the sci guy 

I used the googles and couldn't find a chart...
I'm wondering what the basic equivalency is for a 27 x 1.25 tire is in 700c width? Like 700x32 or 35?

I've got the 27" wheels on one bike and 700c on another and the 700c one is in the market for new tires and I want to go wider. I did a test fit of the 27" on the other bike (since it originally had 27" wheels) and that width fit the frame, so I'm looking to go about that width or just slightly less in a 700c.

Thanks.

BFisher

27x1 1/4 is usually like 32mm, but of course manufacturer tolerances and rim width play their parts. I believe many, if not all, 27x1 1/4 tires are also marked ISO 630x32.

malcala622

On the bike with 700c rims the biggest/widest tires will depend on how much clearance you have under the brake calipers.

What frameset and brake calipers do you have paired with the 700c wheels?

Personally I build all my wheels with 700x23/25 to ensure clearance.

ThermionicScott 

1-1/4" = 32mm

In a pinch, you can multiply/divide by 25.

79pmooney

Roughly, 27 x 1 1/4 is going to be 1.25" X 25.4 mm/in = 31.8mm or 32c. Roughly because tire sizes are like shoe sizes. You have to try them on.

Ben

the sci guy 

Tektro R559 long reach's on a 1984 Raleigh Sportif. Currently have 700x28 in a Kenda tire.
Tons and tons of pics of the bike on my Instagram.

Perfect, thanks. Thinking of getting some Panaracer Pasaleas in a 700x32 gumwall then.

Kabuki12

I have 27" x 1 1/4" Michelin ProTech tires on my Super Mondia and they measure 1.390" after inflation to 90lbs. This calculates out to over 35mm wide! Be careful on brand , I was fortunate they BARELY clear the frame . I backed up the axel stop and I was OK. I have to deflate the rear to remove the wheel. I also have two release levers for each wheel's brake, one at the handlebar lever and one on each cable hanger.That said, I love the tires and I am not afraid of dirt or gravel roads. Joe

ryansu

Roughly:

27 inch size ISO rough metric equivalent width (Note look on your sidewall for the ISO measurement)

27 x 1 630x25c

27 x 1 1/8 630x28c

27 x 1 1/4 630x32c

27 x 1 3/8 630x37c

palincss

There is no such thing as 27 x 1.25. It's 27 x 1 1/4. As you'll see on Sheldon's site, two tires with what should be identical widths, one expressed as a decimal and one as a fraction, will never be the same. What you need to pay attention to is the ERTRO designation, which is the bead seat diameter. 27" is 650, 700C is 622. Regardless of the width, those two tires will never fit on the same rim. It gets even hairier when you get to 26". There are quite a few 26" sizes: MTB 26" is ERTRO 559; 650B aka 26 x 1 1/2" is 584; 26 x 1 3/8" English 3-speed size is 590 and there are several other more obscure sizes called 26".

You may be able to convert your 27" bicycle to use 700C wheels by using longer reach brakes. At that point, the actual width of the tire -- what will actually fit -- should be your guide. In some cases, that may actually be wider than the original.

SurferRosa

You know society is in trouble when "the science guy" can't convert inches to millimeters.

the sci guy 

1.25 is just another way (faster) to write 1 & 1/4.
I wasn't looking to put 700c tires on a 27" rim or vice versa. I have 2 bikes, one with 27" wheels and one with 700c wheels. I liked the width of the new tires I just put on the 27" wheels, and wanted to know the 700c equivalent to pick out new tires for that bike.

I actually didn't know it was literally just doing this. I'm well versed in sciencey science, but still new to bike science. :-)

palincss

No! Mathematically it may be, but in terms of tire sizing, no. The fractional size indicates that it is a different tire, almost certainly with a different bead seat diameter. And it may be wildly different. Today, 26xdecimal = mountain bike 559mm bead seat diameter. 26xfraction almost always means something very much larger. 26x1 1/2 = bead seat diameter 584. Read the Sheldon Brown article I linked earlier about tire sizing systems.

Road Fan

Steve, isn't 27" a monniker for ERTRO 630? 8 mm more bead seat diameter than the 622 mm of ERTRO 700?

