nasanu
(Banned)
- 1,429
- Australia
- nasanu
- nasanu
Faster on grass?
- Thread starter armyk
- 59 comments
- 5,206 views
- 756
- Buenos Aires
- jaycee_xr3
I use to feel me faster on grass...
- 43
Yes, grip has nothing to do with terminal velocity. Terminal velocity is when a falling object stops accelerating. It's based on gravity and drag.
Grip has plenty to do with speed. It's not the only factor but a vehicle needs grip to even move. Grip assures maximum energy is converted to actually useful wheel movement.
If grip has no effect on top speed, then aerokits and wings wouldn't affect it either.
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- 7,669
- Exorcet
- OE Exorcet
- 43
Kits create drag and reduce straight-line speed. I just wanted to throw that out there as it pertains somewhat to top speed while cornering.
A vehicle very well might eventually reach top speed on grass but it'll take a lot longer and a larger percentage of wheel spin will always be wasted as compared to driving on asphalt.
Maintaining top speed requires less energy than achieving it. On a low-friction surface, you will pouring energy in to just getting speed for longer. That energy is already being partially wasted with wheel spin, however.
If you suddenly go off track and hit grass, the sudden loss of grip might cause momentum to play a larger role possibly and slip increases. The result might be an initial speed boost until wheels spin and inertia slows the vehicle.
Grass has more rolling resistance, increasing the vehicles tendency to slow and requiring more energy to move the wheels.
The effect is power wasted trying to move the wheels and when they finally do, there's nothing to provide resistant friction.
As far as real-life is concerned, if vehicles were faster on grass then drag races and land-speed records would be set on grass.
The Bonneville salt flats might be low-friction but at least they have better rolling resistance. Still, tyre-driven vehicles must be tuned to maximize speed. This discussion is about road/track tunes.
I assume PD would simulate basic principles of physics regarding a terrain type present on most every course.
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- 600
- DeolTheBeast
On Indy oval with the X1 you can gain another 20mph going along the grass on the straights. Even with assists turned off it's still kinda weird.
- 7,669
- Exorcet
- OE Exorcet
Well the wheels will only spin if they can't produce enough traction to match the output from the engine. Real grass/soil will produce additional problems beyond traction that could lower speed. But GT's grass is basically slippery road. That's why you'll go faster in GT. I'm not saying that grass will increase speed in reality.
Energy in that paragraph should be power (or energy/time). And yes, it is wasted with wheelspin, but tires won't spin automatically just because they're rolling over a lower friction surface. Downforce can also increase there grip, regardless of the surface.
Momentum can't increase your speed. Momentum and inertia will only serve to work against acceleration of any kind, be it slowing down or speeding up. The wheels may or may not spin once they transition to lower friction surface. If they do spin, the car slows down. If they don't, the car will speed up from reduced friction.
I don't know if real grass has more rolling resistance than asphalt, but I do know that grass and soil deforms more easily. This is probably where most speed loss comes from; the tire deforms the ground and as a result a portion of the normal force pushes back on the car.
GT grass on the other hand doesn't deform. So the additional friction is missing.
I think they did. But only the basic ones. Deforming soil would be complex to code and is probably unnecessary. That's why I think GT grass speeds you up.
- 1,255
- Indiana
- camaroyenko
- No....just no.
On the Indy 500 race I always went into the grass to pick up a couple extra mph xD
- 692
- Brighton/UK
- GTP_janimal
I'm not a physicist, but I have lived with & worked with them for a long time. As far as I understand it this is an absolute.
When driving on tarmac at 200kph let's imagine. If you engage the clutch so no power is transimmted to the wheels the car will begin to slow due to friction, inertia, wind resistance etc... momentum is not preserverd except in a vacuum when not in contact with any other surface.
When rubber tyres hit grass, the grip is substantially reduced. This reduces the amount of power that can be converted into forward motion. Consequently the spinning tyres translate less energy into forward motion and the natural effects of inertia, wind resistance and friction (yes your tyre might be spinning but there is still friction as the spinning tyres slide across the grass) reduce momentum.
