Why is sliding in dirt track racing faster?

[ektrules]

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?

 

[berkeman]

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?

From my perspective, it's because you can step out the back tire, which points your force vector (from the back tire's torque) inward in the turn, helping you to turn sharper than you can if you don't break the back tire loose. There is a balancing point where the back tire (or tires for a car) are out at the best angle to give you the best speed and acceleration through the dirt turn. If you step it out too far, you slow down and go too far into the direction of the inside of the turn. Don't setp it out enough, and you head towards the outside of the turn...

[URL]http://static.etnies.com/site-images/news/article/nick%20wey%20high%20point.jpg[/URL]

http://nyk27.com/wp-content/uploads/2011/01/9A7J0132-548x364.jpg

 

[Curl]

Also from my experience with videogames (Dirt 2) you want to slide so that the car's nose points in the right direction. Basically you swing around the rear of the car so that you are now pointing in the direction of the new path, then floor it.

Of course you only do this if you are going too fast to make a classical turn.

 

[mender]

Think of the dirt or gravel surface as a consumable that needs to be cleaned of loose material first.

 

[gmax137]

... 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...

There's a lot of sliding on asphalt too

 

[AlephZero]

A general way to look at this question is:

Suppose you want to make a turn of x degrees. You have to do two things:
1. Rotate the velocity vector of the bike or car by x degrees.
2. Rotate the bike or car about its center of mass x degrees, so it is pointing in the right direction when you finish the turn.

If you don't slide, the two things are "locked" together by the geometry of the bike or car's steering system. If you slide, they can be indepedent. This gives you more options about the line you can take through the corner.

If you don't slide, the work done by the engine is limited to less than the static friction force, because the sideways component of the friction (which prevents sliding) is at right angles to the direction of travel and is not doing any work. In the limiting case where you at the point of sliding, the engine can not do any work to propel the car, because all the friction force is acting sideways.

If you slide, and the engine is powerful enough, you can apply the full dynamic friction force all the time, and the total amount of work done going round the curve can be bigger than if you don't slide.

 

[mender]

A surface that isn't prone to deterioration through abrasion (asphalt) provides higher grip when the surface is clean. Surfaces that are prone to deterioration through abrasion (dirt and gravel) benefit from being "swept" clean by the tires (i.e. sliding) to get to the intact surface beneath the loose material. The act of abrading the surface also enters into this.

Here's a reasonable analogy: try using a medium grit sandpaper on wood, then compare that with using it on steel. The sandpaper has more "grip" on the softer wood - until the sandpaper clogs up with sawdust and no longer abrades the surface. Cleaning the sandpaper restores that grip.

The preparation of the dirt track surface plays a large part in the amount of traction available. A well-prepped track has a fairly high moisture content that goes down into the dirt. If the track isn't watered properly, the water only penetrates a short way into the surface and it doesn't take very long before the top surface is worn off, the track looses its grip and becomes dry-slick. Wet and tacky makes for fast cornering and enough traction to pick up the front tires down the straights.

 

[mender]

If you slide, and the engine is powerful enough, you can apply the full dynamic friction force all the time, and the total amount of work done going round the curve can be bigger than if you don't slide.

The amount of work done is limited to the amount of force that can be generated, shown as the resultant of the longitudinal and lateral forces.
http://www.auto-ware.com/setup/fc1.htm

Given that the static friction coefficient is lower than the kinetic friction coefficient, generally less work can be done by the tires when sliding.
http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/frictire.html#c1

Applying more power from the engine will make the situation worse by increasing the amount of slippage, heating up the tires and further reducing the available grip. That's why you don't see long trails of tire smoke from the winning cars out of the corners on asphalt, only the loosing ones - and drifters.

 

[Lsos]

I wonder if reaction force of the tire throwing dirt back plays any part in this? I heard from flipping through channels and accidentally landing on the redneck channel, that in tractor pulls they can benefit from the jet effect of spinning the tire as fast a possible and hurtling massive amounts of dirt out the back.

 

[mender]

There will be an effect but I suspect it is fairly minor. I think if one were to calculate just the momentum change of the dirt needed to absorb the amount of power being displayed by those tractors, considerably more dirt than is on the whole track would be needed!

 

[ektrules]

Applying more power from the engine will make the situation worse by increasing the amount of slippage, heating up the tires and further reducing the available grip. That's why you don't see long trails of tire smoke from the winning cars out of the corners on asphalt, only the loosing ones - and drifters.

