Trainer and bike settings
 

Hello. I have an ultegra di2 12v.
On my treainer i have installed my old 11-34. On my bike i'm using the 11-30.
***chain length is correct for both****
i want to know how i can set it up correctly for both because on the trainer the derailleur is skipping a bit.

It is likely that you will have to make minor adjustments to your RD. You're dealing with two different cassettes and different hubs, they are seldom exactly alike.

Trainer is a wheel on trainer or wheel off trainer?

If it's a wheel on trainer, then the bike did shift correctly before you put it on the trainer... or when you take it off?

But I really assume that it is a wheel off trainer. And rccardr gave about the only answer that one can give for the virtually no information you gave us.

A wheel off trainer, can have both wheel cassette and trainer cassette be adjusted, so as to use the same rear derailleur setting.
But nobody said it was easy.

https://zwiftinsider.com/trainer-cassette-spacing/

Barry

You installed your old cassette on the trainer and its skipping suggests to me cassette is worn. Put the cassette that is on your bike right now and switch it to the trainer and see if it skips. If it doesn't you will know the answer. I also don't think your derailleur is skipping but the chain gliding over the top of the cogs unless I am confused as what you are saying. I use four different bikes on my Tacx trainer (all 12 speed) and I never need to adjust for anything.