It's all about mu

[brianc]

the mu in this one is strong.

[s2d4]

Btw, Homer is at best an operator....

[Dave-ROR]

Originally Posted by wparsons

Every wheeled vehicle by definition has a track width and wheelbase (forgetting a segway or unicycle), or are you talking about a track as in a tank tread? If so, why on earth are you bringing up skid steer vehicles in this discussion?

Are you claiming that on a bicycle you need to turn the bars to the left to

My first thought was "UNICYCLE!!!" but you covered that.

As for the bicycle bit, he's sorta right. Although I've rarely done so on a pedal bike, on a motorcycle you countersteer to lean the bike over initially, the steering then corrects and you turn.

FWIW, I can't drive for shit in GT5. I mean granted I can't really drive for shit in real life but I rarely hit walls in real life....

As for understeer being faster I think it, in practical terms, depends on the driver. All drivers have preferences. It's *easier* to drive a car that exhibits slight understeer. This makes for a better endurance car in general as 2 hour stints are tiring enough without needing to be at the limits of a loose car. Our endurance car is neutral with steady throttle (reasonable throttle, 100% is "steady" also but...), at full throttle it can push a bit, lift and it rotates. It's about perfect IMO for an endurance car. My track car has much stiffer springs, but more importantly instead of equal rates all around slightly more rear spring.. most important is a sway bar that's significantly thicker and mounted directly to the chassis (oem on the track car is on standoffs that flexed further reducing effectiveness). The track car can slightly oversteer (technically it's neutral but if you aren't very smooth with inputs it will come around under steady throttle at the limits) steady throttle, is neutral underthrottle and a handful if you lift (or hit a small bump, pebble, drop of oil, etc). The track car is significantly faster around the track too. I also prefer loose cars. :shrug:

[Kevstra]

Suberman is giving Calgarians a bad rep.

You insult others' English speaking skills, and yet, it's you who can't form a coherent argument. You manage to say a lot without actually saying anything. It seems to me your train of though goes thus:

1. There is a thing called Mu. It is constant.

2. Therefore a shift of relative sway stiffness to the front of the vehicle, inducing understeer, makes the car quicker around the track.

That's my best interpretation. Where the hell does your conclusion come from? So there's Mu; what does that matter? What does that tell you about the performance of the car? Mu is only a characteristic of the tire. It doesn't tell you anything about the car's handling or performance. Normal acceleration in a circular travel path (measured in m/s², ft/s², or G's) tells you a bit about what a chassis and suspension setup can do with a given tire, and thus, a given Mu.

Remember that Mu is in fact a function of temperature (among other things) as maloney2 has pointed out. Let's ignore it's relationship to the applied normal and shear stresses at the tires surface, and hold temperature constant. Now Mu is constant (more or less).

So what? It is just a potential grip for a given normal force. How can one make a statement about front-to-back sway balance from that?

Getting around a corner as fast as possible is incredibly complicated, as there are a near limitless number of variables. Ultimately, what you're trying to do in a corner is distribute both the gravitational weight (down) and "inertial weight" (horizontal - both normal and tangential to the path of travel) in a way that makes best use of your available Mu. You then try to design a suspension setup that maximizes the size of the tire contact patches under cornering. THAT subject is one that degrees can be based on (bachelor, master, and doctoral) and is one that you, myself, and most of the people on this forum don't fully understand.

Just because you drive a Subaru in the winter in Calgary and you know about a thing called Mu, you aren't an expert on suspension and handling. You should STFU about your claims and test them if you're so sure about them. Then go and eat some humble pie - ask more questions and don't be so blindly certain about things. The discussion about suspension and handling is an interesting one when the participants aren't making stupid claims based on nothing.

[drewbot]

In before rebuttal

[Dave-ROR]

Originally Posted by wheelhaus

Actually the part about the bike is somewhat correct. On a bicycle, the effect is nearly undetectable because you are 90% of the total weight of the machine and shifting your weight has a dramatic effect on the bicycle and its wheels. At high speed its more noticeable. A motorcycle is a better example, where you are only a portion of the total weight and have to actually steer the bike. To steer a motorcycle at speed you actually "counter steer" by pushing the bars opposite the desired direction of turn, and it steers the tires wide and leans the CG inward. The bike will attempt to right itself due to rake and trail steering geometry, so you maintain or increase this pressure to hold a line in a corner. The angle of steering is minute, bar movement is nearly undetectable. On a bicycle, you're moving pretty slowly, and this weight shift happens almost instantly, that you're then steering into the turn to maintain balance.

Understeer may not be faster per se, but it does help stabilize the car. A car that wants to oversteer more readily can cause driver stress. Sometimes a chassis tuned for a bit of understeer is just enough to reduce driver stress and allow him to push harder or drive for longer stints in an endurance race, which may result in faster times.

Yeah so if I would have read the rest of the thread before replying I could have saved myself some time.

[Dave-ROR]

Oh yeah, I merged these threads since it's pointless to have two...

