Re: "Sliding windows that DON'T" with out a struggle !!!
Alf, sliders are inherently terrible for that. Some of the better ones we install in homes have rollers with bearings and ride on a track. Because of needing a tight seal, operation is always a bit stiff with a slider, as opposed to an awning or casement style window.
One thing one must do is to be diligent about keeping them clean, as the track is exposed to the weather. Once cleaned, a lubricant that dries completely, such as a spray silicone can be used. Don't use anything greasy because it will just attract more dirt.
2017 Escape 5.0 TA (someday soon, I hope
2015 Ford F150 Lariat 3.5L EcoBoost
2009 Escape 19 (previous)
“Most folks are about as happy as they make up their minds to be.” — Abraham Lincoln