Chainset/Derailleur mismatch

[Aroundm21]

My new chain started to slip so looking at gears, I saw the front was very worn and replaced the crank/chainset.

the cheap replacement worked well - except I didn’t spot steel material so the weight.
so I again replaced this Decathlon2449198 - CRANKSET 48/38/28 8S 170MM
https://www.decathlon.co.uk/48-38-28-chainset-silver-id_8517148.html

with this one 4096853 - CRANK SET 7/8/9V ..
https://www.decathlon.co.uk/bike-triplebottombracket-789-id_968556.html

Both triples; same; original bottom-bracket; 8 gears at back. Admittedly different numbers of teeth,

Now the problem is that my Shimano Alterus derailleur will not reach-out for the largest cog on this new one. It covers smallest & medium cogs but will not move the chain from small to medium (it will move down to smallest; and I can move it from smallest to medium by hand).

I’m reluctant to buy a new derailleur as I don’t believe that’s the problem but cannot see why the new chainset is deeper/wider; it certainly doesn’t look it !! The Decathlon chainset were stated as being Shimano compatible.

i also cannot get any slack from the cable shifter; whereas I can adjust the rear gear cable from the shifter !

any ideas appreciated - how do other people ensure compatibility please ?
thanks in advance - Mike.

 

[Ajax Bay]

The Shimano compatibility thing is a red herring and the number of teeth on the rings is, as you imply, irrelevant.
First of all, check you have unscrewed the H limit screw on the front derailleur. This may be preventing the cage from swinging across (by hand) far enough to shift the chain onto the large ring.
The original chainset you fitted needed (I'm guessing by looking at the 'often bought with it' list) a 118mm BB. Don't know how long the "original" BB spindle is - you could measure it, with care.
Chainsets specify the correct spindle length (square taper) and your new one requires a 113mm - this gets the chainline right. So if your old new one was OK and fitted onto a 118mm then the new new chainset would be 3mm to the right (half of 118-113) and the large ring might be too far right for the FD to reach. Solution is to procure a UN-55 BB (link is 115mm) of 113mm with the correct BB shell width (likely 68 or 73mm but must get it right). This will be a better BB than the one you have and should cost <£23. A cheaper BB is the UN-26.
I'd take the chainset off, measure the spindle, and go from there (if over 113mm).

i also cannot get any slack from the cable shifter; whereas I can adjust the rear gear cable from the shifter !

Sorry - don't understand.

 

[Aroundm21]

Thanks for replying.
just to let you know that I asked my local bike shop to look (suggesting a chain extension may help) and they worked out that the chainset hadn’t been fitted properly onto the bottom bracket !

if you’re anywhere near Manchester Uk, then big shout to Ken Fosters cycle logic in Chorlton who got me out of this inexplicable situation and back on the road !
thanks for replies.
mike.