As I've gotten on quite well with the Cheapo i25s from PX on My MTB I decided to order another set (they were down again to £7.50 so why not) with no specific idea about what I'd use them for...
And I'm now half toying with rebuilding the wheels on my Gravel/CX bike using them, they're maybe 50g heavier per rim but are proper TCS rims rather than my current set which are taped up 19/17mm road/touring rims which have been a little bit of a struggle to get tyres onto and inflated in the past.
My question is this, just how narrow a tyre can you mount on a 25mm internal rim?
I reckon my 40mm Summer tyres should be OK but my current winter tyres are 32mm, I'm thinking that might not make for an ideal profile.
I could buy some cheap hubs and use the skinny wheels for winter and new wheels in summer but I'd actually prefer it if I could simply change tyres twice a year rather than have spare wheelsets knocking about.
from what I've read a "Rim to tyre ratio" of 1.4 to 2.2 x internal rim width is considered to be in about the right range, which would mean I should find a 35mm tyre for winter.
So is anyone running a tyres closer to a 1.3ish ratio? And if so how does that affect them grip/ride/sealing wise, especially tubeless?
There's a tech piece from Mavic here with a table that lists the ERTRO recommendations for each internal rim width. For 25C it looks like the narrowest tyre is 44mm
I've currently got 28s tubeless on my 23mm internal i23s (went up with a track pump), having previously ran 27mm tubed. I've used 20mm tyres on a 19mm internal rim before and it was fine (TT bike)
You'll be fine with 32mm, they'll have a great profile and grip fantastically (IME).
If 23c tyres are fine on 17mm internal, why shouldn't 30c be fine on a 25mm internal?
There's been a few different pieces written on rim/tyre combos and I did look at that mavic article, it seemed to be more focused on rolling resistance, Bare in mind Mavic have been a little conservative in their guidance in the past, the idea that you can't mount a tyre below 44mm on a 25mm rim seems a bit odd IMO, I reckon a 35mm-ish tyre might be OK in i25, but I'm thinking a my 32mm tubeless tyre might just be a shade too narrow...
CTC Guidance (and a couple of other places) seem to say 1.4 - 2.2 x rim width: https://www.cyclinguk.org/cyclists-library/components/wheels-tyres/tyre-sizes (as an approximate guide at least) but I bet that's assuming tubes are being used.
Which seems sort of sensible, but then you realise 2.3" - 2.5" (~64mm) on an i25 is pretty common on MTBs (some go even wider now) and would of course exceed the upper limit of that CTC guidance (but match up with mavics)...
Basically I'm just questioning some of the hard and fast "Rules" as they don't all quite sync with real life anymore, especially since people started using TL tyres/rims and ditched tubes...
why shouldn’t 30c be fine on a 25mm internal?
sidewalls might be a bit more like the "Bell" profile (see below):
As described here: https://www.bikerumor.com/2016/08/12/tech-story-match-bicycle-tire-width-rim-width-best-results/
apparently more prone to squirm, odd deformation and potentially worse rolling...
I might just try it and see, it's only a few mm... Right?
I'd argue much less prone to squirm tbh.
The table is probably slightly out of date and does of course depend on what tyres and rim combinations were tested. There'll be some grey area where some tyre/rim combos worked but other tyres of the same width didn't so they've been a bit conservative, which for a general guidance is fine. "Standards" will always lag behind practice to some extent as people try to push the boundaries.
You've also got to deal with the vagaries of manufacture: some 37C tyres are wider than some 40C ones for instance on the same rim at the same pressure so there's almost a sense of "try it and see".
I run 2.35" tyres tubeless on a 19.5 ID rim which is at the limit according to that table and I've not had any problems. I don't think I'd run 2.6" and certainly not 3" tyres on it though. On a different rim (but the same ID) I use 28mm tyres tubed but those rims currently have 37C tyres tubeless.
I'd say you're pushing it somewhat.
I've tried 32mm CX tyres on Crest rims (21mm internally) and it was horrible. The tyre tread ends up so flat you wonder if the center of it isn't actually concave and lifting off the ground! Adn they didn't so much squirm in the corners, more like the feeling you get on a rubbish fat bike tyres when the bike won't go in a straight line, you couldn't ever ride it no-handed, it felt like it would momentarily exaggerate any lean you gave it to turn, then flick right back the other way and try and kill you.
I've just bought some more NOS crest rims for my CX bike which will be used with 38C tyres (or 42c if they fit).
I was using 23c tyres on a 21mm internal rim and have just changed to 25c tyres. No issues with either (certainly don't get any experiences to match thisisnotaspoon, tyre is still round, can ride no handed for miles so that must be down to the CX tyres as I am using road tyres) although from the description above for "the bell" i guess I am losing some comfort and cornering.
Don't go with the sidewall damage comment as the diagrams shapes don't even back that up.
It was probably the tread. The outer knobbles were bent inwards where they should have been vertical. The same problem is probably what made corning so bad, as the bike was leant over it didn't take much lean before knobbly tread and were rubbing the slick sidewall.
The sidewall damage is because more of it sticks out past the tread. On a narrow rim the tread should be the widest part.
Yes, that is all MTB tyre specific and makes sense. For road type tyres it doesn't make any difference, the sidewalls are not sticking out at all. For gravel tyres, all depends how knobbly and what shape the tyre is I suppose.
I’m actually looking to put CX tyres on the same rim( cheap as well from Planet X ) was planning on 42-45c possibly 50
Thanks for this thread, i was on the verge of posting a very similar one. I have my eye on some ztr rims (25.5mm internal diameter) and was hoping that i could get away with Vittoria Voyager 700c 38mm tyres - looks like it could be a goer. Thanks op!
Arch Ex (25mm internal) rims on my cx which I've used with 33 and 35mm tyres with no issues. I'm fact they work really well.