Tyring is't it?

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Spanner Monkey
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Tyring is't it?

Postby Spanner Monkey » Mon Aug 10, 2015 11:57 pm

Black circles.
On the subject of tyres we may as well get into some issues relating to tyres, and how best to combat them.
Pressure:
It is important to maintain the correct pressure in your tyres to ensure the largest contact area with the road. The pressures specified by manufacturer should be considered a start point, but are never usually more than about 4psi out from ideal. To ascertain whether your tyres are inflated correctly, look at (and possibly measure) the tread across the tyre. If the tyre is more worn in the centre then the tyre pressure is too high. If the tread is worn on both edges then the pressure is too low. I stress both edges as track and camber will have an effect on single sides of the tyre. If the wear is even then the tyre pressure is correct.
If you have assessed that the tyres are over or under-inflated, adjust them by increasing or decreasing the pressure by 2psi at a time. Obviously it will then take a good few miles to have an effect on tyre wear before you can re-check, but once you’ve established the ideal pressure for your tyres then it’s easy to maintain.
It’s also important to check your tyre pressures at the same temperature every time you check. As the gas within your tyre will expand (due to heat) and therefore increase the tyre pressure. I always try to check the pressure when the tyres are at their hottest – after a brisk drive on varying road-surfaces.
Talking of gas, I get asked a lot about Nitrogen in tyres. Despite what you may have been told, Nitrogen still obeys Boyles gas law. It reacts to temperature the same way that air does – by expanding. Air contains between 75-78% Nitrogen anyway, so what are the benefits of using (almost) pure Nitrogen in your tyres? Discuss…
If you are ‘at one’ with your car, you should be able to detect a 2psi difference across an axle (i.e. if one tyre has lost pressure).
Load will also have an effect on tyre wear. If you constantly run your car with 4 adults and their luggage then you would expect the tyres to show as under-inflated if the pressure is set to factory settings.
Tyre construction.
In the good old days there were three main constructions of tyres; Crossply, Radial and (mainly US) Bias-belted.
The nomenclature referred to the ‘lay’ of the main reinforcement strands – in a crossply they go across the tyre from bead to bead, in a radial they go around the carcass, following the tread, and in a bias-belted tyre they go both ways (oo-errr!).
As crossplies are old hat for cars, and bias belted are…..well…..crap, we’ll deal with just radial ply tyres.
Radial tyres come in a number of types. There are steel-belted and nylon belted (referring to the material used in the main ‘ply’s).
Tread-wise there is the ‘standard’ symmetrical tread (which can be fitted either way round on either side of the car)
Asymmetric tyres which usually have an ‘open’ tread pattern on the outside and a virtual slick on the inside. These have to be fitted the correct way round, but will work in both rotational directions as they rotate on opposite directions on either side of the car.
Directional tyres will only work efficiently if they are fitted the correct way round. Their technology comes from circuit racing tyres which are ‘laid’ one way, and so the rubber is set to grip better in one direction than the other. See it like a cheese-grater where the grip (grate) only works one way. Having been involved with motor racing in no small way I can attest to the grip levels being substantially better in the correct rotation. Put a set of circuit slicks on the wrong way round and lap times will increase by a substantial margin. I’ve seen +1.5 seconds per mile increase with incorrectly fitted tyres.

