F-body Chasis and Suspension Guide for Road and Auto-X
Please note this was taken from FRRAX.com and written up by Trackbird.
F-Body Chasis & Suspension Guide
Introduction
Setting up and tuning your car’s suspension and chassis is much like setting up your engine. A drag racer, at least not one that wins, would never install just any camshaft without considering his application and researching it as much a possible. His entire engine setup would be a collection of parts not just meant to work together, but also well tuned.
The most basic thing one needs to know when modifying their chassis or suspension is the application. Asking which shock or springs are best is not enough. One needs to know what class or series they’ll be racing in and what the rules allow beforehand. The setup for a NASA AIX car, SCCA Solo street prepared car, and daily driver are going to be entirely different. A drag racer with a trailered car will probably have different requirement for a camshaft than a casual enthusiast that drives his car to work everyday. A serious drag racer will typically not mind a rough idle, sub 3K rpm power, and changing valve springs every few thousand miles. However, the person with a daily driven car obviously will want to give up some peak power for reliability and drivability.
The MORE details you can compile regarding your application the better!
Our GOAL in setting up a car’s suspension and chassis often comes down to one thing-maximizing the grip provided by four patches of rubber. At the same time we want a car that is predictable. A car that doesn’t do anything weird in bump, roll, or under braking will be easier to drive and therefore should be faster. The main goal of this introductory guide is to get you to THINK about your car’s setup and approach it in a scientific way. Never be afraid to think outside the box or tinker with your car. Mistakes provide you with a valuable opportunity to learn, but it is also foolish to not heed the mistakes of others. Too many f-body owners just copy someone else’s setup or part. Just because something isn’t a bolt-in part designed specifically for an f-body that doesn’t mean it won’t work.
As a note this document covers 3rd (’82-’92) and 4th (’93-’02) Gen F-bodies. Every attempt has been made to distinguish between the two in this document. In general the 3rd and 4th gen’s both share rear suspensions and most rear suspension pieces are interchangeable. However the 4th gen uses a short long arm front suspension (NOT strut), the 3rd gen uses a strut suspension.
Basic Vehicle Dynamics & Tuning
Perhaps in the future this section will contain a basic primer on vehicle dynamics and how to tune your car. There are books available which cover this topic better than this article ever could.
The following are suggested for design and tuning/setup:
Tune to Win - Carroll Smith
Engineer to Win – Carroll Smith
Prepare to Win – Carroll Smith
Race Car Vehicle Dynamics –Milliken & Milliken (the “bible”)
F-body Overview
F-bodies have a reputation for being straight line only and poor handlers. However, a properly setup F-body is a fast autocross or road race car. Primary weaknesses from the factory are similar to many other cars. The OEM shocks are trash and the factory springs and sway bars are often too soft. Despite these problems, the F-body has a good suspension design from the factory and arguably has the best production solid axle rear suspension made. The only area where you might get real complaints about the suspension design is the rear roll center height. Some consider it to be too high, but there are solutions available. The first thing you really need to do is consider what springs, sway bars and then shocks you want to run. Skimp on any of these, especially shocks, and you will have an ill-handling car. There are so many parts of your car’s suspension and chassis that are so closely interrelated it’s difficult to tell one where to start. However, shocks, springs, and sway bars are where one will find the biggest gain. Throw in a panhard bar next and one will have a “nice handling” car. Even changing non-suspension parts can throw off your suspension setup. For example changing from a weak differential to a nice, higher-bias one such as a T2R may allow you to stiffen the rear suspension. Many people that are forced to run in a class where the stock diff is mandatory will run softer rear sway bars and springs to band-aid a weak diff.
F-body Specific Parts
Disclaimer: These cars are popular for drag racing and as such many aftermarket parts are designed with that in mind. Using a part for an application it wasn’t designed for is at best an expensive mistake and at worst fatal. Consider every part you buy and modification you make to your car and its possible repercussions. Parts are not to be put onto a car and forgotten about; they are meant to be routinely inspected and checked for signs of fatigue to prevent failure. Ultimately you are responsible for your own safety.
