>>9853430. Why do you assume you can or should be comfortable during strenuous physical activity?
.5. railbeds are shitty riding.
1. Start by running your tires at a lower pressure. Typically tires & rims on an old rigid mtb will get down to 25psi safely, and as low as 20psi risking pinch flats
2. Learn to ride in attack position. Keep your knees flexed, your grip loose, and your upper body relaxed to absorb shocks. You may need to build some basic fitness with normal riding before you can do this all the time.
3. Get MTB bar-ends. The extra hand positions will help.
