horinorbi1993

Hi,

I have a new Giant Propel with tubeless (hookless) wheels. Based on the tire width (25mm) and my weight, the optimal tire pressure should be around 83 PSI or 5.7 bar.

I completely let the air out from both tires and started to inflate them with my existing pump (valve is at 90 degrees, so that the sealant does not block anything). However, as the pump gauge shows only around 40 PSI or 3 bar, both tires are already firm and it is hard to get any more air in them.

Thinking that the gauge of my pump is completely damaged, I have bought a new higher quality pump (Crankbrothers). However, the issue is the same: the gauge shows around 50% of the suggested tire pressure, though the tires are already firm… So the problem is probably not with the pumps.

Apologies if I am doing a basic mistake, but I cannot figure out the cause of the problem. Unfortunately I do not have an electronic pressure tester to doublecheck the values.

Any idea behind this mistery?

Sy Reene

At 50psi, a tire will feel pretty firm if you don't have any prior reference point. What does the gauge go up to on your new pump? Each stroke once you're at 50psi, should be adding somewhere I'd guess between 4-5 psi, so you're about 6-7 strokes away from full at ~85psi ?

Elvo

Are you using a high volume hand pump? Try using a floor pump

WhyFi

Are you really heavy? What calculator are you using? The ETRTO hookless standard has a max of ~73psi.

As far as hitting a wall at ~40psi, I got nuthin'.

Bici Veloce

Try swapping out the valve cores.

Sy Reene

It's definitely curious, but so far, Giant/Cadex is seemingly shrugging off of ETRTO.
https://www.giant-bicycles.com/global/tire-pressure

Branko D

Imo, get a cheap digital pressure gauge, they can be had in most bike shops for very little, because something just seems off; a 25c tire does not feel very firm at all at 50 psi.

The other part of the equation is that if you intend to stick to the 72.5 psi limit for hookless rims, you really should just ride 28c and above. That's just a too low limit for 25c to be as efficient as they can be (Silca recommends about 88 psi for high speed on nice, fresh tarmac for me). That should be far below blow off pressure even for tires more prone to it, but 72.5 psi is what the standard says for hookless right now.
​​​​​
​​​I'm betting any timetrialists on hookless are just going for 25s with tested optimal pressure at their weight and ignoring the standard.

​​​​​If the standard was higher (and wheel manufacturers say they could be) then hookless would see more adoption, imo.

horinorbi1993

I am using the official giant calculator as posted by someone else below. My weight is 76 kg, tire is 700x25c with 19.4 mm inner rim width.

horinorbi1993

I am using a Crankbrothers Gem floor pump, has both high volume and high pressure functions.

horinorbi1993

The gauge goes up until around 40 PSI on the new floor pump, then it becomes hard to get more air in the tires. As I got the bike from the LBS, the tires seemed much softer, as (in my understanding) the case with tubeless should be... Anyways, I have ordered an electronic pressure checker, will see what that says.

horinorbi1993

Thanks for your input. I am afraid the 72.5 PSI limit applies only to Zipp and Hunt wheels. If you check the official Giant guideline regarding hookless tires (see the link above posted by Sy Reene), 70 PSI is the MINIMUM tire pressure. And, if I fill out the fields on this link with my data, I arrive at a suggested pressure of 83 PSI.

Sy Reene

fwiw, the Zipp calculator will tell you about the same 83psi (averaged), but then provides a disclaimer -- probably to adhere to ETRTO.

WhyFi

As noted, the ~73psi limit is an ETRTO guideline; Zipp and Hunt aren't the odd ones out, these are standards that many wheel and tire manufacturers are building to. Just something to be aware of if you stray from Giant's own tires in the future.

