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  1. #1
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    Thinner tire = Smaller diameter?

    Hi all,

    My 84 Motobecane Mirage Sport has little to no clearance everywhere. I noticed this before, but didn't think anything when I ordered new tires in same size as what is currently on the bike. (27 x 1 1/4).

    Basically the Panaracer Pasela TG's I ordered rub the front derailuer, while the Specialized Tri-sports (basically slicks) in the same size have 2-3mm of clearance.

    Talking to people it seems that if I return the Paselas for 27 X 1 1/8, it should resolve my issues, is this true?
    1993 Cannondale T700 - 1994 Specialized Rockhopper - Actionbent T1 (Electrification in progress!)

  2. #2
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    Tire sizes are not exact and they vary by manufacturer. Going to a smaller "size" may help, but is no guarantee the tire will fit.

    It may be better to find something local that fits first before ordering more that turn out to be too big.

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    No one local carries Panaracers. The LBS's only have basic gumwall tires or Armadillos @ $45 a pop, at least in the 27 inch sizes. (I do believe in buying locally, but $90 for a set of tires seems crazy to me).

    I have to send the 27x1.25's back either way, so might bust down to a 27x1.0 (of the same tire) and see if that resolves the issue. With the 1.25's the tire spins but due to me not being able to keep my tires trued it rubs during the revolution, so I don't have to back it off to much.
    1993 Cannondale T700 - 1994 Specialized Rockhopper - Actionbent T1 (Electrification in progress!)

  4. #4
    Senior Member johnknappcc's Avatar
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    This probably seems like too much work, but if your brakes reach why not go down to 700C wheels? I did on mine, and it is awesome! That will give you an extra 4mm clearance before tire, and you will have a much easier (and cheaper) time finding tires.

    The link in my sig is the bike in question.

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    Indeed, as johnknapcc says, if you have some cash to spend, you can upgrade to 700c wheels. You'd need a pair of new 700c wheels - the front one would fit fine, but the rear would come spaced 130mm so you'd need to cold-set your rear dropouts to the new, wider standard. Then, you'd need to install extra-long reach brakes - good news here, those can be had for cheap in dual-pivot versions that are pretty strong. Or, you could make a brake drop bolt (Sheldon Brown has a page explaining how to do that) and reuse your existing brakes.

    It all adds up... If your wheels are good, maybe you can just find a 27x1 1/8 tire which will probably fit...

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    I'd like to convert to 700 wheels, but really that is outside my budget right now.
    1993 Cannondale T700 - 1994 Specialized Rockhopper - Actionbent T1 (Electrification in progress!)

  7. #7
    Senior Member johnknappcc's Avatar
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    Quote Originally Posted by corkscrew View Post
    I'd like to convert to 700 wheels, but really that is outside my budget right now.
    I hear you on that one . . . but if you have a friend that has a 700C front, throw it on your bike, and see about fit and brake clearance. There is no reason you couldn't use a front 700C and a rear 27. I routinely find 700C wheels on CL for dirt (20 dollars or less). They aren't anything special, but for the time, they can get you around (and probably solve the tire clearance issue).

    Before purchasing my wheel set, I bought a cheap Alex rim to test my brake clearance.

    It might be a little mismatched, but like you, I wasn't about to drop 100 dollars on tires to fit old wheels. Also, make sure your rims can handle a less wide tire. You can do the front 700C for cheap, and wait for a really good deal on a 700C wheelset. I got my Mavic Open Pro/Ultegras for under 200.

    EDIT: Didn't fully get the post, about the rear wheel, but I would still keep my eyes peeled for an inexpensive 700C wheelset. We have a few shops around Chicago that sell brand new generic ones for 45 a piece. I would check spoke tension before installing though.
    Last edited by johnknappcc; 05-14-09 at 12:05 PM.

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    Being a contract worker - I already blew my bike budget for the next month or two on the bike,rack trunk, fenders, tires rack, tubes, seatpost, etc. New rims aren't an option unless I can find them for free.

    That being said the 27x1.25 tire clears the front just fine. Its the rear one thats rubbing against the front derailuer (where it is strapped to the seat tube).

    Heres an example of the clearance available to me, mine is the Mirage sport, on the right.
    http://www.equusbicycle.com/bike/mot...becane0005.jpg

    What I need to know I guess, is if I send the tires back to nashbar, and get the same tires in 27x1.125 or 27X1.00, will that back things off at all?
    1993 Cannondale T700 - 1994 Specialized Rockhopper - Actionbent T1 (Electrification in progress!)

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    Stop referring to the tyres with a decimal number. You need 27 x fractional tyres. You cannot blindly change nomenclature, it means something significant in tyre sizing.

    Have you checked the front derailleur is installed properly/non-bent before looking at the tyre as the only solution? Post some pictures of the front derailleur area from the top.
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    Senior Member johnknappcc's Avatar
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    Wow, that does look tight. Tough to say, but I would try the 1's.