Suspension and steering effects.
As a rule of thumb, tracking effects on tyres are as follows:
If both tyres are worn on the inside edge, there is too much toe-out.
If both tyres are worn on the outside edge, there is too much toe-in.
What complicates this is the effect of camber. If your tracking is perfect but you have too much negative camber then your tyres will wear on the inside edge, too much positive camber then the wear will be outside edge. Easy to pick up if it’s only happening on one tyre, but if both tyres on the same axle are wearing equally then it will be a process of elimination to assess what aspect is at fault.
Manufacturers tracking spec’s should be viewed the same as their tyre pressures – as a starting point. When the car is produced with metalastic suspension bushes (which are nicely compliant for ride comfort) an allowance is made in the tracking and camber specifications for the movement that will take place.
Dealing with front-wheel track on a rear-wheel drive car as an example, from the factory the front wheels have toe-in. As the car is driven forward the theory is that the compliance in the mounts will allow the tyres to ‘push back’ into zero toe-in (parallel).
Now fit Nylon bushes. The deflection in these is minimal, so that original toe-in setting is not being ‘pushed back’ as much, so the wheels remain in toe-in when the car is driven. Wear on outside of the tyres.
With Heim joints (rose-joints) the deflection is virtually zero, so the effects of the manufacturers toe-in specs will be substantial tyre wear. “But I’ve had the track checked several times”…
Doing the numbers:
The numbers and letters on the side of your radials all mean something.
For a 235/50R20:
235 is width of tread in mm
50 is the aspect ratio (the height of the sidewall as a percentage of the tyre width)
R = Radial
20 = diameter of wheel rim in inches.
There will also be a letter, and possibly another number. The number is the load index (mainly used in commercial vehicle applications) and the letter is the speed rating as per the list below. The numbers are maximum mph.
N 87 U 124
P 93 H 130
Q 99 V 149
R 106 Z 150+
S 112 W 168
T 118 Y 186


That’s it. I’m tyred now…..

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Re: Tyring is't it?

Postby Mustang Barry » Tue Aug 11, 2015 2:36 pm

Spanner Monkey wrote:Put a set of circuit slicks on the wrong way round and lap times will increase by a substantial margin. I’ve seen +1.5 seconds per mile increase with incorrectly fitted tyres.


Is it just me that's confused with this statement?

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Re: Tyring is't it?

Postby bigkeeko » Tue Aug 11, 2015 3:40 pm

Mustang Barry wrote:
Spanner Monkey wrote:Put a set of circuit slicks on the wrong way round and lap times will increase by a substantial margin. I’ve seen +1.5 seconds per mile increase with incorrectly fitted tyres.


Is it just me that's confused with this statement?


Nope. can`t say I get it either. :?:

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Re: Tyring is't it?

Postby cati » Tue Aug 11, 2015 3:51 pm

He is saying lap times get slow if you put the tyres on the wrong way round - which means less grip and therefore slower times


Monkey mate... nitrogen is good because it does not get absorbed through the tyre walls as does the rest of the gas in the atmosphere, hence you retain pressure for longer. Your are quite right it does obey boils law and does increase in pressure if the temperature goes up and xxxx Lussac has something to say as well.

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Re: Tyring is't it?

Postby bigkeeko » Tue Aug 11, 2015 5:00 pm

aaaaaaahhhhhhhh.. Not something you see much of though. Tyres on back to front. :|

Then again, i don`t do track days so it`s maybe more common than I imagine ( like IMS failure) sorry cati :lol: Joke mate. Relax.

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Re: Tyring is't it?

Postby Spanner Monkey » Tue Aug 11, 2015 9:49 pm

cati wrote:Monkey mate... nitrogen is good because it does not get absorbed through the tyre walls as does the rest of the gas in the atmosphere, hence you retain pressure for longer. Your are quite right it does obey boils law and does increase in pressure if the temperature goes up and xxxx Lussac has something to say as well.


But as air contains 75% (ish) nitrogen, presumably any pressure loss due to absorption would be restricted to the remaining 25% ish?
I only doubt the necessity of Nitrogen as I've had air-filled tyres which have retained their full pressure for years, and I'm sure I'm not alone.
Personally I think the whole Nitrogen thing is snake oil......

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Re: Tyring is't it?

Postby Spanner Monkey » Tue Aug 11, 2015 9:50 pm

bigkeeko wrote:aaaaaaahhhhhhhh.. Not something you see much of though. Tyres on back to front.


I beg to differ........

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Re: Tyring is't it?

Postby bigkeeko » Wed Aug 12, 2015 12:19 am

Spanner Monkey wrote:
bigkeeko wrote:aaaaaaahhhhhhhh.. Not something you see much of though. Tyres on back to front.


I beg to differ........


