GP4000S II Pressure

Sutton_Rider

What pressure should I run GP4000S II 25mm on a Mavic Aksium at? I'm 165 lbs (75Kg), don't race, normal road surface. I think 88psi front and 98psi rear in the dry, would you agree? I've also seen on here a while back now there were issues with sidewalls splitting are these tyres reliable?

mfin

That's higher than I'd be running at your weight, I'd be looking about 80 front and 85 rear. The pressures you're using are too high really and defeat the object of having 25s over 23s really.

andcp

Interesting timing! I'm 77 kgs and run 90/100 for the same tyre, however I'm just about to start running 85/95 to see if I prefer those pressures; at 90/100 I'm not as confident covering as I am when I run 23's at 100/110.

ryan_w-2

107kg - running 115R 110F

88kg - running 100R 95F

trivial_poursuivant

I'm 69kg and have run 25 Conti II at around 6.5 bar (about 94 psi ) rear and 6.2 bar front. I have tried going lower, as far down as 80 psi but feel I get better performance at the pressures I'm running now.

mikecassie

I'm 60kg and run same tyres at 85psi rear & 80psi front. Don't fancy going much lower for fear of pinch punctures.

itboffin

Im usually around the low 160lbs and run 95/90 in the UK mostly for fear of punctures on our terrible roads, currently 175 lbs so I've gone 105/100 and knocked back my speed as my tyre feel more slippery

bondurant

Also those tyres come up more like 28s, so lower would be better. 80/85 sounds about right, maybe even a bit less. Much depends on what rims you have.

Higher pressures make me more nervous about cornering, conversely to andcp. I don't know the physics behind it but it seems to me that a bit more suppleness is a good thing, helping the tyre deform to the road surface more easily.

apreading

180lb - 190lb rider here.

25mm GP4000S size up more like 27mm - about the same as 28mm GP 4 Seasons.

I have run mine at 75 front and 85 rear for years and never had a problem. they often get down to 60-65 before I get around to pumping them back up too although I try not to get them that low.

If you want to run tyres in 90s or 100s then you may as well ride 23mm tyres.

There is a calculator here: I set it for 45% weight on front wheel and 55% rear using the second table. I would select 28mm for the GP4000S 25mm because they are almost that wide.

http://www.dorkypantsr.us/bike-tire-pre ... lator.html

joncomelately

70kg and I don't like the feel of my 28s when they go less than 85 on the rear so am usually around 85/95. I've covered roughly 5-6000 miles on GP4KS without any problems, and as mentioned elsewhere were recommended them by my cousins who ride 5 figures per year and won't ride anything else; so from a personal perspective reliability is pretty good.

animal72

80f85r for me, just under 12st.

N0bodyOfTheGoat

~85/95 (f/r) here weighing ~74Kg (plus ~9Kg bike; ~2Kg kit; ~1Kg liquid; ~1Kg mechanical emergency kit and food in jersey etc.).

Websta24

Im 78/79kg and run 95 front and 100 rear. Sometime 90/95

I did drop them to 85/90 and whilst they where comfortable i started picking up punctures regularly!

supermurph09

71KG 110psi front and rear. Until this year I always ran 120psi, but dropped to 110psi this year, any lower and it feels like they are flat. Each to their own I guess.

And just to add, since going down to 110psi I've had more punctures this year than the last 3 years combined.

pilot_pete

Supermurph09 wrote:

71KG 110psi front and rear. Until this year I always ran 120psi, but dropped to 110psi this year, any lower and it feels like they are flat. Each to their own I guess.

And just to add, since going down to 110psi I've had more punctures this year than the last 3 years combined.

:shock:

Sutton_Rider

It’s been about 24 hours since I posted this thread and I’ve had a good number of replies, and collected information from other sources. I’ve also asked the manufactures Continental, but they couldn’t be bothered to reply.

Rounding slightly up or down to accommodate for the differences in rider’s weights compared to me and then taking the average, I get 87 psi for the front and 94 psi for the rear. Even taking the robust average it comes out as 87 front and 92 rear.

I originally said 88 front and 98 rear so I’ll just try a few pounds less in the rear and see how it feels.

So thank you all for your input.

N0bodyOfTheGoat

NitrousOxide wrote:

~85/95 (f/r) here weighing ~74Kg (plus ~9Kg bike; ~2Kg kit; ~1Kg liquid; ~1Kg mechanical emergency kit and food in jersey etc.).

In light of my "man down" moment on Friday (understeered through downhill right sweeper http://www.strava.com/activities/115162 ... /4166/4983, bike fine, I got scratched up in hedge), I want to revise my suggested pressures for ~87Kg on roads with some lose gravel* in places like those near Butser Hill...

Today I'm going to go with ~70/90, after going back to the FastFitnessTips tyre pressure Google Sheets tool (based on suggested figures for 45% front weight bias and tyres measuring ~27mm).

* Just like my Feb 14th understeer fall locally, I can't be sure what exactly cause the fall, but I've been gradually increasing my front tyre pressure in recent weeks.

chrisw12

So ask a 100 people and you’ll get 100 different answers. Throw in the fact that everyone’s answer will be based on some bias, there’ll be massive inaccuracies in the gauges and everyone’s on bifferent bikes and weights and it’s a bit of a lottery question.

If I could be bothered I’d ride around the local cycle track and measure this properly, it’s doable with a power meter, you use the same ideas as aero testing. But I cant be bothered so I just stick to a number that’s easy to remember...100 front and back

Sutton_Rider

You are quite right chrisw12, but it was not intended to be a scientific research project, I just wanted a flavour of what guys are doing. For myself I started on 85 front and 95 rear, but found the front a little squirmy. Then tried 90 and 100, but have now settled on 90 front and 95 rear, according to my pressure gauge.

thegreatdivide

So will you be adjusting pressure for every ride depending on the weight of your choice of clothing that day? One bottle or two? Number of gels or fig rolls? How heavy your breakfast/lunch/dinner was and how much of it you pi$$ed or dumped out?

It's a bike. Ride it.

Sutton_Rider

No, 90 - 95 will do for all occasions. I came to the same conclusion, it's a bike, not much gets past you does it and I do ride it - A Lot. However, with the lack of info from the manufactures its nice to know that the pressure (in this case) is more or less right for the product.