951, 944, 968 racers. Question about toe settings??
05-10-2010, 04:57 PM
#91
Race Car
Been out of town and away from the computer for a few days, but I've got one line of questions.
Regarding jacking. My understanding (and I'm an engine guy, much moreso than a chassis guy) is that so long as the roll center is below the track surface, jacking is not an issue. Presumably, that is how the factory did it. As we lower the car, we sink the roll center further and further below the pavement surface.
Now to the point about longer pins, which is the only thing I question (and I should point out that I'm not "challenging" Patrick's friend, I'm trying to make sure I understand this as much as possible). Now, typically speaking, in a race car, the roll center moves very little (as mentioned, due to long swing arms and minimized travel). However, as the wheel travels more and more, the instant centers move more and more. By attempting to get the lower control arm back to its more "neutral" position relative to factory settings, we should be alleviating the wild changes in instant centers, right?
Back to the jacking issue. With our cars, the instant center is the intersection of the line that follows the control arm with the line that is perpindicular to the strut (drawn at the center of rotation at the top). As we lower the outer portion of the control arm, we are moving the instant center upwards (bad in the sense that it increases the jacking potential). However, if we set camber by bringing the top of the strut inwards, that should help push the instant center further down. Shouldn't it?
Patrick, any chance your friend might be willing to comment on my line of thought? Again, I think I have a somewhat decent grasp on this stuff, but I'd like to know if I am missing something big.
And I haven't forgotten about your PM.....
Regarding jacking. My understanding (and I'm an engine guy, much moreso than a chassis guy) is that so long as the roll center is below the track surface, jacking is not an issue. Presumably, that is how the factory did it. As we lower the car, we sink the roll center further and further below the pavement surface.
Now to the point about longer pins, which is the only thing I question (and I should point out that I'm not "challenging" Patrick's friend, I'm trying to make sure I understand this as much as possible). Now, typically speaking, in a race car, the roll center moves very little (as mentioned, due to long swing arms and minimized travel). However, as the wheel travels more and more, the instant centers move more and more. By attempting to get the lower control arm back to its more "neutral" position relative to factory settings, we should be alleviating the wild changes in instant centers, right?
Back to the jacking issue. With our cars, the instant center is the intersection of the line that follows the control arm with the line that is perpindicular to the strut (drawn at the center of rotation at the top). As we lower the outer portion of the control arm, we are moving the instant center upwards (bad in the sense that it increases the jacking potential). However, if we set camber by bringing the top of the strut inwards, that should help push the instant center further down. Shouldn't it?
Patrick, any chance your friend might be willing to comment on my line of thought? Again, I think I have a somewhat decent grasp on this stuff, but I'd like to know if I am missing something big.
And I haven't forgotten about your PM.....
05-10-2010, 10:47 PM
#92
Rennlist Member
944CS chris when are you going to send me my book or my $40
05-11-2010, 12:20 AM
#93
Rennlist Member
Thread Starter
My friend will be along presently. He's a busy man at MIT.
I will see if my other friend 'Trucho-951' might comment. He's been very helpful and knowledgeable too.
I will see if my other friend 'Trucho-951' might comment. He's been very helpful and knowledgeable too.
05-12-2010, 11:17 AM
#95
Rennlist Member
Hi all,
'Tis I who has been corresponding with Patrick. Originally, I had intended on responding to the thread, but after I was done with my response, I found it ridiculously long -- so I put it into a PM to Patrick.
Inevitably, however, the message found its way back to the thread anyway!
So, I'll try to answer any questions (particularly those from 67King posted above) --
Simply put, jacking is the vertical component of the force experienced on the wheel. Meaning, there is always an x-component (lateral acceleration, in this case) and a y-component (jacking).
As you would expect, if the Kinematic Roll Center (KRC) were at ground-level, the jacking forces are essentially zero. When the KRC is above ground, the jacking forces are positive (upwards); when the KRC is below ground, the jacking is negative (downwards). The line of thought goes that neither is ideal, as it reduces the amount of potential grip your car can have (because some of the force that could be transferred laterally is being transferred vertically).
