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Tire size and rim width

Old 03-25-14, 11:05 AM
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Tire size and rim width

I have a pair of wheels that are technically marketed as 29er rims and are 622x21c rims.

I would like to ride a narrower slick tire as I will mostly be on road, but if I look at

Tire Sizing Systems
or
Tire Dimensions | Schwalbe North America

It looks like the narrowest tire I can use "Safely" is a 35mm. I really was hoping to use 28 or 32mm, but am worried about potential pinch flats or rim damage doing so.

Are these charts just conservative? How much risk is there with a narrower tire? Anyone have any experience on this?

I have been thinking of ultimately getting another wheelset anyway, likely with a 19mm wide rim and maybe a dynamo, but not really ready to do that immediately (both have no idea what to get and need to wait till I have the $ to spare). I am trying to figure out what tires to put on the bike right now, so was hoping to get some more info on this.
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Old 03-25-14, 11:24 AM
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I think the charts are conservative. I have often used wider tires than they recommend without any problems.

There's a trend these days to use wider rims with narrow tires, particularly 700x23 tires with rims that have a 23mm outside diameter. Typically, those charts would say that's a bad idea, but it's becoming a highly recommended configuration. I can only speculate as to how well that would translate to your rims and preferred tires.

Have you verified the 21mm inner width measurement yourself?
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Old 03-25-14, 11:38 AM
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Before charts came out, the conventional wisdom was tire outside width = 1.5x rim inside width, with plenty of fudge room to either side, especially wider.

If the rim really is 21mm inside width, you can certainly go to 32mm tires, and most likely to 28mm. There are even some who'll go narrower yet, but I won't go there.

Generally, you're good if the tire profile is more like an Omega, than like in inverted U.
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Old 03-25-14, 11:48 AM
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My understanding is that wider tires on narrower rims can lead to the rim splitting/failure due to excessive outward force of the larger tire, which can be limited by running at lower pressure. For example, I have succesfully run 60mm tires on 19 mm rims at 2/3 max pressure (40 psi on a tire that can be inflated to 60 psi). Too low pressure can lead to poor handling, so you need to find the proper balance.

The opposite is true of running narrower tires on wider rims, where the risk of damage is to the tire sidewall, resulting in a tear or blowout, as well as pinch flats and rim damage due to impact.

However, I don't actually know if any of the above is actually correct, so take it with a grain of salt. I do know the tire recommendations are conservative, and are regularly exceeded. Keep in mind that you are going to suffer the consequences, so going way out of the guidelines is probably not the best idea.
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Old 03-25-14, 11:55 AM
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My 29er came with 622-54 tires and I have run 622-32 tires on the same rims without any noticeable issues. I use the Tire Pressure Guide to set my tire pressure which may affect how my narrower tires behave on these rims.
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Old 03-25-14, 11:57 AM
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Originally Posted by alan s
My understanding is that wider tires on narrower rims can lead to the rim splitting/failure due to excessive outward force of the larger tire, which can be limited by running at lower pressure. For example, I have succesfully run 60mm tires on 19 mm rims at 2/3 max pressure (40 psi on a tire that can be inflated to 60 psi). Too low pressure can lead to poor handling, so you need to find the proper balance.....
The outward stress the tire exerts on the rim is called "hoop stress". The formula is very simple, pressure X width, so a 2" tire at 50psi, creates the same stress as a 1" tire at 100psi. Since tires are usually ridden a pressures inversely proportional to their width anyway, it becomes a wash.

That said, I'd be a bit leery of pushing the limit on narrow sport/race rims where the makers were watching weight, and expecting that only narrow tires would be used, especially if said rims were nearing the end of their brake track wear life.

My commuter uses 2" tires on 21mm rims at about 60psi. The brake tracks are well worn at 25k miles, but so far so good. I will say that they can get squirrely if I have a slow leak and pressure drops significantly, but I've learned (the hard way) to watch for that. If the front wheel doesn't have the right "bounce" on road bumps, I check.
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Old 03-25-14, 12:27 PM
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Originally Posted by Andy_K
I think the charts are conservative. I have often used wider tires than they recommend without any problems.

