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Posted by daniel 
January 19, 2009 02:00PM
I still don't understand why a narrow tire and a wide tire with the same sized patch would have different grip...in any direction. It's all about friction and if I have 7 sq in of patch area for example, the grip should be the same since the same physical laws are acting on both. Why should a square patch have more friction than a rectangular on with the same surface area?

It must be something other than patch size that makes the difference. The only thing that my tiny brain can think of is that the patch size changes during cornering with the different tires. The wider tire with the stiffer side walls may keep more of the patch in contact with the road than the taller softer side walled narrower tire.

Bear with me, I'm just trying to wrap my head around this winking smiley



Edited 1 time(s). Last edit at 01/19/2009 02:00PM by Archeo-peteriX.
January 19, 2009 02:49PM
Take a cardboard cereal box standing up on end. Push on it on the wide side, it falls over very easily. Push on it on the narrow side, it requires a pressure point higher up on the side of the box to get it to tip over. The force is different even though the "contact patch" area is the same. The cereal boxes contact patch is more stable when a force is put on it from different directions. Same idea with tires. A wider (even though it's narrower) contact patch will provide more solid grip when cornering before the tire begins to fold over and funky things happen with the tires.

Cab
1990 325i(s)
2004 325XiT
January 19, 2009 03:25PM
since i have felt like i've indirectly been taking some flack for having bigger brakes on my car, i would like to reiterate the fact that, when doing the e36 5-lug conversion, one must use the brakes from the e36 m3. Since they are already on the car, I might as well keep them. I don't think there is much arguing that they are superior to other e36 brakes regardless of the weight of the car. Maybe they are overkill on an e30, but if less braking force is needed to stop with the e36 m3 brakes, then theoretically they should last a bit longer anyway. and as i said before, they look cool B)-

and i am glad that my desire to have wider tires on my car has sparked such an interesting discussion about tires. to me it makes sense that, when moving laterally, wider tires will provide better grip.


January 19, 2009 04:41PM
Quote
Cab Treadway
Take a cardboard cereal box standing up on end. Push on it on the wide side, it falls over very easily. Push on it on the narrow side, it requires a pressure point higher up on the side of the box to get it to tip over. The force is different even though the "contact patch" area is the same. The cereal boxes contact patch is more stable when a force is put on it from different directions. Same idea with tires. A wider (even though it's narrower) contact patch will provide more solid grip when cornering before the tire begins to fold over and funky things happen with the tires.

So the key here is that the wider stiffer tire will retain a larger portion if it's contact area than the narrower tire during 'fold over'...that makes sense. I think smiling smileyo
January 19, 2009 04:44PM
Quote
daniel
since i have felt like i've indirectly been taking some flack for having bigger brakes on my car, i would like to reiterate the fact that, when doing the e36 5-lug conversion, one must use the brakes from the e36 m3. Since they are already on the car, I might as well keep them. I don't think there is much arguing that they are superior to other e36 brakes regardless of the weight of the car. Maybe they are overkill on an e30, but if less braking force is needed to stop with the e36 m3 brakes, then theoretically they should last a bit longer anyway. and as i said before, they look cool B)-

and i am glad that my desire to have wider tires on my car has sparked such an interesting discussion about tires. to me it makes sense that, when moving laterally, wider tires will provide better grip.

No flack Daniel, we're just having a nice thinking session about the merits of wider tires and bigger brakes. There is nothing wrong with your setup and certainly no need to change any of it. It's cool and as you can see, others here might like to have the same or similar setup at sometime down the road B)-
January 20, 2009 06:19AM
There is no intended flak from my viewpoint, I apologize if I've come across that way. In motorsports, bigger brakes are, in general, better than smaller brakes. It's certainly not something I would get rid of if my car came that way. I'm just saying that for my intended purposes, I don't think a BBK is necessary, or even desired. I would like to retain my 14' and 15" wheels, I feel that the stopping power of the stock E30 brakes is more than adequate, and the one negative thing that a BBK adds is more unsprung weight. I don't think that the benefits of a BBK would outweigh (no pun intended, of course) the extra weight. Plus, stopping distances at street and autoX speeds are more dependent on the tires than on the brakes. The main benefit of a BBK is to resist brake fade and overheating on a track. I'm not a track junkie, and don't intend to be, so for me, a BBK doesn't seem worth the extra $1k.

All that said, there's nothing at all wrong with big brakes, and I certainly wouldn't eliminate them if my car already came equipped. I'm glad you are enjoying your car! They certainly do look cool, I know I've taken many pictures of brakes at car shows, a nice brake package really looks great.

Cab
1990 325i(s)
2004 325XiT
January 20, 2009 06:49AM
Quote
Archeo-peteriX
Quote
Cab Treadway
Take a cardboard cereal box standing up on end. Push on it on the wide side, it falls over very easily. Push on it on the narrow side, it requires a pressure point higher up on the side of the box to get it to tip over. The force is different even though the "contact patch" area is the same. The cereal boxes contact patch is more stable when a force is put on it from different directions. Same idea with tires. A wider (even though it's narrower) contact patch will provide more solid grip when cornering before the tire begins to fold over and funky things happen with the tires.

