Why is sliding in dirt track racing faster?

[rcgldr]

Surely the slip angle of the front wheels counts too, towards the direction the can axis points. If the front slip angle is greater than the rear slip angle, the car will be pointing outwards and vice versa.

Yes, I corrected my previous post to explain the car points inwards of the rear tires direction, not the direction of the front tires, or even the direction of the center of mass of the car, unless the car is in an oversteer (drift) condition.

Getting back to the dirt track issue, as previously mentioned, it's better to drift through turns because traction is low. I'm not sure of all the details, other than you get lateral and longitudinal grip when drifting on surfaces with reduced grip.

(Note I was unable to edit / correct my previous posts due to some sort of server error for several hours).

 

[ektrules]

If you could lower a set of driven wheels at the centre of the vehicle which were angled in the direction you wanted, they could be used to push the car inwards, into the curve when needed. But you don't have them so you need to point the car and use the wheels you've got. A vector thrust rocket engine could give the the same result. OMG, I wish I hadn't suggested that. Someone will go out and make one!

This line of reasoning is incorrect based on my understanding of Newtonian physics. There is already a force vector on every tire (rear and front) pointing into the turn when not sliding. That force vector is directly opposite of the momentum (right term?) vector trying to push the car sideways, "up" the track. Static friction > kinetic friction. Therefore, if the tire is sliding, there will be less force pushing the car into the turn, so you'll have to go slower if you want to stay on the track while sliding.

Forgive me for any mistakes in reasoning, or incorrect use of terms. It's been a couple years since I've taken a physics course. Computer science is my field :)

 

[sophiecentaur]

Every car is sliding sideways a bit on any corner (that's what the slip angle describes). It's just that the angle is small on normal surfaces. On dirt tracks there is a lot of slip and, without powering round a bend, the cars would have to go round 'unentertaingly' slowly.
Yes there is a force vector from each tyre but the force on the driven wheels is greater than if they were not driven (obviously?). So an extra set of wheels could provide more force in the wanted direction. Let's face it, if you could go round corners without use of the engine then that's just what drivers would do. I don't watch a lot of racing - particlularly not dirt track - but I see that they are always pointing as far into the bend as they can, subject to controlling an actual spin and they are producing a lot of dust and smoke from the back wheels, showing that there is significant drive coming from them.

I would go so far as to say that the ultimate dirt track vehicle would have four wheel drive plus four wheel steering. I have a feeling that very sophisticated control would be necessary to get it right, though.

 

[Ranger Mike]

ektrules

maybe if you can look at

this thread Race car physics Dec31-08 11:44 AM posted by Ranger Mike
will help..
one huge factor you did not address is weight transfer which adds downforce on the outside tires.,.this dynamic factor shreds a static scoring of tractive force of the tire

 

[sophiecentaur]

This hasn't been discussed as it can only be of secondary importance and doesn't apply at all with motorbikes.

 

[Ranger Mike]

Why is it often a faster way around a dirt track to slide around turns? The simplistic view that I got from Physics 101 says that the static friction coefficient is always higher than the sliding friction coefficient. So, that view would imply that a vehicle would be able to go around a turn faster if the tires were not sliding. That does seem to be the case with auto racing on asphalt, but not the case with dirt track racing. I'm guessing it has something to do with the fact that dirt is not static surface, but more like a collection of particles, almost like a fluid?

the original post asked a question about auto racing...did he not?
regarding the importance of dynamic weight transfer, i submit the following

huge weight transfer to the outside tires

 

[Ranger Mike]

i think the major misconception is that dirt is loose and not a good surface for traction...this is a photo of a sprint coming out of the turn and nailing the throttle...

how is this possible...lots of weight transfer thur the suspension..dynamic and tuneable

 

[Ranger Mike]

Every car is sliding sideways a bit on any corner (that's what the slip angle describes). It's just that the angle is small on normal surfaces. On dirt tracks there is a lot of slip and, without powering round a bend, the cars would have to go round 'unentertaingly' slowly.
Yes there is a force vector from each tyre but the force on the driven wheels is greater than if they were not driven (obviously?). So an extra set of wheels could provide more force in the wanted direction. Let's face it, if you could go round corners without use of the engine then that's just what drivers would do. I don't watch a lot of racing - particlularly not dirt track - but I see that they are always pointing as far into the bend as they can, subject to controlling an actual spin and they are producing a lot of dust and smoke from the back wheels, showing that there is significant drive coming from them.

