anti-squat with an IRS
02-26-2009, 01:03 PM
#1
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anti-squat with an IRS
OK here's the question
Since the uprights don't see engine torque
Can you even have anti-squat with and IRS, any IRS?
They'll see braking torque, since the brakes are mounted on the uprights, unless you running inboards, but engine torque is a 'pass through' deal.
Since the uprights don't see engine torque
Can you even have anti-squat with and IRS, any IRS?
They'll see braking torque, since the brakes are mounted on the uprights, unless you running inboards, but engine torque is a 'pass through' deal.
02-26-2009, 01:59 PM
#2
Race Director
yes, if you draw lines through the upper control arm mounts, and again through the lower control arm mounts, and find the Intersection of the two line....that's your roll center. Depending on whether that point is above ground, below ground, or at ground level, determines whether you get squat or lift upon braking/accelerating.
Aftermarket companies actually sell "lift bars" for mustangs to really JAM the wheels into the ground for drag launches.
I'm no expert on this, but I do know there is a fine line between lift and squat, or you get a car that brakes OR accelerates with great grip, but not the other.
Aftermarket companies actually sell "lift bars" for mustangs to really JAM the wheels into the ground for drag launches.
I'm no expert on this, but I do know there is a fine line between lift and squat, or you get a car that brakes OR accelerates with great grip, but not the other.
02-26-2009, 02:29 PM
#3
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lift bars work on a solid axle, which definitely sees torque reaction, since the diff is part of the axle.
But does that analysis work for an IRS?
But does that analysis work for an IRS?
02-26-2009, 02:36 PM
#5
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St. Jude Vendor Donor '03-'04-'05-'06-'07-'08-'09-'10-'11
OK here's the question
Since the uprights don't see engine torque
Can you even have anti-squat with and IRS, any IRS?
They'll see braking torque, since the brakes are mounted on the uprights, unless you running inboards, but engine torque is a 'pass through' deal.
Since the uprights don't see engine torque
Can you even have anti-squat with and IRS, any IRS?
They'll see braking torque, since the brakes are mounted on the uprights, unless you running inboards, but engine torque is a 'pass through' deal.
Ok, Here is the answer....
oh.....Nevermind
LG
02-26-2009, 03:02 PM
#7
Melting Slicks
The stock C4 suspension bars have quite a bit of anti squat. DRM mounting brackets change the bar angles resulting in a considerable reduction. So much so, that without stiffer springs on a powerful car, the rear suspension bottoms with ease on hard acceleration under 45. Increase power and it gets worse.
02-26-2009, 03:04 PM
#8
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So you think this analysis is correct then?
And this is why I asked
And this is why I asked
do NOT consider the C4 rear suspension as a "4-link", it is NOT...the C4 rear suspension is a "torque arm" (vaguely similar to pre-1955 chev "torque tube" design or recent circle track "fifth coil" designs (less the coil), but differing from those as outer bearings (spindles) are divorced from the torque arm, on the C4...there is NO engine torque input to the C4 rear spindles, thus no chassis reaction (squat) caused by spindle controls (dogbones), under acceleration/decel--the torque arm (aka "C-beam), alone, does ALL "engine torque" input to the chassis...the dogbone/rear spindle does act as a "brake floater" and WILL input during braking, only.
Last edited by BrianCunningham; 02-26-2009 at 03:08 PM.
02-26-2009, 03:16 PM
#9
Melting Slicks
Im not familar with your diagram formula. I remember it like David stated. Draw a line from each bar till they join. The point above/below GC is the result. Cant remember if above or below is more anti-squat. Would need to look at the books.
Did you fab the drawing?
Did you fab the drawing?
02-26-2009, 04:48 PM
#11
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Has nothing to do with the TQ applied on the spindle.
This also applies to drag racing and can be adjusted with ladder bars or torque arms.
02-26-2009, 05:57 PM
#12
Drifting
This month's Race Car Engineering has an article about it.
I suggest you read it.
ETA: that sounded snotty, sorry, didn't mean it that way.
I suggest you read it.
ETA: that sounded snotty, sorry, didn't mean it that way.
Last edited by mgarfias; 02-26-2009 at 06:01 PM.
02-26-2009, 06:36 PM
#13
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I'd just like to see the physics behind it
I'll haven't seen that issue yet
Thanks
02-26-2009, 07:09 PM
#14
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I see where I went wrong, anti-dive/squat have nothing to do with torque
http://en.wikipedia.org/wiki/Suspension_(vehicle)
http://en.wikipedia.org/wiki/Suspension_(vehicle)
Anti-dive and anti-squat
Anti-dive and anti-squat are expressed in terms of percentage and refer to the front diving under braking and the rear squatting under acceleration. They can be thought of as the counterparts for braking and acceleration as jacking forces are to cornering. The main reason for the difference is due to the different design goals between front and rear suspension, whereas suspension is usually symmetrical between the left and right of the vehicle.
Anti-dive and anti-squat percentage are always calculated with respect to a vertical plane that intersects the vehicle's center of gravity. Consider anti-dive first. Locate the front instant centers of the suspension from the vehicle's side view. Draw a line from the tire contact patch through the instant center, this is the tire force vector. Now draw a line straight down from the vehicle's center of gravity. The anti-dive is the ratio between the height of where the tire force vector crosses the center of gravity plane expressed as a percentage. An anti-dive ratio of 50% would mean the force vector under braking crosses half way between the ground and the center of gravity.
