|
bigmac wrote:
Several months ago, Brent had an article ( under Good Tips I believe) on setting pinion angles on driveshafts. I cannot locate it so does any one have a reprint of the article? Thanks in advance
This one?
Good Tips
May 2007
Brent Vandervort
Rear suspension geometry
The rear suspension on our hot rods has two primary jobs: to support the weight of the back of the vehicle, and to control the position of the wheels relative to how the chassis goes down the highway. Independent rear suspension is more complicated and less common than a solid rear axle, so we’ll deal with that another time. Solid rear axle suspension is often called “live axle”, and has to keep the axle square in the car so that a tendency to turn is not induced. You certainly would not intentionally install a rear suspension out of square, but that often happens accidentally due to improper design or installation.
Leaf spring suspension is probably more common on hot rods, but we’ll first look at the equal length four-bar type to illustrate a critical design point. Four bars control the axle by means of four radius rods. The term “radius rod” makes it clear that they operate in a radius, part of a circle, forming an arc as they rotate. In a hot rod, that rotation takes place as the axle moves up and down during normal suspension travel over a bump in the road. Travel also takes place in a turn since the body rolls outboard of that turn. What is going on during body roll is that the outboard end of the axle is moving up relative to the chassis, and the inboard side is moving down. And that’s where problems begin.
If the bars are level, relative to the ground, the radius rods describe an arc that is perpendicular to the ground (true vertical). That motion causes the axle to move forward in the chassis an equal distance on each side, which is why drive shafts have a slip joint. The axle remains square in the chassis and no roll steer effect is induced. Notice that all this good stuff is based on the radius rod arcs being true vertical. What happens if they are not?
Let’s say our intrepid hot rodder is the real careful type, and has real good equipment like a level, maybe even a digital angle one, or an angle finder. He most often sets the chassis on stands, carefully leveling it side-to-side and front to back. This is his first, fundamental, and possibly fatal error. If the chassis is not going to sit level front to back when it’s done, why would you begin the setup procedure that way? Think about that a few minutes before going on.
If our builder made the common error and set the chassis level, he probably also set the bars level, being a careful guy who even reads instructions on occasion. When his hot rod is finished and does indeed sit on the predicted 3-degree rake, his bars will be down 3 degrees and the travel arcs will also be 3 degrees off true vertical. This doesn’t sound like much until we realize that the effect is doubled in body roll. That outboard side of the axle is going up relative to the frame while the onboard is going down. A three-degree error off of vertical will give you about 1/8” horizontal movement based on 2” of travel. And, the effect of the body roll is to bring the axle forward on the inside of the turn and back on the outside, so we introduce oversteer, tightening the turn that the driver had in mind. Now he has to turn the steering wheel back to compensate, the body rolls, and the process starts all over again. These oscillations can be merely annoying and lead to the car being described as “squirrelly”. If all this occurs at high speed in a vehicle with a high center of gravity (read that street rods and SUVs) it can get downright dangerous.
Our guy certainly would not purposely install the axle out of square it but ended up that way by accident. Sway bars would help by controlling the body-roll that upsets the axle position and are highly recommended for any performance vehicle. Get them too stiff to compensate for bad design, and your back will complain as they damage ride quality. Experience has shown that a 2-3 degree rake is common to hot rods. Doesn’t it make more sense to set it up with that rake? Then you don’t have to be constantly making corrections for, or worse yet ignoring suspension, engine, and drivetrain angles. Get this right and it all gets easier.
We’ve been discussing equal length four bars here, since that is the ideal for normal road use. You can also get good results from a four bar that has shorter upper bars to fit under a floor that you’d rather not cut to clear equal length bars. There will be an increasing amount of pinion angle change as the difference in bar length increases, so watch going too far. Turning on the opinion warning light, I would be very leery of an upper bar less than 70% of the lower bar length. Triangulated four bars do have a shorter upper bar since it is angled to provide lateral axle location without the use of a panhard bar. The working length of that bar would not be its actual length, but rather the effective length as seen in a side view. That shorter upper should still be true level and parallel to the lower bar for best handling.
Another variation on the four-bar is the four-link, coming out of drag racing. It uses upper and lower bars that are seldom parallel, or even the same length. Both ends of the bars can be raised or lowered in order to tune the system, changing the “bite” according to track conditions. What’s really happening is that we have an adjustable length ladder bar system. If you make an imaginary line that extends the bars forward, those lines will eventually cross at a point called the instant center. As the angle of the bars are altered, that instant center can be moved up and down to change rear chassis rise, and forward or back to alter the weight transfer. All that is fine for a drag racer, but it doesn’t answer the challenges of a street car as well as a more normal 4 bar. It also tends to take up a great deal more space, so you will often see the trunk area minimized or eliminated. It can be readjusted to have the bars level for the street and altered for drag racing, so it can work well for a dual-purpose hot rod.
