17-inch Rota wheels - some questions about going up two sizes from 15-inch Fuchs...

[island911]

Originally Posted by gunlover05

...
While wider contact patch would be better for AX for sure, ....

Yes, that is conventional wisdom. But ask your self, why? ...why is wider considered better? If both a skinning and a wide tire each are touching the ground with 30psi, and holding up 600_lbs each, then they should both have 20_in² of contact, each. But wait, conventional wisdom says wider puts more rubber on the road. Well, conventional wisdom is often wrong.(though well propagated) ..."wider" does not give more contact... just makes for a cooler running tire. (less deflection)

Think about how the shape of the contact patch... if you're doing mostly tight radius turns, then you hardly want a steam-roller contact patch. A too narrow patch wouldn't be much better. In theory, a circular contact patch would be optimal, for smallest scrub radius.

Anyway, it's all about balance and optimization... it may be that a taller tire shaves more time due to lower rolling resistance than it gives up in worse scrub, rotational inertia, and CG. --there are too many variable for me to say what set-up is optimal. All I can point out is that, contrary to conventional wisdom "wider is not always better."

[dd74]

Have Formula One tires and wheels increased much in height and width within the last 20 or so years, or have the wheels just become lighter (stronger?) and the tires higher in grip? What about kart wheels and tires?

[gunlover05]

fwiw, in the SCCA nationals AX, most people squeeze the widest, stickest tire they can legally and physically put in the wheel well...why? I can only imagine they are trying to get more contact patch....also, in the last Grassroots motorsports, they tested several AX tires all on 8x18" wheels (might have been 17s, but can't remember, but was 8" wide for sure) ...these
AX tires were mostly 275s, with one exception of Kumo, which buckled and they had to move down to 245s...interestingly enough, the 245 Kuhmos tested the fastest. Now most people outside of serious AX would never attempt to put a 275 on an 8" wheel. But serious AX'ers evidently do...

Also in this months Excellence magazine, there is a dedicated 914 AX car highlighted...noticed it had 7x8x15 fuchs, but has Goodyear 9" wide x 23" dia slicks on front and back.

Track tires are another story altogether I'm sure, but is wider tires better in AX? I'm sure there is a limit, but seems like the serious AX guys think so.

[Quicksilver]

Originally Posted by dd74

Have Formula One tires and wheels increased much in height and width within the last 20 or so years, or have the wheels just become lighter (stronger?) and the tires higher in grip? What about kart wheels and tires?

F1 is a weird little microcosm where the rules have made some anomalies. They had to add a rule that says that you have to have a working suspension. They have suspensions that have joints that bend instead of having some sort of moving bearing like the rest of the world. They have 13" wheels and tires with fairly large sidewalls that provide almost all of the suspension compliance in the car.

What they are doing real doesn't have anything to do with what the rest of the world is dealing with.

Originally Posted by island911

Yes, that is conventional wisdom. But ask your self, why? ...why is wider considered better? If both a skinning and a wide tire each are touching the ground with 30psi, and holding up 600_lbs each, then they should both have 20_in² of contact, each. But wait, conventional wisdom says wider puts more rubber on the road. Well, conventional wisdom is often wrong.(though well propagated) ..."wider" does not give more contact... just makes for a cooler running tire. (less deflection)
. . .

The 30psi x 600lb= 20sq" calculation is correct assuming that you are talking about a tire made of a substance that is flexible but complete resists stretching along the surface and doesn't have a compressible surface.

The fact is that a tire doesn't press into the ground at the uniform pressure equal to the internal pressure. The tire is compliant and will have portions that press into the ground at a higher pressure then the internal pressure. Because of the distortion of the shape of the tread carcass and the compliance of the tread rubber changing the size of the tire does in fact make a noticeable difference in the contact patch.

The most effective increase you can make to maximize the contact patch is increasing the diameter of the tire but this introduces a large number of other problems such as increased CG height, weird suspension angles, tire clearance, etc. Because this means designing a car from scratch we basically go for increased width.

The basic flaw in the "size doesn't matter" argument is it uses the flawed idea that a tires performance can calculated using a "coefficient of friction". A coefficient of friction is a standard physics concept taught in any high school so the repeated misapplication of this idea is understandable.

A tire doesn't have friction. It has grip. This is easy to prove. The highest possible coefficient of friction is 1. Tires have had coefficient of grip higher then 1 for decades so obviously it cannot be described as "friction".

