# Fatbike tire footprints?

I am curious if there are any empirical experiments, or theoretical computational results, comparing the "footprint" size of the various fatbike/29er/mountain bike tires?

There are lots of claims about how this tire can get through that material better than this tire. But, at the end of the day, the reason for these differences is the size of the tire makes with the surface of the road/trail/snow/mud/sand.

I get that these things change with tire pressure as well, so feel free to add that as a variable. I weigh 180 pounds, so that's what I'll be using if I do this experiment this weekend.

My lack of results in Google may be due to my unfamiliarity with the right terms. If there's a better term for what I want, I'd love to know that as well.

• The term you are looking for is "contact patch." Jan 23, 2015 at 20:25
• It's a pretty easy calculation based off PSI. If you weigh 180lbs and are running 10psi on your fat bike (high for a fat bike) you have roughly 18 square inches for a contact point - nine per tire. If you weigh 180lbs and run 40psi on your 29'er, you have about 4.5 square inches of contact across both tires. Jan 25, 2015 at 12:54
• So your contact patch increases as you add weight? What weight would be necessary to make the whole surface of the tire the contact patch? Jan 26, 2015 at 17:31

There are more variables that you are missing. Rim width is also a factor as well as the tire "setup" itself (tubeless or tubed). So all the variables you would be looking at are tire width and size, rim width, tire pressure and wheel setup (tubeless or tubed). For any or the data to be meaningful, you'd need all of that data. Generally you will get the widest footprint/contact patch by riding a wide tire on a wide rim at low pressures with a tubeless setup.

• Can you explain why rim width and tube type matters? My current research shows that to at least a first order approximation, all that matters is air pressure. (I'm not talking about shape in my question, just size/area of the contact patch.)
– John
Jan 23, 2015 at 22:48
• A wider rim at the same pressures will "square out" the tire profile and produce a larger contact patch. Snowcats were a first mass produced rim for this. A double wide rim for mtb that produced a larger contact patch. Tubeless setups will also produce a wider tire since tires actually inflate larger when there isn't a tube pulling against that air pressure. The effect is probably unnoticeable on a road tire, slightly noticeable on a mtb tire and very noticeable on a fat tire. TBC Jan 23, 2015 at 22:53
• If you have two same model fat tires on the same rims at the same pressure but one tubed and one tubeless, the difference in size will be clearly noticable. The tubeless setup will look like it's at a higher pressure or "over" inflated. Jan 23, 2015 at 22:55
• Can you explain the physics behind these assertions? Or, do you have a picture you could provide to show the difference? I can't figure out why tubeless or wide rim would be a substantive difference.
– John
Jan 26, 2015 at 17:10
• As for the rim width, increasing it increases the circumference of the air bearing "tube" that the tire/rim combo becomes. The skinniest rim possible will pull the tire into a nearly perfect circle with the smallest contact patch. The widest rim possible (for the tire to still bead) will arch or "flatten" that circle out, producing a larger contact patch for the new larger circumference. Jan 27, 2015 at 16:51

I found this image on "the web." It doesn't give a comprehensive answer to your contact patch comparison request, but it gives a pretty clear idea of the difference.

• I don't think the photos were taken at the same distance, therefore they may not be comparable. Jan 23, 2015 at 20:43
• Assuming these photos were taken at the same distance (they sure don't look like they were to me) then the 3.8" track is 2.5 times wider than the 3.0" track. Is that the message you're sending? That gets it in one dimension, but the ability of a tire to "float" will be a function of the tires footprint in the line of travel as well. I could see that easily being 2x to 4x different, but I don't know.
– John
Jan 23, 2015 at 21:19
• These photos are useless because only two of the five pieces of data needed are listed. The smaller track could have been produced by a high pressure tire on a much narrower rim. It would be very easy to make the 3.0 tire have a larger footprint by manipulating the rim width and tire pressure. Jan 23, 2015 at 21:21