Wheels... 15" all round or 15" front and 16" back!?

[Anonymous]

Hi,

Newbie question coming up....

Ive got an 02 MR2 currently with the standard 15" wheels all round, i was looking at upgrading to 16" all round and adding some lowering springs to the mix. However, its been mentioned that some people are running 15" on the front and 16" on the rear (this may be standard on 03+). Does it cause any issues either way? Will handling improve or become worse with a different set up?

My main question is... what should I do??

Any advice on this would be very much appriciated!

[Anonymous]

The most important thing is same brand tyres all round and correct stagger however adding 16"s to this gives more tyre choice and I run these and feel it gives a bit more grip as wider tyres can be fitted all round.

[markiii]

wider tyres do not equal morre grip

[Anonymous]

Is that just a statement or can it be backed up with anything?

[markiii]

common sense,

 grip is function of tread pattern, rubber compund, contact patch (wider often just changes teh shape not teh size) and to a very large degree pressure

there is no point in havng a lightweight car on massively wide tyres as teh grip will (other points above not withstanding) be determined by weight and area, i.e pressure

as i said wider does not = more grip

[Anonymous]

So if you compared 15" wheels with 205 toyo's and 16" with 225 toyo's then they would both the same amount of grip?

[markiii]

So if you compared 15" wheels with 205 toyo's and 16" with 225 toyo's then they would both the same amount of grip?

the 225 may actually have less, depending on teh pressure calculation and teh contact patch

I wouldn't want to be teh one doing teh calculation, and you may well have acheived having more grip in your particular case.

however staing that fitting wider tyre = more grip is factually incorrect

[Anonymous]

Sorry,I would need to see some proof before I believe you on that one.Even Toyota widened the wheel and tyres on the facelift,I dont think they did that just for something to do.

[markiii]

up to you

prove to me that wider = more grip?

see thats where you misinterpretng me

I didn't say wider tyres can't equal more grip i said teh statement that wider tyres = more grip is incorrect

its quite possible that in the particular instance of the rear of teh mr2 the calculations come out that it does

its equally possible that because teh facelift is heavier (6 speed box) for example means that it affects teh calculations anyway

prefacelift and postfacelift cars don't weigh teh same

the performance of teh tyre will also be influenced by teh extra weight of teh actual wheel, by the different spec of teh rear springs (also changed on teh facelift) and the extra bracing on teh post facelift changing teh way teh chassis reacts

as I said its more complicated than just saying wider tyres = more grip

[Anonymous]

Just makes sense,wider tyres equal more rubber in contact with the ground therefor more grip.

[Anonymous]

Found this which seems to make sense and possibly put this to bed.


As technical as I can get.....

The contact patch on a wider tyre will be shorter but wider for the same load and tyre pressure, but the contact area will be the same size for both tyres.
A wider tyre has a larger contact patch only if the tyre pressure is lower than in the narrower tyre. This is an important fact to remember.

With a larger contact patch the contact pressure per square unit is lower than with a narrower tyre. The narrower tyre also has a contact patch which is long and thin - i.e. it extends round the rim of the tyre, thus increasing the amount of effective tread area in that direction. In snow (and mud even) the higher contact pressure and the longer contact patch helps the tread pattern penetrate the road surface, compacting the snow/mud into the tyre tread and thus increasing the grip; in the same conditions a narrow tyre with no tread would spin like crazy, of course.

As said above, with the correct inflation pressure the larger contact patch on a wider tyre means a lower contact pressure. This means that the tyre can suffer more load added to it before is loses grip than a tyre with a smaller contact patch. This extra load, in racing, comes from braking, cornering forces and to a lesser extent acceleration loads. Of course, you might expect a tyre with a larger contact patch to last longer than a narrow tyre, because of the lower loads per square unit, so read on.......

Because the relationship between the grip level given by a tyre and the load on the tyre is not a linear in the performance envelope of the tyre we are interested in - i.e. right at the limit - there is no simple formula to explain clealry grip versus load. A chap called Pacejka did his best to model this relationship mathematically and his maths has been used in simulators and computer games. If you are into very complicated maths, look him up!

