How can I fix my UNDERSTEER at Auto-X events!

[billy911]

Went for the first time this weekend and had major understeer on my 78SC targa(all stock). Ran 40PSI front and rear on my Comp T/A ZR's, will a autocross specific tire help alot? BTW, it's lowered to Euro spec.

[Colby]

According to specs, you should run less tire pressure in the front (29 front, 36 rear). By raising front tire pressure, you should decrease understeer. So you raised it as much as you can. Next step would be to lower rear tire pressure. Are you able to change sway bars, torsion bars, etc? That would be the easiest way, but may throw you out of your class.

Colby

[makaio]

When I run my tires at the factory settings of: 29 front & 34 rear, my car has just a little understeer. If I bump the fronts up to 32, it balances it perfectly.

I would tinker with tire pressure some, before you start spending $ on new tires, shocks, and sway bars. Unless, your class permits you to, or you want to take your car to the next level.

[emcon5]

In a catolog, published by one of Wayne's competitors is a great chart that lists how handling is changed, and by what. It is more a quick and dirty guide than detailed instructions, but it is helpfull. Here is a digital pic of the chart.



The quality isn't great, but it is readable.

Tom

[Chuck Moreland]

Given that it was your first ax, I would suggest it was driving technique and likely nothing wrong with the setup. It's easy to brake too hard and late (or make some other error) and cause understeer.

Assuming that nothing is wrong with your car (eg. broken sway bar mounts, etc) I would suggest you attend several more ax events and get instructors to ride with you before concluding the car needs some work.

It's 80 percent technique and 20 percent car.

[Jack Olsen]

And 40 psi (cold) tire pressure sounds awfully high.

[Randy W]

I agree with Chuck. 911s understeer in tight autocross corners because they are very light in front, and if not enough weight transfer to the front takes place while you are making your turn, you will get push.

Slowing down enough for the corner and using a trail brake technique should remedy this common problem. Try braking smoothly and carrying the last of your braking deep into the corner while turning in, then start to feed in the gas as soon as you feel the car beginning to rotate. There is a maximum speed that you can carry through a tight corner depending on your cars tire patch. Exceeding this speed will cause understeer, even if you are trail braking. In autocross, sometimes you have to go slow to go fast.

[billy911]

Thanks a bunch guys, that chart is pretty interesting. I'm starting to think it was mostly my technique, I was braking very hard and pushing the limits of the tires through the corner, causing a lot of push, guess I need to smooth it out a bit.

[billy911]

Randy, I notice you said to trail brake into the corner, which is kinda what I was thinking at the track. Seems it would keep the nose down and some weight on the front helping turn-in. However my instructor said not to trail brake a 911, but to brake before turn in or I'll end up going backwards. So I was misled from the start by a guy who's aparently not familiar with our cars. Oh well, I've got a lot to work on next time.

[andyu911]

I think before using the trail braking technique, brake early and acelerate out of the corner is the first step. Then you can try to modulate the throttle to get the tail out through corners. It's fun and that's 911.
Andy
'87 Carrera

[masraum]

Billy, the instructor wasn't nessecarily wrong. Trail braking with a 911 is more touchy than in most cars because of the rear weight bias. If it was your first track (autoX or otherwise) experience he may have been thinking it would be best for you to get the basics down before throwing "advanced" techniques into the mix.

A newbie (which includes me) will not be as adept at catching situations (severe oversteer due to trail braking) when they get out of hand so it might be better to try to avoid those situations until the car control is a little better. I'm not saying don't try it, I do, I'm just thinking that might have been his train of thought.

[cstreit]

Trail braking is a technique best left to those with quite a bit of experience, especially in the 911... I've given it a go a few times and found that if I'm trail braking, it's because I entered a corner too fast, and need that extra front weight to get the car to turn in... Better to enter the corner a little slower and get a smooth clean exit. Slow in, fast out!

[jluetjen]

Expert or not, Autocrosses are the perfect place to explore and practice techniques like trail braking. Killing a couple of cones is a far easier problem to deal with as opposed to going off of a track at high speed or worse hitting a couple of pedestrians.

Regarding trail braking causing 911's to spin I'd argue "Not So!". If you are clumsy and go completely over the top, sure it will happen. If you ease into it gently and find that the rear is starting to get light, you can often accomodate by just opening your line and backing off of the brakes by a little bit until the rear end settles and the car scrubs a little bit of speed. You'll most likely be taking a slightly later apex to finish the recovery. It costs a couple of fractions of a second, but it sure beats going off course.

