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I'm looking at some wider rims to replace my RS80s.  As the RS80 rims are about 15mm internally pretty much anything is going to be wider.

External width is irrelevant I think but internal width on 'wider' options runs from 17 up to 20mm.  I imagine there will be an optimum width for a given nominal tyre size.

I intend to mainly be wearing 25mm Conti GP4k (28s I will use occasionally).  So at the moment I'm thinking 18mm internal width will be a good choice with the 25s (Hunt's Race Aero current favourites) but what is the wisdom of the crowd/herd? 

5 comments

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TypeVertigo [428 posts] 9 months ago
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The trusty fallback is the ETRTO rim width chart. I have wheels with 19 mm internal width, and as per the chart, I shouldn't be running anything narrower than a 28 mm road tire. So far it's worked out for me, as 28 mm rubber has slowly become more available.

Mind you, the rims' internal width seems to have blown my effective tread width to almost 30 mm across from the nominal 28, so there's that as well.

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Nixster [404 posts] 9 months ago
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Good old Sheldon I should have guessed that it would be there. Thanks for the link TV.

The chart suggests 25mm is okay on 17mm rims but not 19mm; I'm guessing still okay at 18mm.

On one of my bikes 28s just  rub at the fork crown so I'm hoping fattened 25s will squeeze under. 

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oceandweller [80 posts] 9 months ago
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The standard rule of thumb the great tech guy at CTC, Chris Juden, used to give was 1.4 up to 2.2 times the internal width, with 1.8 times internal width optimum. So 17mm internal will take anything from 24mm to 37mm tyres with 30mm optimum. I was running 25mm Conti GP4ks on 17mm rims (internal) until recently & they were great.  In the winter I'd switch to CX tyres (33mm, UCI rules) & they worked fine too. I've recently switched to 28mm Schwalbe Pro 1 tubeless for the road & they're even better than the Contis. So, yes, 17mm or 18mm will be fine with 25mm tyres but have you considered going a bit wider on the tyres?

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BehindTheBikesheds [2011 posts] 9 months ago
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well I run open pros (15mm), Open 4s (13mmish)and have used a 32mm easily, on the Open 4s it properly looks like a lightbulb but even cornering around a mini roundabout at 17mph didn't cause any issues.

A 15mm internal width is fine for a 28mm, any wider and you will end up with a 28mm measuring when inflated a fair bit wider, a 17/18mm internal width rim will likely make a 28mm more like a 30mm and a 25mm more like a 27mm.

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Nixster [404 posts] 9 months ago
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Thanks guys.  I have a clearance issue on one of my bikes (Supersixes of different vintages) on the front, which is stopping me running 28s on it, hence asking about the 25s.  Hopefully a 25 on an 18mm rim will be just right!