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Old 14th February 2007, 12:08   #16
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Originally Posted by manveet View Post
I disagree.

Wider tires will obviously have a greater area of contact.
Actually not - the tyre bible has a nice example ...

Car Bibles : The Wheel and Tyre Bible

If there's one question guaranteed to promote argument and counter argument, it's this : do wide tyres give me better grip?
Fat tyres look good. In fact they look stonkingly good. In the dry they are mercilessly full of grip. In the wet, you might want to make sure your insurance is paid up, especially if you're in a rear-wheel-drive car. Contrary to what you might think (and to what I used to think), bigger contact patch does not necessarily mean increased grip. Better yet, fatter tyres do not mean bigger contact patch. Confused? Check it out:

Pressure=weight/area.

That's about as simple a physics equation as you can get. For the general case of most car tyres travelling on a road, it works pretty well. Let me explain. Let's say you've got some regular tyres, as supplied with your car. They're inflated to 30psi and your car weighs 1500Kg. Roughly speaking, each tyre is taking about a quarter of your car's weight - in this case 375Kg. In metric, 30psi is about 2.11Kg/cm².
By that formula, the area of your contact patch is going to be roughly 375 / 2.11 = 177.7cm² (weight divided by pressure)
Let's say your standard tyres are 185/65R14 - a good middle-ground, factory-fit tyre. That means the tread width is 18.5cm side to side. So your contact patch with all these variables is going to be about 177.7cm² / 18.5, which is 9.8cm. Your contact patch is a rectangle 18.5cm across the width of the tyre by 9.8cm front-to-back where it sits 'flat' on the road.
Still with me? Great. You've taken your car to the tyre dealer and with the help of my tyre calculator, figured out that you can get some swanky 225/50R15 tyres. You polish up the 15inch rims, get the tyres fitted and drive off. Let's look at the equation again. The weight of your car bearing down on the wheels hasn't changed. The PSI in the tyres is going to be about the same. If those two variables haven't changed, then your contact patch is still going to be the same : 177.7cm²
However you now have wider tyres - the tread width is now 22.5cm instead of 18.5cm. The same contact patch but with wider tyres means a narrower contact area front-to-back. In this example, it becomes 177.7cm² / 22.5, which is 7.8cm.
Imagine driving on to a glass road and looking up underneath your tyres. This is the example contact patch (in red) for the situation I explained above. The narrower tyre has a longer, thinner contact patch. The fatter tyre has a shorter, wider contact patch, but the area is the same on both. And there is your 'eureka' moment. Overall, the area of your contact patch has remained more or less the same. But by putting wider tyres on, the shape of the contact patch has changed. Actually, the contact patch is really a squashed oval rather than a rectangle, but for the sake of simplicity on this site, I've illustrated it as a rectangle - it makes the concept a little easier to understand. So has the penny dropped? I'll assume it has. So now you understand that it makes no difference to the contact patch, this leads us on nicely to the sticky topic of grip.
The area of the contact patch does not affect the actual grip of the tyre. The things that do affect grip are the coefficient of friction and the load on the tyre - tyre load sensitivity.
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Old 14th February 2007, 12:36   #17
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Originally Posted by shuvc View Post
The weight of your car bearing down on the wheels hasn't changed. The PSI in the tyres is going to be about the same. If those two variables haven't changed, then your contact patch is still going to be the same
But why are we keeping the psi same, isn't that an assumption ??

What happens in case I reduce the psi because I am upsizing?
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Old 14th February 2007, 12:39   #18
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Originally Posted by manveet View Post
But why are we keeping the psi same, isn't that an assumption ??

What happens in case I reduce the psi because I am upsizing?
Well, you could increase the contact area by reducing psi on your existing tyres as well .
The concept is that just having wider tyres does not necessarily ensure more contact area. You could even have less if you inflated them with a much higher psi.

Last edited by shuvc : 14th February 2007 at 12:40.
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Old 14th February 2007, 14:53   #19
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Originally Posted by shuvc View Post
The concept is that just having wider tyres does not necessarily ensure more contact area. You could even have less if you inflated them with a much higher psi.
Got it the first time around dude. I've had my "eureka" moment like the writer said

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Originally Posted by shuvc
Well, you could increase the contact area by reducing psi on your existing tyres as well .
Contrary to what it may appear, I was NOT trying to argue for the sake of doing so. Where I was coming from is that when we upsize our Tires, don't the OE recommendations get invalidated and we are thus supposed to reduce the psi.

I believe that the OEM does a lot of research to come with appropriate psi figures - I realized this a couple of years back while getting air filled in an Accord - Honda has recommended a different psi for EACH of the tires - varying from 28-33.

This may be OT, but I'd really appreciate if anyone could advice if there a scientific method that tells us by how much the psi should change based on the changes we make to tire sizes? Ishan, you there ??