It should not be possible at all for speed to increase as the tarmac, tyre boundary is not slowing you down but actually providing you with forward motion.
There may be some extremely rare environmental conditions that mean that grass can provide a better grip surface than tarmac but I doubt it.
If grass was really faster than tarmac they would make race circuits out of grass right?
[edit] If you look at balistic trajectories you see a similar disconnect between reality and perception. When you ask someone to describe what happens when a ball is fired into the sky at say a 40º most lay people will describe the ball as accelerating until it reaches the peak of the ballistic arc. This however is completely false the ball will be losing momentum from the moment it leaves the barrel. We perceive it as acceleration because it is rising, but in reality it just takes time for the balls momentum to reduce enough for gravity to exert its effect.
Someone earlier in the thread suggested that you might perceive an increase in momentum simply because any braking you are applying to avoid the situation is no longer (as) effective once your tyres leave the grippy tarmac. This sounds quite plausible for a real life explanation of why someone might perceive their speed to increase. Unfortunately I don't think it can explain an 10 - 18kph increase on the speedo in GT. So either the physics isn't quite right [edit- and I wouldn't expect a racing game to have to code the physics engine to this level] or they modelled the speedo measuring wheel rotation (and consequently wheel spin on reduced grip surfaces) rather than velocity.
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- 2,747
- Morgoth_666
The only way grass will ever make you go faster in real life is if you are travelling fairly slowly and then cross over into a downhill section of very wet grass, causing gravity to overwhelm your tires' grip. It's the same thing that happens on ice, and I don't think you'll see too many cars faster on ice than on asphalt.
The cliche simply comes from the fact that when you are trying to slow down and hit wet grass your tires lock and you instantly stop slowing down nearly as fast as you were, so it feels as if you sped up due to g-forces on your body.
I doubt it's in anything but a few rare specific instances(if ever outside a near vacuum) that rolling resistance from the road surface itself is holding you back more than aero drag.
If grass increases top speed in GT5, GT5 is wrong. Again.
The cliche simply comes from the fact that when you are trying to slow down and hit wet grass your tires lock and you instantly stop slowing down nearly as fast as you were, so it feels as if you sped up due to g-forces on your body.
I doubt it's in anything but a few rare specific instances(if ever outside a near vacuum) that rolling resistance from the road surface itself is holding you back more than aero drag.
If grass increases top speed in GT5, GT5 is wrong. Again.
- 43
I didn't mean to imply momentum increasing intentional velocity such as proceeding straight forward from tarmac to grass but instead unintentional velocity increasing momentum such as sliding off a turn or while braking accompanied by a sudden transition from traction to non-traction.
Sorry for any confusion in my writing.
- 7,669
- Exorcet
- OE Exorcet
I'm not sure that I completely understood your post, so this is a restatement of my post rather than a reply to yours.
Acceleration is any change in velocity. Positive acceleration means to go faster, negative acceleration means to slow down. Momentum opposes all acceleration no matter the sign. So the natural response for a car that is not under power that drives onto grass in both GT and real life is to lose speed. The loss of speed is due to air, ground, and transmission friction but not momentum or inertia.
Momentum conservation 1 - Reality
Momentum of a system is always conserved. Even with friction. When a car slows down, it transfers its momentum to the Earth. but the Earth is ~10^20 more massive than the car so no one notices that the movement of the Earth is being altered.
Momentum conservation 2 - GT
The Earth isn't modeled. Instead there is a massless surface (the race track). Although massless, it acts as if it has infinite mass (which is an approximation of the Earth compared to a car). So the momentum of the car can be thought of as lost. But it results in pretty much the same outcome as 1.
When the car is under power, things change. If the wheels grip, the car uses the same amount power to move forward, but there is less friction slowing down the undriven wheels. It will accelerate to a high speed faster than on tarmac. If the wheels slip, the car will slow down because it won't be able to generate as much force to push itself forward.
Thus, grass does not slow down the car because it is a lower friction surface. There must be something else responsible for a loss in speed. That would be energy robbed from the car by deformation of the soil (and possibly the grass itself) that the grass grows in.
