"Having reduced the frontal area of our HPV's by reclining the rider, improved the shape with fairings and improved hill climbing by weight reduction, the next item on the agenda in our search for a 60 km/h Sporting , Commuting, Shopping, Pedal Powered Machine, must be rolling resistance!

HPV's often tend to use smaller wheels than conventional bikes for a variety of reasons.  Sometimes for space reasons, sometimes to give a stronger wheel in side loading (e.g. trikes) and sometimes for lightness.  Long and loud are the discussions as to which is best - big wheels or small wheels!

Conventional wisdom is that large wheels roll better.  In fact, one could imagine that a large wheel would cope better with bumps and probably roll easier than a small wheel.  On the other hand, there is no denying the fact that a large wheel will cause more air drag than a small one.

One Christmas, a couple of years ago, the question in the Sims household became "how much difference is there in rolling resistance between large wheels and small wheels?" as we contemplated the design of our next vehicle for the 1993 HPV Challenge.

Now some time ago, I had noted that there was a lack of reliable data on the rolling resistance of different tyres, and after trying a number of road tests to measure rolling resistance, I could see why.  There were just too many variables - rider position, road speed, wind velocity, gradient, vehicle set-up, etc, etc.  So I decided a lab test was the only way to go, and had half managed to build a tyre testing machine before the demand for trikes put a stop to it.  Now the question seemed more important than the trikes!

THE MACHINE

Power is absorbed as the tyre flexes in contact with the road, so I had intended simply to run the tyres loaded on a roller or drum.  I contemplated using a very large roller to simulate the road, but that was going to be difficult to drive without power losses, and expensive.  Likewise driving the wheel would involve losses.  I decided to use a 4.5" diameter roller as it could be direct drive from a motor I had, and the relatively small diameter would accentuate the difference between the tyres for me.

So the 4.5" diameter roller was mounted directly on the output shaft of the motor, an ex-computer drive motor, approximately 20V DC and about 0.25 hp, for which I had efficiency figures.  It was mounted on a bench and an arm was built to hold the wheels over the roller, with a weight loading system designed to press the wheel into the roller with a force of 30 kg - equivalent to the tyre loading on our trikes.  A pair of 12V batteries and an ammeter accurate to 0.001 A completed the test rig.

A number of wheels were built up on Suzue sealed bearing quick release hubs, and the tyres were tested at a number of pressures.

RESULTS

After allowing for motor efficiency, the figures show the power absorbed by the tyre at a steady 30 km/h.  One thing which is immediately obvious is how much better they roll when pumped up hard!  Many tyres showing HALF the resistance at 100 psi compared with 30 psi!  It amazes me the number of people I see riding on half-flat tyres.  Must enjoy the extra exercise!  Anybody know of a good 200 psi hand pump?

Surprise, surprise, the SMALL diameter tyres roll BETTER than the larger ones!  For example, 20" slicks absorbing 20-25 W against 32 W for two well-known 26" slicks!

I mentioned this to a Moulton man - no surprise!  He showed me a paper by Dr. Alex Moulton.  In trying to find out why his bikes were faster than ordinary racers, he did some careful tests inside an aircraft hanger and found that with the same tyre construction, section size and pressure, 17" wheels rolled 6% BETTER than 27" wheels.  Unfortunately he was unable to account for the difference, and I think a number of people did not believe the results.  The reaction of the bike racing body was, as you might expect, to ban them!

There are three possible explanations I can think of.  One is that with a smaller diameter tyre there is less air volume in it, thus it will allow less deflection at the contact patch.  Two, the smaller diameter means less surface area for heat dissipation, so it runs hotter, causing higher pressure and easier flexing.  Thirdly, the contact patch is more circular than the rather oval patches on larger diameter wheels and therefore, although the area of the contact patch should be the same with the same pressure, the CIRCUMFERENCE of the contact patch will be greater with the oval shape against that of the more circular shape.  Thus the line where flexing takes place will be longer on the larger diameter tyre, hence more power absorption, just like one can expect to get more absorption with a thicker walled tyre!  The last explanation has the most support from other people, and seems the most likely.

A greater surprise was the fact that the 20" x 1 1/8" (28-451) 100 psi IRC Roadlites I got from the States at great expense were not as good as the fat 20" x 1.75" (47-406) Tioga slicks we had been using!  Another case of the round vs. the oval?  I guess I will have to do some calculations to see if the Roadlite's thinner profile and much lighter weight make up for the deficiency in rolling resistance - it's not obvious on the road.

Another surprise was that cheap Taiwanese tyres performed better than the Japanese, American or German tyres!  And best of all was a 16" x 1.75" Indonesian knobby with the knobs ground off!

We are, of course, talking about pneumatic tyres on a smooth, hard surface.  Don't expect your mountain bike to go better in mud or sand with smaller wheels.  Even Dr. Moulton found his bikes were slower in sand!  I guess the smaller diameter tyre would sink lower, causing more work to be done displacing the sand.

So there we have it, not only are we ahead on air resistance, strength and weight with small wheels, but we are also in front with rolling resistance!" - from Issue 45 of the BHPC Newsletter, Spring 1996

"Development of small, efficient wheels is a worthy goal in the field of HPV's because of their clear advantages in terms of weight, packaging and aerodynamics.  The comparison of rolling resistances of small wheels vs. more-common large bicycle wheels is an important step in this process.  Therefore, I was interested in Ian Sims' article "Greenspeed Tyre Testing" (Human Power, vol 12:13, Winter-Spring 1996). Unfortunately, the author's test method does not allow a valid comparison among varying tire sizes.

Ian reports that he used a 4.5-inch-diameter drum as his test "road surface" because it was simple, inexpensive, and could be directly driven by an available motor.  The problem is that running a 27-inch tire on a 4.5-inch drum is much like running a 4.5-inch tire on a 27-inch drum in terms of the contact patch shape and the distortion of the tire body,  which accounts for much of frictional loss.  We would expect that measured rolling coefficients on  the small drum would be substantially higher than on a flat road, and that the differences among various tire diameters would be de-emphasized by this test procedure.  The former is confirmed by a calculation of the rolling resistance  coefficient of 0.008 for Ian's best test case (20 watts at 30 km/h and 294 N load), which two or three times the value typically reported for good road tires.  The latter hypothesis is consistent with Ian's finding that, based on his test method, small wheels are as good as, or better than, the large.  I would love to believe this conclusion, but it is counterintuitive and  contradicts the theory and data that have been developed over many decades.

I am hopeful that, with the hard work of Moulton and others, small wheels with appropriate suspensions will allow our HPVs to be smaller, lighter and more efficient than they are today.  But I'm afraid that Ian Sims' tests do not provide valid confirmation of the superiority of small wheels at present.

Ben Brown
Project Scientist,
Robotics Inst.,
Carnegie Mellon University"

However, for those of us stuck with small wheels on at least one end of our machines, Ian's figures are as good a way as any for us to compare different tyres of the same or similar diameter, and have been reasonably well born out in practice by the findings of coastdown tests, such as those performed by Ed Gin - http://home.earthlink.net/~gkpsol/tiretest.html and http://home.earthlink.net/~gkpsol/tiretesttwo.html