But the real benefit of wider tyres is that because the contact patch is larger, the inflation pressure is lower, allowing the rubber to deform and follow more closely the small variations in the track surface, and a softer rubber compound can be used to give the same rate of wear without the temperature in the tyre increasing to a level where the rubber deteriorates. The same loads on the same rubber compound in with a smaller contact patch would over-heat, which first loses grip and then destroys the tyre tread completely.

Softer rubber has a higher coefficient of friction, and that is where the man gain in wider tyres comes from. But the setting of tyre pressures in wider tyres becomes critical, as that is what balances the size of the contact patch and hence the pressure per square unit of the contact patch with the road, and the shape of the contact patch.

Too much pressure in the tyre and the contact area shrinks, which loses grip. Too little pressure in the tyre and the shape of the contact changes for the worse, which again causes a loss of grip.

Tyres and grip are very complicated subjects. Please consider what I have written as only a very simplistic overview of what is going on between your tyres and the track.

[markiii]

even if you assume the contact patch is bigger (it may be or may not be)

you are applying less weight to it

run your finger over a piece of glass (the  flat side not the sharp edge)

now do it again whilst applying less pressure

oh look less friction  :-) :-) :-)

and thats before you look at dynamic loads

[markiii]

his last statement is key "simplistic"

I'm not saying wider can't give more grip, just that its not a given fact that it will as it depends on a huge number of other variables

[Anonymous]

Call it a draw Mark.  s:lol: :lol: s:lol:

[markiii]

think that may be wise  :-) :-) :-)

to teh OP

it depends as you can see

personally i upgraded to 16" rear with the 15" pre facelift rears used on teh front

I ran 195/50/15  on teh front and 225/45/16 on teh rear and was very happy with that setup. I did it specifically to run teh Yokohama advan neova tyres that only come in those sizes

I beleive Spit also runs those wheel combination but with different tyres.

its not going to be a massive difference either way if you run teh same tyre brand and model, and is down to personal taste, main thing is maintain front to rear stagger, and make sure they are all teh same brand and model

[Anonymous]

That got pretty intense for while there... nothing like a first time post to stir up a bit of banter! Anyway, i think ive got the jist, its not so much the wheel size thats the issue, its the tyre size. So as long as i get good quality tyres with slightly wider / larger on the rear i should be good to go!

Thanks for the advice and i hope you two can still be friends  s:wink: :wink: s:wink:

[Dac]

Mark - I was pretty much brought up on your logic mostly because my dad believed it (and still does!). It seems very common sense, the narrower the tyre, the more load per square cm and therefore more grip making it equal to (or only slightly different) to that of a wider tyre. I think he was suckered into this because of other peoples claims that wider tyres make the car less capable, most probably due to the geometry not being changed to match as well as other variables.

I have now as you can tell learnt the error of this and now believe very similar to what was posted above, that a wider tyre has better heat dissipation properties and therefore better able to maintain "stiction". What I will say in your defense is that by making the tyre wider as also stated above, you change the contact patch shape and this changes the way the tyre deforms changing the handling characteristics. A narrower tyre being more predictable as it deforms more in unison and a wider tyre having a sharper "edge" when it decides to let go.

Regards,
Dean.

[GSB]

I love this argument. It happens in every car based forum you care to name, and usually more than once...



Fat or thin? The question of contact patches and grip

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 of the rubber compound and the load on the tyre. As far as friction is concerned, the formula is relatively simple - F=uN, where F is the frictional force, N is the Normal force for the surfaces being pressed together and u is the coefficient of friction. In the case of a tyre, the Normal force basically stays the same - mass of the car multiplied by gravity. The coefficient of friction also remains unchanged because it's dependent on the two surfaces - in this case the road and the tyre's rubber.
The coefficient of friction is in part determined by the rubber compound's ability to 'key' with the road surface at a microscopic level.
mechanical keying

This explains why you can slide in a corner if you change road surface - for example going from a rough road to a smooth road, or a road surface covered in rain and diesel (a motorcyclist's pet peeve). The slide happens because the coefficient of friction has changed.
So do wider tyres give better grip?

