ChrisB

Okay, lay it on me. Do we want long instant centers, short instant centers, different lengths for different applications (which length for which application), what are the pro's and con's?

How does TA length effect instant center?
How does TA angle effect instant center (not pinion angle - the actual height the TA is mounted at)
How does LCA angle effect instant center?

Thanks,
Chris Bennight

The BeaSSt

iTrader: (1)

We are looking for the same answers. Anyone? Bueller? Anyone?

Dave

Colonel

I don't really know the full definition of "instant center" and I'm certainly no suspension expert but I'll be glad to make a couple of guesses about the effects of these things....

A longer TQ arm would tend to make the car want to bring the nose up easier. The arm would have more leverage over the weight of the car. Better weight transfer would be the result.

I would think more TQ arm angle (if I'm picturing correctly what you mean) would also promote better weight transfer by giving the TQ arm better leverage over the weight of the car.

More LCA angle (front being higher than the back) helps to drive the axle downwards (the tires into the tarmac which is a good thing) but also help to keep the front end down. Kinda like the rear of the car wants to stand on it's tippy toes. <img src="gr_stretch.gif" border="0">

Bob2of3

iTrader: (11)

I'll try to lend my way of think to this but I"M NO EXPERT <img src="gr_images/icons/wink.gif" border="0">

The lighter the car the longer amd lower mounting torque arm you want to keep the wheel stands from getting outragous. In this case you want the lift point to be forward of the CAR's instant center(ie the pro stocker's have their instant center 12 to 16" in front of the car with their 4-link setups)

A heavy car I would think would benefit from a shorter higher mounted torque arm to help achieve so sort of wheel stand(max traction) as that would make the force vector lines from the lift point and the car's IC meet easier

Give me a little time to think about LCA angles and I'll get back to you <img src="gr_images/icons/wink.gif" border="0">

Also I think required reading is Dave Morgan's Doorslammers: The chassis book. This is the ONLY book dedeciated to drag suspensions that I've been able to find and its ALWAYS within reach when I'm planning the next stage of the project

<img src="graemlins/gr_cheers.gif" border="0" alt="[cheers]" />

ChrisB

I'll preface this by saying I do *not* know the answers - I am just bouncing some ideas off in response to what was said.


[quote]Originally posted by Colonel:
<strong>I don't really know the full definition of "instant center"<hr></blockquote></strong>

If we had a 4-link it would be the intersection of the upper links with the lower (if a line along their centerline was extended).

My conundrum is if we apply this to our cars the the mounthing point is irrelevant to instant center - this is what I am thinking, but it goes against popular opinion.

I think the instant center is going to be defined as the point of intersection between the LCA centerline and the TA centerline.

The torque arm mounting point would then be the point of rotation - if we make the TA shorter than the instant center then we are going to achieve anti-squat; conversely if it is longer than the instant center point then we are going to be in a squat situation.

[quote]
A longer TQ arm would tend to make the car want to bring the nose up easier. The arm would have more leverage over the weight of the car. Better weight transfer would be the result.<hr></blockquote>

I can cover this one at least - a longer TA will be hurt anti-squat afaik - it will bring the nose up, but by causing the rear end to squat. You then have weight transferred to the rear of the car, but some is absorbed (initial hit) by the springs, etc. squat is generally considered bad.

Anti-squat would be when the rear actually picks up a bit on launch - at first this may seem bad, but think about it from a newtonian perspective. To lift the rear up something has to apply the force to do that. We know for every force there is an equal and opposite force - the t/a / lca apply the upward force to lift the rear, so the normal force is down on the rear end.

So our initial hit not only gives us the weight of the car pushing down on the tires, but also the extra normal force it requires to lift the rear end up against gravity. And it's this initial hit we are trying to maximize.


[quote]
I would think more TQ arm angle (if I'm picturing correctly what you mean) would also promote better weight transfer by giving the TQ arm better leverage over the weight of the car.<hr></blockquote>

By TA angle I mean the reference from horizontal beteween the front mounting point (tranny mount, new x-member, whatever) and the rear - and I think changing this angle effects instant center; (the center of rotation for a planar body), and changing the TA length changes the moment arm which applies torque around this instant center?

