pstoppani

I haven't seen this mentioned before so I'll share a bit of wisdom I learned recently about the AWD system in our cars and why I'm disconnecting it; after being a holdout against 2WD because I always felt the AWD is an advantage at the race track.

As a bit of background, my car has an UMW 3.6L motor with Kevin's latest turbos; it makes close to 600HP on a cool day on 100 octane gas. The car also has solid bushings, solid engine mounts, rear tilt kit, evo uprights, RS sways, and recently added Motons with 600/800 springs.

Last weekend was the first time out with the Motons after having PSS9s on the car for the past 5 years. The Motons made a noticeable and measurable difference. They inspire a great deal of confidence by eliminating the cars body motions, as one would expect. At Thunderhill I shaved a second off my best lap time and my lap times were considerably more consistent.

But, I also gained some disturbing and very distracting steering wheel kickback, at least, that is what I thought it was until several people pointed out that inside front wheel comes off the ground quite a bit.

Given the cars was so planted and after discussing this with a few race engineers, it seems the suspension setup is fine and the wheel coming off the ground is a common trade off to ensure the rears are optimally planted for exiting corners.

So, what I thought was kickback is actually caused by the 5% power going to the front now going to the wheel that is off the ground. Whenever the wheel touches the ground again, the power going to it causes the jerk in the steering wheel. It is most severe as I'm rolling on the power, which makes sense given the weight transfers to the rear and power is increasing.

At this point, I could try to stiffen the rear or soften the front, but I'd likely be compromising the excellent chassis balance the car appears to have. This would help keep the front on the ground. But, given that so little power is going to the front anyway, unless I cause throttle induced over-steer, it really does not make much sense to transfer the weight to the front.

Conclusion: it seems a 2WD conversion is the right answer.

Here is a video, with an old forum member K24Madness in a turbocharged C4S, that shows the steering wheel flying around in my hands (skip to the second lap at 2:00; first lap is chasing K24Madness and some Mustangs):

http://www.youtube.com/watch?v=lIOoFbxINqw

Spartan

These cars are always going around turns on three wheels

Ditch the awd and you'll be very surprised & happy with the handling esp at turn in and track out. Just roll the throttle on nice and smooth

pstoppani

Somehow, I just knew you'd be the first to respond

Given they are three wheelers, the FWD part is doing nothing anyway!

I'm crossing over to the 'dark side'

beentherebaby

Actually four tires in contact with the pavement will generate higher cornering forces than three and four is the preferred approach. You can correct the problem by using a softer front ARB and stiffer front spring rate. Whenever possible an ARB should be used as a fine tuning device, not the primary suspension spring.

Giving up AWD and using three-wheeling is just going backwards.

pstoppani

Interesting, because softening the front ARB was my first inclination. Decisions decisions. Certainly easier and cheaper to try softening the ARB...

ca993twin

Pete,

You can soften the front ARB or stiffen the rear ARB in an attempt to keep all 4 wheels on the ground. You will probably get a lot of oversteer if you do that. BeenThere is right... you may need to change spring rates, and that will require some experimentation. You may try to contact JJAYB and see what spring rates he was running to save some missteps.



Keep us posted.

bb993tt

Pete, you have the same spring rates Jimmy was using on blackie.

Y65MPH

Lifting the inside front is a sign of poor suspension setup. Three on the ground in never better than four. Revisit your setup. RWD is not the solution to your problem. Removing the AWD will help the steering wheel movement a little bit but every time that inside wheel hits the gound it will be moving at a different speed than the other three which will always cause an unsettled feeling.

Spartan

lol just wait until you get comfortable and start getting at a fast pace with the 2wd.

Its hard for me to explain the difference in driving style between the two setups to people. When driven fast on track with R comps (ie an ture honest fast pace) you will slide the car more. Not top gear type sliding by controlled sliding that you can feel while driving but someone couldn't really see if they were watching from the sidelines. You'll use the rear end to help steer the car more esp at turn in while trail braking.

Hard to explain but you'll get out of the car with a huge smile when you get it just right

I love these things on the track. Fun fun fun fun

Spartan

Also regarding lifting the inside wheel debating over the internet is difficult as we won't know the specific circumstance for where when why it lifts. For those that have been to Road American you can walk down to the inside of canada corner and watch every single car that comes through at speed lift its inside wheels. Cups, RSRs, you name it. If its a 911 variant and being driven quickly it lifts the inside wheel.

My point is regardless of setup you can still lift an inside wheel if the turn is just right.

Pstoppani if you are comfortable then I wouldn't lose any sleep over it

No HTwo O

Plus the weight savings, right, Pstoppani!

IIRC, it's about 150 pounds total. Spartan and Wagongotya can correct me if I'm wrong.

Greg H.

Why is it that 4wd is banned from road racing?

Greg H.

911/Q45

Keeps the costs down, don't want to give unfair advantage to well funded teams.

pstoppani

I will first tune the suspension to keep all 4 wheels on the ground as much as possible; I think there is room for improvement. Might as well take advantage of the various adjustability doodads I've invested quite a few $'s in. Then, we'll revisit the 2WD conversion.

pstoppani

Greg, Don and Tom,

It was so much fun hanging out and driving with you 993TT chaps at Thunderhill! I've got Miller Motorsports Park in July with HOD on my calendar and plan to go; hope to see you guys there. I'm sure I'll be down at THill in the Spring as well; me and the Turbo love that track