What is the difference?
- Friction is the force that is transfered between the surfaces of 2 objects sliding past each other. It is simple to calculate force because if you double the force pressing the two surfaces together you always double the force required to get the surfaces to slide. Very easy to work with and kind of sounds like what our tire do...
- Grip is the force that is transfered between 2 object by an interlocking effect. With an interlocking effect the force required to get the two objects to slide past each other is not linear. It will in fact be a curve like the curve shown on a tire performance graph. For a blatant example of an interlocking effect look at a cog railway or a rack and pinion. Our tires do the same thing as the rubber interlocks with the rough surface of the road.

We have one more bit to add to our tires besides the mechanical interlocking effect with the road: Molecular Adhesion. The molecules of our tires can actually momentarily bond to the road surface as the tire rolls over it. This bond will be torn apart when the tire continues its rotation but if you are loading the tire at very high levels it can cause the bonded rubber to tear off the tire. This can happen without sliding the tire which is why high effort braking (without ever sliding) will significantly effect tire life. (A new one I learned from a tire engineer in the last year!)

The end result is that increasing the width of the tire will significantly increase the grip of the tire even if no other variables are changed. A lower level of loading will increase the coefficient of grip. Our cars have tires that are no where near the limits where additions to aero and parasitic drag will negatively effect performance (except for gas mileage) so put the rubber down. If you don't believe it, well fine, but you are going to get stomped at your next Auto-x if you have any decent competition.

[island911]

fwiw, the "20_in² of contact" example was used for example to help understand a basic ... like those massless ropes and frictionless pulleys used in physics studies. Speaking of frictionless pulleys, you say "A tire doesn't have friction. It has grip." What about the point when a tire goes beyond it's point of static friction and starts to slide? Will the Molecular Adhesion prevent this? . . .is friction never part of "grip"? . ..or is grip never part of friction? -just raz'n ya.

Seriously, by far the largest force our tires have with the road is friction. -usually/mostly static friction. ...the Molecular Adhesion of a hot tire certainly can help hold the tire from sliding, but the problem is, as the adhesive forces start to go up, the Cohesive forces (of the rubber) go down. That is, "wet glue" is slippery. Oh, and "mechanical interlocking" is exactly where friction lives.(there are exceptions) usually at a micro level. ..whereas tire designs strive to maximize the "interlocking" on both micro and macro levels.

For a fun extreme, let's say you have such high adhesive forces that friction is not needed... what happens to the needed rolling torque to pull the adhesively stuck contact patch free? ..with adhesion forces higher than the friction coef=1 implies that 70-something percent of the weight of the car would have to be pulled lose, in overcoming adhesive force ...that would be the same as driving up a 100+% grade. (45°+ hill) TO add to your high adhesion problems, MUCH of the road would be permanently stuck to your tire. THEN you are rolling around with asphalt tires (in a larger radius).. . how much force would be required to turn the steering wheel? ...could turns be made?

Anyway, Are you sure that you want to say "A tire doesn't have friction." ?

...I would be okay with; "Beyond friction, some tires provide some adhesion."

[island911]

Originally Posted by Quicksilver

....

The end result is that increasing the width of the tire will significantly increase the grip of the tire even if no other variables are changed. A lower level of loading will increase the coefficient of grip. Our cars have tires that are no where near the limits where additions to aero and parasitic drag will negatively effect performance (except for gas mileage) so put the rubber down. If you don't believe it, well fine, but you are going to get stomped at your next Auto-x if you have any decent competition.

I do like that you're willing to go out on a limb like that.

Are we anywhere near the limit where scrub radius becomes a factor? ..or is wider always better?

[island911]

Originally Posted by Quicksilver

...

A tire doesn't have friction. It has grip. This is easy to prove. The highest possible coefficient of friction is 1. Tires have had coefficient of grip higher then 1 for decades so obviously it cannot be described as "friction".
...

I just can't let this stand. "1" is NOT the limit for friction Coef's. ..f_s=1 just means that say a sheet of rubber holding on a slope at 45°, where down force and side force are equal, that it is on the cusp of hanging on.

I'm not much of a writer, so maybe this will help:

per Wiki:

Main article: Coefficient of friction

The coefficient of friction (also known as the frictional coefficient) is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used – for example, ice on steel has a low coefficient of friction (the two materials slide past each other easily), while rubber on pavement has a high coefficient of friction (the materials do not slide past each other easily). Coefficients of friction range from near zero to greater than one - under good conditions, a tire on concrete may have a coefficient of friction of 1.7.

When the surfaces are adhesive, Coulomb friction becomes a very poor approximation (for example, Scotch tape resists sliding even when there is no normal force, or a negative normal force). In this case, the frictional force may depend strongly on the area of contact. Some drag racing tires are adhesive in this way.