Yeah, this is the theory I was trying to explain in my question. If the power was not delivered by the wheels, but with a rocket or something, the argument that sliding allows cars to supply more force toward the inside of the turn would make sense. But, I'm talking about normal cars that do work through the friction between the tires and the road. Tires can only handle so much force before it exceeds the static friction force, and then starts sliding. When tires are sliding, the friction between the tires and the road is significantly less, so the tires can't do as much work. This means that you can go through a turn faster when the tires are not sliding.

With all the kinds of racing on asphalt that I know of, sliding is a bad thing, and is known to increase lap times. You only see experienced drivers slide when they make a mistake, or are trying to correct a mistake (coming out of a turn too fast, hitting a patch of debris, etc). Even when racing in the rain, Formula 1 drivers try not to slide around turns.

On dirt track racing, such as sprint car or rally racing, the fastest cars slide through turns all the time. This seems to contradict the above theory. What is different between racing on asphalt and dirt that makes it better to slide on dirt?

(Sweeping debris doesn't really explain it, because drivers slide their cars during the entire race, and sweeping debris does not give drivers any advantage over the other drivers.)

 

[sophiecentaur]

Also from my experience with videogames (Dirt 2) you want to slide so that the car's nose points in the right direction. Basically you swing around the rear of the car so that you are now pointing in the direction of the new path, then floor it.

Of course you only do this if you are going too fast to make a classical turn.

I think that what happens in a video game is not to be considered as evidence in any form. Someone wrote a simulation on the basis of experience and, possibly, theory but that is not a proof. I am ranting about this a bit but simulations are only suitable if the model is totally correct. - good to demonstrate but no use as a proof.

 

[Ranger Mike]

Having run dirt a little in my time..the answer is "dirt is not quicker than asphalt"..no way
its all about tire contact and who can maintain it the longest on any given track.
no way will a half mile track be quicker than an asphalt half mile track...especially if we have significant banking ( over 20 degrees)...
one major factor to be considered is the fact that a professional dirt track event means the track condition will change at least 4 times over the race event. The track will go from slurry wet to hard packed dry condition and this will vary depending upon if the track is watered and smoothed during the event. This will change the set up, tires and handling. Day long events will have track conditions change simply by amount of sunlight present ( ambient temperatures)
another major problem is that the "Groove" or fastest race line will change as the track packs in. The driver has to adjust to this and is constantly looking for the fastest groove during the event...the top of the track will get cluttered with massive quantities of loose dirt thrown by the the tires and force the cars to use the lower groove..
this means you have no consistent race line as you have on asphalt ( the asphalt track groove will also move but not as dramatic as dirt).
Plus it is one messy tough dirty job crawling under a race car at night to work on it...

 

[mender]

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?

So you're looking for the process that provides the traction and the reason that sliding is more effective than not sliding, right? I haven't come up with much on the theory behind that - lots of practical advice on how to compensate for changing track conditions though, like Ranger Mike posted!

Sounds like you're leaning towards a non-Newtonian fluid explanation.

 

[ektrules]

So you're looking for the process that provides the traction and the reason that sliding is more effective than not sliding, right? I haven't come up with much on the theory behind that - lots of practical advice on how to compensate for changing track conditions though, like Ranger Mike posted!

Sounds like you're leaning towards a non-Newtonian fluid explanation.

Yeah, I was wondering why sliding on dirt is effective on dirt, and not asphalt. I was hypothesizing that the reason might lie in the fact that dirt is a collection of loose particles. Honestly, I'm surprised that a physicist hasn't stopped in here, and posted an explanation :) I figured it would be something simple.

 

[xxChrisxx]

Exactly how the mechanics of gripping on asphalt vs gripping on gravel work are black magic. As is everything to do with tyres.

However the main reason that you don't get cars sliding on asphalt is becuase the radii of the corners are much larger.

Compare the tightest hairpin on a track to even a loose hairpin on a rally stage. The rally stage will be realllly tight. In autocross for example, you get people sliding on tarmac beucase they need to pivot the car on the spot.

EDIT: Dirt track racing, never mind.

I think the reason they slide round coners is bacuse they don't have enough grip to do anything else. It sounds tautological but if they could get tyres to grip like they could on tarmac, you'd see them taking tarmac type lines.

 

[sophiecentaur]

Speeds on loose tracks are much lower than on hard 'sticky' tracks. That much is obvious.
The so-called slip angle is much higher on loose tracks. To 'steer' round a bend on a loose track, you would need to be going absurdly slowly because you can't produce enough lateral force by just changing the angle of the front wheel(s) both to reduce speed in the initial direction and give it a lateral velocity. By turning the car / bike into the curve you can increase the lateral force by using the force from the driving wheels as well as the force that the steered wheel produces to push 'sideways' (drift).
The same thing happens, to some extent, for every vehicle going around a corner; the vehicle doesn't naturally actually point in the direction of a tangent to the curve (understeer or oversteer). A normal road car is designed to understeer because your average driver can't cope with the effects of oversteer. All performance cars (or any car with enough poke) are made to oversteer in a controlled way to go round a track as fast as possible (see that silly Top Gear prog with Clarkson in). It's just much more noticeable on a loose track because the angles are more pronounced.

