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j4k3

Almost time to replace my chainrings. A few questions. (lemmy.ca)

submitted 7 months ago by [email protected] to c/bikewrench

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Context: My 90s MTB still has its original Shimano chainring (crankarms say Alivo). The cassette was replaced several months ago, as was the chain and bottom bracket. No wear on the chain right now.

The derailleurs are Alivio, and probably the original ones from the mid-90s. I don't have any reason or intention to replace those, although, I do have a Deore LX on my decommissioned 90s MTB that I could likely drop in, but I digress. LOL

I guess two questions:

What would I need to provide to my LBS as far as measurements to get me a replacement? I understand that I'll likely need another BB and maybe(?) a chain, but I plan to stick with the same gearing.
Ideally, I'd want to find a 3x chainring where I can replace the crank arms and individual chain rings when needed. Will this be easy to find, considering the age and group set?

Thanks in advance.

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[–] calamitycastle 2 points 7 months ago (1 children)

If you are just needing a new one then everything else should be able to stay the same.

Do you definitely need new chainrings? They last pretty much forever unless they're damaged, in my experience. But nonetheless, really all you need is the BCD which is the measurement of the bolt distances:

https://www.wolftoothcomponents.com/pages/how-to-measure-bolt-circle-diameter-bcd

Then you need to know how many teeth your current rings have. Then order the same spec and install.

However if you are going from a single chainring to a triple then yes you will need a wider bracket as well to account from the extra rings. Assuming a square taper bb, it will only have two measurements, the shell, which will be either 68 or 73mm and then the total width (e.g. could be 110mm or 122mm or anything really). The latter will determine the clearance you get for the rings from the chainstays. It is measurable and your LBS could do it. This is ultimately going to determine your chainline, which you want to be as straight as you can (I.e. middle ring up front in line with middle ring on the cassette).

I'm by no means an expert but this is what I'd be looking for

[–] [email protected] 1 points 7 months ago (1 children)

Thanks for the info.

Yeah, the LBS said the middle chainring has some wear, so I'd rather just change it all up.

I'm staying with a 3x, and same gear range. Looking at a few Shimano front derailleurs, the BB will be quite a bit wider than what I have.

Maybe I can pretty-please my LBS to have a look and decide what the best swap would be. I'm also going by what they have in-stock or can get easily, so my options are further limited.

[–] calamitycastle 2 points 7 months ago (1 children)

In that case I guess I would just order 3 new chainrings with the same number of teeth, keep the cranks and bottom bracket, just make sure to order the right number of bolts and BCD. Simple. There's a special tool to help hold the chainring bolt in place while you unscrew it but you can make do with a flathead screwdriver or an old spoke

[–] calamitycastle 1 points 7 months ago

Edit: and the front derailleur should also be fine unless it's damaged

[–] j4k3 1 points 7 months ago (1 children)

My current road crank has around 70k miles on it, just for reference. It doesn't shift great, but that is from the frame design. I've run triples before. They are hard to dial in perfectly. When I commuted with one, working at my first bike shop, I had one absolutely dialed. It took having a fine threaded inline barrel adjuster and the one built into the down tube to get that thing just right.

Chainrings rarely cause issues. Even the ramp shape profiles have very little impact on the actual shift. Almost all of the force of the front shift is handled by the little ramp inserts.

It is extremely hard to quantify wear on chainrings unless you have an identical ring to compare directly. The profiles made to allow the chain to seat earlier and smoothly make the ring wear deceptive visually.

I'm not saying I know better. I'm simply an experienced former Buyer for a chain of bike shops. Some people swear that changing chainrings is important. They are generally a minority. Most of the mechanics that actually ride and race, only change them if they are sponsored to do so.

[–] [email protected] 2 points 7 months ago

I appreciate the insight. I'm not actually having issues with the chainrings/front shifting, but I did with the cassette, which is why I replaced it. I figured I might as well replace the chainring too, since it's over 30 years old and wouldn't be that expensive anyway.

But I guess it's one of those "if it ain't broke" situations, so maybe I'll leave it as is. 😂