24 replies to this topic

[shaun979]

Does a car really reach higher speeds driving down a long straight right next to the pitwall, as compared to driving out in the middle of the track width?

This was claimed by a semi-pro driver and I am trying to understand how it might work - if it does. It seems counter-intuitive in that the more "sides" (4 sides being a square tunnel) you add to a car at speed, the closer it is to pushing a column of air ahead of it which should slow it down.

[Ross Stonefeld]

In American Oval racing, especially NASCAR; its considered that there is a wall of turbulent air up against the wall, and you really want to spend as little time in its area as possible. Exiting the corner is fine, you're trading it in for extra radius, but you go down the straight slightly closer to middle for a 'cleaner' run down the straight.

[McGuire]

It's not only slower...more importantly that's also where the rubber marbles land, not to mention all the nuts, bolts and other debris that falls off the cars. So at the very least you will get rubber buildup and also you stand a good chance of cutting down a tire. Never drive against the wall if it is not on the racing line.

[shaun979]

Thanks for the responses.

I've never seen this "wall effect" mentioned in any book (driving or aero) and I have a few professional driver friends who have never mentioned it. This was the first time I was hearing about it.

The person who spoke it is a lower level semi-pro driver and he specifically recommended it as a strategy to get down the long straight faster, despite being off the racing line - which should be a straight line diagonally across the long straight from T15 exit to T1 brake zone. I don't recall seeing any racecars at this track deviating from the standard line when they're qualifying, or when they're not in a pack.

[Fat Boy]

There was an SAE paper that measured this. They claimed there was an aero advantage by driving next to the wall. I'm talking within a couple inches. I don't remember the actual mechanism that was in effect, but I do vaguely remember reading it.

Mac is right, though. In real life, you just want to be on line. Driving through a bunch of crap is just going to make life _real_ difficult when you get to the end of that straight.

Sounds like advice from a semi-pro driver who is apt to stay semi-pro.

[Ross Stonefeld]

Im trying to think of what benefit being close to the wall would do but not being an engineer I'm coming up with little. Perhaps the NASCAR phenom of moving away from the wall is similar to their bizarre habit of not running the proper radius through the corner.

Im having a vague recollection from an early 90s DTM race (when they did the full N-Ring) that they liked to favor one side or the other on the insanely long final straight, because that side was heavily wooded and they were less vulnerable to crosswinds or something. Not sure that makes much sense, because you'd still have wind from the exposed side.

[Lontano]

I remember hearing something about that during a coverage of a F3000 or Nissan series race, i don't quite remember. Infact, the pitwall was on the "right" side of the track (the first and last turns where in the same direction), and the drivers where indeed getting very very close to the pitwall. Maybe it improves the efficency of the floor of the car (as it has nowhere to go on the pitwall side so it has to exit through the difusser)?

i have no idea of what i'm talking about

[GregorV]

Driving next to a (high) wall in an ideal flow is the same as if there was another car mirrored on the other side of the wall, sans the wall. Thus running next to a wall is similar to two cars running side by side along the track. If the latter configuration shows some aerodynamic benefits, chances are that running next to a wall helps as well.

One effect I can think of is that a larger part of the flow is then going over the roof of the car which is aerodynamically more sound as there is those ungainly wheel openings and wheels inside them at the sides. Then again, the air on the side that is closer to the wall is bound to get accelerated more than the air on the other side (something you may have experienced on a motorway when another car is passing nearby, likely creating an attractive force towards the wall), thus perhaps creating even more drag where turbulence develops so the effect is hard to predict. Nevertheless, I am pretty sure that there are probably quite measurable aerodynamic effects when running close to the wall, be it positive or negative ones.

[shaun979]

To add more detail to the situation...

There is a white line on the track about 2 feet off the pit wall and this is what some are driving next to. They are not getting within a foot or within inches of the pit wall. The pit wall is only about 4 feet high.

The recommendation is being made for and applied to sedan type cars, not open wheel cars with a diffuser or heavy aero. The claimant may have picked this up while he was racing F2000.

There is no crosswind effect because on one side you have the pit buildings and pit wall, and on the other side there are the stands. The large majority of the straight is shielded from cross wind this way.

