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Replacing chainset - already got cranks

Old 09-13-21, 08:33 AM
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Altlandisto
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Replacing chainset - already got cranks

I need to replace the worn out chain set on my bike (aka front cassette). Looking online to buy the 2 parts is difficult. I already have pedal cranks. Do I really need to buy the full chainset and crankset together? Thereby making my current pedal cranks useless.
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Old 09-13-21, 09:09 AM
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No but you need to know the bolt circle diameter for the cranks that you already own. That's a harder measurement to make than you are probably thinking.
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Old 09-13-21, 09:14 AM
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Chainset and crankset are pretty much the same thing. Chainset is typically how people from the UK and elsewhere refer to what we in the USA call a crankset.

How are you meaning it?
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Old 09-13-21, 09:21 AM
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Originally Posted by Iride01
Chainset and crankset are pretty much the same thing. Chainset is typically how people from the UK and elsewhere refer to what we in the USA call a crankset.

How are you meaning it?
It's just the two front sprockets which need replaced. The teeth are worn down, and would damage any new chain.

But rather than being able to just replace these 2 sprockets, it seems that I need to buy a full set of the 2 sprockets AND the 2 pedal cranks, including the bottom bracket.
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Old 09-13-21, 09:30 AM
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Depends on what crank you have and of course whatever is available in the limited inventory that will likely persist well into 2022.

Some cranks, you are almost better off replacing the entire crank with rings. Others it can be quite inexpensive to just change the rings.

I have bought a new crankset and kept the rings for my existing crank and sold the bare crank.

So you have options. But it'd be easier to and less hypothetical if we knew the exact make, model and version of crank and any other part you wish to discuss.
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Old 09-13-21, 09:36 AM
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Originally Posted by Altlandisto
It's just the two front sprockets which need replaced. The teeth are worn down, and would damage any new chain.

But rather than being able to just replace these 2 sprockets, it seems that I need to buy a full set of the 2 sprockets AND the 2 pedal cranks, including the bottom bracket.
The word you're looking for is "chainrings." You can replace just your chainrings as long as you match your bolt pattern (5-bolt, 4-bolt, etc) and BCD (bolt circle diameter).
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Old 09-13-21, 09:51 AM
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https://www.wolftoothcomponents.com/...e-diameter-bcd
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Old 09-13-21, 10:03 AM
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Thank you to all above.

The bike is a BTwin Alur 700 road bike which I got second hand. Btwin is the own brand of a sports shop called Decathlon.
Unfortunately I have no idea what the Bolt Circle Diameter of the bike is. But I found some info online: "Cranks: BTwin chainset 50/34, 12-28" https://www.bikeradar.com/reviews/bi...ur-700-review/
The Decathlon website advises me that the spare parts compatible with this bike are as pictured - i.e. they are not selling the chain rings on their own.
I obviously know the bolt pattern (5-bolt).
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Old 09-13-21, 10:06 AM
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Lots of chainrings will list along the side the BCD or bolt circle diameter, as well as how many teeth, as long as the BCD is listed everything else should be easy, you can count the number of teeth, the number of speeds your bike has and the number of bolts.
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Old 09-13-21, 10:09 AM
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Yeah, look at the chainrings themselves. Are there 5 bolts? If so, and you've got 50/34 chainrings, it's a pretty safe bet that they're 110mm BCD, but they should be marked. If your crankset says Shimano on the side of it, well, get some R7000 5800 chainrings.

Last edited by WhyFi; 09-13-21 at 12:27 PM.
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Old 09-13-21, 10:25 AM
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From what I gather from the bikeradar link is that the bike is a 10 speed 105 drivetrain with a BTwin 50/34 crank.

BTwin probably doesn’t make the crank and it is just a similar FSA, Origin 8, etc. crank. As such there are no published p/n specs like what you would find with a Shimano crank.

As mentioned above, you do need to measure the BCD and look for a matched set of 10 speed 50/34 chainrings that will fit. You have some latitude to mix-n-match, but for your situation just buy a set.

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Old 09-13-21, 10:42 AM
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OP, per you rlink to the bikeradar article...
Crankset (chainset) you have is Shimano 105, 5 bolt 110 BCD
to replace the 'chainrings', you;ll need the proper 'tool' , might be a hex wrench or star wrench - AND a small tool which hold the backside collar as you unscrew the chainring 'bolt' (sometimes can be done using a LARGE slothead screwdriver... but not for all...)
10 spd chain of your choice ... Shimano, or SRAM or whatever... I find KMC X10 works great for me... Around $30-$40 USD, depending where you buy.
Chain may need to have links removed to get proper length for you setup, requires a chain tool which will work with 10 spd chains...
Rear - Cassette, given that you current chain is toast -stretched AND the Chainrings are well worn, I would HIGHLY recommend replacing the cassette. If you like what you currently have , et the same, - Shimano 105 10 spd cassette cog range of choice (artilce says 12-28 is stock) ... a '10 sd chain whip' and a cassette lockring tool ... to remove old and put on new - very easy job.
costs..
2 Shimano Chainrings to match .... prolly $130 to $150 for both... might be had for less with diligent online searching...
OR Newer 105 4 arm crankset (chainset includes crankarms & rings, no BB) - prolly $200 ish + ???) , replacing Crankset is easy, if done properly... 5mm hex and small plastic non-drive side crankarm cap tool
Chain - $30 - $40 online
Cassette - $60 - $80 online
+ tools...
ride on
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Old 09-13-21, 12:08 PM
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If it is a Shimano crank, then the part number will be stamped on the backside of the arm near the hole for the pedal. As do a few other mfr's.

If you know the exact part number you will be able to find out a lot about what you might or might not be able to use with those cranks.
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Old 09-14-21, 07:04 AM
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Originally Posted by Altlandisto
It's just the two front sprockets which need replaced. The teeth are worn down, and would damage any new chain.
Are you certain that what you think is wear is not actually shift ramps to facilitate shifting?

But rather than being able to just replace these 2 sprockets, it seems that I need to buy a full set of the 2 sprockets AND the 2 pedal cranks, including the bottom bracket.
In many cases, the chainrings are available separately. You do need to know the bolt-circle diameter for your crank to get rings that will fit. Some lower-end cranks have the chainrings permanently riveted to the crank arm; in that case you need to replace the whole crank, which may also require replacing the bottom bracket.
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Old 09-14-21, 09:32 AM
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It's just the two front sprockets which need replaced. The teeth are worn down, and would damage any new chain.
I have to ask the same question as JohnDThompson Are you certain that your rings are worn? On modern chain rings and cogs they do a lot of sculpting on the teeth to make them shift better. And there are many teeth that are unique in their shape. You might can go to the website of your cranks maker and maybe find a pic of your crank when new and compare.

Also the chain is way harder than the rings. It'll not be damaged by the rings other than if your rings are worn you won't get as much life out of your chain. But you'd still get a lot of miles out of it. So if low inventories are a problem finding something. Just ride on with the new chain. Eventually something will be there.

Typically if your rings or cogs are worn, then they'll show that by letting the new chain skip over them when you start to put some power into them. Though you still have to eliminate some other causes unless you just first want to spend money for new rings or cassette.
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