I would go so far as to say that the ultimate dirt track vehicle would have four wheel drive plus four wheel steering. I have a feeling that very sophisticated control would be necessary to get it right, though.

?

 

[ektrules]

Every car is sliding sideways a bit on any corner (that's what the slip angle describes). It's just that the angle is small on normal surfaces. On dirt tracks there is a lot of slip and, without powering round a bend, the cars would have to go round 'unentertaingly' slowly.
Yes there is a force vector from each tyre but the force on the driven wheels is greater than if they were not driven (obviously?). So an extra set of wheels could provide more force in the wanted direction. Let's face it, if you could go round corners without use of the engine then that's just what drivers would do. I don't watch a lot of racing - particlularly not dirt track - but I see that they are always pointing as far into the bend as they can, subject to controlling an actual spin and they are producing a lot of dust and smoke from the back wheels, showing that there is significant drive coming from them.

I would go so far as to say that the ultimate dirt track vehicle would have four wheel drive plus four wheel steering. I have a feeling that very sophisticated control would be necessary to get it right, though.

Why would an extra set of wheels help when friction is not determined by surface area?

From what I've read, the "slip angle" phenomenon is not about sliding or "slipping." It's about the deformation of the tire; no sliding involved.

Again, all work to move the car both laterally and longitudinally is done by the friction between the tires and the surface. If there is sliding, then less work can be done because sliding friction is less than static friction.

Man, I've been thinking about this too much :)

I think I'm starting to get the picture of how this works though. What I thought was simple, is actually a very complex maximization problem. Without sliding, the friction vector of the rear tires will be pointed quite a bit more forward than toward the inside of the turn. Although kinetic friction is quite a bit less than static friction, it may be possible to get a vector with a larger component in the direction of the inside of the turn by sliding the rear and providing power to the wheels, even thought the total friction between the rear wheels and the surface will be less (because kinetic friction is less). Add in banking, dynamic weight distribution, and aerodynamic, then you have a very complex maximization problem.

The only reason I can think of that explains why sliding on asphalt is often "slow," and sliding on dirt is often "fast" is because there is less difference between the sliding and kinetic friction coefficients on dirt surfaces.

 

[Ranger Mike]

Your almost there...I think I'm starting to get the picture of how this works though. What I thought was simple, is actually a very complex maximization problem.

how ture..ifin it were easy there would be many Jimmie Johnson champions Without sliding, the friction vector of the rear tires will be pointed quite a bit more forward than toward the inside of the turn. Although kinetic friction is quite a bit less than static friction, it may be possible to get a vector with a larger component in the direction of the inside of the turn by sliding the rear and providing power to the wheels, even thought the total friction between the rear wheels and the surface will be less (because kinetic friction is less).

we will come back to this a little later , if ok with you

Add in banking, dynamic weight distribution, should be dynamic weight transfer, and aerodynamic, then you have a very complex maximization problem. yes
The only reason I can think of that explains why sliding on asphalt is often "slow," and sliding on dirt is often "fast" is because there is less difference between the sliding and kinetic friction coefficients on dirt surfaces.