[quote]<strong>More LCA angle (front being higher than the back) helps to drive the axle downwards (the tires into the tarmac which is a good thing) but also help to keep the front end down. Kinda like the rear of the car wants to stand on it's tippy toes. <img src="gr_stretch.gif" border="0"> </strong><hr></blockquote>

Yep, agree here - since all the forward force from the rear is applied through the lca bolts the forward vector is fixed - so if we make the lca more angular we are increasing the downward vector by necessity.
Also this moves the instant center back, which is effectively the same thing (I believe)

Bob2of3: Yep, I think you are on to it - I was trying to think of the TA length as effecting instant center - but after reading your post I believe it actually determines the moment arm length that works on the instant center. Then everything else falls in place. And that is why high hp cars like longer TA's - just to keep the car in line (and why madman cuts 1.2x sixties with his tiny torque arm, but can't keep the front end down).


Now here high hp would probably be what 650, 700+ rwhp? So my question is, why do we want these long torque arms. It would seem to me at NA/streetable HP levels we want the moment arm well behind the instant center - giving us maximum initial hit (especially six speeds). And this would be why the peteZ bars (SAM, stock eliminator, etc.) work so well - the put the moment arm point of rotation more rearward than event the LCA bolt.

Also going by this you would just want to make sure your moment arm was in front of your rear end (don't see how you could get it behind with our suspension type ) - but if it was behind that would create all sorts of unloading problems.


So does this sound good, reasonable? Where's nineball, he's a ME isn't he?


Thanks!
Chris Bennight

Colonel

I like this discussion!

Chris and Bob, lemme summarize a few thoughts and tell me where or if you disagree (I think you will.) <img src="gr_stretch.gif" border="0">

I think we agree on more LCA angle. It drives the tires down and makes the rear of the car tippy-toe. This is good for the initial hit but keeps the nose from rising as quickly or as high which may or may not be a good thing depending on the car.

A longer TQ arm isn't good for the initial hit because it doesn't make the car want to tippy-toe as much as a shorter TQ arm. But, it tends to make the nose of the car want to come up faster and higher which may or may not be a good thing depending on the car.

A high HP car that really has trouble keeping from dragging the rear bumper would want a shorter TQ arm and more LCA angle.

Did I get it completely wrong about the TQ arm and why?

[ December 16, 2001: Message edited by: Colonel ]

[ December 18, 2001: Message edited by: Colonel ]</p>

Patrick G

iTrader: (14)

The longer the torque arm, the less leverage it has on pushing the body up, and planting the tires to the ground. If the axle gets 400 lb ft of rotational twist and we distribute it over a 4 foot torque arm, we only get 100 lbs of force pushing the body up and forcing the tires down (in theory). If we shorten the torque arm to 2 ft, we now have 200 lbs of force pushing the body up and the axle down. The shorter the better in this case. This is partially how the Pete Z bars work. They take the 400 lbs of rotational torque and distribute it over a 1 ft length, giving a theoretical 400 lbs of separation force. Sounds better to me.

Bob2of3

iTrader: (11)

I'm drawing a blank when I try to figure out what the LCA's do. Are they like the lower bars of a 4-link? I keep picturing them as a locating device which I'm not sure that the case <img src="images/icons/confused.gif" border="0">

Now in my previous post I'm just relating my situation. I am going to have a high HP light weight car on a track that helps rotate the earth <img src="gr_images/icons/wink.gif" border="0"> so I'm going to have a problem keeping the front wheels down cause anything over 6" from the bottom of the wheel is wasted energy.

This sort of relates a little to the pinion angle(which I don't know if there is a universal answer to) but if you have a problem hooking the car things that would help- More negative pinion angle, shorter torque and a lighter shock setting. Now if you don't have a problem hooking a stiifer shock, less negative pinion angle and longer torque arm might produce a better et because it would keep the energy in the linear plane instead of the vertical plane.

<img src="graemlins/gr_cheers.gif" border="0" alt="[cheers]" />

The BeaSSt

iTrader: (1)

So, and I am no expert by any means, a shorter torque arm combined with the traction bars will help keep the front down and transfer that energy to pushing the car forward? What are the advantages of the Billingsley & Madman front adj, torque arm mount? It looks like they have four adj. on the front. Do we need to start experimenting with a shorter, possibly adj. (as in front to rear} torque arm?

Dave

ChrisB

[quote][bI'm drawing a blank when I try to figure out what the LCA's do. Are they like the lower bars of a 4-link? I keep picturing them as a locating device which I'm not sure that the case <hr></blockquote>[/b]

Yep, like the lowers in a 4-link; the T.A. angle would be analgous to the uppers.

