For some reason people of the internet (but surely not you, loyal reader!) lose their shit over the prospect of new options like 27.5/650b and ‘Plus’ tires. I don’t really understand the personal anguish there, but I do understand the feeling of analysis paralysis that comes with deciding on a tire and rim size.
This is a complex topic full of ambiguity and potential gotchas so how do we go about tackling it comprehensively? Well to kick things off I will post some half-assed charts to get the conversation started and then wait for you guys to fill in more tires and tell me where I’m wrong. Sound fun?
First up, above is an interactive chart (click it, tap it) that gives us the equivalent tire width for various rim diameters. The idea is that a smaller diameter rim lets you run a fatter tire, assuming your frame clearance is sufficient. I like to think of it as changing the ratio of tire to rim where the overall diameter of the wheel remains consistent.
Of course, the reality of tires is that you can’t always find the tire you want in the diameter and width that you want. And weight of the wheel is a consideration when going up or down in size. So let’s look at some real world data.
The chart above calculates the diameter of a particular tire (Y axis) and plots it against the weight of the tire (x axis). The size of the dot represents the width of the tire. Now this chart isn’t particularly helpful because all it tells us is that more rubber weighs more than less rubber (remember we’re just increasing the ratio of rubber when we reduce the rim diameter).
But what about when we consider the change in weight from rim size and shorter spokes? Jeez man, do you expect me to do everything here? Well I haven’t started a rim database (thank God) but I have put in some placeholder rim weights (based on Velocity Duallys) in this chart and you can edit these to try out whatever rims/hubs/spokes you’re thinking about running. Here’s the same chart as above with rim weight included:
So the chart above shows that the weight gap between the tires closes when we consider rim weight as well. Remember the ratio: more rubber and less metal. In most cases the reduction in weight from sizing down your rims is more than offset by the increased weight of the tires so you have a net increase in weight. This all depends on the particular tire and rim combo and we need more data to see how the trend scales.
Does this mean that we would all be better off running skinny 700c tires? Hell no, man. Picking a tire is like picking a life partner. You don’t chart that kind of thing. Listen to your heart. Feel the whirr. Listen to the zzzzzip on the pavement. When in doubt, go fat.
Hey! This is just a first stab at this analysis and there is a lot more we can do in terms of understanding how rim width affects tire width, how bottom bracket height is changed based on tire sag, etc… but what I could use help with now is getting more tires and real world measurements into the tire database. Click that link. Add data. Let me know what else you want to see.