- 692
- Brighton/UK
- GTP_janimal
You make complete sense and match my (uneducated) understanding so far
However....
this doesn't seem right and appears to contradict all you said up until this point.
The only thing keeping the car going up until it hits the grass is the energy transferred through the wheels, without that energy driving it forward the natural tendency (unless in a vacuum) is for the environment air friction, wind resistance, air pressure pushing the mass of the veihicle towards the earth etc.. to reduce it's momentum.
The fact that there is less grip on the grass reduces the ability of the power transferred through the tyres to maintain momentum.
You must go slower on grass except possibly in exceptional circumstances.
I'd appreciate any links you can provide to subtantiate the last part of your post
[edit] BTW I am only attempting to understand the real world physics, not how GT implements it.
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- 7,669
- Exorcet
- OE Exorcet
This is the key point.
Less grip does not mean less force pushing the car.
http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html
See the graph on that webpage. For a wheel that grips the surface, the static coefficient of friction matters. Notice how the static friction force changes up to a maximum.
For a car, that maximum is the point where traction is lost.
The force that the wheels generate, which comes from the engine, must be less than the maximum friction force for the wheels to grip. Since the wheel force depends only on the engine, changing the maximum grip force doesn't change the wheel force until the max friction force is moved below the wheel force.
In short, you assume that when a wheel goes onto a low friction surface, that it must spin. But this is not true. It may or may not spin.
If you could find real life grass that someone was laid out as a perfectly flat and undeformable sheet, it would allow a car to reach a higher speed that it could on tarmac (at the cost of less cornering grip).
Unfortunately for speed freaks, real life grass is not a flat and rigid surface. The energy that car loses when driving on grass and soil is the energy that is absorbed by bent/uprooted grass, and compressed soil.
Does that make more sense?
- 336
- Australia
Could it be simplified or compared to say, one wheel with enough grip to transfer the acceleration from the engine (where the other wheels have no traction), is better than four wheels where not all grip is utilized to accelerate the car (because the engine is using most of its power pushing the air). The former having less rolling resistance?
- 692
- Brighton/UK
- GTP_janimal
well to be honest not immediately, but that's why I only have physicist friends rather than being one myself
I am quite good at understanding the low-end common sense stuff but after that I really have to try.
Unfortunately I am drunk right now, so I'll have to check out the link in the morning when I have a better chance of understanding it.
Usually though when it gets this specific I have to ask a physicist to explain it to me in terms I understand - is there a blush icon? Then again they do reckon I am pretty good at picking up this stuff once I can get over the terms & maths that I don't have a deep understanding of so there's hope for me yet.
[edit] having looked at the link there is no chance of me overcoming the language used without assistance. I feel that makes me sound stupid but I do have a degree in Software Engineering and have spent 15 years modelling( simulating) and optimising chemical plants and specific chemical processes. However my speciality is in creating the software architecture and overall design rather than the core algorithms. I do have to understand some fairly complex maths and concepts to design these things, but I spend a lot of time asking domain experts to talk to me like a child, ie from first foundations to explain every principle that underpins the core mathematics.
I definitely want to understand this though so I'll be asking a few Phd's who know me well to help me model it in my head.
Thanks for the robust reply
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- 7,669
- Exorcet
- OE Exorcet
Maybe this modified graph will help
The thick black, green, and orange solid lines represent the maximum friction force for different surfaces. The thick blue line is the force at the wheels at a certain speed. Each of these surfaces (black, green, oranage) has its own friction curve. They all look like the thin black curve that was on the graph at the beginning. The only difference is the height of the peak. Each surface's peak intersects it's max friction value.
The wheel will grip or slip depending on if the force at the wheels exceeds the max force for the surfaces or not.
Clearly, the black and green surfaces can take more force than the wheel is exerting, so if the car drove from:
-Black surface to green surface
-Green surface to black surface
The wheels would refuse to slip and the car would have the same forward force pushing it.
