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Discussion in 'GT4 Tuning' started by Scaff, Mar 18, 2005.
Well if it didnt look rediculous you'd go faster on the straights. read my edit.
Well that can be your little secret maybe you go to a drag strip and try out your theory
You'll have less rolling resistance, fair enough, but sod all else
As the article says its very hard to calculate the PMI for an individual car, and should be taken as a rule of thumb.
A 306 Rallye (great car by the way) would have a fairly low COG, if its lower than that of a Supra I honestly could not say.
What we can say is that if you needed to change direction rapidly, the 306 has a lot less mass that needs to move around the CM than the Supra.
The upshot is that as a general rule a heavier car will normally have a higher PMI and therefore handle transient forces less well than a car with a lower PMI.
As for the 306 being quicker round a track, then it depends on the track.
A highly technical track with short straights would favour the 306, as the higher transient forces in the corners would benefit a car with a lower PMI; while a faster track would benefit the extra power that the Supra has due to the lower transient forces involved in the corners.
In GT4 terms a course like Autumn Ring would generally favour the 306 and a circuit like Fuji would favour the Supra.
However, you must remember that all of this is theoretical (very in this example), but it does serve as a good rough guide.
Well, yes, definitely depends on the track as his top speed is some 20mph higher than mine
A most informative and superb thread.
I read the 1st 6 or so pages, and I have a question. I'm sorry if it has already been asked:
How does downforce affect braking? I know it allows faster corner speeds. Does it influence braking at all?
Welcome, you should have carried on to page 10 (but it is a lot to go through), downforce & braking is something we are just starting to look at this.
Have a look at the following post
SL 65 AMG- 0.3 seconds
I wonder if ever anyone used the data logger to do some tests in GT4? Just to see how accurate (or inaccurate) the simulation is and to see what would be the best method to improve each cars brake distances.
So while playing around with a '03 Viper SRT10, I had the impression that this car seems to have unusually long stopping distances (this was just an impression of course, coming from the fact that it accelerates much faster than many other cars, and thus it's required to brake much earlier when approaching corners).
The data logger shows rather accurate speed-distance tables, so it seems suited to do some comparisons. For this i drove the Viper on the Test Course at 250 km/h (155 mph) and shortly after crossing start/finish applied full brake power.
After coming to a halt, I finished the lap (else the data logger wouldn't show anything).
The Viper was modified with a supercharger and wings (downforce settings 8 front, 12 rear).
Test 1, car weight 1533kg, Sport2 tires:
stock brakes: 175.2m
racing brakes: 175.3m
Test 2, car weight 1533kg, Normal3 tires:
stock brakes: 206.6m
racing brakes: 205.2m
Conclusion: No difference between stock and racing brakes (the data loggers resolution comes to about +/- 1m when reading the table), but a difference of about 30 meters between normal and sport tires.
In another test I stopped the car from 250km/h, accelerated from 0 to 250 and stopped again. Then compared the stopping distances between normal and racing brakes
Test 3, car weight 1533kg, Sport2 tires:
stock brakes: 173m then 176.1m
racing brakes: 175m then 174m
Only minimal differences, unsure if this comes from inaccuracies in the test or if the game simulates brake temperatures. Have to think about other tests which put more stress on the brakes over longer periods of time.
Finaly I purchased Weight Reduction 1, reducing vehicle weight to 1349kg.
Test 4, car weight 1349kg, Sport2 tires:
stock brakes: 175.3m
No difference in braking distance compared to the stock vehicle (1533kg).
//edit: just found fido69's test (this thread is huge, anyone willing to write an index? ) which give similar results
Interesting stuff Lazydog, I also use the Data Logger for these tests, its a handy tool, just a pain that you have to finish the lap to get the results (well its a pain on the test course).
Thanks for the welcome and thanks for the links. I knew I should have read further.
I really laughed at what Fido69 said after carrying out his tests, more specifically : "Emotionally and physiologically, I "feel" that racing brakes improve my lap times. I "feel" that I am able to brake later and better, but I can find no scientific reason to say that they should.
Very well said.
I know what you mean, I have said a similar thing in a post (somewhere in here), its realy hard to describe, but when using the DFP the brakes just seem to offer better feel and modulation when you have fitted sports brakes.
Its a very subtle effect, and seems well done.
Maybe we just get better trail-braking control using racing brakes? I know I get better times whenever I buy the racing brakes, but I don't know why and tht bugs me.
Is it "real" or is it psychological? More testing needed...
maybe you are looking in to this a 'little' too deeply ?
ever noticed brake fade in gt4 ?? I honestly don't know as I just play it as a 'game'...but I doubt it!
Funny, i always buy racing brakes for tougher races and i always did seem to do better. A little odd if there is no difference. Tyres and the right ASM settings obviously have far more of an influence...
there is a big difference in the racing brakes it improves your handeling decreases your tire wear and you have a shorter brake distance
The data logger tests mentioned earlier seemed to show otherwise...
The racing brakes do not improve braking distances, this has been demonstrated by a number of people (myself included) using a range of cars and the data logger. Please read the last 3-4 pages were the reasons for this have been explained in detail.
Improvements in handling, I think this would depend on how you describe handling; but certainly the racing brakes do seem to improve brake feel and modulation.
I'm not sure how the racing brakes would help tyre wear in GT4, any gain from the improvements to feel and modulation would be small gains and one of the major causes of tyre wear through braking would be locking the tyres, and while racing brakes may help you avoid locking up, so would better brake control.
Thank you for your comment and opinion.
However, I (and I believe a good number of others) have an interest in seeing to what level real world physics have been built into the physics engine of GT4. Quite honestly it has suprised me just how much detail they hav managed to get into just the area of braking.
