What exactly is it that causes a surge in foward momentum during the transition from wheelspin conditions back to non-wheelspin conditions?
I realise it's a massive gain in traction, but that traction ramps up greatly long before the tires are rotating at the speed that matches the vehicles velocity. Why is that?
Driving in the snow today it occured to me that this effect is just as pronouced in the snow as it is on pavement....
I am no tire physics guru, so I was hoping someone could explain this to me in some level of detail.
It seems that this is present in LFS but so hard to tell w/o feeling the car under you. Perhaps a GMeter analysis will tell.
Since the tires rapidly loose grip as the speed differance of the tire and the surface it is in contact with increeses, then it only hold true the you will gain grip back at the same rate as the speed differences lessen.
This is not a liner equasion though but a cuve.
At least that is my theory and seems to be correct from seat of the pants feel in the real world.
There are two types of friction; dynamic and static. the coefficient of friction is a number relating the stikyness between any two surfaces. the coefficient of static friction is ALWAYS higher than that of dynamic friction.
think of it this way. when you're pushing a heavy box on the floor, it's hard to get it moving, but once it starts sliding it's much easier to keep it sliding. that's because you've gone from static to dynamic friction conditions.
with a spinning tire, as the tire slows down and is slipping less, more asperities on the tire's surface contact more asperities on the road surface and have more time to momentarily interlock and create a directional force.
did i explain that clearly? it's late and i'm over tired.
BTW, this is why it's faster to stop a moving car by NOT locking up a wheel. once a wheel locks you've moved into dynamic friction between the rubber and road surface and therefore lost a certain amount (approx 1/6 to 1/3) of your frictional force.
I don't believe it's the surge in momentum (acceleration) you feel, I think it's the surge in acceleration. (integral of acceleration, I think?)
The acceleration undergoes a sharp jump and then levels off, (which is in line with what you feel) while the momentum starts increasing more quickly, and continues in that way.
Hope I made sense. It's something I've thought about a bit, though don't have any fancy textbooks handy to back me up... but the human body appears to feel the change in acceleration, much more strongly than 'just' the acceleration, which can mean the game doesn't feel right, because you're not getting 'tricked' by the g forces.
Imagine that you have planted your foot to the floor and keep it there through out the duration of this manouver. At a standstill the power the engine is supplying overcomes the friction generated between the rubber and the tarmac by some considerable degree. Thus the spinning wheels. But as the car starts to move the fricional force between tyre and ground begins to lessen as the speed of the wheel begins to match that of the ground. (Outside of this thought experiment you do of course have throttle moderation and gear changes to consider but that just makes things way to complicated for tyre physics 101 . . .lol)
As the two speeds merge the friction generated by the ground starts to match and supersede that of the tyre and the engines ability to overcome that friction. At which point the tyre will stop spinning and give you that jolt that you asked about.
..to add to that comprehensive summary, when static friction conquers all and takes over, the jolt that you feel is the engine's torque passed through the transmission, causing some flex in the driveshafts and also a bit of suspension movement, all of which give the jolt more of a "boing" than a "bang".
I'm not very technical, and don't have much of an answer.. but I understand the question
Once in a while one questions happenings that you just took for granted for years. I felt that transition from dynamic back to static friction countless times, and it always made sense in my head that it happens (I guess I could say I had an abstract or intuitive understanding of it, but not a technical one). I never questioned the thoery behind it until now. Kind of one of those ".... so why exactly IS that anyway" moments!
I need help to understand this graph. Is it saying grip is optimal when tires are "slipping 20%"? Wouldn't that contradict the fact that static friction is always higher than dynamic friction?
afaik it's because the terms 'static friction' and 'dynamic friction' apply properly to smooth body friction problems, and real life isn't like that. You need a certain amount of slip because the tyres 'press' against the irregularities of the road surface, so it's not only friction.
If tyres were only 'static' and 'dynamic' friction (and I understand exactly what you mean) then it'd be impossible to generate more than 1g without downforce, and drag cars manage close to 7g prior to aerodynamic assistance.
But I don't know enough to elaborate (edit: much) further.
Yeah, with tires there is really never a time when static friction is acting. So what you are feeling is not the transition from dynamic to static friction, but simply the different amounts of dynamic friction you get under different dynamic circumstances.
And yeah, the slip ratio graph is basically the ratio of road speed to tyre surface speed. So you always get your optimum grip when the wheels are spinning slightly.
Usually the speed that we calculate from revs and gear ratio doesn't match. In fact these two never match unless no forces act on the tyres, which means the car is standing. When forces act on the tyre the tyre deforms, wether the friction is static or not, the tyre deforms and thus allows for the difference between the theoretical speed and the speed the tyres travel at.
Now let's increase the forces from zero to whatever the car is capable of:
1. The car is standing, no deformation (apart from the deformation due to the weight of the car, rather uninteresting here).
2. The car accelerats, the tyre deforms slightly. The forces that act on the tyre force it to increase contact with the asphalt, the tyre pulls itself into contact.
3. The car accelerates more. The tyre deforms as much as it will, yet the static friction isn't enough to keep the friction static and the wheel spins.
