Quick - I need some alignment help please

[dickster]

interesting thread!

interesting cause i've got a similar problem.

my car handles great in left handers (slow and tight or sweepers, it turns lovely) but in right handers it "pushes or understeers".

at the track (which had two left handers!) the problem slowed the car - VERY FRUSTRATING.

i've been chasing this thing for months.

i thought the left side front may have positive camber but it seems to be the same as the right (negative).

i've no track experience, but i've read a book or two to try and find a solution. from that i totally agree with what randy and tyson are saying. try "how to make your car handle" or ba's book.

the problem is that understeer or push is caused by any one of a number of things. mine has had new front torsion bars, but i believe the rears are original. the rear bushes are also shot to hell. softness in the rear will contribute to understeer, which is where (i hope!) i think my problems are.

i just gotta locate the f*cker.....................

[David Ceruti]

Thanks Randy - I still hope to hear from Joe as well.

I was also reacting to his mudslinging as well - whenever anyone starts resorting to their experience to back up a claim I start to wonder where their facts are. Also reminds me of Napoleon and his horse!

Nevertheless Joe did get me thinking and he may have a point that his settings are more suited for everyday driving. I know that I enjoy pusing quite hard through tight stuff because its challenging to do well and I'm going slow enough to compensate for anything that may happen (like all those erratic moving chicanes on the roads). I'm too nervous to tackel fast stuff off the track though which, IMHO, is where the more aggressive camber and castor settings seem to come into their own.

Also, the extra braking of less camber is attractive

This is all an attempt at opening up the debate and learning not to give a definative answer (or even opinion)

[Wil Ferch]

Guys:

Allow me a "centrist" view on all of this...

Yep...let's stop the mudslinging and view the facts...

There is no denying that some negative camber will result in superior cornering abilities. Let's also recognize that the USA 78-83 cars were set up higher to simply comply with US bumper height and headlight height laws...and had VERY little to do with what was "right"...form a vehicle dynamics perspective. therefore, on this point alone...we can't assume that what Porsche engineers specified is without criticism.

Yes...increased castor promotes straight line stability...in excess this can lead to the wheel simply "flopping" over when turned and would create understeer or push. Think of a Harley chopper with an extemely extended front fork...and this can be easily visualized.

That said..it IS interesting that the C2 and later cars use a much lower positive castor setting than the earlier torsion bar cars...something like 2.5 degrees vs 6 degrees as I recall.

So....we al had something to contribute and think about...OK?


---Wil Ferch

[stlrj]

Sorry to keep everyone in suspense, but work and family obligations always seem to get in the way whenever things get interesting.

As it happens, my primary test track for testing my suspension tweeks before I head for the hills are the high speed overpasses and freeway onramps on nearby highway 101 and 37. Only when I'm satified with the high speed performance do I move on to our famous Lucas Valley road and on to Mt Tamalpias. So to answer your question, high speed stability is a major concern, in fact, it feels more composed and stable at all speeds. I may be mistaken, but I do believe that Porsche has done some high speed testing of their US ride height 911s in Zuffenhausen before turning them loose on our shores, not to mention all those Road and Track/ Car and Driver reviews on US ride height 911s, raving on how well they handle.

Something that may have been overlooked is when the factory sets the camber positive is the fact it is not static, it does not stay positive for very long but goes negative the moment you turn the steering wheel 90 degrees off center, so a positive initial setting means little due to high amount of caster designed into the 911 suspension that automatically goes into negative territory without you realizing what just happened. At full 20 degree steering wheel lock, you could end up with as much as 2 degrees or more negative when you initially started with 1/2 degree positive with the wheel pointed straight.

What may be causing all that push that everyone complains about is too much negative camber(great for the autobahn)instead of the wheels going positive which is virtually impossible with 6 degrees of caster but it is very possible to have too much negative.

Basically, my ultimate goal was to optimize the tire contact patch for all speeds and all conditions which I have finally done to my satifaction and just as important, I have been able to prove my theory on my track.

Looking forward to the rain for some real excitement.



Joe

[jluetjen]

Dickster;
It sounds to me like your car is not "square". Since winter's coming, if it were me, I'd put it in the garage and do a complete set-up on it. This should include a complete corner weight, ride height and alignment. For the sake of academic curiosity, It would be interesting to look at your initial settings to see if something jumps out that could be causing the problem. The problem that you describe could be caused by bad corner weights, alignment, shocks, springs or tire pressures to name a few.

