Size matters for gravel tires. But why is my tire different from what the label says?

The eternal question for gravel bike owners: “Will these gravel tires fit on my bike?”

A simple question with a complicated answer, because tire dimensions are not fixed. There are 4 main effects:

  • Rim internal width (medium effect)
  • Tire pressure (small effect in the usable range)
  • Manufacturer definition/honesty (large effect)
  • Production variation (medium effect)

Rim internal width

Simply put, as the rim’s internal width gets bigger, the tire will measure wider. After all, the tire doesn’t have to go “full circle”, the center section is taken care of by the rim. The wider the rim, the bigger this center section and the more tire is available for create a bigger “circle”.

In other words, if you have wider rims, you can use narrower gravel tires and still create the same effective tire width.

Gravel tire pressure

This one’s pretty intuitive, as you pump up the tire it gets bigger. So if you want a bigger tire, just pump it up harder. Unfortunately, higher pressure quickly makes gravel tires unridable, so within the pressure range that is useful, this effect is not large.

There is however a related effect that you should be aware of. The first 1-2 days of use, a tire will stretch. That’s why we always let a gravel tire sit before measuring it in order to get the most accurate “in use” dimensions.

Manufacturer definition/honesty

If the label says 700x35c, what can you expect the tire to measure? There is no consensus over this. Most tire makers do not specify on what rim the tire will be this size, or even at what pressure. Additionally, it can be tempting to label a tire bigger than it really is.

After all, most riders only look at the label and never measure the width. So if you take a 32mm tire and label it 35mm, you have a lot of happy customers who think they are riding a super light 35mm tire. Of course the only reason it’s so light is that it isn’t 35mm, but they’d never know. Interestingly, there are also manufacturers who “under-label” their tires.

Production variation

Producing tires is a complicated process and rubber compounds are complicated materials. Even out of the same mold on the same day, two tires can vary quite a bit if production conditions change.

Solving the puzzle

It’s hard to properly engineer gravel frames & forks if you don’t know the exact size of the tires used. So we took a ton of gravel tires and started measuring them on different rims.

To do so properly, we introduced two definitions:

RAM = Radius As Measured

WAM = Width As Measured