Bahhhh ... and 69/96
Originally Posted by mihlbach
Ask this in the BMX forum...thats where this practice originated. My understanding is that using a narower rear tire allows you to run higher pressure, giving you less rolling resistance. And the bigger front gives you better traction in the turns.
A lower pressure on a larger tire will give you the same rigidity.
To answer the original posters question, mudders are fond of smaller rears because they "dig down" better. Larger tires float and mudders don't like that. There is likely a perceived rolling resistance advantage to a smaller tire. On paved surfaces this belief is justified, on irregular surfaces it probably is not as the ability of the tire to conform to the terrain allows the bike to travel straight rather than bouncing. Every bump translates into lost momentum (this is the advantage of larger tires).
Another reason that some prefer a smaller tire in the rear is the perceived "responsiveness" of the bike. Advocates of smaller tires will point out (deceptively correctly) that a larger tire means greater rotating weight and this translates into a larger moment of inertia. They perceive that this will slow down their acceleration. They also perceive it as they ride (quite correctly) as the larger diameter wheel package effectively changes the gearing. They fail to do the actual calculations to discover that the rotating 20 grams difference in the tire compared to mass of their body is practically insignificant. The effective gearing change re-inforces a physics fallacy and hence a smaller wheel in the rear.
Finally, the #1 reason to run a larger front than rear ... because you typically can. The chainstays of a frame are typically more confined than fork stanchions. A lot of people can only run a 2.1" on the rear even though they may prefer a 2.3" which will fit easily on the front.
This perception is so strong that it is leading manufacturers to produce "96er" bikes with a 29er in the front and a 26er in the rear. The theory is that the larger front wheel will reduce rolling resistance over obstacles while a smaller rear wheel provides quicker acceleration. The production of these bikes with medium travel front forks and hard tails is just laughable when you carefully consider it. You place suspension on a better rolling wheel and no suspension on poorer rolling wheel. It's a double shrug when you realize that you can just tweak the gearing of the bike (lose 2 teeth in front or add 2 in the back) to account for the "sluggishness" of the larger wheel). I suspect that the engineers run the numbers and shrug their shoulders as the market perceives this ... so it is so.