lets review just what happens when we turn left( or right)..
for a given set of tires, on an asphalt track, and typical passenger car suspension, everything is ok until one of two situations occur..
the driver nails the throttle in the turn and thus causes loss of traction ( spins the tires too fast) and the momentum of the vehicle, suffering loss of rear tires sticking to the track, spins out...not to likely with todays grocery getter..or
the car enters the corner at too high a rate of speed and the front tires can not provide enough force to steer the front of the car around the corner and literally shred as the tires experience wheel lock and snow plow toward the out side of the turn...( its pushing , or suffering under steer).
Nothing is going to change the direction of the car without the use of the front wheels. To the degree the front wheels can negotiate a turn successfully depends upon many things, Static tables showing Tire coefficient of friction vs slip angle, coefficient of friction vs vertical tire load, coefficient , coefficient of friction vs percent of slip, tire force vs vertical tire load..don't mean a thing if you ca not handle the transfer of sprung weight to the right front tire in the turn.
this applies to dirt track as well as asphalt..the reason we have smaller suspension travel on asphalt cars is because the track is consistent and there are a lot less variables to contend with..those photos I posted of the 1500 pound, 800 h.p. dirt l cars coming off the turn show huge suspension droop, weight jacking, vast body roll..all to negate the varying effects of the changing dirt track.



gotta get some beer..did this help?

 

[ektrules]

Ok, interesting. You're implying that the front-right tire has the most important job of turning the car on a left-hand turn. Makes sense.

 

[Ranger Mike]

you got it!
there arr many other factors but it all comes down to being able to turn the car in, while controlling the diagonal weight transfer from left rear to right front, keeping the body roll to minimum, arriving at the apex of the turn with enough traction ( proper weight on drive wheels) and being able to hook up the tires with the car pointed at the correct path to your next turn point..(drives do not drive ROUND the track they " diamond" the track for the shortest line..but that is another discussion..
bottom line is to control the sprung weight properly with the correct springs and shocks(dampers), anti roll bar, aero package..etc..

 

[mender]

This recently came up in another forum and got me thinking about it again.

Through further discussion, I've come to the conclusion that it is the shearing of the dirt particles against each other that accounts for the force generated by the tire. When the track is tacky (moist) the amount of force generated when essentially tearing a dirt particle away from its neighboring particles will be more than when the track is dry and the material loose. As seen in tractor pulls, it appears that the faster the tires are spinning, the more material is being sheared and the more force is being generated. If each particle can be considered to represent a unit of force generated, spinning the tire at a higher rate shears more particles in a given time span and as a result should generate more force. That seems to match well with what is happening.

The shearing action takes place in the dirt layer, not in the tire so the heat generated by the shearing action is absorbed by the dirt rather than the tire. From what I understand, dirt tires typically don't wear quickly and can last several seasons in some classes (spec tires) because of the lack of heat in the tires.

However, as the track packs hard and dries out the generation of force switches to essentially the same as what happens on asphalt, with gripping/adhesion and interlocking of the rubber and track surface being the main mechanism. The shear force increases in the hard-packed dry surface to the point that the rubber of the tire shears first. Because of that, it is fairly easy to overheat the tires because the shearing action that accompanies sliding now is occurring at the tire surface rather than the track surface. In this scenario, spinning the tires results in a loss of traction as the tire heats up and the rubber at the face tears away with less and less force just like on asphalt when the spinning tires go up in smoke. Not spinning the tires but instead relying on "gripping" gives the most traction under these conditions.

So sliding in dirt track racing is faster because more work is being done by the tires when the tires are spinning, because more particles are being sheared from the track surface; I think it likely that the higher speed that the particles are being sheared also increases the force per particle, doubling up the effect much like the increase in force experienced because of aero drag as a car is moving through the air at higher speeds.

This also appears to work just fine to explain what we see with dirt bikes.

A bit of an aside: sliding on asphalt might also work better than "gripping" were it not for the characteristics of the tire compounds now being used. I do remember seeing pictures of drag racing in the '60s with the fastest cars spinning the tires for most of the quarter mile, indicating to me that sliding/spinning allowed the car to generate more force and therefore more acceleration - and that vastly different tire compounds were being used then! Seems to me that there was a lot more sliding on the road racing tracks as well at the time.

If a tire on asphalt didn't have the heat build-up and subsequent loss of rubber integrity, it's possible that spinning the tires on asphalt would also show a higher force being generated. I think there might be something to that as the traction peak with asphalt is reached with a slip angle or slip percentage, one that might coincide with the rate of heat build-up at the tire surface, effectively making all cornering or accelerating in the kinetic friction range rather than being static friction as most discussions assume. More things to investigate!