Shocks
Many people use Koni shocks because they are a quality adjustable shock that doesn’t break the bank. The front 4th gen Koni’s are twin tube as are the rears. However, 3rd gen Koni rear shocks are monotubes and can be used on 4th gens. The adjustment is different however. Koni shocks are available in single adjustables (SA) that adjust rebound damping and double adjustables (DA) that adjust both rebound and compression or “bump.” The downside, besides being twintube, is that Koni’s are costly to revalve and rebuild. Another option is to adapt Koni 30 series circle track shocks to the rear suspension-the same holds true for many universal shocks. Koni offers two models of single adjustable struts for the front on 3rd gens, the Specials (red) and Sports (yellow). The Sports have a heavier valving and are preferred for most racing purposes.
Bilstein HD’s are a popular shock, they’re monotube and best of all they can be revalved cheaply for your application. Jack at Bilstein and Sam at Stranoparts.com are great resources for valving Bilsteins to suit your application. Shocktec used to convert Bilstein to externally adjustable shocks, but they are currently out of business.
LG Motorsports offers a presetup coilover package using aluminum bodied Bilstein race shocks with springs for the front and rear. This setup uses user-revalveable shocks with rod ends that eliminate the factory compliance bushings. It uses a similar setup to the front suspension with a coilover shock mounted in the stock location, but with a different upper perch. The rear setup mounts coilovers to the stock shock location, but since they are coilovers the springs are now moved to the location of the rear factory shocks. This will change the wheel rate and calculations need to be done before comparing rear rates here. One issue that’s debatable here is that LG places the spring over the shock and thus puts higher load on the axle’s shock mount than the engineers designed it for. There is also one large bolt that passes through the axle shock mount which is placed in single shear. Despite concerns, this is a race proven product which Lou @ LG Motorsports used on his SCCA World Challenge Camaro. Another argument that can be made supporting the LG setup is that the shock mount, although designed for just the shock, probably has a large enough factor of safety built in to handle the added load. As with any part on a race car, constant attention and inspections are necessary.
Unbalanced Engineering offers both a Penske and Bilstein race shocks which can be custom valved to your application. The unique feature of their product is that their front setup uses the stock upper perch which makes them legal in two very popular classes-SCCA Solo ESP and NASA CMC. Like the LG setup these are high dollar setups usually sold to all out race cars.
**You may have noticed I omitted Gabriel, Tokico, HAL (their drag shocks) or KYB This is because their products are not usually suitable for our applications**
Coilover/Springs/Spring perches*
Coilover setups
Ground Control
In the front of 4th Gens GC offers an aluminum threaded sleeve and collar that sits on the front shock body. They allow the car’s height to be adjusted by threading the collar up or down with a spanner wrench-sort of a poor man’s coilover shock. Ground Control doesn’t make a coil over for the front of the 3rd gen, but they do have a weight-jack setup that places a threaded screw adjust assembly on top of a shorted spring that mounts in the stock spring perch. The weight-jack can be adjusted by using a 8-10” ½ inch socket extension that is run up through a hole in bottom of the stock A-arm. For 3rd gens Spohn performance makes a coil over setup.
Global West
Global West’s system is similar to that of GC for the 4th Gen. The main difference is that the collar uses a jam nut instead of an allen screw that tightens the collar. They are also a lot more expensive than GC’s setup.
Allstar
Allstar is a stock car product company. They have threaded spring jacks which can sit on the rear spring perches. Their biggest advantage is they allow the use of standard 5” race springs that are widely available from most any stock car parts vendor. The downside is they are steel and can rust if not clear coated. Another downside is they are adjusted by hand and lack slots for a spanner wrench. However, with stiffer than factory rear springs, the rear springs usually have no preload with the suspension at full droop thus allowing them to be rotated by hand.