So the key here is that the wider stiffer tire will retain a larger portion if it's contact area than the narrower tire during 'fold over'...that makes sense. I think smiling smileyo

The other thing with the shape of the patch has to do with how the force is applied. The tire is attached to the wheel, which is attached to the car at the hub. Basically all the force is transmitted from the road to the car at the hub. So you have a focused force opposite in direction from the way the car is turning that keeps the car from sliding. The force can be represented by a vector (think of an arrow pointed right at the middle of the contact patch). If the vector is centered in the long side of a long and narrow patch, there is less rubber directly in line with the vector, and the force can push the middle of the contact patch more easily than if it is pushing against a shorter but wider patch.

When you're going forward, the shape of the contact patch is less important (unless you're dealing with driving through snow/rain, in which case a narrower tire is better b/c it has less snow/water to have to displace), but for lateral grip, wider gives more resistance to the sideways motion.

Cab
1990 325i(s)
2004 325XiT
January 20, 2009 07:40AM
Quote
Cab Treadway
When you're going forward, the shape of the contact patch is less important
Your explanation makes perfect sense from a physics theory perspective. However, does this imply that a monstrously powerful car like a 500-HP Corvette Z06 should accelerate in a straight line just as quickly with skinny 185s as it does with the 325s that it comes with? Experience would suggest otherwise, i.e. that more powerful cars need wider tires to put the power down to the road. If this is the case, then either the wider tires have a greater contact patch, or the shape of the contact patch is still important in a straight line.

__________
Dave
'91 325iX
January 20, 2009 08:03AM
Quote
Dave_G
Your explanation makes perfect sense from a physics theory perspective. However, does this imply that a monstrously powerful car like a 500-HP Corvette Z06 should accelerate in a straight line just as quickly with skinny 185s as it does with the 325s that it comes with? Experience would suggest otherwise, i.e. that more powerful cars need wider tires to put the power down to the road. If this is the case, then either the wider tires have a greater contact patch, or the shape of the contact patch is still important in a straight line.

I don't know for sure, but my intuition (which is often wrong) tells me that a high HP car like that is going to have substantial weight transfer to the rear under hard acceleration, which will cause a 185 tire to squat so much that it would be extremely deformed, and this probably has an effect on the ability to put down power. It's not just the weight of the car, the acceleration will put more force on the rear wheels than just the car's weight, so the tire has to be strong enough and wide enough to create enough of a contact patch without overwhelming the tire's capabilities...

I don't know...

Cab
1990 325i(s)
2004 325XiT
January 20, 2009 10:41AM
Quote
Cab Treadway
When you're going forward, the shape of the contact patch is less important (unless you're dealing with driving through snow/rain, in which case a narrower tire is better b/c it has less snow/water to have to displace)
That's an important factor. A narrow tire will cut through snow, whereas a wide tire will tend to float up on the snow losing contact with the road surface.

Quote
Cab Treadway
If the vector is centered in the long side of a long and narrow patch, there is less rubber directly in line with the vector, and the force can push the middle of the contact patch more easily than if it is pushing against a shorter but wider patch.
Now we're making progress.

How much a tire will deform under side loading depends on many different factors like sidewall stiffness, tire pressure, camber angle, etc.. When I first started parking lot pylon racing, my tires used to fold over so far that I was grinding the lettering off the sidewalls. Someone suggested pumping the pressure up to 45 psi and that made a huge difference!

But, assuming the two tires otherwise have equal characteristics, the only difference being one has a long skinny contact patch and the other one has a short wide patch. With the car sitting still in your driveway, have someone big and strong to lean against the side of your car. The car will move slightly sideways, trying to stretch the tire off the rim. Given the same sidewall stiffnesses etc, I'm thinking either tire should deflect pretty much the same amount. The big difference is how the tires behave under side loading while the tire is rolling.

The tire is stretched sideways at the contact patch, while the rest of the tire doesn't change shape. So, when the tire is rolling forward under side loading, the new patch of rubber comes straight down, is planted on the road, and subsequently stretched sideways. Even though the rim and the rest of the tire is pointed straight ahead the whole time, because of the way the rubber moves as it comes down, is planted, then stretched, the tire actually rolls forward at a slight angle to the direction it's pointing in. That's called the "drift angle".

Note, do not confuse "drift angle" with the currently fashionable sport of "drifting", which actually involves breaking traction completely and sliding the tires. "Drift angle" occurs any time there is a side load on the tires while they are rolling while planted firmly with grip. Even though the tire is pointed straight ahead, it's actually crabbing off at an angle away from where it's pointed.

A tall skinny tire with a long narrow contact patch will have a higher "drift angle" than a wide tire with a short wide contact patch.

This photo of Jack Brabham at Zandvoort in 1966 shows the extremely high drift angles of the tall treaded tires they used back in those days. The tires are not "sliding". At the very limit of grip, this is the extreme crab angle at which the tires worked best. Wider low profile tires, having a shorter wider contact patch, operate at much smaller drift angles giving the appearance that modern cars are operating on rails.

The advantage of tires that operate best at smaller drift angles is in the response time when transitioning from turning one way to another. Looking at the photo of the 1966 Brabham F1 car, you can imagine there's quite a lag time between having the car set at maximum right hand cornering as shown, and then pitching the car the other way into maximum left hand cornering, whereas a modern car with very small drift angle tires will react instantly to left/right transitions.


January 20, 2009 02:39PM
Thanks Ferd...that was what was missing. I can actually visualize this happening.
Funny how that which seems so obvious can be somewhat incorrect if all the pieces of the puzzle aren't there grinning smiley
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