Suspension and chassis upgrades
- Category: PROJECTS
- Published: Saturday, 17 January 2009 09:19
- Hits: 20896
I'll begin by discussing the goal of my suspension and chassis suspension upgrades. I am not looking for a racing level handling. I want crisp touring handling that is equally comfortable on brisk drives through mountain passes (I live in Colorado).
I'm using a Vredestein Sprint Classic 185/70x15 radial tires on 5 1/2" wide Minilite 8 spoke knock-off alloy wheels. This wheel and tire combination will provide considerably more holding power than the original skinny 5.90x15 bias ply tires and heavy 4" wire wheels. That increase in holding power translates into more stress on suspension components and an opportunity for better handling if they are properly controlled. So what I wanted to provide for the wheel/tire combination was better shock control and better management of the tire attack angles to the pavement.
With that in mind, I decided that Udo Putzke's Fahrspass Bilstein gas shock conversion kit was the best place to start. It's not cheap, but I have come to believe it is a good value, and it has received very positive reviews. It has been in the market for a long time, so it's well sorted out. The Bilsteins are custom valved for Healeys, and the kit integrates the tube shocks in a manner that is very unobtrusive to the existing structure. To carry the style integration a few steps further, I exchanged the customary bright blue dust covers for black dust covers, made custom covers for some of the exposed bracketry, and powder coated all the bracketry to match the stock chassis components. (Pic #1, 2) These few changes make the Bilstein kit look like they might have been a factory option. In the front, the stock Healey shocks remain in place and continue to function as the pivot point for the upper A-arms. However, as directed by the kit instructions, I removed the shock valving. Now the original shocks are not providing any bounce or rebound control. In the rear, the lever shocks are completely removed and replaced with the Bilstein conversion kit. (Pic #3)
For roll control, I replaced the stock 5/8" front sway bar with a 3/4" bar from Tom's Import Toys. Tom recommended this size rather than the 7/8" which he felt I would find too harsh for normal street driving. He considers the 7/8" bar better suited for track work. I took his recommendation. I also purchased a set of his uprated elastomeric polymer bushings to replace all my stock rubber suspension bushings. Tom said these are about 30% stiffer than stock. I believe that will be about right to control my wheel/tire combination, but not as stiff as the urethane racing bushing kits that are on the market. A complaint with the racing urethane bushings (generally a red color) is that they may develop squeaks. Of course, this isn't a big deal in a track car, but it can be annoying in a street driven car. Tom said he has never had a complaint of his polymer bushings squeaking. I have driven my bare chassis around the neighborhood, and it feels tight but not harsh and nothing was squeaking... I look forward to trying it with all body panels in place.
In the rear, my BJ7 has a panhard bar to control side to side movement of the rear axle. However, there was no provision for axle wind-up on braking or acceleration. Under these conditions, braking and accelerating, the axle tries to twist or "wind" the spring... the spring will resist this winding and wants to unwind. If it is under extreme stress, the wind/unwind scenario is repeated and may be violent enough to cause the tires to bounce often referred to as "axle tramp." Of course, bouncing means your tires are not maintaining constant contact with the pavement and control is compromised. Even in less extreme conditions, optimal rear tire control is marginalized by this condition. The goal is to keep the rear tires planted on the pavement under all stress conditions. The improved grip of my tire/wheel combination will tend to exacerbate this condition. The BJ8s came with radius control arms (sometimes called trailing arms, tramp bars, or anti-tramp bars) which assist the springs by preventing twisting under stress conditions. Cape International supplies a nice kit to fit radius control arms to big Healeys. I purchased one of these kits. The only issue with the kit was that both it and the Putzke Bilstein shock kit replace the lower spring pad with one of their own design. So what I had to do was fabricate a lower spring pad that would accommodate both the shock mount and the radius control arm attachment point. I contacted Udo Putzke and chatted with him about this problem. He was extremely helpful, both in providing me some of the raw mounting bracket material (before it was welded up) and with advice on the proper measurements for the new mount. Net net, I have a custom lower spring pad that beautifully handles both the lower shock mount and the radius control arm attachment. (Pic #4, 5) Pic #6, shows the completed rear suspension with new rear springs, radius control arms (you can only see the left one in the photo), the stock panhard bar, and the Bilstein shocks.
Now that the front and rear have upgraded control systems, let's talk about how to steer the package. A major drawback of an Austin Healey's front suspension is that there is no capability for adjusting the camber angle of the wheels (the stock camber angle is 1 deg. positive, meaning the top of the tires tip away from the centerline of the car, like this /). Back in the day, apparently this wasn't too big an issue since I guess the tires were the weakest link in the fight to maintain adhesion with the road... I really can't say. But, modern tires can benefit from some additional negative camber. I would like to see 1/2 to 1 deg. of negative camber angle (meaning the top of the tires tip in toward the centerline of the car, more like this / ). To get the capability to adjust camber, there are currently a variety of kits out there by providers such as Cape International, Denis Welsh Motorsport, Moss Motors, and Tom's Import Toys. However, when I was putting my plans together the best option was adjustable camber shock mounts made by a gentleman in Austraila, Theo Van Heel (I learned of this beautiful piece from Larry Varley's website). His mounts replace the original shock mounting brackets that are welded to the top of the frame towers. Using a tapped plate machined from high strength steel, these plates can be adjusted in and out with two horizontal bolts. I contacted Mr. Van Heel, and he agreed to make me a set of these adjustable camber shock mounts. They are beautifully engineered and allow the stock shocks to be infinitely adjustable for camber. (Keep in mind that the stock shocks act as the attachment and pivot point for the upper control arms and the control arms in-turn attach to the top of the king pin. The shock mounting point determines the camber angle of the wheel. Moss Motors just recently offered a similar kit.) To install these, as I mentioned, you must replace the current front shock mounts with the adjustable shock mounts... not a trivial job. Since I was rebuilding the entire car from the ground up, this was doable. To ensure that the new adjustable shock mount plates were accurately placed, I entrusted the job to Sports Car Craftsmen under the guidance of Paul Dierschow. (Pic #7, 8) Paul and his guy, Dan, carefully aligned the replacement mounts. I now have a fully adjustable front suspension.