Or you can try what the "professional" 911 drivers did which is to pick up the throttle and balancing the rear end using the throttle. You'll find that by getting off the brakes and "gently" picking up the throttle that the weight will transfer from the front to the rear and you'll pick up traction in the back and lose it in the front. Your oversteer will disappear. Of course if you just give it a "bootfull" of throttle, the rear tires will never have a chance to have the weight transfer on them before the HP kicks in and you will spin them at which point you will spin.

[Randy W]

John has done a great job of explaining the details of trail braking and balancing the car on the throttle. Many instructors (including the Bondurant school I went to) tell you not to trail bake in a 911. I think this is safe advice for someone learning to drive the 911 on a track. In autocross, you want to try everything. When you do, you will find that by "braking" the rules you will discover techniques that can actually improve your times when applied correctly. The turns come up so fast in autocross, these techniques need to become intuitive. IMO, the best way for this to happen is to practice everything that has to do with car control, from the beginning, as often as possible.

[911nut]

Dumb question....What is "trail braking"?

[avendlerdp]

Maybe a tad of negative camber in the front will loosen up the rear and get the car rotating better. Alignment is the cheapest and most important part of your suspension set up. You can dial it in to your driving style. You do have to develop you driving tech. first though. ALex

[LeeH]

As an elderly autocrosser once told me after a run... "Son, if you'd just slow down you'd be a whole lot faster."

He was, of course, correct.

[pwd72s]

If it was a really tight and slow speed course? I used to remove a front sway bar link...thus giving me no bar in front. It helped reduce the understeer. Really tight courses tho..the never out of 2nd gear ones...

[Tony'z911]

I went to my 1st autoX of the year last weekend and since they were calling for rain I didn't bother to take my Kumoh's w/ me. It didn't end up raining until later in the day so I was able to drive on dry pavement and I've never experienced sooooo much push in my car! I let a friend co-drive the car w/ his own # and we both felt that something seemed wrong w/ the car. I have adjustable swaybars and they are set up to give me neutral handling on the track w/ 225/50/15's at all 4 corners. I guess with street tires size 205-front, 245-rear I had way too much rear grip and not enough front grip. I did not want to adjust my rear tire pressure since the rear tires were showing wear right up to the edge of the tire, and the front pressure seemed right also.

Do you think adjusting my front swaybar or rear swaybar would have been better to do? I know I would lengthen the front and/or shorten the rear sway, but which one would have made the larger impact to the car's handling?


pwd72s- When do you ever get out of 2nd in an AutoX? I only have a couple times and that was when they held an autoX on a track so there was some run off area. It was still dangerous though since you were hitting a slalom at 60-70 mph down one of the straight where there wasn't much runoff and a couple cars (with experienced drivers) ended up getting sideways and shooting off track. Luckily no one was hurt, but 3 cars had significant damage and this was only an autoX which is supposed to be car friendly!

-T

[J P Stein]

My experience in AX is limited (but growing) and my car is NARP
but..........

I have found that, for me, the best AX set up is one that gets the car to rotate. These are fairly low speed maneuvers and the price of over-rotating is just a few dead cones and some embarassment. I think the NASCAR boys call it "loose" and that is an apt description. Get the back end around, the nose pointed approximately where you want to go and nail the throttle.

An AX "sweeper" is maybe a 75-100 foot radius corner. Loose allows one to throttle steer around.

It does make the higher speed slalom sections more "interesting" and I have to learn to deal with that.

Since most AXs are held on traction limited surfaces that are less than smooth (OK, rough as a cob). Fairly soft suspension seems to work better. Tires with good recovery characteristics are a godsend.The correlation between this type of set up and one for a race track is limited, me thinks.

I have watched guys that are really fast and they look very smooth.....but their cars are seldom pointed in the direction they are actually traveling......marvelous.

There's a difference between sliding and drifting, IMO. A drift puts the car at the raw edge of traction while maintaining the desired direction of travel. A slide is something that needs to be corrected before proceeding and is a time killer.
My slides out number my drifts by a wide margin. I get it right just offen enough to keep me coming back.

I may be going in the wrong direction with my thinking, but I figure it's all part of the learning process. I'd love to have someone that's really good drive my car as I ride along. Prolly need a barf bag, but it would be worth it.
Maybe when I get up to Bremerton.........