Thinking of putting some WTB Nano race at 40c on some planet x model b wheels.
How do i know if they will work ok on that rim? i know 35c work fine but if anyone knows the acceptable upper limit on a road rim that be good to know.

Also how does a 700x40c CX tyre compare width wise to a 29×1.9 29r tyre?

This is the place to look – http://sheldonbrown.com/tire-sizing.html

Check your brake clearance first. OK if running Cantis , or V brakes . Normal road brakes can have small clearances

Well, my road rims are 0.4mm wider externally then the Mavic 717s I’ve got on my MTB, so I see no reason why I couldn’t put a 2.2 29er tyre on them……
Frame clearance is another issue entirely though!

Also how does a 700x40c CX tyre compare width wise to a 29×1.9 29r tyre?

1.9in = 48mm

I had 1.8″ 29er tyres on my magic open pro rims no problems

Well, my road rims are 0.4mm wider externally

internal width is the important one (if it’s important at all – depends on the usage I guess; wouldn’t really fancy cornering at 50mph on a skinny rim with a big volume tyre)

Thanks for the info all.
my canti or miniv will have clearance and so will the front, but the rear at 42mm maybe a bit tight but i could run slimmer back there.

Sheldons width considerations seem to be very cautious, allready outside the range on 35c tyres compared to internal width, so maybe it will work ok or maybe its just pushing the boundaries too far.

So ideally a wider rim is needed, hmm
Maybe now is the time to upgrade the forks to disc and get new wider front rim and keep the rear wheels for now as the frame cant run disc.

It would help to know how wide the rim is.
When you think about it, MTB ers have been breaking these “rules” for years. I bet you happily bung a 2″ + tyre on a rim only a few mil wider than a road rim.
More practically I use a 38mm road tyres on 16mm inner width rim with no hassles. Go to the tyres maximum pressure of 80psi as well. It didn’t sit well on a 13mm inner width rim.

When you think about it, MTB ers have been breaking these “rules” for years. I bet you happily bung a 2″ + tyre on a rim only a few mil wider than a road rim.

MTB tyres are a bit more forgiving though, you’re far more likely to slide on the ground at any reasonable tyre pressure than the carcase is to roll over on the rim.

I’ve got 38mm tyres on my tourer/commuter and that’s got ridiculously narrow rims!