Not that I think that any of this is the source of your problem. I don't see how something like faulty cores would prevent you from going beyond 40psi and the likelihood of two pump gauges being in agreement, while being so wildly wrong, is exceptionally low.

edit: one thing that you could try immediately, just to rule out the cores, is to remove the core and inflate to see if you still hit a wall at 40. All of the air is going to rush out when you disconnect the pump head, obviously, so be sure to keep the valve away from the pooling sealant inside.

horinorbi1993

Thanks for your answers, much appreciated. I will also try to inflate without the valve cores, see what happens. If the recommended tire pressure cannot be safely achieved with these rims and a larger tire size is recommended, it is a mistery for me why Giant then mounts 25 mm tires on the bike (or did I misunderstand something in your comments?).

It will take a bit until the electronic pressure checker arrives, but I will report the results. Maybe some other readers have the same issue..

Sy Reene

I'm wondering how the actual ETRTO reads. I have a feeling maybe that max psi could be dependent (like maybe it should be) on the inside rim width. Enve, similar to Giant with its 19mm wheels, departs from the 73psi, but only for its 21mm width wheels (the 45/65 models).

WhyFi

I haven't seen any deviation in that max spec. We're fully in the weeds and purely speculating, but I don't know that ENVE is terribly concerned about ETRTO; they've had hookless wheels on the market for four years or so, well predating the standards.

Iride01

Are you pushing the pump handle to the bottom of it's stroke or just stopping when the compressed air in the pumps piston starts fighting back? I don't know about your pump, but pumps that push more volume of air per stroke will get quite difficult to push the handle down that last 1/4" they need to go.

If you suspect the gauge in error, then go get a hand held gauge that will fit your valve stem and check if it shows a big difference from what the pump gauge shows.

horinorbi1993

I’m pushing down the handle completely, though it doesn’t make a difference in my opinion. I could definitely get a bit more air into the tires, I guess I could maybe even go until 60 PSI. But already at 40 PSI (based on the gauge at least) the tires are quite firm, so I don’t want to risk a blow-off or damage by forcing additional air into the tires.

Iride01

Well what does the tire look like when you sit on the bike? If it looks like the contact patch is about right and the sidewalls aren't bulging, then maybe that might be another indicator that your gauge is off. And on 25 mm wide tires, I'd have to say that 40 lbs of pressure would feel very soft to me when pinched.

Though two or more gauges will probably leave you wondering which is correct, they'll still let you know if one is way off or not. So getting that extra gauge will let you make a more informed decision about whether to simply keep adding more air or not.

And you don't have to get the expensive one either just to see if they somewhat agree.

horinorbi1993

So I was at the LBS and I felt a bit ashamed as it turned out that the only problem was that I should have pumped more air in the tires..

As I originally took the bike from the shop, the tires were much softer and I thought that should be the normal. Now I know at least how firm the tire should feel at the optimal pressure…

Thanks for your insights anyways!

Badger6

I thought you couldn't pump more air in the tires. Anyhow, glad you got it figured out. Now go enjoy your new cushy tubeless setup!

RNAV

FWIW, I suspect there are very few people who can determine tire pressure with any sort of repeatable accuracy using only feel . . . at least at the pressures most folks run 25mm tires at. 85 psi vs 75 psi makes a noticeable difference in ride quality, but I doubt most folks could tell the difference by pinching the tire with their hands. Takeaway: use a gauge, don't go by feel.

Chandne

Do your wheels have a maximum PSI recommendation? The recommended PSI you posted, seems reasonable to me. They probably are using hooked-bead rims in their calculations, so I'd go with the max recommended if it is lower than the 80-85 you posted. Otherwise, the tires may blow off the rim. I like the idea of hookless on gravel and MTB but unless the suckers are really wide and using really wide road tubeless tires, I am not sure I want to run 70ish on the road with skinnier rims.

eduskator

Don't worry. It happens to the best of us. Sometimes, it's hard to trust hardware! You did good by checking with your LBS. Overinflating a tire, especially on hookless rims, can lead to serious harm.

80-85psi is indeed the good range. FYI, I use 28mm on same setup and I ride comfortable on 65-70pis (Giant's recommendation).

I can be indeed hard to push down for the last few PSIs. It depends on a few factors. The pump being the most important one.