In regards to a locating device - think of it this way. The tires turn and the rear end is pushed forward, and so pushes the car forward. All the forward force is transmitted to the body through the LCA (the T.A. can not be fixed in a fore/aft relationship (or must at least have some freedom) - so can't transmit any forward force).

Now if your LCA's are totally level all the force from the rear end pushes the car forward directly.

If your rear LCA mounts are lower than your front mount then there are different components. Now the axles is still pushing forward just as hard, so we still have to have the same reverse force in the horizontal direction (basically your car goes just as fast) - but as your push forward there is also (because of the LCA angle) a vector component pushing up on the car, which is in turn pushing down on the rear end. This downward force is what causes the tires to plant harder (more angle).

Since the instant center is defined by the intersection of LCA and TA angles this moves the instant center back also.


[quote] Now if you don't have a problem hooking a stiifer shock, less negative pinion angle and longer torque arm might produce a better et because it would keep the energy in the linear plane instead of the vertical plane<hr></blockquote>

Backwards! <img src="gr_stretch.gif" border="0"> patrick g got it right - a longer ta will lessen the initial "hit" - which is good if your car is pulling the front wheels to high. but up until this becomes a problem I think a shorter TA is better.

As for the planes - see above. Basically your rear end is pushing forward with a certain force regardless of your suspension - if your car didn't move forward at the same rate then the rear end would cross the finish line first <img src="gr_stretch.gif" border="0"> The forces we are re-distributing really aren't drive (forward) forces, but rather rotational forces as a result of torque from the rotation of the gears-axle transfer. Sort of the counter force if you will.


[quote]So, and I am no expert by any means, a shorter torque arm combined with the traction bars will help keep the front down and transfer that energy to pushing the car forward?<hr></blockquote>

If you are pulling the wheels to much you may need a longer TA. Otherwise a shorter one is better (if you car isn't uncontrollable).


[quote] What are the advantages of the Billingsley & Madman front adj, torque arm mount? It looks like they have four adj. on the front. <hr></blockquote>

If you change the height of the front mount you change the instant center. Moving the mounthing point closer to the ground will shorten the instant center, just like making the LCA more angular will.


I still think a shorter TA is better *untill* your car becomes to much of a wheelstander.


Chris

Matt98SS

so LCA brackets which relocate the LCA's rear mounting point lower are good at planting the tires but not so good for wheelstands?

if so then I planned correctly a while ago

TQ arms are pretty mysterious to me as well. Thoughts in my head about which one to get are always going thru.

this is a good thread anything else?

Smoke_ur_5.0

iTrader: (2)

[quote]More LCA angle (front being higher than the back) helps to drive the axle downwards (the tires into the tarmac which is a good thing) but also help to keep the front end down. Kinda like the rear of the car wants to stand on it's tippy toes<hr></blockquote>

This would explain why I can't pull a friggin tire for the life of me!

I have the rear control arm brackets with the LCA's set on the bottom hole..the car is lowered about an inch or so in the rear but the angle of the LCA's is definitely higher to the front of it. Now everytime I have pulled a high 1.5 60 foot time with the 6m I am like oh that had to of looked sweet! I must have pulled at least the drivers side tire....I go around and ask and everyone says "No but it looked really close!" Then after reviewing it on video it's like Colonol said...it's kind of on it's tippy toes.

You might ask what does it take to pull a tire with a 6m? Well I have seen it done on a completely stock suspension and a 1.7 60 foot time.

I would have thought that a longer T/A would be better but can see exactly what Chris is talking about now...the shorter T/A would have a better angle to drive the rear end down. Hmmm? Glad I haven't ordered a T/A yet <img src="gr_stretch.gif" border="0">

