Little Tire Traction

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How to Set-up a Stock Suspension Car, Part 1

 

Stock rear suspension drag racing is without question the big buzz today, and we’re not talking NHRA-legal stuff either. Instead, the current fervor is all about quick street cars. This is the sort of car that dominates, in sheer numbers, any of the myriad fastest street car organizations. But in truth, it’s also a wonderful combination for bracket racing, grudge racing or just to have fun. It’s also perfect for the Pinks All Out format, but that’s another story.

Here’s the rear end setup before the gas tank installation and before the brakes and other pieces were completed. We spent a lot of time considering the suspension geometry on the car. As noted, we’re not big on altering the instant center by a huge amount. We’ve seen so-called “lift bars” actually oval the holes in suspension mount points. Chassis experts figure this is because the lift bars are actually attempting to rip the rearend out of the car.

This shows upper trailing arms from both TRZ and AutoFab, both top quality parts. TRZ pieces (those installed in the car) are at the top. In this case, we decided to stick with one manufacturer for rear suspension hardware instead of mixing and matching.

What’s appealing about these pieces is their ease of adjustablity. In the case of a car such as our Buick (stock-style GM triangulated suspension), the upper bars are used to center the rearend housing from side to side (by adjusting one upper). The upper bars are also used to set the pinion angle (by adjusting both uppers in the same direction and same amount). The adjuster for the top trailing arm bars is shown here.

The beauty of a stock suspension car is that anyone can put it together and tune.  For the most part, it’s a pretty basic bolt-in deal. If you can swing a wrench, you can piece one of these suspension systems together. On the other hand, if you have a car with limited wheel well room, there are a few complications.  In our case our project car is an ‘86 Buick Regal. GM G-bodies such as this have tons of aftermarket support, they’re easy enough to find, and they’re reasonably affordable. The only big downside to these cars is the fact the frame needs some massaging in order to fit some decently sized back hoops. For a closer look at the frame fix, see the accompanying sidebar.

What about the actual suspension arrangement? Big power will usually smoke the tires in an early A-Body or a late-model G-Body or a Fox-body Mustang.  How do you make it work? When it comes to the back end of any of these cars three major pieces are needed to make them hook: trailing arms, shock absorbers and an anti-roll bar.

We removed the top bushings and bushing sleeves from the rearend housing and installed a set of spherical bearings from TRZ Race Cars. Actually, we used the billet bearing retainers from TRZ and swapped out the spherical bearings for pieces of superior quality from Aurora Bearing.

When it comes to trailing arms, GM, for example, used U-shaped stampings. As you can imagine, this isn’t the most conducive configuration when it comes to laying down big E.T. numbers, especially with some big grunt under the hood. Boxed replacements are available, but that’s more of a crutch. The real answer is to install tubular components with adjustment capability. Why is adjustment capability important? One major concern is to establish a working pinion angle, however that’s only the beginning. The truth is, the coil suspension found on an A, G or Fox-body is nothing more than a 4-link with angled trailing arms. By incorporting angled bars, the manufacturer saved money. It wasn’t necessary to include a Watts linkage or track locator in order to keep the rearend from falling out of the car. It also meant the car could be a bit lighter, particularly from an unsprung weight perspective.

The lower trailing arms we used on the Buick are also from TRZ. The TRZ are on top and the AutoFab pieces are on the bottom. The big difference is the Delrin bushing on the frame side and the adjuster on the rearend. The Delrin-equipped bar is more street friendly than one with rod ends (less harsh and less noise). Delrin material doesn’t deflect like poly, which means it’s a far superior material for a street driven car. Some poly bushings cause “stiction”—the bushing momentarily seizes—and that causes all sorts of traction tuning issues. The lower bar adjustment shown in the last photo of this sequence is designed to set the wheelbase on the car (and obviously attempt to simultaneously center the wheels in the wheel well). This setup allows for minute wheelbase changes.

 

Instant Centers

The other potential dilemma is the instant center (IC) location. It’s way out there, and in some cases, ahead of the car or near the engine. Stop right there. What is “instant center”? Instant center or IC is an imaginary point about which the chassis or a suspension member rotates in a given position. This all sounds complex, but it’s not.  IC is found by projecting lines along suspension members to a point of intersection. Where those lines intersect is the instant center. IC acts as a pickup point for the rear suspension, even though the points are imaginary.

We didn’t use lift bars, airbags or trick springs (the stock six-cylinder springs are in the car), and we didn’t use revised suspension geometry either. The key to making these things hook is in the anti-roll bar and the shock absorbers. To pre-load the suspension in an A, G or Fox-body Mustang, a sway bar is most often used. When the car is driving straight down the road, an anti-roll bar is effectively in a neutral position, meaning it has no affect upon the way the car feels or works. It also has no effect upon ride height. Get on the throttle though, and the anti-roll bar counteracts torque rotation. Increasing the diameter of a stock rear bar or using a weld-in anti-roll bar allows you to tune the amount of torque rotation the car has. The beauty of the weld-in ARB is that you can physically get underneath the car and adjust the links that locate the bar to the rear axle housing (see inset photo). That means you can add or subtract pre-load at will, and it’s pretty easy to disconnect or disable should it be desired.

