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SVT Shelby GT500
Panhard Bar vs. Watts Link
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<blockquote data-quote="Norm Peterson" data-source="post: 16491875" data-attributes="member: 193960"><p>No way to make this short . . .</p><p></p><p>Seems neither one of those articles ever mentions 'rear axle roll steer'. This is where the axle actually steers slightly as the body rolls. This is not the same effect described as the rear of the chassis moving sideways relative to the rear tire footprints. With the possible exception of autocross (where nimble handling is a plus and speeds are low so the risk of poo happening is minimal), you really want this effect to be minimally understeerish rather than oversteerish. This is not an either understeerish or oversteerish concept; it can be quantified with numbers as to how much (of either) exists with any given stick-axle suspension linkage arrangement.</p><p></p><p>I think it's important to realize that axle steer is generally just a little bit out of step with your steering wheel inputs unless you're particularly smooth with your steering inputs. That's because roll steer has to wait for the roll to actually happen, which takes up to a few hundred milliseconds after you've done some steering to get the cornering started. Has to, and while this may be on a subliminal level it is something a driver can pick up on, especially on the oversteerish side when the rear axle is actually steering itself toward the outside of a corner and the car is basically giving him more steering than he asked for. At autocross, that can be fast, as long as you can keep up with it. At higher speeds too much oversteerish roll steer can be spooky to downright scary, and on the street, risky because you do need to stay on top of it. People who have aggressively lowered their LCAs without lowering their cars may understand this, because of pushing the rear axle suspension into roll oversteer that way.</p><p></p><p></p><p>They both missed the matter of bushing compliance, and even polyurethane has some, whether we're talking about a PHB or a poly-bushed Watts. At 2° of roll, this component of lateral axle shift could be on the same order of magnitude as the shift due to the PHB arc at 1". While this is more of a street driving concern, it's still something to think about. After all, if you're building up a street driver, you probably want at least a little NVH isolation in your suspension. And if you're actually racing, the rules for your racing class may dictate what you can and cannot use (like NASCAR not allowing the Watts link). Autocross and wheel-to-wheel road course racing generally aren't "run what you brung, and hope you brung enough" like drag racing tends to be.</p><p></p><p></p><p>On the matter of differential mount vs axle mounted Watts link 'propeller', missing entirely is any mention of whether you want to hold the amount of lateral load transfer carried through the roll centers constant and let the amount of roll vary slightly with ride height (diff-mount Watts), or hold the amount of roll constant with ride height and let the geometric (roll center) load transfer do the varying (chassis-mount Watts). I'm not convinced that either way is better than the other, only that they're slightly different.</p><p></p><p></p><p></p><p>A lot of the rest reads like nitpicking between proponents of the different methods of locating an axle, with each side obviously having their own agenda.</p><p></p><p>Yeah, the geometric rear roll center for the Fox/SN95 chassis is way up there at about where the upper links attach to the pumpkin, and in the S197 it's closer to axle height. MM's writer apparently dropped the ball on that one, but it's not the clear indication of MM not understanding the problem that Griggs would have you believe it is.</p><p></p><p>And while Griggs makes a point about it being possible to locate the Watts link geo roll center down really low (almost as low as front suspension geo-roll centers tend to be), the solution they described for the Trans-Am racers is not something that anybody makes for public consumption. As far as bolt-on solutions for a street-ish car go, the Trans-Am solution is basically a unicorn.</p><p></p><p></p><p>Norm</p></blockquote><p></p>
[QUOTE="Norm Peterson, post: 16491875, member: 193960"] No way to make this short . . . Seems neither one of those articles ever mentions 'rear axle roll steer'. This is where the axle actually steers slightly as the body rolls. This is not the same effect described as the rear of the chassis moving sideways relative to the rear tire footprints. With the possible exception of autocross (where nimble handling is a plus and speeds are low so the risk of poo happening is minimal), you really want this effect to be minimally understeerish rather than oversteerish. This is not an either understeerish or oversteerish concept; it can be quantified with numbers as to how much (of either) exists with any given stick-axle suspension linkage arrangement. I think it's important to realize that axle steer is generally just a little bit out of step with your steering wheel inputs unless you're particularly smooth with your steering inputs. That's because roll steer has to wait for the roll to actually happen, which takes up to a few hundred milliseconds after you've done some steering to get the cornering started. Has to, and while this may be on a subliminal level it is something a driver can pick up on, especially on the oversteerish side when the rear axle is actually steering itself toward the outside of a corner and the car is basically giving him more steering than he asked for. At autocross, that can be fast, as long as you can keep up with it. At higher speeds too much oversteerish roll steer can be spooky to downright scary, and on the street, risky because you do need to stay on top of it. People who have aggressively lowered their LCAs without lowering their cars may understand this, because of pushing the rear axle suspension into roll oversteer that way. They both missed the matter of bushing compliance, and even polyurethane has some, whether we're talking about a PHB or a poly-bushed Watts. At 2° of roll, this component of lateral axle shift could be on the same order of magnitude as the shift due to the PHB arc at 1". While this is more of a street driving concern, it's still something to think about. After all, if you're building up a street driver, you probably want at least a little NVH isolation in your suspension. And if you're actually racing, the rules for your racing class may dictate what you can and cannot use (like NASCAR not allowing the Watts link). Autocross and wheel-to-wheel road course racing generally aren't "run what you brung, and hope you brung enough" like drag racing tends to be. On the matter of differential mount vs axle mounted Watts link 'propeller', missing entirely is any mention of whether you want to hold the amount of lateral load transfer carried through the roll centers constant and let the amount of roll vary slightly with ride height (diff-mount Watts), or hold the amount of roll constant with ride height and let the geometric (roll center) load transfer do the varying (chassis-mount Watts). I'm not convinced that either way is better than the other, only that they're slightly different. A lot of the rest reads like nitpicking between proponents of the different methods of locating an axle, with each side obviously having their own agenda. Yeah, the geometric rear roll center for the Fox/SN95 chassis is way up there at about where the upper links attach to the pumpkin, and in the S197 it's closer to axle height. MM's writer apparently dropped the ball on that one, but it's not the clear indication of MM not understanding the problem that Griggs would have you believe it is. And while Griggs makes a point about it being possible to locate the Watts link geo roll center down really low (almost as low as front suspension geo-roll centers tend to be), the solution they described for the Trans-Am racers is not something that anybody makes for public consumption. As far as bolt-on solutions for a street-ish car go, the Trans-Am solution is basically a unicorn. Norm [/QUOTE]
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