As far as Stlrj's suggestions, if it were me I'd try to get my hands on tire pyrometer and take some tire temps after a run up his favorite hill. (I've got one, but I'm on the wrong side of the country.) I've always found that set-ups which provide even temperatures front to back, and across the tread of each tire with a slight (~20-30 degree) increase in temperature on the inside of the tread provide the best handling. Without that data on his set-up, I'll hesitate to comment on it directly. I can only comment on the settings and changes which I've seen or done myself. I also agree that Stlrj's driving style (once again sight unseen) could certainly have an impact on his car's handling and the settings that make car and driver happy.

[David Ceruti]

Will - some nice points made and Joe is your car at the US or European ride heights?

on the point Will made about the setup of the newer cars, it makes me wonder whether the difference in wheel offset has anything to do with it. I know from my limited experience with fwd cars that it was beneficial to have the kingpin axis as close to the centre of the tyre contact patch as possible (which is why most wheels today have the dish on the inside). Now this looks as if the new porsches (C2 onwards?) also seem to have these type of wheels so I'm guessing that the may have changed the front wheel geometry in this direction and this may effect the camber etc.

Excuse the rambling and poor explanations

Good thread though

[bento]

so i took my car in to the shop and had them bring the ride height up by about 3/4 of an inch in the front via the adjustment screws. then i went off to the alignment shop and had them set the toe properly (steering wheel was off center previously) and check the front alignment. the car is allegedly at 1 degree of negative camber at the front. the car does feel better but i still hear the "clunking" sound on a hard right hard turn. i'm running 7.5 inch wide rims in the front of my narrow body car, do i need camber plates to clear them properly? i still don't quite understand why this would happen post installation of new shocks when i had no such problems on my blown out boges.

dickster, you having similar issues?

db

[Leland Pate]

Wow, i'm so glad I did a search on "caster" before I started a new thread!
I didn't even know this thread was going on!


Ok, so here's the scoop. Some of you might remember my "Got beat by a Honda" thread... and some of my "alignment woes" threads.
The situation is as follows: My car was "pushing" horribly at last weekends race (where I was slower than my roomates civic). I had originally thought that the problem stemmed from the difference in the front and rear camber (the rear was mistakenly set to -3.5 deg and the fronts were a mystery) It was visibly obvious that there was allot more neg. camber in the rear than in the front. At the time i thought this was causing the understeer I was getting on the track. Well now i have it set to:

1.6 in the rears
And L 1.5, R 1.6 in the front.

But this hasn't stopped my understeer problem.
While the car was on the alignment rig, I found that the "caster" was sitting at 5.2 degrees of positive caster on both sides.
I wanted to ask you fellas about this... now I've seen some pretty good explainations and suggested settings, but, I have to ask.... how the hell do you move the shock tower back and forth????? Are you just supposed to smack it with a hammer? I couldn't get it to move at all, front to rear...

[stlrj]

Dr Pate,

You may not be able to remedy the problem you have with push unless you could duplicate the alignment settings of the 99 Civic SI: caster 1.66 degrees, camber 0 degrees. I managed to get my caster down to 2.5 degrees after modifying my camber plate and relocating my strut outside the bushing. The stock plate has very limited adjustment.

Just Joe

[Tyson Schmidt]

Optimal settings will depend entirely on how the rest of the car is set up. With tire spec being the most important.

If you have very sticky tires, you will want very aggressive settings. ie: lots of negative camber, positive caster.

More grip from the tires equals more load placed on the sidewalls. More load on the sidewalls makes them roll under more, necessitating more negative camber.

Stickier tires will also cause more body roll, and with it, camber loss. Most cars have at the most a 3/4 ratio of camber compensation built into the suspension. This means that if the car rolls 4 degrees, the camber is only compensated 3 degrees, resulting in 1 degree of camber loss. That is why 1 degree of negative camber is a good setting for a mildly set-up car. Play with the big dogs, and you'll need all the negative camber you can get. But your driving skill has to improve along with the set-up, or the car will feel odd-handling to you until you learn car control, and can push the car hard enough to exploit it's level of grip.

Reducing the caster will quicken the steering because more of the angle will be dedicated to the steering direction and less to the camber change. But in a set-up car, you want the extra negative camber that the extra positive caster gives you much more than the quicker steering response, since stiffer rebound on the shocks and swaybar tuning can get you so much more turn-in that it isn't necessary.

