13C rims

[Revellinho]

My 700C rear wheel has a 13C rim width and from reading a few 'recommendations' tables from manufacturers, it seems that 25mm tyres are the widest that should go on it.  With some huffing and puffing I can get a 28mm beaded tyre seated and have ridden it, usually at around 90 psi.  Am I tempting fate or is it a case of 'if it goes on then it will be OK'?

[LittleWheelsandBig]

It'll be fine. Think about how wide MTB tyres and rims are.

If you are really pushing the width, the practical maximum and minimum pressures for your tyre get quite close together.

[hubner]

I think if the tyre is thick, stiff, hard, like a touring tyre or off road tyre, it would be ok on a narrower rim.

But for lightweight supple tyres, I think it would best for the max tyre width to be only a bit wider than the rim, eg max 25mm tyre on a 20mm rim (outside width). I think there might be handling problems if there's too much "overhang". rather than any problems with fitting the tyre onto the rim.

[Revellinho]

thanks you guys!

[Samuel D]

People talk about handling problems with a wide tyre on a narrow rim, but I’ve never noticed any difference in feel from the relative widths of rim and tyre. Only the measured tyre width and pressure have caused noticeable differences in feel. Perhaps I haven’t gone extreme enough in terms of relative widths or pressures.

Spa Cycles have assured me by phone that I can put 28 mm tyres on my Exal XR2 rims (13.5 mm internal width) if I want. They probably know from experience that this works.

[Brucey]

you should be aware that there is a hidden issue with using a wide tyre on a narrow rim, and that is that the static pressure load of the tyre may overload the rim, causing it to splay prematurely, or even split in the well of the rim.

Most rim manufacturers will issue guidance about tyre width vs permissible maximum tyre pressure. Wider tyres cannot be inflated to such high pressures without blowing the rim apart.

If you want to see how this works Mavic have a chart that shows this info for their rims. Their rims also come with this info written on the rim in some cases. BTW the tyre will also have a maximum pressure rating, and you should use whichever is the lower as the maximum for your installation.

If you want a rule of thumb, then if (say) the rim is OK with 100psi in a 25mm tyre, then reduce this maximum rating by 25/28 if you are installing a 28mm tyre.

Also, a wide tyre will tend to roll around on a narrow rim. Fundamentally this happens because the tyre contact is no longer centred on the tyre once the bike is leant over; to a first approximation, the whole affair will be unstable once the contact point is outside the rim width. The sensation is of the tyre 'folding over' and is very unsettling indeed.  The wider the tyre, the worse this is, and the narrower the rim, the more the tyre will move, with less force required.  The pressure at which this occurs will vary with the rim width.  Some riders never find this limit, others might find it instantly.

 Personally I think it is a bit dangerous if a soft tyre is first signalled to a rider by it rolling around on the rim. It is certainly alarming.

Between these two things, the range of acceptable pressures for a wide tyre on a narrow rim may be considerably less than if you fitted the same tyre on a wider rim.

cheers

[LittleWheelsandBig]

For something like a 35+mm tyre, sure, but 28mm isn't really excessively wide for a 13C rim. I like to corner hard and have ridden 28mm Paselas (flexible sidewalls) without any problems on narrow rims.

Lower the maximum tyre pressure a bit (so you don't blow out a part-worn rim) and it'll be fine. Lower the tyre pressure too much and the rim will wallow over the tyre contact patch but you have to let it down pretty far (think slow puncture behaviour) for that to happen.