Just wondering if you've had any problems running at high pressures? Tyre spec says they will go up to over 100psi but my rim spec says not to use tyres under 1.5" or over 60psi(dt rims). Who's recommendations should I go with?

The rim manufacturer is setting an upper limit. Stick to it.

I didn't know GPs came in a 1.5"?

I usually run tyres really hard to avoid punctures so they would not be the rim for me

mavic rims just come apart at high pressures…so beware

The GP's are 1 1/8". Its DT that are recommending not to go under 1.5".

I ran my 1.2" GPs at 90 PSI on Sunn rims which I'm sure would have had a similar rating with no issue.

i ran conti ultra gators on mavic 717s at 120psi for a while with no problems

was hellish uncomfotable though, so dropped it to about 90

I used to use the 1" GP's at over 100psi, if you're using rim brakes you'll soon find out if the braking surface is too worn, wear earplugs and safety goggles when pumping them up 😀 my ears are still ringing now. 😳

Contacted DT and the official response was not to use them. They aren't designed for it and rims would not be covered by warranty if they do give way.

mavic rims just come apart at high pressures…so beware

Do they? What, all of them?

Shit! I've been running Mavic rims on my road bike, with pressures of well over 100psi, for years! To think, just how much I've cheated death all this time… 😯

I shall change them immediately. What should I replace them with?

My audax bike uses 26" wheels with not very special CR18 rims – been running 25m GPs at 115psi without problems for a couple of years

Haven't had any problems with Mavic xm 317 & Conti GP's at 105psi yet 😕

High pressures and large volume tyres = lots of force pushing rim walls apart
High pressures and low volume tyres = not quite so much force pushing rim walls apart

IIRC my Mavic rims give two recommended pressures, one for narrow tyres and one for wide!

High pressures and large volume tyres = lots of force pushing rim walls apart
High pressures and low volume tyres = not quite so much force pushing rim walls apart

Eh? I don't get that. Pressure is pressure, no? Regardless of the space it's in?

IE, 100psi in 10cc is the same as 100 psi in 1000cc?

No? Am I missing something?

I thought larger volume = less pressure needed, at the expense of greater weight and rolling resistance.

I'm confuddled now… 😕

Pressure is force divided by area, therefore if you have the same amount of pressure (e.g. 60psi) acting on a larger surface area (e.g. the surface of a 2.1" tyre as opposed to that of a 1" tyre), then you will have a greater total force acting on the tyre bead….

At least I think that's the way it works.

No, I still don't think that's right. I don't think the volume has anything to do with it.

A skinny road tyre is designed to hold much higher pressure (up to 140psi), because a skinny hard tyre is mechanically more efficient; smaller contact patch, much less rolling resistance. If you factor in the additional pressure caused by the riders weight, and sudden spikes caused by bumps etc, then it would be safe to assume the tyre and rim are designed to withstand even greater pressure, maybe 200psi+.

A fatter, larger volume tyre is designed for comfort and shock absorption, rather than optimum mechanical efficiency, so lower pressures are sufficient. This would surely allow for the greater bump forces suddenly increasing the pressure within. If you land from a jump, then the pressure inside the tyre could spike from 40 to 80 psi, or perhaps even more. So, the tyre and rim would surely be designed to withstand higher pressures than the indicated guide. So, I suppose in theory you could have a rim of the same dimensions/materials/proportions as a road tyre, and a fat tyre which could withstand the same pressures as a road tyre. It just wouldn't be very comfortable off road.

I'm at a loss as to how increasing volume somehow would put more strain on tyre and rim, if the pressure remains the same.

Here's my reasoning…

If you have a tyre that measures, say, 50mm from bead to bead and is 26" in diameter, it will have a surface area of (approximately) 0.033m2.

If you have a tyre that measures 25mm from bead-to-bead and is 26" in diameter it will have a surface area approximately half this amount.

If you inflate both tyres to the same pressure, the larger tyre will have double the total force acting on the surface, since force=pressure x area. The area is double for the wider tyre. That is fairly fundamental physics, I think.

Since the pressure will be acting normal to the tyre surface, some of it will be trying to expand the tyre radially, and some will be acting lateral to the wheel (i.e. push the rim walls apart).

This basically means that for two tyres of differing volume, under static conditions and inflated to the same pressure, the larger tyre will exert more force on the rim.

This would explain why my Mavic 719 rims has max. safe pressures of 48psi for 2.3" tyres and 113psi for 1" tyres. (see here: http://www.mavic.com/mtb/products/xm-719.323318.2.aspx)

If that theory were true, then the force on the rim would be approx. proportional to the tyre width for a given pressure. This would mean that the max force should be constant, so 2.3*48 should be about the same as 113*1, which it almost exactly is. Hurrah.

If you inflate both tyres to the same pressure, the larger tyre will have double the total force acting on the surface, since force=pressure x area.

Surelyt that's a contradiction? The pressure remains the same, regardless of the surface. The only thing that changes is the volume.

Imagine you have a tin can, of 500ml. You fill it with 100psi. Then, get another can, this time 5000ml, and fill it with the same pressure. The same forces are acting within both the cans. So, they could both be constructed of the same materials with the same properties and dimensions.