Last edited by manveet : 14th February 2007 at 14:56.
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Old 14th February 2007, 17:16   #20
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Originally Posted by manveet View Post
don't the OE recommendations get invalidated and we are thus supposed to reduce the psi.
Come to think of it, should we not increase the psi for wider tyres, to maintain the same contact area ? I mean, the width is anyway increasing, so we've got to reduce the length which should be possible by increasing psi ? When I changed from 185/65 to 195/60, the tyre shop pumped in 30psi. The recommendation for the OEMs was 25-29.

Quote:
I believe that the OEM does a lot of research to come with appropriate psi figures - I realized this a couple of years back while getting air filled in an Accord - Honda has recommended a different psi for EACH of the tires - varying from 28-33.
.. do Accord owners on the forum follow this ?

Quote:
This may be OT, but I'd really appreciate if anyone could advice if there a scientific method that tells us by how much the psi should change based on the changes we make to tire sizes? Ishan, you there ??
It should be possible to calculate based on the car bible formulae. We can probably assume a weight for the vehicle and back-calculate.

Btw, OT, does Ishan really have something to do with tyres ? I recall a vehement denial in his signature sometime ago. Must say am not aware why he did so.

Last edited by shuvc : 14th February 2007 at 17:17.
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Old 14th February 2007, 20:53   #21
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Originally Posted by manveet View Post
Ishan, you there ??
Yes and I shall respond to this later. Am a bit tied up right now.

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Originally Posted by shuvc View Post
Btw, OT, does Ishan really have something to do with tyres ? I recall a vehement denial in his signature sometime ago. Must say am not aware why he did so.
No Shuvc I do not have anything to do with tyres apart from being a user & enthusiast. I've always had a fascination with tyres, no idea why though! There has been an impression amongst some people on the forum that I'm a tyre dealer since I have a little bit of knowledge about tyres and I was actually asked this question when a member from down south specifically called me upto clear this doubt, so just for fun I had put on a signature denying that!!

So now you know the truth & the reason behind the signature
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Old 14th February 2007, 21:49   #22
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Yes and I shall respond to this later. Am a bit tied up right now.
Thats ok. The statement was made in jest.
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Old 14th February 2007, 22:17   #23
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What you guys aren't cosidering is the lower profile of the wider tyre. As the profile reduces, the tyre lowers it's chances of the sidewall flexing under hard braking, this makes sure that the wider (lower profile) tyre retains a larger percentage of it's contact patch when subjected to braking forces.

When rolling, the diff in contact path maynot be much but from my point of view, this is a good thing as it doesn't affect the rolling resistance of the tyre.

C'mon guys, do you really expect a 165/80 R13 tyre to provide as much traction as a 205/50 R15 (on the same car with the same tyre pressure)? It's just not possible!!!

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Old 15th February 2007, 07:11   #24
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Let's assume for a moment that we're testing the brakes in a laboratory, where we can just consider coefficients of friction and normal forces. For two identical cars, the normal force (or vehicle weight) is the same, so this is a non-factor in affecting the braking distances. That leaves only the coefficient of friction. If we assume that the two tyres are constructed of the same material, then they should have the same friction coefficient, right? Well, yes, except that the wider tyre has more surface area than the narrower one. I'm talking about the "circumferential" area around the outside of the tyre. This is given as Pi x Diameter x Width. Pi and Diameter are constant, but the Width's are not, by definition, the same. Therefore the wider tyre has more surface area.

Why is this important? Well, it turns out that the coefficient of friction of rubber on asphalt is dependent upon temperature (actually, it's usually measured as a function of sliding velocity, but it ultimately is a function of the heat generated at the sliding interface). In our case, during threshold braking, we get instantaneous, "micro" sliding action between the rubber and the road. This causes heat to build up, which drops the coefficient of friction. A wider tyre, having more surface area, can more easily dissipate this heat energy than a narrower tyre.

So in a nutshell, wider tyres can improve the braking distance BUT remember that this is co-dependent on many other factors.
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Old 20th February 2007, 09:48   #25
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kpzen-whats your opinion about dunlop sp9000,205/50/r15?using right now on my baleno.
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Old 20th February 2007, 11:04   #26
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The stock rims for accent viva are 5J steel. You can not go beyond 185 width on these rims unless you plan to change them. A 185/60 may reduce your ground clearance. 185/65 seems to be a better bet for you, the speedo error and reduction in ground clearance are minimal. Bridgestone site shows 185/65R13 in B650.
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Old 20th February 2007, 13:35   #27
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Originally Posted by desigNation View Post
kpzen-whats your opinion about dunlop sp9000,205/50/r15?using right now on my baleno.
No personal experience of Dunlop SP9000 but the thread pattern is amazing......
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