Any Engineers Care to Explain in Laymans Terms, INSTANT CENTER?
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Joined: Jul 2000
Posts: 520
From: MD
Madman… I’d love more details… but I’m at a sticking point at what you just wrote (I’m guessing a typo?)
In neither TA design, sliding front link or pivoting front link, will the instant center ever be in front of a line perpendicular to the TA through the front of the TA (in other words, it cannot be further forward then the length of the TA). Effectively the IC of your setup (again, based on the pictures on your site, I have no idea how current they are) will always be the font pivot point of your TA. With a sliding pivot you could put it in the same location if you wanted to, but it would involve some messing with the LCA’s.
I do find it interesting that you’ve found that your cars (much faster then anything I’ve had the opportunity to build) do seem to like about the same antisquat that I’ve found works (yours works out to about 57%, and I usually like to see about 60%, and like I said, many aftermarket setups end up with significantly more then that which I feel upsets the launch).
Rear shock tuning on these things is admittedly something that I haven’t gotten a good handle on. Seems to me that they only control the rate of rise of the rear of the body on the launch and how fast it settles as it’s going down the track. It would seem to me that really, jounce/compression rate on them should be fairly stiff, and the rebound/extension rate on them should be fairly soft, but that isn’t even close to what most people run and I’m not sure why something else would work.
What do you do to limit wheel stands with the front suspension?
In neither TA design, sliding front link or pivoting front link, will the instant center ever be in front of a line perpendicular to the TA through the front of the TA (in other words, it cannot be further forward then the length of the TA). Effectively the IC of your setup (again, based on the pictures on your site, I have no idea how current they are) will always be the font pivot point of your TA. With a sliding pivot you could put it in the same location if you wanted to, but it would involve some messing with the LCA’s.
I do find it interesting that you’ve found that your cars (much faster then anything I’ve had the opportunity to build) do seem to like about the same antisquat that I’ve found works (yours works out to about 57%, and I usually like to see about 60%, and like I said, many aftermarket setups end up with significantly more then that which I feel upsets the launch).
Rear shock tuning on these things is admittedly something that I haven’t gotten a good handle on. Seems to me that they only control the rate of rise of the rear of the body on the launch and how fast it settles as it’s going down the track. It would seem to me that really, jounce/compression rate on them should be fairly stiff, and the rebound/extension rate on them should be fairly soft, but that isn’t even close to what most people run and I’m not sure why something else would work.
What do you do to limit wheel stands with the front suspension?
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Joined: Jan 1999
Posts: 86
From: Baton Rouge
To measure instant center on any type of rear suspension you take the upper mount on the rearend and draw a line to the front of the car. Then you take a line from the lower mount and run it forward. Where these lines cross is where you start to measure i/c. You measure the point out and up and there is your i/c.
Using the front suspension to control wheelstands has been alot of trial and error. First we always run travel limiters. The front of a f-body has 9 inches of travel. In a perfect world that would be 4.5" up and the same down. I usually limit the front to 3"to 2.5" down. Also I use a lighter spring than most to kill stored energy. Most cars have 350lbs per inch springs on them and they are compressed 5 inches. This puts 2100lbs per spring of stored energy in the front. I run a 275lbs per inch spring which puts 1650lbs per spring in the front. This will kill the tendency to trow the front of the car airbrone.
On my shocks I use AFCO double adjustables. I run the top which is extension at 3 sweeps(there are 6 sweeps) and the bottom which is compression on 4 out of 8. This is the rear. On the front I run them at 2 on the top and 6 on the bottom. I change only the top on the front depending on track conditions.
Using the front suspension to control wheelstands has been alot of trial and error. First we always run travel limiters. The front of a f-body has 9 inches of travel. In a perfect world that would be 4.5" up and the same down. I usually limit the front to 3"to 2.5" down. Also I use a lighter spring than most to kill stored energy. Most cars have 350lbs per inch springs on them and they are compressed 5 inches. This puts 2100lbs per spring of stored energy in the front. I run a 275lbs per inch spring which puts 1650lbs per spring in the front. This will kill the tendency to trow the front of the car airbrone.
On my shocks I use AFCO double adjustables. I run the top which is extension at 3 sweeps(there are 6 sweeps) and the bottom which is compression on 4 out of 8. This is the rear. On the front I run them at 2 on the top and 6 on the bottom. I change only the top on the front depending on track conditions.
