tire sizes
 

I am brand new at riding.I got a rossin to work out/ride to work on.I don't know anything about bikes.I have to replace the tires on it so I was going to try to go with something a little bigger.it currently has mavic ma40 wheels with continental 700 x 18c tires on it.what is the biggest tire I can fit on these wheels?thanks

I have never seen 18mm tires. Get a metric ruler and pump the tires to 100 psi. Use the metric ruler to measure the full width side to side. I seriously doubt your tires are less than 21mm wide.

Have your neighborhood bike shop fit your new tires to make sure they are suitable for your rims. Most road bike rims work very well with 25mm tires (most 25mm tires have a MEASURED with of 23mm or 24mm) and 25mm tires are a good choice for a rider under 200 pounds who rides on smooth roads.

If you weigh more than 200 pounds and ride on dirt, gravel, or rough pavement, 28mm tires would be better, if your bike shop thinks they are a good match for the rims.

all I know is what is on the side of the tire.it is a contanental 700 x 18c.they are super small.that is why I am asking.I am a big guy 6-4 280lbs so I am looking for something with some give.I just didn't know if these tires were all that would fit these rims.

If you weigh 280 pounds, you need 32mm tires or at least 28mm tires. If your current tire is really just 18mm wide, a 32mm tire is too wide for your rim. You will need a rim designed for hybrid bikes, as such rims are designed for wider tires.

The reason that you need a wider tire is NOT that they are wider. It is because the most common size, 23mm, has just a tiny amount of space between the rim and the road. When a 280 pound rider is riding fast and hits a pothole, the rim bottoms out against the road, resulting in an instant flat. A 32mm tire doubles the height and width of that air cushion, providing about 400% more cushion than a 23mm tire.

At your weight, even with a 32mm tire, you need to keep it at the maximum PSI marked on the tire. If the tire were at, say, half the maximum PSI, the same thing: hit a bump riding fast, and you will have an instant flat.

You need to work closely with a good bike shop. The tires you select must also leave clearance between the fork and the tire, and the chainstays and the tire, plus empty space for mud and debris to clear without getting jammed up. Some modern frames do not work with 32mm tires.

So, visit your neighborhood bike shop.

IF your rims hold an 18MM tire, they probably won't take anything larger than a 25MM.

I don't know what size the rims are all thats on them is mavic ma40.the tires is what I am getting the 700 x 18c from.do the ma40 come in 18mm?

There was a trend back in the early 90s to use really thin tires, as thin as 18mm.

You could probably go has wide as 28mm.

If you really have MA40s (I thought Mavic stopped making them years ago), you can go as big as you want, or at least as big as will fit in the frame and between the fork blades. I have an old set of MA40s on my singlespeed, and they've held tires as big as 700x41. No problems, no wobble, no trouble at all. I run 700x35s all the time.
Check the frame and fork, though--I doubt there's room for anything that wide. But you're smart to get rid of the skinny rubber.

There is simply no way that a rim that provides a GOOD fit for a tire that measures a "true" 18mm wide can provide a GOOD fit for a tire wider than 28mm.

There is a reason bike shops exist, and that is to get help from people who are experts. At the shop, the staff can hold the rim and tire in their hands and inspect them. You need to go to a good bike shop, and ask them for the widest tire that is a proper fit for your rims, and that will leave safe clearances at the fork and chain stays.

well I took it to the shop, and I went with 25s.after reading on hear I wonder if the guys at my shop know what there talking about.they said that my weight would be fine on a 23, but I went with the 25 to be sure.

Tire sizes
 

I am trying to learn about this tire-size issue. I bought a used Schwinn Searcher and it has 700/38c tires with a fairly deep tread. They look like hybrid tires. I would like to try a thinner tire that is smooth, because I have a long commute and I think the hybrid tires are increasing the road resistance & making me exhausted quicker.

I don't have a metric ruler but I think the rims are about 1" wide. Road conditions are a paved bike trail and my weight is 110. So I am wondering if a thinner tire would be appropriate for me & if so, what dimensions (and brands) do folks recommend?

You'll probbly like a narrower, smoother tire a lot better. You need to remove a tire and carefully measure the inner width of one of your rims. Then consult the Sheldon Brown website to see how narrow of a tire will work. If you try to install too narrow of a tire you'll get frequent "snake bite" flats.

I went to the bike shop & they ordered me a 28C (I think). They also told me about 'heavy duty inner tubes" so I'll never get flats. I never heard of those! Anyone know if this claim is true?

My experience has been good tires make a bigger difference with regard to flats than anything else. If your running toilet paper thin tires, puncture proof tubes will help but nowhere near as much as switching to some tougher tires.

There are "Thorn Resistant" tube which are basically twice as thick. They can help IF you have thorns, since the thorns have to penetrate a little more.
There's a noticeable weight penalty however.
Slime filled tubes may seal enough to get you home.
They also have a weight penalty and can mess up your pomp if the Slime "spits back" when attaching the pump head. That happened to me.
My floor pump that used to be good for about 60 PSI is only good for about 40 PSI now.