Maybe I should have said `Not something I see much`

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Re: Tyring is't it?

Postby Spanner Monkey » Wed Aug 12, 2015 2:26 am

bigkeeko wrote:
Spanner Monkey wrote:
bigkeeko wrote:aaaaaaahhhhhhhh.. Not something you see much of though. Tyres on back to front.


I beg to differ........


Maybe I should have said `Not something I see much`


:)

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Re: Tyring is't it?

Postby waylander » Wed Aug 12, 2015 9:52 am

question for SM...

standard tyres on the 05-10 are 16", 17" or 18" - quite a lot of people up-grade to 19"/20" with tyres ranging from 245 - 315.... how do you decide what the best pressure is?

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Re: Tyring is't it?

Postby cati » Wed Aug 12, 2015 4:03 pm

Mr Monkey

yes you are right about the 25%, but oxygen can diffuse through the rubber where as nitrogen does not - cant remember the exact science if it is simple diffusion, or oxidative pressure .... been a few years since uni, but thats the theory. Just like party balloons and helium, cheap ones deflate during the party, as it leaks out through the membrane and expensive foiled ones it doesn't

But if you loose 25% of the volume, Prof Boyle says you will loose pressure.... or is it xxxx lussac cant remember which

Tyres round the wrong way - i agree it is shocking. Worth checking out next time your are out - bikes and cars often have them on the wrong way despite the big arrow saying rotation direction...

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Re: Tyring is't it?

Postby cati » Wed Aug 12, 2015 4:06 pm

Waylander - why should the pressure be different? just a punt but seems logical

pressure = volume x area

In your big mahossive gangsta wheels - the side walls are smaller, the rolling radius is probably the same and the difference is taken up by the larger rim size ?

so your volume and area changes in proportion and over all ratio remains the same ?

so the pressure is probably the same ?

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Re: Tyring is't it?

Postby Mustang Barry » Thu Aug 13, 2015 3:56 pm

Mustang Barry wrote:
Spanner Monkey wrote:Put a set of circuit slicks on the wrong way round and lap times will increase by a substantial margin. I’ve seen +1.5 seconds per mile increase with incorrectly fitted tyres.


Is it just me that's confused with this statement?


cati wrote:He is saying lap times get slow if you put the tyres on the wrong way round - which means less grip and therefore slower times




Oh yeah! I had to read that again and again until my brain figured out I was being a massive idiot.

Makes perfect sense now!

Coat retrieved. :roll:

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Re: Tyring is't it?

Postby waylander » Thu Aug 13, 2015 4:57 pm

cati wrote:Waylander - why should the pressure be different? just a punt but seems logical

pressure = volume x area

In your big mahossive gangsta wheels - the side walls are smaller, the rolling radius is probably the same and the difference is taken up by the larger rim size ?

so your volume and area changes in proportion and over all ratio remains the same ?

so the pressure is probably the same ?



problem is that in some cases the RR is not the same (eg I'm running 295/30/20's for drag racing, obviously a specialist sub-requirement, some are running 15's with a different RR)....

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Re: Tyring is't it?

Postby Spanner Monkey » Thu Aug 13, 2015 8:34 pm

waylander wrote:question for SM...

standard tyres on the 05-10 are 16", 17" or 18" - quite a lot of people up-grade to 19"/20" with tyres ranging from 245 - 315.... how do you decide what the best pressure is?


I'm afraid the only truly foolproof method is in my text above:

To ascertain whether your tyres are inflated correctly, look at (and possibly measure) the tread across the tyre. If the tyre is more worn in the centre then the tyre pressure is too high. If the tread is worn on both edges then the pressure is too low. I stress both edges as track and camber will have an effect on single sides of the tyre. If the wear is even then the tyre pressure is correct.
If you have assessed that the tyres are over or under-inflated, adjust them by increasing or decreasing the pressure by 2psi at a time. Obviously it will then take a good few miles to have an effect on tyre wear before you can re-check, but once you’ve established the ideal pressure for your tyres then it’s easy to maintain.


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