**Also: see “Shocks” above**
Factory replacement springs
These are usually reserved for people that are interested in appearance or want to make a compromise between cost, handling, and driveability or any combination. Factory replacement springs are proprietary and limit you to a fixed ride height and the rate the designer chose. There are a few decent ones available. Eibach’s ’98-’02 pro-kit (will work on ’93-’02 cars and is better than the ’93-‘-97 prokit) for autocross or LG’s G2 springs for road course use are decent. Many replacement springs are too low for their spring rate or way too stiff in the rear. Coilovers and spring jacks allow you the freedom of swapping to a wide variety of rates and allow easy adjustment of ride height.
Sway/Sta-bars
For fourth gens you have front bars ranging from 28-36.5mm and rear bars from 17 to 25mm. Rear sway bars are interchangeable on fourth and third gens.
GM offers in addition to the stock V6 and V8 bars 1LE sway bars. All ’93-’02 1LE’s came with a 32mm hollow front bar. The ’93 1LE’s came with a larger 21mm rear bar whereas the ‘94+ 1LE’s used the standard V8 19mm rear bar.
The trend in recent years is moving towards a larger front sway bar for 4th gens. Suspension Techniques makes a solid, but HEAVY 35mm front bar which is cheap and readily available. SLP makes a hollow 35mm front bar, but it is overpriced and the wall thickness is rumored to be too thin. Sam Strano sells a hollow front 35mm bar made for him specially by Addco. It’s nearly as stiff as the solid 35mm bar, but without the weight penalties. For about $50 more Hotchkis sells a hollow 36.5mm front sway bar that is a bit stiffer and is powdercoated. Hellwig also produces a 33.34 mm or 1-5/16” solid front sway bar.
4th Gen Front Sway Bars:
Manufacturer OD (mm) Solid/Hollow
GM 28, 30, 32 Hollow
Addco 32 Solid
BMR 32 Solid
LG Motorsports 32.5 Hollow
Hotchkis 36.5 Hollow (6.25mm wall)
Spohn 32 Hollow
Suspension Tech. 35 Solid
Stranoparts/Addco 35 Hollow (6mm wall)
SLP 35 Hollow
Hellwig 33.3 Solid
On the front of 3rd gens GM offered 32mm, 34mm and 36mm hollow bars, the 36mm was standard on the 1LE’s. Hotchkis offers a 36mm (hollow I think) front bar and Spohn has a 33.3mm (1-5/16) solid front bar.
3rd Gen Front Sway Bars
Manufacturer OD (mm) Solid/Hollow
GM 32, 34, 36 Solid
Spohn 33.3 Solid
Hotchkis 36 Hollow
Hellwig 33.3 Solid
Rear bars for the 3rd and 4th gens are widely spread. Factory 4th gen rear bars included 18, 19 and 21mm rear bars. 3rd gen factory rear bars included sizes such as 23 and 24mm. Several companies sell a solid 25.4 mm rear bar and Addco as well as Spohn makes a 22mm solid rear bar. Most sway bars are actually in in inches so when we mention 25.4mm bar we also mean 1". Since the factory Hotchkis sells a 25.4 mm rear bar which is hollow. Recently Sam Strano Jr. has started selling a 25.4 mm bar with a 4mm wall for a very reasonable price.
Manufacturer OD (mm) Solid/Hollow
GM 17, 19, 21, 23, 24 Solid
Addco 22 Solid
BMR 21 Solid
LG Motorsports 21.5 Solid
Hotchkis 25.4 Hollow (4mm wall)
Spohn 22, 25.4 Solid
Suspension Tech. 25.4 Solid
Stranoparts/Addco 22 Hollow
Hellwig 25.4 Solid
Stranoparts/Addco 25.4 Hollow (4 mm wall)
Subframe Connectors
Welded or bolt-on? (hot topic) Many argue that bolting SFC’s will result in loosened bolts or elongated holes over time. This argument will never end. The simple approach to this is just get the things welded. It’s not that expensive and you’ll never be left second guessing yourself or worrying about it at night. You cannot go wrong having them welded unless you catch your car on fire.