I should note that the KRC on the vast majority of cars (including the 944) is above ground. Meaning, when we lower the car (and consequently lower the KRC), it's still above ground, but at a slightly lower static point. And, in my previous post (on the last page, I think) I argue this is a good thing, but only as long as we can do something to affect the rear (and get the Kinematic Roll Axis to where it should be). In other words, while lowering the roll center of the front suspension is a good thing, it could have detrimental effects when not mated to the proper rear roll center. The entire vehicle is a giant system.
The concept of a below ground KRC has been around for a while, but only in recent years have suspension experts been employing it, and even then only in limited doses. Personally, I don't see much use to it; I've heard a number of theories both for and against -- many people say that it counteracts Elastic Weight Transfer (EWT -- i.e., body roll) but I'm not so sure. From what I've been able to make out, I feel it has the potential of overloading the tire's grip level and therefore make the under- and oversteer gradients much more steep.
So, whenever I design a suspension I tend to keep the KRC at or just above ground level (keeping the lower A-arms parallel to ground at ride height is the easiest way to do this).
The reason I'm fairly nonchalant about the exact position of the KRC height is because as soon as the car becomes a dynamic system -- i.e., it starts driving -- the KRC location is all over the place. I've seen suspensions where sure, the KRC is in a great place, but at 1 degree of wheel travel, it's now 35 ft. outside the track of the car, and below ground!
The KRC is a great way to start the design of a suspension, and a very simple-yet-effective method of calculating EWT. But, it still only remains useful in a STATIC system -- and once the suspension moves, all hell breaks loose.
Which is why the current trend in suspension design for the past eight years or so has been the look at the Force-based RC. Without going into too much physics, the main point is that once a suspension is loaded into a turn, it instantly becomes an asymmetrical suspension. So as we enter a corner, the KRC moves (i.e., away from the centerline of the vehicle). While the KRC location itself doesn't really help us much at this point, we can use it to find the Force Application Points, which are the two points on the centerline of the vehicle that intersect with the lines drawn from the respective contact patches of the tires to each of the Instant Centers. It is from the FAPs that we can determine the actual Force-based Roll Center.
This is getting very down-and-dirty into suspension geometry, so I'll stop here.
Patrick, you originally asked "if I could change anything on the 944, what would it be?"
I can think of good improvements to the front immediately, but the rear will take a bit more thought... Anyway broadly speaking, I'd first change the front suspension to a dual A-arm pushrod type. In my opinion the pushrod is absolutely essential for fine-tuning the suspension. Also, I'd like to do some simulations on adding more Ackerman, as I feel this is the main reason (coupled with the high polar moment of inertia) why these cars have so much trouble turning-in.
Oh and I'd want driver-adjustable sway bars, too
Like I said, I honestly haven't given the rear suspension much thought...and to be honest I'm not particularly thrilled of semi-trailing arms.
I"m very sorry for the lengthy response, y'all. This is why I don't post much anymore -- it takes too darn long!
67King, are you a GT alum? Great school. I'll be finishing my graduate degree up here in Baaahsten by July, assuming all goes to plan...
'Tis I who has been corresponding with Patrick. Originally, I had intended on responding to the thread, but after I was done with my response, I found it ridiculously long -- so I put it into a PM to Patrick.
Inevitably, however, the message found its way back to the thread anyway!
So, I'll try to answer any questions (particularly those from 67King posted above) --
Simply put, jacking is the vertical component of the force experienced on the wheel. Meaning, there is always an x-component (lateral acceleration, in this case) and a y-component (jacking).
As you would expect, if the Kinematic Roll Center (KRC) were at ground-level, the jacking forces are essentially zero. When the KRC is above ground, the jacking forces are positive (upwards); when the KRC is below ground, the jacking is negative (downwards). The line of thought goes that neither is ideal, as it reduces the amount of potential grip your car can have (because some of the force that could be transferred laterally is being transferred vertically).
