Graham Shrive’s Clarity on tyre clearance

There are few people who have had more of an impact on what Factor Bikes has become than Chief Engineer, Graham Shrive. With over 20 years of design and engineering experience, Graham has been essential in ensuring Factor and Black Inc products continually live up to our motto: Never Status Quo.

Continually focusing product development on what is best for the rider above all else, Graham and his team of engineers and industrial designers are committed to assisting Factor riders to make educated decisions regarding product choice according to each individual’s goals, environment, and needs.

In the present piece, Graham has provided guidance in the process of choosing the appropriate tyre size for your Factor. As with so many material choices in cycling, the quick answer to which tyre size is best is: “It depends.” But in the following text, Graham explains that there are ways to make educated decisions.

There are several factors that impair our ability to say specifically that your frame will fit XYZ sized tyre, aside from the tyre that came specified on your bike. One of the most obvious is that there are a great variety of tyre manufacturers making products people from around the world have different reasons to purchase. While we want riders to feel free to fit whatever tyres they would like, riders need to understand that what one manufacturer calls a 50 mm tyre may not be the same size as another manufacturer’s 50 mm.

Some brands run very true to marked size when inflated, while others can be quite a bit larger. Care needs to be taken when using a tyre other than what is specified as OE with the bike at time of sale to ensure that they match the dimensional assumptions made by our team during the design process.

Know your “as measured” tyre size

These dimensions are noted below. It's important to recognize that we base these dimensions on ACTUAL MEASUREMENTS taken by our design team, across a range of rim widths. This is what we refer to as our design size tyre. In addition to your design size tyre’s dimensions, ISO regulations stipulate that there must be a minimum of 4 mm of debris clearing gap between the rubber and the frame.

Riders should take care that this 4 mm of clearance is sufficient for unusual conditions including mud, grime, and road debris that may result from fresh pavement patches or newly laid chip-and-seal. It’s important to note that paint damage and frame wear brought about by these situations is not covered under warranty. Additional considerations should be made for frame and wheel flex for riders that find themselves having issues rubbing under heavy loads, etc.

In addition to the variation in matching the marked tyre size to the actual tyre size at maximum width, there is also the matter of the rim width variable. As the rim width increases or decreases, the shape of the tyre carcass distorts depending on where the plies are stiffer in the tyre. That is to say, it is not uniformly round in all situations. This anisotropic behaviour of the carcass can lead certain tyres to become much taller when rim width is slimmed down, others may stay largely round and not get as tall.

Keep in mind tyre drift

On top of all the above, we’ve recently noted a very interesting phenomenon when measuring MTB tyres (i.e., 2.2-inch, 2-inch, etc. MTB tread carcasses). We were inflating the tyres to pressures that they typically wouldn’t see in use on a mountain bike, as much or greater than 40 PSI, we’d measure them, and often leave them for a few days and come back to measure them again.

We noted that over the course of a few days, the tyres could grow by as much as 2 or 3 mm. In addition to this, at our Taiwan office, the effect was much more pronounced than in Canada, as it was 35+ degrees Celsius in Taiwan and 20 degrees Celsius in Canada. The amount of tyre size drift varies from brand to brand. We have observed as much as 4 mm and as little as no drift, depending on the version of the tyre, as well as the brand.

Do your research

All these considerations need to be taken into account when a rider is contemplating a new tyre purchase. Unfortunately, as a bike manufacturer, we can’t measure every tyre in existence, so we will refrain from commenting on what does or does not fit. That said, there are an ever-growing number of resources available, both online and in your local shops.

This issue is well understood, and riders have varying degrees of acceptance of things like scuffs on your frame from picking up rocks or debris, so each individual case needs to be considered on its own, and with a pair of callipers in hand.