cellery

I got these Club Roost Cross Terra 27 x 1 3/8 (630 x 37c) tires on my cyclogrinder whatever you want to call it bike. They're rated for 40-65 psi and I've run then down to 45 back/40 front but the ride is still a little harsh on mixed road/dirt conditions. I've considered going down to 30psi on them but a little concerned about pinch flats. I'm not a heavy guy at 150lbs, so is running that low a valid concern for 37c tires or should I go for it? They will be driven on fire roads, washboard and similar. Also, I've got some Mr. Tuffy tire liners in there - could those be the cause of the harsh ride? I didn't experience this with higher pressure Vittoria road tires on the same bike before I put the cross tires on and didn't have the liners in with the road tires. Thanks.

Andy_K 

I guess it depends on how bad a pinch flat would be.

I weigh 200 pounds and I've run 700x30 Michelin Mud2's as low as 30 psi in CX races and gotten away with it. With 700x35 Schwalbe Racing Ralphs I've pinch flatted at about the same pressure.

There are so many variables. The only way to really find out how low you can go is to keep dropping the pressure until you flat then you know that was too low.

lasauge 

I weigh the same and I'd guess the lowest I'd go with tires of that size would be around 40/45psi front/back, below that I suspect the handling would become squirrelly especially when climbing out of the saddle. But of course that's just a guess, since as Andy_K mentioned there are several variables at play here, what I suggest is that you make your decision based on sight/feel - I like to see my tires bulge out just slightly when I'm riding on a hard surface in my primary riding position, personally.

As for the harsh ride, I would guess that's the tires themselves. I haven't ridden these myself, but the specs available online show those tires have a low TPI count in the casing, and enough rubber for two tires applied in a very lumpy pattern. They're cheap for a reason.

halfbiked

40 to 30 psi is a big drop. Head in that direction, in smaller increments.

cellery

Low TPI count makes sense - I hadn't thought of that. Sucks there's a limited range of wide tires for 27 inch. I'll drop in low increments toward 35 to see how much I can get away with. It's also a very heavy bike compred to my other light road bike so that is probably factoring into my perception of ride comfort.

Cynikal 

Also, find tubes with removable valve cores and put some Stan's in there. I have run 32's down to 28psi and I'm 165 doing this.