[ December 17, 2001: Message edited by: Smoke_ur_5.0 ]</p>

Bg Drg

Chris,
Excellent analytical thinking! Just a few thoughts: The theory of the IC being a fixed point drawn between the TA and the LCA will only work if the rear of the TA pivoted on the housing, which it does not. This is not to say that the IC is TOTALLY FIXED at the TA mounting point. As torque is applied to the housing and the housing attempts to rotate clockwise in the car, force is transferred forward through the LCA, BUT the housing cannot twist because the TA won't allow it. I used to think of this as a simple ladder bar system, but this discussion has opened all of our eyes as to what exactly happens in the rear of these cars. So, if we analyze this further, in a ladder bar or 4-link, what is the IC? It is the point of rotation of the housing. Because the ladder bar is fixed to the housing, the IC is the length of the bar. In a 4-link, the IC is an imaginary point drawn outwards from the upper and lower bars. As the suspension travels up and down through it's range of motion, the IC height will vary, but the length remains basically the same. In a ladder bar, the IC remains fixed in height and length, similar to our TA. Although our IC length will vary a nominal amount through it's travel (hopefully if it is allowed to), it is basically fixed. HOWEVER, because the TA does slide, it cannot be analogous to a ladder bar. Therefore, I hypothesize that the TA length/height will dictate most of the suspension characteristics. The LCA rear location will have a minimal effect on the initial hit of the tire, especially if the TA has ANY flex at all. The length of the TA will dictate how much force is used to lift the car and push down the rear axle, and how quickly this happens. A short TA/non-pivoting LCA will cause a quick, hard planting suspension, while a long TA will allow a high HP car to control the violence by distributing that energy foward in the car closer to the center of gravity. Because our system is designed for this type of car, we utilize a full length TA. For a high 10 sec. or slower car, however, I agree the shorter style TA will most likely perform better. For this reason, we are currently designing a more user friendly system with unprecidented adjustability. <img src="graemlins/camaro.gif" border="0" alt="[Chevrolet]" />

Pro Stock John

iTrader: (34)

I think the torque arm does more on our cars since stock length LCA's in stock locations in back just locate the rearend.

BgDrg, remember when we talked at US41 about doing adjustable LCA mounts on the 9", with like 5 locations? I still think it would be interesting to experiment. Also, is your tq arm longer than the RT tq arm?

ChrisB

Mr. Billingsley: Excellent point, and thanks for the feedback! I see what you are saying now - since in a 4-link the upper LCA would have a rotational degree of freedom, and only be fixed in respect to it "binding" against the other. (which is where the i.c. would be the intersection of the upper and lower links).

Sine the torque arm effectively binds against itself (in this context) it determines the characteristics. It actually makes sense!

Just wanted to say thanks for clearing that up, and that it's obvious why your car runs as well as it does (a sharp mind behind it!) I will be giving you a call tomorrow to find out about T.A. status!

Chris Bennight

MADMAN

iTrader: (4)

I agree with BgDrg BUT the lca are not doing anything at the launch and under power EXCEPT keep the rearend under the car. The T/A is providing the lift and traction advantage. The lift point is where the upper bar and lower bar intersect. That is why Jays arm and mine have different holes to atttach the arm. You can move the bar to compensate for different tracks.

TwoFast4Lv

iTrader: (8)

hehehe you guys are funny <img src="gr_images/icons/wink.gif" border="0">

BTW There is a nice write up in one of the monthly rags about this. on the stand's now!

SOme beter read up <img src="gr_images/icons/wink.gif" border="0">

Colonel

Madman, I've talked a good bit with Jay about my combo. I'd MUCH appreciate your input on what would get me closest to 1.2 60 fts.

So far, 1.38 is my best 60 ft time without the transbrake. I plan to have it working consistantly soon. I desperately need perfect traction to reach my goals of mid 9 second timeslips on motor.

Thanks! <img src="gr_stretch.gif" border="0">

MADMAN

iTrader: (4)

Obviously we all want the PERFECT run. That is more of a challenge than racing someone. In my opinion the perfect traction would be the car leaving the line and the rear tires planted enough to move the car forward(not up) without bogging the motor. We are doing some intense R & D with the Emmons brothers Stocker.
On their cars one has the Pete Z bars and the t/a is disconnected. The other has one of my S/E legal t/as. Both cars have identical motors and weights. Both cars started out with HAL shocks.
What we have learned so far IS, the Pete Z bars hit the tire hard the car consistently ran 1.43 60fts, 11.15 1/4 times. Adjusting the car was limited to tire pressure and shock adjustments.
On the other car first pass was identical within a couple hundreths of the lift bar car. After making some shock adjustments (tightened the front one click, tightened the rear 2 clicks. The car ran 1.37 60ft and a 10.99. Second pass the car ripped the license plate off. The t/a was set at 3 dgrees pinion and the bar ran up hill4 degrees. I moved the bar down in the car so we had 0 degrees in the bar and still had 3 in the pinion. Tightened up the front 1 more click and off we went to try it. The car 60ft the same 1.37 but it was hazing the tires according to the video. HERES the catch the front tires were only off the ground about an inch. Next we installed a set of dbl. adjustable AFCO shocks on the rear and set them at 2 sweeps in the top and 6 on the bottom. The car ran 1.34 and 10.95.
We are going back this weekend to try some more adjustmenst and put the AFCOs on the other car.