 

[Ranger Mike]

sophiecentaur

may i digress on a few points..please...
The same thing happens, to some extent, for every vehicle going around a corner; the vehicle doesn't naturally actually point in the direction of a tangent to the curve (understeer or oversteer).

Almost true ..but.. real round track cars are intentionally set up with offset castor. If the driver takes his hands off the steering wheel the car will automatically turn left due to the caster offset. In the rear tire set up you run a lot of stagger so the car will naturally steer left..think of a sytrofoam coffee cup on its side, it will automatically roll toward the side the small end is pointing..If you really have the car dialed in for the track then car will almost round the turn on its own because of this set up.

A normal road car is designed to understeer because your average driver can't cope with the effects of oversteer.
I am not so sure about this. the main reason stock production cars ' push " when going over the limit of normal speed limits while rounding the corner is due to the engine being over the front drive wheels and this causes a weight bias ( usually 55% front end weight) that gets magnified if there is any braking ( weight transfer to the front). It is simply a fact of economical production.

All performance cars (or any car with enough poke) are made to oversteer in a controlled way to go round a track as fast as possible.

Again, not so sure about the design intent. Once your pass the performance point of HP to weight ratio, you can spin the tires at will due to the available horsepower. This is something the average grocery getter does not have, instant power.

By turning the car / bike into the curve you can increase the lateral force by using the force from the driving wheels as well as the force that the steered wheel produces to push 'sideways' (drift).hummm...turn into the corner...I don't think so.. would cause the car to spin out... and I need to do a little DIGGING to find my set up notes on DIRT tracks..sorry for the terrible pun..

 

[Ranger Mike]

when driving dirt , you " steer with your Toe" ..by this i mean you run down the front straight as fast as you can until you see God, then crank the wheel to the out side ( turn right ) and goose the throttle with your toe until you get the car thru the apex and can straighten it out and nail the throttle...
this pic shows a couple of things...dirt ( once the track reaches the hard packed condition) does give you a tractable surface..
dirt track cars have a lot more suspension travel ( compared to asphalt cars) to compensate for the varying condition,
aero down force is huge...

 

[Ranger Mike]

this may be the point where you might want to think about straightening the car out

 

[rcgldr]

understeer ... oversteer

These terms just explain which end of a car slides more or slides first when reacing the limits of grip (also referred to as being "tight" or "loose"), or the other definition is when the rear tires have more slip angle than the front (this would be drifting as seen on dirt tracks).

When turning, the rear tires generate a lateral force, which is only possible if the rear tires are also oriented inwards of the direction of travel when in a turn. Because of this, all cars will end up oriented slightly inwards of the direction of travel when in a turn.

Getting back to dirt track racing, from articles I've read the main issue is the low amount of traction compared to pavement, combined with the limitation on how easily tires can deform at the contact patch which is related to how much lateral force can be generated. Theoretically, if a tire could be made with extremely soft and tall sidewalls, it could be possible for a dirt track car to corner normally, but apparently it's not possible to do this, so instead the cars are drifted thorugh the turns instead. Another factor could be that kinetic friction isn't much less than static friction at lower amounts of friction and/or certain types of tires (like bias-ply tires).

 

[Ranger Mike]

rcgldr..
When turning, the rear tires generate a lateral force, which is only possible if the rear tires are also oriented inwards of the direction of travel when in a turn. Because of this, all cars will end up oriented slightly inwards of the direction of travel when in a turn.

do you mean Toe In?Another factor could be that kinetic friction isn't much less than static friction at lower amounts of friction and/or certain types of tires (like bias-ply tires).can you re-state this in other words because you lost me...

 

[sophiecentaur]

Ranger Mike

If a domestic car goes into oversteer then your average driver just couldn't cope. Reverse Lock?! what the hell's that . . . . crash. This is what happens when eejits in well powered cars put their foot down on a bend with a wet road (rear wheel drive).

The angle of the car axis wrt the direction it is traveling can be into or out of the curve, depending on what the driver is doing with the steering at the time. The optimum angle may well be a bit 'into the curve', though.