Another note: Lowering the car a great deal will cause poorer initial turn-in due to bump-steer. That is why many people feel better percieved turn-in when they raise the front suspension. Ideally, you would lower the car, and install the dropped outer tie-rod ends and the steering rack spacers to compensate.

All that said, a 911 will always understeer in the slow speed stuff. It's the nature of a rear engined car. It's one of those epiphany moments when you realize you have to slow down to go fast. Some day you will get it Leland, and the Civic will be far behind. Carry too much speed into the tight turns, and you will have to do so much undoing that you will lose seconds in each hairpin. Watch the fast guys in the turnarounds. They are smooooth. They stay in tight, and keep a steady smooth arc, modulating the arc with the throttle and almost no steering. The whole time they are looking way ahead to the corner's exit point.

[CamB]

I get this vague feeling that there are just too many OTHER variables going on....

Tyre pressure, sway bar settings, standard or otherwise torsion bars... blah blah blah.

No a Porsche, but it will do - the BMW - I turned the Koni adjustable front shocks 1 full turn tighter (on rebound) and it understeered like a PIG into tight corners. It is also extremely sensitive to tyre pressures.

Its a package - I'm not sure who said it in the posts above - but you need it all to be set up together.

[David Ceruti]

Cam

The change in your beemer's handling is because the shock's compression setting has a big effect on the transitional cornering of your car - it basically controlls the rate at which the car rolls and takes up the steady state cornering "stance". This will be felt especially in tight turns because everything happens much faster.

My guess is that the stiffer setting overloaded your outside front tyre and the inside tyre was too light to help - result understeer!

If you dont want to back it off (the obvious solution) because it helps on other parts of the track you can try to reduce the roll bar stiffness at the front

Also - and this is just a guess - getting on the power earlier may lighten the front a little and bring out the back a bit and compensate for the front shock setting

Enjoy the playing!

[Randy W]

Tyson, you did a great job of explaining the inter-relationship of all that is potentially adjustable in the 911 suspension. I also agree that the very best drivers will want the car set up so that it is way too twitchy for most people. By cranking up shock compression, dialing in lots of negative camber, and running race tires you are not only asking more of the car, you are also asking more of yourself. A novice will get in the car, complain about it pushing in the slow turns and snap oversteering in the slaloms, when someone like Greg Fordahl will get in the car, run ten seconds faster, and jump out of the car remarking "It works for me." From my own experience having autocrossed seriously for six years, and graduated up the ranks (winning a National Tour and two regional series championships), the way you set up your car will continue to change as you gain more experience in what the 911 will do and what it won't do when driven at the limit. Becoming truly confident in yourself and the car, so that when you are going faster everything you do slows down in the car, has alot to do with it.

[jluetjen]

Here's a couple of other facits of the rubics cube that you need to keep in mind:

1) The roll center of a McPherson strut suspensions will move quite a bit in compression and droop. If the roll center goes below ground level (where all of the forces are coming into play) you will have a lot of handling headaches. If you lower the front end of your car so that the front A-arms are lower at the inside mounting point then the outside, or even enough that this happens when the suspension is compressed, you will have problems. If you really want to burn some brain cells on what this means, check out this thread. Keep in mind this is why the factory raised the spindles of the RSR. This allowed them to lower the front end of the car without putting the front suspension into this severly compromised situation.

2) Cam; I think that you may have done the exact opposite change to what you ineeded to cure the understeer. As a rule if you are going to change shock stiffness in rebound, you want to do it at the opposite end of the car. You might want to try resetting your front shocks to their original settings and then increasing the stiffness of the rear shocks in rebound. This will slow down the transition of the car's weight to the front-outside wheel when turning and increase the transition of the car's weight to the outside rear wheel. This change should decrease push/increase oversteer when transitioning into the corner. As Dave points out, you did the opposite. Give it a shot and let us know what happens.

3) I'm not convinced that "fixing" the castor is going to make a big difference in a car's handling as long as it is even on both sides. If everyone's problem is similar to Lelands, it sounds like the understeer is happening long before enough lock is put into to make caster a factor. I think it's a question of ride height (see above), shocks, T-bars and sway-bars. And most importantly driving style.

4) While in general I agree with what Tyson said, I'm not of the belief that a 911 can't be made to handle well in slow tight corners. I just think its a question of figuring out the suspension settings and driving inputs which are required to make the car operate up to its full potential. That's the fun of working on a race car.

