Suspension, High Performance modifications

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Suspension, High Performance modifications

Postby Larry Jowdy » Wed Sep 19, 2007 6:55 am

The 550 Spyder Suspension By Doug Snyder
Edited by racerdave and Larry Jowdy

This write up is intended to give you a basic understanding of the suspension on a Replica Porsche 550 Spyder that uses VW style suspension parts in a configuration similar to the original 550.
First, we should be aware of what we have to deal with. Be advised, I’m not being overly negative but just stating the facts. Below, you will fine two quotes from “Tune to Win” by Carroll Smith. If you aren’t familiar with Carroll Smith he is the savior of the amateur racer/car builder. If you own no other books you should at least have a copy of the four “Win” titles he wrote. #1 Turn to Win #2 Engineer to Win #3 Prepare to Win, and, #4 Drive to Win. In my opinion, no other author explains these subjects as well as Carroll Smith.

The Swing Axle, according to Carroll Smith.
“The swing axle is an abortion, it should never have been invented: today its use would not be considered by any automotive engineer let alone a racing car designer. It is of interest only to those fanatics involved in Formula V, where its use is a requirement. Its disadvantages include: a very high roll center, extreme jacking, extreme camber change, and almost total lack of adjustment. It has no advantages.

Trailing link front suspension, according to Carroll Smith.
“Trailing link front suspension has a minimum number of parts-all arranged so that in order to withstand the loads involved they must be truly massive. The wheel paths are very bad indeed. It is a fit companion to the swing axle.”

Just based on Carroll Smith’s description of the suspension on a Replica 550, it’s obvious that at best, it is archaic and in many instances, unsafe.

Right from the start, you can see we have a lot to contend with. If we were all sane and just used our toys for Sunday drives and never, never, disobeyed the speed laws, there wouldn’t be a problem but, with a car that looks this good it just has to be driven fast doesn’t it?

What I intend to do here is describe how the suspension works, its bad manners and how to control the inherent negative characteristics.

As with all things, there is a trade off, the better the car handles the harsher the ride. The reason for this is that a major strategy to improve handling is to limit the suspension movement, which in turn, means higher spring rates.

In front we have parallel trailing arms. Two massive arms extend out from the torsion tubes. Inside the tubes the arms are connected to the torsion bars, which are composed of a stack of flat leaves. Together these leaves form a bar, which extends through the tube to the other control arm and is the torsion spring. Midway between each torsion tube is a set-screw that secures the torsion bar from rotational movement. The torsion tubes/suspension is bolted to a stock Volkswagen pan and welded to the chassis of a Replica Porsche 550.

At the end of the arms are ball joints (pre-1966 front ends use king pins instead of ball joints and, I’ll leave that discussion to someone else.) The ball joints connect the arms to each hub/spindle. The upper arm ball joint end goes through a hole in a round conical plug that fits in a larger hole in the hub. The hole the ball joint goes through is off center so that when you rotate the plug (camber eccentric) in its hole the hub will move in and out. This allows for camber adjustment. The arms are normally leaned back a bit, that is the upper arm is leaned toward the rear of the car compared to the lower one. The amount of this lean is the caster. (This is adjusted via shims on a stock VW but is not adjustable on a Replica 550 Spyder due to the front end being welded to the frame.
Last edited by Larry Jowdy on Wed Sep 19, 2007 7:22 am, edited 3 times in total.
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Postby Larry Jowdy » Wed Sep 19, 2007 6:59 am

Now the whole idea of suspension is to make the ride as comfortable for the passengers as possible while maintaining the best possible handling. One must decide the relative importance of these opposed objectives. To handle well, the suspension must keep the tire in contact with the road surface. In other words, it must maximize the area of the tire tread that touches the ground. Where the tire meets the road is called the contact patch.

With a trailing arm suspension the wheels move up and down exactly perpendicular to the chassis, when you enter a turn the car leans and unfortunately the tire leans exactly the same amount. This makes for bad camber control and reduces the contact patch.
The stock method of controlling this is with a camber eccentric, which increases static camber and caster, which increases dynamic camber. Unfortunately the stock values are not adequate for high performance. So what do you do? Here are several ways to help the situation.
There are two basic things that need to be done, increase the camber and reduce the body roll. There are two way to increase static camber, the easy one is an after market eccentric which has about double the range and can increase the static camber to about 2 to 2.5 degrees. The second is to weld up the hole in the hub and redrill the hole further over. Obviously this is a lot more costly and time consuming and probably only the hard core would consider it. If this method intrigues you, go to a shop that sets up the suspension of off highway desert racecars. The other way to increase camber is by increasing caster. You add a caster wedge behind the lower torsion tube between the tube and cradle. When you increase caster you get what is called camber gain. Camber gain acts as the suspension compresses. When this happens, your camber actually increases.

The purpose of all these measures is to increase negative camber. Negative camber causes the top of the tire to lean in toward the center of the car. The reason for the increased camber is as follows. Let’s say you’re making a left turn. As you turn the car, the weight transfers to the right side of the car. As the weight transfers, and if you have several degree’s of negative camber, the tire contact patch will increase when the tires rolls toward the outside of a turn due to the shift of weight. Essentially the car rolls onto the tire.

