Halolll

I have a 26” mountain bike 21 gears and I need to buy a replacement back alloy rim, the problem is I’m not real sure which type of rim I need and I want to buy a second hand one maybe on EBay if possible. I’ve been told there are two different types of rims. Any help to resolve this problem would be greatly appreciated.

Scooper

There are more than two "26 inch" rim sizes, so you need to be more specific. Sheldon Brown has a good explanation of tire and rim sizing HERE.

Generally, 26" mountain bike rims will be ISO 559mm, although some Schwinn 26" rims are ISO 584mm. You also need to know the width of tire you'll be mounting to determine the appropriate inner rim width.

Finally, different rim manufacturers have different effective rim diameters (ERD) for the same ISO size, so if you want to reuse the same spokes with the new rim, the ERD of the new rim will have to be the same as the ERD of the old rim.

This all assumes you want to lace a new rim to the existing hub rather than just buying a new wheel.

barba

Unless you are looking for a project or have an uncommonly good hub you want to save, it is probably going to be cheaper to buy a new wheel. Figure the cost of the rim, all new spokes and the labor to build the wheel (assuming you are not doing it yourself) into your calculations.

halfspeed

Stupid question: Are you talking about =rims= or =wheels=? Some n00bs don't know the difference. Buying rims and building them into wheels is not a novice project.

operator

I'm going to venture a guess that he wants to buy a new wheel - the 2 "types" he's talking about are steel and AL. Either that or post #2

Bill Kapaun

Or a Free Hub vs Free Wheel?

Halolll

I’d be more inclined to say my question is one of ignorance, but your response is pure stupidity.
Yes I need and new back wheel, sometimes I say wheel people think tyre, I tried writing rim and still there is confusion.
I’ve been told my wheel is of a ratchet type, would that be correct? Is there a ratchet type?

halfspeed

Sorry, I meant my question might seem stupid. Not yours.

On a bicycle a wheel is made of a hub, spokes, spoke nipples and a rim. These parts are available separately and some experienced cyclists will assemble wheels from those parts. That's part of why your question was confusing. On a bicyle a 'rim' is never a complete wheel.

A tire is a separate component and a tube is installed inside it.

"Ratchet type" isn't very meaningful. Multi-speed rear hubs are either freehubs or freewheels, both of which ratchet. There are also one speed 'fixed' gear bikes. So it's still not completely clear what "two types" you're talking about. My best guess is that you're trying to determine if you have a freehub or a freewheel.

The best place to go to learn the terminology is here: https://sheldonbrown.com/

Also, read this to find out if you have a freewheel or a freehub: https://sheldonbrown.com/free-k7.html

Finally, note that most discussions will refer to 5,6,7,8,9 and 10 speeds. This refers to the number of rear sprockets, not the total number of gear combinations. So your bike is '7-speed'.

Scooper

OK, let's start over from scratch. What brand, year, and model is your bike?

Since it's a 21 speed, your bike probably has a triple crank chainring and a 7 cog cassette on the rear wheel. Is that right?

Rear wheels with multiple gear cogs generally use either older "freewheels" with the ratchet mechanism inside the freewheel, or they use newer "freehubs" in which the ratchet mechanism is more or less integral to the hub. Both use ratchet mechanisms, but there are substantial differences between the two.

Knowing the brand, year, and model of your bike would be helpful in trying to pin down what wheels would work.

EDIT - halfspeed beat me to it by a minute.

Halolll

The bike is near 10 years old on the hub is written Parallax Shimano Alivio FH MC10 VIA and I bought it from a bike shop and must of been build by them because it had no brand name.

Bill Kapaun

You have a Free Hub.

Thing is, it doesn't really matter if your replacement wheel is Free Hub or Free Wheel. The cog spacings should be the same. Therefore, either type should slip in/out if set up properly AND are both 7 speed.
The Free Hub is more desirable however.

Halolll

So if I’m reading this right, I can buy any Free hub wheel and if they have more or less gears all I need to do is exchange my original cluster for the one on the wheel I buy?

Black Shuck

Not really, but you can fit a 7-speed cluster to a 8 or 9 speed hub if you put a 4.5mm spacer behind the cluster, or cassette. There should still be 7-speed freehubs available but 8/9(it's the same) will be easier to find. Your local bike shop should be able to provide you with the spacer you need.

Mr. Underbridge

Might dropout width be an issue if he goes that route?

oldster

But we really dont know what he needs...Whats kaput???Why does it need something????
Or ,(probably), am I pizz'n up the wrong rope???
Bud

Halolll

A spacer behind the cassette worked well! Thanks to all for your input.

I_bRAD

It does if you want to re-use your cassette or freewheel!