12 replies to this topic

[kevinq]

Guys,

I was reading through one of Franklin's posts (I can't remember which one) and saw a comment on top fuel car rear track widths. The post said that a narrow rear track was an advantage and I am just curious as to why. The only thing I can think of is a reduction in drag as the frontal area would decrease with a narrow track. The change in normal force on the rear tyres would be worse for a narrow track (with the torque reaction from the engine) wouldn't it?

Regards,

K

[Franklin]

Originally posted by kevinq
Guys,

I was reading through one of Franklin's posts (I can't remember which one) and saw a comment on top fuel car rear track widths. The post said that a narrow rear track was an advantage and I am just curious as to why. The only thing I can think of is a reduction in drag as the frontal area would decrease with a narrow track. The change in normal force on the rear tyres would be worse for a narrow track (with the torque reaction from the engine) wouldn't it?

Regards,

K

There is also a belief that with the rear tires close together there is less steering effect if one gets more traction than the other.

[Wuzak]

Originally posted by Franklin


There is also a belief that with the rear tires close together there is less steering effect if one gets more traction than the other.

Perhaps they should go for one big wheel in the middle, and have outriggers to stabilise the car!

[McGuire]

Obviously, traction between two drive wheels can never be 100% equal. Since the wheels are located some distance from each other, there is a torque or couple across the axle between their drive forces, acting at the plane of the tire contact patch. In order to acclerate the vehicle's mass forward, this pair of forces must act through the vehicle's CG, located some distance ahead of the rear axle and above the pavement. So to whatever extent the drive forces are unequal, the resultant torque will attempt to steer the car by pushing the car's CG in a direction other than precisely forward.

This effect is further magnified if the car is equipped with a spool (locked axle, no differential).  Here the only means of distributing torque between the two wheels is friction. If one tire loses traction, the torque to the other wheel is doubled. A fuel dragster weighs 2175 lbs and develops over 8000 lbs of wheel thrust, 4000 lbs on each rear wheel (we hope). A relatively small difference in traction between drive wheels can make the car impossible to drive in a straight line.

Narrowing the rear track width simply decreases the displacement between the two drive forces, reducing the moment arm of the force couple accelerating the car's CG forward.  Very roughly put, the two drive wheels now each have less leverage on the CG  and the car will now drive straighter under power. Seems like a very queer approach to dynamics; from an oval or road racing perspective downright counterintuitive...but it really does work. Obviously there are implications in lateral roll with such a narrow track, but since drag cars don't have to go around corners it's not a first order concern.

[McGuire]

Originally posted by Franklin


There is also a belief that with the rear tires close together there is less steering effect if one gets more traction than the other.

LOL, is this another "belief" about vehicle dynamics Franklin does not share?

BTW, this is another of the reasons a ground-effect underbody won't work on a dragster. With a suitable rear track width, there's no place to put it.

[dosco]

Originally posted by Wuzak


Perhaps they should go for one big wheel in the middle, and have outriggers to stabilise the car!

Yeah, with a WING shaped BEAM AXLE. Should be able to go head-to-head with those fancy LSR VEHICLES. Too bad Gabelich STOLE the idea ALREADY.

[Franklin]

Originally posted by McGuire


LOL, is this another "belief" about vehicle dynamics Franklin does not share?

BTW, this is another of the reasons a ground-effect underbody won't work on a dragster. With a suitable rear track width, there's no place to put it.

On a conventional Top Fuel dragster you put the tunnels under the headers.

(Comments by Dale Armstrong from page 42 of National Dragster, February
12, 1992 on Mike Magiera ground effects package also run by Joe Amato)
"It appears that the ground effects start sucking the car down too
aggressively too early. A couple of hundred feet out it was really
crunched down on the tires. That would be good at a track where the
traction was marginal, but at most tracks you don't need that much
downforce that early. At 300 feet the conventional wing is pushing down
and putting the full 17 inches of rubber down to the track. If you can
get a good footprint like that, it will stayed hooked up at most tracks.
But the package was sucking down too hard and taking the crown out of
the tires and putting a big wrinkle in the tires. It's absorbing so much
energy the car is laboring against it." Additionally, said Armstrong,
the extreme effects of the package pulled down hard enough to bow the
chassis., meaning that a different type of chassis might have to be
built, one that might not work as well overall as a traditional chassis.

[Franklin]

Originally posted by McGuire
Obviously, traction between two drive wheels can never be 100% equal. Since the wheels are located some distance from each other, there is a torque or couple across the axle between their drive forces, acting at the plane of the tire contact patch. In order to acclerate the vehicle's mass forward, this pair of forces must act through the vehicle's CG, located some distance ahead of the rear axle and above the pavement. So to whatever extent the drive forces are unequal, the resultant torque will attempt to steer the car by pushing the car's CG in a direction other than precisely forward.