Unfortunately the Beck/Chamonix and Vintage have their torsion tubes welded to the frame and thus you can’t increase caster except by cutting and re-welding. This will require very precise measurements to ensure that the front end is perfectly square when you re-weld it back to the chassis.
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Postby Larry Jowdy » Wed Sep 19, 2007 7:05 am

Another area of concern is to reduce as much body roll as possible. To accomplish this, you need to install an anti-sway bar. The stock VW bar is not adequate so I suggest you investigate several aftermarket sources. The best method of affixing the anti-sway bar is to attach the bar to the frame and the ends of the bar to the trailing arms. This should be done using urethane bushings as they don’t compress at the same high rate as does rubber bushings. The bushings that attach to the trailing arms where they meet the torsion tubes should also be replaced with urethane. Desert racers “box in” the trailing arms (welding a metal shell around each arm) and may be a good idea for this project. VW welded a brace between the top of the shock tower and the torsion tube on their Type 181 (Thing) and this too may help in keeping the front end rigid.

The rear swing axle is more problematic and to make your car handle better, you must accomplish two things. First, limit the droop of the axles to prevent what is called jacking. Jacking occurs when you apply the brakes or when turning a corner, one or both rear wheels go into extreme positive camber (top of the tire leans towards the outside of the car) and severely reduces the contact patch.

The second is to decouple the rear suspension from body roll so that the tires remain flat on the ground even when the body leans.
From an earlier post.
“What we want is to move all the roll stiffness to the front of the car, that's one of the reasons for such a big bar up front. Adding roll stiffness (sway bar) in the back aggravates the issue as it causes more camber change when we want less. The opposite of a roll or sway bar is a Z bar. The difference is that instead of both arms projecting the same direction, one of them goes the opposite direction and of course it has the opposite effect, it decreases the roll stiffness. What's happening here is the swing axle pivots from a single joint near the center of the car as the car leans each axle pivots (one up, the other down) and thus causes camber change, not good. By reducing the roll stiffness you lessen this effect. The body can lean with out the axles having to follow. The other thing you really need to do to the rear is to limit droop which is the other way you get big camber change (the so called jacking effect). What is happening here is as you transfer weight forward (brake) both axles droop.”

The other way to solve this problem is with a Zero Roll as commonly used on Formula V racecars. Formula V, for those of you unfamiliar with this term, is an open wheel race class in which the rules dictate that VW suspension be used. The class has been around for a long, long time and about every conceivable trick has been tried. With a zero roll, the wheels are suspended against each other instead of to the chassis. Arms extend up from the axles on each side and a spring and shock are placed between the arms so that when the axle pivots it applies force to one end of the spring/shock and thus compresses the spring and applies force to the other axle. The beauty of this is that the body roll is now decoupled from the suspension. The body is free to lean about without affecting the contact patch.
Last edited by Larry Jowdy on Wed Sep 19, 2007 7:58 am, edited 2 times in total.
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Postby Larry Jowdy » Wed Sep 19, 2007 7:05 am

In conjunction with Zero Roll, you must add something to prevent axle droop. To accomplish this, rods are attached to the arms that run underneath the coil spring from each side. At the center, these rods go through a washer and a rubber donut and attach to another washer on the far side. So, you have rods from each side that pass each other and capture a rubber donut in between. When the axles droop, the slack is taken out and the washers eventually squeeze the rubber donut and then stop any further drooping of the axles. This whole set up is adjustable by changing the length of the rods. You set the zero roll so the tires never go into positive camber.

Here is a quote from Racerdave describing the difference between a Z bar and zero roll.
“ I have had plenty of experience with the swing arms with both the Z-bar and the zero-roll. With the swing arm, It has been my experience to worry most about droop, or in other words keeping from going into positive camber.

Both of these methods work, as does the anti-sway bar setup that Steve has designed. With the Z-bar working at it's greatest potential the car can stick really well. However there is one attribute that can cause some apprehension.
As you drive thru a turn at 10/10 the cars rear tends to ratchet around the corner as the zbar loads and unloads from one tire to the other. Sometimes it's hard to distinquish between 10/10 and 11/10 when the car breaks loose and you're off mowing the grass.

With the zero roll, it is much smoother. The car is much more predictable in the corner, but it depends on a large sway bar in the front.
If you jack the back end off the ground, you can ride the back tires like a see-saw.

On the zero roll there is a limiter to keep the droop at neutral (0 camber)
On this limiter there is some sort of snubber. Either a die, spring, or some really soft shock snubber. This really makes for a smooth transition to the end of suspension travel under braking. Without this, even with the zero-roll, you would be mowing grass. “


Now I’m sure some of you are saying why do we need all this? The original 550’s didn’t have it and people drove them fast and isn’t it contrary to the idea of a replica to add all this stuff. Don’t we want to be faithful to the original?

I can’t argue with this point of view. If authenticity is your goal then so be it, and yes people did drive these cars to the absolute extreme. Those people were mostly superior drivers and with enough talent you can overcome a lot of adversity. But. for us mere mortals who can’t stand to have a car that looks this fast and then watch a late model Carrera pull along side and give us a snide look and not be able to dust him, well here’s to you kid!

I’m sure there is more to be said but maybe this will do for a start.

I will start a new topic which will allow you to post comments.

Look for “Suspension” in the technical section.
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