Interesting read from the Del Sol guys about tires

[phzoinkMKII]

I came across this on the del sol forum and figured because it mentions mostly hondas I would put it here. Mods if you feel inclined to move this thread please feel free.

There is a common misconception afloat that a taller rim will always provide increased lateral grip, but typically result in a weight penalty that hurts acceleration and deceleration.

Well, this is partially true, but it also has to do with the height of the sidewall and its ability to flex enough to hook up with the pavement.

A 17" wheel with a tire sized to fit within the 5G Civic wheel well (for instance) is going to have such a short sidewall that the lack of material to flex is going to have a negative impact on aceleration and deceleration. It could have a positive effect on cornering at steady state on a smooth surface, but in the real world one rarely encounters these conditions. Typically the car is in transition 95% of the time, and road surfaces are rarely smooth. As a result the lack of sidewall will have a negative impact on cornering more than 95% of the time.

Then consider the distance from the rotational axis of a 17" vs. a 16" wheel...even if the 17" wheel was the same weight (which would be unusual, but possible if one was willing to shell out the bucks for an extremely well engineered wheel) then you still have to deal with the fact that the actual outer shell of the wheel was a half inch further from the axis of rotation all the way around. This means it requires more energy to change the velocity of the 17" wheel (either braking or accelerating).

But what about the belief that a shorter sidewall will flex less and provide better handling?

Well that is just not always true. There is a limit. It is just another iteration of a well-travelled myth people who don't know any better recite over and over again.

Yes, a 50 series tire will provide less sidewall flex than a 75 series tire. But that does not necessarily provide better handling...just a different feel, and depending on the construction of the tire carcass, the compound, and the road surface, the 75 series tire could provide better traction. Ever notice that WRC cars use a taller tire on the rough surfaces than they do on tarmac? Do you think that is because they can't afford to buy taller wheels? Or maybe they just don't have the resources to tune the suspensions to handle the rough stuff? Yeah right.

The truth is that one quickly approaches the fine point where the tire's lack of compliance actually results in less lateral grip than a taller sidewalled tire would. I contend that a 215/35-17 (the size tire that would be closest in outer diameter to the stock 175/70-13 tire - on a 5G - and would fit under the fenders of Civic) while providing less flex and a firmer ride than the 215/45-15, will not provide superior predictability or enhanced control. Firmer ride, maybe, but firmer doesn't always mean better.

What does improve handling is increasing the size of the contact patch and/or increasing the coefficient of friction between the tire and the asphalt. The latter can be done with softer tire compounds and things that happen at a molecular level as a particular tread compound breaks down due to friction at the tire/aslphalt interface...but that's not what we are talking about here.

What I am primarily talking about here is maximizing the size of the contact patch and keeping it in contact with the road.

A contact patch's shape is not a constant...it varies depending on the forces (latitudinal and longitudinal) applied to the contact patch due to the amount of torque applied to the wheel by the motor (accel and decel) and the difference between the direction the tire is pointing vs. the direction that corner of the car is travelling (a.k.a., the slip angle). Based on the compliance of the tire (and the tread blocks - this is sometimes referred to as tread squirm), these forces can allow the contact patch to deform in a number of ways that either increase or decrease the total area of tire tread that is in contact with the road surface. We seek to find the way that maximizes the amount of tread that is coming in contact with the road surface.

As a car corners, the suspension compresses on the outside and droops on the inside, which results in all sorts of variations in the orientation of the tire surface to the road (note, we are not focusing on the camber angle which is the angle of the tire relative to the chassis, but rather just the orientation of the tread relative to the road surface).

In a perfect world the outside suspension would compress and add just the right amount of negative camber to that wheel so that it remained perfectly oriented to the road surface such that the contact patch size was maximized (neither the inside nor outside edge would pull up from the road). Likewise the inside wheel would do the same thing but in the opposite direction. Unfortunately since the forces that cause the body of the car to roll, thus creating the compression and droop actions at each side of the car, are actually being applied slightly in advance of the body's ability to adjust and roll as the car is entering a corner and the suspension is yet to reach its steady state, or on corner exit in the opposite degree of compression/droop, we rarely find ourselves in that perfect world.

