Race Prep. and Repairs
Overview and Checklist
Post Race Disassembly
Post Race Inspection
Repairs and Preventative Maintenance
CV Polishing and Prep.
Suspension Tuning
Suspension Tuning
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General background It seems that in most forms of motor sports, horsepower is viewed as the single most important aspect of vehicle performance. While other systems are important, the engine seems to be the recipient of the majority of R&D effort. In desert racing, at least in Class 5/1600, suspension plays an even more important role than the engine in overall performance. We strive to wring as much performance as we can from our "class-limited" engine but we are more often "suspension limited" than "horsepower limited". In other words, we usually have more horsepower available than we can use due to suspension limitations. The terrain in desert racing is some of the most inhospitable imaginable and no matter how much horsepower we have available, the terrain and our suspension's ability to "soak it up" dictate how fast we can go. Simply put, the more suspension travel we can make available, the faster we can go. The class rules that we compete under limit what we are allowed to modify from "stock" when we build a car; SCORE's Class 5/1600 rules dictate that we retain a "torsion bar" type suspension and that we keep the original track width, wheel base (up to +1") and inner IRS pivot geometry. The rules also limit modifications to the body that would tend to minimize tire/body interference problems (body "lift kits" NOT allowed). Those rules, combined with the physical properties of the actual parts, restrict how much suspension travel is possible/practical. The goal then, is to design, build and tune a suspension system that allows us to utilize all the horsepower we have available, in all terrain types while remaining within the class rules.......
Having finished building our car just just two weeks before our first race, the 2008 San Felipe 250, we ran that race before we had an opportunity to tune our suspension. A short time afterwards, we met with our shock supplier's rep. in the desert to do an initial "suspension tune". By the end of that day, we were 30% faster than when we arrived. We started the day thinking that we'd realize much smaller gains; by the end of the day though, we were amazed at the huge improvement in speed/handling. Since then, we've refined the tune further and are now nearly 50% faster than at our first race. Having raced for several years now, we are better able to "feel" what the suspension is doing and to know what adjustments are needed. More importantly though, we spend considerable time testing changes to find what works and what doesn't. We use video extensively, allowing us to actually see what the suspension is doing and then making changes as necessary. What
we attempt to do is to set our spring rate, ride height, compression
resistance and rebound resistance such that body movement is
minimized while the suspension keeps the tires in contact with
the ground as much as possible. Our "bypass" shocks allow us
to do this over a wide variety of speeds and terrain types; we
can tune for both high frequency/low amplitude and low
frequency/high amplitude movements at the same time. As part
of our pre race prep., during our test/tune sessions just prior to
the race, we alter our suspension tune to suit the anticipated race
conditions. Class rules
that limit modifications from "stock" We can see quite a few rear suspension details here, if we look closely. The first thing you can see is that the drive axles are at a fairly significant angle from horizontal. The CV's (and the geometry of their relationship to the transaxle and trailing arms) dictate how much "droop" we can get before the CV's begin to "bind". We determine where that point is and then restrict "down travel" by installing limit straps (just visible alongside the shocks). Also visible are the scuff marks where the tires are rubbing the fenders when the suspension is fully compressed. The tires will actually travel a bit further up before they are stopped by interference with the body. Some cars use purpose built "up-stops" but we simply let the body stop upward travel. Looking very closely, you can see that we have a stock torsion housing and that we use "long" torsion bars. Also, if you know what you're looking for, you can see that we have "micro-stubs". Micro-stubs allow us to reduce CV angles over a "stock" wheel bearing housing arrangement, allowing just a little more down travel (and they are stronger than stock). There is a fair amount of "positive camber" visible at full droop; the camber changes from positive at full droop to nearly zero at ride height and then negative at full compression (caster changes also, but you can't see that in this photo). The camber change is an unavoidable result of the basic VW rear suspension design that we are not allowed to modify in this class. I'm not claiming an exact rear suspension travel number but you can see how much distance there is between the tire and the scuff mark, then add a bit for body interference to stop upward travel...... We could get more total suspension travel if we were allowed to do a "body lift" (a commercially available 3" lift kit would give us 3" more "up" travel, a huge advantage over cars without one) but they are not permitted under SCORE's 5/1600 class rules......
