When pumping up your tires… Would it be more accurate to look at PSI with a rider on the bike? Or am I missing something here? Surely there is a difference if the rider weighs 250lbs as opposed to 125lbs. Right?
I think you are missing something. The volume of the tire doesn’t change very much when you add a rider, so the pressure in the tire doesn’t change much either. What does change appreciably is the contact patch, i.e. a 250 lb bike / rider will have double the contact patch of a 125 lb bike/rider given the same tyre pressure.
Yeah, I just realized that. Forget this post. 
I thought I was on to something. Hey what about helium in our tires? Eddy B said that it saves some grams.
What does change appreciably is the contact patch, i.e. a 250 lb bike / rider will have double the contact patch of a 125 lb bike/rider given the same tyre pressure.
Are you sure about that? I think someone should start a thread entitled “Post here your contact patch area and your weight” in order to see if that is true or not. I nominate Rappstar to do it.
Opposition / equality of forces and all that. I may be wrong on the “double” bit, there might be a different relationship between the weight of the bike rider and contact patch of the tyres, but I am sure you’ll correct me.
I do know that the tyres squish a bit (increasing the contact patch) when I sit on the bike. and if I have a lower PSI, they squish more.
I think another valid question would be, “do manufacturer recommendations acount for rider weight (or some specific allowance for load, say 175 lbs)?”
As mentioned already, the tire deforms as the load above it increases, increasing the contact patch (and rolling resistance). Generally it’s recomended that heaver riders use higher pressures to resist this deformation (and i would guess to somewhat protect the rims, as a secondary benefit).
We need to make sure. We need to start a thread to prove it.
Aren’t my measurements accurate enough? “Squish a bit” vs “squish a bit more” is pretty scientific and repeatable.
I think what psi you’re talking about plays a role here. At some extreme of psi and weight the tire doesn’t deform anymore, and just blows off the rim.
of course there are bounds. too low and the rim rides on the ground.
At some extreme of psi and weight the tire doesn’t deform anymore, and just blows off the rim.
Are you sure about that? that the tire that would not blow off the rim without a rider on it will blow off the rim with a rider on it due exclusively to the internal pressure change?
Do you mount from the left or the right?
Depends if I am on the pink bike or one of my other bikes 
.
Yes, I’m pretty sure. Think about what’s going on where the rubber meets the road. The reduction of volume where the tire is deforming between the rim and the road is increasing the pressure exterted on the rest of the tire wall, due to the displaced air being forced into the rest of the tire (like what happens when you collapse a full styrofoam cup in your hand - where do the contents go?).
Net pressure stays the same as when the tire was unloaded, but when loaded there are differential pressures throughout the tire. The differential pressures are what’s important when a tire blows. If they exceed the max pressure the tire can stand, the whole thing goes regardless of the net pressure (sort of like stepping on a balloon). There is some zone, granted it’s a small one, then, where an overinflated tire that won’t blow off the rim when unloaded will blow when the rider gets on the bike.
Then again, I’m a bio guy. Physics isn’t my strong suit.
So do we just always inflate to max PSI?
Or are there factors that would call for going a little under? A lot?
Course? Weather wet/dry? Rider weight? Handling skills?
I have a friend (135-40lb) about to race a hilly, tricky descent filled course, on a new pair of Mich Pro 2Race which specify max PSI 116.
Pump to 110, maybe 105 if wet roads? Why? Why not?
Questions, kweshuns…
As you increase the pressure in the tyre, the amount of tyre deformation and pressure differential of loaded-unloaded will decrease as well. Technically, you are correct, but the PSI range you are talking about is really small and I think it would be lost within the noise of manufacturing variances.