hi guys, just wondering is there any consequent if i use 25mm tire on 23mm rim?

If you're talking about 23mm external width, that's becoming an exceedingly common combination and it work pretty nicely. If you're talking 23mm internal width... well, I'm not sure how it'll work out, but it's probably safe to say that it's not the combination that the rim designer had in mind.

Scroll down to the red & green chart near the bottom.
But be aware the rim sizes are referring to internal width, not the external size.

https://www.sheldonbrown.com/tyre-sizing.html#width

If you want the tire/rim combo to be as aero as possible, that is a nearly perfect match.

25mm GP5000 tubeless tires on Light Bicycle 56 mm hoops(23mm internal) end up being about 28.2mm wide.

#105 % rule

That chart is so outdated its laughable.

why is it outdated and why laughable? The chart was created with safety as priority, not aero as priority. If you use a 25mm tire on a rim with 23mm internal width, sure this gives a good tire/rim aero profile... but on the flip side, you know what's gonna happen if the tire gets a catastrophic flat right? That rim is gonna slam into the ground so hard and so fast that there's a good chance that it'll be damaged beyond usable and good chance of a crash too. A wider tire would at least increases the chance of the rim surviving since it's got more rubber to roll on in an event of a catastrophic flat.

A rim with 23mm internal width means that its brake track width is probably around 28-29mm wide (traditional rim with hook).
A 25mm wide tire is not going to be able to spread out over the width of this rim in a flat situation, kiss rim bye bye.

Oh, is that was the 105% refers to? Doesn't that really depend on the external width, too, though? Internal:External seems to vary quite a bit between manufacturers; some of the Fulcrum/Campy offerings, for instance, are 23mm+ wide while still maintaining a 17mm interior.

Yes it does.

The 56mm LB hoops are 30mm wide(23mm internal), so that 25mm tire becomes an actual 28.2mm when inflated. The hoop is supposed to be a touch wider than the tire for the 105% aero rule.

Keep believing whatever you want.

According to that chart, I should be using 40mm - 50mm wide tires on the Light Bicycle 56mm hoops

If this is the sort of thing you worry about better switch to tubulars.

keep believing whatever I want? Really man, is that really the most scientific reply you could come up with?
Did you not read the intention of the chart with safety as its priority? You do you know that a 25mm wide tire is physically impossible to be spread across a 28mm-30mm wide rim (measured at the external brake track) in a flat situation, right?

Note. I did not ask you to use a 40-50mm tire nor am I saying that a 40-50mm tire would give the best aero combination. I'm merely presenting my logic as to why I think the table exists the way it does, and in the process rebutting against your proposition of using a 25mm wide tire on a 28-30mm wide rim (brake trake). But that's ok man, ignorance can be blissful too.

That chart exists to show ideal rim widths for given tire widths, completing the circle of the tire profile if you will. One can go (and many have gone) outside of the recommendations without dying, it just becomes less optimal for the reasons given.

The standard (ISO 5775 or somesuch) has also been revised recently, to account for people running skinny tires on wide rims. Sheldon's page still has the old chart.

Here's why I say that chart is outdated.

"If you use a very wide tyre on a narrow rim, you risk sidewall or rim failure. This combination causes very sloppy handling at low speeds. Unfortunately, current mountain-bike fashion pushes the edge of this. In the interest of weight saving, most current mountain bikes have excessively narrow rims. "

Please show me some current mountain bike that use narrow rims.

"If you use a very narrow tyre on a wide rim, you risk pinch flats"

This is just false

I did not want to mention tubular since tubular is sort of an extreme case that most people will not use, mainly for it hassle. But yes, 3 of my 4 bikes ride on tubulars. And yes, in a past situation where I had used the Bontrager D5 tubular wheelset, 27mm wide at the brake track, with the Bontrager 23mm wide tubular tire for the best aero recommendation (per Bontrager). Then I had a catastrophic front tire flat on a high speed descent where the tire blew out instantly due to a rock cut, thus causing the rim to slam into the tarmac at almost 40 mph. The tire wasn't wide enough to spread across the rim, so rim ate tarmac good and was toast. But at least the tire stayed glued on allowing me to come to a stop. If I was on a clincher combo, that tire would have come off the rim in a heartbeat as soon as that rim rolls on direct tarmac like that.

It's from this experience that question "noodle soup" usage of too narror tire on a too wide rim in a clincher setup. But apparently he thinks aero trumps safety.

Things have changed since that chart was posted on Sheldon's website, but maybe you are unaware of that.

I love how you assume that I'm using a narrow tire on wide rims because aero trumps safety.

FTR, I'm using 30mm Schwalbe G-One Speed tires(31.4mm actual width) on 30mm wide(23mm internal) rims.

Comfort is my main concern, not aero.

alright I'll address a couple of distinct issues here

1. mtb rims have gotten wider over the years, yes that's true, but mtb tires are also have been getting wider too, and mtb tires are still much wider than their rims when compared to a road or gravel setup. So while wider mtb rims have alleviate and somewhat makes the above statement a little less to be of a concerned, the statement is still a valid one since there are still a lot of mtb bikes on older generation rims.

2. having said that, you then use the above statement that you think is outdated to justify your extreme use case: narrow tire on wide rims. You do this for aero reason, which i did not deny. But at the same time you've also failed to address the question about safety which I have clearly laid out in simple terms.

Look man, if aero is your priority, so be it. I get it. Then you should have said something like "I risk a little of safety in exchange for a little of aero advantage". This would have been a fair statement, no?

I do and did switch to tubs on one of my bikes.

Now I just have to worry about the glue holding up.

alright man, cool. Let's see, a 30mm wide rim has 31.4mm of real estate of tire to play on in a catastrophic flat. That's .7mm on each side. Personally i wouldn't risk that setup and would go at to at least a 32mm wide tire, especially if comfort is what I want

Mountain bike tires haven't grown that much(unless you are referring to +size bikes), but the rims have gotten huge compared to the Bontrager modified Mavic MA40 hoops that were once popular. 2.1" - 2.35" is still the most common range of mtb tires



BTW, you didn't respond to the false claim that "very narrow rims and wide tires causes pinch flats".

You do understand that the contact area gets wider when deflated, right?

this is not a false claim (practically speaking). Back in the day, guys would be using wide tires like 2.3" (remember before this they were using 2.0-2.2") and low pressure so they get some better traction. Well as it turns out, it was the lower pressure that was causing the pinch flats. I myself didn't experience much pinch flats under this combo because unlike most guys I run my pressure about 10 psi more (most were running 25 psi, I was running 35 psi). Of course today this is much less of an issue in mtb now because of the switch to wider rim (more volume) and tubeless.

But see man, just because some components of the chart is outdated, it doesn't mean
1. the chart is useless
2. nor is it completely outdated because there ARE still people running old equipment. I still have a Specialized 2007 Enduro running old school wheels!

yes I do, and more than that, I also understand (and experienced) that in catastrophic flat at 40 mph, the rim is gonna be floating all over the place more than that deflated contact patch or even the whole deflated tire. I alluded to this in my tubular setup discussed above.

So low pressure caused the pinch flats(not rims that were too wide for the tires)? We all know that.

1. I never said the chart is useless. I said it was laughable.

2. The chart is outdated, unless your bike is also outdated

Sheldon's chart is definitely outdated.

The following link shows the new updated chart as per ISO4210 and ETRTO redefined regulations for safety. Good article. See section 4 Safety to view the chart.

The Right Tyre Width on the Right Rim. Mavic: Engineer's Talk