There's a trend these days to use wider rims with narrow tires, particularly 700x23 tires with rims that have a 23mm outside diameter. Typically, those charts would say that's a bad idea, but it's becoming a highly recommended configuration. I can only speculate as to how well that would translate to your rims and preferred tires.

Have you verified the 21mm inner width measurement yourself?
My impression was the numbers in those charts was the inner width of the rim, not outer. Is that not correct?

I am not clear what a 23mm outside width would translate to on inside width.

I haven't tried measuring the rim myself yet, though I could certainly try to do so, maybe this evening. I was assuming it was 21 mm inside width.

My concern isn't wider then the recommendation, as this seems to be quite common with MTB tires. My concern is narrower tires then recommended. I don't want to risk sidewall blowout or pinch flats.
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Old 03-25-14, 12:29 PM
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Originally Posted by FBinNY
Before charts came out, the conventional wisdom was tire outside width = 1.5x rim inside width, with plenty of fudge room to either side, especially wider.

If the rim really is 21mm inside width, you can certainly go to 32mm tires, and most likely to 28mm. There are even some who'll go narrower yet, but I won't go there.

Generally, you're good if the tire profile is more like an Omega, than like in inverted U.
Good to know about looking at the profile, that makes sense. I don't think i would try narrower then 28, nor do I really want narrower on this bike, but I am a bit nervous about 28 based on these charts.
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Old 03-25-14, 12:37 PM
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I would imagine that you could easily run 32mm or probably even 28mm tires without much issue. The big thing in road tires lately is 23mm wide rims matched to 23mm tires. These are typically 17-18mm inner width (Kinlin XC279, Pacenti SL23) and seem to run fine with 23-25mm tires.
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Old 03-25-14, 12:51 PM
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Also, I run 35 mm tires on 13 mm rims at 15% below max tire pressure rating with no issues whatsoever. And, yes, you need to base calculations on the inside rim width, not the outside. So, as a rule of thumb, no problem going 3x the inner rim width, as long as you keep the tire pressure at around 75% max pressure rating.
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Old 03-25-14, 12:54 PM
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I've used Vittoria Zaffiro 23 tires on Ambrosio Elite 19 rims, no problem (YMMV).
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Old 03-25-14, 12:58 PM
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Originally Posted by alan s
... as long as you keep the tire pressure at around 75% max pressure rating.
Consider this a quibble rather than a correction.

The rim neither knows or cares about the tire's pressure rating. It only cares about the actual hoop stress the tire pressure causes. Rather than focus on the pressure rating of the tire, which can vary tremendously tire to tire of the same width, consider the high end of the spectrum of pressure ratings of tires the rim maker would expect you to use. (ie. the pressure ratings of the tires in the center of the charts.

Then adjust that based on the width ratio of the tire you're using to the rated tire to find the maximum pressure from the rim's point of view. Subject to the tire's rating, this would be the max. leaving the same margin for safety as would the narrower tire. Of course you're free to use lower pressure balancing ride and handling with comfort.

By example, if the rim is expected to handle a 25mm tire at 140psi, it could equally handle a 50mm tire at 70psi, not that you'd want that.
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Old 03-25-14, 01:13 PM
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Originally Posted by mstraus
I have a pair of wheels that are technically marketed as 29er rims and are 622x21c rims.

I've been running 25 mm tires on 29er XC rims with 18-19 mm internal rim width for many years so I have given this a lot of consideration (light 700c disc wheels were once impossible to find).
IMO, you really need to contact the manufacturer and ask them the maximum pressure this clincher rim can accommodate. If it's some ultra-light butted scandium rim you might find yourself endoing as the rim explodes and gets caught in your fork.
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Old 03-25-14, 02:10 PM
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Originally Posted by mstraus
My impression was the numbers in those charts was the inner width of the rim, not outer. Is that not correct?

I am not clear what a 23mm outside width would translate to on inside width.
Yeah, the charts are talking about interior width, but that number isn't always listed in specs, especially with road rims. Mountain rim manufacturers seem to be a lot better about it, presumably because it matters more to MTB riders.

