Chain skipping down chain rings

[mattb]

Hi All
I have just started home maintenance and have replaced both my rear cluster and chain. Now if I pedal backwards when in largest front and largest rear chain rings the chain will drop down to the second biggest rear chain ring. Whilst this isn't something I will often do I am concerned it is indicative I have done something wrong, like my chain is too long. Ultegra and Dura Ace mix drivetrain.
Any help would be appreciated.
Thanks
Matt

[DaveM]

I'm not super familiar with the latest high end drive trains. But I suspect this is not a problem. Being in both large chain rings is a gear you really want to avoid anyway. It is a middle range gear (high in front, low in back) that is better achieved by using your middle chainring. This is called 'cross chaining' and puts a lot of stress and extra wear on your drive train components.
It's possible your front derailleur adjustment is not perfect since I would normally expect it to prevent this. And if your chain is jumping around in other combinations, there may be a problem. But if only happens backpedaling in that combination, I doubt it's an issue.

[mattb]

Thanks Dave, appreciate the help.
I don't tend to ride cross chainring and also think its ok. Maybe it used to happen before and I just never noticed! I can't make it do the same thing in any other combination so I think I will just get on with riding.
Thanks again
Matt