Crank/chain ring compatibility advice.

[smutchin]

2.55in (64.7mm) between adjacent bolt holes = 110mm BCD

 

[smutchin]

VM ok 175

175 will be the crank length in mm - has no bearing on chainring sizes but something to consider if you ever replace the chainset.

 

[Pale Rider]

What I want to establish is exactly what I need for when there is an offer on at some retailer.

Seems odd to me to buy a spare chainset for a new bike.

Chainsets last for thousands of miles, so unless you are doing a very high mileage - split between your two bikes - you aren't going to need a new chainset for years.

If replacement time ever comes, you might want to change the gearing, which is another reason not to buy now.

It might make some sense to keep an eye out for a good offer on an 11-speed chain.

You will almost certainly need one of those at some point, and the more speeds, the dearer the chain.

 

[Ajax Bay]

So, my question now is am I stuck with this 5 arm chain ring? When I eventually wear it out, what other bits would I need to change to accommodate the 4 arm chain rings-both large & small?

As @si_c says: "Expect it to last a long time before needing to think about replacement."
The OP's question has been answered but after 20,000+km, if the crankset is still going strong but (probably one of) the chain rings need(s) replacing, buy a replacement chain ring (NB BCD = 110mm and 5 bolt). Because chain rings have to be replaced so infrequently, there is minimal merit in seeking to achieve compatibility and significant expense.

 

[night cycler]

Seems odd to me to buy a spare chainset for a new bike.

Chainsets last for thousands of miles, so unless you are doing a very high mileage - split between your two bikes - you aren't going to need a new chainset for years.

If replacement time ever comes, you might want to change the gearing, which is another reason not to buy now.

It might make some sense to keep an eye out for a good offer on an 11-speed chain.

You will almost certainly need one of those at some point, and the more speeds, the dearer the chain.

It was just that I picked up some 105 34T rings for the Rose.

Yes I will keep this in mind about the chain. I have no idea what chains cost generally.

What sort of price would be considered a good offer for an eleven speed chain?