Bicycle Mechanics - Max tire? Replacing Mavic GP4s with CXP22s

I am new to bike riding and am rebuilding a late 70s/early 80s Dawes from CL for an exercise bike. It came with Mavic GP 4 rims & very punky sew-ups. The only markings on them that I could find say "Hutchinson S". I was able to get a Conti Sport Contact 700C (28 x 1 3/8 x 1 5/8) between the stays and in the right place for the brakes (yeah for steel!) but the front derailleur assembly hits.

I already have a set of Mavic CXP22s & I'd like to use them, with a fairly fat tire. They take up 32mm tires, I believe. Since I don't know what size the current tires are, how do I figure out how big I can go . . . short of going to the LBS and trying on wheels & tires? Sorry if this is a dumb question.

Buy a pair of calipers from the hardware store and measure them. They're cheap and no one will ask what you're going to use them for.

Yes, one measurement is worth a thousand words. And know that lables are not measurements.

But more to your question, only actually fitting said tires and rims to your bike will give you real info. How much tire/frame gap/clearance you allow is a judgement at best. So no real black and whites here. Andy.

Measure the inside of rim bead/hook to inside of rim bead/hook in millimeters...

Most 700c road rims for example will be 14mm, 15mm and sometimes 16mm.

Then go here...

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

...use chart near bottom.

=8-)

Ha, well I do have a caliper set, so after deflating the old tire but not unglueing it, I was able to measure the old rim at 13mm inside. According to Sheldon, it could take a tire from 18 to 25mm. However, the new rims can take anything from 20 to 32mm, so I'm still at sea on what will not interfer with the deraillier mech but be as large (& tough) as possible. I think the prudent thing is to research what might be a recommended tire type here at the Forum, then go to my friendly LBS and try some on. They should be happy to help since I'll be buying them there. Thanks again!

Your question was what size of tire will fit in your bike without interference, right? Pump up the tires, measure the width of those, then you'll have an idea of what tires will fit. That's what my earlier post meant.

Since the issue is chainstay or FD clearance then it isn't a rim fit question and there's nothing you can learn measuring the rim. Re-inflate the original tire mount the wheel and measure the clearance.

Then you can use a tire with an actual width greater than the original by less twice the clearance. Note that the nominal tire widths in the catalogs is only a rough (very rough) approximation, so leave room for error, or mount inflate and measure before buying.

If you cannot measure the original wheel clearance, use the one you have and trim the FD until it just touches as a gauge, then see how much farther you need to move it, and subtract twice that.