The guys at Brompton aren’t all that strict
the brompton guy at the bike show was – I had seen a brompton with a riser on like that – which looked like a more comfortable and stable ride – and asked the rep about it – he said it would invalidate the warranty…
More likely they wanted to promote their new high rise H stem…
Actually think brompton have missed a trick to rationalise stems and bars..
currently there is the M bar and stem, the P bar and stem, the S bar and stem and now the H bar (Mbars) plus new H stem..
I think they should of moved to the S stem and offer a range of bars on the one stem – they could cut down on the number of cable options they have etc..
anyhow as for warranties – not an issue – can’t go through life not doing stuff just in case you invalidate a warranty.
Here was my old bike and 2″ riser Sunline MTB:
Current 3″ is an SJS comfort bar – better shape – both had to be cut down though.
For taller riders it seems to be the best way – the new H stem makes you VERY upright plus you get the rubbish flexy M bars..