If one were to take the theory that less sidewall flex resulted in better handling to its extreme, then we would be seeking zero sidewall flex, right? Well, if the sidewall doesn't flex, and the suspension is not able to keep the tire tread perfectly oriented relative to the asphalt, guess what...the tread will lift and only the shoulder of the tire will remain in contact with the road surface. So now the contact patch area has decreased dramatically and you are losing traction.

If you reach and remain in steady state, then a stiffer sidewall can translate into enhanced lateral grip assuming the suspension does utilize spring/damping rates that work with the spring rate of the tire and that it provides sufficient camber correction as well as ackerman to deal with the body roll and the difference in diameters of the circles described by the inside and outside tires (BTW, there is currently no chassis that does that perfectly...certainly not our beloved Civcs which - let's face it - were designed for fetching groceries first).

But in the real world you do not remain in steady state but for a fraction of the time. Even around a skidpad one is typically making adjustments with the wheel or the throttle in an effort to keep the circumference of the circle constant, and these adjustments will cause shifts in the degree of body roll.

Now throw in a bump...let's say there is a .5"x.5" strip of wood lying across the surface in the corner. As your tire encounters this piece of wood, the whole tire lifts off the asphalt and is now resting on the wood...the contact patch has been reduced to whatever the approximate width of the contact patch was before, but only half an inch long. This will produce less lateral grip.

If you've ever experienced a "jittering" sensation when your car was cornering, then you know what it feels like when the tire and suspension is unable to comply quickly enough with variations in the cornering forces or road surface, and the jittering is the result of the tires hooking up momentarily, then losing traction, then hooking up again, etc.

Alright, so let's say we stay away from extremes...let's think about the two sizes I mentioned above (215/45-15 vs. 215/35-17). The 15" tire will have nearly an additional inch of sidewall than the 17. Take weight out of the equation if you wish, and assume that both wheels and tires are the same width filled to pressures where they have the same contact patch at rest.

Once you start applying the forces I described that are present in transitions, and once you throw a few bumps or surface irregularities into the equation, and then factor in the imperfections in the camber curve and ackerman present in the Civic suspension (basically factor in the "real world") you will find that the additional compliance present in the 15" tire will result in improved lateral grip over the 17" tire. At the very least it will provide an enhanced level of predicatbility and control.

So why do all those touring cars have tall rims and short sidewalls? I believe it is primarily due to the desire to run huge friggin' brake rotors (which are free under most touring car rules). These cars make tremendous amounts of horsepower and the drivers do the majority of their passing in late-braking maneuvers going into corners. Nearly every corner entry is a battle for who can break the latest and hardest. Larger brake rotors give them more leverage against the rotational inertia of the wheel, more pad surface, and a greater heat sink. This allows them to charge down the straight, balls to the wall, wait until they see the other guy brake out of the corner of their eye, and then jump on the binders as quickly as possible.

Ever notice how easily a touring car spins? A touch of nerfing and they break loose like tops, and its nearly impossible to catch the cars once they start to break loose. Know why? Because the sidewalls on the tires they run are so damn short that they have very little tolerance for being upset...very difficult to control and can be somewhat unpredicatable (which IIRC was your contention that short sidewalls provided). Its not because the suspensions are set up too stiff...the teams we are talking about - especially the works teams - have development resources coming out the yin-yang. Surely they could design a suspension that was capable of taking a wack from a competitor without causing the car to irreversably lose traction - hell, even NASCAR teams have figured out how to do that some of the time. Nope...the touring car builders are faced with a compromise - since the maximum rim diameters and outer tire diameters are constrained, they have to figure out how to get the most braking capacity built into the cars as possible. In exchange they sacrifice some stability and predictability.