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Joined: Oct 2002
Posts: 2,931
From: Upstate NY
Originally posted by madman69
To measure instant center on any type of rear suspension you take the upper mount on the rearend and draw a line to the front of the car. Then you take a line from the lower mount and run it forward. Where these lines cross is where you start to measure i/c. You measure the point out and up and there is your i/c.
To measure instant center on any type of rear suspension you take the upper mount on the rearend and draw a line to the front of the car. Then you take a line from the lower mount and run it forward. Where these lines cross is where you start to measure i/c. You measure the point out and up and there is your i/c.
Well, not quite.
You may get some differences of opinion on this, madman.
It helps a lot to use diagrams, but I haven't figured out how to post them.
My $.02
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Joined: Nov 1998
Posts: 71,094
From: Hell was full so they sent me to NJ
From my understanding of the definition of instant center, it is the point about which the rear axle assembly is rotating at any given point in time. With a 4-link suspension, its pretty easy to visualize.... the intersection of the extensions of the upper and lower links. But for a torque arm suspension, it gets confusing, at least for me. While you could argue that the lower control arms represent the bottom part of the 4-link supsension, and the TA represents the upper portion of the 4-link, there is an additional constraint on the system that appears to invalidate this assumption, at least with respect to the TA.
The axle's rotation is limited by the fact that the TA-to-axle connection is not hinged.... it is rigid. And thats the rub, in my limited knowledge type brain. While the axle is rotating around the front hinge of the TA, and also around the body hinge of the LCA, the fact that the axle can not change its angular relationship to the TA sort of makes me wonder if the concept of IC is even applicable. It is this rigid connection of the TA to the axle that puts the suspension in bind without the flexible link somewhere in the TA.
Perhaps in the Madman design, running the front mounting brace between the SFC's provides enough flexibility in the mount that the TA is free to move slightly. Couple that with keeping the LCA movement in a narrow range from slightly below to slightly above horizontal, and the front-to-back differential displacement of the TA and LCA's becomes less of an issue.
Brian.... when I said your TA "wasn't going to work", it was in this context, and in the context that WS6 TA brought up... when you fix the length of the TA, you have created a new form of suspension. And it will induce compressive forces in the TA, something that isn't possible with the slip joint or "linked" design. It isn't working in the same way. And if we define "working" as keeping the rear axle assembly from rotating into the wrong position due to the torque reaction, an arm that tears itself apart is technically "not working"....
. And obviously the massive weight of the car in question may have contributed to that failure, since at the time, I doubt it was making more than about 750flywheelHP.
I know you have put down some fantastic 60-foot times, but it isn't unreasonable to ask if they couldn't have been even better with the flexible link. Or maybe you have changed the design since I saw it.... dunno.
I'm just trying to understand the complexities of your design.
Peace
The axle's rotation is limited by the fact that the TA-to-axle connection is not hinged.... it is rigid. And thats the rub, in my limited knowledge type brain. While the axle is rotating around the front hinge of the TA, and also around the body hinge of the LCA, the fact that the axle can not change its angular relationship to the TA sort of makes me wonder if the concept of IC is even applicable. It is this rigid connection of the TA to the axle that puts the suspension in bind without the flexible link somewhere in the TA.
Perhaps in the Madman design, running the front mounting brace between the SFC's provides enough flexibility in the mount that the TA is free to move slightly. Couple that with keeping the LCA movement in a narrow range from slightly below to slightly above horizontal, and the front-to-back differential displacement of the TA and LCA's becomes less of an issue.
Brian.... when I said your TA "wasn't going to work", it was in this context, and in the context that WS6 TA brought up... when you fix the length of the TA, you have created a new form of suspension. And it will induce compressive forces in the TA, something that isn't possible with the slip joint or "linked" design. It isn't working in the same way. And if we define "working" as keeping the rear axle assembly from rotating into the wrong position due to the torque reaction, an arm that tears itself apart is technically "not working"....
. And obviously the massive weight of the car in question may have contributed to that failure, since at the time, I doubt it was making more than about 750flywheelHP.I know you have put down some fantastic 60-foot times, but it isn't unreasonable to ask if they couldn't have been even better with the flexible link. Or maybe you have changed the design since I saw it.... dunno.
I'm just trying to understand the complexities of your design.
Peace
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Joined: Jan 1999
Posts: 86
From: Baton Rouge
I am trying a new design right now but we havent seen any difference in 60ft times. We are seeing the need for a upgraded unit. All of the bars we are building now are 1 1/4 chromemoly and have alot more gussets. The bar that Baxter broke was taken care of for him and we learned off the broke pieces. Our new bar is made of 1 1/2 chromemoly lower and has a new style front mount. We also are offering more adjustment holes. I have this bar on my car and if the 60ft times can be bettered and we can get more control of the launch then this bar will be offered.
