You can fit 28c tyres on the Langster, it's just depend on what tyres really, i.e. Marathon Plus is a big no-no as the sizing is closer to 30-31c than 28c, whether the Conti Gatorskin 28c will fit neatly.
my advice on the Plug is to change the tyres for a decent one, like conti gatorskin 25c or 28c as stock tyres are not the best.
You can fit 28c tyres on the Langster, it's just depend on what tyres really, i.e. Marathon Plus is a big no-no as the sizing is closer to 30-31c than 28c, whether the Conti Gatorskin 28c will fit neatly.
my advice on the Plug is to change the tyres for a decent one, like conti gatorskin 25c or 28c as stock tyres are not the best.