SnowmanSteiner

From what I see the equations look right. In physics you are taking the maximum force of friction and subtracting from the direction you are moving. There are two different measures of friction though, static and dynamic. Dynamic is the one in concern here as the tires are already moving and we don't have to deal with overcoming the static force of friction before we can move. The equations are very basic:
1st you have to establish a coordinate system, consisting of x, y, z, where x is the lateral direction, call it forward and backward, y is the height, and z is the left to right.:
sum fx- this means the sum of the forces acting in the x direction. m=mass ax = acceleration in the x direction
sum fy - this means the sum of forces acting in the y direction where m=mass ay=acceleration in the y direction and g=gravity (9.81)
and I believe the third one is the sum of the moments about a point G

- Steiner

edmcguirk

Mostly right. when you try to give a simple answer to a complicated question something will allways be missing.


1 - Wider tires do not put more rubber on the road. The rubber on the road is determined by air pressure and corner weight. If you've got the same air pressure, you've got the same amount of rubber on the road. When you change the tire size you are changing many things: sidewall height, tire/rim width ratio, tire weight, but most of all - breakaway characteristics. Some will be better and some will be worse. In general putting a wider tire on the same rim will result in a tire with less support. The contact patch will be shorter and will probbably break away more suddenly. That will probbaly feel like more traction but it's usually not much different.

2 - More weight on the tire results in less traction. Because the coefficient of friction for rubber goes down with increasing load, if you double the weight you get like 180% of the traction not double the traction. Because the weight of the car didn't change, that weight had to come from somewhere. So with half the weight on the other tire you get something like 60% of the traction. That is better than half the traction but the traction you gain on the light tire is not enough to make up for the traction you lost on the heavy tire. So if you stiffen up the rear of the car, not only will you lose traction in the rear, you GAIN traction in the front. Unfortunately it's a little less total traction then you had before. But maybe the balance is enough better to make you faster overall.

3 - Adding camber is often better than stiffening the car but not always. Sometimes the car is just leaning over too much. A little stiffness will lessen the need for camber. If you need more than 3 degrees to get the traction you need, maybe more stiffness would be better.


The whole wider tire advice always gets my goat because it's an oversimplification usually based on wrong assumptions. A narrow tire on a wide rim will support the tire carcass well enough so that you will probably need less air pressure. Less air pressure will give you more traction. (unless the pressure is too low to support the tire properly).

When I road race I use a 205/50. I get great traction and great tire wear. The tire is lighter - good for acceleration. The tire is shorter - great for acceleration. Now if I go to a 3 hour endurance race I will consider a 225/50 because the bigger tire can handle a bigger load than a 205/50. The all around handeling might be slightly less but the tire wear will be slightly better. It more than pays for the time for a pit stop to change tires.

Outside of a stock (or IT) class, you've always got a lot more HP and a lot more speed and bigger brakes. A 195 tire just can't stand up to that kind of load and big brakes won't fit inside the rims for a 195 tire. The traction is perfectly adequate, the tire just falls apart. That's why the pro's go with bigger tires.

But for your original question, it's a complicated trade off:

More camber in the front will get you the most traction but your tires will wear the inside edge.

Stiffer rear will give you a harsher ride.

Lowering front air pressure will give you a risk of bent rims on potholes and tire failure if you go too low.

You decide.

ed

edmcguirk

Actually it's still static friction. Dynamic friction is sliding friction. I believe dynamic is always less than static. That's why you don't want to slide your tires.

ed

scathcart

Makes sense to me. I am in fourth year mechanical engineering, so yes, I actually understand this stuff.

RX-Heven

iTrader: (1)

Ever hear of a slip angle?

gerbraldy

I have 205/60/15's on the front of my vert..I understeer sometimes, but I know most of it is my big *** ugly tires, they fold like a mF

wpgrexx

more camber in the front. Bigger anti-sway bars in the rear, strut tower bar in the rear, rear steer eliminator bushings. Adjustable end links for the rear

Valkyrie

The cheapest way to eliminate understeer is to use your brakes

edmcguirk

Slip angle is still static friction. The front of the tire meets the ground at zero rubber stretch. As the tire rolls forward, the rubber begins to stretch sideways. That creates the slip angle. If the rubber stretch exceeds the limit, the tire begins to break away at the back of the contact patch where the rubber is stretched the most. That's why the steering wheel begins to feel lighter as the tire begins to break away. It's because the center of the contact patch is moving forward (the back of the contact patch is losing traction). That moves the center of the contact patch closer to the center of the steering axis which effectively reduces the trail and the centering force.

ed

VietFC

you people argue too much... just try out differnt things and see for yourself.... there's truth in some of the things everyone said... but jeez... you don't have to try to fight with everyone because you think you're right... just give some advice and it's all good... he may try it out and see which is best in his opinion...

SnowmanSteiner

The Static friction coefficient is what is needed to move an object from rest to motion, this will always be higher. Dynamic friction is the amount of force that is opposing motion once it is moving. Since the tire is already moving there is no static friction, only dynamic friction.

- Steiner

edmcguirk

I'm not really trying to be a pain in the *** but...

The car and tire may be moving but the rubber on the contact patch is moving at zero miles per hour relative to the ground. It's static.

ed

rs_1101

thank you. unfortunatly understeer vs. oversteer has many many theories.
the generally accepted version is that understeer is a loss of front grip, and that oversteer is a loss of rear grip. but there are so many many different scenarios where this doesnt apply.
take drifting for example. now drifting, in appearance looks like a controlled oversteer correct? however, how many times have you heard the word 'LSD' used in reference to drifting? strange.. why would i increase rear grip to make my car oversteer? is drifting really a function of understeer? it gets confusing really really quickly.
so lets cover whats been suggested here
tire pressure adjustment- great for fine tuning, cheapest of all
front camber change- probably the best way to go, will give your car excellent turn in capabilities
rear sway bar change- in experience alittle less effective. if you go too stiff, your car will have irregular understeer-oversteer transition. either get the whole set, or (if you have a base model) go with a turbo2 rear bar. it should be about 200% stiffer that the stock one.
the rear sway works by transferring load to the front outer tire. which would bring us back to the tire width discussion.
in reference to that, please remember that drag cars usually only go in a straight line. acceleration grip seems to function differently than turning grip. im running 195 falken azenzis sports on my se, and i have taken down just about every car you can think of on a freeway offramp. including an m3.