LBS says

alistairrs said:

after replacing my 8 speed cassette with a shimano 10 speed 12-27 ,that it is normal/ok for me not to be able to use the first gear (largest cog) on the back with the large cog on the front is this [censored]?

Not BS.
Do not run large to large or small to small. The same ratios will be available using other combinations that do not run the chain at an extreme angle.

alistairrs said:

after replacing my 8 speed cassette with a shimano 10 speed 12-27 ,that it is normal/ok for me not to be able to use the first gear (largest cog) on the back with the large cog on the front is this [censored]?

I'm not very knowledgeable but you're not supposed to cross the chain from the large chainring to the large cog or from the small chainring to the small cog if you do it will rub on the FD like crazy. On triples you can do all cogs with the middle chainring. So, with the large chainring you can use your 12-21 cogs fine and on the small chainring you can use your 15-27 cogs just fine (that is without rubbing on the FD). Hope it helps.

sp103154 said:

I'm not very knowledgeable but you're not supposed to cross the chain from the large chainring to the large cog or from the small chainring to the small cog if you do it will rub on the FD like crazy. On triples you can do all cogs with the middle chainring. So, with the large chainring you can use your 12-21 cogs fine and on the small chainring you can use your 15-27 cogs just fine (that is without rubbing on the FD). Hope it helps.

You are correct in your recommendation, but the real reason to avoid the cross-chaining is not front derailleur rub. The FD can usually be adjusted so that it does not rub. The main reason to avoid cross-chaining is that it accelerates wear on the chainrings, chain, and cassette cogs and there is increased frictional energy losses as well (albeit small). As was already mentioned, these gear ratios are nearly duplicated anyway.

a couple things come to mind with this question.

1. do you have rear suspension? if you do, the suspension movement may cause problems with chain slack etc and so the shop did the best they could.

2. did you upgrade everything - rear der, cassette, and in particular, crank (and therefore chainrings)?

3. what are the chainrings in front? what is your rear derailleur model? are you exceeding the capacity of the rear derailleur to take up chain slack?

it might be that the chain is too short. or that the shop did what shimano recommends which is to make the chain 2 links longer than the minimum required to reach around the large ring and large cog. from my experience this makes the chain as tight as possible - and causes premature chain wear, excess noise, increased friction, and poorer shifting performance.

if you have a choice, I would do it a bit differently - use the longest chain possible in the small/small and see if that will work with the large/large combo. this will work *only* if you are using a derailleur capable of taking up the slack. you have a 15T difference in the rear, you may have a 20 or 22T difference in the front, so your derailleur will need to be able to take up at least 35-37 links of slack. if you have a double crank, you may be dealing with a total of 29T slack.

the chain length technique described will give you a chain about 4 or 6 links longer than than the Shimano recommended method. it greatly reduces friction and makes things a lot quieter in the large ring and larger cogs. the drawback is the chain will bounce more in the small ring. and if you have an inappropriate rear derailleur (which cannot handle the chain slack), my method will simply cause you to shorten your chain too much.

finally, based on your new, wider, 10s cassette, the chain angles will be extreme enough that, for a given smaller ring, you may see/hear the chain hit the next larger ring when using the 12 or even 13T in back. my standard 39/53 crank + standard 10s cassette sees me rubbing in the two smallest cogs.

however, in the case of using the small/small or large/large, on regular bikes, with no funky alignment or other problems, you can use those ratios in a pinch. It's not ideal but sometimes you'll find it useful if it avoids shifting the front derailleur back and forth. I've installed numerous chains on road bikes, hardtails, and full suspension bikes and never had a problem with my "small-small" method (I used to be on the LBS side of things).

hope this helps
cdr

I queried this exact point when I got my new bike with 3x10 derailleurs. I've since learnt that it is best to regulate your gear selection to the following combo,

Large and middle chainrings - Only use the smallest 6-7 cogs
Small chainrings - Only use the largest 3-4 cogs

Running is now much quieter and smoother. I am sure it'll also pay off in terms of durabilty of the running gear.