Triangulated rear 4 bars can work very well, especially with Airride. They eliminate the lateral locater (Panhard) bar by using the diagonally mounted upper arms to control side-to-side movement. In some applications this different mounting makes more room for exhaust, better floor pan clearance, or a perfect spot to mount coil springs or air springs. Drivability is excellent, as good as a “five bar”. The only real downside is that they are designed to mount to a weldable axle housing, so guys with 10/12 bolts, Dana, and Ford 8.8 axles will have a problem. Don’t think that you can use some magic rod to weld suspension parts to cast iron axle housings!
I just mentioned the “five bar”, which is a term I use to describe a parallel four bar with an additional lateral locater called a Panhard bar. The French Panhard cars in the pre WWI era used this bar to control axle side sway on springs, just as we do. It’s probably true that each bar works at its best since the loading is all compression and tension with no side loads, but that doesn’t seem to show up in drivability. The Panhard bar must be as long, as level and as high as possible to work at its best. That often causes problems with floors and exhaust as mentioned above. Too short a bar will see more sideways axle movement, as will an angled bar, for the same reasons we discussed a four bars need to be level.
Since most older cars have a relatively high center of gravity (CG), it is better to have a fairly high roll center (RC). That sounds wrong until you realize that it is the relationship between them (called the roll couple) that does the work. A high CG with a low RC will have severe body roll. Equal height center will have no roll, which is why tether cars have adjustable height cable anchor points. If you can get the RC above the CG, the car will transfer weight to the inside of the chassis in a turn. That is one reason why McPherson strut suspensions can handle so well. The height of the frame side attachment of the Panhard bar largely dictates the RC height. That’s why you see the NASCAR guys adjust the rear bar to “tighten” or “loosen” a car during a race.
Speaking of NASCAR, to this day they use a trailing arm rear suspension very similar to GM pickups from the ‘60’s and 80’s. It has a front mounting point way forward and very close to the centerline of the chassis. It works great for both traction and handling, and is very simple to install. I think the main reason you don’t see them more is a space issue. Where the pickups have a lot of room under the floor, most hot rods and resto-mods don’t. There are some excellent kits on the market, but you have to be willing to do the floor mods to get the needed space.
Finally, parallel rear leaf springs can be made to operate much like a four bar. The GM engineers made a very clever mod to the ’59-’62 Corvettes that added an upper bar to that system. The front half of the leaf spring acts as the lower bar, with the back half only taking vehicle weight rather than axle control. With the extra bar to absorb energy, axle wind up is eliminated. We have added similar upper bars to a number of hot rod rear leafs with much success. The “three link” using a single long upper bar near the chassis centerline and a Panhard bar is a variation of this type that can work well if you have the space in the driveshaft area of the floor.
Going back to the whole discussion concerning the importance of a level bar controlling a rear axle, we can extend that concept to leaf springs. The point where the axle housing connects to the spring, or a lowering block, is connected to its forward bushing center to determine the “bar angle”. A highly arched spring with a tall lowering block will produce a steeply angled “bar”. Major league axle hop often results. It would be better to de-arch the spring or use a 2” tall maximum block to keep the “bar” more level. The real proof of this is seen on ’34-’54 GM cars which originally used a high arch spring. When you jack one up, the axle moves way forward. That makes centering the axle in a hot rod fender more difficult. It also illustrates how important this entire level bar lecture is to those who think it’s not important.
We used all the info above when we developed Fatman’s Wonderbar rear suspension for the Early Camaros and Mustangs. It was our opinion that sufficient space was lacking for a proper length upper four bar link, and not enough floor-pan room for a trailing arm or three-link system without major floor rework. Triangulated systems are out there that we have seen on the track, so they obviously can work OK. We have also installed systems which insert an upper bar link into a pocket welded into the floor. They allow a longer upper bar with little loss of rear seat space, and work really well with high horsepower drivetrains that really need help hooking up. We wanted to design something that would have all the handling and traction without the cutting.
Our Freightliner and other heavy trucks have used a single tapered leaf with Air springs for years. The spring is flexible enough to allow one wheel to rise over a bump while the other side stays in place, yet still enough to add a little roll resistance. It is mounted level to keep the axle squared, and provides chassis rise on acceleration to plant the tires. Coilovers or Shockwaves® are used to support the car’s weight. Since the tapered leaf does not have to absorb energy supporting weight and serves only to locate the axle and absorb axle torque, it can handle wheel hop control easily. This design doesn’t replace a four bar in 500HP plus situations, but works surprisingly well in uncut hot rods with more reasonable engines. Our ‘34 Ford Highboy sedan is using this system with Shockwaves® for excellent ride and handling. Please don’t take this info as a sales pitch, but rather an illustration of how you can take all the theory and use it to come up with innovations.
When you have done a bunch of cars and found out what works well, it makes one appreciate the aftermarket kits more than ever. Most manufacturers have really done their homework on proper design. They wouldn’t survive long in the market without good word of mouth advertising. Different designs exist for different purposes and opinions. We can talk over any question in the seminars offered at select Goodguys events. Collect the catalogs, check the websites, and talk first hand to the vendors at the next Goodguys show to find the system that suits you best! See ‘ya out there!
|
