Help Me Understand the Importance of Rim Width
 

Dunno if this is the right place.

My 1987 Trek 520 was originally supplied with 27"x1 1/4" tires. I changed them out for 1 1/8" tires, almost immediately, and have used that size ever since. With the current supply shortages, I feel lucky to have been able to find 1 1/4" tires.

The rim inner bead width is 16.5mm and the tire inflates to 34mm. From what I read this is suboptimal, but not outside of a useable range. I'm running Michelin Protek tires at 65psi. front and 70psi. rear.

What I'm experiencing is a tendency, of the bike, to "fall into" turns. If I turn at a leisurely rate, the bike steers predictably. But, if I turn a bit sharper, the bike seems to turn sharper than expected.

Is this tendency being caused by using too narrow of rim width? Too low of inflation pressure, instead of the manufacturer's recommended and rock hard 87psi? Or, is it all in my mind?

Thanks for any help, or movement of the question if it should be elsewhere.

My personal, conservative formulas is 2.2X rim width.
Maybe you need a bit more pressure? Try another 5 PSI and see what happens.

I would be far more inclined to blame that on the general geometry of the bike than the rim width.
My first MTB used rims that were - at best - 1/3 of the tire width. That one could corner a bit squirrely at low pressures. Say 1.5 Bar or below. No mystery to that, one could easily see the tire fold over and squirm.

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...87psi is not exactly "rock hard." Why not try it and see what happens ?

I don't see anything wrong other than you just need to try different pressures. Even though wider tires don't need as much PSI to support rider weight as their narrower versions, as a kid I often found that running really wide tires on a narrow rim required more PSI to keep them from "rolling" sideways in a turn and feeling squirrely.

And you said that bike came with that size tire originally, so I doubt Trek would have put a dangerous size tire on that rim.

I first tried the rated maximum. Then 60/65, 65/70, and 70/75. I decided that I liked them most at 65/70. I put them back at 65/70 and that's when I thought of doing the slalom test. Maybe I'll try the test again, at 70/75. But, I don't expect to keep them there, regardless how it might turn out.

I don't know if it means anything, but it's always been hard to push the bike by the seat. The front end wants to flop over.

I've never noticed the falling into turns effect, with the old tires. Maybe it's always been there, but I only noticed it because i was paying attention.

Yes. That's probably not the right way to describe it. Maybe super bouncy. Like, basketball bouncy.

...try adjusting the headset so it has more preload. This can make a profound difference in handling.

I’m with dabac that it’s the bikes geometry but 65-75 psi seems a tad low for 1 1/8” tires if you’re not a light person or carrying a rear load, The tires max pressure has no bearing on optimal pressure. 65-75 is more what I’d use for 1 1/4” tires but again it all depends on the weight the wheels are carrying. There’s a world of difference between optimal psi range for unloaded riding or loaded riding esp. for the rear tire. When I was light and light touring on 1 1/8” tires I’d pump the rear to 80psi figuring it would be down to 75 in a couple days. One time touring with 1 1/4” rear and 1 1/8” front I just pumped them to the same psi.
When I had my shop in the 80’s it was interesting to see the ranges of psi people toured on. Folks on 25 mm tires touring on 120 psi, others on 1 1/4” with 45-60 psi. “Did you know your rear tire has 50psi?” , “I thought it was soft, I filled it last week”

Sorry. I have trouble with making coherent posts.

The tires, being spoken about, are 27x1 1/4", per the label. They actually measure about 34mm, on my rims.

I'd try them at lower pressures, but the manufacturer only rates them to 58psi minimum.

I have bikes that do that. It's normal when they have a lot of "trail."

I think you're just noticing things a little more now that you've put on fresh tires. Unless your headset is loose, I wouldn't tighten it. Just keep riding and learning how to work with your bike.