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tyre psi
NB: Originally posted elsewhere on the Global Riders Network and appears via syndication.
hey guys
as you all know im a newbi lol but the shop i got the bike from told me the tyre psi i should be useing is 45psi for road riding and 60 to 65 for off road is this right
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It'd be the other way around, more likely ...
Lower pressure when off road to help soak up the gnarly bits (rocks!), and to spread the tyre out a bit when climbing in loose stuff.
(The greater the contact patch, the better the grip you'll have. Like when you're trying to cycle on beach sand. So go soft off road.)
Higher pressure on road, because a firmer tyre rolls faster. Much faster.
Enjoy the riding!
What tyres are you using and what type of riding are you doing? Even 45 PSI sounds way too high for mountain bike riding.
45psi too high? So I shouldn't admit that mine are at 60? "It's the snakebites what I fear, Cap'n!"
This does give a very buzzy ride, and I lose out some in climbing. On the other hand, going through the really rough stuff is a breeze. (The rockiest and fastest sections of Anderson's and Oakes.)
I bomb through that at my top speed, with the comfort that the tyres have enough space to soak up hits without pinching. Puncturing and losing tyre pressure when flying down the rough stuff ... not fun.
I started off riding higher pressures as I got sick of snake bites. Found that where I ride and for my ability 40psi was trouble free initially.
As I got better (better at picking lines, landing softly, lifting back wheel, unweighting bike through rocky stuff) I found I could start dropping the pressures.
Unless you are riding mainly dirt I'd run higher pressures than a racer and be trouble free (punctures and damaged wheels are no fun). After a while and you haven't had punctures in a number of rides try dropping it off a bit. If you start flatting all the time then increase it a bit.
Try varying it by 3psi increments (may not sound much but is 7-10% in this pressure range) and always check the pressure before each ride. Find the sweet spot for where and how you ride.
I'd recommend a track pump with gauge. Mine was a cheap one at $40. The roadies gave it a bad rep as it doesn't pump above 120psi and the gauge isn't that accurate. This doesnt really matter if what reads 40 on mine is 38psi on someone else's. The point is it reads the same each time and I can be consistent. Don't trust servo gauges. These are massively inconsistent between servos.
Lower pressures will help you corner better, give more grip and a plusher ride.
Increasing pressure reduces rolling resistance. I'm not one for too much pedaling and might well run higher pressure on the Oaks...
im 95kg running black jacks 26-2.25 so yea i need to lose wieght im 1.5cm off 6ft lol but solid lol
I run tubes with 38 psi in the front and the back. Can't remember the last time I got a pinch flat and I ride on the Oaks often. Also I'm 105+kg.
Better go and check my spare tube and pump now I said that.
For dirt riding,
Anthem - Front 35psi Back 40psi
Reign - Front 32psi Back 38psi
Intnese M9 - Front 22psi Back 28psi
Riders lacking in skil and good line choice need to run higher presures to avoid pinch flats but you suffer with less grip because of the higher pressures.
As you can see on my downhill bike I run alot lower pressures as it gives alot more grip.
I want to know what shop told you this.
Road bike is 110- 120psi, mountain bike is generally a personal choice and dependent on the surface. In my experience, 30-40psi is about normal for Perth.
The more air you put in your tyres, the lighter the bike becomes. This is due to the buoyancy effect. At ground level, air pressure is around100 kPa and so you can equalise with the ambient air pressure with only 14 psi in your tyres. So if you pump them up to 60 PSI you have made a ratio of about 4:1 between your tyres and the surrounding atmosphere. This pressure supports the bike, reducing the amount of weight you are required to propel forward. As an added bonus, this offset in weight you have gained is rotating mass, and so the effort saving effect is enhanced (the wheels will spin faster and bike will go further, for the same input effort).
Increasing air pressure also has the effect of increasing the height and distance you achieve when doing jumps, but be careful not to put too much air in if you do a lot of jumping as you may find it's difficult to come back down to the trail in time to make the next corner etc.
