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PostPosted: Sat Dec 13, 2008 12:46 am 
Moderator /Lincs, E & S Yorks Deputy AEC
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On my Modern Full Susser I run from 25psi-55psi in my 2.1's, depending whether I'm off road or on road, or what conditions I'm riding in.

I find that generally the lower the pressure the better the grip (off road), I always have the rear a few PSI higher than the front though.


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PostPosted: Sat Dec 13, 2008 8:12 am 
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[quote="IbocProSX"]25-27 psi for 2.4s
28-31 psi for 2.25s

lower rolling resistance.
quote]

wrong
lower pressure will allow the tyre to deform and flatten out giving a bigger contact patch with more grip but also with increased rolling resistance
lower pressure for off road higher on it
well there is my 2p
and i run my pressures high but then im a fatty :wink:

rob


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PostPosted: Sat Dec 13, 2008 8:47 am 
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I ride a tubeless 2.1 tire, so I can lower pressures down to 30 or so and never get a pinch flat-I haven't flatted in 3 years now on tubeless- so when I leave home on a ride I start out at 50-60 pounds for the road leading to the dirt, and if the big hill that day is a smooth fire road I leave them there until the top, where I lower the pressures down to 30-40, depending on how rough the rest of the ride is gonna be.


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PostPosted: Sat Dec 13, 2008 10:32 am 
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MAXTHEROTTI wrote:
IbocProSX wrote:
25-27 psi for 2.4s
28-31 psi for 2.25s

lower rolling resistance.
quote]

wrong
lower pressure will allow the tyre to deform and flatten out giving a bigger contact patch with more grip but also with increased rolling resistance
lower pressure for off road higher on it
well there is my 2p
and i run my pressures high but then im a fatty :wink:

rob


This debate is raging on on many a bike forum. The theory of larger contact patch sounds plausible and is widely adopted. I think real life is not so straightforward.

For one thing there are some magazines out there that actually measured rolling resistance (you guessed it, the Germans) and they have concluded lower is faster. A simple test I've read about is to see the free rolling distance from an elevated point onto flat: no pedaling, same start height, same tires, same path different pressures and see which one rolls the furthest. Probably need a few runs each for statistics. Or google the results of others who have done this already

One theory is that by deforming and conforming to the surface less energy is lost ripping the surface apart, less digging, less moving soil and gravel about. And IMHO the principle is not without boundaries, there must be a low pressure limit where the tire deforms to the point it does not work as intended. And running 2.4s also adds rotating mass to my wheels which slows me down. There must be a crossover point somewhere.

Meanwhile I just love the ride of my big tires as they have bags of grip and allow me low pressures though I'm a fatty too :wink:


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PostPosted: Sat Dec 13, 2008 11:58 am 
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IbocProSX wrote:
And running 2.4s also adds rotating mass to my wheels which slows me down.


Uphill yes, downhill no (unless you're pedalling a lot)

IbocProSX wrote:
Meanwhile I just love the ride of my big tires as they have bags of grip and allow me low pressures though I'm a fatty too


Ditto :lol:


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PostPosted: Sat Dec 13, 2008 4:16 pm 
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fair points iboc
but simple test time
put your car tyres up to 45 psi and push it 10 meters
now do the same again at 20 psi and tell me the resistance is lower :wink:
all tho the size will no doubt have much more of a impact on this than 20psi tyre difference we are on about :wink:
and thank god im not the only fatty :P :P


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PostPosted: Sat Dec 13, 2008 6:11 pm 
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Reading this makes me think that I run my tyres to hard. As an old roadie and used to having tyres at 100psi+ I've tended to use about 50-60 off-road on largish tyres (not sure of the size, probably 2.3?). I've always been wary of having 'snake-bite' punctures with to low a pressure.

Weight wise I'm not over heavy, now around 12 - 12.5 stone but I have been a lot heavier.


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PostPosted: Sat Dec 13, 2008 6:13 pm 
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Pressures depend a great deal on make and model of tyres,type of terrain,rider weight etc.I run Maxxis Crossmark 2.1 tubeless at 25psi whilst Schwalbe Nobby Nic 2.1 tubeless seem better at around 35psi,I weigh 71 kilos.On the road I experimented with very high pressure for time trialling and found anything over 120psi a waste of time,too rough a ride and scary handling under braking.
Try different pressures and see what works for you


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PostPosted: Sat Dec 13, 2008 6:26 pm 
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from when I actual road my bikes properly I would run with as high a pressure as I could get away with...

If I found the tires would bounce of objects and make the ride un-smooth they where to high, if the tires where dragging they where too low.

This is just for normal riding in the tracks or fast riding. Having it higher also helped on the roads as we had to ride to and from our riding grounds.

iirc for Smoke and similar era tyres it would be around 40 to 45PSI at my weight (then) of just less than 10stone. Though I always ran 1.95 or thinner.

I would run the front harder ans the suspension forks would take the battering.


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PostPosted: Sat Dec 13, 2008 7:08 pm 
Moderator /Lincs, E & S Yorks Deputy AEC
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Ideally you should run the front softer, a softer tyre will have a greater contact area, giving more grip.

I'd rather have a slide at the rear than the front, as a rear tyre slide is much more controllable than having the front end wash out on you.


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