The last element of the equation was the steering. As we were rebuilding the steering box, it was discovered that there was a need for some machine work to correct the years of wear to the main steering shaft worm gear. Before doing that, I did some investigation and learned that there were limitations with the machining capabilties of the day. The original steering worm gear had a dead spot which was supposed to be positioned in the straight ahead locatio... meaning when you were going down the road straight, the "numb" or dead feeling was the center. In actual practice that was rarely the case. The dead spot is generally off to one side or the other. The orginal steering worms also got looser as they got to the ends of their travel (closer to lock). Modern equipment can eliminate these steering inconsistencies. So rather than spend money on fixing my old worm gear, I decided to purchase one of Denis Welch Motorsport's new and improved constant clearance steering worms gears. The new steering shafts are made on computer controlled 5 axis machines which result in worm gears that are considerably more accurate than the original. The big benefit of this technology is that there is a constant steering feel throughout the range of movement... much like a rack and pinion steering. That is the steering shaft I put in my car.
In addition to these modifications, I replaced all the wear related components: king pins, cross rod assembly, side rod assemblies (also called tie rods), and, of course, every bushing in the suspension with the polymer bushings mentioned earlier.
Lastly (okay, actually the first thing), I strengthened three areas of the chassis. Specifically, (1) the engine mounts, (2) the center tunnel shroud support for scuttle shake, and (3) the original rear shock mounts. First, when I had inspected the frame, after it was media blasted, I discovered that just about every frame connection to the engine mounts was cracked... I mean all of them! I had heard these were weak spots but in a low mileage car, as this is, I really didn't expect to find so many stress related cracks. I believe I've resolved this issue with four gussets on every engine mount connection point. (Pic #9, 10) Secondly, scuttle shake, or shroud shake, is a common issue with Healey's. In an effort to eliminate this weakness, I have strenghtened the connection points from the frame to the center tunnel support. I'd like to give credit to Larry Varley, as once again, it was Larry's website that first put me onto this modification. At any rate, I've run angle iron struts from the frame up the center tunnel and across the top. My understanding is that this additional shoring up of the framework will fix the problem. I'll report back on whether or not it was successful. (Pic #11) This picture shows the center tunnel after the additional bracing has been tack welded into place... note, in this photo the chassis is on a rotisserie and is pictured on its side. I later final welded the bracing along its full length. The third area that I strengthened was the original rear shock mounts. As noted earlier, I've installed a Bilstein shock conversion kit. This kit uses the original shock mounting brackets for its attachment point of the rear tube shocks. Since the Bilstein tube shock will apply more leveraged pressure to the stock mounting locations than they were original subjected to, I decided it wouldn't hurt to add gussets to these original brackets to resist the twisting motion that they will be subjected to. (Note, Udo Putzke has not said this is necessary, this was something that I felt would benefit the stress the bracket would be put under.) (Pic #12) This photo shows the additional gusseting prior to welding. It provides a good view of what was strengthened on the original bracket.
Update July 24, 2010 - Suspension upgrade
The radius control arms originally installed and described above, could well be defined as fixed radius control arms. The point being that these arms are a specific length which doesn't change. Therefore, you're radius control arms and springs are in tension with one another whenever they move up or down. Why? They are attached to the spring pad and chassis at different locations so their pivot points are different (one above the other). The result is that they are moving in different arcs. (Refer to Pic #5) This will naturally cause them to be in tension with one another when they move. While this operating tension is well known, it is still better than the alternative, which is the spring winding up as described in the earlier paragraph. The rubber mounting bushings do a bit to mitigate this situation, but the condition remains. The solution for this is active or dynamic radius control rods. These are effectively radius control arms that change length and, yet, still provide resistance to spring wind-up. Udo Putzke designed a set of these for Austin Healeys using custom Bilstein components. Udo calls them "anti-twist shocks." I have a set of those on my car. Pic #13 The anti-twist shocks place no resistance to gentle oscillations as are present in normal every day driving. But, given a more aggressive attempt to twist as would be the case under hard acceleration or braking, the active units resist. The results are that they impose no additional harshness under normal conditions but they still provide the needed opposition to wind-up under stress. I understand that 2009 Mustangs with straight axle rear suspensions have this technology... oh, and, yes, Austin Healey's have a straight axle rear suspension. I am very grateful to Udo for sharing this development with me. (Link to Putzke)
This project paper was originally posted in April 2009.
This project paper was updated on July 24, 2010.