Hard-top vs T-top? (hot topic) The roof of a 4th gen might have a t-top structure underneath the roof skin, but it does have two longitudinal braces one each side that connect the A and B-pillars. It can be argued that since these braces are just sheetmetal they don’t add any significant stiffness. However, that overlooks the fact that a unibody car’s structure is mostly sheetmetal. Visualize the rear axle welded to the ground and the front end of the car being twisted and then make your own opinion. Hardtops are also easier to prep for wheel-to-wheel racing, but that is another topic. 3rd gens are reportedly in greater need of subframe connectors than 4th gen cars.
SLP sells triangulated boxed SFC’s for 4th gens that are available in both welded and bolt-on. They feature large tall rectangular frame rails and are generally an excellent choice. They tie in laterally at the tunnel brace. Triangulated SFC’s, however, may not be legal in some roadrace and autocross classes. Check your rulebook first.
Kenny Brown’s 4th gen SFC’s are very similar to SLP’s, however while Kenny Brown’s wall thickness is greater the rails have a tiny cross-section which hurts their stiffness. Nevertheless, KB’s SFC’s are a good choice.
Global West sells tubular subframe connectors for 4th gens that just go front to back and do not brace laterally. GW’s aren’t that popular, but feature 2” tubing so they are worth looking into where rules may not allow lateral connections.
LG Motorsports offers 4th gen SFC’s brace the car longitudinally with tubular steel. LG also offers an x-brace kit that bolts onto these SFC’s to add stiffness.
Spohn, Kenny Brown and Global West make SFC’s for 3rd gen T-top and hard top cars that run a long the parameter of the underbody, just inside the pinch weld. Alston Racing also makes SFCs that can be use on any 3rd gen including convertibles. The Alston SFCs mount directly to the front sub-frame and runs diagonally to just in front of the rear suspension pickup point, and can be bolted or welded. There have been a few that have started installing both type a SFC and adding bracing between them to create a double ladder frame under the car.
There are also SFC’s available from the usual suspects most of which are just variations of the same design. As always nothing prevents you from having your own SFC’s fabbed up by a local shop.
Strut/Shock Tower Braces
The effectiveness of STB’s on 3rd and 4th gens is small and debatable. That being said there are a variety of 2 pt STB’s available from all the usually suspects. The only ones that really differ from the cookie cutter 2-points are the 3pt Kenny Brown, Edelbrock and the 3 pt and 4 pt LG Motorsports brace. Hotchkis also makes a 2-pt which is unique in that is separates from it’s mounts on the upper control arm/spring perch mounting bolts. Edelbrock is the only company that I know of that makes a STB designed for 3rd gen TBI and carbureted cars, but it is a 3pt brace and may not be legal for some racing classes.
Front Steering Brace
AKA “The Wonder Bar”. This bar is a front suspension brace for 3rd gens that helps reduce flex in the front part of the chassis around the steering gearbox as the steering wheel is turned. A chassis stiffening bracket came on some models of 3rd Gen F Body cars, which is no longer available from GM. This aftermarket unit is a good addition to anyone who autocrosses or road races their car. They usually bolt between the frame rails and the front sway bar mounts and tie the left and right frame rails together.
Units are available from most of the vendors that make aftermarket suspension parts for the 3rd Gen., including Spohn, Top Down Solutions, and others.
Bumpstops
Bumpstops are a valuable tuning tool and are available from a wide variety of vendors such as koni. When using a non stock bumpstop it is important to make sure that not only your car isn’t riding them at static height, but also that they are not too short and allow shocks to bottom out or suspension parts to hit the chassis. See “Tuning” above.