So in conclusion, when selecting new wheels and tires for your car, keep in mind that there is a point at which a taller rim and a shorter sidewall begin to be a detriment to your performance...and not just the ability to put down the power or the extra rotational inertia or the extra unsprung weight either, but rather in the predicability of the car in transitions.

[TomsMR2]

ill take a 215/40/17 and 235/40/17 over a 195/60/14 and 205/60/14 any day of the week, under all conditions, for anything. id also wager that the increased tire width will turn faster times in real world conditions. compliance and predictability aside, its faster. on the street you should be pushing so hard that the predictability of your tires is really a concern, even with big 18 inch rims.

[91hardtop]

really intelligent post, you should go more in depth and include tire widths on a given size rim, but just so you know taller sidewalls will fold expecially with sticky tires, and that is not predictable

[EL PAALO]

you only need a big enough wheel to fit over the brakes. going bigger than that is generally a waste.
The issue for the mkii is tire options for smaller wheels. 15's and 16's are the sweet spot for the mkii, but the tire options blow.

[RePete]

^ Exactly. I'd love to mount 225/45-15 front and 265/40-16 rear but try finding that in street tires, so I had to go up a size in both. Sometimes you have to go with what's available.

[mthreat]

Good article. I don't know enough to say I agree or disagree, but interesting read.

Notice that formula 1 doesn't have really low-profile tires. But I'll bet they have some STIFF sidewalls! And also, they have almost no body roll -- virtually none.

[91hardtop]

i agree that 15 or 16s are good but why would you want that wide of tires, wouldnt that just enduce snap oversteer unless you have really wide rims

[Rich3sgte]

dude....that was a looooong post but speaking of a del sol...check out this sick video i found!

http://www.youtube.com/v/qkzgrv1aby4&rel

[TreyMR2]

^^ GOOD GOD!! what the hell did he have in that?

[91hardtop]

more like what didnt he have, im talking about weight. its pretty stupid though since they didnt show it spinning the tires through the first two gears

[Oteck]

Oh common snap oversteer a bad excuse for poor driving skills

Originally Posted by 91hardtop

i agree that 15 or 16s are good but why would you want that wide of tires, wouldnt that just enduce snap oversteer unless you have really wide rims

[PearlJammzz]

I had snap oversteer in my car because of the 215/40/r17's that came with the car. After I spun out in the canyon I went to the local mall parking lot late at night when no one was there. I turned the wheel and spun the car out. It took 2.5-3 revolutions before I could regain control, I was astonished.

I then tried it in my snow tires (205/50/r15 or 55's IIRC) and the car actually gripped, it didn't spin out uncontrolably (spelling?). I then sold those ****** 17's and got me some stock 93+ wheels w/ the appropriate sized all-season tires. I can now control the damn car when it looses traction. 17's are horrible for the handling of the MR2, 15's or 16's are your best bet. I would imagine a wider 17 w/ more sidewall than my 215/40's would handle much better (perhaps a 255/40) and not have such ****** traction.

[RePete]

I think those 17" tires you originally got with your ride are to blame, not the actual size. A parking lot isn't sufficient to demonstrate real world speeds and handling/cornering. Nose to nose, increasing width of tire increases traction and grip with everything else being equal.

[91hardtop]

Originally Posted by Oteck

Oh common snap oversteer a bad excuse for poor driving skills

fine supposing that your a good driver you would still have less lateral grip when your tires are wider than your rims

the reason he spun out was he had too much grip and not enough lateral grip. thats why i think that 15" rims are the best because you actually have more contact patch, and your tires can still flex. if you dont believe me about the contact patch being larger than just imagine putting 700 pounds directly over both rims/tires.

[MX450248]

I have 17" 255/40 and 225/40 Kumho MX on the rear and front repectively. I love the grip that I get with them. I have not found the edge of control with them yet and they hook up so well. Plus they are designed for the speed that i like to see in my car on a normal basis, so i dont have to worry about them failing at high speeds.

[EL PAALO]

you mean a proper stagger works well on the mr2?