I wasn't sure where to post this, but I figured MTBers would have the best information about cycling on unpaved surfaces. I'm putting together a hybrid for use on the recreational trails in state and national parks, rails to trails, gravel and dirt roads, etc. I'm using an older steel 700c rigid fork hardtail frameset similar to a cyclocross frame. There is lots of room for the back tire but the fork isn't as generous. I'm thinking of using the Schwalbe Smart Sams https://www.schwalbetires.com/bike_ti...ires/smart_sam. I can easily fit the 45mm on the back but will probably be limited to the 40mm in the front.

Is there any advantage to having a larger tire in back or should I just pick both tires based on the largest size that will fit the fork? The geometry of this frameset is relaxed enough that I'm not worried that a couple mm taller in the back will adversely affect steering or stability, but would I gain anything?

I like having a fatter tire in the front. It holds better in turns and loose material. The back tire can dig in and push with less resistance. If you find you are spinning out and losing traction then you need more tread on the ground. I weigh 225 so I put plenty of pressure on the back tire.

Sounds like a nice build you are putting together. A rail to trail bike, sounds like we need a new name. Post some pictures.

I'll post some pictures as the project progresses. My tire issue is that my fork has less tire clearance than my rear stays, so my choices are to pick the tire size based on what I can fit up front, or to put a 40mm tire up front and a 45mm in the back.

Like Daspydyr said, you generally want the same size tires, or a wider tire in front depending on your situation. A wider tire in rear can be a problem in the wrong situation. If you get onto uneven ground, or are going around a turn on loose terrain, you want the rear tire to have equal or less grip than the front so that it slides out from under you first(assuming a tire slides out). If the front slides out first, you're almost certainly hitting the ground shoulder/face first, and many many people have broken collar bones from that type of accident. If the rear slides out, you have a much better chance of staying upright and if you do go down then it will probably be on your side rather than face first. In your case, I'd definitely recommend just finding out the widest tire you can fit in the front and then using that same tire in the rear. You'll probably want to put a little more pressure in the rear tire.

Ah, thanks, that helps.