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I had a new chain and cassette fitted yesterday. But today, hills which I could easily climb two days ago are now horrendously challenging. What was a pleasure has just become a nightmare. Why is this happening? What should I do?

The bike is a Specialized Tricross, about 4 years old. Never had new rear cassette before, but has had new front cogs, new derailleur, and previously one new chain. The bike appears to work OK when on a rack. See my comment below, please.

Edit. Update with new information: Counted teeth as follows: front 50/30, rear 28/11. There are eight rear cogs. The front cogs were changed a year ago, so I have been accustomed, and the shop advised at the time, original size was now obsolete. It would seem that the rear cassette changed ten days ago is a different size to original, and it seems this may explain my struggling to ride with same positioning of gears as previously. Each of the numbers I have quoted here I counted twice. Seems a different size of cassette was fitted

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    Is this problem only present when climbing, or is your downhill-speed also impeded? How does "horrendously challenging" manifest itself, is it some kind of extra resistance, or is it something like cranks turning too slowly? Do you feel extra resistance when you have your wheel off the ground (bike upside down or suspended somehow)? Do your wheels turn freely when off the ground? Please add some of these details, perchance you will answer your question yourself when you try to determine the answers to my questions... – cmaster Jun 29 at 11:34
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    Does the new cassette have identical gear ratios to the old one? Hybrid bikes often come with a cassette whose biggest cog is much bigger than the next one (I forget what Shimano calls this; the informal term is a "granny gear"), but these often get replaced with a cassette with closer ratios. – David Richerby Jun 29 at 12:35
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    @DavidRicherby Shimano MegaRange is what you are thinking of – Argenti Apparatus Jun 29 at 12:44
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    @DanielRHicks agree - OP should take bike back to where the chain and cassette was fitted and ask 'what's wrong?' or 'what changed?' – Argenti Apparatus Jun 29 at 17:46
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    Just to point out: There should be no “bed in” required. Drivetrain friction goes down very slightly after after a few hundred km, but we are talking <1% losses here. – Michael Jul 2 at 11:36

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