I should note that the KRC on the vast majority of cars (including the 944) is above ground. Meaning, when we lower the car (and consequently lower the KRC), it's still above ground, but at a slightly lower static point. And, in my previous post (on the last page, I think) I argue this is a good thing, but only as long as we can do something to affect the rear (and get the Kinematic Roll Axis to where it should be). In other words, while lowering the roll center of the front suspension is a good thing, it could have detrimental effects when not mated to the proper rear roll center. The entire vehicle is a giant system.
The concept of a below ground KRC has been around for a while, but only in recent years have suspension experts been employing it, and even then only in limited doses. Personally, I don't see much use to it; I've heard a number of theories both for and against -- many people say that it counteracts Elastic Weight Transfer (EWT -- i.e., body roll) but I'm not so sure. From what I've been able to make out, I feel it has the potential of overloading the tire's grip level and therefore make the under- and oversteer gradients much more steep.
So, whenever I design a suspension I tend to keep the KRC at or just above ground level (keeping the lower A-arms parallel to ground at ride height is the easiest way to do this).
The reason I'm fairly nonchalant about the exact position of the KRC height is because as soon as the car becomes a dynamic system -- i.e., it starts driving -- the KRC location is all over the place. I've seen suspensions where sure, the KRC is in a great place, but at 1 degree of wheel travel, it's now 35 ft. outside the track of the car, and below ground!
The KRC is a great way to start the design of a suspension, and a very simple-yet-effective method of calculating EWT. But, it still only remains useful in a STATIC system -- and once the suspension moves, all hell breaks loose.
Which is why the current trend in suspension design for the past eight years or so has been the look at the Force-based RC. Without going into too much physics, the main point is that once a suspension is loaded into a turn, it instantly becomes an asymmetrical suspension. So as we enter a corner, the KRC moves (i.e., away from the centerline of the vehicle). While the KRC location itself doesn't really help us much at this point, we can use it to find the Force Application Points, which are the two points on the centerline of the vehicle that intersect with the lines drawn from the respective contact patches of the tires to each of the Instant Centers. It is from the FAPs that we can determine the actual Force-based Roll Center.
This is getting very down-and-dirty into suspension geometry, so I'll stop here.
Patrick, you originally asked "if I could change anything on the 944, what would it be?"
I can think of good improvements to the front immediately, but the rear will take a bit more thought... Anyway broadly speaking, I'd first change the front suspension to a dual A-arm pushrod type. In my opinion the pushrod is absolutely essential for fine-tuning the suspension. Also, I'd like to do some simulations on adding more Ackerman, as I feel this is the main reason (coupled with the high polar moment of inertia) why these cars have so much trouble turning-in.
Oh and I'd want driver-adjustable sway bars, too
Like I said, I honestly haven't given the rear suspension much thought...and to be honest I'm not particularly thrilled of semi-trailing arms.
I"m very sorry for the lengthy response, y'all. This is why I don't post much anymore -- it takes too darn long!
67King, are you a GT alum? Great school. I'll be finishing my graduate degree up here in Baaahsten by July, assuming all goes to plan...
05-12-2010, 01:32 PM
#96
Race Car
Unfortunately, I'm in a hurry to head up to New Jersey this afternoon. I look forward to reading more on this thread when I have a break. Love threads like this, and hoped to see more of this kind of stuff in this forum than I do.
05-12-2010, 03:10 PM
#97
Rennlist Member
Wit the high spring rates I use I have always run my car close to as low as I can get it, limited mainly by the rear spring height adjustment on the drivers side. Starting from that point I have always tried to run the sills level with the ground to get my basic ride height starting points then adjust the corner weights spreading the ride height changes across all four wheels to minimise change. Doing so has always put my front A-Arms sloping slightly upward from the inner mounting towards the wheels, even with the long pins I have, and based on the consensus parrallel to the ground is the best geometry I recently corrected it by raising the front ever so slightly.
