25mm vs 28mm on aero wheels

Everything about building wheels, glueing tubs, etc.
dricked
Posts: 184
Joined: Tue Jan 09, 2018 1:57 pm

by dricked

What’s their to “deal with”? Pumping up your tires before each ride is about all they need and really should be done anyway...

by Weenie


willmac
Posts: 158
Joined: Thu Aug 25, 2016 11:33 am

by willmac

dricked wrote:
Sat May 11, 2019 1:24 am
What’s their to “deal with”? Pumping up your tires before each ride is about all they need and really should be done anyway...
I found them a bit annoying to mount with potential for pinch flats when installing tires which occur a lot easier than with butyl and they do not like heat. Carbon clinchers with rim brakes and latex tubes is considered a terrible mix.

dricked
Posts: 184
Joined: Tue Jan 09, 2018 1:57 pm

by dricked

Ok gotcha. I don’t have much issue with heat build up where I ride mainly so it wasn’t a consideration for me.

RocketRacing
Posts: 869
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

YeAh, i run rim brakes, carbon clinchers, and latex tubes. But i am 59kg, and i live in more of a hilly area. Any steep climbs are under 1.5k.

I also can quite easily get the desired comfort with 23c (measure closer to 24-25mm) for my aero wheels. So i get to optimize comfort and aero without resorting to super wide aero rims. (Mine are 25mm at brake track, 28mm at widest... so they will cut pretty well).

I guess my point is to remind lighter riders to scale the numbers that are most often developed for 75kg riders. Same goes for heavier riders too. Figure what rider weight the data comes from, Do the math and adjust your pressures/tire width needs accordingly.

Bike rolling resistance data for example is based on a near 90kg rider i think.

Sram has a great free app called “tirewiz” that lets you input rider weight, bike weight tire size and width... and it outputs a suggested ideal pressure. It might be lower than you used previously, but i am assuming josh had a hand in influencing best practices. I find it is a good starting point, and pretty near spot on for me. I use it for everything from my fat bike to my road bike.

igs417
Posts: 28
Joined: Fri May 03, 2019 3:44 pm

by igs417

So many variables in this... :noidea:

Looking at Silca graph on previous page, how to factor in different riders weights and tire width?

For example, what would be better compromise between comfort and speed for a 95-100 kg system (rider+bike) on medium to poor conditions asphalt, assuming that both tire/rim combos are equally optimized for aerodinamics:

tires 700x25 + rims 23C internal, 30 mm outer width, 56 mm deep
or
tires 700x28 + rims 25C internal, 32 mm outer width, 50 mm deep

And, not less important - at what pressures?

GP5000 tires in both scenarios. Willing to go tubeless, if needed.

emotive
Posts: 176
Joined: Tue Nov 01, 2016 10:40 am
Location: Melbourne, Australia

by emotive

igs417 wrote:
Mon Jun 10, 2019 10:44 am
So many variables in this... :noidea:

Looking at Silca graph on previous page, how to factor in different riders weights and tire width?

For example, what would be better compromise between comfort and speed for a 95-100 kg system (rider+bike) on medium to poor conditions asphalt, assuming that both tire/rim combos are equally optimized for aerodinamics:

tires 700x25 + rims 23C internal, 30 mm outer width, 56 mm deep
or
tires 700x28 + rims 25C internal, 32 mm outer width, 50 mm deep
I think given the medium to poor asphalt, and total system weight, the larger air volume of the wider tyres and rims would give you a more comfortable ride, and ultimately less fatigue, and potentially a faster overall ride on long events.
And, not less important - at what pressures?

GP5000 tires in both scenarios. Willing to go tubeless, if needed.
The 25mm GP5000 will measure about 28mm wide on the 23C inner. SRAM/Quarc TyreWiz app recommends 64/67psi
The 28mm GP5000 will measure about 31mm wide on the 25C inner. SRAM/Quarc TyreWiz app recommends 57/60psi for 30mm, so I'd go with 54/57psi.

If you go Tubeless, you can safely drop those recommended pressures another 5-10 psi.

igs417
Posts: 28
Joined: Fri May 03, 2019 3:44 pm

by igs417

emotive wrote:
Mon Jun 10, 2019 12:27 pm
The 25mm GP5000 will measure about 28mm wide on the 23C inner. SRAM/Quarc TyreWiz app recommends 64/67psi
The 28mm GP5000 will measure about 31mm wide on the 25C inner. SRAM/Quarc TyreWiz app recommends 57/60psi for 30mm, so I'd go with 54/57psi.

