Tuesday, February 03, 2015

Your Fat Bike Is Really A 29"er (Or Bigger!)

A typical 29"er rim/tire is, well......29 inches in diameter!
Recently there were a few comments online that indicated to me that many folks don't really understand the relationship between the nomenclature for tire/wheels and how that determines diameter overall, and furthermore, how that in turn affects geometry of a bicycle.

For example: Most people know that fat bikes are based off a rim diameter used for 26 inch wheeled mountain bikes, but they may not realize that nothing about their fat bike is 26"s at all!! That's right. Nada. Nuthin about a fat bike wheel has any dimension measuring 26 inches. So, you may wonder why in the world fat bikes are dubbed 26" wheeled bikes. Yeah......it doesn't make any sense at all. Well........actually it is a big mistake to call fat bikes 26"er based bicycles. If we would stick to long standing tradition, your fat bike would be called a 29"er, actually! Here's why.....

Without getting into a long winded discussion about wheels, just keep in mind that the diameter of a wheel is determined by the overall height of a rim and the tire that fits on that rim when they are mounted together. So, let's take a road bike wheel, as an example. One could mount a typical 23mm tire on that rim, and then take that and compare it to the matching rim with a 38mm tire mounted to it. You would notice that the rim with the 38mm tire would be taller when compared to the rim mounted with the 23mm tire. Its diameter would be bigger. 

This prototype 3.7" tire is just shy of 29"s on a 70mm rim.
  However; the example above uses a different tire to achieve a difference in diameter, but that isn't the only way to alter overall diameter. Of course, one could use a smaller diameter rim, with a really thick, tall tire, and make the overall diameter the same or larger. My example of the fat bike tire here shows this. The tire I used is a prototype marked as a 4.0"er, but in reality it measures 3.7". It is mounted with a tube on a 70mm rim and falls just shy of 29" by a few tenths. A "true" 3.8" tire would be spot on 29"es and of course, a bigger 4.0 tire would be slightly larger than 29"s on this rim, more than likely.

Your 4.8"er tires? yeah......those are about 30.5" in diameter on 100mm rims!

The point is that one can swap out their 3.8"-4.0" fat bike tires for a Summertime set up on 29"er rims with fat bike hubs and 2.25"-2.4" 29"er rubber and have almost no change in geometry. The bottom bracket height stays the same, all angles work the way they were designed to, and the only difference is a lighter, faster to spin wheel set. However; let's say that you want to do a 29+ wheel set, (29"er rims with 3 inch wide rubber), and you wonder if that would work. Well, assuming it fits your fat bike, (and it may not), using 29+ raises the bottom bracket significantly, and changes the way that the bike would handle in a significant way as well. Why? Because those bigger. wider tires make the overall diameter bigger. About 31 inches, in the case of 29+. So, you will raise the bottom bracket by half the difference in diameter between the two wheels, or in this example, by about an inch.

Using 29+ wheels on my Ti Muk raised the BB too much for my tastes.
What if you were to get into a 27.5"+ tire? (Mid-fat- B+, or whatever you want to call it!) Well, the overall diameter of these wheels is slightly smaller than 29"er wheels, so, about spot on if you are running 3.8" tires on 80mm rims. This would be a great swap to keep the geometry similar.

Keeping in mind that some fat bikes were developed with 4.8" tires from the get-go, you may find that sort of fat bike best swaps over to a 29+ wheel set since those wheel diameters match up better. (31-ish inches, as you may recall.) So, in the case of my Salsa Cycles Blackborow DS, it would make more sense for me to build up a 29+ wheel set for Summer than a 27.5"er/B+/Mid-fat one, unless I wanted a lower bottom bracket.

Whew! It can get confusing, what with all the "inch-that", "plus-this" nonsense. Just keep in mind that it is all about diameter! Just like with cars, motorcycles, trucks, and trailers. In fact, the system they use would make more sense for bicycles. See the article HERE if you want to know more about that. However; it won't change because as fast as cycling seems to change, the little bit of traditional stuff we do have is not let go of so easily. Unfortunately, this wheel and tire craziness is part of that.


Rob E said...

Currently have a Pugsley with Marge Lite rims and 3.8" Knards, in your opinion would a 27.5+ wheel set keep the BB height the same or would it lower it slightly? I know that there are a number of variables that could play into this but just a general ball park guess is all I'm really looking for. Thanks.

Guitar Ted said...

@Rob E- Depending upon which B+ tires you decide to use, yes, it should be in the ball park for diameter in comparison to the described wheel/tire combo. Of course, variables with regard to 2.8 vs 3.0, rim width, and combinations thereof can affect this comparison.