Quote:
Originally Posted by choneofakind
Go ride a mountain bike trail on 55 psi tires. You'll be fine.
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On uneven dirt and rock trails, lower tire pressure helps a ton because the tire is able to deform and reach objects on the ground that it otherwise wouldn't be able to with a higher tire pressure. Example: if you run over a pointy rock with higher psi tires, you're only touching a small piece of a single rock, which is more likely to roll or move or give out. With lower psi, your tire is spreading its contact force (area) over more objects, and is more likely to provide a stable footing, and might even able to provide lateral stability on the sides of the rock, etc.
Again it's related to street bike tires' contact patch area and how it affects traction.