If you go Tubeless, you can safely drop those recommended pressures another 5-10 psi.
Isn't that a bit low? Also, looking at the app, it doesn't ask for either rim inner diameter, nor for the tubeless/tubes options. Doesn't those two factors play a significant role?

Continental says min. 95 psi for both 25/28mm tires in regular version, and min. 65 psi for tubeless version.

I currently run 25mm GP5000 tubed on 19C rims (Ksyrium Elite UST) at 95 psi, hesitant to go lower due to Continental recommendations. Considering total weight, should/could I go lower? Why Conti puts a 95 psi limit? Also, why for both 25 and 28 mm version - should there be a difference?

Coming from Mavic Yksion UST 28 mm on the same rims at 80 psi (tubeless), GP5000 feels like a lot faster ride on a good/medium roads but somewhat harsher on medium to poor roads.

AJS914
Posts: 3465
Joined: Tue Jan 28, 2014 6:52 pm

by AJS914

I run 25mm GP4000s @ 75/80psi on 18C rims. I'm also a big guy and we have rough roads around here.

If they fit my bike I'd love to have those 30mm wide / 56mm deep wheels on my bike. Sounds like a great compromise between comfort and speed. I'd probably run them at 70/75psi.

emotive
Posts: 176
Joined: Tue Nov 01, 2016 10:40 am
Location: Melbourne, Australia

by emotive

igs417 wrote:
Mon Jun 10, 2019 1:32 pm
emotive wrote:
Mon Jun 10, 2019 12:27 pm
The 25mm GP5000 will measure about 28mm wide on the 23C inner. SRAM/Quarc TyreWiz app recommends 64/67psi
The 28mm GP5000 will measure about 31mm wide on the 25C inner. SRAM/Quarc TyreWiz app recommends 57/60psi for 30mm, so I'd go with 54/57psi.

If you go Tubeless, you can safely drop those recommended pressures another 5-10 psi.
Isn't that a bit low?
That's in line with modern recommendations. I've been running these pressures for 18 months in my 28mm labelled/30mm measured tyres. They are lower than what most people used to run. To understand why recommended pressures are now lower, have a listen to Josh Poertner (ex Zipp Technical Director and now owner of Silca) talk about optimal tyre pressures. He has helped 8 out of the last 11 winners of Paris Roubaix optimise tyre pressure, so he knows where to set tyre pressures for optimal speed and rolling resistance https://marginalgainspodcast.cc/asymmetry-part-1/
Also, looking at the app, it doesn't ask for either rim inner diameter, nor for the tubeless/tubes options. Doesn't those two factors play a significant role?
Yes they do. Internal rim widths vary greatly at the moment, some people are still running 15C wheels, while others are on 25C All road wheels. The measured width is affected by the internal rim width. Use the measured width, not the number printed on the side of the tyre.

Only a small percentage of road riders are on tubeless at the moment, so this is not a variable in the app, so assume it is for tubed tyres, and adjust downward, because tubeless tyres are not going to pinch flat if you hit a pothole. In the future apps will be add tubeless as a variable, as road tubeless goes mainstream in the next couple of years.
Continental says min. 95 psi for both 25/28mm tires in regular version, and min. 65 psi for tubeless version.

I currently run 25mm GP5000 tubed on 19C rims (Ksyrium Elite UST) at 95 psi, hesitant to go lower due to Continental recommendations. Considering total weight, should/could I go lower?
Absolutely. Try dropping pressure in small amounts each week, such as 5psi, and ride a familiar route to compare segment speed and comfort.
Why Conti puts a 95 psi limit?
Harder tyres feel faster. People buy tyres that feel faster. Conti wants you to buy more Conti tyres. But that hard feeling is not actually as fast as a tyre pressure which can absorb the road irregularities, rather than transferring them into your body to absorb. The Hysteresis is explained well in the podcast above, and later episodes go into more detail. It's pretty geeky and technical, but if you want to understand, this is the best place to learn how to optimise tyre pressure.
Also, why for both 25 and 28 mm version - should there be a difference?
It makes no sense that a 28mm tyre with 30% more volume than the 25mm version would need the same pressure.
Coming from Mavic Yksion UST 28 mm on the same rims at 80 psi (tubeless), GP5000 feels like a lot faster ride on a good/medium roads but somewhat harsher on medium to poor roads.
95psi will be faster than 80psi on smooth roads. 95psi will feel faster on the medium quality road, but may actually be slower due to the energy loss from hysteresis. Validate with your Strava times. 80psi will definitely be faster on medium to poor roads than 95psi. Try roll down the same hill at 80psi and 95psi to compare times and see for yourself.

Post Reply
  • Similar Topics
    Replies
    Views
    Last post