This effect is further magnified if the car is equipped with a spool (locked axle, no differential).  Here the only means of distributing torque between the two wheels is friction. If one tire loses traction, the torque to the other wheel is doubled. A fuel dragster weighs 2175 lbs and develops over 8000 lbs of wheel thrust, 4000 lbs on each rear wheel (we hope). A relatively small difference in traction between drive wheels can make the car impossible to drive in a straight line.

Narrowing the rear track width simply decreases the displacement between the two drive forces, reducing the moment arm of the force couple accelerating the car's CG forward.  Very roughly put, the two drive wheels now each have less leverage on the CG  and the car will now drive straighter under power. Seems like a very queer approach to dynamics; from an oval or road racing perspective downright counterintuitive...but it really does work. Obviously there are implications in lateral roll with such a narrow track, but since drag cars don't have to go around corners it's not a first order concern.

Didn't I just say that? "There is also a belief that with the rear tires close together there is less steering effect if one gets more traction than the other."

[Franklin]

Originally posted by McGuire


LOL, is this another "belief" about vehicle dynamics Franklin does not share?

BTW, this is another of the reasons a ground-effect underbody won't work on a dragster. With a suitable rear track width, there's no place to put it.

http://www.draglist..../POD-102000.htm

The Arivett brothers found plenty of room for a huge tunnel.

What they couldn't find was the funding to adequately test and develop it.

[McGuire]

Originally posted by Franklin


http://www.draglist..../POD-102000.htm

The Arivett brothers found plenty of room for a huge tunnel.

What they couldn't find was the funding to adequately test and develop it.

Well Fwanklin, this car is OBVIOUSLY an example of extremely POOR DESIGN, since it lacks a VERTICAL STABILIZER of any KIND. Clearly these guys had their HEADS stuck up their ASSES.

[kevinq]

Thanks guys. The variation in tyre thrust does seem like the most important consideration when setting the rear track width.

Along with the change in rear ride height as the car accelerates down the track, the narrow track would cause a problem if trying to fit a conventional style rear diffuser IMHO.

K

[McGuire]

Originally posted by Franklin


On a conventional Top Fuel dragster you put the tunnels under the headers.

(Comments by Dale Armstrong from page 42 of National Dragster, February
12, 1992 on Mike Magiera ground effects package also run by Joe Amato)
"It appears that the ground effects start sucking the car down too
aggressively too early. A couple of hundred feet out it was really
crunched down on the tires. That would be good at a track where the
traction was marginal, but at most tracks you don't need that much
downforce that early. At 300 feet the conventional wing is pushing down
and putting the full 17 inches of rubber down to the track. If you can
get a good footprint like that, it will stayed hooked up at most tracks.
But the package was sucking down too hard and taking the crown out of
the tires and putting a big wrinkle in the tires. It's absorbing so much
energy the car is laboring against it." Additionally, said Armstrong,
the extreme effects of the package pulled down hard enough to bow the
chassis., meaning that a different type of chassis might have to be
built, one that might not work as well overall as a traditional chassis.

The problems are incredibly obvious when you think about what Dale learned here, Franklin. First, the chassis must be several times stiffer to take advantage of ground-effect aero...which is exactly what they learned in F1, CART, etc. many years earlier. However, in TF and FC they have an added complication: the chassis also serves as the suspension. To make ground effect work properly, they would first have to make the chassis several times more rigid, and then they would have to incorporate suspension componentry into both ends of the car. You now have a totally different vehicle, with far more parts, considerably more weight, and a whole list of new development issues. You can't just slap sidepods and undertray on an existing car and expect it to work. While many have tried, all have failed.

The next problem: Ground effect underbodies are especially sensitive to ride height and pitch. However, FC/TF tires grow over 8" in diameter over the course of a run. If the bodywork is devised to provide sufficient downforce at the far end with the car 4" above static ride height, at the near end it will develop far too much downforce. Here Dale found he couldn't even get the car up on the tire at 300 feet. And you will never get the underbody in a position to exhibit decent pitch sensitivity, because the rear tires are in the way.

And all this is moot anyway, because ground effect bodywork is not permitted in NHRA. You seem to have a thing for purely hypothetical race cars that can't run anywhere. If you are going to design race cars, the first thing you need to read is the rulebook. The car must meet the rules as surely as it must comply with the laws of physics. Is reality too constricting for you?

[uri]

Originally posted by Franklin


Didn't I just say that? "There is also a belief that with the rear tires close together there is less steering effect if one gets more traction than the other."

From Britannica.com:

Belief:
a mental attitude of acceptance or assent toward a proposition without the full intellectual knowledge required to guarantee its truth

I think there's a little difference in what you said and what McGuire said.