Right out of the gate, the first respondent spotted the difference between our two 2008 Cadillac CTSeses, despite a self-admitted alcohol impairment at the time (at least we're assuming it was alcohol.)
The brake calipers on the RWD CTS (the red one) are located behind the front axle centerline, while those on the AWD/black CTS are situated ahead of the front axle centerline. But no one grasped the significance of this difference.
With respect to the front axle centerline, brake calipers are located opposite the steering arm and tie rods. This means the RWD CTS has a forward-mounted steering rack, while the AWD CTS has an aft-mounted one.
A forward-mounted steering rack is far superior when it comes to steering precision, so most serious RWD machines use this placement. Having the control point (the tie rod ball joints) ahead of the tire contact patch means that forces at the contact patch create smaller unwanted toe-changes. Positive control is also helped by having smaller angles in the steering shaft u-joints (a straighter shot from the rack to the steering wheel, if you will). The RWD CTS features a new ground-up front suspension design and GM incorporated the forward rack placement from the start.
But the AWD CTS uses a carry-over front drive sub-assembly from the AWD Cadillac STS, and that car has a rear-mounted steering rack. I drove this particular AWD CTS, a US-spec car, in Germany at a GM-sponsored event (yes, the photo was taken at the Nurburgring). Despite having the same all-season tires and FE2 level suspension as our red CTS, the AWD car's steering clearly felt less immediate and a bit more vague. There was more steering kickback, too.
Front wheel drive machines with a lateral engine and transmission layout have to use aft rack placement because there's no other choice. And some high performance machines with longitudinal engine and transmission placement (most notably our Nissan GT-R) use the less-favorable rear rack placement if the underhood packaging is particularly crowded.
Dan Edmunds, Director of Vehicle Testing
joefrompa says:
09:19 AM, 03/ 2/09
IIRC Subaru's have forward mounted steering systems, no steering drivetrain kickback (due to symmetrical AWD).
I'm guessing the AWD CTS's are just accepted to be for less sporting individuals (which I think is fair, since you can't get them in a manual transmission anyway, at least AFAIK).
I've really come to love the subaru drivetrain design. The amount of reduced parts, low COG, and better balance took me awhile to appreciate from an engineering standpoint. Once I get my weak-arse dampers under control, I'll be able to appreciate it that much more.
Thanks for explaining the downside to that caliper placement.
Just curious: One commenter pointed out the front airdam appears to be matte black plastic in RWD trim and painted black plastic in AWD trim: Any comments, or just photo differences?
Joe
gibot says:
09:33 AM, 03/ 2/09
That's really good info Dan. I've been trying to find out the reasoning behind brake caliper placement
jederino says:
09:47 AM, 03/ 2/09
yes, nice info! I think I would tend to avoid AWD in a performance sedan, despite it's occasional advantages in the Seattle area.
subytrojan says:
09:50 AM, 03/ 2/09
Thank you, Dan! This clearly explains to me why brake calipers on some vehicles are mounted fore and aft of axle centerlines. This is the best technical blog entry I've read in some time! :o) :thumbsup:
joefrompa says:
09:51 AM, 03/ 2/09
I love my AWD when starting from a stop on even gravelly roads, wet roads, snowy roads, and putting the throttle down in turns. And right now I feel confident with 6" of snow on the ground.
I love it when I feel no torque steer and when I don't want the wheels to break loose.
But I don't like it when it's an extra ~150 pounds on my car, adding parasitic drivetrain loss, and makes the drivetrain a bit rougher (since there's alot more to synchronize).
Just my .02
actualsize says:
09:54 AM, 03/ 2/09
I should finish one point I didn't drive home: Brake calipers occupy the space that remains after the steering rack placement is determined by the engineers. And on production cars that left over space is always opposite the steering.
kurtamaxxxguy says:
10:08 AM, 03/ 2/09
Nice technical info, Dan. Thank you!
clarkma5 says:
10:35 AM, 03/ 2/09
For once edmunds has offered me technical info. that I didn't already know. Huzzah for learning!
stovt001 says:
10:58 AM, 03/ 2/09
Fantastic, I can now justify reading the LT blogs instead of studying for my Strategic Management quiz. Either way, I'm learning.
roar02ram says:
12:14 PM, 03/ 2/09
Thanks, Dan! Great writeup.
redliner says:
12:18 PM, 03/ 2/09
Wow! Very informtive post. Good job Dan, keep it up!
jahfakin says:
12:29 PM, 03/ 2/09
That is by far the best damn post in a very long time!
I'm not a fan of AWD, while I can understand their purpose on performance cars and on the track. For me, I think it's a waste, especially with traction and stability control standard on most cars.
If I was going to get an AWD vehicle, it will have to have the new e-4WD system developed by Nissan...off course, it's not yet available in the States... because we only get new technology after it gets played out in Japan.
http://www.nissan-global.com/EN/TECHNOLOGY/INTRODUCTION/DETAILS/E-4WD/
altimadude00 says:
02:43 PM, 03/ 2/09
The plastic air dam underneath the black car is the same matte black air dam under the red car. It has not been painted. It is an optical illusion created from the black painted body and different lighting and atmospheric conditions between the two pictures.
bbechtel16 says:
05:28 PM, 03/ 2/09
Wow! Thanks for that technical insight! I too have always wondered the reasoning of caliper placement.
cwc1 says:
05:42 PM, 03/ 2/09
Wow, very interesting and informative. This is another reason too, why rear wheel drive is preferable for performance, since it allows for the optimum rack location.
neumayerds says:
11:24 AM, 06/22/09
I know this old, but hopefully somebody sees it and can provide me a quick answer. Is the AWD system in the CTS using the actual subframe from the STS or just all of the suspension and steering components from the STS? If so, does that mean I could use the CTS subframe in my STS to translpant a LS motor in it? It has an opening and front-steering linkage that would accomodate a rear sump pan that the rear-steering linkage presents some interesting issues for me. Thanks