Let's continue with the copy-paste. Ceilidh on Road Racing and Shine all the way to the end....



6. Road Racing, and the Shine Real Street Suspension



Let’s take a brief look at the general characteristics of a decent road-race setup, after which we'll touch for a moment on a setup that gets a lot of press in these pages: the Shine SRSS.



Backdrop



If you can remember back to the Autocross installment, we started there by considering what the ideal setup would be if we ran on perfectly smooth roads (e.g., lower the car to drop the CG, stiffen the springs and bars hugely to reduce roll, go to negative camber to keep the outside tires upright, install ultrastiff shocks to control body movements during transitions, etc.); then we looked at how the unavoidable presence of bumps -- even on a smooth autocross track -- forces us to dial back on those "ideal" suspension mods; and finally we concluded by observing that, for autocross at least, the jury's still out on what's the ideal compromise....



If there's room for Golf/Jetta IV variety in Autocross, it'll be because of a tradeoff between the simplistic benefits of lowering & stiffening vs. the more subtle ability to maintain traction and control on bumps and rough surfaces. Because Autocross surfaces are in fact fairly smooth, and because the low vehicle speeds in Autocrossing make the bumps seem even smaller, it's conceivable that a low, very stiff setup might actually work (we'll leave it to experts to comment on whether such setups do or do not work in reality -- all we're saying here is that if such setups are to work at all, it'd be in Autocross...).



When we get to road racing, however, the much higher speeds and more varied terrain mean we can no longer even pretend to ignore the bumps, and that seems to pretty much rule out the ultrastiff, very low setups for the Golf/Jetta IV. In the Autocross installment, we touched on some of the reasons why that might be so, but let's now look at it with a little more detail:



Spring Rates and Mechanical / Rough Road Grip



As discussed earlier, for every combination of road surface, vehicle weight, unsprung weight, tire stiffness, and vehicle speed, there is a certain suspension spring rate (and shock damping) that will maximize tire grip. If the spring is too stiff, the vehicle is thrown into the air with every bump; if the spring is too soft, the vehicle might ride smoothly, but the inertia of the unsprung weight will carry the tires upwards and off the ground on each bump, and the springs will be too weak to force them back down. Both situations will lead to the tires unloading on the back side of bumps (in the too-stiff case, because the whole vehicle is rigidly leaping about; in the too-soft case, because the tires alone are hopping up and down), and traction is therefore lost.



(As an aside, the above is one of the several reasons why traditional sport utilities & trucks ride and handle so badly on rutted pavement: an old truck-based SUV has very heavy solid axles with a lot of inertia, and that causes the axles to leap about on rough roads. Soften the springs for a better ride, and the axles bounce around almost uncontrollably; stiffen the springs to force the tires back onto the pavement, and the whole truck gets jolted with every bump. It's partly for this reason that almost all the new generation SUVs are going over to independent suspension: one advantage of independent suspension (on a driven axle) is drastically reduced unsprung weight...)



So, in theory one has to carefully choose the spring rate on a car to maximize traction on a real road or track, as too-stiff and too-soft are both problematic. But in practice, at least with Golf/Jetta IV's, we don't have to worry about too-soft. Our independently-suspended cars are heavy, and the CG (even on a slammed car) is high relative to the track and wheelbase; hence if we start softening the springs, we run into problems with body roll, dive, squat, heave, etc. long before the unsprung weight gets out of control.



Or to put it another way: to control the body motions on our cars, we have to stiffen the springs enough that they are almost always too stiff for optimal mechanical grip. Hence, in practical terms, we have a more or less clear-cut tradeoff: stiffen the springs to control roll (and pitch, heave, etc.), or soften them to improve the tires' grip on the track.



Implications of Reduced Spring Rate -- Why Lowering Causes Problems



Thus because of bumps, we can't arbitrarily stiffen the springs to control roll. (And remember from the earliest installments: if the car rolls, the stock Golf/Jetta suspension is intentionally designed so as to progressively lean the outside front tire, which reduces grip and causes moderate to heavy understeer.) So once we leave the smooth, low speeds of the autocross track (and some would argue even *on* the smooth, slow autocross tracks), we have to soften the springs. And that softening causes a whole litany of problems for a lowered car, of which we'll highlight two of the biggest:



1) When we earlier discussed the "ideal" setup on a "perfect", smooth autocross track, we rather naively imagined that we could stiffen the springs to the point that the chassis no longer moved: our "perfect" setup wouldn't roll in corners, and neither would it pitch, dive, or squat under acceleration or braking. Whether or not that's a reasonable approximation on an autocross track is something we'll let the autocrossers debate, but once we're on a real road-race track, with real bumps and realistic spring rates, we can no longer pretend the body isn't moving around. It will move relative to the wheels as the car accelerates, decelerates, corners, and encounters bumps. And that movement has some big geometric implications, particularly at the front suspension:



For a variety of geometric and packaging reasons, it's almost impossible to design a production car front suspension that doesn't "bump steer": when the front wheels move up and down relative to the chassis, they don't stay pointing straight ahead -- instead, at some point they will begin to toe-in or toe-out.