If you try to get your i/c off of the lower control arms AND the t/a front mount as some do then your i/c is actually in front of the car about 3ft and higher than the camshaft. I beleive that the lower control arms only locate the axle for and aft like the panhard does side to side.
If you try to get your i/c off of the lower control arms AND the t/a front mount as some do then your i/c is actually in front of the car about 3ft and higher than the camshaft. I beleive that the lower control arms only locate the axle for and aft like the panhard does side to side.
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Joined: Jul 1999
Posts: 219
From: College Station, Tx, USA
At the risk of jumping in in the middle of a conversation -
The LCA's in an f-body suspension would be "for location only" only if the front TA mount wasn't a sliding design (like yours from what I understand) *and* if the arc inscribed by the ta was the same as that of the LCA's. With a nonsliding front mount, as has been pointed out, the LCA's can put the torque arm in a bind, preventing any kind of rotation. Depending on the busings used, give of the metal, etc you will get some rotation though. When you get flex in these mounting points - assuming the give is in the LCA's - then the instant center is effectively the torque arm mount itself. Infact it always is, you just cant rotate about it if the LCA's are in a bind.
With a sliding front it's still not like a 4-link though (IC intersection of upper a lower mount), because - as Injuneer pointed out - the rear end isn't free to rotation around the torque arm mount (disregarding the lower mount) as it is in a 4-link setup. The bolt hole of the lca mount on the rear rotates around the lca mount, but the pinion must always point towards the torque arm mount... and without doing a diagram I am not sure exactly where that leaves instant center - you really have two rotations - one of the centerline of the body around a point, and one of the body itself around a centerline, so I guess it depends on how you define "the axle" in the context of instant center.
Chris
The LCA's in an f-body suspension would be "for location only" only if the front TA mount wasn't a sliding design (like yours from what I understand) *and* if the arc inscribed by the ta was the same as that of the LCA's. With a nonsliding front mount, as has been pointed out, the LCA's can put the torque arm in a bind, preventing any kind of rotation. Depending on the busings used, give of the metal, etc you will get some rotation though. When you get flex in these mounting points - assuming the give is in the LCA's - then the instant center is effectively the torque arm mount itself. Infact it always is, you just cant rotate about it if the LCA's are in a bind.
With a sliding front it's still not like a 4-link though (IC intersection of upper a lower mount), because - as Injuneer pointed out - the rear end isn't free to rotation around the torque arm mount (disregarding the lower mount) as it is in a 4-link setup. The bolt hole of the lca mount on the rear rotates around the lca mount, but the pinion must always point towards the torque arm mount... and without doing a diagram I am not sure exactly where that leaves instant center - you really have two rotations - one of the centerline of the body around a point, and one of the body itself around a centerline, so I guess it depends on how you define "the axle" in the context of instant center.
Chris
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Joined: Dec 1969
Posts: 472
From: Mobile, Alabama
MadMans "star wars" version won't bind. I'll let him talk about it further if he wants, but my point is that it won't bind due to different arcs of the LCA's and torque arm. That may have been an issue with some of his earlier stuff, but not new stuff coming out.
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Joined: Feb 2000
Posts: 1,248
From: Bountiful, Utah
I see no wrong in MADMANS stuff. What's so wrong with a three link rear?
One less bar than a 4link, same concept.
Yes it will be harsher on the road but thats really a non-issue for a horsepower junkie. Great thread all!
Just remember to go out and buy "Dave Morgans Door slamer Chassie book"
OUTSTANDING reading on this subject and it has 'Diagrams'.
ps..I run the shackel'd front type of T/A with high 1.3 to low 1.4 sixtys.
One less bar than a 4link, same concept.
Yes it will be harsher on the road but thats really a non-issue for a horsepower junkie. Great thread all!
Just remember to go out and buy "Dave Morgans Door slamer Chassie book"
OUTSTANDING reading on this subject and it has 'Diagrams'.
ps..I run the shackel'd front type of T/A with high 1.3 to low 1.4 sixtys.
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Joined: Jul 2000
Posts: 520
From: MD
Well, since no one seemed to be getting it (with the possible exception of OldSStroker), here are some quick scribbles (pardon the fact that my f-body doesn’t look like an f-body, my TA doesn’t look like a TA… I was in a hurry).
