You need a tire that can effectively absorb rocks and road shock like washboards without getting a pinch flat or damaging your rim and allow you to keep control of the bike. The heavier you are and the more gear you like to carry the larger volume tire you’ll want.
What a bigger tire is... More comfortable, less prone to ‘pinch’ flats (big in the dirt), more control, better traction, easier on wheels. What a smaller tire is... Less traction, less comfort, potential flats or wheel damage, and a lot Faster.
The larger the tire the less air pressure you can run, so a bigger tire gets more comfortable at lower pressures. If you have say a 25c you can only reduce the pressure to around 85 psi before you’ll have problems with flats. A 35c can handle pressures from 40-60 psi, so it’s a much smoother ride.
Another factor that you have to consider when selecting a tire is your ability. Fast means nothing if you can’t relax or end up on the ground. A skilled rider can get away with a smaller volume tire because they’re usually better at control and avoiding and/or jumping obstacles.
So, why not just always run as big a tire as you can fit into your frame for everything? A fast tire is a fast tire, even on the dirt, especially when your 60 miles into your ride and starting to suffer. Sometimes getting to the finish line quicker is better than taking much longer even if you’re riding in temporary comfort, because it’s only a matter of time before some type of fatigue sets in.
 |
Schwalbe big apple's on a 29er, my vintage Paramount with 28c Ruffy Tuffy's on South Table Mountain, Golden Co.
|
I personally like road tires in the 28c for everything, I can ride them anywhere and fast, on the dirt with slightly lower pressures around 75 up front and 85 in the rear, I’m 185lbs. I’ve ridden many dirt century’s with tires that size, from cheap Kenda’s, Rivendell’s Ruffy Tuffy’s and now my Continental Gatorskins without any problems. I buy tires with a belt for flat protection, the expensive ones have no real weight penalty, and there’s no glory in a flat repair on the side of the road. I’ve never had a problem with expensive road rubber breaking down, or wearing out any faster on the dirt either. I’ve done a lot of dirt miles on what I like to call a 50/50 ride, road paved asphalt and gravel roads, on Michelin Pro Race 3’s and now my Continental Gran Pre 4000’s in the 25c size.
When I explore new dirt roads I use a 32-35c Cross tire with tight and fast knobby’s, I want the traction to explore single track trails, climb mountains and ride cobble stone paths without any problems, also works well in limited traction conditions like sand and snow. If I’m setting out to ride 100 miles on dirt roads I don’t need a slow tire holding a good man down so I look to my 28c. A 28c fast road slick, the default tire, the tire that can do it all in a pinch, something like the Continental Gatorskins, good for the road and 99% of all dirt roads suitable to drive a car on. A fast 25c road slick, for longer 50/50 rides or dirt detours on maintained dirt roads.
 |
| There is a place for Fat tires. |
For the record I have never ridden anything larger than a 33c in a road tire. Up until a month ago I didn’t own a bike that would accept anything larger 33c, except my 29er, and there was no need. I did ride Rivendell’s 33.3c Jack Brown tires for a summer on dirt roads all over Elizabeth County east of I25 and between Castle Rock and Colorado Springs. An extremely comfortable tire, but I went back to the 28c tires. If you prefer comfort over speed this is a fantastic tire choice, and I doubt you will ever get a flat. My rides tend to be longer than shorter, if you aren’t riding huge miles you might consider a larger tire like this.
 |
| Single track trails like this are best suited to 35c cross tires or greater. |
A couple years ago I rode the Dirty Century in Elizabeth. I show up with my Schwinn single speed riding on a set of 28c Ruffy Tuffy’s... everyone looked skeptical. There was also a group of riders there to represent one of Denver’s ‘cool’ bike shops, all on sweet bikes like the Surly Long Haul Trucker. I mean these things were equipped right, all the cool stuff including racks with leather and canvas bags and of course the proper huge Schwalbe adventure touring tire. Although they looked like the authorities at the start, they never made it to the finish… all of them bailed at some point and returned to the start. They were the first alright, first to bail and first to get the after party started. Here is something else to consider, for years before all these fat tires were available there were 3 basic tire sizes to choose from for your road bike. 27x1”, 27x1-1/8”, 27x1-1/4”. Equivalent to 25c, 28c, and 32c by today. And there was a lot of kids including myself riding there Varsity’s, Continental’s and Paramount’s all over rural America. Moral of the story, big tires are cool, but not that cool.
Tire air pressure is everything on the dirt, when you have it right any tire can be reasonably comfortable. Drop a few psi when going from the pavement to the dirt and it will improve your ride, mellow out the road harmonics that vibrate all the way through the bike and handle bars. You should see the tire deflect under your weight, slightly mushrooming, for all tire sizes. If you let out too much air out your bike will feel sluggish, especially in the rear, and you’ll be prone to pinch flats over pot holes or large diameter rocks in the road. A pinch flat is when the tire meets the rim with such force it pinches the tube and slices it, looks like a snake bite, and ruins a tube. Can be patched if needed but more difficult than a goat head, best to carry extra tubes. I like to carry a good hand pump as well, nice for dialing in the air pressure if you let too much out and for going back to pavement, with a good pump it only takes a couple minutes. Remember your 7 P’s; Prior Proper Planning Prevents Piss Poor Performance!
Thanks to Robert for encouraging me to post more info!
