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

with this one 4096853 - CRANK SET 7/8/9V .. (Unfortunately I cannot post web links)

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 chainsets 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.

shelbyfv

More info is needed but if your cranks use a square taper bottom bracket they may require different lengths to achieve the correct chainline. More specifically, your new crank may need a shorter bottom bracket. The crank specifications should tell you which bottom bracket length is correct. If it all worked before, it's unlikely the problem is with the shifter, cable or derailleur. Hope this helps and welcome to BF

dedhed

Sounds like you used the existing bottom bracket whose spindle length is longer than required for your new crankset.
Cranksets require a specific spindle length to achieve the correct chainline. Your existing spindle is likely too long moving the chainline out beyond the reach of the FD.
A crankset will specify a specific BB length to be used.

dsbrantjr

A quick measurement of the old and new cranksets' chainlines will verify this and guide you in choosing a different BB spindle length if (probable) needed.

Barry2 

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Barry

dsbrantjr

I believe the OP is experiencing difficulties with their front derailleur.

tomtomtom123

I think his confusion came from the op calling the chainrings as cogs which I believe most people will think of cassettes.

Barry2 

Nope - I was just asleep at the time.
Can't blame the OP on this one.



Barry

Aroundm21

Thanks for many replies so soon.

i get the theorey that the BB may be too long. The Chainset with issues states “minimum 113mm” — I find it strange that there’s no maximum stated since that is presumably my issue (too long so pushed out a bit). There’s certainly no visible gap between the chainset & frame, however the issue is probably only 3-5mm.

unfortunately the chainset that did work does state anything about the BB on the website. If anyone can suggest how to compare, I’d appreciate it — removing the BB and buying new is a pain for a few reasons I won’t bore you about! Usually ai’d cycle to Decathlon and ask them.

(the bike is a Riverside 500 but I don’t know anywhere that states specifications such as BB length).

By the way, “cogs” meant the front chainset; I have no issues at the back.

I’ll also look into chain length given different numbers of Teeth.

thanks again.

dedhed

We'd need better information on what the cranksets in question are. Brand? manufacturer? model? part numbers off back of arms?

70sSanO

So you removed the old crankset and installed the new one.

Did you loosen the cable from the derailleur and are-set the limit screws for the new crank?

Barrel adjustments at the shifters are just a convince to be able to fine tune the indexing. They do not adjust cable slack or setup a derailleur for a crank.

You need to loosen the cable attached to the front derailleur, adjust the low limit screw for the inner chainring and re-attach the cable. You will probably have to adjust the high (outer chainring) limit screw. After that is setup, you can fine tune the setup at the shifter.

Jihn

tomtomtom123

You said 113mm minimum. But did you measure the spindle on your bike? There is no way to compare unless you find out what's already on the bike.

frankenmike 

As stated above, first things first: remove the cable from the derailer. Now set your limits using your hand to push the derailer as you pedal.(this is tricky but not impossible). If the derailer still won’t reach after limit adjustment, then a shorter spindle is indeed required.

Aroundm21

Noting some ideas I took it to a local shop (who had not supplied anything, and I knew would be booked-out for repair work) and they very kindly reviewed what I really needed.

turned out that the crankshaft wasn’t fully on the bracket - he refitted it in a minute and it works fine. No visible difference.

so big thanks to Ken Fosters Cyclelogic in Chorlton, Manchester.

And thanks for replies here to guide me and identify possibilities.