Alan_F

Is anyone using the Continental Grand Prix 5000 S TR (tubeless) on their tandem? If so, have you pushed them beyond the rated max pressure without any issues?

We've been riding Spinergy FCC 32's with Conti Grand Prix 4 Seasons - in 28mm for many years and recently the 32mm wide version. We generally ran the 28mm tires at about 90 to 95 psi, and the 32mm tires at about 80 to 85 psi.

I just got a new set of Spinergy FCC 47 wheels and decided to go tubeless for the first time on the tandem. I didn't notice until after I had ordered the tires that the 5000 S TR is only rated for a maximum 72 psi in the 30 and 32 mm widths, even when using hooked rims. This seems a little odd, as all the 25mm and 28mm sizes have higher max pressures when using hooked rims than hookless (all of the widths are 72 psi max for hookless which is the industry standard for any hookless setup), but once you step up to the 30mm, they list 72 psi as the max regardless of rim type.

I rode them for the first time today and set the pressure very close to 72 psi before the ride. I have TyreWiz tire pressure monitors, so I can see the pressure on my Garmin during the ride. The good news is that the tires (with about 20mm of Orange Seal in them) are holding pressure without any issues, but the front tire felt just a little too soft. We are about 315 lbs of rider and about 35 lbs of bike.

During the ride I added a little air to the front tire, bringing it up to about 76 psi. That was better, but maybe still a little too soft for me. Ideally I think I'd like to run these at about 80 psi, but I'm leery of exceeding the maximum pressure. That would only be about 10% above, but I don't like taking chances with something as critical as the tandem tires. The tubes GP5000's are rated 100 psi max in 32mm, so if I can't find a suitable pressure for the tubeless, I may just switch back to tubes and run the regular 5000's at about 80 to 85 psi.

DangerousDanR

I run the TL tubeless right at 90 PSI also on Spinergy wheels. I am surprised that the TR are rated so low.

Fully loaded we run over 450 pounds, so running 70 PSI is a non-starter for us. We ran the TL tubeless until they were showing the casing. Then I ran some backer board screws into them just to see if they would seal up. They did

Alcanbrad

The industry practice for the max tire pressure rating is to rate it at 1/2 the pressure the tire blew off the rim during testing. A tire with a max rating of of 72 psi was likely tested to blow off the rim at ~144. Running 10 to 20 lbs over the max rating should not be a problem, or at least one to worry about being out of margin.

I believe, but do not know for sure, that the lower max pressure on tubeless compatible tires is a result of testing with hook-less rims. The advantage of hook-less is one of lower manufacturing cost that comes with a less reliable fitment on the rim. I believe that this is why the max pressures are all of sudden lower on many tires.

There was a very interesting Cycling Tips Nerd Alert Podcast from a year or two back that went into a deep dive of Tubeless tire and rim designs, what the standards bodies are doing, and what is all means in practicality. It is an interesting and informative listen if you can find it.

Alan_F

2x the rated max pressure used to be the claimed industry standard, but it looks like with tubeless (and hookless) designs, that may have changed. I found this in a 2020 article about tubeless: "Enve’s testing protocol demands that a tyre must achieve 1.5 times the maximum pressure of either the rim or the tyre, whichever is the lower number, without blowing off. For Enve’s SES 4.5 AR, for example, which is rated for 80psi, that means a tyre must hold 120psi without coming off the rim."

That's still a pretty good safety margin, so I probably have nothing to worry about if I run these at 80 psi. It's just odd that Conti has different max pressures for hooked rims at 25mm and 28mm, but then drops to 72 psi for both 30mm and 32mm. Whatever the max pressure is for the 32mm, I would expect the 30mm to have at least a slightly higher rating, but that isn't the case.

DangerousDanR - What sealant were you using when you tested you tires? Some of them seem to have difficulty sealing punctures at road-tire pressures.

Alcanbrad

Here is the link to the podcast I mentioned discussing tubeless rims, tires, and all the issues surrounding them. They do discuss the 72psi rating topic.

https://cyclingtipsnerdalert.libsyn....eels-and-tires

DangerousDanR

I use Orange Seal Endurance or Orange Seal Subzero for my fat tire bike.