Engine theke max koto power output kora hocche tar upor deped kore cooling deya hoy.. R15/MT/XSR er 155 cc engine theke around 20 hp power output neya hocche. So water cooling needed. Royal Enfield er 350cc engine theke same around 20hp output neya hocche. So water cooling needed nah. With water cool 350 cc Engine theke I guess more then 30hp output paoa jabey. Amader koto cc engine egula care na kore.. bike tar max hp and torque koto eta dekha uchit.
Jokhon liquid cooled yamaha mukhe angul die ghumachilo tokhon manush classic 350 nie ladakh jeto. 5000 km er trip e tahole I guess air cooled engine gulo melt hoye jeto. Eto gorome bhoy pele car use kora better AC chaliye.
I ride an air-cooled bike, and if the bike's top speed is 100, you can cruise at 70 km/h for 24 hours without the engine overheating. But if you push it to 100, you'll run into heat issues.