Thoughts?

 

[mender]

Here's an article that appears to confirm the friction model:
http://www.circletrack.com/chassistech/ctrp_0810_dirt_racing_tire_prep/viewall.html

 

[sophiecentaur]

It strikes me that speedway racing is done for 'effect' and for entertainment not for the highest speeds. The fastest way to get round a track would surely be using a 'normal' road surface and racing tyres - else would they not have all roads made of grit?

 

[mender]

Racing's racing, and people will race just about anything anywhere. So far I've raced on asphalt, dirt, and ice; hard to say which one is more fun but they all have their good points.

The fact that other people like to watch us tear round in circles doesn't bother us in the slightest, in fact it allows us to subsidize our habit by charging them an entertainment fee!

 

[haulindirt]

In asphalt racing the coefficient of friction is greater than on dirt. To maintain static friction on dirt and actually be able to turn your car you would have to slow down so much that it would actually be a disadvantage to you. Those who hold it straight have problems and end up in the outside wall unless they significantly slow down. This is why you hear drivers say the car is too tight, or it needs to be loosened up.

That being said, if you had a very high banked race track, you could possibly hold the car straight and be faster than sliding, but there aren't really any tracks with that sort of banking. Also, you're going to have to sling your car more on a flatter track or a track with tighter corners than on a high-banked track.

 

[Luey]

The original question "Why is sliding in dirt track racing faster"? Is actually a false statement. It is not faster to slide through the corners with a dirt car. However it is a compromise for a car that will not turn because the rearend is locked and has no differential. I have been building dirt late models for 25 years and have over 200 wins with various drivers. All of these cars suffer from understeer on corner entry. Therefore most drivers have to pitch the car sideways on entry or turn with 3 wheel brakes etc. At that point they have actually put the rear tires into a slide in order to negotiate the corner. If the drivers do not turn the cars in this manner they will understeer so bad on corner exit the driver will have to lift off the gas to negotiate the corner exit. Recently we have been able to develop setups that allow for neutral steer on corner entry all the way to the apex. The drivers never have to drift the cars through the corner and they are much faster than the old setups. As others discover these developments it will be the end of dirt cars sliding through the corners. Then the discussion can go back to slip angles etc.

 

[A.T.]

I'm guessing it has something to do with the fact that dirt is not static surface, but more like a collection of particles, almost like a fluid?

If the power was not delivered by the wheels, but with a rocket or something...

As you point out in your first post, the tires are accelerating a lot of dirt backwards, so they are a bit like a rocket. This doesn't happen on asphalt. You would have to do some math to see if the momentum change of the expulsed dirt is significant compared to the momentum change of the bike.

 

[mender]

A.T., reread my last post; I think that the amount of force needed to shear a dirt particle from the surface would be much higher than the force from the momentum change imparted by accelerating the particle after shearing. Just my thoughts.

 

[sophiecentaur]

It's pretty normal to find that static friction is higher than sliding friction. ABS works like magic when it prevents the wheels from locking and the situation is pretty much the same.
Downside is that there's much more of a spectacle with grit and stones flying everywhere, even though the car would be going faster without all that fuss.

 

[A.T.]

It's pretty normal to find that static friction is higher than sliding friction. ABS works like magic when it prevents the wheels from locking and the situation is pretty much the same. Downside is that there's much more of a spectacle with grit and stones flying everywhere, even though the car would be going faster without all that fuss.

On dirt ABS doesn't increase the average braking force:

http://en.wikipedia.org/wiki/Anti-lock_braking_system

ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces for many drivers; however, on loose surfaces like gravel or snow-covered pavement, ABS can significantly increase braking distance, although still improving vehicle control.

 

[A.T.]

A.T., reread my last post; I think that the amount of force needed to shear a dirt particle from the surface would be much higher than the force from the momentum change imparted by accelerating the particle after shearing.

Maybe, but either way, the more dirt you shear off and accelerate, the more force you have. So sliding with fast spinning wheels does makes sense on dirt.