Suspension Roll Centers Explained on my Porsche 911

Suspension Roll Centers Explained on my Porsche 911


hey there so last Saturday I installed
the custom camber boxes in the rear suspension of my Porsche 911 and it was
a lot of work for two extra holes to be able to relocate some of the trailing
arm components so other than camber being in the name I didn’t really
explain all the implications of what modifying or messing with the suspension
actually does so I’m gonna do that today so who’s ready for some bench racing
theory all right the modifications I did to
this car changed three major things it changes the camber so why it’s called
camber box and it changes the roll center and it also changes the
anti-squat characteristics other car so I’m going to be explaining each of those
three number one is the camber so the camber is how much the tire leans in
from the vertical when looking down here a negative camber is leaning in positive
camber is leaning out so sports cars always want the tires leaning in factory
ways to adjust the camber is at the junction between the banana arm and the
spring plates so you have the spring plate and banana arm attaches kind of in
a triangle like with my shoulders and when you change the angle of one
relative to the other it actually changes the angle that the axle points
out from the car so if I rotate one relative to the other it sort of turns
it this way this way changes it that way that’s how the factory adjustment works
and that is all great there’s a small range of adjustment because the holes
are slotted there’s an eccentric bolt in there that helps you kind of move things
around but when you lower the car you sort of run out of range of adjustment
and then you’re stuck with a negative camber of like two degrees three degrees
and that’s just not ideal for grip so one thing you can do is you can just
lock the holes bigger and you know get the adjustment you need and that works
there’s aftermarket spring plates that do that there’s the people that weld and
flip and do all kinds of things to the spring plate and that’s fine
another way to do it is to relocate the angle of the banana
so lift my shoulders like this just you know raise one up and that is what I did
that gives a bigger range of adjustment so the the rough adjustment is on the
inside because I use three distinct holes and then the fine adjustment is on
the spring plate so now I can get on a lowered car I can get the camber to
whatever angle I want half a degree one degree one and a half degrees whatever’s
suitable for my driving style all right the other thing it does is it changes
the roll center and the roll center is kind of a confusing topic which is why
I’m making a separate video about it so I didn’t want the last week’s video to
be even longer than it was so the roll Center is a fictitious point in the rear
suspension where the suspension acts on the chassis so it’s a geometry
geometrically controlled place and I’m going to show you what that is it’s not
that different than a center of gravity so if I have a block now here’s my
trusty sanding block if I have a block it’s pretty easy to understand kind of
what the center of gravity is it’s kind of where you can support the block the
center of gravity is kind of in the middle and then it has a center of
gravity in this axis too so that’s kind of easy to visualize what’s more
difficult to visualize is when you put wheels on the block how does the
suspension attached to the block so let me show you on a little little little
diagram okay here’s a diagram of my my my sanding block and it’s the rear view
so this is the block and this is you know just the fundamental kind of
understanding of this center of gravity is in the middle of the block and then
it’s got their two rear wheels here and then this red is sort of how you attach
the wheels to the block you can attach them in various ways this is kind of a
double wishbone setup but we’ll talk about the Porsche in a minute this is
attached relatively low to the car these red lines are attached relatively low
and then on this version it’s kind of the same block same ride height but the
suspension is attached much higher up on the car this is exaggerated but these
two cars are going to behave differently around a corner based on how the block
is supported by the suspension so this car if it’s turning to the right
it’s gonna have a 1g load you know call it 1g load to the left so this car is
going to roll this way it’s going to turn it’s gonna roll like this versus
this car also is turning right and it has a similar 1g load on it but because
of the way the suspension is it’s not gonna roll as much so these dots here
represent the roll centers and this is a way to compare different suspensions and
to calculate how much roll a different car will experience so fundamentally
that’s you know real quick explanation as to what the roll centers are now that
has nothing to do with my Porsche 911 that is a double wishbone suspension
nothing like the trailing arms on this car so I did that for explanation but
now I’m gonna go through and actually measure the suspension arms and also try
to calculate where the roll centers are both in the stock form and in my
modified form and then also look at the turbo Porsche for the 930 from the 70s
also did some fussing around with the suspension here so you might say you
know what are you doing messing around with a great handling Porsche anyways
lowering the car is not just for appearance lowering the car reducing the
height of the center of gravity does a lot for improving the cornering
capability so one lower center of gravity means there’s less load transfer
to the outer tire so actually both tires are carrying a little bit more load as
it goes around the corner and it also reduces the roll as well okay I’m here at my computer now and I
have made a model in my three dimensional software called SolidWorks
I’ve made a crude model of this torsion tube with the brackets and the three
various holes you can see here these are this is the stock hole these are the two
that I added and then this is the hypothetical hole this is what I think I
guess on the location here but the turbo that I mentioned earlier that car has a
hole instead of going up it goes this way and so I want to understand you know