Such toeing would be pretty undesirable for fairly obvious reasons: it means that the toe setting changes when the car pitches forward under braking, or when it pitches back under acceleration, or when it rises and falls over bumps. Even worse, if the car rolls, one tire might be toeing in while the other is toeing out (because one wheel is rising while the other is falling), which steers the car to one side (called "roll steer"); or if one wheel hits a bump but the other doesn't, the bumped wheel can toe in or out while the other keeps pointing straight, which again steers the car (called "bump steer"). Mix and match these various situations -- e.g., let's simultaneously decelerate and roll the car via trail-braking, and then hit a one-wheel bump on corner entry -- and the combined roll and bump steer effects can be extremely entertaining and ever-changing, thereby inspiring the driver to generously & politely compliment the race engineer for the wonderful setup (drivers really love race cars that dart about unpredictably in corners)....



As even production car suspension designers don't like to be shouted at, a good deal of engineering time is spent making sure that bump steer is rarely an issue in the normal life of a car. In practice that means specifying a geometry that crams all the toeing to the far limits of the suspension motions: so long as the wheels are moving up and down to positions reasonably close to the static load position, the toeing is negligible; only when the wheels move close to full jounce (all the way up) or full rebound (or all the way down) does the toeing become pronounced (if you look at a graph of toe vs. suspension movement, the graph is often a straight line (essentially zero toeing) for a good distance above and below the static load position, but then takes a pronounced hook as it approaches full jounce or full rebound). In this way bump & roll steer are ordinarily non-issues: under normal braking, accelerating, and cornering, the car handles fine, and bumpsteer only shows up on bumps so enormous that any steering effect is swamped out by all the other violent things that must be simultaneously going on.



So what happens when you drastically lower a production car? In severe cases, you move the static load position (the position where the suspension sits and works around) right into the region that contains all the bump steer. And so the car bump steers. It also roll steers. And the turn-in characteristics (which are greatly affected by toe in and toe out) will vary wildly depending on how hard you brake during corner entry, or whether you're hitting 2-wheel bumps. In short, your "race car" will adopt many of the delightful handling characteristics of a 1940's Buick, minus the comfortable ride and cool hubcaps.



(2 notes here: one is that the rear suspension will bump & roll steer as well, though the effect is usually less pronounced than in the front; the other is that it's standard practice to "bump steer" (meaning, to "reduce the bump steer effect on") a radically lowered car by changing the positions of the steering rack and steering arm pickup points; but I've never seen any discussion of such on the VW forums -- probably because the next point (below) makes the issue moot)



2) The second effect is one that's been discussed to death elsewhere on this forum, and which we'll only repeat here so as to put in the context of the earlier installments: Almost everyone following this thread will have already read somewhere that lowering a McPherson strut suspension will increase the tendency to roll. Some readers, however, might have wondered why that's an issue: if the car tends to roll more, why can't we just stiffen the springs to compensate?



The reason, as it should be clear by now, is that we can't arbitrarily stiffen the springs. If we stiffen everything so as to reduce roll, we lose mechanical grip over realistic bumps; if we soften the springs to increase mechanical grip, we roll the outside front tire into adverse camber. Catch-22. What we need, then, is a way to reduce roll without going crazy on the spring rates. And we can do that by keeping the roll center nice and high.



Such the reason for Dick Shine's oft-repeated assertion (oft-repeated not because he's been unclear, but because so many people seem unwilling to believe him) that lowering the front end of a Golf/Jetta IV will destroy the handling. If you don't keep the roll center fairly high, you lose grip either through increased roll & adverse cambering of the outside tire, or else through having to control the roll with overly stiffened springs. (Note: we'll not discuss drop spindles and major component swaps here).



Or to summarize it another way: there are three big means of increasing/ maintaining the cornering ability on our cars: (1) keeping the tires more upright by reducing roll; (2) retaining mechanical grip by not letting the spring rates get out of hand; and (3) reducing the total lateral weight transfer by lowering the CG. On a Golf/Jetta IV, #1 is the most important, and #3 is the least. Hence optimizing #3 while compromising #1 and/or #2 is not the way to go.



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