This is a sliding front link setup:
This is a pivoting front link setup:
Madman, like I’ve already said, the question isn’t if your setup works in your car, but why it works. After reading the descriptions and seeing the pictures I suspect that overall you’re raising your antisquat some, and secondly, I think that the binding that you will get with your setup sorta acts like a limiter, limiting the total motion and the speed of that motion. I bet that if you converted to a sliding link that had the same antisquat you’d need stiffer rear shocks to work the same.
(damnit, this would be 1000x easier if this board played nice with attachements)
This is a sliding front link setup:
This is a pivoting front link setup:
Madman, like I’ve already said, the question isn’t if your setup works in your car, but why it works. After reading the descriptions and seeing the pictures I suspect that overall you’re raising your antisquat some, and secondly, I think that the binding that you will get with your setup sorta acts like a limiter, limiting the total motion and the speed of that motion. I bet that if you converted to a sliding link that had the same antisquat you’d need stiffer rear shocks to work the same.
(damnit, this would be 1000x easier if this board played nice with attachements)
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Joined: Jul 2000
Posts: 520
From: MD
Originally posted by Hot Rod Hawk
I see no wrong in MADMANS stuff. What's so wrong with a three link rear?
One less bar than a 4link, same concept.
I see no wrong in MADMANS stuff. What's so wrong with a three link rear?
One less bar than a 4link, same concept.
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Joined: Jan 1999
Posts: 86
From: Baton Rouge
WS6TA you are very close in your theory. BUT on my arm IF you say the lower control arms are part of the instant center theory then the lines from the lower control arm and the t/a should cross. Where they cross would be the i/c.
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Joined: Jul 2000
Posts: 520
From: MD
That's been my point from the beginning... if you have a front pivot rather then sliding joint the lower arms have no effect on IC.
FWIW, if they did, your design would actually have less antisquat then the stock setup at a stock ride height.
FWIW, if they did, your design would actually have less antisquat then the stock setup at a stock ride height.
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Joined: Dec 1969
Posts: 1,147
From: Phila., PA
Wait, wait, wait. I thought I had it but now it's slipping away from me again.
WS6- I'm wondering if it's just the opposite. If the sliding front link at the front of the TA can't take any lateral force (front-to-back) then the LCAs just do the front-back locating. The TA takes all the rotational force of the axle, since it's bolted solidly to the rear end like a big lever.
In my ignorance I still KNOW that in the real world that's how the forces are divided on a stock F-body setup: The TA takes the twisting, the LCAs do the pushing. You just ahve to look at it and you KNOW that's what the factory guys were intending when they designed it. I won't comment on non-stock setups becuase I have never seen or used them and am not qualified to comment, therefore.
What would be REAL helpful is if someone would give the TEXTBOOK DEFINITION of the IC. Is it the point the housing tries to rotate around when torque is applied or does it have to do with the arc through which the rear end travels as it moves up and down. Should be able to get down to the bottom of this pretty quick if we define terms precisely.
WS6- I'm wondering if it's just the opposite. If the sliding front link at the front of the TA can't take any lateral force (front-to-back) then the LCAs just do the front-back locating. The TA takes all the rotational force of the axle, since it's bolted solidly to the rear end like a big lever.
In my ignorance I still KNOW that in the real world that's how the forces are divided on a stock F-body setup: The TA takes the twisting, the LCAs do the pushing. You just ahve to look at it and you KNOW that's what the factory guys were intending when they designed it. I won't comment on non-stock setups becuase I have never seen or used them and am not qualified to comment, therefore.
What would be REAL helpful is if someone would give the TEXTBOOK DEFINITION of the IC. Is it the point the housing tries to rotate around when torque is applied or does it have to do with the arc through which the rear end travels as it moves up and down. Should be able to get down to the bottom of this pretty quick if we define terms precisely.
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Joined: Oct 2002
Posts: 2,931
From: Upstate NY
Originally posted by Damon
Wait, wait, wait. I thought I had it but now it's slipping away from me again.
WS6- I'm wondering if it's just the opposite. If the sliding front link at the front of the TA can't take any lateral force (front-to-back) then the LCAs just do the front-back locating. The TA takes all the rotational force of the axle, since it's bolted solidly to the rear end like a big lever.
In my ignorance I still KNOW that in the real world that's how the forces are divided on a stock F-body setup: The TA takes the twisting, the LCAs do the pushing. You just ahve to look at it and you KNOW that's what the factory guys were intending when they designed it. I won't comment on non-stock setups becuase I have never seen or used them and am not qualified to comment, therefore.