what the effect of the role Center is on the turbo car versus the modification
that I did and I’ll discuss all those different options I did measure you know
sort of the rough lengths you know the height of the hub relative to the
centreline and this is sort of measured with a tape measure so it’s not
perfectly accurate but it is relatively close to what the actual measurements in
the car are so this is going to allow me to calculate where the roll centers are
and then this giant ball here in the background is sort of my estimation of
what the center of gravity is and this is hard to get I don’t know exactly what
that is I think I estimated it around 24 inches but what I’m really after is the
relative differences and so that’s that’s what that’s for so to get the the
roll centers it depends on the way that this triangle sort of hinges about this
pivot point in this pivot point that’s where the bushings go so these are the
lines at which that thing pivots okay so there’s the stock line the middle hole
the upper hole and then the turbo hole and then where those lines sort of meet
the plane of where the wheel contacts the road creates these intersection
points and then those lines when they come down to the road area they
intersect along the center right here these four dots zoom in these four dots
rep the different role centers so the turbo
role Center is the third from the bottom this is the lower hole this is the
middle hole this is the turbo and then this is the upper hole so let me show
you in a different way where I can actually put some dimensions on it okay
no these lines are probably confusing a little bit messy but I’ll try to explain
you know what it is that I’ve done this drawing here this upper one is showing
the rear view of the car with the left wheel on it it’s at European ride height
and these roll centers are variable based on where the trailing arms
attached that’s what all these lines are for now if I turn some dimensions on I
can look at even more lines these lines are the height of the roll center
relative to the ground so the ground is at zero the factory location for the
roll center is three point three inches the first hole the medium hole going up
on the trailing arm – it was five point to the turbo which moves that that
shorter trailing arm moves it back is five point four five and then if I use
the highest hole on my new bracket that would be about a seven inch height of
rural center that’s a higher roll center so that’s important to know now on this
view right here this one has the car lowered one inch lower than European
ride heights so when you lower the car you lower the roll center so the height
of the roll center if you lower the car one inch is 33.0 where it was three
point three up here so you lower the car one inch you lower the roll center about
a third of an inch now you can combat that by as you lower the car you lower
the roll center well you can move the trailing arm up higher and that goes
back more to a factory setting so what was three point three two if I move it
to the upper hole it’s four point nine so you’re actually overshooting a little
bit if you compare it to what turbo had the turbos roll Center was
almost 5 inches in lowered form if I choose the middle hole I get 4.9 so
that’s really close to what the turbo roll centers were and that’s one of the
goals it’s not to sort of go outside the realm of what the Porsche engineers had
had planned for the turbo which is you know a more responsive car now
the other thing that’s important to know I’m going to turn these dimensions off
what’s also important to know as I showed earlier in the fundamental
diagram with my sanding block this is the center of gravity these are the
rural centers the further these two are apart the more the car is going to roll
so if I turn the dimensions on for what’s called the roll couple you can
see that in stock form it’s about a twenty three point four inch roll couple
that’s the distance between there when you lower the car it goes down to twenty
two point three so you reduce the roll couple by about one inch that means when
you lower the car even with the suspension geometry accounted for it
will roll less one because the center of gravity is lower in two because the the
roll center doesn’t go down as far as the ride height does so that’s why the
net result is your car doesn’t roll as much that may not be true for the front
it’s a totally different suspension geometry and I haven’t analyzed that yet
but I will certainly get to it one thing to do to combat the lower roll center is
to you know raise it up that reduces the roll couple something similar to what
the turbo had so if I choose the middle hole it’s right on top of the turbo hole
which geometrically are very different one moves the the location back with a
shorter trailing arm the other one moves it up with the stock you know
conventional trailing arm but you can get a roll center that’s almost
identical to the turbo and that’s why I selected the hole positions that I did
and that’s what I’m after now there is another
difference between these locations and let me let me switch to a different view
okay so when you look at this stock location from the rear you can see this
tire we move this way a little bit this tire actually the camber is changing
over its suspension travel it starts with about zero and then it goes to
something that’s a negative camber now that’s good as the car rolls it’s going
to get additional camber due to the suspension geometry and and that camber
gain is something that you need to pay attention to because where this is
attached is going to kind of change how quickly the camber goes from kind of a
vertical location to kind of leaned in location so that was for the stock form
I’ll play it again also if you look from the top view not only does the camber
change in this direction but it actually crease causes the toe-in to change a
little bit too I’m not sure you’re able to see that in in the way this is
showing it but when when it goes up the cam the toe in turns in so that’s just
the way this technology is I mean this was created in the 60s and that’s what
you get you get a camber change and you get a toe in change I’m not sure if it’s
