Why was I slower? How to read a comparison of two rides

You rode the same course twice and the clock says you were slower. Fitness, the wind, the bike: they can all be true at once. The compare’s job is to say how much was each, and how sure it is.

Here’s how to read one, on a real answer: the Nice 70.3 bike course ridden seven years apart, a TT bike the first time and a road bike the second. The second ride was 19:22 slower.

Reading the bars, biggest first

Start with the biggest bar. Drag: 10:26, give or take 0:51, high confidence. Drag is everything that makes the air harder to push through: position, drafting or wind. Here most of it was the bike, which is the story the Nice post tells.

Then power: 4:54, give or take 2:37, medium confidence. Fewer watts, measured by the power meters on both rides and turned into time by the model. The band is wider, because it carries what two power meters could disagree by.

Timed, not explained

Some reasons aren’t modelled at all: they’re timed. Different road: 2:31, spent on road one ride covered and the other didn’t. Descending: 1:18. The model has no brakes, so it can’t explain a descent: braking would look like drag. So descents are timed, not explained, and the page marks them with squares rather than dots.

“Can’t tell” is an answer

Now the honest one. Weight: 1:00, give or take 1:54. The band reaches both sides of zero, so the page says it can’t tell whether the weight cost time or saved it. That isn’t a failure. It’s the most useful thing on the page, because it stops you blaming the wrong thing.

Air density: −1:04. The second day was warmer, and warmer air is thinner, so it gave about a minute back.

And the wind?

And the wind? The page reads each ride’s drag by the way the rider was heading. If drag was highest in one direction, that looks like a wind from there, and the page says so in words that match the evidence. When the pattern doesn’t look like a wind, it says that too, and leaves it in the drag rather than guessing.

Read your own

To read your own you need two files of the same road, and a power meter on both for the full breakdown. Nothing is uploaded: the files are read in your browser.