1

I need to change my crankset and bottom bracket, is is possible to upgrade to 48 tooth from the stock 42? The front derailleur is shimano Alivo and has a clamp. It could be moved up to a higher position to accommodate the bigger crankset. Will the upgrade work? Or will I have to buy a new derailleur?

Thank you for your valuable responses, I will stick to my stock 42 tooth option. The risk of damage/danger is higher that the merits of high gearing at the top. Presently I am able to use the total gear range without any problem. I don't want to mess with the usability of the system.

Improve this question
2
  • 1
    48t is a very big chainring for mountain bikes, and I'm not sure it's even usable on MTB terrain. Consider that traditional 2x cyclocross gearing is 46/36t in the front. Are you mainly riding this bike on the road? – Weiwen Ng May 21 '20 at 16:33
  • Yes I am riding the bike on road mostly. – Antony Manuel May 22 '20 at 18:29
5

What you need to do is look up the total capacity specifications of both your front and rear derailleurs.

For a front derailleur the total capacity is just the largest difference between the largest and smallest chainring tooth counts the derailleur can handle.

The total capacity is for a rear derailleur is the largest (difference between the largest and smallest chainring tooth counts) + (difference between the largest and smallest rear sprocket tooth counts) the derailleur can handle.

If the crankset with larger rings exceeds the front or rear derailleur capacities, it will not work, however, Shimano's specs are quite conservative, so you may get away with a couple of teeth over the specified max numbers.

You can look up Shimano derailleur specifications here: https://productinfo.shimano.com/#/

Another concern is chainstay clearance. Larger rings may hit the chainstays, especially as you are thinking of rings that are on the large side for a mountain bike.

Improve this answer
1
  • I would also like to point out from experience that some modern front derailleurs from Shimano are only compatible with the max chainring size from the cranksets in the same groupset, as some have a tighter arc design to lessen chain drops. (therefore not accommodating bigger-diameter chainrings) – Gregory Leo May 21 '20 at 18:26
2

The profile of most triple MTB front derailleur cages is set to match a 42 or a 44. Pair it with a 48 and you'll have to raise it up for it to clear, and then you can have problems with bad performance, chain drop, and the chain rubbing on the cage in some of the small ring combinations. There are Shimano front derailleurs that have a cage profile that matches better with a 48 but still work with MTB type shifters. They're usually identified as "Trekking" models. Ideally you would get one of those for things to work well.

You'll also need to address the total capacity of the two derailleurs. Just going to a 48 without changing the small ring as well may exceed the rear derailleur's ability to tension the chain in all gear combinations (likely will unless you're running a 28t large cog in a setup that could handle a 34t). Likewise, leaving the small ring alone would likely result in the chain dragging on the FD cage in some small/small combinations. There are many questions here about total capacity. Cheating it to get more total gear range at the expense of not being able to use the small/small combinations is an option if you can handle the risk of consequences should you shift into those gears accidentally.

Use the specifications tables on si.shimano.com to figure out what current production front derailleur has numbers that meet your needs.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.