The car is sitting with both front and rear suspensions fully "drooped" - the limit straps are stretched tight at this point. Once we get the engine mounted and some fuel in the fuel cell, the rear suspension will settle some and you'll notice less "rake" (the car will sit more level). The front suspension is sitting higher than we want it at this point due to the new torsion packs we installed. They will "settle" some during our test and tune session; we'll make the final pre-load adjustments after they do so. Same with the rear suspension; we want to run the car around the desert a little before we make final adjustments. A little of the "rake" you see here is normal for a 5/1600 car, but not quite this much - the rear will probably be set 3-4" lower and the front 2" lower come race day, depending upon our best guess as to the condition of the race course.......
This is the driver's side rear trailing arm and spring plate assembly - the rear wheel mounts via our "microstub" assembly to the aft/outer end (upper right of center in this photo). The assembly pivots around the two tubes at the front of the assembly (bottom left and right in this photo); the small tube on the left is the "inner IRS pivot" and the long tube on the right is part of the "spring plate" where the torsion bar is inserted to provide the "spring action" - this end is mounted in urethane bushings and is free to rotate but not move otherwise. As you can see, the two pivots are not "coaxial" and, though you can't see it here, the line between the two pivot locations is also not perpendicular to the "fore/aft" centerline of the car...... This is why there are both camber and caster changes as the suspension cycles up and down on a 5/1600 car that is in compliance with the class rule set. The only way to eliminate the camber and caster changes is to change the geometry of the inner pivot - and the rule set specifically disallows those changes. It is possible to make the wheels sit perpendicular to the ground at ride height by building the trailing arms to do so - but the camber/caster changes will still happen (you are just moving the problem from one part of the suspension travel to another) if the inner pivot geometry remains unaltered. Having the wheels perpendicular to the ground at ride height is a good thing; as with everything in racing though, there are compromises to be made and rules to follow......
There's quite a bit to see here also; first off, you can see that we retain a traditional VW style front suspension beam (though it's an aftermarket, heavy duty version). We have stock width/length trailing arms but they are much heavier duty than stock, as is the "King Kong" king pin/spindle assembly. We also have heavy duty link pins tying everything together. Stock VW parts, even reinforced VW parts will not survive here if you're really racing...... In the left side photo you can see that the trailing arms are able to droop almost vertically, they could do the same in the upward direction. That being the case, you might think that almost 18" of front travel is possible; while that's physically possible, several problems will force you to limit both the up and down travel to more like 10" total. The first problem is the amount of "twist" the torsion packs are exposed to - even with 10" of total travel the torsion packs see in excess of 90 degrees of twist - and that's very hard on them. The second problem is that it becomes very difficult to eliminate "bump steer" beyond about 10" of total travel without relocating the steering box by cutting away the frame head for clearance (not allowed in the rule set beyond 1" of clearance for tie rods). If you look at many 5/1600 cars, you'll see that rule violated often....... The third problem is that the king pin/trailing arm geometry does some strange things at the extreme ends of travel and you really don't want to operate there. Finally, the front shocks must be attached to the trailing arms somewhere and allowing an extreme range of travel complicates that issue. If you look closely at the left side photo, you can see our "up stop" - a 1" diameter pin welded to the end plate between the upper and lower trailing arms. The lower trailing arm stops traveling up when it contacts the pin. By the time it gets to that point, the trailing arm is generally not moving very fast so there isn't any damage done. The right side photo shows our "down" limit strap, we just felt it was easier than making a mechanical stop of some sort. You can also see our King "Race Series" bypass shock and how the cage structure was built to support the upper end. The two tabs you see welded to the upright to the rear of the shock are for steering dampers. We built the car without power steering originally and the dampers were an attempt to reduce feedback to the steering wheel - it didn't really help though. Power steering (at least the Char-Lynn type) is WAY better at isolating the steering wheel from impacts that would have previously torn the steering wheel from the driver's hands.
Here you can see our front suspension at both the upper and lower stops at the same time. The right side is fully compressed and the Driver's side is fully "drooped". The torsion packs are anchored at the center of the beam by the torsion adjusters and permit both wheels to act independently. There are no "sway bars" on our car, that allows us to do what you see in this photo (otherwise the driver's side tire would be off the ground here).
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