Panhard bars
This is a location where rod ends add little to harshness even on a daily driver. It’s almost a no-brainer to use a double rod ended piece. Poly bushing’ed PHBs, particularly BMR’s, are prone to failure in these applications. Rod ends also limit deflection and enhance the feel of the car and prevent wider rear tires from rubbing the fenders under side loading. Look for a PHB with high quality ¾” rod ends-preferably 3-piece rod ends that run about $30 a pop. You can also make your own PHB with parts from stock car part vendors. Aluminum is also suitable for this application and can save weight over steel, but either choice will work fine.
LG Motorsports sells nice quality steel and aluminum phbs. Unbalanced Engineering, Spohn , UMI, Stranoparts and others also sell steel phbs with quality rod ends.
Avoid designs, BMR, which use an adjuster in the center of the phb. Under axial compression this is where stress is the greatest and is the worst possible place to put an adjuster.
Panhard bar lowering brackets
The rear roll center height on 3rd and 4th gen F-bodies is high resulting in an inclined roll axis. When you lower a late model f-body the roll center height in the rear doesn’t drop as much as the front, therefore the roll axis becomes even more inclined. One reason that GM did this perhaps is to allow for softer rear springs so the ride quality would be improved. Since the rear roll center height is determined by where the phb crosses the axle centerline we can very easily change the rear roll center height by lowering the phb. This is accomplished by adding on phb brackets. This is nothing new, although Unbalanced Engineering released their prefabbed kit a few years back which has thrust this controversial issue into the spot light.
As covered under “basic vehicle dynamics”, the roll center height is a basic fundamental of the car’s suspension. A change in the roll center height will change the distance between it and the cg where centripetal force acts upon. Lowering the rear roll center height therefore increases the rear roll couple by increasing the lever arm length. This will increase rear body roll and if made alone the car will push. To counteract this stiffer rear springs and sway bars must be added. Then the shocks must be revalved. The basic goal in revalving them is to lessen compression damping to keep the car compliant over bumps (despite stiffer springs) and to increase rebound damping to control the stiffer springs. Popular shocks for this are Koni DA’s, Bilstein HD’s and other custom valved race shocks such as aluminum bodied Bilsteins or Penskes. Some users have made their brackets adjustable for different roll centers and leveling the panhard bar.
Torque Arms
There are a wide variety of torque arms available on the market. In general you have two types: stock length torque arms that connect to the transmission or transmission crossmember, and shorter arms that connect to the floorpan or a separate crosmember. If using a shorter torque arm it’s generally a good idea to have subframe connectors that tie in laterally like Kenny Brown’s or SLP’s since the floorpan isn’t very strong.
Random Technology, Spohn, BMR, Addco, SLP and others make a basic stock length tubular torque arm with solid rod ends at the rear end and a poly torque arm bushing that sits in the factory mount. The main advantage these arms offer over stock is the ability to adjust pinion angle and their added stiffness. The added stiffness and long length helps prevent brake hop, which ‘98+ cars are especially suceptable to. LG Motorsports offers a similar stock length torque arm, except it replaces the poly bushing up front with a solid spherical bearing.
Spohn and BMR both make shorter torque arms which connect to the tunnel brace behind the transmission. The reduced length results in greater traction on acceleration. Global West also makes a shorter torque arm which varies from these two. Cons of these torque arms is that the short arm length really hurts under braking making the car more succeptable to wheel hop. Since the point about which the rear axle rotates during bump changes with a shorter torque arm you can add rear spring rate to compensate.