Are there any phyisicisticianists out there can clear this one up, or am I going to have to start a whole new thread?

Force=Newtons
Pressure=Newtons/Metre^2

To get the force you integrate pressure over the area of the vessel, be it a tyre or tin can.

Since the pressure is the same for both cases, but the surface area changes, the total force (in Newtons) acting on the surface will also change.

I think i've said all I can on the matter now!

I still think you're wrong. I think the pressure guidelines are more to do with the design of different types of tyre than they are to do with the design of the rim. MTB tyres aren't designed to withstand the higher pressures of road tyres. If you could construct a fat tyre of the same properties of a road tyre, then surely it would be able to withstand the pressure?

We need a proper scientist on here. Where's that Barnes, he's quite clever.

I am happy to be proven wrong though, and I apologise if I'm wrong at all.

It's bugging me now, and I need to know!

Bigger tyres do exert a greater force on the rim/bead for a given pressure. This should make it clear.

http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/pressure_vessel.cfm

Fair enough! I think pressure guidelines for rims are telling you the safe operating conditions for the rims, whilst pressure guidelines for tyres are telling you the safe operating conditions for tyres though.

To answer the question without jabbering on about high school physics, yes i've put those same tyres on my mountain bike to use as a commuter. I run them at 80psi and it's very fast on the road if a little harsh. My rims are the original Mavics on a very old skool XC bike, they were probably a bit narrower back in the day so more suitable. Of current 26" Mavic rims I think I only spotted the XC717 that would accept down to a 1" tyre. Hope I don't ever puncture, they were harder to get on the rim than a normal mtb tyre.

I don't understand that stuff, ac282. 🙁

I still don't understand how a simple increase in volume somehow puts a tyre or rim under more strain.

Poppa is right. The surface area inside the tyre is greater on a fat tyre and force = Pressure x area. The force is trying to separate the 2 beads. The only thing resisting this force is the rim.
It is the force not the pressure that is trying to split your rim apart.
If you have 2 pneumatic rams, 1 small 1 large, at a given pressure the large one will exert more force. Your tyre is effectively a annular pneumatic ram.
HTH

I wouldn't worry too much anyway, just stay within the safety margins that the rim and tyre manufacturers recommend and you should be fine.

If you have 2 pneumatic rams, 1 small 1 large, at a given pressure the large one will exert more force.

How?

100psi is 100psi, regardless of the size of the ram, surely?

i ran some 1" slicks on both mavic 317, 717 and some cheapo aluminium unbranded rims at over 100 psi with no problems for 2 years.
Rims were only changed after crashing into a pedestrian who ran across the road!

100psi is 100 pounds per square inch. If your ram has a piston of 1 sq in in area the force is 100 x 1 = 100lbs force
If your ram is 2 sq inch in area the force is 100 x 2 = 200 lbs force.

That still doesn't make sense, in relation to tyres. The 2sq. in ram is still only pushing 100psi.

If you get a set of scales, and put a 100lb weight on it, that is 10cmx10cm, it still weighs the same as a 100lb weight measuring 100x100cm.

Or; if your weights are both 1×1", but one is only 10" long, and the other 100", they still apply a pressure of 100psi.

Can anyone explain this in Idiot please, because I'm struggling here… 🙁

psi doesn't matter to the rim. Force will break the rim (assuming that it's going to break from bending rather than bursting), not pressure – force is the result of pressure x area.

A bigger area (bigger tyre) will result in a greater force applied to the rim.

In your analogy you would have to say that the scale's area is 10x10cm and that if you applied a fixed pressure and then wanted to double it, you'd have to double the weight – eg twice the force.

I'm still not convinced at all.

I want a simple, clear explanation in language I can understand, and preferably graphs and diagrams.

Leg pulling now, shurely?

Well, the question I want answering, is 'If the tyre volume is increased, but the pressure remains the same, will this put the rim under more strain?'.

This has not been answered sufficiently. Just a load of confusing and conflicting babbling. 🙁

Nice graph though! 😀

Talkemada,
Forget about the scales in your recent post.
Back to those rams. The 2 sq in one does, as you say, see 100psi. But that's 100 pounds force on every inch, 2×100=200 pounds force.
(likewise, a 1sq inch ram will also see 100psi, but have a force of 1×100 = 100 pounds force.
Pressure isn't force (unless area is 1)

2×100=200 pounds force.

On what? The same square inch? No; its the same psi, isn't it?

And what's this 'force' business? P = F/A, so P = 100 psi.

So, by my reckoning, the pressure in a 1" tyre would be applying the same force on any given part of a rim, as the same pressure in a 2" tyre. So if a rim can withstand 100psi, the volume of the tyre is therefore irrelevant, surely?

I'm sticking with my theory, until someone can prove otherwise.

On what? The ram, or the rim.
PSI isn't pounds per sq inch. It's pounds force per sq inch. Hence 100 pounds force per square inch is a total of 200 pounds force if the area is 2 sq inchs.

Lord Above… 🙄

Yes, but 100psi will only exert 100 pounds force per SQUARE INCH of the rim. Not 200, not 3, not banana. 100.

Soooooooo……..

Will a larger volume tyre at the SAME PRESSURE be theoretically possible on the same rim?

I refer you to the many answers above.