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Tyre pressure


Andy m

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Andy m

Found this on Twitter

 

Loved the simplicity of the test. Look at the elongated contact patch. 

 

For road riders think how that extra rubber is warming up and using fuel and causing wear. 

 

For off road, more importantly , look at how much you need to drop it. All those GSclub loonies fighting over 30 psi vs 28 are just silly. 

 

Andy

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Griff

Just curious as to what weight of bike was used ? 

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Andy m

The tread honestly looks like a pedal bike, but the proportions remain the same. 

 

Andy

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SteveThackery

We are wrong to assume that a bigger contact patch = better traction.

 

The larger the contact area, the lower the contact pressure.  In fact, if the basic laws of friction apply (they don't always) then the nett effect in terms of friction between the two surfaces is zero.

 

It's obviously more complicated, but the complications are just that: very complicated and by no means the same in every case.  Other factors which determine whether a bigger contact patch = better traction include the type of surface, its hardness, whether its sheer strength varies with pressure (like snow, but not like mud), and other factors I can't remember right now.  :-)

 

If you look at the formula for calculating how much torque a multi-plate wet motorcycle clutch will handle before slipping, the surface area of the friction points is not one of the variables in the formula, believe it or not.

 

 

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fred_jb

I imagine that if the increase in traction is not linear with increased contact pressure and/or the traction reaches a maximum value at some pressure level below what a smaller contact patch provides, then there could be some benefit in increasing the size of the contact patch.

 

Just a thought - I expect Steve will shoot me down in flames!

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MatBin

Not just about contact patch on a vertical wheel though is it, I wouldn't fancy going round a corner at 8psi, tbh I probably wouldn't fancy it at 16 either, the sidewalls would be flexing all over the place I reckon.

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SteveThackery
6 hours ago, fred_jb said:

Just a thought - I expect Steve will shoot me down in flames!

 

No, not at all!  :lol:

 

I'm just saying it's wrong to assume that a bigger contact patch automatically means more traction.  After all, that's why skis have such an enormous surface area in contact with the snow - to minimise the "traction".  And yes, I know snow is different from tarmac and mud!

 

Incidentally, I think one reason to have a larger contact area is due to the nature of the rubber tyre.  I can imagine it gets hot and soft when all the contact pressure, and all the power, is concentrated into a small patch.  It's complications like these that make the basic friction equations too simplistic to be useful.

 

I reckon the dynamics of exactly what happens at the contact patch between tyre and road is extremely complex with loads of relevant variables.  I think it would be useful to do a bit of research on this.  Somebody, somewhere will have published a paper that explores this area extensively, and then we can all read it and become insufferable bores.  :)

 

 

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Just fitted my new Pirelli Scorpions on Saturday 32 front 36 rear as I do with all the bikes I've had around the 200kg mark.......to date it works without any adverse affects!

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Lower pressures and increased contact patch give increased grip off road (proven fact, not open to debate ;) ) and Andy’s post actually illustrates why. At the higher pressure 5 knobs are available to grip/dig in and at the lowest 7. Massive difference. Very interesting to see it so clearly illustrated. On the road it’s a different story, naturally. 

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At 8psi the sidewalls would overheat pretty quickly and destroy the tyre. However, the Dunlop Trials rear tyre construction can run at those low pressures due to their tall sidewalls.

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Andy m

The beginners mistake is usually thinking some sort of compromise works. 

 

On the road you want a roller, the air is there to absorb bumps. Adhesion is by microstructure. This is why slicks work or steel wheels work on iron rails. The contact patch on a train is how much steel deforms, but the force is large and the mu value (from the materials) is acceptable. 

 

Once you give up on microstructure because the surface breaks up there are two choices, dig in or float above. You can move an 80 ton tank across marshland by getting the ground pressure low enough. Power helps you stay on top but starts to bring in forces more complicated than a caterpillar track. Once you meet something that won't take this you sink in until your contact area increases and the ground pressure drops according. The air is only there to keep the tyre on the rim. This doesn't work on a hard surface because the tyre is like jelly. You have the same steering difficulties as a bulldozer. 