However, the orange surface's maximum force is well below the wheel force. If the wheel went front black or green to orange, the wheel would begin spinning and the car would lose some of its propelling force.
EDIT
Also, reading the whole webpage led me to a section on rolling resistance. It's not very long or detailed, but it did remind me that a perfect circle rolling without slip on a surface produces no friction because the contact patch is zero. Real tires deform and have a non zero contact patch. This is what produces rolling friction - which is essentially static friction but not traction. And since static friction decreases with decreasing coefficient of friction, it supports my statement that a slicker surface increases speed.
The thick black, green, and orange solid lines represent the maximum friction force for different surfaces. The thick blue line is the force at the wheels at a certain speed. Each of these surfaces (black, green, oranage) has its own friction curve. They all look like the thin black curve that was on the graph at the beginning. The only difference is the height of the peak. Each surface's peak intersects it's max friction value.
The wheel will grip or slip depending on if the force at the wheels exceeds the max force for the surfaces or not.
Clearly, the black and green surfaces can take more force than the wheel is exerting, so if the car drove from:
-Black surface to green surface
-Green surface to black surface
The wheels would refuse to slip and the car would have the same forward force pushing it.
However, the orange surface's maximum force is well below the wheel force. If the wheel went front black or green to orange, the wheel would begin spinning and the car would lose some of its propelling force.
EDIT
Also, reading the whole webpage led me to a section on rolling resistance. It's not very long or detailed, but it did remind me that a perfect circle rolling without slip on a surface produces no friction because the contact patch is zero. Real tires deform and have a non zero contact patch. This is what produces rolling friction - which is essentially static friction but not traction. And since static friction decreases with decreasing coefficient of friction, it supports my statement that a slicker surface increases speed.
- 692
- Brighton/UK
- GTP_janimal
Maybe this modified graph will help
The thick black, green, and orange solid lines represent the maximum friction force for different surfaces. The thick blue line is the force at the wheels at a certain speed. Each of these surfaces (black, green, oranage) has its own friction curve. They all look like the thin black curve that was on the graph at the beginning. The only difference is the height of the peak. Each surface's peak intersects it's max friction value.
The wheel will grip or slip depending on if the force at the wheels exceeds the max force for the surfaces or not.
Clearly, the black and green surfaces can take more force than the wheel is exerting, so if the car drove from:
-Black surface to green surface
-Green surface to black surface
The wheels would refuse to slip and the car would have the same forward force pushing it.
However, the orange surface's maximum force is well below the wheel force. If the wheel went front black or green to orange, the wheel would begin spinning and the car would lose some of its propelling force.
I understand this bit, well I think I do but maybe my inablity to relate it to going faster on grass might suggest otherwise.
So which of these lines would represent grass and which would represent tarmac?
I can see how this might relate to wheel spin, but I don't how it relates to the following.
1) Without the energy being transferred from the tyre to the road the car would lose momentum - ie pull in the clutch, the car eventually stops
2) So the car is only maintaining forward momentum by transferring the rotational force of the tyre into linear movement along a surface.
3) Assuming - and I am assuming here, that grass has a lower static friction force ....
oh wait, I think maybe I understand.
The mass (car) already has momentum (not sure how to describe the kinetic energy already applied to the vehicle by n minutes of x energy transferred from the tyre to the road surface before it transfers from one surface eg black line to another surface eg. orange line )
Whether or not you are applying throttle the static friction force of the surface always applies
If the orange line is far enough below the black line then the force required from the engine / tyres to overcome the static friction of the black line (required to drive it forward) is ....
ah bugger I can't quite explain it in words (which is why people normally express these concepts in equations & graphs obviously) but yes, I can see it in my head now.
I think I really do see what you are saying.
If I can make that understanding drunk I'm confident I can apply it to the graph sober tomorrow.Thanks for that, much appreciated.
- 692
- Brighton/UK
- GTP_janimal
Maybe this modified graph will help
The thick black, green, and orange solid lines represent the maximum friction force for different surfaces. The thick blue line is the force at the wheels at a certain speed. Each of these surfaces (black, green, oranage) has its own friction curve. They all look like the thin black curve that was on the graph at the beginning. The only difference is the height of the peak. Each surface's peak intersects it's max friction value.
