antisquat %, IC height & length discussion - S197 mustang

[lindertw]

Lots of great info in here. Lets keep this thread going. Anybody got any stats to add?

any chance you took measurements before you went all mini-tub on us?

 

[luv2cheat]

What rpm did you leave at?

I am not exactly sure. If I were to guess 2k

 

[CPRsm]

Just a small thing to add. 100% AS will give you the least/no movement in suspension on that hit. The farther back the IC, the more dramatic the % change will show. Basically if you have two plots and both are 110% AS, the shorter IC will throw the rear end from the car harder.

 

[fdjizm]

With no movement in the suspension on launch is there no weight transfer?

 

[CPRsm]

There will still be weight transfer. It just won't be amplified to the tires by the suspension.

 

[BMR Tech]

Correct.

Here is a good example of weight transfer, but no squat or rise (rear) upon launching:

http://s455.photobucket.com/user/bmrfabrication/media/2011%20Mustang%20Project%20Car/VIDEO0063.mp4.html

Notice the gap in the rear does not change, yet, the front rises? Pretty subtle 1.4 sixty foot pass, huh?

Next time I get this car on the lift, I will try and get some measurements to share. Our car HOOKS, and hooks HARD. We haven't gotten the tires to "break loose" since 2011.....lol

 

[CPRsm]

I know "every setup is different" gets thrown around a ton but applies here also. A stick can't create its own "hit" to the tires and needs less help from the AS of the suspension. It will change with tire and car height. Foot brake cars need the most help from the suspenion because they hit the tires the hardest. Sometimes you have a good IC, but you aren't hitting it hard enough. Especially IC's w some length. So because it doesn't work at 3k doesn't mean you shouldn't try dropping the clutch at 5k

 

[Norm Peterson]

With no movement in the suspension on launch is there no weight transfer?

If I only had a dollar for every time I've heard people wondering about that . . .

Suspension movement does not cause "weight transfer", it's really the other way around, where suspension movement is only the visible evidence that load transfer is actually happening.

You have to have the load transfer happening first, and you either add or subtract load from the suspension springs in order for them to compress or extend, respectively. Mainly that happens during acceleration, braking, and cornering.

Of course, in the case of 100% anti-squat, the rear suspension movement in squat is zero . . . which means that the suspension geometry is handling all of the load transfer, leaving the springs with nothing to do but hold up the weight of that end of the car.


Norm

 

[luv2cheat]

Correct.

Here is a good example of weight transfer, but no squat or rise (rear) upon launching:

http://s455.photobucket.com/user/bmrfabrication/media/2011 Mustang Project Car/VIDEO0063.mp4.html

Notice the gap in the rear does not change, yet, the front rises? Pretty subtle 1.4 sixty foot pass, huh?

Next time I get this car on the lift, I will try and get some measurements to share. Our car HOOKS, and hooks HARD. We haven't gotten the tires to "break loose" since 2011.....lol

What rear tire is that? It looks tall

 

[luv2cheat]

I changed my rear springs, lowered the front ride height, and installed BMR relocation brackets. Note I took ride height measurements from the bottom of the wheel arch this time. i am going to the track tonight, I will edit this post with 60' time later on.

Antisquat %: 106.48
IC length: 67.61
IC height: 14.28
springs front: Afco 14"150#
springs rear: Afco 14"125#
airbag: NO
ride height front: 27-1/2" (measured from floor to bottom of fenderwell arch)
ride height rear: 30-3/16
tire dia front: 26"
tire dia rear: 28
strut type front: QA1 single adjustable
strut type rear: Afco double adjustable
wheelbase: 107.1"
center gravity: 21-1/4" (crankshaft bolt + 6")
LCA relocation brackets: BMR
LCA position: Top
UCA bracket: YES,BMR
UCA position: Lower hole
pinion angle: -1.0deg
Weight rear tires W/driver 1340
Weight front tires W/driver 1558

Best 60' so far 1.75

So anyone know how much is this going to change if i replace the 28" rear tire with a 26"? Will I need to make any adjustments?

 

[Norm Peterson]

For a quick and dirty approximation . . .

If the diameter of front tires is also being reduced by 2", take an inch off of all of the rear suspension pivot heights and an inch off the CG height as well.

If the front tires are staying the same height as before, take an inch off the rear (axle side) upper and lower control arm bolt heights, about 0.92" off the forward (chassis side) upper bolt height, about 0.83" off the forward (chassis side) lower bolt height, and only drop the CG height by about 0.46".



FWIW, I probably wouldn't much worry about changes in antisquat that are less than about 2%, and I wouldn't bother with the decimal percent parts of the answers at all.


Norm

 

[kdanner]

So anyone know how much is this going to change if i replace the 28" rear tire with a 26"? Will I need to make any adjustments?

You're going to lose 5% or so AS, can't be exact because you'd need to reweigh. If you get the performance trends 4 link calculator you can simulate these changes easily. Also it will be more accurate, as it doesn't make the invalid assumption that the upper and lower axle bracket bolt locations are exactly on the rear axle centerline like that baseline calculator does. Right now it shows your AS way forward of ideal if the car ET is something like 12 flat, which is just a guess on my part. As ET gets quicker, ideal IC will move forward. An upper mount with a hole 1.25" lower than the one you're currently using would put it right in the recommended area like below, whether you have a 28" or 26" rear tire.

