Gear

Technical FAQ: Max tire PSI, outer chain stops, falling arches

Lennard Zinn answers questions about max tire pressure, how to stop chain drops, and the Tour de France's falling 1km-to-go arch.

Dear Lennard,
I just acquired a pair of 26” Roval Control SL (carbon 1200g pair) wheels on huge sale. I plan to put on 26 X 1.10” tires, specifically a Michelin Wild Run’r folding tire, with stated minimum pressure of 58psi. Photo attached.

However, the rim sticker said max pressure 45psi, which at that pressure the 26 X 1.10 tire still feels way too soft to ride. It has been my understanding from the DT Swiss website that max pressure depends on tire width even for a carbon clincher .

My question is: is the warning sticker on the Roval carbon rim 45psi an absolute number, or is the max pressure depends on the width of tire? Please enlighten me on the technical aspects (legal department aside).
— Ardi

Dear Ardi,
The stress that can burst a bicycle tire is called “hoop stress,” or stress in the circumferential direction, as opposed to the stress along its length. This is also the stress that is trying to blow it off of the rim. The hoop stress of a bicycle tire can be approximated by the equation for a thin-walled, cylindrical pressure vessel, namely:
Hoop stress = (p * r)/t , where:
p is the internal air pressure, r is the inner radius of the tire, and t is thickness of the tire wall.

You can see that the hoop stress on the tire wall is not only a function of the tire pressure and the thickness of the tire, but it is also directly proportional to the width of the tire. This means that the bigger the tire, the greater the hoop stress at the same pressure.

Besides the fact that an MTB tire pumped up to 100psi or thereabouts will ride horribly, it will likely either explode or blow off of the rim. And if it does blow, it doesn’t mean that the tire is less well-made than the road tire running for its entire life at that pressure, or that the relative sizing of the rim and tire bead were less accurate (these are both assumptions I have heard people voice). Rather, it means that the hoop stress is so much higher on the big tire at the same pressure that it can’t possibly hold it.

MichelinSpecialized

That’s why you don’t (or had better not) pump a mountain bike tire as high as a road tire. This explains the big drop-off in pressure with tire width in the DT Swiss chart you linked to. It’s not just more comfortable and produces better traction to run a 3.2-inch tire at below 29psi, but it also is necessary to avoid damaging the tire.

This Silca test is another way of illustrating this; it shows that a bigger-diameter tire is stiffer than a smaller-diameter tire at the same pressure.

So, a short answer to your question is that the 45psi on the warning sticker on the Roval carbon rim is not an absolute number; rather, the maximum pressure depends on the width of tire (and the bigger the tire, the lower the pressure should be).

― Lennard

Dear Lennard,
Why do road chain catchers only catch the chain to the inside? My front derailleur throws the chain to the outside sometimes, even when I adjust it so it just barely rubs the chain in high gear.
— Todd

Dear Todd,
I hope you’ve gotten good at easing off and pedaling your chain back on before it either gets jammed or you have to stop and get chain grease all over your hands wrestling it back on. It is not a good idea to try this with an electronic front derailleur, though.

The answer is that road chain catchers that work with a front derailleur and still prevent the chain from going off the outside as well as the inside do exist. WickWerks has one, and it is being used in the Tour de France right now. This is a good thing, because we’ve many times seen a pro throw the chain off in major races, even with a Di2 front derailleur and an inner chain stop.

TLJ-sentinel-inside

The WickWerks Sentinel provides the function of a standard inner chain stop, but, unlike other inners stops, it also has an outboard prong to prevent over-shifts as well as dropped chains. It is adjustable both inside and outside independently from the front derailleur to deal with variations in chainline among road cranks. Team LottoNL – Jumbo is using the Sentinel during the Tour de France with its Shimano Di2 electronic front derailleurs.

WickWerks’s lead engineer, Eldon Goates says in the Sentinel press release, “You want to push the chain hard against the inside of the large chainring during an up-shift. When the front derailleur is set for super-fast shifting, sometimes it will over-shift. Using the Sentinel will enable the LottoNL – Jumbo team riders to have super-fast shifting without ever having to worry about over-shifting.”
― Lennard

Lastly, lots of people have asked me variations on, “What’s up with the inflatable arch falling on the riders in the Tour?” I had never thought about it before, but it does seem crazy that the only thing preventing those things from falling down on the riders is maintaining air pressure in the arch from air compressors that are plugged in, and generally into portable generators.

Given the amount of security in the last couple of kilometers of a Tour stage to prevent spectators colliding with the riders winding up to finish, it is surprising that a fan can accidentally bring down the arch supporting the red kite right on top of the riders.

The Tour said there would be security to prevent this in the future, but when Tom Dumoulin rode under the 1km arch before the ski station of Arcalis in Andorra, it sure looked like there was nobody near it, and nobody to prevent somebody from yanking on the cord or the legs.

I think it needs something akin to either a fender or a reflector bracket extending out from the fork crown to prevent a cantilever straddle cable from falling down on the tire if the brake cable were to break. In other words, a scaffolding on either side of the road with a big cable or rope going under the arch could prevent it from dropping on the road in the case of sudden deflation.
― Lennard