 

What about using the old “no hop” bars that were so common in the ‘70s?  The problem is, by altering the angle of the bars, they tend to create a very short instant center. This usually works pretty well in a car with low power, but as you feed more and more torque to it, the car becomes increasingly violent.  And in some cases, the top mount bars (that raise the upper trailing arm location on the rearend housing) have a tendency to hammer the trunk floor when you get on the throttle.

Years ago we spoke to race car builder Jerry Bickel about the subject of IC points, and he noted that many very successful Pro Stock drag cars have instant centers way ahead of the vehicle, as do any number of cars fitted with torque arm suspension (for example, any of the late-model Firebirds and Camaros). Those cars are known to hook pretty much on grass, and they have decidedly long instant center points. That’s why it’s not necessarily a bad thing to leave the actual suspension mounts in the stock location.

 

Pinion Angle and Preload

Including adjustable suspension members means you’re provided with the opportunity to fine-tune the suspension and to set the static pinion angle. Why is pinion angle so important? If the pinion angle isn’t properly set, the operating angles of the driveshaft and U-joints can’t be controlled. Pinion angle is measured between the pinion gear flange and the driveshaft. As the suspension in a car wraps up, the pinion is driven upward. In order to insure that the pinion is correctly oriented while under power, it is typically set nose down static. For most applications with solid rear suspension bushings or spherical bearings, the pinion angle should be set between to -1 to -2 degrees (negative angle or pointing down). If the suspension incorporates OEM-style rubber suspension bushings, then a pinion angle of -3 to -4 degrees is preferred. The reason is due to the added deflection in rubber bushings.

The other big key to adjustability at the rear of the car is the shock absorber arrangement. We wanted a high quality double adjustable shock. Strange Engineering is doing just that, so it was our choice. Strange has both adjusters (rebound and compression) for each of the back shocks located at the base. Because of the adjuster size and location, we had to rotate the shock at the lower mount–first and last photo. 

 

So far so good, but what about preload?  Preload is a major tuning tool in traditional drag race 4-link cars, but when it comes to a triangulated 4-link, it’s not quite the same. Here’s why: In a high horsepower combination, engine torque tends to roll-rotate the chassis. This means the car bites better in the right rear. That extra bite tends to move the car toward the left. In a low powered combination, the forces that rotate the rearend tend to provide more bite to the left rear tire. In either situation it’s possible to counteract the forces with preload. In a conventional, non-triangulated 4-link, you can shorten the upper right bar to increase the preload on the right rear tire. If you can lengthen the same bar, then more load is placed on the left rear tire. Sounds easy enough, but it doesn’t work that way with a triangulated 4-link.

Shock absorber adjustment is super easy to set. We set the rebound first, and for a baseline, we set it to the hardest (full clockwise) position, making the shock difficult to extend. The body won’t separate easily; it’s a very conservative starting point.

We set the compression adjuster to full soft (full counterclockwise) position. Tnhe shock will be easy to compress, and the car will droop at the back (very conservative start setting). At this point, we can tune the car for various drag strip conditions. When the car is set-up for street use, it’s easy to turn both adjusters (rebound and compression) to four or five clicks back from full hard. If there’s a need to firm up the handling for some reason, just turn both of the adjuster knobs clockwise. It’s not necessary to remove the shocks from the car for adjustment. Simply crawl underneath and twist knobs—no tools required.

On cars equipped with triangulated upper control arms setting preload isn’t possible with the upper bars. When one upper control arm is adjusted then you’re actually shifting the rearend housing left or right. The folks from TRZ Race Cars point out that the upper bars on a triangulated 4-link are used to center the rearend housing from side to side. The upper bars are also used to set the pinion angle. Adjustable lower bars are used to center the tires front to back in the wheel well and to make minute wheel base dimension changes. Then how can you tune or preload the rear suspension in one of these cars? Use an anti-roll bar to do the job. Then adjustable shock absorbers, front suspension limiters, and to a lesser degree, specific spring rates are used to get the car to hook.

The accompanying photos detail a bolt-in suspension arrangement we installed in a G-Body Buick. Check it out.

 

TIRE TUCK

Notching a Frame to Accept Big Rolling Stock

GM’s G-bodies are perfect race candidates, at least on the surface. When initially planned, the inspiration was weight reduction and everything, including the glass, got thinner. The bodies arrived fully framed with relatively spacious engine compartments that could accommodate anything from a V-6 to a rat motor. Unfortunately, there’s a fly in the ointment, lack of rear tire room.

In stock form, a P275-60R15 drag radial is as big as it gets. You’ll struggle to fit a 9 x 28-inch slick. The problem spots within the wheel well are the leading and trailing edges of the frame rail. Here, the factory frame moves outward toward the exterior quarter panel and that’s where big tires encounter interference.

The fix is to notch the frame. Once the frame is properly notched though, a G-body can accept tires ranging from P325-50R15 radials to 11.5 x 28.00-inch slicks, and that’s with the stock inner wheel well untouched. Here’s a short thumbnail sketch of how it’s accomplished.

Text and Photos by Wayne Scraba

 

 

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