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I have a mountain bike looking frame with wheels that currently have road tyres. The bike is classified as a hybrid.

My tyres state: 28" x 1.6 (42-622)  
The rim: ETRTO 622 x 18

I am having difficulty locating any mountain bike tyres for size 28".
Can I use size 29"?
Would I need a different set of rims?
Also, do I need a different tube for these tyres?

My goal is to do some dry trail mountain biking.

Lots of question. Thank you

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1  
Schwalbe has some good info about tyre sizes and also about dimensions. One other thing to watch out for is frame and chain clearance. If your current tyres work fine (42-622 ETRTO), then be careful about going wider than 42. A 50-622 tyre may not clear the frame and chain even if it goes ok on the rim. Check how much clearance you have with your current tires, both in diameter and width before going wider. Being a hybrid it is probably not designed for very wide tyres. –  Jason S Oct 27 '11 at 1:45
    
Great info, thanks –  Valamas Oct 27 '11 at 3:26
1  
So you've got some answers about getting tyres suitable for mountain biking on your hybrid bike. I think you may also want to take into consideration the difference in core purposes. The hybrid frame isn't quite designed for the battering that riding proper single track can deliver. If you're only planning a few rides to see if you like it, then you will probably be fine, however if you're thinking of the hybrid as a longer term option, I would err on the side of caution. –  Dan Oct 28 '11 at 9:32

2 Answers 2

up vote 6 down vote accepted

There's a bunch of weird standards for naming the size of tires and rims, and there's an actual ISO standard.

The best thing to do is ignore the big letter printed names that have inconsistent conventions, and look for the ISO standard stuff, like "42-622".

The ISO standard size of your tires and rims is 622. (18mm wide rim and 42mm wide tire)

ISO 622 is the same as "700c" (common road size) and 29er (new larger mountain tire).

In other words: yes, a 29" tire should work fine, as long as it's not too wide.

Try to stick with something close to the same width and they'll work fine with your existing rims. Mountain (off-road) tires with their knobs can require more clearance at the same size.

As long as you keep to about the same size tire, your existing tubes should be fine.

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True, as far as it goes. A 29'er tire is called a 29'er even though it fits a 700c rim (or a 28" rim) because of the additional volume of the tire itself. I'm not aware of any hybrid frame which will accept a 29 inch tire, and if the tire fits in the frame, you will still need to watch out for brake clearance issues, as the brake arms (assuming rim brakes) may touch the tire before contacting the rim, due to the additional rubber width. –  zenbike Nov 9 '11 at 4:47

28, 29, 700c, all the same.

[all the same rim diameter, with wildly varying tire sizes. The tire will fit the rim, but is unlikely to fit in the frame, if it is mch larger than what came on the bike.]

As freiheit pointed out ISO is the same on all of those tire sizes as well as your rim. The thing to be worried about is when your rim width gets really close to or greater than your tire width (ie running a skinny road tire on a wide mtb rim) or when there's a significant difference (wide mtb tire on a road rim). You can run a tire in the 30-50mm range on pretty much any rim width you want, so you're safe there. Regarding the tube to use, the one you currently have should have a measurement on it, inches for mtb or mm for road but beyond size one is no different than the other. It should read a range, something like 700x35-42 or 29x1.0-1.5. If your tire falls within that range you should be fine. Too big a tube and it's going to be hard to cram in the tire and you'll probably pinch it when mounting it- too small and the rubber potentially gets stretched too thin and the tube pops.

EDIT: Also, if you want to know more about rim/tire sizing, go visit the late great Sheldon Brown's site though it may cause more confusion than clarification.

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I think my tube says (very hard to read): 700x35/43c. Im not sure about that c since it is difficult to read. Does this tube fit in range? –  Valamas Oct 26 '11 at 3:08
    
You haven't purchased a set of tires yet, right? Until you have a the set of tires you want there's no way to know whether the tube will work or not unless you're planning on limiting your purchasing options to something with a width that fits your tube. That would be silly, btw. Tubes are cheap. –  joelmdev Oct 26 '11 at 3:13
    
No, I have not purchased. And from your advice just now, im going to pickup a new set of tubes too then. Thank you so much –  Valamas Oct 26 '11 at 3:26
    
If you're bike is from this millennium, don't worry about finding the 'c' in '700c.' It's been a long time since there were any other 700 variations on the market. –  joelmdev Oct 26 '11 at 3:29
    
@Valamas, no problem. Don't forget to accept an answer whether it be mine or someone elses. –  joelmdev Oct 26 '11 at 3:34

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