TC rod FAQ by JMortensen

9 minute read

TC rod FAQ

Compliments of JMortensen @ http://forums.hybridz.org

1-30-2006

TC Rod FAQ

The TC rod is an integral part of the Z suspension which locates the front control arm and controls the caster. It can be modified in various ways and these modifications are the subject of this post. Bushings can be changed, a nylon/aluminum cup assembly can be added, or a new adjustable arm can be substituted. This FAQ will try to address the common issues with these parts and hopefully will help you decide whether it is a part worthy of buying or not.

WHAT IT DOES

The TC rod takes the braking loads from the front suspension and transfers that load to the frame rails and into the structure of the car. The stock rubber bushings will give, and this give can be felt as a wandering under very hard braking. If you dont intend on racing your Z you will probably never experience this feeling, and substituting any parts to fix this problem is probably a waste of money for you.

ALTERNATIVES TO OLD STOCK RUBBER BUSHINGS

New Rubber Bushings--The most obvious alternative is NEW stock rubber bushings. New bushings will help a lot, and the stock suspension is designed with the compliance of rubber bushings in mind. Installing new rubber bushings is an easy and safe way to improve your Zs performance under braking.

Polyurethane: Polyurethane bushings are a very common upgrade for Z cars. In the TC rod area they might not be the best choice however. In a control arm bushing the polyurethane allows the control arm to pivot up and down but prevents deflection. Deflection in the control arm bushings causes a mushy feeling and camber change in the front and can cause that same mushy feeling, camber change, and toe change in the rear. The TC pivot works differently. Unlike in the control arm bushing where the metal inner sleeve rotates inside the polyurethane, the TC rod pivoting through its arc FLEXES the bushing to allow movement. Since polyurethane is so much stiffer than stock rubber, moving the suspension through its range of motion puts a lot more stress on the end of the TC rod and there have been cases of the TC rod actually snapping due to these stresses over a period of time. Failures seem to be more common in the ZX and 510 suspension where the rod is in tension, but we have some reported cases at hybridz of Zs having the same failure where the rod is in compression under braking. Possible fixes for this problem are to use a poly bushing in front and a stock rubber bushing in the back of the TC rod, since the front takes the braking load and the rear rubber bushing will allow the arm to pivot more easily, or even drilling holes in the poly bushing to give it some more compliance.

G Machine Aluminum/Nylon bushing replacementThis system creates a joint that takes the place of the front bushing. This joint is made from aluminum cup and a nylon ball which pivot freely as the suspension moves. This is a pretty good option. The bushing in the back is not important for taking the load under braking, so using a rubber bushing in the rear ensures that the force required to move the suspension though its arc shouldnt be more than stock. This does move the pivot slightly, which has other deleterious effects on the suspension, read more below to see what this involves.

Adjustable TC RodsOne way of making the TC rods adjustable is to take the stock rod, cut it, and add a turnbuckle in the middle somewhere. If you choose this method you can use any of the bushing configurations above. The benefit to adding the turnbuckle is the ability to adjust caster. The stock rod is about the perfect diameter for running a 5/8" die down it. However when I tried this modification it became apparent that my rods were already bent. Your mileage may vary.

Adjustable TC rods have been available through Arizona Z Car for years. Also one of our own moderators Mike Kelly used to make some (but does not any longer) and a new company called Techno Toy Tuning is selling these parts on ebay. There are others available as well. They all work basically the same way, although the designs differ slightly. The benefit to this style TC rod is adjustable caster, and that is a very significant benefit.

The rear end of the TC rod uses a metal to metal rod end joint that attaches to a clevis. This clevis bolts in place of the stock front bushing. The rear bushing makes no difference at all and is usually (always?) eliminated because the pivot is provided by the rod end in this style setup. My understanding is that some of these use a polyurethane bushing behind the clevis. This would introduce a little flex into the system, and is done to reduce road noise and increase compliance. Most eliminate the front bushing as well and rely on the metal to metal joint to allow the suspension to move.

There are some issues to be concerned with the heims jointed TC rods:
1.Wear. These are really race car parts and I don’t believe that any manufacturer of these arms recommends them for street use. They use a metal to metal joint that needs to be inspected regularly for wear.
2.NVH transfer to the chassis. I have had this type of TC rod in my car for years and I just inspected my frame rails and had no cracks, but there have been reports of cracking frame rails around the TC rod area, and the speculation is that this is due to the NVH transfer to the rail. If you install suspension parts that have no compliance it is necessary to inspect the suspension arms and chassis regularly.
3.Binding of the control arm bushing. As caster is added the control arm pivots forward. When you add a whole lot of caster the angle of the control arm can get pretty severe. This can result in binding in the control arm bushing. The bushing is pretty much straight at the factory caster setting of 3 degrees. If the goal is to add a whole lot of caster it is a good idea to use a custom lower control arm which also uses a rod end and allows for the control arm pivot to move freely. Torquing the lower control arm pivot by using poly and adding caster is a bad thing.
4.Binding of the bolts that locate the front of the TC rod. The bolt holes where the TC rod bolts to the control arm can also bind. When adjusting caster (always with the vehicle on it's wheels) it is a good idea to loosen the two front bolts when adding caster, then adjust the caster, then lock the two bolts down again. Doing this ensures that the end of the arm isnt preloaded. It is theoretically possible to add so much caster that loosening the bolts and tightening them down doesnt alleviate the load on the end of the arm. In this case it should be possible to open up the holes a bit to alleviate that pressure. The Arizona Z Car arms and some others come with a clevis at the front to attach the turnbuckle to the end of the arm which bolts to the control arm. The clevis is loosened, caster adjusted, then tightened. This eliminates any stress on this area of the arm from caster adjustments but relies on the smallish 3/8" bolt to connect the two pieces.
5. Semi-leading arm? The clevis type attachment that is universally used in these aftermarket arms extends the pivot out an inch or an inch and a half from the stock pivot location if bushings are used. This seems fairly inconsequential, but actually this turns the front suspension into a semi-leading arm. This causes the suspension to compress under braking loads.
6.Caster curve change. The shorter arm will also result in a more dramatic caster curve. As the TC rod goes past horizontal under braking or as the suspension compresses caster starts to fall off.
7.Camber curve change. Since the caster curve causes the caster to go positive as the TC arm goes past horizontal, it also affects the lower control arm as well. The control arm does not swing perpendicularly to the centerline of the vehicle. Instead it swings slightly forward until the TC rod is level then slightly back as the TC rod goes past horizontal. This change is more dramatic in cars using a clevis and a rod end, because the shorter arm requires more angle to move through its complete arc.
8.Dynamic wheelbase changes. Because of the changing caster curve, the wheelbase will actually change during the suspensions movement as well.

Benefits of the adjustable TC arm:
1.Adjustable caster. The stock Z front suspension is reluctant to turn in. Adding caster will make a very noticeable improvement in the vehicles ability to turn in, and the car will require less negative camber in the front end to keep tire temps consistent across the tread.. In my experience this benefit far outweighs the problems listed above. Some may not agree with my assessment, but that has been my experience.

Benefits of the rod end TC arm:
1.Reduced stiction. Rod ends move freely throughout their range of motion. Unlike the rubber, polyurethane, or even the GMachine bushings, the rod ends have very little stiction in them and do not bind when installed correctly. The more freely the suspension moves the better the tires will stick to the road.


OK, thats about all Ive got. You all found some errors in my last FAQ, so please point out anything amiss in this one. As always you work on your car at your own risk, and using racing parts on a street car is not condoned or recommended.