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Can I upgrade my bike with a METREA Urban crankset with chain guard 1x11-speed if my current system is Shimano FC-M171, 42/34/24 with bottom bracket Shimano BB UN26?

And can I install CS-5800 cassette if now is Shimano CS HG31, 11-32, 8-speed with Rear Hub Shimano FH-RM 35, 32h? Thanks for information!

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Yes, Metrea cranks will fit on your bike, but you need a new bottom bracket. You have a threaded bottom bracket shell with a cartridge BB, Metrea cranks work with outboard Holowtech II BBs in the threaded shell. The Shimano spec pages says use an SM-BBR60.

Whether you can get an 11 speed cassette on the freehub depends on the size of cassette you use. Road 11 speed freehub bodies are slightly longer than 8-10 speed ones to accommodate the wider cassette, however, if the largest sprockets are big enough the cassette spider can overhang the hub a little and not interfere with the hub body or spokes. Many larger cassettes come with a spacer that is used on 11 speed freehub bodies and left off on 8-10 speed freehub bodies.

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The Metrea crank will first of all need a different bottom bracket, which is no problem. What you need to worry about is frame clearance. Depending on how your bike is set up now, it may put its single 42t ring where your 34t is currently, a big jump. Whether the frame will be able accommodate it without rub isn't something a forum can help you with.

To figure it out, you'd pull its chainline measurement off si.shimano.com and compare that to a firsthand chainline measurement of where your current 42 from your triple is falling. From that comparison you may be able to easily see sure, no problem, or you may find that you have to look more closely. If the latter, use a ruler or caliper to get a sense of whether it could go in as much as it's going to and still give about 3mm of clearance minimum, usually considered the smallest reasonable number to accomodate flex.

CS-5800 will not go on FH-RM35. No 11-speed road cassette will.

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