If the contact patch remains the same size and the coefficient of friction and frictional force remain the same, then surely there is no difference in performance between narrow and wide tyres? Well there is but it has a lot to do with heat transfer. With a narrow tyre, the contact patch takes up more of the circumference of the tyre so for any given rotation, the sidewall has to compress more to get the contact patch on to the road. Deforming the tyre creates heat. With a longer contact patch and more sidewall deformation, the tyre spends proportionately less time cooling off than a wider tyre which has a shorter contact patch and less sidewall deformation. Why does this matter? Well because the narrower tyre has less capacity for cooling off, it needs to be made of a harder rubber compound in order to better resist heating in the first place. The harder compound has less mechanical keying and a lower coefficient of friction. The wider tyres are typically made of softer compounds with greater mechanical keying and a higher coefficient of friction. And voila - wider tyres = better grip. But not for the reasons we all thought.

What about lateral force in cornering?

In terms of the lateral force applied to a tyre during cornering, you eventually come to a point where slip angle becomes important. The plot below shows an example of normalised lateral force (in Kg) versus slip angle (in degrees). Slip angle is best described as the difference between the angle of the tyres that you've set by steering, and the direction in which the tyres actually want to travel. As you corner the lateral force increases on your tyres, and at some point, the lateral force is going to overcome the mechanical grip of the tyres and that point is defined by the peak slip angle, as shown in the graph. ie. there comes a point at which no matter how much vertical load is applied to the tyre (from the vehicle weight), it's going to be overcome by the lateral force and 'break away' and slip. So why do wider tyres perform better when cornering? Well apart from the softer rubber compound giving better mechanical keying and a higher coefficient of friction, they have lower profile sidewalls. This makes them more resistant to deforming under lateral load, resulting in a more predictable and stable contact patch. In other words, you can get to a higher lateral load before reaching the peak slip angle.
A slip angle graph

In reality, trying to figure this out using static examples and reading some internet hack's website is all but impossible because what's really important here is dynamic setup. In reality the contact patch is effectively spinning around your tyre at some horrendous speed. When you brake or corner, load-transfer happens and all the tyres start to behave differently to each other. This is why weight transfer makes such a difference the handling dynamics of the car. Braking for instance; weight moves forward, so load on the front tyres increases. The reverse happens to the rear at the same time, creating a car which can oversteer at the drop of a hat. The Mercedes A-class had this problem when it came out. The load-transfer was all wrong, and a rapid left-right-left on the steering wheel would upset the load so much that the vehicle lost grip in the rear, went sideways, re-acquired grip and rolled over. (That's since been changed.) The Audi TT had a problem too because the load on it's rear wheels wasn't enough to prevent oversteer which is why all the new models have that daft little spoiler on the back.

If your brain isn't running out of your ears already, then here's a link to where you can find many raging debates that go on in the Subaru forums about this very subject. If you decide to read this, you should bear in mind that Simon de Banke, webmaster of ScoobyNet, is a highly respected expert in vehicle dynamics and handling, and is also an extremely talented rally driver. It's also worth noting that he holds the World Record for driving sideways...........

If you decide to fatten up the tyres on your car, another consideration should be clearance with bits of your car. There's no point in getting super-fat tyres if they're going to rub against the inside of your wheel arches. Also, on cars with McPherson strut front suspension, there's a very real possibility that the tyre will foul the steering linkage on the suspension. Check it first!
Holy crap that's complicated. Isn't there a shorter answer?

Yes.
Choose the dimensions of your tyre according to the 'comfort/cornering speed' ratio that suits you. Lower profile/series = more precise cornering. Higher profile/series = more comfort. To increase the contact patch, lower the tyre pressure a little.

Read more:  m http://www.carbibles.com/tyre_bible_pg3 ... z1OnDVzX8J m

[Dac]

Very good post GSB - Now lets not get into the debate about whether bigger discs increase braking power   s:lol: :lol: s:lol: