drmiller100 Posted February 7, 2009 Share Posted February 7, 2009 Lets talk about Antisquat and Antidive. Antisquat is also known as Pro Lift. Antidive is talking about what happens when you apply the brakes. If you design the suspension in a specific way, the brake torque or forces are directed to Lift that end of the suspension. Here is a side view of Anti-dive. The car is heading left, assume the brakes are on. And it is an SLA suspension, unlike our struts. http://www.tonyfoale.com/Articles/Dive/Dive_5.gif The forces MUST go up the arms. Depending on how the arms are positioned, the chassis could be "lifted", neutral, or "sucked down." There is an excellent writeup here: http://www.tonyfoale.com/Articles/Dive/DIVE.htm I'm having a tough time finding a good pic of antisquat. The drag racers call it changing the instant center, which is indeed a better way to describe it. Milliken has a terse description of it in their pamphlet. I found this, which maybe gives the idea with a live axle. http://www.wallaceracing.com/coilspring.htm Basically, you are using the forces to LIFT the back of the car on acceleration. The drag racers and oval track racers use it to get "bite" on acceleration. The traction advantage only occurs while changes in torque are occuring. So, when you hit the throttle, you get a moment of "bite" before the forces all level out. But that moment can be HUGE on getting the jump on your competitor!!! To summarize for z-cars, I don't know how to get antisquat on a zcar, and antidive isn't worth the effort. Quote Link to comment Share on other sites More sharing options...
260DET Posted February 10, 2009 Share Posted February 10, 2009 Re anti squat, for a circuit/road course car you don't want it, keeping in mind that a lot of acceleration is done in corners. Something to do with it effectively and drastically increasing rear spring rates to such an extent that grip can be lost and the car may spin. With the various model Skylines and other Nissans with multi link rear suspension its common to space the front of the rear cradle down to get rid of the anti squat. I think that the S30 is neutral at the rear as far as anti squat goes. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 10, 2009 Share Posted February 10, 2009 Re anti squat, for a circuit/road course car you don't want it, keeping in mind that a lot of acceleration is done in corners. Something to do with it effectively and drastically increasing rear spring rates to such an extent that grip can be lost and the car may spin. I think that the S30 is neutral at the rear as far as anti squat goes. Eh, I think you need to take that one on a case by case basis. I know the Z32 300ZX has a lot of anti-squat built in. Apparently this was done with an eye towards road racing and has the side effect of making it a poor drag racer. You could argue that not having very much anti and running stiffer springs would accomplish the same goal and might work better, but again, case by case... Quote Link to comment Share on other sites More sharing options...
260DET Posted February 10, 2009 Share Posted February 10, 2009 Yeh Jon I'm not sure about exactly what anti squat can do in a circuit/road course situation but it is something to do with the loss of rear suspension compliance. Most of the info around is in relation to drag cars which I can understand may benefit from a traction bite. I do know for a fact that its common, in AU anyway, for the multilink to be tipped as mentioned with circuit Nissans. Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted February 10, 2009 Administrators Share Posted February 10, 2009 Re anti squat, for a circuit/road course car you don't want it I hear Vette's work pretty well I'm having a tough time finding a good pic of antisquat. Not sure what kind of pic you're looking for, but here are a couple... C4 5-link... C5 Double A-arm... Notice the whole cradle is 'rotated'. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 10, 2009 Share Posted February 10, 2009 Yeh Jon I'm not sure about exactly what anti squat can do in a circuit/road course situation but it is something to do with the loss of rear suspension compliance. Most of the info around is in relation to drag cars which I can understand may benefit from a traction bite. I do know for a fact that its common, in AU anyway, for the multilink to be tipped as mentioned with circuit Nissans. A couple things anti-squat can do for you: Keep the chassis level. Might be less likely to pull a front tire. Keep toe from changing. If the suspension has a lot of toe change, anti-squat would help minimize that negative aspect. Keep the ass end suspended. If the car were really softly sprung or really powerful, neutral or pro-squat might leave you with the suspension bottomed on acceleration. Quote Link to comment Share on other sites More sharing options...
johnc Posted February 10, 2009 Share Posted February 10, 2009 A couple points: 1. Braking torque reacting through the caliper (assuming the caliper is behind the spindle) generates a small amount of pro-dive through the lower control arm. 2. Anti-dive is a suspension reaction to sprung mass pitch. You can get anti-dive reactions from a bad downshift, lifting off the throttle, etc. Quote Link to comment Share on other sites More sharing options...
260DET Posted February 11, 2009 Share Posted February 11, 2009 Another consideration is that on a race car you can control dive and squat by other means eg springs and dampers. Road cars are relatively softly sprung and damped so may need anti's for comfort considerations. The other thing is that there are degrees of anti dive and squat, its not all or nothing. Quote Link to comment Share on other sites More sharing options...
drmiller100 Posted February 11, 2009 Author Share Posted February 11, 2009 If a rear suspension has antisquat, the rear suspension will probably have pro squat when braking, which is a bad thing. However, the antisquat in and of itself generally outweighs braking issues. One thing circle track guys do is put the calipers on bearings, and run a separate rod to the chassis to get "pro lift" on braking, and "prolift/antisquat" under acceleration with the 4 link. It is really hard to get antisquat on a rear wheel drive without affecting the rear axle steering in a negative way. In a perfect world, the rear axle would turn the rear tires in the same direction as the front tires for fast transitions and good exit speed. When you go antisquat, the rear tires probably steer opposite the front, which causes slower transition. The vette pics are great. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 11, 2009 Share Posted February 11, 2009 If a rear suspension has antisquat, the rear suspension will probably have pro squat when braking, which is a bad thing. Not always. Semi-trailing arm for instance usually has pro squat when accelerating and braking. And it's not necessarily a bad thing (especially on a car that tends to nosedive a lot under braking). If the rear squats when braking that levels the car and prevents it from nosediving as much. That is often preferable to the rear jacking UP when braking. Ask anyone who races a car with leaf springs about that. I know when I traded out my shackles for sliders when I was autoxing a pickup truck it made a big difference in stability under braking. Again, case by case is the way to judge what is "good" and "bad". However, the antisquat in and of itself generally outweighs braking issues. One thing circle track guys do is put the calipers on bearings, and run a separate rod to the chassis to get "pro lift" on braking, and "prolift/antisquat" under acceleration with the 4 link. I haven't seen what you're talking about on cars, but I have seen it on mountain bikes. It's purpose is not to get "pro lift" but it is to free up the suspension while under braking. If a car is set up with "pro squat" under braking, that means that the braking torque's effect on the control arm is to make the suspension lengthen as far as possible, and then to keep it there while that braking torque is applied. For semi-trailing arm suspension with the control arms pointing down from front to back for instance, that means that when you hit the brakes the rear control arm wants to level out, dropping the rear of the car down under braking. Until you release the brake it wants to stay in that level position, reducing suspension movement and essentially creating a bind. This is very similar to a single pivot rear mountain bike suspension, and the elimination of this effect is what the floating caliper is trying to achieve. http://www.therapycomponents.com/floaterfaq.htm From here: http://www.stockcarproducts.com/techtips.htm BRAKE FLOATERS - Brake floaters remove brake torque from suspension components by directing the brake forces through separate radius rods. By adjusting the angle of the brake radius rods, brake influence on handling can be tuned. I don't know for sure, but my SWAG is "brake influence on handling" means that you would set one side different than the other to get the rear axle to steer around the track. It is really hard to get antisquat on a rear wheel drive without affecting the rear axle steering in a negative way. Why? If you use upper and lower control arms, or even single control arms and struts like the Z, it is easy to set the suspension up to have NO toe changes. You might have a slight change in track on one side or the other as the suspension compresses, but those effects are present on a Z anyway due to the TC rods in front, and you don't hear too many people complaining about it. Certainly not like the solid axle guys complaining about rear steer. In a perfect world, the rear axle would turn the rear tires in the same direction as the front tires for fast transitions and good exit speed. Which is why people so eagerly remove HICAS from 300ZX's and Skylines when they are raced, I guess. Quote Link to comment Share on other sites More sharing options...
johnc Posted February 11, 2009 Share Posted February 11, 2009 In a perfect world, the rear axle would turn the rear tires in the same direction as the front tires for fast transitions and good exit speed. Having driven a 300ZXT with HICAS as an instructor during a racing school I can tell you that (at least Nissan's implementation) it induced mid-corner understeer and was very unsettling at turn-in. If it was my car and I raced it, I too would disable HICAS. Quote Link to comment Share on other sites More sharing options...
