Dialing out understeer?

[Hotwatermusic]

What's the most prescribed method of reducing the amount of understeer on a 911? I have a '74 running 205/50's front and 225/45's rear on 16" Fuchs, Carrera front struts, stock front and rear sway bars, and stock torsion bars sizes. It's been lowered from U.S. height by about an inch as well. I find that the car just plows way too much during moderate to hard cornering. I would prefer a far more balanced, if not oversteer-leaning, ride. Where should I start?

[wwest]

The nature of the "beast", live with plowing or die from QUICK/Sudden over-stear.

"Anchor" the front and the rear wants to come about, pivot around, even moreso.

[Flieger]

Stiffer rear anti-sway bar.

Start learning to trail-brake and use trailing throttle oversteer to tuck the nose in.

[hesaputz]

So right; get the front planted and turning, on then off the throttle to rotate, back on as smooth and strong as you can so's not to plow. Real stiff in the rear, pliable in the front.

[Wil Ferch]

Much too much to say here......

A couple of thoughts, nonetheless.

....if the understeer is "low speed" understeer...meaning the steering input angle is cranked-in rather large, then there is only so much you can do. "Technique" helps here as to when and how hard you apply throttle during such circumstances. Applying throttle to quick and too soon usually promotes grinfing understeer.

If, however, you are focused on mechanical changes.... in general, stiffening up the rear roll stiffness or softening up the front helps. Also...widening the front track or narrowing the rear track helps. All-in-all....whatever mechanical modifications you do....be careful of what you then dialed-in as to the car's behavior at high-speed turns.....where for the same amount of cornering "G"'s....you have cranked-in much less steering wheel angle, and the front tires are running much less deviation from "straight-ahead" angles than with low speed corners of the same high-G's.

My point?---> if you dial out understeer at low speed ( high-steering angle) situations.....be prepared for high-speed ( low-steering angle) situations to get you more neutral or even to pucker-factor inspiring oversteer! The best compromise is to alter technique for low speed, autocross-type work because dialing out understeer for that kind of work can backfire on you for high speed turns where you inadvertantly dialed-in some oversteer.

[burgermeister]

What Flieger said.

Tires & alignment can also make a difference - if the tires are sensitive to too much positive camber, and you have your front & rear camber maxed out, you'll end up with less positive camber in the back during cornering, which promotes understeer. Switching to a tire that doesn't care about the extra degree in front will even it out a bit more.

I ran my stock 88 (-0.8 / -1.8 camber) with Bridgestone RE760's, and it was reasonable on track. Switching it to RE11's made it plow like crazy until I wore the outside edge off (I had an instructor drive the car to verify it wasn't just me - his comment was "this car does NOT want to turn"), at which point it was OK. Aftermarket sway bars cured it completely.

Wil makes a good point too - careful dialing too much oversteer in! Though I think the track change reccommendation is backwards?

[RSTarga]

1) Do you have the same age, brand and type of tire front and rear?
2) Are your tire pressures 29 front 34 rear?
3) what carrera shocks do you have front and rear?
4) have you had the car aligned and corner balanced? Camber and toe in can affect under/oversteer quite a bit.
Basic rule is the softer and lower you go the more it sticks. That applies to tire pressure , roll bars Tbars/ shocks.
Do you have a big rear wing with no front spoiler?
There are so many variables we need more info.

[911st]

All has mostly been said but here is my two cents:

First it seems we are very front camber challenged on a 911.

Thus, more neg camber first. Change front strut tops, remove shock hats and go for as much as you can (apx -3 deg). Better yet, have the struts modified for more camber and the axel raised as much as your rim size will allow. This will improve camber gain.

Less shock rebound up front. Most front 911 shocks have to much rebound. This has the effect of stiffening the front on turn in.

Stiffer rear or softer front spring rates. A stiffer rear sway is the quickest way.

Wider front rims. Stiffens the side walls and better supports the tires.

Wider front tires, but only if you can get enough neg camber.

If for AutoX can run toe out.

----

The cheapest thing to do is lower the front and get the front tire air pressure right. This has the effect of softening the fronts effective spring rate, increasing neg camber, and helps it to bite a little better.

[stlrj]

Originally Posted by Hotwatermusic

It's been lowered from U.S. height by about an inch as well. I find that the car just plows way too much during moderate to hard cornering. I would prefer a far more balanced, if not oversteer-leaning, ride. Where should I start?

I also have a 74 911 that had an understeer problem for many years which has finally been corrected.

The first thing you have to do is to stop listening to all the advice that you have been getting up to now and do just the opposite of what is being suggested or you will never get it sorted out.

Second, you need to raise your front end back up to U.S. height in order to get your camber back to spec. Then have it aligned to bring the toe back to where it should be.

As soon as you take it on your first test drive with the alignment settings it was designed to have you will understand the reason for factory specs.



Cheers,

Joe

[wax105]

Hotwatermusic,
It depends on your ultimate objective for the car?!? I think all the advise given has value, just depends on the application. If your objective is to have a nice daily driver listen to "stlrj" or if you're getting serious at the track and want to evolve in this direction, all the other suggestions should be studied.
By lowering you changed the geometry and other changes need to be made to correct it! But in the end they will make the car faster...

My 2cents

[Flieger]

Lowering the front decreases the center of mass height, which decreases the total front load transfer. It also makes the suspension softer in roll stiffness. The lowered ride height also increases the negative (good) camber.

These effects combine to make the front tires more evenly loaded in a turn and so the front has more grip.