Spohn also makes several kits for the 3rd Gens that move the front attaching point to the transmission crossmember. This front mount is a large rod end. The advantages of this system is it moves the front mount off of the fragile transmission tailshaft and onto the transmission crossmember, it is a solid front mount, removing some compliance from that end, and is movable in all three axes. The rear mount is solid rod ends and are, like the other aftermarket arms, allow for pinion angle adjustments. Additionally, there are Spohn kits for several different transmission combinations, so the user is not limited to what came with the 3rd Gen from the factory. The disadvantages of these arms are that they can transfer more road noise to the interior of the car, the front mount tends to rattle and require constant monitoring and servicing, they are heavier than the factory torque arm/crossmember combination, and there is some loss in ground clearance due to the construction of the crossmember. They are strong units, though, and for those running non-stock transmissions, they might be a viable alternative to other suspension solutions.
Unbalanced Engineering offers the most unique solution-a decoupled torque arm for the 4th Gen. This provides one with the best of both worlds. You get a long torque arm (effectively twice as long as the stock one) under braking and a short one under acceleration to provide maximum anti-squat. The only downside is the additional tuning time. A 3rd gen decoupled torque arm is rumored to be in the works.
Upper Control Arms (4th gen Only)
Global West offers an upper control arm with either delrin or del-a-lum bushings which fixes the lack of available caster and camber with the stock setup.
LG Motorsports offers rod ended tubular upper control arms which also allow for increased caster and camber over the stock setup.
The usual suspects also offer tubular UCA with the usual caveat about rod ends.
**Bushings: See “Lower Control Arms (Front)”**
Caster/Camber Plates & 3rd Gen A-Arm
3rd gens use a a-arm and strut configuration on the front.
The front a-arm is more than adequate for racing once the bushings are replaced with Grobal West’s del-a-lum bushings or standard poly bushings. The a-arms can be replace with a tubular arms, like from Spohn, that will save a couple of pounds, but some think it’s a lot of money for very little gain.
The strut portion of the suspension can be modified with replacement caster/camber plates. Ground Control make an assembly that can increase the caster and camber by replacing the bushing in the stock strut plate, but this does requires some modifying and welding of the stock strut place. Spohn makes replacement strut plates, but I don’t know it they can increase the caster or camber any.
Lower Control Arms (Front 4th Gen)
Many front LCA’s offered are tubular and are only suitable for drag racing. GM’s 1LE 4th gens came with special lower control arms. The difference was stiffer bushings. The actual 1LE bushing is rumored to be the same as the moog replacement.LG Motorsports makes the only tubular lower control arms for the front suspension that are suitable for road race or autocross usage. They feature a beefy rectangular cross-section much like the factory stamped steel LCA’s, but they allow the use of ¾” rod ends which eliminate stock compliance bushings. The use of rod ends also allows added adjustability which will increase the available camber and caster over stock.
**Bushings for stock front LCA’s. Moog replacement bushings are a proven replacement. Energy suspension makes a poly set of bushings for the front upper and lower control arm bushings that have a poor design for the caster bushing. The rest of the set is fine, but their caster bushing has been proven to fail repeatedly. Prothane uses a different poly caster bushing which is currently being tested by some on FRRAX.com The Prothane caster bushing should be much stronger, but that comes with a cost-there is some increased bind which seems to be inconsequential at the present time.
Lower Control Arms (Rear)
GM’s 1LE 4th gens came with special lower control arms. The main difference was the standard star shaped rubber bushings were replaced with solid ones for added stiffness. The actual 1LE bushing is rumored to be the same as the moog replacement. These are the same as 3rd gen rear LCA’s.
The other big choice is either an aluminum or steel lower control arm with ¾” rod ends. As with the panhard bar, good quality 3-piece QA1 or Aurora rod ends are a must to provide decent life and reasonable quietness. Unfortunately, unlike a rod-ended PHB, rod-ended LCA’s will hurt ride quality. Good rod ends will keep them quiet, but that doesn’t mean they won’t rattle everything else in one’s car. LG Motorsports makes nice aluminum or steel LCA’s in rod/rod, poly/rod, or with poly/poly configurations on each end. Unbalanced Engineering, Spohn, and UMI also make decent rear LCA’s. Like PHB’s, it’s quite easy to make your own rod ended LCA’s with stock car parts.
