Question about Oversteer/Understeer and changes that affect this
 

I recently replaced the front bushings (ER Rubber), front swaybar bushings, and 22mm torsion bars. I have Koni yellows front & rear. Car is a '74 with 20mm front and 18rear adjustable sway bars.

So, now that I fixed/modified the front, I'm able to increase the stiffness of the front shocks with much improved ride. I noticed that the steering is stiffer (to a good level) and the car feels great.

However, last night well I was out cruising around, I took one of my favorite U-turn type onramps fairly fast and the rear kicked out - way more than usual. It caught me off guard since I drive this road all the time.

I'm going to be doing the rear suspension next with inner/outer bushings and new torsion bars. I was planning on using 26mm bars but I'm wondering if this is a good match for the 22mm front and will stiffening up the rear bring the oversteer down a bit?

I'm new with the 911 so any advice would be great........Vern

If you have oversteer, and you stiffen the rear, you will have more oversteer. If you have understeer, and stiffen the front, you will have more understeer.

Basically, if you want to reduce oversteer, you need to stiffen the front or soften the rear. If you want to reduce understeer, you need to soften the front or stiffen the rear.

Scott

hmm. Maybe my car kicked out because we had a lot of clouds and humidity yesterday. And, maybe I was going faster than normal due to my new smoother suspension. i can definately say I had a better adrenalan rush than normal.

You've stiffened the front of the car quite a bit with the torsion bar change.

What were you doing with the throttle when you had the rear end step out?

JR

For a 911 you should have more like 29mm rear to match the front 22s.

I was on it but probably let off when it started to slip. I know that you're supposed to ride it out....I didn't brake, just let off the gas and it corrected.

I'm thinking that this year, I will do some DE events.....cuz that was fun.

I agree with Flieger you need to go to minimum 27 rear tbars to balance the front out. 29 is the correct balance but for street too stiff. 22's are too stiff for the street too IMHO 21-27 would be great.

Softer rear/Firmer front to help oversteer (kick out).

EXECstudio.com - Tech: How to modify Oversteer and Understeer

You increased the front stiffness so that should have helped reduced the oversteer. What was the rear setting for the new sway bars? Bolt hole/mount point closest to the bar centerline is firmest. You might want to adjust the rear sway for less firm.

Cliff

You're probably right. I came across the 22mm bars on the classified page an the price was right so I bought them. At the time, I had no idea that my bushings were binding and I originally felt that the stock torsion bars were too weak for the Koni Sports. I had a similar experience on my 944 when I added Konis and the right got too rough and then when I increased the spring rate, the ride smoothed out.

So, I think my ideal setup will be to reduce to 21mm an get 26/27 for the rear. In time, I'll get it right. All I can say now is that the car is 100% better than it was 2 months ago and I can put up with it until I can afford to make some more changes.

For a quick solution, loosen up the rear bar.

Just put 22mm front and 27mm rear Tbars on my '80SC/3.6 widebody (rear track same as Turbo) as well as new Bilstein shocks, and refreshed bushings all around. I also added 22mm frt sway bar and 21mm rear. I am using 7.5"/17 frt and 9"/17 rear wheels. I couldn't be happier with the handling and ride. Very neutral handling, if anything perhaps a bit biased to a little understeer, or as the NASCAR guys call it "a little tight".
I actually prefer the bit of understeer, rather than neutral. After having nine 911s since my first '68 911, I have "hung out the rear end" and in fact spun more times than I care to remember. Some understeer gives a bit of a cushion before you get to the hairy edge. It took me a while to learn to "stand on it" when the rear end starts to go however, rather than the instinctive reaction to let off the gas! It's still quite hard to disipline yourself to do it though!

You mentioned the sway bars are adjustable - how were they set?

I tend to agree with Tobluforu - perhaps soften off the rear sway a notch or two.

On the track, one thing that I learned about oversteer is this:

We all hear about sudden off-throttle oversteer that these cars are known for. The common thinking is that the rear need more tract and that the front needs less in order to balance the car. But is that really it? What if the front doesn't have enough traction?

Here is what:

You are screaming through a nice sweeper and the front end feels a bit light (understeer).
The rear is hooking up really well though and you are modulating your steering with throttle to compensate for the understeer. A little less throttle and you get a bit more front end traction.
SInce it is all about the low front end traction at this point, there is no concern about the back end coming around because it has massive grip compared to the front.

THEN, you have this brainy idea to get off the throttle completely as you think about breaking for the corner. What happens next is text book.

The front, (understeering vigorously up until now) suddenly gets more weight shifted to it, providing new massive amounts of front traction eliminating the understeer. The front tires that up to this point were understeering and pointed slightly more into the corner than the nose of the car, as if now on rails, take the nose of the car in the full direction they were pointing, and the nose darts across the track, thus leaving the tail end (and unsuspecting 911 driver) hung out to dry.

Moral: be careful of too much understeer. It can lead to massive oversteer once the front is able to hook up from changing throttle or road conditions.

wayner,

Actually, you have it backwards. The dynamic is that lifting the throttle causes the rear to lose vertical load and, if the vertical load lost is enough so the lateral load is too much, the slip angle will increase dramatically. Couple that with the slip angle staying the same or decreasing at the front, and you have the recipe for a spin out.

Scott

Sometimes when the rear is too soft the car rolls over and hits the shock bump stops=instant oversteer. Counter intuitive.

Or, he just used too much throttle and kicked the rear out....

JR

Thanks for all the replies.

As I've been reading the posts, I've also been trying to remember how exactly I was driving. I think part of the problem is that I've been so happy with the new bushings that driving the car has become a lot more fun. And so I've been making several trips around town to all my favorite on/off ramps; that's really the only way to have fun in city driving around here. In addition, it's been 100+ for a couple of weeks and then we had some cloud cover and rain the other day - this made for great evening driving as the temperature was perfect for open air driving (windows down in my case). You could tell the driving was good because there was hardly a car going under 80 - everyone out there was enjoying the drive.

So, after an hour or so of spirited driving, I went back for one more trip on my favorite on ramp. This has a straight-away for about 1/4 mile and then a sharp right hand u-turn and onto the freeway. It's a high speed curve so it's a lot of fun to get on it.

All that said, I think I was going too fast into the turn and ended up letting off the gas "before" I broke traction. I'm not positive but that's what I think now. The kick out was not huge, but enough to know that if I'm not careful, I could be going backwards in no time. And, this was the first time the car has done this to me. I'm thinking that with my new bushings, I've been driving much faster than normal just because the car is riding smooth agian.

Regarding the Rear Suspension, I don't know exactly how it is set. The last time I made an adjustment to the Koni Yellows, the adjust button on one side wouldn't work. I couldn't make any change and I can't tell how it is set. I may try again and see if it has fixed iself. it did work before.

Lastly, I'm not an expert on the rear sway bar settings so here are some photos - maybe you can tell me what the setting means. It's a Welmeister 19mm adjustable and the front is 20mm stock, under suspension type.

http://forums.pelicanparts.com/uploa...1342816152.jpg

And, here's my car:
http://forums.pelicanparts.com/uploa...1342816292.jpg

Tidybuoy,

That sounds like too spirited of driving for the street......

Scott

I have the same bar on my 72 and by the looks of it compared to mine, it's too tight. Now, all cars are different so comparing it to mine really means nothing, but try sliding the clamp another inch toward the end of the bar. When i use to setup my car for auto-x I tested it over and over on the same corners right next to my house until I hit the sweet spot.

+1

What tobluforu said.

Cliff