Interestingly my perception has been that it hasn't feel quite as good since, so maybe it's not my imagination and the jacking effect Collin described is actually noticeable. Hmm...
Interestingly my perception has been that it hasn't feel quite as good since, so maybe it's not my imagination and the jacking effect Collin described is actually noticeable. Hmm...
05-13-2010, 08:56 AM
#98
Rennlist Member
Thread Starter
Well I sort of feel like someone who started an event by dropping a flag and watching it go in interesting tangents. Glad to see it still going. Interesting stuff indeed. I love that concept that once a car is moving and under cornering load that the KRC could be 35' feet away and subterranean. Ha, that's a weird but somehow humorous concept!
05-13-2010, 02:47 PM
#99
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Great discussion guys, I love it!
It is possible for a car to experience jacking, even with stiff spring rates, once it runs out of free travel.
IMHO, much of havoc can be introduced, unexplained by theoretical suspension calculations once the car runs out of suspension free travel (either in bump or droop). I mean, the theory is valid as long as our “assumption” that the car’s suspension is moving freely is also valid, however, for a lowered 951, that is not always the case (especially given the numerous aftermarket parts combinations available.)
It is possible for a car to experience jacking, even with stiff spring rates, once it runs out of free travel.
IMHO, much of havoc can be introduced, unexplained by theoretical suspension calculations once the car runs out of suspension free travel (either in bump or droop). I mean, the theory is valid as long as our “assumption” that the car’s suspension is moving freely is also valid, however, for a lowered 951, that is not always the case (especially given the numerous aftermarket parts combinations available.)
05-14-2010, 04:37 PM
#100
Rennlist Member
Wit the high spring rates I use I have always run my car close to as low as I can get it, limited mainly by the rear spring height adjustment on the drivers side. Starting from that point I have always tried to run the sills level with the ground to get my basic ride height starting points then adjust the corner weights spreading the ride height changes across all four wheels to minimise change. Doing so has always put my front A-Arms sloping slightly upward from the inner mounting towards the wheels, even with the long pins I have, and based on the consensus parrallel to the ground is the best geometry I recently corrected it by raising the front ever so slightly.
Interestingly my perception has been that it hasn't feel quite as good since, so maybe it's not my imagination and the jacking effect Collin described is actually noticeable. Hmm...
Interestingly my perception has been that it hasn't feel quite as good since, so maybe it's not my imagination and the jacking effect Collin described is actually noticeable. Hmm...
That's interesting. While I wouldn't necessarily jump to the roll center conclusion 100% right away, it does sound like this could be the issue. As you said, it sounds like with the car super-low, the roll center was between ground level and chassis bottom, which would have helped the way the car handled. And, in raising it back up, the roll center was then higher, increasing the jacking forces experienced by the suspension.
It's not a bad thing to have some rake to the car (in fact, it is almost a requisite with aero-based cars). I would take a look at the rear roll center of your car and see what's happening to your roll axis. Don't forget, changing front ride height also will affect the weight distribution as well as the rear ride height, which is why the whole system must be considered.
When's your next track day/practice? It'd be interesting to get some back-to-back testing, if possible!
05-14-2010, 04:55 PM
#101
Rennlist
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Thanks for getting involved with the thread Collin. I follow a good bit of what you are saying and it does help to confirm my "empirical" understanding of these suspensions work.
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Larry Herman
2016 Ford Transit Connect Titanium LWB
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Past Flames:
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CLICK HERE to see some of my ancient racing videos.
Larry Herman
2016 Ford Transit Connect Titanium LWB
2018 Tesla Model 3 - Electricity can be fun!
Retired Club Racer & National PCA Instructor
Past Flames:
1994 RS America Club Racer
2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
CLICK HERE to see some of my ancient racing videos.
