Different tyre size, different psi?

[ST for me!]

ok, so if your bike runs a 190/50 from the factory and you choose to run a 190/55 or even a 200/50, do you run a lower psi to obtain the same contact patch? the reason i ask is because the 190/55 and 200/50 both have a higher load rating, meaning stiffer construction, no? if you are runing a stiffer constructed tyre, wouldn't one have to drop a few psi to maintain the same contact patch/pliability as the factory 190/50?  

[Kraz]

I don't know it's much in the differences you're talking about, but I think LARGE tires (tractors) run MUCH lower PSI... but that might be for other reasons... and that's totally unconfirmed, and what I heard last time I read a thread like this.  

[miles]

No.  PSI in motorcycle tires is unrelated to tire size.

[ST for me!]

how about tyre load ratings?

[miles]

how about tyre load ratings?

Yeah, that makes a difference because it indicates different carcass construction.  

Your initial supposition was wrong about contact patch, though.  A larger tire naturally has a larger contact patch because less deformation is needed in a larger circle to achieve the same patch area.

Imagine two tires, exactly the same in construction  but wildly different in size.  The small tire has to deform its small circle by a given amount to achieve the desired contact patch, but the much larger circle of the bigger tire means the deformation is far slighter.  With the same deformation (which is purely a result of tire pressure in this example, because the construction of the tires is the same) the larger tire will have a much larger contact patch.


Here's a graphic:

[ST for me!]

I feel like, smarter & stuff now. Many thanks!

[bomber]

Miles covered it nicely -- no fiddling the the air pressure will make a differently sized tire have an identical contact patch to another size tire -- fashion hjas it's price, as always ;-}