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Changing cassette size

Old 04-16-21, 08:20 AM
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mistagregory
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Changing cassette size

Question for any bike mechanics out there, or those with the knowledge: I currently run an 11-40 cassette with a XT Di2 derailleur on my 1X gravel bike. Can I change to 11-42 without making adjustments to chain length, and will the Di2 derailleur adjust itself to shift properly to the new size? 11-40's are impossible to find but some online vendors have 11-42 so I'm thinking of going that route.
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Old 04-16-21, 10:02 AM
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Welcome to Bike Forums.

Pointer, there is a Mechanics Forum here on BF that is better suited to this question.
However it will certainly get enough reads here in General to allow it to find help.

All Rear Derailleurs have a "Capacity" spec expressed as two numbers.
1. The largest size rear cog permitted.
2. The tooth difference between the largest and smallest rear cogs. + tooth difference between the largest and smallest front chainrings.
Obviously with your 1X (single front chainring) you are only counting rear cog teeth difference.

If your chosen cassette falls within these specs, the rear derailleur is good to go.
If it falls outside the spec's..... well there are many experienced opinions here to help with that.
All Shimano Specs available here
I think yours is on the far right of this page


As for the chain length.
We can't tell how long your chain is, how long your bike is, and the size of the single front chainring.

Best practice when changing a Cassette is to also change the chain. They tend to wear as a pair.
So one of the many "How to" videos on determining a replacement chain length will suite you well.

All the best

Barry

Last edited by Barry2; 04-16-21 at 10:15 AM.
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Old 04-16-21, 10:15 AM
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Thanks Barry. I just put a brand new chain on, so that's not an issue. I was just wondering if I could upsize to a 11-40 from 11-42 without making any other adjustments.
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Old 04-16-21, 10:22 AM
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I think I found your RD spec, see the new edit to the Shimano page in my first post.

Chain is likely too short, but ONLY you can determine if the chain is of a suitable length, You may have already cut it on the short/long side... I can't tell.

Barry
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Old 04-16-21, 10:32 AM
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Originally Posted by mistagregory View Post
Thanks Barry. I just put a brand new chain on, so that's not an issue. I was just wondering if I could upsize to a 11-40 from 11-42 without making any other adjustments.
I'd just try it and find out. Perhaps you'll need to add a (addt'l?) quicklink, or not.
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Old 04-16-21, 10:37 AM
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Originally Posted by Sy Reene View Post
I'd just try it and find out. Perhaps you'll need to add a (addt'l?) quicklink, or not.
Reene,
Please do tell the OP what happens to his RD if he hops on his bike the chain is too short?

Thanks

Barry
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Old 04-16-21, 10:41 AM
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Originally Posted by Barry2 View Post
Reene,
Please do tell the OP what happens to his RD if he hops on his bike the chain is too short?

Thanks

Barry
Umm Ok. I'll spell it out.. Try out the chain while bike is in the stand, pedal slowly while trying to shift into the largest cog.
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Old 04-16-21, 10:43 AM
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Thanks all, appreciate the advice! Happy to be here on BF, should have joined years ago. Cheers
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Old 04-16-21, 10:59 AM
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Barry, I can't access private messages because I'm a newbie.
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Old 04-16-21, 11:05 AM
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Sorry, I thought noobs could still read PM's.

The PM simply states... please be careful.
If the chain is too short and you attempt to ride the bike, bad things can happen.
Be sure to test the shifting to the large cog when NOT riding the bike.

Didn't want you to bend or break anything.

Barry
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Old 04-16-21, 11:26 AM
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Originally Posted by mistagregory View Post
Thanks all, appreciate the advice! Happy to be here on BF, should have joined years ago. Cheers
This might provide some guidance as well. Rule of thumb? Add this many teeth and add this many chain links?

If you feel like it, you could also take a picture of your rear setup shifted into the smallest cog, and another in the current cassette's largest (40) cog and post here for a looksee. Make sure you include the full derailleur and a jockey wheel assembly.
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