Tag Archive: chassis

Most of us have heard about sway bars, and can recognize them when we see them. But what do they do? Quite simply they control the transfer of weight from one side of the car to the other. When does this happen? Why when the car goes around a turn. When the car leans over, from the force of inertia, the stiffness of the bar, which is the only component connecting both corners of the car, resists this action. That is why they are commonly called roll bars.

When you look at the tube frame GT1/Trans-Am cars you immediately notice that the sway bars are different from your street car, or a stock car. Not only do they look different but they are in a different location. That is partly by necessity, partly from design even thought they serve the same function. We will look at each of the two separately. For the purpose of this discussion we will only be talking about the front bar.

On the stock car the bar is typically mounted as low as possible, in front of the wheels. Earlier designs were mounted in two blocks on the bottom of the frame rails, like your street car. Newer designs place it inside a tube which connects the frame rails. This allows the car to be run at a slightly lower ride height. The arms are removable and slip onto splined ends on the sway bar itself. These arms connect the bar to the lower control arm. This design, while not adjustable, is simple and effective.  Its placement is made possible by the “kickup” on the front subframe.

The first thing you notice about the swaybar on a tube frame GT car is its placement. Unlike the stock car, it is mounted high, above the upper frame rail, and generally in front of the wheels. (see the picture above) The reason for this location is that these cars use a flat frame rail. Thus there is no room under the car to mount the bar. The bars are generally mounted in one of two ways. The traditional way is for it to be sandwiched in a pair of split blocks bolted to the upper frame rails. Some newer designs have a heim joint threaded through the bar itself, this is simply attached to a bracket on the upper frame rail with a bolt. Regardless, it like the stock car design is connected to the lower control arm. In this case be tubular bars with heim joints in either end for adjustability of length.

The ends on these  bars are a mixture of fixed and rotating ends. (see the picture for an example of a fixed arm) The fixed arm, while it has different mounting locations  for the links, is not adjustable once attached. The rotating end is unique in that it is driver adjustable while the vehicle is in motion. It consists of a flat bar which is bolted to the sway bar. Mounted inside a bearing on the sway bar, it is free to turn.  The other end of course is, like the non adjustable end, attached to the control arm via means of a link. Driver adjustablity is by means of a cable run back to a quadrant mounted inside the cockpit. By moving the lever the cable causes the blade to be rotated from the vertical. This reduces its ability to resist the vertical forces on it, thus reducing the resistance to roll. While this provides the driver with an ability to reduce handling  problems during the race it is not perfect. The change is not linear. There is no constant amount of change per movement of the bar.

This of course is just a simplistic explanation of sway bars, and I welcome your comments.

Example of sway bar mounting on 2011 Trans-Am car.

Chevy Beretta Trans-Am type car

While running around getting ready for Christmas I thought I would throw this picture up.This is a car that was built to be a spare car for the team. Never finished, it was recently sold to someone who hopefully will complete and put it on the track. Very nice piece though.

We haven’t forgotten the post on sway bars, but it may have to wait until after Christmas.

Merry Christmas to all, be safe and spend as much time as you can with family and friends.

Another photo of the Capri

As millions of us get ready to take to the highways, remember safety first!

Left front of Trans-Am car

Left front of Trans-Am Car 2010

For years now one of the trademarks of a first class SCCA Trans-Am, or IMSA GTO car was the centerlok wheels. Just by a glance at the wheels you could tell who was a serious contender, or a pretender. Generally those wheels were made by BBS, although Jongbloed and a couple of others were occasionally seen.

What was the reason that the centerlok, more properly center knockoffs, rather than the almost universal 5 x 5 pattern wheels were used? Basically there were three reasons.
1. Unsprung weight. The suspension components and hubs were lighter than on an equivalent 5 x 5 assembly.