"but be careful not to put too much air in if you do a lot of jumping as you may find it's difficult to come back down to the trail in time to make the next corner etc"
Excellent advice Max, that explains why those dirt jumping kids can get so high. they must be like 200psi....
Wanting to get some more air I tried your advise. I'm now deaf and need a new tyre, they must have been defective......
32/36 works well on my Yeti running Schwalbe USTs
Back to school i think Max. Higher pressure raises more of the tyre knobs off the ground, decreasing rolling resistance but reducing grip. The sweet spot for grip and rolling resistance is about 30-40PSI on a MTB.
More air = more weight in tyres, but as air weighs 1.225g/litre at 1 bar, the typical 2 bar in a MTB wheel is 3/5s of fk all. PLease explain to me your buoyancy theory, and you do realise that if your tyres are equalised with ambient air pressure they are flat?
As you own a Tallboy, which are not cheap, you must have means and therefore are clever. Are you taking the piss?
Not at all!
I studied TEE Chemistry, where I learnt about Boyle's Law. An interrelationship concerning Pressure, Temperature and Volume. Bicycles weren't invented back when ole Robert Boyle was conducting experiments, so there's no direct application recorded in the literature, but I have performed measurements. Temperature is of course relevant, and so for example here in Perth you can reap even greater benefit from more air than, say, in Melbourne on a winter's day (or a summer's day). I left this notion out earlier as I didn't want to overly confuse the issue.
I could try to explain the theory, but it's easier just to point at hot air balloons - they float, and clearly do not require any explanation to prove it. You're right about the knobs (which is probably why hot air balloons don't have knobs, as they have no traction requirements). But this is precisely why mountain bike tyres DO have knobs - as you benefit from increasing air pressure and thus increased buoyancy, the knobs allow the tyre to maintain some grip on the trail.
so would i be safe to say. that if im riding road higher and if im riding dirt lower say around 35 front and 40 back sound right.
Gekness it also depends on tyre size. A 2.35 inch tyre has about 16% more area on the ground than a 2.1 inch tyre, so you can run it at 16% less pressure, since force is pressure times area. Also, if you went to a 1 inch road tyre, you'd have to double the pressure for the same force. I did the Karri Cup on 28 PSI front and rear, which I found to be about as low as I could go to get good rolling resistance and maximum grip. I picked that pressure after timing myself over the same course multiple times with different pressures. I went with 2.35 inch tyres because I knew there'd be a lot of sand on the course, which there was, (stage 2).
I thought maxfacta was taking the piss when I read his post(I hope he is?) I am pretty sure that lower tyre pressures equal less rolling resistance on soft trail surfaces much like our pea gravel in WA. I guess its the same as 4wd tyres on the sand. Less air means less rolling resistance. If one was riding very hard pack trails or on the road then more air may help. On pea gravel and softer type trails you want the tyre to float over the surface. I run about 26 psi front and 28psi rear around Perth. I find this roll the best and maintains pretty good traction. I will dig up the test results from one of the tyre mobs asap.
Ok so what about filling with helium? Will that also increase the air I could get by reducing rolling restistance and make my bike weigh less on the scales by almost levitating. I think one of the car tyre shops ( ian Diffans)offers nitrogen as an alternative to standard air. What difference does that make?
From the schwalbe website. I think the test has some holes in it but its give you an idea.
"The truth about rolling resistance:
Wide tires offer more control, more comfort and more fun. While this has become more widely accepted over the past few years Sport Scientist Peter Nilges, was not too interested, he was more concerned with speed.
The avid, German League road racer wanted to determine the best tire width and air pressure combination for fastest performance. He completed 300 test-rides with SRM cranks to establish the exact energy consumption for his thesis "Road Rolling Resistance". He tried three different tire types in three widths at 20 psi, 30 psi, 45 psi and 55 psi on road, dirt track and cross-country The results were clear.
