Different rubber size fronts vs back??

[Alex]

I've been considering getting wider wheels for a while now but every time a thread comes up everyone seems to state "You won't get a larger contact patch, just a different shape" this statement (and ones like it) seems to imply that you need a larger area to get more grip. Then there is the whole tyre pressure issue that pops up every now and then so I decided to see how the area (magnitude) would change with pressure and came up with some numbers
Pressure (psi) / Area (cm^2)
28 / 12.70
29 / 12.27
30 / 11.86
31 / 11.47
32 / 11.12
33 / 10.78
34 / 10.46

So my contact patch will be ~20% smaller if I go to 34psi from 28psi, if grip levels are all about area then that means I'll pull ~20% less G's in my car around a corner, which would be a noticeable difference you would think. (forgetting that the coefficent of friction is not dependent on the area). I've messed around with pressures and found that while there is a difference I'm sure it's nowhere near 20%. (But there is a definite difference, especially in how the car handles/feels)
Which leads me to believe that the shape of the contact patch is more important that the actual area. Hence yes it will make a difference if you get wider wheels/tyres.

But IMHO if you are worried about grip the best way to deal with it would be to get better tyres, since different tyres have different compounds (hence coeffiecent of friction).
the following values were used for the data above
Conversion factor from psi to Pa = 6.895 x 10^3
Load = 250kg
Gravity = 9.81m/s^2
now to hope I did the calculations properly and didn't make a total fool of myself

[Fatty]

there is a good diagram in the "miata performnace handbook" explaining the way the contact will change shape, but not size, with wider tyres.

[Sasso]

Why don't you get stickier rubber rather than wider rubber. I have skinny ass 185/60R14 Toyo R888's that are basically bald on the rear and they can still hold up 12psi of boost in first gear without slipping. They are brilliant everywhere, even in the wet, they don't need to warm up they are just fantastic. You just use normal 32psi on the street and drop it for track days (so they are around 30psi hot) if you do track days. They don't wear very quickly on the street either if you're worried about it, my mums Re050A's on the MPS6 wear out quicker than my R888's and I drive hard.

Drive someones car with a set on and you will never look back, the sheer grip and stabilty on the street is unmatched, and they're cheap for what you get, especially if you have small rims, sidewall flex is greatly reduced since its reinforced so don't worry about low profile.


Oh yeah, I know you're anal about weight, the grip produced from stiffer sidewalls and stickier rubber more than offsetts the added weight of the semi slick tyre. Also don't think that lower profile means less inertia is good because bigger diameter tyres produce more grip, which is more desirable. If your car is too slow add more power and grip, creature comforts can remain, tyres are the most important part of the setup of the car.

[Sasso]

See, not much flex on the highly loaded corner.

[Fatty]

here is a not so great diagram showing contact patches for wide and skinny tyres. this is just a generic diagram off the net, but it illustrates my point. remembering that our cars are light so they don't put much weight / downforce on the tyre as you can see, the SHAPE of the contact patch will change (from rectangular to square) , but the SIZE pretty much remains the same. therefore, due to the lightness of our cars big fat tyres do not guarantee more grip. i'd argue that there are more negatives than positives in going to bigger tyres. if you had a big heavy car squashing those fat tyres down , sure you'd get a bigger contact patch and it would be worthwhile.

[matt9111]

So does that mean that the shape of the contact patch is more important than the actual amount of rubber on the road?

I'm so confused....a wider tire must add more grip somehow, otherwise, everyone would use 185's. Look at Porsche, Audi, BMW, all their performance cars use tires from 245's to 285's.

Hahaha, yeah, im confused

[Fatty]

no, the shape is not as important as the size. you are just hung up on the notion that wider must be better.

yes all those cars you mentioned use wide rubber but i bet those cars weigh a lot more than an mx5.

wider rubber does have other advantages tho, slip angles and what not, but that is above my level of knowledge.

[matt9111]

Haha, fair enough.....in your opinion is it better or worse to run a wider tire on the MX5?

[Adam_NAclubman]

A wide, thin contact patch would probably be better for lateral forces?

[Damo]

A wider tyre has more rubber so you could use a softer compound without the tyre overheating more quickly. Seeing as these tyres will be intended for heavier/more powerful cars they will most likely use the same compounds so you won't likely see the benefits. That's my theory - sound logical?

[Gazbo]

Have a read here http://www.carbibles.com/tyre_bible_pg2.html re:contact patch area (half way down page)

[Hellmun]

As I understand it more rubber gives you a larger heatsink. Remember Race tyres especially need to be kept in a certain heat band to be at their best grip (compound dependant). The heavier the car and the more heat generated the larger the heatsink. I believe as someone else has already ascerted that a contact patch wider than it is tall will produce more lateral grip at the expense of Braking/Acceleration as well. I know I spend more time at the limit of lateral grip then I do braking and especially accelerating.

I'm keen to try some 225/50/15 semi-slicks as I've been seeing over 90C in my semislicks after half a lap cooldown which seems to be past their comfortable range.

[irwin83r]

semi slick tyres having a stiff side wall allows you to run a lower pressure, which increases contact patch, while maintaing side wall stiffness, as apose to a soft side wall which will bulge, become laggy in response to movements between the tyre contact patch and wheel angle and increase tyre tempratures due to an increase in rolling resistance and more so tyre flex. this being the case the contact patch is increased but it cant get any bigger side ways as the side walls do not spread out onto the road. there fore the contact patch grows forwads and backwards becoming more of a square then standard. surely there fore to get back to the standard contact patch shape you have to increase the width of the tyre. how much is way too much math then what im prepaired todo. this compinded with the want of a more rectangular patch then standard would make me want to run wider tyres.

but as helmund has pointed out with a lower tyre pressure the tyres work harder, through the side walls resisting flex, and the extra resistance to rolling created by the slightly flat tyre there fore the tempratures created in the tyre are increased and the tyre temp comes into play. this again as pointed out by helmund is dulled by having more rubber to desperse the heat but its really a balancing act to stay in the heat range of the tyre and mantaining a good foot print.

also worth noting if the sidewall of the tyre is a supporting member ie the tyre pressure is not high enough to support the cars downward force alone the side wall stiffness will be supporting some of the mass of the car this, and the resistance to opposite forces between the wheel and the tyre contact patch from cornering or accelarating/braking is the flex i am refering too creating more heat in the tyre.

[rjastra2]

So does that mean that the shape of the contact patch is more important than the actual amount of rubber on the road?

I'm so confused....a wider tire must add more grip somehow, otherwise, everyone would use 185's. Look at Porsche, Audi, BMW, all their performance cars use tires from 245's to 285's.

Hahaha, yeah, im confused

Yes... but the tyres in question are usually on 18-20 inch diameter wheels. Meaning the tyre circumference is much larger than the 205/45x16 found on an NB8B. This has a major affect on the contact patch as well.

205/45R16 is common sized tyre on warm/hot hatches of the weight of a NB8b. So it is probably the best compromise for grip/comfort etc.

[16bit]

according to the good people at 949 racing you should go wider. For 15's on an 8 inch rim they reccomend 225 45 15's. The wider the tyre the greater load it can take, the less pressure you need which = more grip. having a wider tyre on the same size rim is always going to result in more contact patch. If you are serious about doing it email the guys at 949racing.com and see what he says. I would try and get the opinion of someone who has done it and tested it on the track.