2. Suspension geometry. Most designers are proponents of the “zero scrub” design. This requires getting the wheel mounting flange as far outboard as possible. In other words a typical three piece centerlok front wheel will have a 1″ or 1.5″ outer rim half. Because of its spindle design a 5 x 5″ wheel has to use a wider outer rim half, perhaps 3.5″ or 4″.
3. Pit stops. With the single nut, the time to change tires was less. Admittedly this was the least important of the three factors.

But nothing is without drawbacks, and this is no exception. The price of a centerlok suspension assembly is far more than that of a 5 x 5. How much, did you say? Well it varies but it is hundreds of dollars per corner. Wheels for the 5 x 5 are far more readily available and at a fraction of the cost. And lastly since most races no longer require pit stops the speed of tire changing is pretty much irrelevant.

I should note that there is a hybrid design, used back in the 90’s by Peerless and a few others that adapted a 5 x 5 hub to use centerlok wheels. This was neither fish nor fowl, didn’t solve the geometry issue, and added about one pound of weight per corner.

So which is the right way to go? Depends on your wallet I guess. If money is the most important thing than by all means go with the 5 x 5. But if performance, and looking the part, are critical than the centerlok is still the way to go.

View of the naked cockpit - Riley chassis TA car circa 2010Well, congratulations to Tony Ave, the 2011 SCCA Trans AM Series Champion! With his win in the second race at Brainerd yesterday he clinched the championship.

So lets take a look at these cars, again. Its interesting that many of these cars have been around a pretty good while. And that in one way or another they all go back to a common theme. The tube chassis built by Pratt and Miller back in the day. That was a quantum leap from the unibodies and stock car chassis that had gone before. In one fell swoop those were obsolete.

Before you go any further let me say that the following is MY OPINION, based on conversations with professionals. And hard to quantify given the constant changing of tires and car weights, engines, etc.

Even today I think most people would say that the Pratt cars are the Cadillac of all the modern TransAM cars. Now I know that some people will say, “NO!” that Rocketsports cars were the dominant cars. But I disagree. Rocketsports became the organization that P&M had been, and still are, but they weren’t better.

But the P&M car was one that was for the expert, not the average driver. And this led to the Riley and Scott, which was more comfortable for more drivers, and in the right hands just as fast.

And of course dont forget the many cars that Pancho Weaver built that were successful as well. These even more than the R&S were for the bulk of the competitors.

And I don’t mean to overlook the Roush cars, which had more success than any of them. But they had the benefit of more testing, more data collection than any team before or since. And, of course, absolutely first class fabrication and maintenance. They are deservingly collectors items today.

These were just the most widely known of the chassis, there were and are others. Peerless,Riggins, and  some teams built their own, particularly backup cars, and then the chassis Tony Ave is building has to be considered.

I invite any opinions or discussion you have about the subject.

Next time we can talk about the hardware. Transmissions and engines.

Chassis front

For several years I have had a bare GT1 style chassis lying around the shop. And admittedly I have been trying to sell it. Well, to be candid, I wasn’t exactly overrun with takers.

So after some deliberation I decided to go ahead and build it into a vintage GT1 style car. So after some consultation, I sent the chassis to Scott McLearen of 2M Engineering to lay the ground work.

First let me say that although I have the parts to make it into a true TA style, centerlok hubbed ride I choose not to yet. The reason being that I may use it for track days, or rentals first. And its cheaper to use 5 x 5’s. I can always swap to the centerloks.

Scott did a great job of fabricating the suspension and getting the car up on its wheels. He made brand new control arms and uprights, all the links, and trailing arms, etc. No used parts in the suspension at all.


Now I get to try my hand at fitting the body. The body I choose, is the correct one for this vintage of chassis, an 1987 Mustang carbon kevlar piece. This particular body was used, as you can see on the #65 AER car in 88. Still has the four National event win decals in place.
I’m just finishing trimming out the steel roof, required for these bodies. Mounting begins next week.

If you need good fabrication, or assistance at the race track I strongly recommend Scott.

Front Bodywork