Higher pressure was only faster on the road. Off-road rolling resistance was lower, the wider the tire and the lower the pressure. This was similarly true for dirt tracks, soft forest roads or cross-country and up to 40 Watts could be saved in extreme off-road conditions; poor acceleration caused by higher tire weight being generally compensated for. Explanation: A tire at low inflation pressure adapts better to uneven surfaces. It sinks into the ground less. Overall it suffers less retardation.
Narrow tires are only recommendable for lightweight riders, weight saving or extremely muddy trails".
Some of you guys are so far off the mark.
maxfacta, have you ever ridden a bike on flowing single trail with to much pressure in?? You can only be out by 5psi and you loose all your grip.
Less pressure = more rolling resistance.
Put 20psi in and go for a ride then put 60psi in and feel the difference.
More pressure = less grip on dirt. On a donwhill bike 22-25psi is perfect anymore than that and you start sliding around.
Its the same for an XC bike you you also need to take into account rolling resistance as you are doing alot of upright riding that doesnt require much grip.
LOL maxfacta, too many people here think you are an idiot - woooooosh!
The interesting thing about mountain biking is how the laws of physics seem not to operate. Contact patch for cars and road bikes is irrelevant - grip is a function of friction and weight, not area. Yet off road, because the ground under the tyre is not uniform we have contact patch and nobs as completely relevant.
on a uniform designed tyre, good rolling resistance and grip are opposite ends of the same equation - choose one - but we tend to judge grip by what happens in cornering, so design can be different for going straight versus turning
I'm assuming Max is taking the piss, surely? The more air pressure means you have more air in the same volume. Air has weight therefore higher pressure has higher weight which is the reason they weigh scuba tanks to see if they're full
On the point about contact patch not being relevant for cars etc, that's only in theory but not in reality. In reality a wider tyre will give you more grip because rubber is not linear and at some point will start to tear apart and lose traction. Wider tyre = more rubber to spread the same lateral force over = less chance of slip. To a lesser extent the same applies in MTB
Trolls live under bridges, not in hot air balloons...
"On the point about contact patch not being relevant for cars etc, that's only in theory but not in reality. In reality a wider tyre will give you more grip because rubber is not linear and at some point will start to tear apart and lose traction. Wider tyre = more rubber to spread the same lateral force over = less chance of slip. "
Your first part is right, the second disagrees with physics.
The reason wider sport tyres have more grip is because the compound is softer - the compound is softer because the width allows greater wear and less heat buildup. A narrow tyre could indeed provide exactly the same amount of grip on a ferrari, but may only last one lap. The second point is incorrect because physics does indeed describe the real world and the only 2 variables are the co-efficient of friction and weight - the width is very important however to the tyre manufacture and what compounds can be used to survive the temperature and wear parameters - i understand this is so in the face of how things seem to be but there you go - the softest compound tyre has the most grip in dry conditions
BTW, it took me ages to get my head around this concept.
Per mountain biking - it follows that the most important aspect to grip is weight over the tyre - and given that a slipping back wheel doesnt usually result in an off - its the front wheel that counts - so soft compound on the front, and concentrate on weighting the front wheel into turns . However for the most part the physics above doesnt apply as much because the smooth surface is assumed, which works for road but not for gravel, mud etc
Wider tyres on a car most certainly do mean more grip- of course there are other factors too but the 10 inch slick on the back of a V8 supercar is going to be quicker than running a 7 inch one.
And race cars will fill with nitrogen as its more stable so the tyre pressure doesn't change as much when the tyres get hot. Its a complete waste of time on a road car as your tyres will never get as hot as on a racetrack.
On topic, i'm about 82kg and feel comfortable at about 26psi front/ 28psi rear on 29er Racing Ralphs.
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LOL this is such a funny thread, and I am adding to it DO'H!!!