drmiller100 Posted February 12, 2009 Author Share Posted February 12, 2009 Not always. Semi-trailing arm for instance usually has pro squat when accelerating and braking. And it's not necessarily a bad thing (especially on a car that tends to nosedive a lot under braking). If the rear squats when braking that levels the car and prevents it from nosediving as much. That is often preferable to the rear jacking UP when braking. Ask anyone who races a car with leaf springs about that.. Lets say we have power induced antisquat. That means we probably have brake induced antilift. When we stab the brakes, the rear brakes pick the rear tires up off the ground, causing you to spin out. It is pretty easy to design a suspension with zero toe, that still has plenty of roll steer. The ZX is one of them. If a car tends to nosedive under braking, then you might consider antidive in the front. When all done, if you don't have enough horsepower to roast the tires on corner exit, you probably don't need antisquat. However, on autocross and circle track with big horsepower, I think there is a place for it. I know my IMCA modified had a bunch of antisquat and floater brakes, and it was pretty old school 8 years ago. Quote Link to comment Share on other sites More sharing options...
johnc Posted February 12, 2009 Share Posted February 12, 2009 When we stab the brakes, the rear brakes pick the rear tires up off the ground, causing you to spin out. OK, I see a trend here. You're confusing weight transfer with brake behavior. Brakes torque or brake application do not lift tires or suspension on any modern vehicle. The sprung mass moving around is what causes the effects you're describing. Here's a good primer: http://en.wikipedia.org/wiki/Weight_transfer Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 12, 2009 Share Posted February 12, 2009 OK, I see a trend here. You're confusing weight transfer with brake behavior. Brakes torque or brake application do not lift tires or suspension on any modern vehicle. I think he might just be misunderstanding a phenomenon that is actually happening. With some rear suspension designs, hitting the brakes does indeed compress the rear suspension, and it IS actually the brake torque that does it. But what it does not do is lift the rear tires off the ground. There is still plenty of gravity to hold that back end of the car down when the brakes are applied, until the center of gravity is high enough, the wheelbase is short enough, or the deceleration g's are high enough. To me this idea is analogous to people thinking that Cary's limiters will cause the inside tire to lift off the ground. They don't. They cause the suspension to stop moving. The two are entirely different things. If the rear suspension stops moving, that is very likely to make that end of the car (or mountain bike) loose, because it can no longer follow the road surface. That is exactly why those floating brakes exist. They allow braking without jacking the suspension around or binding it up. They allow braking AND suspending to happen simultaneously. The reason that Cary's droop limiters are helpful is because they prevent an even more negative effect on traction (excessive roll), and the suspension stops moving on the least loaded tire. Quote Link to comment Share on other sites More sharing options...
johnc Posted February 12, 2009 Share Posted February 12, 2009 On the rear of the S30, brake torque twists the LCA and the upper strut mount. Looking at the driver's rear from the outside of the car, that twist is in counter-clockwise direction. Because of the equal and opposite reaction thing, there is an upward force at the rear of the LCA and downward force at the front of the LCA and a forward force at the top of the strut mount. All of these forces tend to cancel each other out so any net affect is small or maybe even unmeasurable. While dyno testing my old race car at Superior Automotive in Anaheim we had to use the brakes to slow the rear wheels because downshifting through the gears in a sequential transmission was the only way to get to neutral. No jacking effect on the chassis was noticed by me or anyone else when the brakes were applied. There may have been some, but its really not a consideration in the context of anti-dive or anti-squat. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 12, 2009 Share Posted February 12, 2009 Right. Those floating brakes would do no good on the rear of a Z. ZX is a different story, but the Z doesn't suffer from brake jacking. Quote Link to comment Share on other sites More sharing options...
johnc Posted February 12, 2009 Share Posted February 12, 2009 I agree that a semi-trailing arm rear suspension can jack itself all over the place when you hit the brakes. Quote Link to comment Share on other sites More sharing options...
drmiller100 Posted February 13, 2009 Author Share Posted February 13, 2009 I agree that a semi-trailing arm rear suspension can jack itself all over the place when you hit the brakes. Ok, will you agree the zx also has pro squat under acceleration???? Quote Link to comment Share on other sites More sharing options...
johnc Posted February 13, 2009 Share Posted February 13, 2009 Ok, will you agree the zx also has pro squat under acceleration???? Sure. Never said it didn't. Quote Link to comment Share on other sites More sharing options...
drmiller100 Posted February 13, 2009 Author Share Posted February 13, 2009 I To me this idea is analogous to people thinking that Cary's limiters will cause the inside tire to lift off the ground. They don't. They cause the suspension to stop moving. T. Lets say I put suspension limiters on the front suspension. So you get in a left hand corner, and the left front limiter gets tight. So now you go faster, and the whole chassis leans even more. What keeps the left front tire on the ground? Quote Link to comment Share on other sites More sharing options...
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