3.2 Carrera and SC handling - Page 3

911st · 10-25-2010, 06:53 AM

Here is what is represented as a factory poster of a new 89 911 (note euro bumpers).

Looks like they may have raised the ROW up to match the US.

stlrj · 07:12 AM

Quote:

Originally Posted by 911st

Looks like they may have raised the ROW up to match the US.

Nope. Only 9mm ( less than half an inch ) separates US height from Euro per Spec Book.

US height: (a-b)=99mm.

Euro height: (a-b)=108mm.

Cheers,

Joe

Zeke · 10-25-2010, 07:29 AM

Quote:

Originally Posted by 911st

I agree. Dose not seem right to me either.

It might be the numbers I used would achieve the lowest race setting one might run at.

However, for euro look we do lower the front more than the rear to achieve the 1 deg of rake that has been adopted as part of the look.

I had 25" tires on my 85 and with the front fender at 25.25" my a arms were significantly slanted.

When the cars were set at the same height, did they raise the ROW or did the US soften there requirements?

Seems like tire size will have a lot to do with this discussion. In the 70's and 80's we know we did not have the aspect ratios we see commonly today on our cars. I think the measurements taken at the a-arms should be preferred over wheel arch height.

To me, it is possible that some USA cars have been lowered as much as 3" if both smaller diameter tires were installed and the suspension was lowered to the minimum height as recommended by the spec book.

A long time ago on this forum, this a-arm issue was discussed at length and people like Steve@Rennsport told us that a-arms must stay level at the lowest. Use raised spindle struts if you need more. Your shocks will love you.

911st · 08:55 AM

Why do we lower the front of a 911?

Two main reasions. For the look and or to improve performance.

If for the look, we just do not want to set ourselves up for any problems.

The first issue is bottoming out. With stock suspention and wheel size (aspect ratio changed over the years but the height seemed to stay close to 25") if we go to low with the stock spring rates we might bottom out. Most have found there is not much problem if we say around the 25.5". Rasing the axel on the strut as well as shortining the strut can let one run lower without risk of bottoming out. So can stronger springs.

The other issue is potental for bump (toe) steer. I think burgermeister and one other person mesured for this. I belive if you install the inexpensive rack spacers and this effect is minimized. The A arm angle does not seem to matter much as the tie rods follow to match the angle change to a large degree. At 25.25" on my car I did not have any issues that I was aware of.

If lowering it to improve performance, our goal is to achive a lower center of gravety relitive to the tire contact patch and to get the neg camber so the wheels will be at there best angle to the track. On an unmodified 911 we will never be able to get as much camber as we need.

If the rules do not alow anything more than adjustment we get the most neg camber if in a turn the A arm can can approach a right angle to the strut and when the caster is set as its max.

This is why I suspect introducing more rake into the 911 seems to increase front bite. It gets the A arm closer to a 90 deg angle to the strut pushing the wheel out further for more neg camber. It also has the effect of lowering the front's effective spring rate as side loads push against the a arms. Softer front spring rates also make for more front stick.

With a lot of caster we get more neg camber as we turn the wheel.

On my car I could almost get to about -2 deg static and picked up about another half deg from the caster effect. However, with the car lowered this put me at a point in the camber curve that was very flat with little addation camber gain in a corner.

How can we get more effective neg camber?

One way is rasing the spindle on the strut. The more the A arm angles down toward the wheel, the more neg camber we will gain with compression as it pushes the wheel out faster with compression.

For a street car this also lets us run less static camber for better tire ware and still achive a higher level of effective camber in a corner.

Back to bump steer.

On the track when my car was fighting for grip I did experance some action at the steering wheel. Some of that might have been bump steer. However, as the amount of effort required to hold a turn was signifficant, I belive most of this was the caster effect against the varing front wheel traction. Some toe steer is ok as it ballances against the rear. If we over correct for toe steer it is reported to make the car oversteer or turn more that the steering wheel angle would suggest.

