Info Needed on TA Height 1

[racerdude]

In my last post Greg and Norm got me to understand the IC on a GM torque arm suspension. I think I am up to speed in the IC,thanks Norm and Greg.

I am still failing to understand what the TA height(raising or lowering) does to the IC when you have parallel lines with the LCA's or they just wont intersect for a mile.

If you raise the TA from the stock imaginary line or lower it what does it do to the IC if ya also move the LCA's so it's line is parallel with the TA change and the perpendicular line off of the nose of the TA is not moved much.What do you use for a reference point the CG? Or the anti squat line?
What does all this do to weight transfer an a drag car? You might want to refer to my previous post also.

THANKS
Larry.

 

[UnbalancedEngineerin]

For a traditional TA raising or lowering the TA (at least within reason) does not really change the IC location. The reason for this is that the IC is located by drawing a line normal to the TA axis of compliance (bushing in most cases) and locating this lines intersection with the extension line of the LCAs. If you raise or lower the TA the location of the IC doesn't change as long as you are not also changing the angle of the bushing (which can cause other issues). Hope this helps.

Jason S.

 

[racerdude]

Thanks Jason


I moved the TA up 1-1/2" and the car stood on the bumper and came down on the wall. Nothing else was changed including the pinion angle. This leads me to believe that the height of the TA has a lot to do with bite. The LCA's were lets say running a parallel line to the TA and had been running there for some time.
I have another post with all the details if you will look, it may give you more info as to what happened without making another long post.

 

[Joest]

Larry,
From my experience, I'd have to agree with Greg and Norm. Changing the TA front mounting height does nothing in terms of instant center or antisquat. Obviously you are concerned with damaging your car again, but this could simply be a case of a very sticky track that particular day. You really need to get more data points to be able to pin the problem on any one parameter. If you are on the edge of wheelstands anyways, the chassis could be very sensitive to small changes like weather, tire pressure, track surface condition. It might not be easy to pin down the problem considering testing could be damaging. I would install wheelie bars and then go to a testing session. Once you determine the problem you can take them back off and be class-legal again. Otherwise you are going to have to limit launch RPM and avoid the possibility of a reoccurrence.

Cheers,

Joest

 

[UnbalancedEngineerin]

Did you move the whole TA up the 1 1/2"? If not then it sounds like the TA was no longer parallel to the ground in the bushing which would certainly have an effect on the line normal to the bushing axis and therefore the IC as well.

Jason S.

 

[racerdude]

Thanks Joest,

The training wheels are already on.
This car has been very consistent over the past couple of years.We have run this track most every weekend so that's a given. The car was the same as it's been for the whole year with nothing changed but air pressure. We went up 2# so it would spin and not stand up,guess what.
There is something going on with the TA height in the car and I don't understand it. The car runs a 1.32 60' consistently and it will pull the wheels for about 15' and about 12" high and set them back down gently. I was looking to get in the 1.2's 60' but that will have to wait until next year (repairing)
I don't know if the reference point is the CG or the anti squat line.The TA was above the AS line so it should not have put it on the bumper. What I was after was the whole car to raise and plant harder so I could go up on the launch RPM's. The front end has limiters also and they let it travel about 2". Sooo if I cant get to understanding how to adjust it I will have to go back to the way it was.
This isn't the first TA late model Camaro I have seen do this but I didn't raise the TA as much as them and the effect shouldn't have been as much.

Any help is much appreciated.

 

[racerdude]

Jason,
I made longer links for the front,which has a heim end,and raised the front mounting point 1-1/2" from where the original mount was for the aftermarket TA. Then I readjusted the pinion angle back to 2*. The TA has never been parallel to the ground,it has almays been angled up at the front.

LR

 

[Joest]

What you want to look at is the angle of a line drawn from the rear axle centerline through the TA front mount (your virtual TA angle). This is what is critical, not the actual TA structure angle. Another point that needs clarification is that the front TA attachment is not the IC. The IC is located at the intersection of the trailing arm axis and a VERTICAL plan going through the front TA attachment point. Look at the car in the side view, draw vertical line through the TA front attachment, draw a line through the trailing arms forward. The spot these two lines intersect is the IC. If you are still unclear on this then look it up in Racecar Vehicle Dynamics, by Milliken. If all of this makes sense to you then how can the effect of front TA height effect the location of the IC? The only effect I see is if the front of the TA is raised significantly (several inches) and the virtual TA angle is sloping upward to start, then raising the front mounting point will have a tendency to shorten the TA. If the TA angle slopes downward to begin with the effect will be to length the TA. Most TA aren't short enough for this to have much of an effect. You will be lucky to change the actual TA length by more than 1/8". Does this make sense?

 

[racerdude]

Thanks Joest,

I have all that down pat.What if the lines do not intersect? If ya draw a vertical line from the TA down to the line of the LCA's this is the IC,correct. OK but at what height is it? Is it the attaching point of the TA or is it some wear along the line?

Correct me if I'm wrong but as I see it you have to know the CG of the car. Then you point the TA AT the center of gravity with the projected line through the center of the TA from axle center to attaching point. Then you try to get the LCA angle to intersect the front TA mounting point.To me this would be ideal if the car will bite.If the car wants to stand up which way do you go? Do you bring the TA down,more angle on the LCA's,or both to get the IC further back in the car.
From that point if you raise the TA or take upward angle out of the LCA's far enough for the lines to never meet then where is the IC. The thing that has me confused is if the lines never meet going to infinity and do you want the TA line parallel with the ground below the anti squat line. This should make it wheel stand more if the LCA line is such that they never intersect.

 

[UnbalancedEngineerin]

Ah hah! The traditional torque arm sliding mount has been replaced on your car by a heim jointed setup with an intermediate link. Correct? If that is the case, then the verticle line through the front mount does NOT locate the instant center for your setup. I haven't drawn up the kinematic diagram, but I believe you need to draw a line along the short link and the intersection of that with the LCA line will be your IC.