[KPDoubleX]

Originally Posted by 7thgear

But.....but......how do those flipflops stay on

[IronSean]

Originally Posted by maloney2

Hi Everyone,

This is my first post here so I figure I should introduce myself. My name is Sean. Last year I finished my masters in mechanical engineering studying vehicle dynamics and tire modelling. I have attached my thesis for the curious reader and to hopefully have somebody actually read the damn thing since I spent 2 years developing it! I have been participating in grassroots racing for nearly 10 years driving a wide variety of cars including a Hyundai accent, Toyota mr2 Spyder, Toyota Celica GT4, Mazda Rx-7 and now the BRZ.

I would certainly not say I am an expert, but I do have some insight on this topic and after reading several posts from suberman I would like to clarify some of the false information.

First I would like to make something abundantly clear

THE COEFFICIENT OF FRICTION IS NOT CONSTANT UNDER ANY CIRCUMSTANCES!

The effective coefficient of friction for a given tire is a function of not only temperature, pressure, age, and road conditions, but most importantly normal load. That's right the coefficient of friction is dependent on the normal load

MuFn = Ft
Where, Mu f(T,P,t,Fn)

Where Mu = coefficient of friction, T = temperature, P = Pressure, t = time, Fn = Normal load, and Ft = Tire lateral force

I have attached a typical plot of lateral tire force response as a function of slip angle. Each line represents a different normal load. starting at a normal load of 200 Newtons and incrementing by 200 Newtons up to 1000 Newtons. (The maximum normal load is relatively low because this is a 13" FSAE tire)

Several things can be determined from this plot.

1) The peak lateral tire grip occurs at a different slip angle depending on normal load
2) The reduction in performance after the tire is fully saturated is more dramatic at higher normal loads
3) The increase in lateral grip with increased normal loads is a situation of diminishing returns. This is most easily seen by looking at the right side of the graph after each tire has saturated, but this is also true when comparing the peak lateral force at each normal load.

The third point is very critical and is the reason roll bar tuning allows for the designer to change the balance of the vehicle between oversteer and understeer.

As the vehicle corners, weight is transferred from the inside tires to the outside tires due to the roll moment generated at the center of gravity. Now if the coefficient of friction were the same than it wouldn't matter how much weight transferred since the sum of the lateral grip between the left and right sides would always be equal.

Since the coefficient of friction diminishes with increased normal load, we can tune the amount of relative weight transfer between the front and rear axle in order to force one end of the vehicle to generate more lateral force and thus generate a yaw moment. This diminishing coefficient of friction is also why a lower center of gravity increases overall grip.

Assume every tire is exactly the same at the same temperature,age, etc.

Also assuming the chassis is a perfectly rigid body, the roll angle must be the same between the front and rear axle of the vehicle. This means that if the roll stiffness of the rear of the vehicle (roll bar plus spring) is higher than the front of the vehicle than more weight transfer will occur on that axle. Because of the diminishing coefficient of friction, the rear axle will generate less force overall and cause the vehicle to yaw into the turn. The opposite is also true, more front axle roll stiffness will cause the vehicle to yaw away from the turn.

This is only one of several factors that affect the handling of the vehicle, but to attribute the handling characteristics to one factor as suberman has in regards to the rear wheel geometry of the vehicle is utter non-sense. The overall vehicle handling is a factor of all of these things and the relative weight of the suspension characteristics, tire characteristics and environmental characteristics on the system response must be evaluated in each handling situation in order to understand how much one of these factors contributes to the purportedly undesired oversteering tendency of the vehicle.

suberman if you feel the vehicle has too high a tendency to oversteer this can easily be remedied by changing the roll stiffness of the vehicle, changing tires, wheel alignment, or rear differential tuning, depending on the situation you find the vehicle oversteering too greatly in.

I can assure you the suspension is not set up from the factory to oversteer inherently as this would result in too many accidents and lawsuits.

The one thing Subaru did to allow a bit of fun with the car is put on those skinny tires so that the car can power over with its chassis twisting 150 ft-lbs of torque.

I hope this post was informative. Here is a link to my thesis for anyone interested.

http://137.207.14.230/vdc/downloads/...loney_masc.pdf

Sean

I had to do a double take, since I'm also Sean, also have an Engineering background, and was coming here with some similar information.

The only other tidbit I was going to add was that Mu is the biggest fudge factor we have in physics and engineering projects for a lot of reasons, some of which you mention: It's rarely constant, hard to accurately predict, depends on a foolishly large number of factors, isn't actually that well understood. In it's current form is literally "we found that these two substances usually produce this proportion of frictional force compared to Normal force."

[Kazuya]

This thread is leaking out of the Canadian forums.

[wbradley]

So we now have several expert opinions, including two from engineers refuting Suberman's claim than Mu is constant. I wonder with whom the judge will rule?

Suberman I suggest you refrain from soliciting your professional services on this forum as that would be tantamount to ecoli offering catering services. LOL

[wparsons]

Don't worry, he'll claim everyone is still wrong, including the guy that actually races his car a lot, and include lots of nonsensical quotes that have zero relevance to any point he is trying to make!

[7thgear]

Originally Posted by Kazuya

This thread is leaking out of the Canadian forums.

finally, something useful coming out of Canada

[Dave-ROR]

Originally Posted by 7thgear

finally, something useful coming out of Canada

Excuse me.. Modern ice hockey..