Avoid having poly bushings on both ends. Poly bushings will not allow the LCA to rotate during body roll and this will cause bind. The following LCA’s are acceptable for our uses: rod/rod aftermarket, rod/poly aftermarket, rod/rubber aftermarket, or rubber/rubber stock. Looking at the stock star bushings you can see how they were designed to be stiff in compression or tension, but soft in twist. If you cannot visualize why poly bushings are bad try the following exercise. Go look at the LCA mounting brackets front and rear, now imagine a poly/poly LCA in place and the rear axle glued to the road, now visualize the car in a right or left hand turn, the body rolls, the brackets will twist relative to each other.
Some will argue that poly/poly LCA’s act like a spring and you can tune around them by softening the rear suspension. However, it’s difficult to predict how much stiffness poly bushings add and how it changes during roll, therefore it makes much more sense to tune with springs, sway bars and shocks. Bind is never good.
Lower Control Arm (Rear) Relocation Brackets
There are both bolt-on and weld on LCA relocation brackets. Welding the brackets on is the preferred method. These are brackets welded onto the rear axle which allow the rear of the LCA’s to be lowered. Lowering the LCA’s increases anti-squat. Many drag racers will lower them as low as possible. The downside to this is lowering the LCA’s will cause roll oversteer which will make the car hard to control on course and can be very dangerous. It’s preferred to keep the LCA’s as close as level to minimize the amount of roll steer. In fact it’s preferable to have them slightly inclined towards the front to provide roll understeer which helps in applying power on corner exit.
Roll steer in the rear suspension is caused by the fact that the rear LCA’s scribe an arc as they move. When the car rolls in a turn one side will squat and the other lift causing the LCA’s to move in opposite directions-which they do and in an arc. The effective length of the LCA’s being longer on one side and shorter on another points the axle in a direction non-parallel to the car. If they rear axle points out you get oversteer and if it points in you get understeer. Roll steer is unavoidable but can be minimized by keeping the LCA’s as close to level as possible. Drawing a picture with the LCA’s inclined down, level, and up, along with some imagination, is the best way to visualize rollsteer
K-member
As far as road course or autocross usage you will be stuck with the stock k-member. The factory geometry is so good that there isn’t much to fix with an aftermarket k-member. K-members currently on the market are lightweight tubular pieces that aren’t as strong as the stock k-member and are prone to failure under our uses and sometimes even daily driving! The factory K-member is plenty strong, and by the time you get most tubular K-members gusseted and reinforced enough to stand the rigors of racing, they weigh almost as much as a factory unit.
LG Motorsports built and released photos of a prototype 4th gen k-member suitable for autocross and road course duty, but it has never reached production.
PA Racing produces a 4th gen k-member (not listed on their site) that they claim to have modified for road course usage, but there are still doubts about it in the community.
Motor & Tranny Mounts
In addition to solid motor and tranny mounts made by various vendors for race cars we also have poly mounts.
Motor Mounts: Energy Suspension and Prothane both make stiffer poly motor mounts. Although rumors of these causing false knock are abundant, many people have noted no KR after the installation. They also don’t seem to add near as much noise and vibration as the poly tranny mounts usually do.
Tranny Mounts: Poly tranny mounts are known for adding vibration and noise when used on 4th gen cars. However, removing the preload plate from the energy suspension mount can reduce noise as well as grinding down its poly nubs in the bottom. Prothane also make a poly transmission mounts. The main advantage of these mounts is they are the strongest available. The GM 1LE mount is another option which adds strength without the noise and vibration, but some people still break these.
Rag Joint Eliminator
This is a product made solely by Unbalanced Engineering. It’s basically an aluminum disk that replaces the fiber “rag joint” in ’93-’97 cars. The rag joint wears out over time and develops play in the holes for the rivets. It also breaks down quickly from the added heat of headers.
