28mm wide rims on rim-brake frames

[torger]

As a poor person who likes somewhat light and fast bikes, I'm still on rim brakes and look into upgrading to a second hand last-of-the-rim-brakes race bike, like a 2016 - 2019 Scott Foil. These last of the rim brake era bikes have direct mount rim brakes and official 28 mm tire clearance.

I have bought into "wider is better" trend, so I'd like to push the rim/tire to the limit. I build my own wheels, so I'm looking into lightbicycle aero rims which have wide rims also for rim brake. They had 30 mm wide rim-brake rims for a while (with 23mm internal), but it seems to have been too wide (many had troubles fitting them) so now they have 28 mm wide brake track with 30 mm wide at the widest point. The internal is 21 mm. It seems like a 28 mm tyre will balloon to about 29,3 for the narrowest to 31 mm for the widest on a 21 mm inner width rim.

I'd like to hear experiences from this type of setup, and how likely it is to succeed. I'm okay with if I have to get different brakes (which to get?), but swapping out the frame would be harder of course. Haven't bought a frame yet, so if these rims only work with some few select frames it would be nice to know which ones.

My current assumption is that if a frame's official tire clearance is 28 mm, a 28 rim should also fit, and even if the tire baloons to 31 mm it is probably ok as official fit requires 4 mm space on each side, and I'm prepare to drop that to 2.5 mm. It needs to fit into the brake caliper though. Maybe special brake pads is needed?

[mrlobber]

Successfully pushing outside of the maximum specified tyre widths likely depends on a combination of several frame parameters, such as fork / chainstays clearance (sometimes even seattube clearance), caliper clearance and so on; you're unlikely to find out the limits without experimenting yourself. I run the LB 28/21 rims on a SL6 with EEbrakes, and there's plenty of clearance for the rims, however, I've not pushed the tyre clearance at all, and still use some worn Turbo Cottons which measure around 26.5mm on those rims.

28mm tubulars already have visibly less room left, and a 30mm WAM tyre, depending on its height, actually might not fit the calipers anymore, on my setup. But as said, yours might differ.

Btw that official 4mm clearance, don't quote me on this, might not have actually been around when the last gen rim brake frames were designed.

[blaugrana]

I use these Light Bicycle 28mm wide rims on a Cannondale Supersix Evo from 2017 (the last model with the horizontal top tube), and I'm having no issues. I would say that 28mm is really the maximum width you can probably use on Dura Ace brakes without needing low profile brake pads, so I can see why they are not offering the 30mm width option anymore.
I am using 28mm Continental GP5000 clinchers and there are no issues with clearance. I haven't measured it, but it's not particularly close. But I would say this is probably the widest setup (both for rim and tyre) that you can use without running into issues, needing non-standard brakes, etc.

[torger]

Just to add some info to the thread, I did a quick surf now on the clearance thing, it seems like the 4 mm comes from ISO standard 4210-2 section 4.10.2.

The previous version of this standard is dated 2015, which has been superseeded by the current version from 2023. Unfortunately this standard costs money so I can't see the exact wording, but I find some references to it by Cervélo that states that it's 4 mm, including in a manual from 2017, so already the 2015 version of the standard was 4 mm. The 4210 has been around for a long time though, and in its 1996 version the required tire clearance was actually just 2 mm.

The indian standard IS 10613 follows the ISO standard closely and for that one can find the cleartext on the web, it was there I found out the old versions just had 2 mm. With the latest it seems like racing bikes have a requirement of 4 mm, while mountain bikes, young adult bikes and touring bikes have 6 mm.

The large manufacturers often refer to ISO 4210 in their manuals so I assume that they comply with it.

[jdms2k]

I have LB AR45 that are 21 internal and 28 external. New GP5000 28mm tubeless tires are surprisingly running at 28.5mm. I use DA9200 calipers and black prince pads.

[Hexsense]

Some naunce points. But modern tire of the same labeling size are actually less tall than the age of Continental GP4000sII.

Back then, tire labelled 28mm have huge casing. But it mounted on a narrow rim. The tire form near complete circle, thus tall.
Now, the tire labelled 28mm have smaller casing. But it mounted on a wider rim. So the width measure out similarly but we crop the smaller part of complete circle. Thus, it is less tall.

So, A frame that say 28mm max tire clearance back in the old days may have more height clearance than you expect.

Chart version. See 28mm tire on 15mm rim is taller but narrower than 25mm tire on 19.5mm rim?

Exagerated drawing of what happen when casing shrink but rim width widen. Such that tire end up at the same width.

[torger]

Thanks for the feedback, I think I feel confident enough to just assume that it will work. There's a bit of adjustment as 28 mm tires comes in various actual widths, and the rim itself should clear. Most likely I will need to get low profile brake pads, and in worst case get new brakes I suppose, but a frame with official 28 mm tire clearance should clear a rim with 28 mm wide rim brake track. As I'm not racing and mostly ride on my own or with just a few friends I can live with the wheels being fiddly to remove if that would become an issue.