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How big is my tire? – Gravel Bike Tech

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

Posted by Gerard Vroomen - 01 June 2020

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

The results have been quite enlightening as you’ll see in part 2.

◄ Previous Story What tire do I need? – Gravel Bike Tech
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9 responses to “How big is my tire? – Gravel Bike Tech”

  1. M

    Mcct says:

    29 October 2021

    Hi Gerard
    Do you think the Schwalbe Rocket Ron 27.5 X 2.1 with an internal rim of 25mm would fit the Exploro Race?

    Thanks

    Reply
    1. Gerard Vroomen says:

      29 October 2021

      Still fits on this blog too :-). and the 2.25″ width fits too.

      Reply
  2. C

    Caspar Gebel says:

    18 August 2021

    Hi Gerard,
    when it comes to narrow road tires, what kind of handling is to expect? Would you say there’s a minimum tire width that works with the geometry of the Exploro Race?
    Thanks,
    Caspar

    Reply
    1. Gerard Vroomen says:

      18 August 2021

      Hi Caspar, you could run a 28mm tire but the question is, why would you? Once you have the tire clearance, why not take advantage of it? Especially since there are so many good tires nowadays that are bigger (those didn’t exist a few years ago). So it may make more sense to run a 32mm Conti GP5000 or a Pirelli Cinturato 35mm or something along those lines. Great rolling resistance, great comfort and even able to do some mild gravel without getting a lot of punctures.

      Reply
  3. J

    Josh Deetz says:

    12 August 2020

    Great blog for demystifying tire section problem. Having worked with nearly every tire maker at a mold design level I can say one more input that makes a big difference in tire section is the standardized rim width for given section within the international bodies that define what is allowed. Over the last 3 decades the suggested rim width for a specific section of tire has changed quite dramatically. This resulted in, at any one point in time, most tires available from a tire maker were not in compliance with the norms. From the ’80’s through to ’90’s the suggested rim width for a specific section were graded in fine width differences. What I mean is as a tire changed 5mm in claimed width, the rims should be changed. As ETRTO was the only remaining functional or living standard for bicycle tire and rims, and being European, the standards were set from a logical and controlled standpoint of what should be proper OEM, and essentially homologated. Some effort in the 1990’s resulted in a reduction of expected rims widths and a loosening of the tire width variation to rim width allowances. The net affect was broader tire widths were suggested as compatible with fewer rim widths.
    Now we are back to a rim width revolution and even making rim widths rarely seen before. To make things more complex as engineering staff changes over time, internal standard designed rim widths also change. I’ve sat on sidelines for 40 years watching this slow motion fender bender. It’s time to stop this mess, and here is how I proposed we do this.
    Adopt a labelling convention that identifies the tire’s contribution to final section width, as well as the rim width’s contribution. These can be and must be labelled individually. An inflated mounted tire section is never determined by one component, it is clearly a sum of two values, and current standards or guidelines fail to define results with accuracy needed.
    All the work has been done and proven for over a decade, but it is known by far too few and as yet not adopted.

    Reply
    1. Gerard Vroomen says:

      12 August 2020

      There are several working groups on this as you maybe know, but of course that’s another way of saying it might take another 10 years. That’s why we started measuring the tires instead of waiting.

      Reply
      1. J

        Josh Deetz says:

        07 October 2020

        Get me your address and we will send you sample tools for use once we finish the new ones. my email is in your system when registering, use that address and we will get a tool over ASAP.

        Reply
  4. W

    Weslley says:

    30 June 2020

    Hello, how are you? My name is Weslley and I currently live in Los Angeles, I am a cyclist and I have been involved in this sport for over 20 years. I am Brazilian and soon I will be back. Brazil is a great bicycle market and, consequently, an opportunity. Gravel is still a baby. There is no better time than now. I am interested in being a graduate, ambassador, etc. brand in Brazil and, consequently, participate in a portion of the sales caused. I’m not an investor. Are we going to take this brand to Brazil? I am a partner. I have a project and knowledge for people to scale and leverage as sales. thank you and big hug. success to you.

    Reply
    1. Gerard Vroomen says:

      30 June 2020

      Hi Weslley, it’s not my side of the business, but I think they have already plans in place for Brazil. Thanks for your offer to help.

      Reply
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