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700x28C Gran Prix 2000S II Pressure?

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700x28C Gran Prix 2000S II Pressure?

Old 08-04-17, 04:14 PM
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diabloridr
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700x28C Gran Prix 4000S II Pressure?

We've been running 25C Gran Prix's on the Macchiato at 115PSI.

Team + Bike weight = 300 pounds.

Decided to throw a set of 28C GP's on as an experiment.

Anyone have suggestions or experience on what PSI I should run?

I'm guessing 105 or so, but open to guidance.

Last edited by diabloridr; 08-04-17 at 04:48 PM.
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Old 08-04-17, 04:45 PM
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Assume there is a typo and you mean GP 4000s II tires, not 2000s II.

Because the 28mm are so big (to over 31mm on our rims), we run the 28mm at around 14psi less than the 25mm tires on the same rims. Try that to start.

Note the new tires will take a week to reach their maximum width. So if you start with a higher psi, as the tires grow you may find a lower pressure works better later on.

The height on our 28mm has crept up a bit over time too. Initially we had pretty good clearance to the fork crown, now it's very minimal.

The ride quality on the 28mm is so much better that we do not ever want to go back to the 25mm.
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Old 08-04-17, 04:51 PM
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Originally Posted by twocicle View Post
Assume there is a typo and you mean GP 4000s II tires, not 2000s II.
Yes, 4000s!

Thanks for the feedback.
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Old 08-04-17, 09:52 PM
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I agree with Twocicle. The 28mm is a beast and I ran much lower pressure than the 25mm, at only 90psi to be exact. But keep in mind the rim width has a large impact to the desired tire pressure. I was using a very wide rim, Hed Belgium Plus, at 21mm internally and 25mm externally. The wide rim increases the tire's internal volume. A larger internal volume will need lower pressure to maintain the same tire 'stiffness'. So depending on your rim's internal width I would experiment a bit but 90psi is probably a good starting point.

I've since switched to a narrower rim with 18mm internal width. Consequently I've switched from the 28mm to the 25mm, and now I use 110psi on the 25mm for reduced rolling resistance.

The GP4k II is the best tire ever made! But I also understand that some people ride on roads that are more prone to flats. So GP4K II may not work well for them.
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Old 08-05-17, 01:24 PM
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Our team + bike weight is about 315 lbs. I like to reference the Berto Tire Pressure app on my phone. (See https://play.google.com/store/apps/d...pressure&hl=en for Android phones.) It supposedly calculates the pressure that will achieve the ideal 15% tire height drop when the bike is loaded.

Based on that app, I run 100 psi in front and 120 psi in back for our GP4000SII 622-28s.

As others have mentioned, the tires are actually larger than advertised, and my feeling is that we could run a lower pressure, maybe something like 90 / 110.

If I had more time, I'd use our digital bathroom scale to figure out how much weight there is on each of our tires when we're on the bike. Then I'd use something like the Berto Tire Pressure app (or an online equivalent) and start with pressures that are a little higher than that. Then I'd go down 10 psi in each tire and go for a short ride. I'd repeat that until we could feel that the pressure was too low. The front tire will push (understeer) while cornering and the handling in general will be sloppy. Try not to get to the point where you're risking pinch flats! Somewhere in there you'll find your own "Goldilocks" pressure.
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Old 08-05-17, 07:29 PM
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Originally Posted by pdlpsher View Post
The 28mm is a beast and I ran much lower pressure than the 25mm, at only 90psi to be exact. But keep in mind the rim width has a large impact to the desired tire pressure. I was using a very wide rim, Hed Belgium Plus, at 21mm internally and 25mm externally. The wide rim increases the tire's internal volume. A larger internal volume will need lower pressure to maintain the same tire 'stiffness'. So depending on your rim's internal width I would experiment a bit but 90psi is probably a good starting point.

I've since switched to a narrower rim with 18mm internal width. Consequently I've switched from the 28mm to the 25mm, and now I use 110psi on the 25mm for reduced rolling resistance.

The GP4k II is the best tire ever made! But I also understand that some people ride on roads that are more prone to flats. So GP4K II may not work well for them.
Absolutely. Rim width is a huge factor effecting how much air volume you have and then at what pressures to run. I didn't post our pressures for that reason... everyone's setup is quite different.

Adding up our tandem and single bike miles and years on GP 4000s, I can't exactly recall the last time had a flat with these. I think on my single back in 2012 there was one with the 4000s not the II (Chili compound). This year in the Pyrenees with the 4000sII on my single I clipped a large sharp rock @ 60mph on one descent. That cut the sidewall, but not much. Close call!
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Old 08-07-17, 09:52 AM
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I adjust pressure according to condition of road surface on a certain route. Also, we stand a lot and AFAIC this requires a bit more air in the front. We only use 28's.
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Old 08-15-17, 08:46 PM
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We weigh maybe 330 at ride start and run 95 front and rear with 23mm rims. Never had a pinch flat.
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Old 08-15-17, 08:54 PM
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Take this however you like but, I'm 260 lbs plus 25 pound bike and plus 15 lbs of books, laptop, clothes and lunch during my commute. I have the 28 GP 4000sII (awesome tire by the way) that I run at 80 front and 90 rear. No pinch flats for me with my stuff in panniers or over the rear wheel. Hope that helps.
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Old 08-16-17, 03:44 PM
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Total weight for me plus stoker, tandem and stuff is ~350 lbs. Lately I've been running these 28mm tires at 80 psi front, 90 back, on rims with 18 mm between the internal bead lips. Actual tire width is 32mm. We just returned from a ride where I focused on a pot hole, only to hit another next to it that I didn't see. Still no pinch flats. Tires feel comfortable and smooth, still roll plenty fast.
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