How to change fitness signature for selected activities

I’d like to have a different fitness signature for activities using a TT bike. When I change it using advanced MPA tab it updates my global signature, how so I change it only for selected activities?

Out of curiosity: why would you want that? I have a TT bike and a ‘normal’, bike, but I have never considered that.

I’m definitely faster on the TT bike, but I cannot produce more power while using it, compared to what I can on the other bike…

Your fitness signature on a TT bike would not really be a fitness signature - it would just be a record of steady state efforts. A true fitness signature would cover a good range in the power demand curve - a TT ride doesn’t really utilise 5 minute power and you definitely wont hit peak power.

I have one signature for TT and normal riding and I am quite happy with that.