Thumbs up to maxfacta. This is the best popcorn thread in awhile
This is a bit technical, so either bear with me or skip to the next post
While wider and softer tyres in general do generate more grip. However, as the grip available from the track surface itself increases, losses through unwanted friction from squirm and tyre deformation can actually make the car lap slower. At its extreme, the car starts to get "out of shape" on the track and can actually tuck a tyre under - or even traction roll - and crash off the circuit. There is an upper limit to usable traction. Above that and the car is slower, not faster.
Narrower tyres/wheels widen the vehicle's track (so long as the outer edges are in the same place as the wider wheels - track is measured from the centre of the tyre contact patch), and combined with less grip mean vehicle stability is returned. So if the track surface is sticky, the car is more stable and can lap faster with less than the maximum available grip.
I was enjoying the stuff on mtb tyres, but all this stuff about car tyres sounds a bit tediously real.....
Yair, sorry about that.
Normal programming will resume shortly no doubt
wow lol all i asked was a simple question and i get all this lol. screw asking how high i should have my seat lol
At the start of the thread I said I timed myself with different pressures around the same circuit, that way the conclusion is drawn from real life experimentation rather than theory at the keyboard.
The problem with such experimentation is that temperature fluctuations at the different times the trials were conducted result in a modified relationship between atmospheric pressure and the tyre pressure. Hence the buoyancy effect won't be directly comparable. Such tests will lead to inconclusive results. Ideally such tests would be conducted in a thermotron chamber or similar, although in practice it can be difficult to temperature-control an entire mountain bike circuit. My control mechanism of choice is a thermostatically controlled heating element for each tyre. These line the inside of the tyre, so as not to interfere with traction. They do impinge upon available volume and so negatively effect buoyancy, however the volume difference can be calculated and true results can be extrapolated via graphs.
Reading Fear and Loathing in Las Vegas by Hunter S Thompson I think you'll find he prefers around 90 psi in his car tyres. Jumps in Maxfacta for a ride...
"I hate to advocate drugs, alcohol, violence, or insanity to anyone, but they've always worked for me"-Hunter S. Thompson
Now THAT guy knew what he was on about.
Me, I'm full of it. The coffee has worn off now so there'll be no more of this nonsense from this quarter.
That explains it.
Personally, I find it's good for a 7% improvement in my time over any set distance
Same time tomorrow then?
...and 1 pedant to point out that "URLs" shouldn't have an apostrophe.
25psi in the front of my Trance, 30 on the back - I weigh 88 kgs. Grrippy. [edit: tubed]
FWIW, I just don't notice the rolling resistance issue in a centimetre of pea gravel and red dust.
A modern full suspension is just so much more forgiving with regard to lower tyre pressures.
BTW, I run 120 psi plus on my road bike (I always worry the tyres are going to explode or unbead, but those kevlar Vittorio Rubinis have never let me down - not one puncture in 4 years of commuting. Those tyres are awesome.)
on the mtb I run 28 psi tubeless front and 45 psi with tubes rear and can't remember the last time I had a flat?... but then again I can't recall the last time I rode on dry dirt as well
road.. I run 120 psi.
I used to run tubed 45psi off road (to prevent pinch flats), 50-60 on road (less rolling resistance). I read the schwalbe study after seeing lots of forum stuff and dropped the off road pressure to around 35psi (tubed). I'm sticking with around 35 off road, haven't had any pinch flats (yet) and seem to have more grip, the schwalbe report leads me to believe there is less rolling resistance with a reduced pressure when on gravel, backed with some scientific testing. Still run 50-60 on road, but that's usually with mtb slicks anyway. All that's with a dually, HT might be different, but I would experiment and find what's best for ME
i run 30 in the rear and 25 on the front with tubes on an anthem 29er for the KC. i'm 80kg. i pinched flatted at 25 in the rear so i bumped it to 30 to be safe.
a mate of mine is a fast competitive rider and he runs 25 in the rear and 23 or a bit less in the front with tubes on a 26er hard tail at 70kg.