Following these thoughts the ideal point of tyre deformation would be where the tyre is being deformed as much as possible without going to purely dynamic friction. Tyre deformation leads to slipping. Thus there can be a link between slipping and ideal grip.
I always remember reading that while under acceleration/braking, optimum grip comes when the wheel is spinning at a speed approx 11-15% faster/slower than the speed of the car against the road.
Wouldn't have thought that was strictly true mate. Due to centrifugal force (Which doesn't exsist by the way, but lets not get into that now) the tyre deformation actually will reduce the contact patch with the road surface. Watch a Drag super alcohol or what ever they are called car light up it's rears. The deformation there reduces the contact patch by half. But the combination of soft rubber, heat and a very sticky road surface means that the drag cars can still utilise the available grip and get to do a 1/4 mile in 14 seconds or whatever they are doing it in now. But all that is why race cars have low profile rubber, less rubber walls to the tyre the less deformation you get in cornering, acceleration and braking. At least in closed wheel cars. Why open wheelers have larger wheels I don't know.
There are several factors affecting choice of profile for tires. You meant to say the open wheel cars have big tires and relatively small wheels.
Low profile tires are by no means better in longitudinal accelerations than higher profile ones. In fact, the reverse is usually true.
Reasons to use lower profile tires:
-Heavy weight of car makes developing an appropriately compliant yet also laterally stiff sidewall very difficult at large profiles. The race cars on low profile tires are to my knowledge all quite heavy.
-Larger diameter tire/wheel assemblies increase in weight in a non-linear fashion. Importantly, large wheels combined with low profile tires are far lighter than the opposite combo. Heavy rotating things are bad for race cars, as are heavy unsprung things.
-Large diameter wheel/tires add more frontal area to the vehicle.
-Heavy cars need large diameter brakes, which demands a large diameter wheel.
-edit: Another important one: Tires are relatively undamped compared to race cars. Bigger profile tire means more undamped suspension travel. Perhaps not as big of an issue for high downforce lightweight open wheel cars as it is for big saloons that find proper damping absolutely critical to achieving good grip.
Open wheel cars don't have these problems. Light cars with carbon brakes don't need big brake diameter. The tires can achieve massive lateral grip without rolling over, but remember that the lateral load on an F1 tire at 3g is still less than a big saloon at 1.2 or 1.4g
Watch more closely and often. They light up the rears in an attempt to heat the tires. When actually launching the car down the track, lighting the tires is a bad, bad idea. When used appropriately, the tire maintains a relatively low profile until centripetal acceleration stretches the thin sidewall as the car reaches the big end of the track. This has a beneficial constant change in gear ratio but also a detrimental effect on traction. The downsides to the thin wall are nothing compared to the advantage of slingshot launch.
If the tires get tall at the little end of the track, there is wheelspin and very little traction, and if not quickly corrected, a ugly mess.
As to the times, try 4 seconds. I've got a stockish front-driver that runs 14's...
The effects Vain describes are well known but perhaps not well understood outside of proprietary tire test data. Very, very fast superbikes often show a rear wheel speed much higher than a front wheel speed, despite no obvious (and catastrophic at those speeds) signs of wheelspin. The drag racing tire with its thin sidewall displays in a overall sense the opposite phenom because its diameter grows so wildly.
I think Vain's analysis is spot on. There is a point between pure "static" friction and pure "dynamic" friction that tires can sustain the largest forces in. That point is not easily defined and may well change rather dramatically on different surfaces and under different conditions.
Ever see F1 races where someone's launch control manages to pull four or five whole carlengths on the field in moments? If it were simple, no one would be doing that. A drag racer can tell you that the power required to pull several carlengths on a field of other racers vastly exceeds the power differential between the different teams. It has to come down to more subtle (but incredibly important) differences between teams.
The fact that who gets the best launch (amongst folks with similar fuel loads) seems to change very frequently suggest just how poorly understood or implemented these features are.
Makes sense, otherwise dragsters would have massive rims and wee little tires!
erm, I know you meant to say centrifugal (yes we all know it doesn't exist LOL)
Really! How much of a difference are you talking?
Finding the perfect launch on such a light car must be exceedingly difficult since it has such a comparatively low inertia and responds rapidly to small changes. I can appreciate the delicacy there!
Yea, but there you are entering into all the 'other' things that goes into getting a race car of the line. F1 is actually a bit unusual in not copping out into a rolling start for such powerful machines. (Although, as I found out earlier watching the qualifiying that they have 2 cylinders and 200 bhp less this year.)
Getting these cars off the line in anything like a competative fashion is a highly complex operation as highlighted above.
But I would still like to learn more about tyres. Obviously there is so much more to them than just friction and deformation.
There really is a huge amount to learn about tires. It seems that the tire industry is one of the most tight-lipped of automotive industries. Probably the best way to get a really great knowledge of how these things work is to get a job in the industry.
I find tires to be fascinating, and would definitely like to learn more about how they work.
Intrestingly enough, my mum did a lot of research on them for her course at university. Mr Goodyear died early because of how tyre's are made. The chemicals used to make it work when mixed make a toxic vapor, which damages the nervous system.
Bearing in mind my non-techy-tyre knowledge 