In turn, the rear will be doing more work. This means more oversteer.

You can accomplish a similar re-distribution of roll stiffness by adding/steiffening the rear anti-sway bar.

What strlj has described will not make the car faster for track type driving. The factory specs little camber and more toe-in. Race cars cannot get enough negative camber, generally run lots of caster, and run zero to positive toe-out.

[winders]

Originally Posted by Flieger

Race cars cannot get enough negative camber...

This is true for race cars running radial tires. Not so true for race cars running bias-ply slicks.

Scott

[Flieger]

Well, I will defer to you on that. But I think they may be near the lower limit even for bias-ply if you still have stock camber plates and ball joints without any slotted holes or such.

[Joe Bob]

Matched tires, alignment, corner balance, adjustable sway bar and shocks....

[DanielDudley]

You do not steer a Porsche. You drive it.

Take it to someone who knows Porsches, and ask them to give you an opinion. BTW, LSD will make this more pronounced.

[DanielDudley]

Originally Posted by 911st

All has mostly been said but here is my two cents:

First it seems we are very front camber challenged on a 911.

Thus, more neg camber first. Change front strut tops, remove shock hats and go for as much as you can (apx -3 deg). Better yet, have the struts modified for more camber and the axel raised as much as your rim size will allow. This will improve camber gain.

Less shock rebound up front. Most front 911 shocks have to much rebound. This has the effect of stiffening the front on turn in.

Stiffer rear or softer front spring rates. A stiffer rear sway is the quickest way.

Wider front rims. Stiffens the side walls and better supports the tires.

Wider front tires, but only if you can get enough neg camber.

If for AutoX can run toe out.

----

The cheapest thing to do is lower the front and get the front tire air pressure right. This has the effect of softening the fronts effective spring rate, increasing neg camber, and helps it to bite a little better.

This is a great post. BTW, how old are your tires ?

[stlrj]

If more negative camber is the solution, then why do 911s continue to understeer? I have owned too many un 911s that do not understeer that also do not have negative camber.

Automotive engineers always dial in a bit of understeer by simply dialing in a little negative camber.


So, why is it so difficult to look outside the box?

[RSTarga]

No wonder some lose their way on chassis setup!
First start from stock baseline ( european not US standard)
Check condition of all components including tires.
Proceed to test drive, then adjust one thing only, test drive again, adjust one thing, etc. etc. etc.
I see some very confusing advice above, we still don't know anything really about his car other than he says it understeers.

My suggestion: Go to bookstore and buy Carrol Smiths book on chassis setup. I forget the title "engineer to win" maybe? This is truly a very good reference book that should be in any enthusiasts library. It's old but still valid.

[911st]

Originally Posted by stlrj

If more negative camber is the solution, then why do 911s continue to understeer? I have owned too many un 911s that do not understeer that also do not have negative camber.

Automotive engineers always dial in a bit of understeer by simply dialing in a little negative camber.


So, why is it so difficult to look outside the box?

I think a stock suspention 911 sways about 5-6 deg's up front. It has maybe 1 deg of camber gain from suspention compression and the caster effect, and about the most neg camber we can dial in is about -1,5. Thus, we can get to about -2.5 effective camber when we need about -52. to -3.5 of camber.

This puts our tires at a less than ideal angle to the track and we could use about another 3 to 3 deg's of camber to let the tires work as they were designed.

This is significant. Run your stock car at the track for a day with as much caster and neg camber as you can dial in and at the end of the day the outside of your front tires will show much more ware than the rest.




911's may seem to understeer at slow speeds like on an autoX course but believe me, they oversteer big time at the limit.

Thus, Porsche has set them up to transfer as much traction to the rear as possible.

How did they do this?

By making the front stiffer than the rear.

Thus, we have about a 40f/60r weight distribution with about a 55f/45r% effective spring rate distribution.

Putting this another way, we have 50% less weight on the front wheels with about a 20% higher spring rate.

Again, on top of this there are things like the shock rates and suspension design that further exaggerate this stiffening of the front.



I agree with the points made about learning to drive a 911.

The track is much different than is AutoX. On the track you can very much steer the car with the throttle. If you get good at trail braking, the front will turn as much as you could ever want it to.

With AutoX it dose seem like it is difficult to get a car to turn. This is usually because one is over cooking the corner more than anything. However, most AutoX racers do stiffen the rear to get the front to bit and help the car rotate. They also run a big amount of static camber up front.

JMO

[Hotwatermusic]

Originally Posted by RSTarga

1) Do you have the same age, brand and type of tire front and rear?
2) Are your tire pressures 29 front 34 rear?
3) what carrera shocks do you have front and rear?
4) have you had the car aligned and corner balanced? Camber and toe in can affect under/oversteer quite a bit.
Basic rule is the softer and lower you go the more it sticks. That applies to tire pressure , roll bars Tbars/ shocks.
Do you have a big rear wing with no front spoiler?
There are so many variables we need more info.

I am running Yokohama tires front and rear. They're probably at 5/32", wear-wise. I have Bilstein shocks front and rear. Stock TB's, stock sway bars. No rear wing, but I do have a front chin spoiler. I have a strut tower brace as well. The car has been recently lightened: spare tire, compressor, tools, interior carpet, back seats, rear deck sound pad removed; muffler-swap dropped 15lbs out back, Recaro front seats. Tire pressure is 29 and 35, give or take 1psi. It is a Targa, in case there are theories about that causing understeer as well.