 

The trouble is you are constantly dropping down to 8 psi to get going then either need to stay at walking pace or need to come back above 25. The military have needlessly complex and unreliable systems that can do this on the move. 

 

Trail tyres will stand 8psi all day at walking pace, but you have to be desperate to stand all the work with the pump if you then find a bit where you can speed up. 15psi gives a hot wobbly wheel effect and no track effect. 

 

Andy

Edited by Andy m
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Spindizzy

10% off pressure can double the temperature of an aircraft wheel at landing speeds. Heat from increased sidewall flexing. Can cause blowout or fusible plug failure. I never liked hearing pilots with opinions on alternative pressures.  My road bike I run to the book as the pressure matches the tyre and loading characteristics. Car I use upper end of what is now called 'eco' pressure. Off road I will leave to those that actually do it and know their stuff from experience. 

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Griff

I normally stick with the recommended front tyre pressure of my road going motorcycles. However I do vary the rear. I am of the firm belief that 42psi in a rear tyre of a bike that is being ridden solo, is too high. My reasoning is that not enough heat gets into the tyre at that pressure solo, especially in the wet. I like to lean a tyre over close to its limits because that is an aspect of motorcycling that I have come to very much enjoy over the years. So I drop the rear pressure to about 38psi and that gives me a degree of comfort if nothing else. So far that procedure has never let me down but again I will touch wood. A nice bit of pine.  Two up I revert to the recommended 42. 

 

I also ride offroad and in that discipline it is a balance of how low You can go within recommended limits without incurring a pinch flat over rough ground. 

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  • 2 weeks later...
larryblag
On 22/06/2020 at 19:20, Griff said:

I normally stick with the recommended front tyre pressure of my road going motorcycles. However I do vary the rear. I am of the firm belief that 42psi in a rear tyre of a bike that is being ridden solo, is too high. My reasoning is that not enough heat gets into the tyre at that pressure solo, especially in the wet. I like to lean a tyre over close to its limits because that is an aspect of motorcycling that I have come to very much enjoy over the years. So I drop the rear pressure to about 38psi and that gives me a degree of comfort if nothing else. So far that procedure has never let me down but again I will touch wood. A nice bit of pine.  Two up I revert to the recommended 42. 

 

I also ride offroad and in that discipline it is a balance of how low You can go within recommended limits without incurring a pinch flat over rough ground. 

Dropping both tyres by 2 psi on the NC significantly improved solo ride comfort. With pillion I used the recommended rear tyre pressure though. 

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  • 1 month later...

To resurrect an older thread... I have had my NC about a month and checked/changed most things as you do - except tyre pressures! Not an oversight so much as an act of faith as the valve caps were of the pressure detection type and a solid green. Bike comfort has been a surprise too - really good - though a tendency to fall into turns a bit at the front.

 

Finally thought I better check pressures to find 26 psi front 32 psi rear which is way below recommended. Pumped up to 36F and 42R but the ride is quite harsh. I am a lightweight btw.

 

Now, why are the recommended pressures so high and the same solo as 2 up? Not seen that before so I intend to try (as one or two of you have) modified solo pressures of 32F and 36R. Any thought or words of experience with NC pressures?

Edited by Bugsy
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Andy m
55 minutes ago, Bugsy said:

 valve caps were of the pressure detection type and a solid green. Bike comfort has been a surprise too - really good - though a tendency to fall into turns a bit at the front.

 

Finally thought I better check pressures to find 26 psi front 32 psi rear which is way below recommended. Pumped up to 36F and 42R but the ride is quite harsh. 

The leak may well be due to the magic eye tyre valve caps. The one I broke open (I'm supposed to sell TPMS at work) was just two pressed monkey metal cylinders with an O-ring rolling between. O-rings are not designed to be compressed radially nor roll up and down. I would not risk my neck on such Chinese rubbish. If the cap came off the cylinder you would effectively have a blow out. 

