Are knobby tires really that much slower?
by Alison Powers, ALP Cycles Coaching
A few weeks ago my friend quarantine teammate, ALP athlete, and ALP designer Daphne, and I did a fun and interesting test. We did this test because the past few times we had been riding bikes together, she was dropping me on the downhills; fairly hard and I wasn't just sitting around coasting. There were a few differences that could be the reason for the dropping. Clothing: I had on baggies and she was wearing spandex. Bikes; we were both on CX/Gravel bikes however, I rode a bike with flat handlebars and she rode a bike with drop bars so she could get into the drops. TIres: I had knobby tires for a mix of single track, loose dirt, and snow. She had tires with no knobbies.
We decided to do a test. Let's limit all the other factors minus tires. Riders agonizing time and brain drain over tire selection for racing. We were going to get to the bottom of it. Are knobby tires really that much slower?
For the testing we both:
- Wore the same clothing (not each other's clothes, but the same of our own clothes...)
- Used drop bar CX bikes
- Used the same width tire (38mm) at 35 psi.
- Weighted the same (I was 7 lbs more so she got a small dumbbell to ride with)
- Started the racing the same way and held similar positions on the bike while testing.
Our testing strip was ~300 meters long of fairly hard packed dirt with some gravel and bumps. We had two lanes - right lane and left lane. For each test, we tested in each lane to make sure our conclusions were accurate.
Test 1:
- I had knobby tires and Daphne had no knobby tires. She won both times.
Test 2:
- I traded the knobbies for the no knobby tires. Toward the end of the first round, I thought I was going to catch and pass her when I hit bumps that instantly slowed me down. But the fact that I was closer to her was proof that these tires were faster.
- Round two - we switched lanes and I won. This time she hit the bumps that caused her to slow.
The end result was tire selection matters - and kind of a lot (when it comes to speed). Also, our right lane was significantly faster, and how much those bumps slowed each of us down really drove in the point that line selection when riding dirt/gravel roads matters. Smooth is fast. Be smart where you decide to put your tires.
For fun, we decided to test a few other things.
- Position on the bike - for one of the tests, with the same tires, I bent my arms more and got in a more aerodynamic position. I was much faster.
- Road bike/clothing - for our last test. I switched to my road bike and spandex. I handily won, even in the slow lane, and the speed difference was amazing to feel.
In conclusion, everything we learned:
- The Specialized Sawtooth is faster than a knobby CX tire.
- Line selection really matters. If looking for free speed, ride smooth lines.
- Position on the bike. For free speed, get aero. Ride in your drops, bend your arms, get low.
- Clothing. All the times you ride and race with a vest or jacket flapping in the wind, you're getting slower and wasting a lot of energy. Tight-fitting spandex with no loose flapping will save you energy and give you free speed.
To make the testing even more fun, we videoed it. You can watch it below:
Remember our ALP Cycles Climbing Challenge is May 22-24. You can win coaching, a customized training plan, schwag, and other cool prizes. Plus you can use your hard-earned fitness in a fun event.
Happy Riding!
