For trials I know you want tire pressure to be pretty low. Should it be low enough that you can really feel the tire buckle sideways somewhat in a sideways hop?
The short answer is no. Pump your tire up untill the side wall stops folding over. Ideally you want your tire as stiff as possible such that you can get spring out of it. Just play around with it unitill you find something you like and your tire doesnt bottom out or fold over on.
If you can’t get any spring out of it, then you can’t hop as high. You need to use the compression of the tire to your advantage, to spring yourself up.
I advise having the PSI somewhere between 22 and 28. That’s what works best for me.
I change it depending on the kind of stuff I’m doing. Low is good for soft landings and hops, but then you’ll hear evil CLANG noises doing drops and gaps get weird when the tyre starts folding on take-off or landing. High feels rather solid so isn’t as smooth or bouncy, but is a more solid platform for gaps and drops.
Of course, a lot of it also depends on whether I’ve remembered to inflate it before setting off…
On the whole it wavers around 25psi or so.
Phil
As I’ve mentioned before, you can’t give out tire pressure specs without including your tire size, and your weight. If you have an “entry level” Trials uni it might have a much smaller tire, and need much higher pressure. Also your weight is a big factor.
For most things, the object is to keep it as soft as possible without your rim hitting the ground. You’ll have to determine how much air that is depending on how hard you’re landing and doing other things. You also want to avoid the tire folding over on side hops. You may need to add some air to get good spring action for jumps or high hops.
I ride at about 22 that is best all round as the name says i am thinuniking so i don’t weigh much!just 1 psi can make alot of difference
Ben
I run between 15 and 20. maybe 25 if i am gapping some really skinny rails or sometimes doing natural trials.
john
Thanks for posting this question. I obviously do not run enough air in my Trials tire, as it folds over often on side hops. (at least 10 times yesterday).
I will add air to it tonight. Then I will be the highly inflated --chirokid-- 
Just as I was ready to take my 29er on a ride last evening, my family arrived at home. I asked my 9 year old daughter if she would like to go for a uni ride with me and she quickly went and put on her tennis shoes. Since she rides on a 16" Jugglebug (which I added a Muni tire to), I decided to put up my 29er and take the 20" Summit instead.
We had a great ride together. She has really been practicing hopping, and last night she put some of it to the test. She would side hop up onto speed bumps, the off the other side. I was so proud of her.
As a way to keep this on topic, just last week I had to patch a hole in the Jugglebugs tube because she was hopping with to little of air in the tire. When I got the tube off, it was all chewed up, so make sure to run enough air in your tires to prevent this. --chirokid–
That’s a good point for all you parents and coaches out there. Keep an eye on the tires under your little charges. Kids are about as aware of tire pressure as they are aware of how dirty they are (in other words, oblivious). I notice a too-low tire instantly. It amazes me how kids, even near-beginners, can jump on and ride all over the place with the rim basically on the ground.
Here’s what I like:
Rail riding - Tyre pressure as low as possible without hitting the rim on drops up to about 1.5m (I don’t like riding rails that are higher if they’re skinny ones and the low pressure only applies to skinny ones for me).
Gapping and high hopping - I like to not be able to feel the tyre folding. When I go for sidehops that would cause it to fold more (like about 1.7m-ish) I still take off with the wheel just a bit past perpendicular to the direction I’m going. Although this is mainly because I suck at hopping forwards I think it’s also because I like to use the sideways spring of the tyre. I think if you’re hopping to the side you can get away with a higher pressure and still get good spring out of it.
Narrow surface gapping - I like to have the tyre completely fold around the surface (I’m talking about surfaces like 20-60mm wide) but never fully hit the rim. I find getting the right pressure for these sorts of gaps tricky.
Limit-pushing / scary drops - Tyre pressure so that the rim doesn’t hit the ground, then a little bit more is my preference.
Andrew