The force of friction is always exerted in a direction that opposes movement (for kinetic friction) or potential movement (for static friction) between the two surfaces. For example, a curling stone sliding along the ice experiences a kinetic force slowing it down. For an example of potential movement, the drive wheels of an accelerating car experience a frictional force pointing forward; if they did not, the wheels would spin, and the rubber would slide backwards along the pavement. Note that it is not the direction of movement of the vehicle they oppose, it is the direction of (potential) sliding between tire and road.

The coefficient of friction is an empirical measurement – it has to be measured experimentally, and cannot be found through calculations. Rougher surfaces tend to have higher values. Most dry materials in combination have friction coefficient values between 0.3 and 0.6. Values outside this range are rarer, but Teflon, for example, can have a coefficient as low as 0.04. A value of zero would mean no friction at all, an elusive property – even Magnetic levitation vehicles have drag. Rubber in contact with other surfaces can yield friction coefficients from 1.0 to 2.

[mtbguy]

I'm in the same situation right now - just bought some 17" wheels to use on my 87 Carrera narrow body- the wheels are supposed to have correct offset for my car. I use it as a spirited daily driver- no DEs or track events.

My dilemma is mostly with the FRONT tires size........on my "new" wheels.

The current tires on these 17s are: FRONT- 205/50 17s REAR 255/40 -17s.

I'm trying to decide between sticking with the 205/50s up front or switching to 225/45-17s instead. The current tires seem to fit fine without rubbing (standard factory ride height) but the tires are old and weathered and I don't want to really push it driving ten year old tires around corners to really test them out.

What are peoples thoughts on this? 205s or 225s?

I want the car to be light on its feet with steering but also well balanced with minimal understeer or oversteer. Also wondering if the 225s will stick outside the fender too far.

Thanks for any advice on this!

[mjshira]

Originally Posted by dd74

Interesting you say this. Tyson Schmidt stuck 17-inch Lindsays on his old car, and perceived, as I remember, very little power loss. Who's to say? Some can't tell the difference between 20 or 30 horsepower difference. Still, for me, it is a concern. .

then I added a 3.6 and there was really no difference noticed ;-)

[Moses]

Originally Posted by mtbguy

I'm in the same situation right now - just bought some 17" wheels to use on my 87 Carrera narrow body- the wheels are supposed to have correct offset for my car. I use it as a spirited daily driver- no DEs or track events.

My dilemma is mostly with the FRONT tires size........on my "new" wheels.

The current tires on these 17s are: FRONT- 205/50 17s REAR 255/40 -17s.

I'm trying to decide between sticking with the 205/50s up front or switching to 225/45-17s instead. The current tires seem to fit fine without rubbing (standard factory ride height) but the tires are old and weathered and I don't want to really push it driving ten year old tires around corners to really test them out.

What are peoples thoughts on this? 205s or 225s?

I want the car to be light on its feet with steering but also well balanced with minimal understeer or oversteer. Also wondering if the 225s will stick outside the fender too far.

Thanks for any advice on this!

I have two sets of 17" wheels. For the track I run 225s in front without issue. For the street, I run 215s.

[mtbguy]

Moses- thanks for the info. I am now leaning towards using the 205s since it will not be tracked, and what I have now is almost inline with the fender lip (laterally)- I think if I go to the 225 it might be an inch or so wider and then be sticking out past the outside of the car- probably would not look so good and be much noisier I would think.

You mentioned that you use 215s for the street- when I used an online tire size conversion (from miata.net) it looked like the 215s would lower the front of the car because dimensionally they are shorter than the 205/50 or 225/45. Anything you have noticed with this?

Thanks again!

[Moses]

Originally Posted by mtbguy

Moses- thanks for the info. I am now leaning towards using the 205s since it will not be tracked, and what I have now is almost inline with the fender lip (laterally)- I think if I go to the 225 it might be an inch or so wider and then be sticking out past the outside of the car- probably would not look so good and be much noisier I would think.

You mentioned that you use 215s for the street- when I used an online tire size conversion (from miata.net) it looked like the 215s would lower the front of the car because dimensionally they are shorter than the 205/50 or 225/45. Anything you have noticed with this?

Thanks again!

Nope. The 215's look great. If you can get the tires you want, I'd go with 215/45/17's

[autodoctor911]

a 225 width tire will only stick out 10mm, or about 3/8 of an inch more than a 205 width tire, since it is only 20mm wider and half of that will stick out to the inside.