My main point is that there is no shortage of ability of a vehicle to get going far too fast for entering the next bend. When it gets to the bend, what is needed is as much force normal to its initial path as possible, to make it go round the corner. With sufficient skill and control, this can come from the rear drive wheels, which have to supply as little tangential force as possible as the vehicle is already going as fast as it can cope. This can be even taken to an extreme, when the car is almost pointing (driving) radially inwards to help keep it on its path. Not good value on tyre rubber, though. 30.000 miles on one set? I think not. This general principle applies in all / most circumstances but, on a very loose track, everything is exaggerated.

The situation with front wheel drive may well be different in detail but still I think the driven wheels are used best when they actually help with the cornering by providing force in the appropriate direction. Did anyone ever design a front / two wheel drive motorbike? That would have been interesting.

 

[Ranger Mike]

sophie..that was great...especially the part about the eejits
anyway.. a good conversation and i get your point..and i think we both can agree driving dirt is not for the timid or for the average eejits
rm

 

[sophiecentaur]

I remember I was that eejit, once in 1968 (ish). I lost it on a wet patch at low speed in a Morris 1000 and just waltzed round and round until I came to rest in a hedge. If I'd had my wits about me I would have just put the wheel the other way and emerged with no scratches.

 

[Ranger Mike]

sophie ,,morris 1000
watch this


may bring back a few fond memories..
glad to know there is another gear head across the pond!
rm

 

[rcgldr]

When turning, the rear tires generate a lateral force, which is only possible if the rear tires are also oriented inwards of the direction of travel when in a turn. Because of this, all cars will end up oriented slightly inwards of the direction of travel when in a turn.

do you mean Toe In?

No, I mean slip angle. In order for the rear tires to generate lateral force, they need an overall slip angle. This means an entire car will be oriented slightly inwards (radiallly) of it's actual direction (tangental) when it's turning.

clarification... entire car will be oriented slightly inwards of the rear tires actual direction. Perhaps better stated that slip angle at the rear tires is like having the rear tires steer slightly outwards.

http://en.wikipedia.org/wiki/Slip_angle

Another factor could be that kinetic friction isn't much less than static friction at lower amounts of friction and/or certain types of tires (like bias-ply tires).

can you re-state this in other words because you lost me...

That sliding friction isn't much less than non-sliding friction when grip factor is low and/or depending on the tire (like a bias ply tire).

In the case of a dirt car, the rear tires generate both lateral (sidways) and longitudinal (forwards) forces relative to the tires orientation when the car is drifting through a turn.

The grip factor seems to be the main issue. Rally cars drift a lot on low grip surfaces (dirt or snow), but not on high grip surfaces like pavement.

All performance cars (or any car with enough poke) are made to oversteer in a controlled way to go round a track as fast as possible.

Formula 1 race cars are setup with more relative front downforce than rear downforce. They'll oversteer at slower speeds, but at higher speeds the downforce is setup to understeer a bit to avoid spins due to snap oversteer when pulling 4 g's in turns at speeds around 160 mph (like the turn Pouhon at Spa).

 

[sophiecentaur]

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.

But this thread is split into two, now. The situation is very different for dirt track, slow speed and for formula 1. I don't think we can come to just a single conclusion.

 

[ektrules]

(see that silly Top Gear prog with Clarkson in). It's just much more noticeable on a loose track because the angles are more pronounced.

I love that show.

If a domestic car goes into oversteer then your average driver just couldn't cope. Reverse Lock?! what the hell's that . . . . crash. This is what happens when eejits in well powered cars put their foot down on a bend with a wet road (rear wheel drive).

I hate driving front wheel drive cars in the winter. If you hit a patch of ice, the car just keeps going straight, and there's little you can do about it. In a rear wheel drive car, you can tap the gas a little to slide the rear end around.

The British say "aluminum" funny :)

No, I mean slip angle. In order for the rear tires to generate lateral force, they need an overall slip angle. This means an entire car will be oriented slightly inwards (radiallly) of it's actual direction (tangental) when it's turning.

http://en.wikipedia.org/wiki/Slip_angle

That sliding friction isn't much less than non-sliding friction when grip factor is low and/or depending on the tire (like a bias ply tire).

In the case of a dirt car, the rear tires generate both lateral (sidways) and longitudinal (forwards) forces relative to the tires orientation when the car is drifting through a turn.

The grip factor seems to be the main issue. Rally cars drift a lot on low grip surfaces (dirt or snow), but not on high grip surfaces like pavement.

Cool, I didn't know about the slip angle phenomenon, and I didn't know there was a field called, "vehicle dynamics." "Pacejka's Magic Formula" looks interesting too. Gotta love magic, lol.

Yeah, the only thing I can think of that seems to explain why sliding on dirt is "faster" is that the static and kinetic friction forces are very close on low-grip surfaces. I think things like dynamic weight distribution and banking might have something to do with it too.

 

[sophiecentaur]

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!

 

[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.