Maximum tyre pressure on carbon rims

[chrisbainbridge]

I am looking at using a 29er XC carbon rim with internal rim width of 19mm with 28mm tyres. However they generally quote a max pressure of 60psi.
Is this generally because of the high energy impacts on XC bikes and would you expect me to be OK running a road wheel at 80psi?

[LittleWheelsandBig]

Wider tyres put more load into the rim, so narrower tyres should work at a higher pressure. If it is rim-braked, be aware that many epoxies are affected by heat.

[Ningishzidda]

Is a '29er' a 622?

It can be used for road tyres, eg Conti Supersonic 20mm which have a reccomended inflation of 150 psi.

If it blows apart, take it back for a refund.

I have Shimano RS80 wheelset with carbon rims. Conti Supersonic at 150 psi.

If your chosen rims can't manage this, they shouldn't be on sale.

PS 28mm tyres go to 100 psi for the road. Lower pressures on CX cus they want grip in mud.

PPS Tyre pressure is exerted in ALL directions. Rim, rim walls and tyre casing. Put wider tyres on and inflate to 85 psi as opposed to 100 psi for 23mm tyres, and the load on the rim walls is 15 psi less.

[PeeJay]

PPS Tyre pressure is exerted in ALL directions. Rim, rim walls and tyre casing. Put wider tyres on and inflate to 85 psi as opposed to 100 psi for 23mm tyres, and the load on the rim walls is 15 psi less.

Not sure about that,  pressure is load per unit area.  A bigger tyre has a taller side wall and therefore a larger area for the air to push against.  Therefore more load for the same pressure.

[mrcharly-YHT]

PPS Tyre pressure is exerted in ALL directions. Rim, rim walls and tyre casing. Put wider tyres on and inflate to 85 psi as opposed to 100 psi for 23mm tyres, and the load on the rim walls is 15 psi less.

Not sure about that,  pressure is load per unit area.  A bigger tyre has a taller side wall and therefore a larger area for the air to push against.  Therefore more load for the same pressure.

Changing the size (width) of the tyre alters the direction of the force against the 'hook' of the rim. Wider rims are subject greater outward forces, all else being equal.

[Ningishzidda]

The neck of a balloon is thicker rubber. It doesn't stretch as much as the round of the balloon, but has the same pressure against it.

Anyway, the 'Safety Factor' of a bicyle rim is probably 500% at least.

Anyone had a brand new, Quality Passed rim split apart due to tyre inflation?

[LittleWheelsandBig]

http://velonews.competitor.com/2013/04/bikes-and-tech/technical-faq/tech-faq-all-about-carbon-clinchers_284201

Note the effect that heat from braking has on many epoxies and subsequent retention of tyre beads.

[PaulF]

If it's a 29er XC rim I'd expect it to be disc only which means that heating effects are probably moot?

[LittleWheelsandBig]

See my first post in this thread.

[PaulF]

My point was that it might not have a braking surface for rim brakes - as it's described as XC & 29er I assumed it was a MTB rim...

[LittleWheelsandBig]

Correct but how pedantic do you want to be? Probably not rim braked but if so, etc, etc.

My second reply was more for Ning asking about failing carbon rims.

[bikenrrd]

Here's some anecdata - I have some DT Swiss 29er rims on my 'cross bike, with disc brakes.  With 28mm Conti GP 4 seasons.  I commute on it.

I only pump them up to 60 PSI, as above that they start to feel a bit noodley and squirmy.  This could be for any number of reasons, and could be entirely in my mind.

I think I'll probably rebuild them with H-Plus Son Archetype rims for next winter.  These are anodised on the rim braking surface, so with disc brakes they will never be word and look bling for years.

You can buy road specific carbon rims with 19mm internal / 23 mm external width.  Look for cyclocross ones, or road ones with U shaped profiles.  Not all of them are high profile either - you can get 25mm ones.  These will definitely be able to take the pressure (and might just be the MTB rims rebadged - but at least they are sold as road rims with a maximum pressure to match).

[chrisbainbridge]

You can buy road specific carbon rims with 19mm internal / 23 mm external width.  Look for cyclocross ones, or road ones with U shaped profiles.  Not all of them are high profile either - you can get 25mm ones.  These will definitely be able to take the pressure (and might just be the MTB rims rebadged - but at least they are sold as road rims with a maximum pressure to match).

This is for disc specific use so rim temperature not a problem. I am trying to get as light as possible on the rim and use a dynamohub.
I have had difficulty finding light rims with a wide internal rim width and 32 spoke for the dynohub.

I will look at cyclocross specific a bit more.
Thank you

[recumbentim]

So do 29er carbon rims , like reynolds xc ,  take 100 psi on a road tubeless tyres  ? The Wheels are a stronger build 28 spoke 26mm outside size ect and ideal for audax as I have a disc set up. The advantage to me is rotational weight .

[mrcharly-YHT]

Check manufacturer specifications. I tried looking these up online. Reynolds are crap. Utterly useless at giving out technical info.

Probably not and why would you want to anyway?  I would doubt it would be sensible.

[Frank9755]

Why is rotational weight an advantage? Audax is mostly about riding along at a steady speed so rotational weight is not really an issue.

[recumbentim]

Rotational weight for me is about saving my energy.  Using new technologies without sacrificing comfort or durability is MY objective in my cycling. The gains maybe marginal but I don't think I would go back the wheels I was using on recumbents 25 years ago. I also think they have gone over the score on weight loss and spoke sizes on road stuff, thats why I am looking at 29er. Possible my best option is to build or get built with the components I want.

[andrew_s]

The tension on the tyre sidewall increases with tyre size (like PeeJay said), as well as acting in a more outwards direction.
If you use a 23 mm tyre at 105 psi and swap to a 25 mm tyre, if you keep the same pressure the tension increases to 25/23 times the previous value, or if you want to keep the same tension, you've got to reduce the pressure to 23/25*105 = 97 psi.

If a rim manufacturer quotes a maximum pressure, hopefully they will also give a tyre size and you can re-factor the value for a different tyre. Be a bit cautious if the size is very much bigger, due to the more outwards load direction.
If you want to factor in the change in load direction, you could assume that a 28 mm tyre makes a 28mm circle that sits on the rim flanges and do some geometry, but I'd probably fit the tyre and measure the angle between rim and sidewall (which would also give a real rather than nominal tyre size).

[Frank9755]

Rotational weight for me is about saving my energy.  Using new technologies without sacrificing comfort or durability is MY objective in my cycling. The gains maybe marginal but I don't think I would go back the wheels I was using on recumbents 25 years ago. I also think they have gone over the score on weight loss and spoke sizes on road stuff, thats why I am looking at 29er. Possible my best option is to build or get built with the components I want.

It's up to you what you do, and there is nothing at all wrong with getting a nice new set of wheels purely for the undeniable pleasure that they give.  However, in terms of energy saving the gains are more non-existant than marginal.  In rolling countryside, where a lot of audax is, they may actually be negative as you would lose speed very slightly quicker on the climbs and gain it slightly quicker on the downhills, pushing your overall wind resistance up.

The reason to reduce rotational weight is not to save energy but to enable you to accelerate marginally quicker out of a bend in a road race.  In audax, you don't need to do that.

[recumbentim]

Thanks for that Frank I have obviously been peadling under a few misconceptions for a while. Andrew I just found a  a chart by Easton that said it was Ok to run their narrower carbon 29er rims with 28mm at 100psi .