 

[Whiskey11]

For a quick and dirty approximation . . .

If the diameter of front tires is also being reduced by 2", take an inch off of all of the rear suspension pivot heights and an inch off the CG height as well.

If the front tires are staying the same height as before, take an inch off the rear (axle side) upper and lower control arm bolt heights, about 0.92" off the forward (chassis side) upper bolt height, about 0.83" off the forward (chassis side) lower bolt height, and only drop the CG height by about 0.46".



FWIW, I probably wouldn't much worry about changes in antisquat that are less than about 2%, and I wouldn't bother with the decimal percent parts of the answers at all.


Norm

Question for you Norm. If you were to say, install a Torque Arm and were curious to know what your %AS would be after the installation and you know the length of said Torque Arm can you approximate the %AS by changing the angle of the UCA in that calculator to get the IC length to the length of the torque arm?

 

[luv2cheat]

Thanks for the quick replies!

 

[Norm Peterson]

Question for you Norm. If you were to say, install a Torque Arm and were curious to know what your %AS would be after the installation and you know the length of said Torque Arm can you approximate the %AS by changing the angle of the UCA in that calculator to get the IC length to the length of the torque arm?

While it might be theoretically possible to do that for the static position, your bigger problem would be in modeling the UCA pivot co-ordinates to put the SVIC in the same place that the torque arm does, which is determined by a rather different-looking construction (that's not quite the same if the TA is connected at the chassis by a short link instead of through a bushed sleeve). Dynamically, SVIC migration with a TA differs from what it is with either a 3- or a 4-link, so this approach would be of limited value.

If you have a Millikens' "Race Car Vehicle Dynamics" book handy, there is a picture and some descriptive text for torque arm side view geometry (Fig. 17.41). Be careful how you read it, as the construction line from the TA chassis side pickup point is not necessarily a true vertical line (and generally isn't, quite).

FWIW, I've also done up spreadsheets for four variations of the "TA" arrangement (quote marks intentional), and they all do axle steer as well.


Norm

 

[Whiskey11]

While it might be theoretically possible to do that for the static position, your bigger problem would be in modeling the UCA pivot co-ordinates to put the SVIC in the same place that the torque arm does, which is determined by a rather different-looking construction (that's not quite the same if the TA is connected at the chassis by a short link instead of through a bushed sleeve). Dynamically, SVIC migration with a TA differs from what it is with either a 3- or a 4-link, so this approach would be of limited value.

If you have a Millikens' "Race Car Vehicle Dynamics" book handy, there is a picture and some descriptive text for torque arm side view geometry (Fig. 17.41). Be careful how you read it, as the construction line from the TA chassis side pickup point is not necessarily a true vertical line (and generally isn't, quite).

FWIW, I've also done up spreadsheets for four variations of the "TA" arrangement (quote marks intentional), and they all do axle steer as well.


Norm

In reality it's nothing more than mental self pleasuring anyway since that program will only show static %AS numbers rather than dynamic ones. It is, at best, an approximation anyway. The value that matters to me (coming out of a corner) is far too complex for this engineering school drop out to try and compute!

What I do know is that my "static" %AS is very low (between 5% and 10%) and the "static" %AS with the Torque Arm is closer to 30%-35%. That is using very rough numbers estimated with my drop and your stock numbers Norm. That of course assumes that a torque arm's IC doesn't vary fore/aft by a whole lot by raising or lowering the car. That also assumes that my measurement of the Cortex Torque Arm (47") is accurate to where the IC will actually be...

In other words, tons of assumptions made which means not very accurate!

 

[Norm Peterson]

In reality it's nothing more than mental self pleasuring anyway since that program will only show static %AS numbers rather than dynamic ones. It is, at best, an approximation anyway. The value that matters to me (coming out of a corner) is far too complex for this engineering school drop out to try and compute!

Not as hard as you might think.

A first cut might be to develop the anti-squat and roll steer curves as a function of sprung mass ride height. This won't capture the effect of pitch rotation on chassis side pivot points, but it will give you some idea about how fast those parameters vary.

Just to complicate matters a bit, on corner exit the left and right rear wheels generally won't be working with the same anti-squat value.


Norm

 

[Radar Doc]

Thought I'd post a link to some 3 and 4-link calculators I ran across while surfing a few months ago:

"3,4-Link" Link

The calculators in Excel format are at the bottom of the page. A couple good points about these calculators are:

1. You can enter pivot point positions at other than axle centerline (like KDanner mentioned).

2. There are inputs for lateral positioning so roll-center, etc is calculated.

3. The best part....FREE.

Mike

 

[06stangia]

How do I find the center of gravity on a lowered car?

 

[Norm Peterson]

Camshaft height on a pushrod motor or crank bolt height + 6" (or whatever) will still be close enough.

You could argue that the wheels, tires, brakes, and axle don't drop at all. But using camshaft height or whatever is not in itself fussy enough to justify being picky about what gets lowered and what doesn't.


For some older cars, there is a NHTSA inertia database that gives much more precise CG heights, and you could get fussier about changes if you started from that. But you'd have to track all the things that were different.


Norm