Could someone enlighten me regarding chainwrap (or point me to a good online resource)?

I understand the concept that a rear mech. can only take up a certain amount of slack chain, and that a chain that’s long enough to be used with the big ring of a triple might be too long in the granny ring and certain small cogs for the mech. to accommodate the slack (I’ve been told that if I used a 48, 38, 28 triple and an 11-36 cassette I couldn’t use cogs below 15 teeth whilst in the granny ring), I’m just not sure how to do the maths.

Years ago I used a Deore 48, 38, 28 trekking chainset and an 11-32 (or possibly 34) toothed cassette in a 3×9 setup without issue.

Bump for the morning crowd.

(Big ring – granny ring) + (large cassette sprocket – small cassette sprocket) = required capacity.

Capacities are:

For Shimano:

SS – Short Cage Road Double – Maximum Cassette Cog is 27 and Total Capacity is 29

GS – Medium Cage MTB/Road Triple – Maximum Cassette Cog is 34(MTB)/27(Road) and Total Capacity is 33(MTB)/37(Road)

SGS – Long MTB – Maximum Cassette Cog is 34 and Total Capacity is 45

For SRAM:

Short – Maximum Cassette Cog is 34(MTB)/28(Road) and Total Capacity is 32(MTB)/31(Road)

Medium – Maximum Cassette Cog is 34 and Total Capacity is 37

Long – Maximum Cassette Cog is 34 and Total Capacity is 45

Taken from Here, but it’s a bit out of date, capacity is greater than stated. An SGS (long) MTB mech should see you right.

Thanks.

From an earlier thread though:

SGS (long cage) officially has a chain wrap capacity of 43T, (48-26)+(36-11)= 47

and

If the chain wrap spec. is 43T, then you simply can’t use any sprocket smaller than 15 with the little ring, so presumably all but the smallest two sprockets (assuming you set up the chain length so you can use big-big to avoid accidents)

Whilst I understand that with the small chainring and smallest cogs there’ll be too make slack chain for the mech. to handle, how do you work out that a 15 tooth and and 26 tooth ring will be fine but anything less wouldn’t.

(I might just stick with 42, 32, 22 and 11-32 and be safe)

Can anyone help an old duffer work it out?