My Dahon folder is fitted with Schwalbe Kojak 20 x 1.35 (35 - 406) tyres - bought new and ridden less than 25 miles (with new correct size Presta valve tubes) .. The bike lives, folded up, in the back of my car.
Both front and rear tyres have a "flat spot" - one is opposite the valve, the other is at about 3 o'clock from the valve. I've checked that the beads are seated and tubes aren't pinched etc - pumped them up to correct pressure and let them down again several times.
Spinning the wheel in the frame looks like an oval wheel - rims are OK - it's visible on the tyre casing.
I get a sort of "rolling undulation" ride caused by the flat spots.
All ideas why welcome please . . . .
Rob
Edit - I should add that the tyres I replaced (whatever came with the machine) were fine - I changed to the Kojaks as they are supposed to be a bit quicker