Jason S.

 

[racerdude]

Thanks Jason,

Yes it is an aftermarket TA. The front mounts where the tunnel brace was mounted and on top of the cross tubing with 2 links that pivot. It is about the same angle as stock but don't mount to the tranny. The rear mounts in the same way as stock,except it is mounted to a 9" Ford rear instead of the 10 bolt stocker. It is adjustable for pinion angle also.
The rear has LCA relocation brackets that allow me to adjust the LCA angle. Now they are at a slight uphill andle from rear to front(2*-3*). I can get more but to get less I have to adjust the ride height of the car which is do able. It has adjustable coil overs in the front with 300lbs per inch springs,with travel limiters if needed,no anti roll bar and adjustable Koni shocks in the rear with coil springs of 175lbs per inch and height adjusters and the factory anti roll bar.

LR.

 

[Joest]

Ok, that might changes things a bit, but fundamentally, the way you are laying out your IC is not correct. Your vertical line and the line going through your trailing arms will always intersect. This is what determines the height of the IC. The intersection point determines both the longitudinal location and the height. You are correct, to get the antisquat value you most certainly do need to know the CG. To determine the antisquat you need to draw a line from the rear tire contact patch (axle centerline at the ground) forward through the IC (not the TA front mount, since they may not be the same spot), continuing all the way up to the front axle center line of the car. Now, at the front axle, measure up from the ground. The antisquat amount is number you measured divided by the height of the CG. If you want it as a percent then multiply it by 100. To adjust the antisquat you change the angle of the LCAs. Angling them up toward the front increase antisquat (hence, bite) and angling them down reduces antisquat (bite).

If what Jason is saying is true about the intermediate link, then raising the TA front mount up or down will change the angle of the intermediate link effectively lengthening or shortening the TA. I'd suspect that with an intermediate link, instead of drawing the line at the front of the TA vertical you will draw it through the pivot points of the intermediate link. If the link is short the articulation will be high and a slight changes in TA front height will significantly change the effective TA length. A shorter TA will, of course, increase bite by increasing the antisquat.

Joest

 

[racerdude]

Thanks Joest,

If you run a string from center of rear axle through the heim end center on the front of the TA, then run another for the LCA's the lines will never meet carrying it to infinity. Actually the angles are going away from each other. The LCA's are a slight uphill angle(2*) and the TA has a 8-10* angle up at the nose. It is about the same angle as the stock one just doesn't go as far forward. The links at the front mounting point for the TA are 90* to the ground or car.They are straight up and down.

 

[racerdude]

See post above also-- this was an after thought.

When I drop the line straight down from the TA front mounting point and it intersects the LCA line that is the front to rear IC. At this point how high is it and what do you use as a reference to measure the height? Ground to ? CG height? If I angle the LCA's severely and get the TA string and the LCA string to cross behind the TA front location point,is that cross point the IC? If I take angle out of the LCA's to make the strings cross in front of the TA front mount (4" ahead) is that the IC Just further forward?

LR

 

[Joest]

I don't understand you argument about the lines not intersecting. Lines (in mathematics) are infinitely long and will always intersect unless they are parallel. The point of intersection is your rear suspension IC in the sideview. I don’t understand your question about front or rear IC. We are discussing the rear suspension right? You can use anything convenient as a reference, but I usually use the ground. If the line you dropped from the TA front mount is vertical you cannot get the lines to cross behind the TA front mount because the intersection point one the vertical line you drew which is directly below the front mount. Assuming you have a sliding link (not the intermediate link) the IC is always located directly below the TA front mount. By changing the angles of the LCAs you can only change the height of the IC. Your TA length determines the distance the IC is from the axle centerline. Please draw out and consider everything I said line by line in my last post and everything should become very clear.

 

[racerdude]

Thanks Joest
The strings would be running forward from the rear axle.In the car the lines running forward are going away (farther apart) from each other on account of the angles. The string for the LCA's is a slight angle and the string for the TA is at way more of an angle upward. I am putting the strings like you would measure a 4 link.Am I doing it wrong? There is no argument from me I am just trying to learn how to figure the adjustments for a TA setup and get it straight in my mind.

I think the light JUST came on!!!!!!
See if this makes sence in a country boy manner.
The LCA's determine if it stands up or not.
The car is being PICKED streight UP by the TA at the front mounting point.
The angles of the LCA's determine if the rear of the chassis is pushed up to keep the front down (the front will rise but not stand up if there is sufficent angle upward on the LCA's)The whole car will raise instead of the rear trying to run under the front wheels(wheelstand).If the angle of the LCA's is right at parrellel with the ground then when the TA LIFTS the LCA's try to push the back of the chassis down and helps it wheelstand.Because of the TA not able to pick the front end off the ground without the LCA's helping it, this plants the tires harder.

Am I even close???

 

[Joest]

I have layed out the TA geometry for your reference. Assume the numbers to be arbitary. Pay particular attention to the vertical line at the TA front mount, the IC location, and the line from the rear tire contact patch through the IC that extends up to the front axle (this is how the antisquat is calculated).

-Joest

 

[racerdude]

Thanks Joest,

Man it is simple when you can see it. I want to thank everybody that responded and helped with my questions.

Now maybe I can run with the training wheels pinned up.

Thanks again everybody.


LR

 

[UnbalancedEngineerin]

Hey Joest,

The diagram you posted is for a typical slider mount TA. The diagram is different for a TA with a short verticle front link. It turns into more of a 4 bar mechanism instead of a slider mechanism.

Jason S.

 

[Joest]

Jason,
Yes, I agree. I wanted to start with the basics first.