===========

Gregor V, I don't think running next to a wall is like running next to a car going down the straight. Unlike a moving car, the wall is not displacing any air and leaving a wake.

[McGuire]

Originally posted by shaun979
To add more detail to the situation...

There is a white line on the track about 2 feet off the pit wall and this is what some are driving next to. They are not getting within a foot or within inches of the pit wall.

Nope, I can't see how that would have any aero benefit.

[Lontano]

see? i didn't know what i was talking about

[GregorV]

Shaun, indeed in the far wake the mirroring may not work anymore, but generally the concept should be applicable. Enforcing the boundary condition of an ideal flow at the wall, which is that the perpendicular component of the flow velocity with respect to the wall should vanish, is exactly equivalent to removing the wall and placing another car of the same shape as if the wall was a mirror. The same principle applies when e.g. studying the effects of a wing in proximity to the ground. True, with a real viscous flow the boundary condition changes a bit (the velocity at the wall is equal to the free stream velocity and a boundary layer may occur at the wall as well) but generally the approximation should work rather well as the flow is close to ideal, apart from the wake. Only far behind the two cars, where their wakes are about to merge, would such an approximation fail severely as the two turbulent flows behind the two cars can not be considered as symmetric anymore, but that should not affect the overall forces on the car much.

[GregorV]

For an example, check this:

http://www.mh-aeroto...oils/jf_wig.htm

[ZRob]

I actually remember Dale Jr. talking about how there was a disadvantage of running close to the wall as well as two cars running side by side. It only makes sense that running against the wall would build up air in front of the car and lower the pressure behind it. The same effect is true in the case of two car side by side, but NASCAR drivers, according to Jr. , do have to be aware of the turbulant area along side a car at places like Daytona and try to keep separation. I am just adding my two cents, if anyone cares, but I'm pretty sure I'm right. I also see GregorV's point and agree fully. Running a car close to a wall is like running it close to the ground. Aerodynamists using symetrical models in windtunnels placing an identical car inverted under another car has been proven an accurate way of sumulating a car running on the ground. The aero forces of two cars running side by side would push against each other effectively mirroring each other. I many be wrong, but I pretty sure they are the same thing: a car side by side and against a wall

[275 GTB-4]

Cough!! Bernoulis Theorem Cough!

[wati]

Originally posted by shaun979
Thanks for the responses.

I've never seen this "wall effect" mentioned in any book (driving or aero) and I have a few professional driver friends who have never mentioned it. This was the first time I was hearing about it.

What are your friends professionals at? Nascar racing 2003 (the game)? That sim gives you this effect, there was a debate in some sim forums if driving close to the wall is cheating or not. Maybe that's where your friends got that idea.

[shaun979]

I said "who have never mentioned it"

[wati]

Yeah, my mistake.

[Paolo]

I was asked about it years ago, by a pilot I knew.
It seems to be a legend carried on in race circuits.
Aero theory states quite clearly it will slow the car down (mirror effect, as mentioned).

[Ray Bell]

I was once told that this could be used as a method to shake a slipstreaming car behind you...

[phantom II]

King Richard began driving close to the wal toward the end of his carreer because it didn't hurt as much when he hit it.

[Fat Boy]

'King Hiro drove there because he didn't know any better.

[Ross Stonefeld]

Nice.

[275 GTB-4]

Originally posted by 275 GTB-4
Cough!! Bernoulis Theorem Cough!

It would be nice at this juncture to hear from some real aerodynamiscists....whilst Bernouli says that the air rushing through the gap will be faster than the unloaded side......I have no eye-deer what the accompanying drag effect is....or indeed whether the "cushion" of air aids forward progress.

Can someone please blind me with science on this one????

[shaun979]

Originally posted by 275 GTB-4


It would be nice at this juncture to hear from some real aerodynamiscists....whilst Bernouli says that the air rushing through the gap will be faster than the unloaded side......I have no eye-deer what the accompanying drag effect is....or indeed whether the "cushion" of air aids forward progress.

Can someone please blind me with science on this one????

I second this.

Sounds like the car would get pulled towards the wall once it got sufficiently close.