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I just got an old MTB, I need to replace all the parts except the frame since the parts are in broken condition, except the sprocket and cassette I'm a total newbie and LBS here cost crazily high, so I'm trying to replace all the parts by myself

The o.g parts are :

  1. Shimano tourney SIS 3x6 - rusty and broken
  2. Tire size is 26x2.125"
  3. Crankset is rusty and broken too
  4. Brake is also rusty and broken

So the question is

  1. If I replace the group set to - let's say - Micro shift 3 x 9, will the group set work well? Do I need to change the cassette or I can stick with the original one which only have 6 speed and I can't/mustn't shift to 7th gear?

groupset microshift

  1. Which rim size will suit the tire? Is it also the 26 inch rim?
  2. If I want to use narrower tire, 26 x 1.5", do I need to find a special rim size which will accommodate the tire width or any 26 inch rim will just do fine?

Thanks

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  1. Just replacing the groupset is not enough. 9 speed cassette has different spacing from 6 speed, so the derailleur steps won't match the cogs. For 9 speed cassette, you'll need a new hub, and the new hub is wider than 6 speed so you need to spread the frame too.

  2. There are many different 26" sizes, none of which actually measures 26". For 26 x decimal inches, the correct one is 559mm

  3. See above and What is the maximum or minimum tire width I can fit on my bicycle

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Normally a bike frame is designed for a specific wheel size and it's not really usual to change that size unless you go for a very specific intent.

If your frame has only v-brake or cantilever brake mounts (two types of brake that squeeze the rim in order to slow down) you must stick to the same rim diameter.

If your frame has disc brake mounts, you can fit other rim diameter, however, there may be other limiting factors, for example tire clearance or toe overlap if you get a bigger combo of rims and tyres, and pedal clearance if the new combination is smaller.

Regarding tire width, most "regular" width MTB rims will allow anything ranging from 1.5 inch to 2.3 inch tire width, so unless your rims are wider than normal, you can easily fit a 1.5 tire.

I have done such thing myself: used a hardtail MTB for commuting and riding on the road by mounting 26 x 1.5 slick tires to it. (The same rims I normally use with 26x2.2 knobby tires)

Small width tires have a smaller outer diameter, so a 27.5 rim with a narrow tire may be similar to a 26 rim with a very wide tire, but it's more common to use a 27.5 tire with a very wide tire (near 3 inch) to achieve a similar outer diameter to a 29 inch wheel. (this is called "plus tire") Always consider that for this to work the frame must be designed for that or have the necessary clearance.

In my case, I currently have a frame that is designed for 26 inch wheels but has a lot of clearance, which, after many measurements and research, I determined it would accept a 29 rim with a very narrow tire (I used a 700c x 38 mm, 700c and 29 inch rims have the same bead seat diameter, that is, the same ETRTO size.).

This only worked because the frame has disc brake mounts and I swapped the suspension fork for a rigid one (the original one would not fit the 700c x 38 tire) I did this to have a hybrid commuter. The diameter increase from a 26x1.5 tire to a 700x38 made me gain about an inch of pedal to ground clearance.

For your case, I'd recommend sticking to 26 rims in regular width. You will be able to use 26x1.5 but you can also get wider tires that are slick (good for pavement) or have very little knobs which are a good compromise if your routes include dirt paths and sandy or muddy terrain.

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I already restored the bike, yes it's hella expensive, but still I'm having fun :)

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  • This is a comment, so please don't post it as an answer.
    – DavidW
    Dec 17 '20 at 15:42
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    Could you expand this with edit to show what issues you found and how you resolved them? That will make this into an answer. Photos would be very interesting too.
    – Criggie
    Dec 17 '20 at 19:46

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