05-15-2010, 12:22 AM
#103
Rennlist Member
Thread Starter
No, but I wouldn't know where to start. I have checked out some video* of the last track day in question and I'm sure most people would suggest that not much seems apparent from this film. I'm sure having done quite a few changes to the car since that we should have an improvement next time out. I've bought some Racers Edge camber plates which will allow us to lower the car without reducing the free travel. So this will also be a nice improvement. We have also been running with some retarded ign so there will be a nice improvement to the responsiveness of the motor as well.
*As for uploading the files to Youtube, I am really having a hard time doing this. I have changed nothing in my setup yet Youtube keeps rejecting the video....and after 8+ hours too which only increases the frustration. I have even split the video up into 2 clips of about 4 mins but have only been able to load one. There's not much to see but here you go anyway. If anyone has any tips how to upload more successfully please pass them on because I'm stuck with part 2 not loading amongst other holdups.
http://www.youtube.com/watch?v=3qyd6Dbmx38
*As for uploading the files to Youtube, I am really having a hard time doing this. I have changed nothing in my setup yet Youtube keeps rejecting the video....and after 8+ hours too which only increases the frustration. I have even split the video up into 2 clips of about 4 mins but have only been able to load one. There's not much to see but here you go anyway. If anyone has any tips how to upload more successfully please pass them on because I'm stuck with part 2 not loading amongst other holdups.
http://www.youtube.com/watch?v=3qyd6Dbmx38
05-15-2010, 04:15 AM
#104
Rennlist Member
Thread Starter
Ok, got Part 2 uploaded although at a reduced quality.
I think part of my problem was just not having done many laps at this track and I can clearly see quite a few places I need to improve. Also chasing the GT3 in front makes it tricky in that he can drive out of some places faster than me, but overall I was a couple secs quicker than him over the weekend. The nature of our events is that it is under the level of door to door racing so we can't pass unless on certain places or if they signal you. If we get too nose to tail the marshalls will pull you in too. Excuse the gear crunch too please. That's being attended to in the workshop today. No 2nd gear syncro.
It's more fun chasing someone around but your times suffer.
http://www.youtube.com/watch?v=YX99Ac_Yie4
I think part of my problem was just not having done many laps at this track and I can clearly see quite a few places I need to improve. Also chasing the GT3 in front makes it tricky in that he can drive out of some places faster than me, but overall I was a couple secs quicker than him over the weekend. The nature of our events is that it is under the level of door to door racing so we can't pass unless on certain places or if they signal you. If we get too nose to tail the marshalls will pull you in too. Excuse the gear crunch too please. That's being attended to in the workshop today. No 2nd gear syncro.
It's more fun chasing someone around but your times suffer.
http://www.youtube.com/watch?v=YX99Ac_Yie4
Last edited by 333pg333; 05-15-2010 at 07:20 PM.
05-15-2010, 06:11 AM
#105
Rennlist Member
That's interesting. While I wouldn't necessarily jump to the roll center conclusion 100% right away, it does sound like this could be the issue. As you said, it sounds like with the car super-low, the roll center was between ground level and chassis bottom, which would have helped the way the car handled. And, in raising it back up, the roll center was then higher, increasing the jacking forces experienced by the suspension.
It's not a bad thing to have some rake to the car (in fact, it is almost a requisite with aero-based cars). I would take a look at the rear roll center of your car and see what's happening to your roll axis. Don't forget, changing front ride height also will affect the weight distribution as well as the rear ride height, which is why the whole system must be considered.
When's your next track day/practice? It'd be interesting to get some back-to-back testing, if possible!
It's not a bad thing to have some rake to the car (in fact, it is almost a requisite with aero-based cars). I would take a look at the rear roll center of your car and see what's happening to your roll axis. Don't forget, changing front ride height also will affect the weight distribution as well as the rear ride height, which is why the whole system must be considered.
When's your next track day/practice? It'd be interesting to get some back-to-back testing, if possible!
Our season finished in April and starts again in Octber. The engine is now out of the car for a rebuild .