Anti-squat is the counterpart to anti-dive and is for the rear suspension under acceleration.
Anti-dive and anti-squat may or may not be desirable depending on the suspension design. Independent suspension using multiple control arms can be an issue if the percentage is too high (say over 30%). A percentage of 100% in this case would indicate the suspension is taking 100% of the weight transfer under braking instead of the springs. This effectively binds the suspension and turns the independent suspension into no suspension like a go-cart. However, in the case of leaf spring rear suspension the anti-squat can often exceed 100% (meaning the rear may actually raise under acceleration) yet because there isn't a second arm to bind against and the suspension can freely move. Traction bars are often added to drag racing cars with rear leaf springs to increase the anti-squat to its maximum. This has the effect of forcing the rear of the car in the air and the tires onto the ground for better traction.
Anti-dive and anti-squat are expressed in terms of percentage and refer to the front diving under braking and the rear squatting under acceleration. They can be thought of as the counterparts for braking and acceleration as jacking forces are to cornering. The main reason for the difference is due to the different design goals between front and rear suspension, whereas suspension is usually symmetrical between the left and right of the vehicle.
Anti-dive and anti-squat percentage are always calculated with respect to a vertical plane that intersects the vehicle's center of gravity. Consider anti-dive first. Locate the front instant centers of the suspension from the vehicle's side view. Draw a line from the tire contact patch through the instant center, this is the tire force vector. Now draw a line straight down from the vehicle's center of gravity. The anti-dive is the ratio between the height of where the tire force vector crosses the center of gravity plane expressed as a percentage. An anti-dive ratio of 50% would mean the force vector under braking crosses half way between the ground and the center of gravity.
Anti-squat is the counterpart to anti-dive and is for the rear suspension under acceleration.
Anti-dive and anti-squat may or may not be desirable depending on the suspension design. Independent suspension using multiple control arms can be an issue if the percentage is too high (say over 30%). A percentage of 100% in this case would indicate the suspension is taking 100% of the weight transfer under braking instead of the springs. This effectively binds the suspension and turns the independent suspension into no suspension like a go-cart. However, in the case of leaf spring rear suspension the anti-squat can often exceed 100% (meaning the rear may actually raise under acceleration) yet because there isn't a second arm to bind against and the suspension can freely move. Traction bars are often added to drag racing cars with rear leaf springs to increase the anti-squat to its maximum. This has the effect of forcing the rear of the car in the air and the tires onto the ground for better traction.
02-26-2009, 07:33 PM
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here we go, just what I was looking for
http://sccaforums.com/forums/347654/...ad.aspx#347654
from that thread
http://sccaforums.com/forums/347654/...ad.aspx#347654
Short answer - you can have anti-squat with IRS, though in practical application you are limited to a fraction of that available in a live axle.
Bit of a discussion on it here:
http://www.pro-touring.com/forum/showthread.php?t=12696
Bit of a discussion on it here:
http://www.pro-touring.com/forum/showthread.php?t=12696
The geometric construction is only slightly different from what's been posted above, but it generally represents a huge difference in launch behavior (as you already know). The anti-squat line for IRS passes through the rear axle rather than the rear contact patch.
See the ** note in the November 28 post in this thread. Redraw the constructions to suit and you'll probably find the A/S line is at a fairly shallow slope. Perhaps somebody has C5/C6 rear suspension data; I suspect that it runs a bit more A/S than the RX7s did.
Other than doing things to make a big change in SVIC location - maybe some rear shocks with lots of bump damping will help?
Norm
See the ** note in the November 28 post in this thread. Redraw the constructions to suit and you'll probably find the A/S line is at a fairly shallow slope. Perhaps somebody has C5/C6 rear suspension data; I suspect that it runs a bit more A/S than the RX7s did.
Other than doing things to make a big change in SVIC location - maybe some rear shocks with lots of bump damping will help?
Norm
Last edited by BrianCunningham; 02-26-2009 at 07:49 PM.
02-26-2009, 08:09 PM
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I'm sure it's just me, but I came to Corvette Forum to try to learn something about my car. Seems just as much a study in human psychology.
Oh yea, I almost forgot to put one of these in:
02-27-2009, 10:02 AM
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The bold text sentence from Norm is what has always made sense to me. With a snapshot picture of the (C4) vehicle while it's accelerating, you can model the rear wheel/tire as having the tread/road interface as the base (as this part of the tire is "fastened" to the ground). The axle/hub line is where the forward accelerating force is imposed on the vehicle. The virtual trailing arm formed by the trailing links would then determine the accelerating force vector, and the c/g location would determine the mass resistance point. The offset of the force vector and c/g location would then determine the "attitude" (of the body reaction/rake, not the mood) during acceleration.
The only way things make sense to me is that while a live axle may have anti-squat at zero speed (entirely due to torque input), an IRS can only have anti-squat during actual acceleration.
I welcome polite corrections to my post above. I'm a EE design engineer, and a vehicle dynamics enthusiast, but college statics/mechanics class was a few years ago.
The only way things make sense to me is that while a live axle may have anti-squat at zero speed (entirely due to torque input), an IRS can only have anti-squat during actual acceleration.
I welcome polite corrections to my post above. I'm a EE design engineer, and a vehicle dynamics enthusiast, but college statics/mechanics class was a few years ago.