I am thinking of getting a disc equipped road bike, and have been thinking of what wheels to use. I will be looking at running tubeless 25mm tires and would like a nice light set of rims, and carbon if at all possible.

So is there any reason why I couldn’t use the Light Bicycle 29 XC light rims?
I’ve heard people say that the tire will blow off the rims as they are not made for high pressure, but that is not backed by the physics as I understand it.

Pressure = Force / Area
so Force = Pressure * Area
Area
25c tire = 0.000490874m^2
50c = 0.001963495m^2

Pressure
30PSI = 206842.5 pascal
100PSI = 689475 pascal

So the force of a 2.0″ tire at 30PSI = 406.13N
For a 25c tire at 100PSI it is 338.45N

So why would something putting out less force not work on the same rim, unless I am missing something in my calcs?

I dunno what your calcs are but the point is the force the bead of the tyre puts on the rim and I don’t see how your calc relates to that.

I doubt this will be lower at 100psi on a narrower tyre.

You seem to be basing it on the entire surface area of the tyre, as said it’s the pressure exerted by the tyre bead which is virtually a constant, so taking your cals this is the pertinent bit:

Pressure
30PSI = 206842.5 pascal
100PSI = 689475 pascal

So it’s 3.5 times more pressure on the sidewalls of the rim.

I’d ask them, but really, why bother? Carbon road rims don’t have a contrasting brake track that looks silly or anything, so I’d just get some of their wider road rims and have done with it.

Pressure is not a measurement of force, so why is it 3 times more?

Also, I do not want deep rims. I ride on the Isle of Man where it gets a bit breezy so want shallower section wheels.

Think of the force which is creating the tension in the carcass of the tyre. This is what is trying to pull the bead off the rim. It is higher at a higher pressure.

I’m not quite sure of what you’ve calculated frankly, but the surface area of the whole tyre, which you seem to have calculated, is irrelevant for what you want to know, which is the force being exerted on the sidewalls (and spoke bed) of the rim. This isn’t offset in any meaningful way by running a different sized tyre.

I don’t know why you’d not just use these, 24mm deep (so very similar depth to the MTB rims, and shallower than the hookless ones), lighter than the MTB rims. Just not sure what you’re trying to achieve?

While I know that you can run tubeless with a conversion kit, I would prefer a tubeless ready rim.

Not many of those except for the Light Bicycle U shape 45mm

Which is why I said to ask them, they’re amenable to doing things they don’t necessarily advertise.

If you’re wanting to run tubeless at >100psi I’d be even less inclined to get an MTB rim.

You see, force is not pressure as pressure is made up of the force applied over an area.

So from my understanding, to if the pressure goes up, so does force if the area stays the same. But the area is not staying the same so a higher pressure in a smaller container is exerting less force that a lower pressure in a bigger container.

This is also now just a mental exercise as I want to understand this.

Have you just been ignoring what we’ve written?

Try reading and considering it.

Bye.

I have read, but the physics doesn’t work that way, from what I understand?

Unless you can explain why I am wrong in the physics, what are you upset about?

But the area is not staying the same so a higher pressure in a smaller container is exerting less force that a lower pressure in a bigger container.

The area the force is being exerted on – the bead of the tyre/interface with the rim – doesn’t change though.

I think what they are getting at is:

The area of both tyres should be the same. Its based on contact with the rim not contact area with the floor.

The force on the rim is greater at 100psi compared to 30psi.

edit: beaten to it.

Ok, but the force is not only on the bead-hook of the rim, A lot of the force is taken by the tyre, which is why tubeless ready tyres have no stretch beads, hence the no bead-hook rims.

So in my mind, the force is not completely applied to the rim, but the tire does act as a cylinder in some respects.

I’m probably getting too complicated, but I don’t like it when there are statements like you can’t use a MTB rim as the tyre will blow off. I don’t see why a MTB rim is so different in strength to a road rim to be able to take those pressures, as I don’t think the forces are so much higher as 3X more.

Fundamentally Shred is correct – the force pulling the tyre off the rim is lower with a narrower tyre. However the calcs are wrong, as you appear to have got a squared term in there when the area of the tyre is actually directly proportional to the width, not proportional to the square of the width (area ~= pi * width * circumference).

It is also higher with a wider tyre. Look at it like this, imagin chopping the tyre in half at the point the sidewalls are vertical – with a 25mm tyre the upward force due to the pressure at this point is pressure * 25mm, which is shared between the two beads. With a 50mm tyre the upward force due to pressure at this point is pressure * 50mm, which is still shared between the two beads (OK I know we should be looking at an area not a linear dimension to convert pressure to force, but the circumference dimension does not change for these calcs – not only that, but the tension in the carcass is a force per unit length rather than a spot force). This is the tension in the carcass which directly translates to the force at the bead.

Does nobody here own a Notubes rim? Have you not spotted that the max pressure varies with tyre width – this is the reason.

Though to come back to the original point, there is no way I would run a tubeless road tyre at 100psi on a rim not designed for it.

which is why tubeless ready tyres have no stretch beads, hence the no bead-hook rims

They have reinforced beads to deal with the greater forces, they’re not inherently “no stretch”, hence you can use ‘normal’ tyres on a hookless rim, that came about because tyre tolerances are far better than they were back int’ day.

The example I was thinking of is the Enve XC 29er rim.

Their pressure chart is on the specs page
http://www.enve.com/wheels/mtb/29XC.aspx

Is there any reason why I couldn’t use the Light Bicycle 29 XC light rims?

yes max tyre pressure is 70psi.

and i think you want to run 100 psi

but the wider 700c 38mm clincher carbon rims for cyclocross bikes a road bikes come in upto 36hole and

will take 160 psi they are located in the road section and can be ordered without brake track and weight 30g more.

Avoiding deep section though.

Isn’t it more about bead/rim wall spec than pressure? Stans rims are made for easy tubeless and low pressures, start sticking high pressure in and the tyre will blow off the rim – probably. My CX tyres are rated for 75 psi, when I accidentally stuck ~100ipsi n the there other day it blew off the (standard) rim and exploded my innertube which was highly amusing for co-workers but a pita for me.

Use stuff in a way that it is not designed to work and you may well have problems no matter how good your maths.