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Looking to upgrade my tyres on an old roadie mash up to something chunkier so I can handle some off road sections on a ride coming up, currently it has a 27 x 1 1/4 front tyre and a 27 x 1 3/8 rear with plenty of clearance all round*.

I've got a set of 700c rims (internal width 15mm) that I am looking at putting on in place of what's already there because the tyres are cheaper for it, so would the wider 47c tyres be suitable for these rims? They have had a set of 32c tyres on them for ages no problems but it still looks like they'd fit a wider tyre quite fine, Just want to make sure first.

Also if wouldn't mind knowing what would be a minimum size tube to use, hoping to avoid having to buy new ones because I am on a $50 budget to get the bike suitable for the ride. At home I already have a number of thinner tubes that I doubt would work (up to 28c I believe) but also a pair of 27 x 1.25" tubes, Would these work for the 700 x 47c tyres? or would I need fatter tubes also?

*should be noted that the tyres already on the 27" rims are slicks or near enough and are falling apart due to being left out in the weather for many years.

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    Your main difficulty (assuming that there is clearance in the frame for the wider tires) is getting your brakes to work with the smaller rims. Presumably this bike has some sort of rim brake, and the 700c rims will be enough smaller that the brake pads are going to rub on the tire, not on the rim.
    – Daniel R Hicks
    Commented Nov 18, 2015 at 3:43
  • I'd be surprised if tyres that big fitted into a road frame. None of mine will take 25mm.
    – user400
    Commented Nov 18, 2015 at 7:20
  • @DanielRHicks From what I saw of the previous set of tyres there is plenty of clearance on the frame and brakes to go wider, and the brakes (I think) have just enough adjustment to fit the 700c rims and work properly.
    – Lampephysics
    Commented Nov 18, 2015 at 10:54
  • You understand what I'm saying? The rim has a smaller DIAMETER (not talking about it's width). The pads will have to be moved downward, closer to the axle, by about 4mm. Some brake systems can accommodate this, some can't. And this will change the leverage of the brake system, depending on the brakes making them "harder" or "softer", and certainly making adjustment touchier.
    – Daniel R Hicks
    Commented Nov 18, 2015 at 13:47
  • @DanielRHicks yeah I understood, hence the "and the brakes (I think) have just enough adjustment to fit the 700c rims and work properly."
    – Lampephysics
    Commented Nov 23, 2015 at 0:40

2 Answers 2

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To fit 700c (ISO 622) wheels into a bike built for 27" (ISO 630) you only need to shift the rim brake pads down 4mm. If you're exceptionally lucky your current brakes will have enough reach and you'll just have to adjust the pads. That's the first thing to check, because if that doesn't work there's no way you're going to do this on a $50 budget.

2" wide tyres should be ok on the 700c rims. Not ideal, and probably outside the manufacturers specifications, but it should work. I've done that with 406 wheels before and it's worked pretty well.

The real problem is the tubes. Again, new tubes overstretched like that would work in a pinch, but they're be extra vulnerable to punctures because of the stretch. But usually if you're replacing aged tyres I'd suggest new tubes since they're likely to be in similar condition. Trying to over-stretch old tubes is very likely to fail immediately, and if not will almost certainly fail shortly after you start riding.

If you can get the 700c wheels to work, I'd go with a 38mm tyre rather than 42 or 48, just because I'm really wary of damaging the tubes. I assume on your budget you can't afford to carry a spare. But really, if you're doing a ride of any length I think you should buy at least one spare, unless it's a ride you can bail out of and get someone to carry you and your bike home. Even a cheap, shitty spare tube is better than nothing. personally I would cheap out on the tyres in order to buy a couple of tubes. It's not worth going out on a ride and having to go home because your old tubes give out.

If you can't make the brakes work I suggest buying the fattest 27" tyres that will fit your tubes, which is likely to be 1 3/8". I'm sure I've seen 1 1/2" x 27" tyres but I can't find any now.

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    Sorry I should have explained better, I have one set of 700c rims and another of 27" rims so I won't need to fit a 700c tyre on the 27" rim. As for the tubes, so if I were to use my current tubes I understand that the 700c and 27" tubes are roughly interchangeable. Do you have a recommendation on the max width tyre (in 700c size) that I could 'safely' run the 27 x 1 1/4" tubes in? I'm not certain but I think the 1 1/4 lines up roughly with a 32c, so in my head I could probably stretch them to a 38c or maybe a 42c tyre but probably not the 47c? Thanks
    – Lampephysics
    Commented Nov 18, 2015 at 3:30
  • Sorry, I missed that. Edited my answer
    – Móż
    Commented Nov 18, 2015 at 3:52
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Schwalbe's website would say no. Your maximum tyre size on a 15mm clincher rim is 32.

To carry a 47mm tyre properly you want a 15mm rim to 27mm rim

I've run big tyres on rims too small, and after a while the tyre sidewall failed at the edge of the rim.

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    It's interesting to note, though, that my Novara Randonee was delivered from the factory with 35mm tires on 13mm rims. And I've never had any problems that one could attribute to this combo.
    – Daniel R Hicks
    Commented Aug 18, 2016 at 22:47
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    @DanielRHicks perhaps those particular tyres are more robust on the sidewall - the above chart was a Schwalbe production and is based on their product and testing. So its not an absolute, and probably errs on the side of safety. But its a starting point that is beyond anecdotal evidence such as "it worked for me" or "I rolled the tyre off the rim"
    – Criggie ♦
    Commented Aug 19, 2016 at 0:19

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