 

I also had an argument with Carole Nash about the same. They sent me them as a free gift. Their insurance was possibly invalid if you fitted them and had said blowout!

 

If you want TPMS, get one with sensors mounted inside the rim. They can't cause any sort of leak. 

 

The book pressures are what they are. Probably set from a curve of load to pressure for the OE fit Dunlop sidewall thickness. I can't recall if I ran my NC at these pressures. I recall deciding to change them. At 72Kg I'm not the heaviest. 

 

Andy

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I run 34-36 front and 36-38 rear on the Strom and X-Adv depending on the type of terrain and conditions where I plan to ride.  36F and 42R are only used two up. My former NC700X developed a slow leak on one of the wheels just prior to parting with it. The culprit was blistering paint with corrosion on the sealing aspect of the rim. Something to look out for.  

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I realized the valve caps might be the culprit as I unscrewed the first one - puff of air is the giveaway - only the cap is keeping the pressure in! They clearly were not accurate either. In the bin.

 

I will try some lower pressures when solo as it seems many riders here do experiment. 2 up the recommendation seems appropriate.

 

Thanks!

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Rev Ken

When I queried from a manufacturers tyre technician why so many bikes, especially Hondas, seem to have 36/42 as recommended pressures, he told the assembled audience that these pressures were agreed between them and bike manufacturers. He said that recommended pressures were for both solo and two up. So at least it is a recommended starting point and it it is the same from almost all popular tyre makes. I've often wondered if you depart from these pressures substantially whether insurance firms would use that as an excuse to turn down any claims. (I am not heavy but have always run my bikes with the recommended pressures.)

Edited by Rev Ken
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Its a nightmare isnt it?

i dont pay much attention to car racing but read this weeks report of Hamilton and that race - it seems that the whole F1 race is completely governed by tyres! Forget the engine and driver -its that plastic stuff that makes all the difference!

So if the millions spent on tyre development can't get a racing car's tyres to last a race -well thank goodness we don't have 1000 hp to burn ours out!

what a subject-we want

  • them to last forever
  • stick like glue in the wet dry ,rain and snow( well ok ice then)
  • not go funny when we lean into bends
  • stop on a sixpence -or whatever the modern rexpression is
  • cost as little as possible

This thread could run forever - unlike my tyres -but i stick to the Pilot 4s thanks

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2 hours ago, Andy m said:

The leak may well be due to the magic eye tyre valve caps. The one I broke open (I'm supposed to sell TPMS at work) was just two pressed monkey metal cylinders with an O-ring rolling between. O-rings are not designed to be compressed radially nor roll up and down. I would not risk my neck on such Chinese rubbish. If the cap came off the cylinder you would effectively have a blow out. 

 

I also had an argument with Carole Nash about the same. They sent me them as a free gift. Their insurance was possibly invalid if you fitted them and had said blowout!

 

If you want TPMS, get one with sensors mounted inside the rim. They can't cause any sort of leak. 

 

The book pressures are what they are. Probably set from a curve of load to pressure for the OE fit Dunlop sidewall thickness. I can't recall if I ran my NC at these pressures. I recall deciding to change them. At 72Kg I'm not the heaviest. 

 

Andy

more sense-thanks Andy

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outrunner

I am sure I have mentioned somewhere else I run my tyres at 34 front and 38 rear, this set up suits me but may not be good for you so try a few pressure mixes to see what suits you best and remember to set pressures when tyres are cold and do a few miles to heat them up before making comparisons.

 

 

Andy.

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Wedgepilot

I use 34/38 as well, it just feels better. 42 especially feels harsh. I'm about 90kg fully kitted up, so not the lightest.

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outrunner
15 minutes ago, Wedgepilot said:

I use 34/38 as well, it just feels better. 42 especially feels harsh. I'm about 90kg fully kitted up, so not the lightest.

Me too!

 

Andy.

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