And, just as in the real world, an understanding of the physics can and will help a person become a better driver.
If I (and others) want to look into this deeply, why would this be a problem, if its not of interest to you, don't read it.
Sorry if the above sounds a little short, but I fail to see how your post has helped further this thread.
well, as I said before brake fade is THE problem when doing trackdays, GT4 doesn'y model that....does it ?? (honest question)
look in to it as much as you like, as you say, was just my opinion....
Exactly how much brake feel and modulation apply to the primitive PS2 controls is debatable
I can't imagine they'd have included the racing brakes mods if they didn't make any difference. They must do something.
I wish they had included brake fade in the physics but I don't think they have. Nor have they simulated glowing brake discs (that I'm aware of)
Maybe we'll get that in GT5?
The change to 'feel' and 'modulation' with the DS2 is not noticable (to me any way), but with the DFP a difference can be felt, but it is subtle.
Brake fade I am reserving judgement on at the moment, I want to fully test this before I make my mid up, I doubt it is included, but only one way to find out.
Not noticed glowing discs (but have not looked for them), can't believe they have missed that one!!!
Could you expound on this further? Assuming there are no aerodynamic aids (downforce), I don't think this to be true. The normal force on the tire doesn't increase as velocity increases. I don't see how you need more force on the brakes in order to lock them at higher velocities. I'm also assuming that weight transfer is the same at 100mph as it is at 40mph. (That should be correct, right?)
I'll try. (But I'm trying to be good at work, so I'll make it short...)
The main difference that I see is that at 100mph the tires and wheels are spinning much faster than at 40mph. Duh.
[snip] (I had launched into a whole thing about angular momentum and torque... but realized it was bunk)
In all our previous talk, we completely ignore the fact that the wheels are spinning objects and that the brake pads act on the spinning disk. This method of friction is fundamentally different from the tire-asphalt interaction. The tires ROLL along the ground. The brake pads SLIDE across the disk. I.e., the brakes act using dynamic friction, the tires use a kind of static friction.
The dynamic coefficient friction is lower at greater rates of speed, but as you slow down, the coefficient increases and approaches a static friction coeffiecient, and therefore you don't need to press on the brake pedal as hard to create the same amount of force at the tire.
Did that make sense?
I'm sure I can find graphs of dynamic friction versus velocity somewhere...
Of course it's more complicated than I stated:
See the plot in this paper.
Initially, the CF decreases with increasing velocity, but then it starts to increase again. So depending on at what velocity your car is at, the braking force needed may increase or decrease as you slow down. Or, it may actually increase and then decrease! Braking is not an easy skill!
I wish GT5 would turn off the automatic traction control for both braking and accelerating, and let us sim freaks have fun looking for the proper gas/brake pressure to optimize accel/deceleration. What fun that'd be!
Excellent stuff, and thanks for the reply Fido, had a 14 hour day with 4 hours travel, and would have been too beat to have put together a coherant reply.
No problem. It was a good question. I enjoyed thinking about it.
Hope you got some sleep. GT4's got me waaay into sleep depravation. And now I have some more ideas on racing brakes to test, too. Perhaps the sensitivity of the DFP brake pedals is increased with racing brakes? Then the dynamic application of brake force will increase (i.e., press the brake pedal down 50% results in 90% brake force?). This would result in faster application of braking.
During my testing of the 100-0 braking distances, I purposefully eliminated this effect by starting at ~102mph and then using the data logger to give me 100-0 braking distance. I wanted to eliminate the "variable" of my stomping on the brakes differently between different runs...
hmm, see? Experimenter ignores results he didn't like.
Good point. I completely overlooked that fact.
Brushing up on my rusty physics, IIRC F=(m*v²)/r.
m and r will be constant at 40mph and 100mph, so that leaves us with F ~ v²
10000 is 625% greater than 1600...
Which would agree with what Scaff originally said.
I speculate that that additional 625% angular force is rather insignificant compared to the force needed to decelerate the rest of the car. With further speculation, it would seem that it's not significantly more difficult to lock the wheels under braking at 100mph+ speeds.
With regards to dynamic friction coefficients, I guess someone could indirectly do a experiment plotting acceleration (deceleration) vs. brake pedal pressure and see if it holds true for the specially formulated car brake pads we use on our real world cars...
I don't think you have the formula right. I tried doing the same thing but when I searched the formulas I found:
torque (T) = angular acceleration(alpha) x moment of inertia (I)
for a wheel of radius R,
alpha = a / R
I = k x mass of wheel x R^2
(where k is some constant based on the geometry of the wheel. Constant assuming all sorts of stuff about the stiffness of the wheel, temperature, and other variables, of course)
Since our 'a' (deceleration) is constant (we assume; based on the tires) between the car at 40mph and the car at 100mph, alpha is constant. Since alpha is constant the Torque is constant. Constant torque means constant amount of brake pedal force, assuming the CF stays constant...
Which it doesn't.
BTW, the wheels have both angular momentum AND linear momentum which is why it is doubly important to reduce their mass. Not only do you have to accelerate them linearly, you also have to spin them up to speed.
This is well known to bicycle riders, who follow the rule, "1gram from the wheel is worth 2 from the frame".
This was posted by myself a while back by myself. However with some new info I don't totally agree with it anymore.
While CF does not directly depend on surface area at all ( If you dont agree, google it) , It does depend on load. And since CF will generally decrease with load then 4 tires supporting 25% of a given load will have more traction then 2 tires supporting 50% each. This also applies to wider tires since there will be less pressure (psi, Nm^2) applied on the contact area, and therefore they will have a higher CF. What I said in my original post is still valid to a certain extent; this simply adds to it to make it more correct.
Anyways I'm just bringing it up again cus I don't like to post inaccurate information.