michaelknight00

Hi!!!
I have been pondering this hard choice. 25mm or 28mm tubeless tyres (GP5000S TR) for my hookless rims? They are 19.4mm internal width.
25mm should be more aero, and Giant (my bike and rims brand) says 25c should be 28mm.
But does it make sense to push to 28c then?
Thanks for your inputs!

yaw

I have 19.5 internal rims and opted for 25 over 28 GP5k on the front for the aero profile because the 28 bulged out and the difference in comfort is not noticeable enough for me. But I use 28 on the rear.

25 front measures 27
28 rear measures 29
(after. ca 1000km)

So if you're mostly on good surfaces, I'd go 25 to give the wheel a better frontal profile. But if you otherwise need the extra width or lower pressure then 28.

michaelknight00

Thank you!
I guess where I live roads are overall OK. It's either OK, or really bad, but then I would avoid bad section.
Thank you for the advice!

scottfsmith

It all depends on your roads. On my regular rides my wild guess is it is about 10% 23mm-optimal, 30% 25-optimal, 30% 28-optimal, and 30% 32-optimal. I run 30s.. being on the big side gives a little extra comfort.

WhyFi

I'd personally run 28s - my roads are ****, I'm not a featherweight, and any aero gains of 25s would likely be more than offset by suspension losses and fatigue on many of the longer rides that I do.

That said - they're consumables. No need for handwringing - choose one but tell yourself that you'll try the other in a few months after you're run the first set down to the cords.

PeteHski

I've been running various 28, 30 and 32 mm tyres on my Giant SLR1 rims (17 mm internal hooked). Our local roads are pretty rough and full of potholes. I don't notice any aero difference between them, but the wider tyres are more comfortable. They do bulge out a fair bit from the rim, but I don't let it bother me. Unless your roads are super-smooth I would go with 28 mm.

Iride01 

If you ride a lot you will be needing new tires in a year or two anyway, so just pick one and then try out the other when it's time to change and then you'll know for yourself what you prefer.

ThermionicScott 

I'm not planning to ride anything less than 28mm, ever again.

Chandne

Well, my 25s measure 28 so I'll vote for 28s.

EDIT- I don't know if they are faster everywhere. On rougher roads, I do think they are. On smoother roads and especially long climbs, I think 25s are slightly faster though not by much. The 28s are faster on those same descents so it is a wash. I'll go for the comfort on the 28s given my bike has no Iso Speed etc anyway.

sw20

I've always ridden 28s but recently upgraded to 19mm internal and 27.5mm external wheels. I went with 25s and can say I haven't noticed any difference in comfort whatsoever.

vespasianus

Hookless? Those can only be pumped to a certain pressure that might be too low for a 25mm tire. How much do you weigh?

ratell

Pirelli says only their 28 and 30mm Race TLR tires are compatible with hookless rims I believe for this reason.

PeteHski

Yep, IIRC I think it's 73 psi max for hookless rims.

michaelknight00

Thank you for your answers!
My roads are either very good (I'd cruise at 30-35kmh on those) or very bad (but then I would try avoiding them when I can).
I am quite lightweight, 56kg so hookless and pressure are fine for me
Actually funny story, the Giant store where I bought the bike initially pumped the Gavia tyres at 100psi. I was expecting those to blow off but never happened. So somehow that means Giant was able to deal with tight tolerances pretty well. I now usually keep pressure between 75 to 85psi and all is well

Calsun

28mm tires have greater volume and allow for a 15% lower PSI than 23mm tires and it is the air drag of the rider that is the overriding factor and not tire rolling resistance. I have one road bike that has 28mm tires but my 20 year old road bike has rims that are too narrow for anything wider than 23mm. Not worth replacing its wheels to be able to use 28mm wide tires on it but if I rode it more it would be very tempting, especially with the terrible roads in California.

Hiro11

28s should be slightly less aero but offer more grip and less chance of pinch flatting. I'd go with 28s. Also, I agree with the poster above: I'm never riding anything below a 28 again. I'll bet we start seeing road bikes coming standard with 30s soon.

Bike Gremlin

I'm happy to see road bike tyres (slick profile) being available in a 28 mm width. It wasn't always so, at least not in my country. Don't feel like going back to any slimmer ones. Not racing or competing though, so the drag and mass difference between 23 and 28 mm is hardly noticeable or measurable as far as I'm concerned (your tastes and priorities may differ). Hell - on rougher roads, 28 mm tyres still roll when out of the saddle, while the 23 mm ones would make the rear wheel bounce and slip a bit on cobblestone climbs.

michaelknight00

Thanks everyone for.your inputs!
I see overall more ppl focus more on the comfort and peace of mind benefits from the 28.

msu2001la

Just going to throw it out there that unless you are regularly banging out rides at 25mph+ avg speed, any measurable aero difference between 25s and 28s is going to be very minor.
The aero benefit from reduced frontal area will also be offset by differences in rolling resistance.

There is also the question of rim width. You mention your wheels are 19.4mm internal, but what is the external width? depth? From a purely aerodynamic perspective, the optimal tire width would be 5% less than the external width of the rim (105 rule). If the maximum external width of your rim is less than 26.25mm, it's going to be sub-optimal for aerodynamics with both 25mm and 28mm tires. Is 28mm more sub-optimal? Yes, but I'm guessing the difference isn't that dramatic between the two.

All that said - At 56kg - the Zipp tire pressure for 25mm on 19mm i.d. hookless shows 64-68psi. For 28mm those numbers go down to 55-58psi. I am 75kg and run 28mm on road at around 60psi (hookless, tubeless 21mm i.d.). If I were 56kg and running narrower rims, I'd probably run 25mm.

prj71

I used to ride 28s. Now ride 32s. Way more comfortable ride. No way I'd go back to 28's again.

michaelknight00

​Thank you! Indeed my rims are "only" 23mm external width, 42mm depth (basically the Giant SLR2 disc wheels).
If I had wider ones I wouldn't think too much between the 2 options, but here I am

yaw

So knowing both options will measure a couple mm wider on that internal dimension and thus far exceed the external width, leaving not much chance for that airflow to stay attached, does that make a good enough argument for new wheels

michaelknight00

Ahahah I was trying to avoid that kind of investment... But you may be right that both options aren't great aero wise, so I may just go with wider tyre to get full comfort benefit

GhostRider62

Will 25mm tires blow off hookless rims at 74 psi? 79? 84? The correct answer has to take rider weight and proclivity for risk into account.

I'm running 25mm tubeless at 90 psi......with hooks.

Calsun

The only advantage of a narrower tire is being able to use a small tube that takes less space in the saddle bag. A rider is working to overome air drag and the rider is the primary cause of air drag followed by the bike frame and any attachments. The wider the tire the greater the air volume and so a lower PSI can support the same load of bike and rider. My rims on my Trek bike only allow for 25mm wide tires and for me it is not worth buying new wheels to be able to use 28mm tires.

In terms of load supported for larger tires the PSI can be lowered as the volume of air increases - and more than most would realize.

700x23 at 100 PSI
700x25 at 85 PSI
700x28 at 65-70 PSI