      Tony

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

Join 42 other followers

%d bloggers like this: