Switched from 23mm to 25mm... settled on the same pressure! Why?

Discuss light weight issues concerning road bikes & parts.
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robertbb
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by robertbb

I just switched from 23mm GP4kS2's (which I'd been happily using a long time) to the 25mm version, on the same wheels (Zonda C17's).

(Yeah I'm a slow adopter, but the 23mm's measured 25mm in reality, matched the rim width well and felt great @ 85PSI front and rear so I didn't feel a need to change).

Everything I've read tells me on these new 25mm tyres I should be dropping pressure considerably... only when I run anything less than 85PSI the handling SUCKS (tyre feels unstable and sloppy beneath me descending and generally feels slow). These only measure 1.5mm wider at 26.5mm wide, but they are taller and look huge by comparison to the naked eye.

Anyway, at 85PSI it seems to click and the handling returns, but with noticeably more comfort (which is obviously nice). So all good, I'm happy with them but I'd like to know what gives? :noidea:

TLDR; Why am finding exactly the same pressure is best for me in the wider version of the same tyre? I certainly don't think I was running those 23mm tyres at a pressure that was too low at all...

I'm 72kg @ 178cm.

by Weenie


AJS914
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by AJS914

I also just switched from 23mm to 25mm GP4000s. I was running 85psi on the 23s which was on the low side for me - really just barely enough pressure. I was running them that way because I just moved to NM where the farm roads I ride on are rough. Previously in Washington state on much smoother roads I was running 100 psi.

Anway, I settled on to 70/75 psi for the 25mm GP4000s. This is on 25mm Farsports rims with an 18mm internal width. They don't feel sloppy. They feel like they have more air than the 23s @ 85psi.

A few years ago I tried 25mm GP4000s on some Campy Proton wheels. They were probably C15 or C14 rims. The tires were shaped like light bulbs on those rims and handling was horrible even at 100psi.

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pdlpsher1
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by pdlpsher1

I think the wider tire being also taller, and the tallness will give you an extra suspension effect. So you counteract the suspension effect you are using the same pressure to make the tire feel 'right'. I don't think there's anything wrong with this approach as long as the difference between the narrow and wide tires are not dramatic. In your case it's only a 1.5mm difference. If you go from, say 25mm actual width to a 30mm actual width, then keeping the same pressure will lead to a super stiff ride on the 30mm.

robertbb
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by robertbb

@AJS: What do you weigh? (just for context)

@pdlpsher1: Interesting. I see what you're getting at. I wasn't talking about suspension effect though - what I meant by "handling sucks" is exactly that... when cornering hard the tyre seemed to collapse side to side, like the sidewall wasn't holding its shape, and it felt floppy. I started at 70PSI (based on what I'd read) and added a few PSI at a time and landed right back at 85PSI as the lowest pressure the tyre would hold its shape properly.

Catagory6
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by Catagory6

ok, i must be a complete boorish rube. but how do you tell the difference in ride quality between 2mm of tread? and doesn't tire pressure depend on rider/bike weight?

AJS914
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by AJS914

I'm 195 lbs.

I wouldn't worry about it too much It's not like 85 is a super high pressure. Like I said, I used to run 100psi on 23s when I was able to ride on smooth roads. 85psi for my weight on 23s was right on the limit. I even got my first flat in years recently and it was a pinch flat from hitting a pothole I didn't see on a group ride.

Try 80psi next time. You'll probably get used to it.

robertbb
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by robertbb

Catagory6 wrote:
Thu Jan 10, 2019 5:33 am
ok, i must be a complete boorish rube. but how do you tell the difference in ride quality between 2mm of tread? and doesn't tire pressure depend on rider/bike weight?
Just feels more "cushioned" under me... particularly on bigger bumps or sections of road that aren't smooth.

Again, comfort was not really the characteristic I was focused on. The handling went through the floor with the bigger tyre at a lower pressure - that's what surprised me. I suspect that with a non-vulcanised open tubular style of tyre this would have been even worse as their sidewalls are softer.

TobinHatesYou
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by TobinHatesYou

My pressures depend on the ride. If I’m just going out without targeting a segment for a PR, then I run 28mm tires around ~60psi. If I’m going for that PR, I run closer to 85psi on a 28mm. If I know I’m going to be pushing a descent, I stick with lower pressures. 60psi on 28mm isn’t squirmy at my weight (61kg/135lbs.)