easy to see on camera there but that’s something to pay attention to if you
look at this angle here it’s becoming more severe as the suspension
articulates up ok the wheels bouncing up and down right now it’s going to show a
plot of the angle of the wheel as a function of time so you can see it’s
sort of alternating between almost minus one degrees and two degrees these
negative signs are flipped it’s leaning in more that would actually be a
negative camber but you can see it varies between sort of you know kind of
three degrees of angle as it goes up and down it’s suspension travel so I’m going
to remember this plot locate this in this uppermost hole and then we’ll look
at this again and see if anything is dramatically different okay here is the
same analysis with the suspension arm attached to the uppermost
hole it’ll be the most extreme probably not even going to use this hole but for
the purpose of understanding what’s happening you can kind of see how the
suspension does the same thing it a gains camber as it articulates up and
it’s also towing in as it goes up just like it did before the initial camber is
a little bit different I don’t know why the scale is so different but the
difference between taht kind of top and bottom is still three degrees so it’s
it’s pretty similar to the stock form and and that’s just because the length
of the arms are basically the same it’s just attached at a higher point at
no point does the wheel go through any sort of positive camber at least at this
ride height which is pretty low I don’t think it’s ever gonna go to positive
camber so that’s important to know yeah I didn’t do a model with the trailing
arm attached to the turbo location I’d have to change some geometry and I
actually don’t even know the distances for the turbo I’m basically just
guessing based on pictures but I do know that the effect of reducing this arm is
going to cause the camber to ramp more quickly it’s going to increase in
negative camber as the ride height goes down the camber is gonna escalate more
quickly okay guys the third thing that changes with this modification is what’s
called anti squat and I think this one is the least significant and I’ll try to
explain why so this is a side view of the car there’s the rear tire this is
the torsion tube this is the estimated center of gravity and when you
accelerate powers coming from the rear wheels and pushing the chassis forward
so it pushes along these two lines one is the line in which the spring plate
attaches to the torsion bar and the other is the line at which it attaches
to the inner mounts which is in my case this is the third hole as I put it at
the highest hole and the way that these forces act relative to the center of
gravity causes the chassis to rotate down so this distance between these two
lines creates a torque on the chassis so when you
accelerate it squats down most of the load is is given to this line right here
and I did not change the location some people do change the location right here
but if I rotate this bottle down most of the load is driven into the torsion bar
right here because it’s so close to the wheel not much load is driven by this
one so if I look at this angle again mo there’s a lot more load on this line
than there is list line so I don’t think that the anti-squat characteristics
really changed that much you could argue that the anti squat is reduced because
this line is this distance here is reduced but in reality I don’t know that
it makes any difference in fact it could be worse because if the wheel is further
outward of this one and forces are coming here this one actually might be
pulling this way but anyways the thing significant I don’t think the squat
characteristics are gonna change one way or the other if you really want to
change the anti-squat characteristics then you should move this point up that
would reduce anti-squat but because I didn’t play with this one at least yet
I’m not gonna say that anti squat is very different so if you’re still
watching this video I’m actually really impressed that’s a lot of mumbo-jumbo
for you know how to position some extra holes and I just want you guys to know
that I did go through some analysis to figure out what the hole spacing should
be and is it within the boundaries that the Porsche engineers used when they
made the turbo that’s what it comes down to the Porsche Turbo moved those
locations further to their rear and I moved my locations further up that
avoided for me having to buy the turbo trailing arms they’re extremely
expensive and you know whether they’re better or not I don’t know it really
comes down to do you need more camber gain or do you not that’s the advantage
of the turbo arms there’s more camber gain there’s not a lot of camber gain in
the front wheels so I don’t know that you need more camera gain in the rear I
have a feeling that the Porsche engineer were somewhat constrained as to what
they could do they couldn’t move the whole torsion bar up because that
requires retooling the whole chassis they probably weren’t allowed to
encroach into the rear seats like I did it’s a four person car and that changes
a bunch of regulations if you if you do what I did you get rid of the back seats
and I think that ruins a lot of mass mass-market appeal so the the response
that you will will achieve that the turbo achieved also is that the car will
feel more responsive it’s it’s it’s gonna roll less in the back so any input
you’re given its gonna respond quicker it also means that it’s going to be more
unforgiving so with less roll means you have less sort of warning when the back
end is going to break free now the Porsche 930 turbo was known as
the Widowmaker and people think it’s because of the turbo and it’s because of
the extra power I mean that’s true but it could also be because of the roll
center making it more aggressive I’m putting my car at I could go back to
stock I could go anywhere in between that’s something the turbo can’t change
it’s just got a fixed location so I like the adjustability that’s why I did it
it’s within the sort of engineering boundaries that Porsche did it’s not
reinventing the wheel even though it might look like it so that’s the that’s
the gist of what I did and the analysis that I did thank you for watching guys