The inside width is typically about 3mm smaller than the exterior width, but that can vary.
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Old 03-25-14, 02:14 PM
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Originally Posted by Andy_K
...

The inside width is typically about 3mm smaller than the exterior width, but that can vary.
Yes it varies, but not by enough to get worked up over. A straight side rim has sides about 1.5mm thick so 3mm does it. OTOH a hook edge rim (most modern HP rims) probably have walls about 2.5-3mm at the hook, so 5-6mm difference inside to outside.

You're not dealing with rocket science, and as the charts show there's plenty of fudge room, so pick a number and go with it.
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Old 03-25-14, 02:33 PM
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This was my reference:

Stan's Alpine rim:


Stan's lists the ISO size as 559x20 for this rim.

What I see much more often is something like this (Velocity A23 in this case):


Velocity lists the rim "width" (no qualifier) as 23mm.
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Old 03-25-14, 03:29 PM
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Originally Posted by FBinNY
You're not dealing with rocket science, and as the charts show there's plenty of fudge room, so pick a number and go with it.
there have been many anecdotal reports of clinchers being blown off BST tubeless rims. since tires are designed for hook bead rims early adoption has it's risks...
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Old 03-25-14, 03:39 PM
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Originally Posted by spare_wheel
there have been many anecdotal reports of clinchers being blown off BST tubeless rims. since tires are designed for hook bead rims early adoption has it's risks...
Part of the problem is that there seems to be a vague (or ignored) standard for the inside shape of hook edge rims. The s=Stan's rim in particular seems to have a smaller hook arrangement than most. This might be intentional to force use of his tire (ala Schwinn years ago).

But you're right, early adoption of anything has it's risks. I'm a late adopter by design, and never , for example, buy a the 1st year of a new car model. I prefer to let others do the bata testing. OTOH- a number of regular suppliers use me as a bata tester because I seem to have a knack for finding creative, unexpected ways to screw stuff up.
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Old 03-25-14, 05:40 PM
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Originally Posted by Andy_K
What I see much more often is something like this (Velocity A23 in this case):


Velocity lists the rim "width" (no qualifier) as 23mm.

The A23 is exactly the first thing I thought of during the OP. I would have probably stuck 28s on the bike and rode off without a second thought.
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Old 03-25-14, 06:17 PM
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Originally Posted by mstraus
... It looks like the narrowest tire I can use "Safely" is a 35mm. I really was hoping to use 28 or 32mm, but am worried about potential pinch flats or rim damage doing so. ...
i would first decide if "Safely" as stated or implied by the articles and charts you are referring to, although the content seems to be focused on the practical aspects of even getting a tire on or staying on at all, were more or less important than experiencing excessive pinch flats or rim damage. they aren't equivalent, IME.

any comment on what i think is safe or not regarding rims and tire combinations, although i have my own opinions, would be useless, but i can say that i wouldn't be very concerned about excessive pinch flats or rim damage, in as much as the remedy is trivial.

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Old 03-25-14, 07:47 PM
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Originally Posted by Andy_K
This was my reference:

Stan's Alpine rim:


Stan's lists the ISO size as 559x20 for this rim.

What I see much more often is something like this (Velocity A23 in this case):


Velocity lists the rim "width" (no qualifier) as 23mm.
I have noticed this exact same thing while trying to look at Rims. Its very frustrating to know what you will get. I wish manufacturers were more clear on this and gave both dimensions.
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Old 03-25-14, 07:52 PM
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Originally Posted by Andy_K
I think the charts are conservative. I have often used wider tires than they recommend without any problems.

There's a trend these days to use wider rims with narrow tires, particularly 700x23 tires with rims that have a 23mm outside diameter. Typically, those charts would say that's a bad idea, but it's becoming a highly recommended configuration. I can only speculate as to how well that would translate to your rims and preferred tires.

Have you verified the 21mm inner width measurement yourself?
So I can say that 21mm is not the outside dimensions, which is about 25-26 mm wide. I am assuming the 21mm inside measurement is accurate but will confirm when I pull the current tires off.

Right now I am leaning to 35 or 32mm tires to be somewhat cautious.
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