Effect of raising rear of car?

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MopaR&D

MopaR&D

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I'm wondering from a general theory perspective how raising the back of older RWD cars makes them handle and accelerate differently. I know it improves aerodynamics but I don't understand how it improves launch or traction i.e. on drag cars with the back end all jacked up. Can someone explain how this works?
 
Actually, generally, "it doesn't." Most cars handle better with more weight biased to the rear, and jacking the rear obviously hurts this. Most cars end up high in the rear mostly for tire clearance.
 
If you look at proper (not amatuer) race cars from back in the day they usually sat high in the front. On a drag car you want to start with the front end up some to help with your weight transfer. When aligning a drag car you generally set the suspension at "track" height. Generally other than to clear improperly sized tires the only reason to jack the back up is because someone thinks it looks cool. Ultra strong rear leaf springs have been known to raise the rear, but you don't have to have the back end up to have that amount of rear spring. I always refer to the high rear stance as the "Stink bug" stance.

I would like to see some documents that state that aerodynamics are improved by raising the rear. The best improvement for aero is lowering the car and installing air dams or underbody shields.

drag car **** follows

http://www.moparaction.com/article/super_stock/113.jpg
http://image.moparmusclemagazine.co...wedge_engine+four_door_max_wedge_race_car.jpg
http://www.cars-on-line.com/update209/images/vankecarstopbanner1.jpg
http://www.prostockhemi.com/gallery....DownloadItem&g2_itemId=555&g2_serialNumber=1
 
I lowered the rear of my 72 Duster by installing the Cal Trac set up a few years back and i'll tell you I picked up huge gains on 60 ft times. I might add I hooked alot harder while street racing some buddies as well.
 
It changes your caster angle also to more negative which is generally not an advantage.
 
They raised the back to clear the tires, pure and simple (been there in the 60;s), It turned out that it actually hurt et. Then along came Dick Landy, and others, with the lower link bar; the forerunners of the ladder bars, 4 links, and the cheap traction bars..

think about weight transfer- front end raises, puts weight on rear tires. Raises too much- all that energy took time to transfer to the year- bad 60 foot time.

Keep the rear low, install whatever, and start adjusting the center of gravity, until it starts to hook good. Now, the shocks are probably too short, causing a bounce. Lengthen. Actually, buy double adjustables.

If the rear squats on launch; shocks too soft. If it lifts 2"- dead on the money. All the weight is now planting the tires.
 
Thanks this all makes a lot of sense guys and it was mostly what I was thinking anyways, I can clearly visualize how having the rear lower and front higher would transfer weight onto the back on launch. My own '70 Duster sits at about factory height but the springs are pretty worn, I want to get new ones and bump it up an inch over or so but I also plan to add 275/60R15 tires at the same time...

dmsbroker are you saying that just from adding Cal-Tracs your rear ride height went down? How does it make that happen?

Also about the aerodynamics part I got it from a documentary on NASCAR in the '50s-70s, it said how racers found that raising the rear slightly so the angle was just a few degrees up (nothing drastic or even very noticeable) made the car much more stable at high speeds...
 
You raise the back end of the car because it's just not cool to have your wife sitting on a pillow when she's behind the wheel on cruise nights. :joker:
 
Jacking the rear end with elongated shackles will cause the car to become tail happy, or in other words more prone to over steer under braking and acceleration. This condition is not desired in a street car where it becomes unstable under hard braking in traffic.

If you are now thinking leaving rear at factory height, and lowering front from factory ride height by cranking down the torsion bars, that will make for crappy handling and ride as well.
 
wjajr said:
Jacking the rear end with elongated shackles will cause the car to become tail happy, or in other words more prone to over steer under braking and acceleration. This condition is not desired in a street car where it becomes unstable under hard braking in traffic.

If you are now thinking leaving rear at factory height, and lowering front from factory ride height by cranking down the torsion bars, that will make for crappy handling and ride as well.
Click to expand...

I want to raise the rear an inch or so but not with longer shackles, rather new stiffer springs that don't sag like the originals. I'm doing more of a road-racer type build for handling and the new 1" torsion bars in front made a HUGE difference but the back end now feels very "floaty" and loose since I haven't changed the back springs yet, not to mention all the shocks are pretty much gone too. Gotta save up for a set of those nice Bilsteins...
 
Raising any part of the car will also raise center of gravity. You would ideally want to have front and back of the car at equal heights for serious road racing, all else being equal.

As far as aerodynamics are concerned, you ideally want to minimize total frontal area in relation to the "relative wind" to achieve the least coefficient of friction. The more body area that directly contacts the "relative wind," the more drag. This can be countered to a degree by helping the "boundary layer," that is the air moving along the body panels, to ride smoothly along the skin and not separate, which causes low pressure areas (i.e. drag). Raising the rear of the car will provide more down-force, but it is negligible until you hit high speed.

Chrysler did some major testing on car bodies for high speed NASCAR racing in the late 1960s. I have read their wind tunnel testing reports for the Charger Daytona and the Superbird. They found that raising the read end increased drag. The nose cones smoothed air flow and greatly helped the boundary layer. They also changed the rear glass to that of the Charger 500 to reduce the low pressure area behind the stock glass. The biggest secret was the wing, which obviously increased down-force on the rear tires, but the greatest asset, and which Chrysler kept a deep secret, was the tremendous lateral stability increase provided by the wing's vertical stabilizers.
 
I think you are probably right about this detail. Mainly, they were trying to avoid the nose lifting at speedway speeds. My Mopar Suspension book says something similar about raking the car 1 or 1-1/2 degrees, though I think they recommended dropping the nose. But most aerodynamic tweaks don't really apply to most street cars at realistic speeds.

FWIW, my Barracuda fastback generates a lot of nose lift at speed, due to the airfoil silhouette. I just grabbed a plastic air dam off an 80s GM to try mounting under the rad support, on the recommendation of a guy I met at a show.

MOPEkidD-3 said:
...
Also about the aerodynamics part I got it from a documentary on NASCAR in the '50s-70s, it said how racers found that raising the rear slightly so the angle was just a few degrees up (nothing drastic or even very noticeable) made the car much more stable at high speeds...
Click to expand...
 
Yup. I believe thast the downforce from the Superbird/Daytona rear wings didn't begin to produce downforce until around 60 m.p.h. Raking a car down on the front end will produce more downforce at speed, but also more aerodynamic drag. It is a trade-off and is best adjusted with trial and error on the track. On the street, however, any effects will be negligible until you get to highway speeds.

Handling and road racing cars are generally as low to the ground as possible to keep the CG as low as possible, which helps getting around turns in the road. Body roll is not your friend when trying to get a car to handle.
 
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