911st · 10-25-2010, 09:03 AM

Looked up 185/70/15's which was on the early 911's and they should be 25.2". A 206/55/16 should be 24.9"

Wil Ferch · 10-25-2010, 09:11 AM

Let's not fool ourselves that 24.9" vs 25.2" is a meaningful difference. It's not. Probably can affect diameter more by altering tire pressures for one of these sizes.

Strlj also says--" It is not so much a caster issue when it comes to heavy steering feel "....
I have to vehemently disagree here too. Caster angle has a profound affect on "heaviness" of steering, and high caster angle makes for heavy steering. No doubt camber plays too....as positive camber ( or neutral) will cause less steering effort than negative camber....but all-in-all, not like an extra degree or 2 of caster.

Milt says... "A long time ago on this forum, this a-arm issue was discussed at length and people like Steve@Rennsport told us that a-arms must stay level at the lowest..."

Ahh...isn't that what I've been saying here a number of times too? Correct. Camber curve gets so mixed up at "lower than flat" levels that the camber curve becomes ( I believe) an "S" shape...truly evil.

Wil Ferch · 10-25-2010, 09:22 AM

Milt:
You ask.."When the cars were set at the same height, did they raise the ROW or did the US soften there requirements?.."

Judging from the tech/spec books data I published here....notice that all markets were the same in 1974 ( 108 mm)....then things started getting goofy with different numbers for the later years. By later 1983 and certainly by 1985...all cars we set to Euro standards again ( 108 mm).

Zeke · 10-25-2010, 09:53 AM

Quote:

Originally Posted by Wil Ferch

Milt:
You ask.."When the cars were set at the same height, did they raise the ROW or did the US soften there requirements?.."

Judging from the tech/spec books data I published here....notice that all markets were the same in 1974 ( 108 mm)....then things started getting goofy with different numbers for the later years. By later 1983 and certainly by 1985...all cars we set to Euro standards again ( 108 mm).

That wasn't me that said that and yes, you have stated what you said you did many times. I did not even try to list all of the suspension gurus simply noting Steve as many here consider his word to be gospel.

Sure, there are many others such as yourself that fully understand the nuances of 911 suspension. I regret that I didn't mention than more succinctly.

Wil Ferch · 10-25-2010, 11:21 AM

Jees, terribly sorry Milt.....wasn't that you in post #43 saying... " A long time ago on this forum, this a-arm issue was discussed at length and people like Steve@Rennsport told us that a-arms must stay level at the lowest. Use raised spindle struts if you need more. Your shocks will love you...."

...wasn't trying to be a nanny...just trying to relate answers to previous people's posts... it's not a big deal anyway. Sorry if this point got in the way of meaningful dialogue. Back to scheduled programming..

911st · 02:40 PM

I learn from these exchanges and appreciate all inputs.

In my opinion, until the a arm goes past a right angle to the strut we are gaining camber. How far that would be I do not know. I did a lot of work to figure how high to set my car so that would not happen.

With a budget and a class that allows it, working the struts to raise the spindle, decamber the axle, and correct for the problems that could create for bump steer issue this is probably a much better idea. Especially for a street car where we want near zero camber at rest and need something around 5 deg of static plus suspension camber gain in a corner to max tire grip.

However, if not the lower we set our front the more static neg camber we can dial in. The lower it can go without the strut going past a line between the center of the torsion bars and the CL of the ball joint, the more gross neg camber we achieve. The more caster we have the more neg camber we can achieve.

Still, on the track it will be no where near enough to keep from wearing out the outside of the front tires on a stock 86 suspension (thicker sways and rear torsion bar).

Even if we do go past the most flat part of the camber curve, it stays very flat past that for a while.

Again, not the best way to do it but for other reasons.

There is a reasion euro look height is so well accepted. It improves handling, makes the car work well and with the rack spacers dose not add any bad habits with stock spring rates. With stiffer springs (not sways) we can even go a little bit further.

A bigger issue seems to come about if one puts to much bump steer correction in. It seems about 15mm gets close to not over-correcting for bump steer on stock struts. Some toe steer seems to be desirable as it makes for a little gain in under-steer.

I am not a suspension expert but I do love learning about these cars.