Wheel hubs
In general most people have the best luck with genuine GM hubs, although they are pricey and the parts store cheapies can be returned and replaced many times under their warranties. The best bet for a parts store replacement is probably Autozone. They sell Timken hubs. Timken is well known and supplies oem bearings on many GM products. The same applies to bearings in the 3rd gen hub/rotor assembly.
One board member bought a no-name hub and had it shear off while on course. It’s probably best to stick with GM or Timken for replacement hubs and stay away from used hubs. Used hubs on ebay could have any amount of mileage on them and there is no telling the supplier. Save money on the non-critical items.
Ball Joints/Tire Rods, et al
Moog is generally regarded as the best supplier of replacement ball joints and tie rods.
Rod Ends/ Heim Joints
As mentioned previously in the article, good quality rod ends are important. Quality rod ends will last many times longer than cheap rod ends and be much quieter. Look for a quality 3-piece rod end from a company such as QA1 or Aurora. Both companies also sell cheap rod ends so beware of low quality cheap 2-piece rod ends. Expect to spend about $30 for a ¾” rod end of good quality.
Fasteners
Most fasteners on an F-body are now metric. It is important to recognize and be somewhat familiar with classes. Standard/english bolts are ranked by SAE grades such as 2, 5, and 8, with grade 8 being the strongest. Common Metric classes included 5.5, 8.8, 10.9 and 12.9. These are NOT interchangeable with SAE grades. An 8.8 class metric bolt does not equal a grade 8 SAE english bolt. However a 10.9 class metric bolt is a close approximation to SAE grade 8. 10.9 class bolts are commonly see used in suspension and chassis parts. 12.9 class, although stronger is subject to hydrogen embrittlement, and shouldn’t be used on suspension/chassis parts. There are charts available online and in industry which list the allowable shear and tension for bolts based upon size and grade or class.
Be aware of the importance of following factory torque values and the max torque values for a specified bolt. Commonly we torque bolts down using a torque wrench which measures the amount of leverage we are placing on a bolt in either N-M, lb-ft, or lb-in. The goal here is to obtain a certain preload in the bolt, however since we are doing this based upon torque we’re assuming a certain amount of torque is used to overcome friction in the threads and the nut against the mating surface. Dirt, grease, oil, thread locker, anti-sieze, and even the plating on the bolts all change the friction and will throw off a torque value. Be aware that torque values are usually stated for dry clean threads or clean threads with a light oil. Using a factory torque spec for a bolt you applied anti-sieze will result in over torquing it. There are many charts and tables available online and in industry that will supply you with torque values.
Also be aware that every time you torque a bolt you stretch it and often times it should not be reused. LS1 owners that have ever done a head swap are familiar with this from using torque to yield head bolts. These bolts are stretched basically go under a plastic (permanent as opposed to elastic) deformation once torqued. The same thing applies to all bolts, although some can be used more than just once.
Using no-name bolts from a hardware store is a big no-no. Often times they are made in China and have cut as opposed to rolled threads. Use a reputable source for bolts such as McMaster-Carr or ARP when you need something more specialized. If you’re in doubt as to the strength required of a bolt, just use grade 8 or class 10.9 for just a few pennies more.
Final Note
If you pick up anything from this know one thing-never be afraid to challenge the same school of thought. Never be afraid to experiment (scientifically of course). And most important of all is that just because something isn’t available from a vendor doesn’t mean you can’t go out and have it fabbed. If you see a problem, research and fix it.
Suggested Reading & Resources
Tune to Win by Carroll Smith
Engineer to Win by Carroll Smith
Race Car Vehicle Dynamics Milliken & Milliken
Drive to Win – Carroll Smith
Nuts, Bolts, Fasteners and Plumbing Handbook - Carroll Smith
FRRAX.com F-body Road Race & Autocross Forums
Corner-Carvers.com Advanced Race Car Design/Setup covering all vehicles and applications