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pdlpsher1
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by pdlpsher1

I run a very high pressure for my weight (62kg or 135lbs). GP5K 28mm at 85/90 psi on 17C rim. Pressure is a very personalized choice. I also ride a tandem and I use GP4K 25mm 95/95psi on 21C rims. The rim width makes a huge difference in tire stiffness. At 95psi on 21C rims the tire is rock hard.

robertbb
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by robertbb

TobinHatesYou wrote:
Thu Jan 10, 2019 6:10 am
My pressures depend on the ride. If I’m just going out without targeting a segment for a PR, then I run 28mm tires around ~60psi. If I’m going for that PR, I run closer to 85psi on a 28mm. If I know I’m going to be pushing a descent, I stick with lower pressures. 60psi on 28mm isn’t squirmy at my weight (61kg/135lbs.)
Yeah, I'll still play around a bit with it. But from what I've read, if one needs to lower the pressure for rides with big descents or for wet weather it means they're running pressures too high at other times.

Hexsense
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by Hexsense

Probably this (exaggerated) picture answer your question. But judging that your 23c 4000sII measure only 25mm while on my previous wheelset the 4000sII 23c measure 27.3mm, and my newest wheelset is even wider:
It's your narrow rims. Your rims probably couldn't provide stable enough tire platform for bigger tire at lower pressure for cornering force you ride.
Image

rides4beer
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by rides4beer

This is exactly why I went with 23mm GP4Ks2s on my 17c wheels, I didn't want to sacrifice the handling. I run a lil more pressure, but I'm also 185lbs. I'd rather sacrifice a lil comfort (even tho it's perfectly comfortable imo) and have no doubts in my handling. I nailed a big pothole on a group ride last weekend doing about 25mph, knocked a water bottle out and rattled the heck out of me, but didn't pinch flat. Bike feels solidly planted taking downhill curves at 45mph. I won't go up to 25mm tires unless they're on wider wheels, but as always, ymmv.

robertbb
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by robertbb

rides4beer wrote:
Thu Jan 10, 2019 4:40 pm
This is exactly why I went with 23mm GP4Ks2s on my 17c wheels, I didn't want to sacrifice the handling. I run a lil more pressure, but I'm also 185lbs. I'd rather sacrifice a lil comfort (even tho it's perfectly comfortable imo) and have no doubts in my handling. I nailed a big pothole on a group ride last weekend doing about 25mph, knocked a water bottle out and rattled the heck out of me, but didn't pinch flat. Bike feels solidly planted taking downhill curves at 45mph. I won't go up to 25mm tires unless they're on wider wheels, but as always, ymmv.
Yep. All the marketing literature from Campy/Fulcrum (and others) on 17c wheels are about how much more suited they are to a 25mm tyre, which is misleading in some ways. That, combined with the "wider tyre = lower pressure" mantra, makes me think a *lot* of people aren't running optimal setups and just going with what the marketing tells them.

The only reason I had to try 25mm GP4kS2's is because I picked up a set of C17 Bora clinchers which are wider at the outside (24.2mm) I wanted a bigger tyre that would protect the expensive rim. I decided to mount them on my C17 Zonda's first and have a play with pressures.

I havn't taken the set-up to any real mountains (that's the plan this weekend), but from testing closer to home 85PSI seems like it will work well enough to switch to 25mm on all my wheels.

But the experiment was quite eye opening!

AJS914
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by AJS914

robertbb wrote:
Thu Jan 10, 2019 9:47 pm
Yep. All the marketing literature from Campy/Fulcrum (and others) on 17c wheels are about how much more suited they are to a 25mm tyre, which is misleading in some ways.
In Campy's defense, the 23mm GP4000 is a 25mm tire. The 25mm GP measures 27mm+. Campy can't be responsible for the tire manufacturers that mislabel their tires.

by Weenie


robertbb
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by robertbb

AJS914 wrote:
Thu Jan 10, 2019 9:57 pm
robertbb wrote:
Thu Jan 10, 2019 9:47 pm
Yep. All the marketing literature from Campy/Fulcrum (and others) on 17c wheels are about how much more suited they are to a 25mm tyre, which is misleading in some ways.
In Campy's defense, the 23mm GP4000 is a 25mm tire. The 25mm GP measures 27mm+. Campy can't be responsible for the tire manufacturers that mislabel their tires.
This is true. Although my 25mm conti's measure up at 26.5mm. I suppose they'll stretch a bit...

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