What would be REAL helpful is if someone would give the TEXTBOOK DEFINITION of the IC. Is it the point the housing tries to rotate around when torque is applied or does it have to do with the arc through which the rear end travels as it moves up and down. Should be able to get down to the bottom of this pretty quick if we define terms precisely.
Wait, wait, wait. I thought I had it but now it's slipping away from me again.
WS6- I'm wondering if it's just the opposite. If the sliding front link at the front of the TA can't take any lateral force (front-to-back) then the LCAs just do the front-back locating. The TA takes all the rotational force of the axle, since it's bolted solidly to the rear end like a big lever.
In my ignorance I still KNOW that in the real world that's how the forces are divided on a stock F-body setup: The TA takes the twisting, the LCAs do the pushing. You just ahve to look at it and you KNOW that's what the factory guys were intending when they designed it. I won't comment on non-stock setups becuase I have never seen or used them and am not qualified to comment, therefore.
What would be REAL helpful is if someone would give the TEXTBOOK DEFINITION of the IC. Is it the point the housing tries to rotate around when torque is applied or does it have to do with the arc through which the rear end travels as it moves up and down. Should be able to get down to the bottom of this pretty quick if we define terms precisely.
"The word "instant" means at that particular position of the linkage. "Center" refers to a projected imaginary point that is effectively the pivot point of the linkage at that instant."
Damon, your thinking is correct with "the point the housing tries to rotate around..." Injuneer also said the same thing.
Your gut feeling for how the stock F-body rear suspension works is also pretty much correct. "Lateral" really refers to side-to-side, and "longitudinal" to fore-aft, but that's minor semantics.
WS6 TA correctly described how to locate the IC in a stock F-body rear suspension in an earlier post in this thread:
On f-bodies, with their sliding link TA the IC is computed by drawing a line perpendicular through the sliding joint at the end of the TA and then a second line through both pivots on the LCA’s, and the intersection of those 2 lines is the IC.
He also diagrammed it correctly:
This is a pivoting front link setup:
http://users.erols.com/mpikas/temp/I...erTA-pivot.jpg
I'd like to comment on modifications to f-car rear suspensions discussed in this thread, but without observing them directly, I cannot.
FWIW, the basic f-car torque arm suspension is similar to that used on the Type 35 Bugatti in 1924 which also had 8-spoke cast aluminum wheels.
My $.02
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Joined: Dec 2000
Posts: 766
From: GARDNERS,PA
F-Body Instant Center Observation:
I was at the track this weekend and had some close up video taken of my rear suspension and rear tire during launch. I'm running an M6 car, 4500 RPM launch on slicks at 13 PSI. My suspension is a Spohn (Generation 1) Torque arm and Lower Control Arms. My rear is fitted with lowering brackets and I have the LCA's on the bottom hole.
We clicked the video through frame by frame and the view was actually pretty spectacular. At launch, the body remained very stable (perfectly flat). The rear and tires were immediately forced down to the pavement. I would guess the max deflection downward was about 2". This absolutely crushed the tires! The shocks couldn't dampen the rebound and the tires unloaded. The suspension went through about 3 of these cycles until things started to settle out. So my setup clearly has a bunch of Anti-squat. So this week end I'm going to move the LCA's up one hole on the rear. I'll also look into getting some Hal's to try to critically dampen some of the suspension dynamics. Looks like I'd need more jounce resistance to the shocks to keep the tires planted after the suspension separates.
Can't say enough about getting some good video footage!
I was at the track this weekend and had some close up video taken of my rear suspension and rear tire during launch. I'm running an M6 car, 4500 RPM launch on slicks at 13 PSI. My suspension is a Spohn (Generation 1) Torque arm and Lower Control Arms. My rear is fitted with lowering brackets and I have the LCA's on the bottom hole.
We clicked the video through frame by frame and the view was actually pretty spectacular. At launch, the body remained very stable (perfectly flat). The rear and tires were immediately forced down to the pavement. I would guess the max deflection downward was about 2". This absolutely crushed the tires! The shocks couldn't dampen the rebound and the tires unloaded. The suspension went through about 3 of these cycles until things started to settle out. So my setup clearly has a bunch of Anti-squat. So this week end I'm going to move the LCA's up one hole on the rear. I'll also look into getting some Hal's to try to critically dampen some of the suspension dynamics. Looks like I'd need more jounce resistance to the shocks to keep the tires planted after the suspension separates.
Can't say enough about getting some good video footage!