Road Fan

My understanding is that there are two basic designs of 630 rims: hooked-bead and smooth-bead. Smooth-bead was the original, and nearly perfectly fits modern wired-bead 27" 630 tires. The more modern hooked-bead rims may be marked 1.25."

wrk101

Not in the bicycle world. Decimal will not equal fraction. If you want a real headache, try 26 inch tires sometime. There's the 559 26'r, typical old school MTB which will be decimal sized (like 1.5). Then you have the 571, which will be fractional, then you have the 584 which can be 26- 1 1/2, then you have the 590. Last is the 597. Five different sized 26 inch rims, four of which are fractional sized. Had a bike once with a 590 rear wheel, and a 597 front. Both used 1 3/8 inch wide tires! (One was the old Schwinn standard). So two different sizes for 26 - 1 3/8 tires.

Then you have 29'rs, which are smaller than 27'rs. Go figure.

verktyg 

This chart shows a comparison of the various 26" tires and rims with the ERTRO designations at the top, inch sizes and metric equivalents in the middle and "650" variants at the bottom. Note: the 650 lettered sizes were originally French designations but other European countries used those call-outs too.

Comparative tire cross-section sizes - note, 1 1/4" should be 32mm not 31mm.



THEY'RE FRIGGIN 700C's NOT 29'ers!!!!

The root of my rant goes back to around 1975-76 when the revered clowns of Marin County, California started mounting derailleurs on 40 Lb. 26" balloon tired "kids" bikes and ridding them down Mt. Tamalpais on the "Repack Rides". They were "getting a lot of ink in the bicycle rags". In SoCal guys were doing the same thing but calling them "beach bombers" and riding around Venice Beach and other seaside communities.

In 1976 we started "Rough Stuff" riding in the British manor: off road riding on road bikes in northern New Mexico and southern Colorado. (we viewed ourselves as purists)

Initially there were only a handful of us crazy enough to ride sewups in the woods, dirt roads and mountains. We used 30-32mm wide d'Alessandro and Milremo badged versions of the same tires. There were several quality levels with either coarse pitch, medium pitch or fine pitch cotton casings. They all had tough treads with a coarse diamond pattern and a puncture resistant belt.

Several of the guys rode 27" x 1 1/4" clinchers as 700C wheels and tires were just coming into the US market. I started riding cheap Wolber 700C x 35 "de ville" clinchers for gravel grinding. They were made for the French town bike market and had about 25 threads per inch cotton casings. They were the only larger size 700C clinchers I could find in the US back then. We got them from Mel Pinto Imports.

Now to really muddy the water, some of the northern European tire makers labeled their sewups 27" x an inch size width. Continental was one example. I bought this Conti cyclocross sewup marked 27" x 1 1/8" about 1977-78. It still holds air and is actually 700C x 30 not 27".




Getting back on topic, there is an 8mm difference in rim sizes between 27" and 700C (ERTRO 630 vs 622) but wheels with the same cross-section tires will sit approximately 4mm higher or lower from the ground.

700C sized clinchers started becoming popular around 1975-76 when riders who used sewups wanted to switch back and forth between those and clinchers without having to readjust the brake blocks every time.

verktyg

Phil_gretz

[stunned] Um, multiply inches by 25.4 to get millimeters. Right, @the sci guy ?

rumrunn6

google does conversions just type in "32mm = inches" & it will give you 1.25984 or conversely type "1.25 inches = mm" & it will give you 31.75 which should be a good ballpark # so you can choose the tire size you want

JohnDThompson 

And so-called 27.5" are just 650B. What's wrong with people?

It would be so much easier if everybody used the ETRTO size designations.

BFisher

Agree 100.25% (or maybe 100 1/4)!
The tire size situation in the bike world is beyond absurd.

the sci guy 

..

ThermionicScott 

Was there ever a competing 27" decimal standard, though?

gugie 

There are lies, damn lies, and tire widths matching what the sidewall sez.

Add to that the rim used will change the actual width, new vs old tire...

3alarmer 

...I might not know tyres, but I know what I like.

Andy_K 

27" is bigger than 29".
29" is the same size as 28".
All three are bigger than 27.5".
And, of course, 27.5" is about halfway between 26" and 27".

It's quite simple really, once you release your tight grasp on reality.