Bicycle Mechanics - Tire confusion

Having read lots of articles and posts on tires, I am baffled how to determine in some scientific method the largest tire I could put on my 89 Trek 400. And then how to verify the measurements of the actual tire before I buy it.

Seems like I need to know the following bike measurements:

- clearance between the brake pads
- clearance between chain stays
- Maximum radius clearance from the axle center to the seat tube or bottom or brake
- Interior rim width at the bead.

For a potential tire I would need to know:

- maximum bead width
- width of inflated tire at its widest point
- maximum radius of inflated, mounted tire

I am no doubt making this too complicated, but if I wanted to put cyclocross tires on my road bike for commuting on a mixture or gravel, dirt and pavement, how big could I go? There has to be some rational way to figure this out?

Thanks as usual for entertaining my ignorance.

B

In most cases that I've messed with the limiting factor has been the distance between the brake pads and the ultimate answer has been 28mm.

Use the real scientific method: Keep trying stuff until you get hurt. then stop.

It depends a lot on your particular frame and size. More often than not a 28mm will work on a road bike. Maybe you could borrow someone's wheels with 28's to see if they will fit.
There are size variations from one tire company to another. A Michelin 23 is about as big as a Continental 25.

It depends a lot on your particular frame and size. More often than not a 28mm will work on a road bike. Maybe you could borrow someone's wheels with 28's to see if they will fit.
There are size variations from one tire company to another. A Michelin 23 is about as big as a Continental 25.

I guess this is my point. The variables here can't be put into some formula for a set of measurements that would define the bike's capacity, and then measurements that could be made on a deflated tire in the store to verify it would fit that capacity? I'm an engineer, my wife would tell you its a social disease I am sure, but I am looking for something finite and not based on experimentation if possible since I don't own a bike store and don't have access to lots of tires.

I have an 89 660. I don't know for sure but I bet that the chainstay clearance is identical. Just eyeballing it, 28mm looks like the biggest you could get in there.

I'm an engineer, my wife would tell you its a social disease I am sure, but I am looking for something finite and not based on experimentation if possible since I don't own a bike store and don't have access to lots of tires.
So find someone that does.

Any good bike shop would not only allow you to try a few tires on your rim, but would gladly sell you the one that fits.

If you can get tire manufacturers to standardize and publicize every technical aspect of their products mounted on every conceivable rim combination of the past, present and future...and most of all, be honest about them...then perhaps you'll make some headway with your engineering approach. Of course, once the perfect tire combo is achieved, it will inevitably go out of production in favor of a new model with entirely undocumented specifications, you'll rip a sidewall and be back to the drawing board.

I know what you mean. I'm a bit anal myself but have learned to expect a wide array of measurement variations in the cycling world. Clothing is worse than tires, I can wear a small jersey on one brand and a large in another. My son had a 52cm frame with the same stand over height as my 56. I have a medium helmet that is smaller than my older medium-small helmet of the same brand.
I guess we just have to roll with the punches.

Al

I was just noting that my European jersey is a XXL, which fits tighter than my Voler L, which fits tighter than my medium size Gap polo shirt....

I guess this is my point. The variables here can't be put into some formula for a set of measurements that would define the bike's capacity,

When you buy shoes do you put your complete faith in the size listed on the box or do you try them on?

When you buy shoes do you put your complete faith in the size listed on the box or do you try them on?

Well I didn't say I was happy about that either...but even at the shoe store they have measuring devices to get you in the ball park for length and width. I have a ruler and a micrometer, but I am not sure what I would measure to estimate maximum "properly labeled" tire capacity of a specific frame.

Thanks for all the responses, I guess I knew the answer before I asked the question.