For comparison, my car has 6 inch wheels all around, stock T-bars, 19mm front and rear sway bars, and stock shocks. I've got a hair-pin at the corner of my property which I drive around fairly often. My car has nothing resembling understeer in dry situations. In fact it rolls around the corner fast enough that I can feel it in my neck and the steering is heavily loaded. Here's my technique:

1) Complete most of the braking in a straight line.

2) Initiate the turn-in as I roll off the brake and move my foot to the gas. Think of it as going from 100% brake to 100% turning in equal amounts. So I've added 10% steering when the brakes are at 90%, 20% steering while braking 80%, etc.

3) Since it's a hairpin, I'm taking a late apex. Prior to reaching the apex I'm already on part throttle. I'm giving it just enough throttle to hold my line. More throttle if I need to open my line, less if I need to tighten it up.

4) Once the car is mostly straight and approaching the apex, I start to feed in the thottle. I can usually feel the rear end squat and the rear suspension geometry steering the rear end around. As I take out steering lock, the car straightens out and rockets down the straight. OK, it doesn't rocket since I need to immediately get off the gas again in order to turn into my driveway, but you get the idea.

How does that compare with the way that others are driving hairpins???

[Randy W]

Sounds good, John. Left foot braking makes the 100% total that is split between brake and throttle easier to modulate, but it can be done if you are really good with doing both with your right foot. Also the presence and type of limited slip will influence this ratio somewhat, with a Quaife allowing less entry understeer and easier hook up on exit.

[Leland Pate]

So..... it's not "killing" the car to be running 5.2 degrees of caster???

[jluetjen]

Leland;
According to my Tech Spec book, my car is supposed to have between 6 degrees 45' and 6 degrees 5' of caster. The smaller number was a change that Porsche made starting in '70 to reduce the caster. In order to reach that setting , they had to modify the upper strut mounts slightly on the body. This was done to reduce kick-back in the steering as well as lesson the effort when at full lock.

You have less caster then that and your suspension is essentially the same as mine, with the exception of wider tires and a heavier car. So no -- your not "killing" the car. In reality, unless your running on a oval, caster really doesn't make a huge difference to a car's handling unless you are running at the limits of the cars ability. The most important thing is that it be consistant side to side, otherwise the car will pull.

Randy;
I agree with the left-foot braking making the process simplier. Given that I'm driving on a public road, I generally don't left-foot brake since I many need to stop due to pedestrians or traffic and I don't want to get my feet crossed up while trying to find the clutch etc. On the track, my objective would be to do a clutchless downshift while left foot braking. But even that would depend on my total commitment to the corner, so any sort of traffic would put me back to right foot braking.

[CamB]

I don't really want to muddy the waters with BMW handling traits! My point was that camber and castor aren't the only things affecting understeer.

Thanks for the tips guys. I had tightened the rebound up in an effort to get more rear traction (no LSD ... yet). It worked perfectly at achieving this goal at the expense of understeer. Unfortunately my swaybars aren't adjustable (and neither are my back shocks). I think I am already at max camber too. More money...

I've put the front shocks back to where they were - when stiffer, other than being able to put the power down properly out of the hairpin at the track I was at, there was no benefit. Well, I suppose it felt incredibly reassuring in fast sweepers.

In fact, since it was hard to get the car to turn into the hairpin well, the extra traction probably made my times worse. It is almost impossible to tell because I am learning and it was my first time there, so of course I got faster...

Thanks for the help. I really need to buy and try an LSD. I have to ship one over from the US though, because they are unobtanium here. And I've spent all my money!

Cam

[Randy W]

John, if you can (and want to), try practicing switching between right foot braking (when using the left foot for clutchwork - recommended) and left foot braking (when in a single gear for a corner or a series of corners). Once you get comfortable with this, you can left foot brake when you need to settle the car under throttle and still use the clutch for shifting - no problem, even on public roads. It takes some time to master, but it is faster on the track. This is the way all the rally guys used to drive before sequential shifting was introduced.

[David Ceruti]

Dickster

I had another look at your post today. Another factor is that your weight in the car will keep it more level in left hand turns (assuming it is a LHD car). See if it handles the same with a passenger in the car and it will give you more info.

What to do about it? That will take a bit more thought or suggestions from some of the gurus lurking on the board. Maybe have the alignment checked with you in the car. But I think that the unequal L/F weights is one of the unsolveables in saloon car racing. I know that some guys imported LHD cars to race over here at Kyalami because most corners on a counter-clockwise track favour LHD cars

Regards

DC