Monday, March 15, 2010

Tire Clearance

JPTwins writes:

"I've been posting recently on your blog as JPTwins. I noticed in your recent craigslist ad that you talked about having plenty of tire clearance.

Well, here's my bike:

I'm rolling 27" Schwalbe marathons 35mm on this bike and there is just barely enough room for fenders. I like the wider tires since Boston roads are pretty bad, but also because I pull my trail-a-bike with this. I was wondering what limits you had used when installing tires and have you thrown a 700c wheel on your 80s FUJI? Is that just a matter of getting longer reach brakes?

Thanks for the advice, and I'd admit that I'm a bit jealous of your move. Not of having to sell the bikes though -- good luck with that!"


I like the S-12-S LTDs, but I'm a sucker for black paint and chromed stays and forks. In any event, that model pretty much represents the best of the S-12-S line.

Running 700c wheels on 27" bikes is pretty easy. Lots of times you don't even have to get longer reach brakes. A 27" inch rim is 630mm in diameter, a 700c wheel 622mm. This 8mm diameter difference gives a radius difference of 4mm, which is the additional brake reach that is necessary. So unless your brake pads are at or near the bottom of their travel, you may not need new brakes. Even if they are, some people file additional clearance in the arms, but I'm not sure how good an idea that is, unless it were just a tiny smidge.

Simple way to test this to borrow a 700c wheel and install it, see if you can adjust the brake pads to work. That's what I do, anyhow. So far, I've not run into any lack of brake adjustment travel, offhand, I'd guess that you won't on yours, but that is just speculation.

For tires, the size is actually the height of the tire - 700c x 35 indicates that the height of the tire is 35 mm. So going to a 4mm smaller radius rim means, in theory, that you can run a 4 mm larger tire. However, I said that this is in theory because tire sizes are usually nominal, meaning for categorization, rather than a hard engineering spec, so there is some variation.

In principle, it does mean that if you are running 27" 35mm (1-3/8") tires on your bike, you could run 700c x 38. In practice, since you mention that the 27" tires are a tight fit with fenders, you could run into problems. Me, I'd probably go to 700c x 35mm and enjoy the better fender clearance. A 35 tire running at 50 psi or so can be a pretty plush ride - it is what I'm doing on my Fuji Gran Tourer these days.

Amongst my bikes, I swap 700c and 27" wheels fairly often. Both ways, even - my Fuji America came with 700c and until recently I was running a nice set of 27" wheels I had around.

It would be nice if you could get 700c x 38 on your S-12-S, but I'd guess that the fender clearance will be as much of an issue as it is now.


Ali said...

Hey man love the bike, I have an old Fuji touring series IV. I was wondering though, I wanted to switch over to a 700c wheel is that going to be tough? Reason I ask is a bike shop I called said it can't be done because of the brake clearance and because my bike is a 12 speed 6 clot. Any suggestions?

robatsu said...

Shouldn't be a problem, although if you have cantilever brakes, sometimes this gets a little complicated.

Google "27 to 700c wheel conversion". You'll find tons of info.