Why 38mm tires? Below is a summary of my understanding of tire rolling resistance:

There are two schools of thoughts on tires. One long held view is that the higher the pressure in a tire the less rolling resistance and the faster the tire. This is borne out by many tests of tires on a drum apparatus that can accurately measure rolling resistance under controlled conditions. As I understand it, an inflatable tire deforms around its contact patch it absorbs some energy. The higher the pressure the smaller the contacts patch and the less energy is absorbed resulting in a faster tire. The desire to have high pressures leads to narrower tires which can hold more pressure than a wider tire made with the same materials. When testing all tires at the same pressure the faster tires are thought to be those with the most flexible sidewalls because those tires take less energy to deform around the contact patch.

The second school of thought feels that a smooth steel drum does not accurately test a tire real world rolling resistance over an uneven surface. The purpose of an inflatable tire is to absorb road irregularities so that the entire mass of bike and rider does not have to be lifted over each bump. Lifting the mass of the bike even a very small distance uses energy and reduces speed. Testing tires on a smooth steel drum ignores the road irregularities which can be significant on a rough road. The tests performed by this group are on real roads but lack the controlled conditions of the steel drum tests. This leads many to doubt their results because they can be affected by rider position, wind, temperature and other factors.

We ride on fairly rough roads and I started riding a tandem with tires at 120 to 130 psi and then went to 32mm high quality tires at 90-100 psi and found them faster. Now we ride 38mm tires at 70-75 psi which I think are slightly faster on our roads and as a bonus more comfortable. For our team the added comfort also seems to help us keep the power up after a few hours in the saddle.

I believe that any high quality flexible tire is fast regardless of its width. Tire construction is the most important factor and there are cheap slow tires of all widths. On rough roads the difference is small when comparing good tires. While I do believe wide tires can be faster, on good roads I am not sure there is much of a difference. To put it all in perspective a little wind or a little stronger riders will outweigh the difference in tires. I think speed is mostly determined by the motors so I wouldn’t sweat the tires too much.

Other Modifications:

We are a moderately powered 280 lb team that likes a flexible frame. Our desire to try a more flexible frame led to the choice to ride a steel Santana because its design allowed me to calculate and test frame stiffness. Ultimately this led to the removal of the lateral tube. Liking flexible frames puts into a very very small minority of tandem riders. We arrived to that view by riding thousands of miles on tandems of varying stiffness so we know what we like. I don’t think flexible frames are for everyone but we seem to have an easier time staying on top of a gear when climbing.

Below is a picture of our bike with 40+mm tires mounted. I went down to 38mm due to tire clearance issues.

Wayne