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    Senior Member DieselDan's Avatar
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    To diectly answer the OP's orginal question: Yes, different tire widths will have a slightly different overall diameter.
    Bikes use brakes to stop.

    If your bike has breaks, don't ride it.

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    Senior Member johnknappcc's Avatar
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    Quote Originally Posted by operator View Post
    Stop referring to the tyres with a decimal number. You need 27 x fractional tyres. You cannot blindly change nomenclature, it means something significant in tyre sizing.

    Have you checked the front derailleur is installed properly/non-bent before looking at the tyre as the only solution? Post some pictures of the front derailleur area from the top.
    Stop spelling tires wrong . . .

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    Thats the closest shot of the Front Derailuer I have @ work. Thats with the old tires on.

    Thanks everyone, I'll be sending the 27 x 1 1/4's back for some 27 X 1's, if that doesn't fix things then I'll look into other options (Different tires or 700 wheels).

    That being said, can someone explain to me how 27 x 1.25 is different from 27 x 1 1/4?
    1993 Cannondale T700 - 1994 Specialized Rockhopper - Actionbent T1 (Electrification in progress!)

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    Sounds like the 1 1/4" tires are very close to fitting and just rub lightly. If that's the case then I'd expect 1 1/8" tires to work fine. Reducing the width by 1/8" is a bit over 3 mm and since that's the diameter, it would increase the clearance by a little over 1.5 mm on each side and 3 mm in the center assuming the tire maker is consistent in their sizing.

  15. #15
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    Quote Originally Posted by corkscrew View Post

    That being said, can someone explain to me how 27 x 1.25 is different from 27 x 1 1/4?
    Here's a better question. Is a 26 x 1.5 tyre different than 26 x 1 1/2 ? Also is the wheel all the way back into their respective dropouts?
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    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by johnknappcc View Post
    Stop spelling tires wrong . . .
    Not everyone speaks American...

  17. #17
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    Quote Originally Posted by operator View Post
    Here's a better question. Is a 26 x 1.5 tyre different than 26 x 1 1/2 ? Also is the wheel all the way back into their respective dropouts?
    Here is a link, courtesy of Sheldon, which covers almost everything you might want to know about bicycle tire sizing.

    http://www.sheldonbrown.com/tire-sizing.html

    Much less confusing to use the ISO/ETRTO sizing convention IMO. The old tire sizing conventions are confusing as all get out and can lead to problems with getting the wrong size tires for an oddball rim.

    As the OP has discovered different manufacturers tires of the same theoretial size can vary considerably in actual diameter and width, enough to cause problems if clearances are tight. This is one of the reasons that Cateye recommends doing an actual rollout test to determine wheel & tire true circumference for most accurate setup of their cycling computers.
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    Thanks for the link - tis a convoluted area for sure.

    Looking at that site, and reading the comments I'll try one size down next, EG the 27 x 1 1/8 tires. As I don't think I'll need new tubes, and my rims are 27x1, so I'm worried 27x1 tires would be to narrow for the rim. The bikes primarily going to be used for pleasure rides and commuting, so comfort is a bit of an issue also.

    To answer that question, the moto has vertical dropouts, with not a lot of room for play. I seat the tire at the inside end of the dropouts.
    1993 Cannondale T700 - 1994 Specialized Rockhopper - Actionbent T1 (Electrification in progress!)

  19. #19
    Senior Member johnknappcc's Avatar
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    Quote Originally Posted by DannoXYZ View Post
    Not everyone speaks American...
    LOL, I was just kidding, my wife is Canadian and she spells stuff funny too . . .

  20. #20
    Senior Member Bikedued's Avatar
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    Panaracers run bigger than normal, which could be part of your problem. The 27x1 1/8" will probably be comparable in size to a normal 27x1 1/4.,,,,BD
    Last edited by Bikedued; 05-14-09 at 03:05 PM.
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    Good to know, thanks!
    1993 Cannondale T700 - 1994 Specialized Rockhopper - Actionbent T1 (Electrification in progress!)

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    Unless you really want the narrowest possible tire, I would also suggest 27 x 1 1/8" as a good bet since you had been using 27 x 1 1/4" in a different brand.

    Going to 27 x 1" may be too narrow for some rims... but if the wheels on your bike are what came with it, based on the close clearances, I think the rims might be OK for narrow tires.

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    Yes, I still have the same 27x1 rims that came with the bike. Think that would be ok with a 27x1 tire?
    1993 Cannondale T700 - 1994 Specialized Rockhopper - Actionbent T1 (Electrification in progress!)

  24. #24
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    Quote Originally Posted by corkscrew View Post
    Yes, I still have the same 27x1 rims that came with the bike. Think that would be ok with a 27x1 tire?
    they should work
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