About the Author: Michael Flood

13 Comments

  1. Thanks for watching this mid-week video! I try to bring you the best tech tips about vintage Porsche 911's. Please don't forget to subscribe and hit the bell notifications, Like this Video and share it with a friend.

  2. Brilliant ! Very well explained. By the looks the suspension can only work if there is some flex in the joints. Is that why there is rubber in the torsion bar joint?

  3. Class is in session with Professor Tom giving the finer points of Porsche rear geometry. Excellent demonstrations for us to see.

  4. How many degrees of camber are you looking to get once it's lowered. So, is the point of all this to have stock camber settings at lowered height, with rapidly progressing negative camber on turns?

  5. Great job explaining the "why" behind the modification. I now have much better understanding of the 930 short arms. Now looking forward to seeing how it drives.

  6. Another great vid! Modeling this in SW was really helpful. At the end you mention Turbo geo…the theory about the internal compartment makes sense. Was the turbo torsion and shorter arm also for added strength? You also mentioned camber gain under load, do you mean anti camber? As in when it accelerates and anti squat occurs, turbo geometry eliminates the camber gain otherwise found in non turbo geometry…not sure I understood that correctly. i.e a turbo engine in a standard susp geo car makes it the widow maker +… b/c there is less tire patch on the ground during heavy acceleration.

  7. Another great vid. Nice job on the SolidWorks animation; cool to see the camber and toe-in change through the range of motion. The part about the limits set on the old German engineers is funny- I can just picture the debate about how moving the mount points up would somehow threaten the the 911’s status as a “4 person car” 🙂 Good stuff!

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