The wheel will grip or slip depending on if the force at the wheels exceeds the max force for the surfaces or not.
Clearly, the black and green surfaces can take more force than the wheel is exerting, so if the car drove from:
-Black surface to green surface
-Green surface to black surface
The wheels would refuse to slip and the car would have the same forward force pushing it.
However, the orange surface's maximum force is well below the wheel force. If the wheel went front black or green to orange, the wheel would begin spinning and the car would lose some of its propelling force.
EDIT
Also, reading the whole webpage led me to a section on rolling resistance. It's not very long or detailed, but it did remind me that a perfect circle rolling without slip on a surface produces no friction because the contact patch is zero. Real tires deform and have a non zero contact patch. This is what produces rolling friction - which is essentially static friction but not traction. And since static friction decreases with decreasing coefficient of friction, it supports my statement that a slicker surface increases speed.
Ok you are confusing me again.
Essentially as I understand the world, to increase speed on a given surface requires an increase in energy. The velocity & momentum don't come from nowhere they have to be imparted from some other form of energy. In the case of a car that is from the engine driving the wheels against a surface.
If the friction at the tyre / surface boundary is decreased, less energy can be transferred resulting in a reduction of speed due to the environmental factors that act against the conservation of momentum.
Or am I talking gibberish here?
[edit] Actually I just noticed it is 4am here when the missus went to the loo quite noisily. I'm sure she knows someone was either wrong or interesting on the internet but still - I'm old, 4 am is way past my bedtime. g'night.
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nasanu
(Banned)
- 1,429
- Australia
- nasanu
- nasanu
Note that I avoided saying terminal velocity to avoid confusion. I was referring to top (terminal) speed and not saying anything about a falling object.
Grip will determine how fast a car can reach its top speed, but has nothing to do with determining it given a normal road and a normal car. No car has the power to spin the wheels on a normal road approaching its top speed. If it has so much power in reserve then it would not be near its top speed (things like gear ratios are really not relevant here).
Your last sentence is rather thoughtless. Aero kits affect top speed in various ways, both positive and negatively. Again an aero kit can provide down force and help a car accelerate faster from a medium speed, but we are simply talking about pure top speed given an infinite distance to achieve it. Grip is really not relevant as there is no car in GT5 that will not produce ample grip near its top speed.
I may be in need of correction here, but my understanding of friction necessitates that two objects must move against each other for there to be friction, ie the wheels must be spinning relative to the road for friction to come into play. Of course in a car at speed the wheels are still pushing against the road. However given that they are not exceeding the resistance induced by friction (spinning the wheels) the road surface does not matter, enough grip is enough grip and the low friction road will produce exactly the same speed as the high friction.
I think people are confusing friction with bumps in a surface. People do speed runs on salt planes not because of low friction but because they are flat.
- 598
- Tanzania
- 2,747
- Morgoth_666
- 7,669
- Exorcet
- OE Exorcet
Are you even thinking?
That is not true. For there to be a loss of energy transfer to the car, the friction must be decreased below a specific amount.
The coefficient of friction on perfectly dry road isn't actually constant. The road surface isn't perfect, so there are tiny variations, and at some point a tire will go from a higher grip section of tarmac to a lower grip one. Although the lower grip surface creates less friction, the wheel does not spin because the decreased friction peak is still above the tire's current level of grip.
It is only possible to have a frictionless wheel under two conditions:
1 The wheel is perfectly round
2 The coeff of friction is 0
We know 2 isn't the case in reality. Neither is 1. A tire deforms and creates a non zero area contact patch. The entire contact patch must be stationary in relation to the ground. But geometry says that for a round object, only one zero surface area spot is supposed to be still relative to the ground. The extra non zero area contact patch of the tire is what's creating the friction. So the tire is always producing unwanted friction, slipping or not.
Spin the wheels all you want, the speedo won't read anything other than the cars forward component of speed. It's pretty obvious.
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