Wil Ferch · 10-25-2010, 06:23 PM

911st.... I just love it whenever your stream-of-consciousness flood gates open up ... I never know where you're going but the journey is enjoyable

911st · 10-25-2010, 08:02 PM

Thank you.

911st · 10-25-2010, 08:28 PM

For what it is worth I have some camber curve graphs for a 911 done by a racer and pulled them.

From stock height over about 30mm of travel the camber gain is almost .5 deg

Assuming the car is lowered about 30mm the camber gain over the next 30 mm on a stock suspension is almost flat. 30mm more of travel and there could actually be .25 of camber loss.

However, if the hub is raised 35mm, the camber gain over 30mm of travel at most points of suspension travel is close to .5 deg.

Basically we just do not have a very aggressive camber curve. Lifting the hub gets us back to near stock on a lowered car but it can not make up for the sway we see on the track.

Or we can just go to something like the Camber King strut top and add another .5 deg of static camber. and not get to crazy lowering the car.

For reference, full race 911's with very stiff suspension run about -3 or more neg camber up front.

stlrj · 09:07 PM

Quote:

Originally Posted by Wil Ferch

s.
Strlj also says--" It is not so much a caster issue when it comes to heavy steering feel "....
I have to vehemently disagree here too. Caster angle has a profound affect on "heaviness" of steering, and high caster angle makes for heavy steering. No doubt camber plays too....as positive camber ( or neutral) will cause less steering effort than negative camber....but all-in-all, not like an extra degree or 2 of caster.

I agree that caster contributes to heavy steering effort, but from my experience negative camber seems to have a more profound effect. Regardless of how you look at it, reducing both caster and camber will reduce steering effort resulting in a more enjoyable driving experience.

If we stand back and look at it from a different angle, we all know how to make our 911s steer heavy, understeer and have our front brakes lock up with little effort...just crank up the caster and dial in all the negative camber we can get to minimize our tire contact patch...

Cheers,

Joe aka stlrj

Flieger · Flieger's Garage

Caster makes the steering heavy in two ways: 1. It causes camber change in a turn as you turn the wheel (both wheels lean into the turn). This means that you are tilting the car and this will be the dominant effect at low speeds. At high speeds, the lean is going with the inertial reaction forces so it should not be felt as much. 2. Caster causes mechanical trail, like scrub radius in a longitudinal direction. The mechanical caster adds to the pneumatic trail present in all tires due to deformation (more apparent with low tire pressures). This effect will be apparent at low speeds and high speeds. The pneumatic trail self-centering force decreases with increasing tire velocity but mechanical trail stays somewhat constant.

Cars with high caster values and lots of mechanical trail will feel odd at the limit of front tire adhesion because the feedback will be pretty constant and the front end will wash out without much warning. Cars with low caster values and little mechanical trail will loose more and more steering force/feel as the limit of front tire adhesion is approached because most of the trail is pneumatic.

So, I would say that caster has more effect than camber alone, even the two are linked in that caster causes camber.

High caster values will not cause the understeer and brake lock-up you describe. That is the beauty of caster. If you brake in a straight line, the tire will not have large amounts of static camber. It only comes in as you turn the wheel. The tire contact patch is nice and large and flat for both braking and turning events, though one must be careful to trail off the brakes at a high percentage of the rate of increase of slip angle.

prebordao · 10-26-2010, 01:11 AM

Getting back to the heavy steering issue...
I thought that my height was stock Euro, but it turned out it is slightly lower.

At the front, I measured the distance between wheel center and torsion bar center and came up with 125mm. According with "good" book it should be 108, so I have a 17mm difference (about 3/4").

Without any spacers inserted below the steering rack, is this difference enough for being felt as a heavier steering in corners ?

I like the car as it is, both in looks and in handling, it's just the steering that I would like to improve. If I insert the spacers below the steering rack, should they have the same height as the difference that I found (17mm) ??

Thanks

911st · 10-26-2010, 05:55 AM

No they are not that tall. Somthing like 8mm but I am not sure.

I would just get the height where you want it and see how she feels. It was designed to work at 108 so there should not be any noticable problem.

stlrj · 10-26-2010, 06:14 AM

Quote:

Originally Posted by Flieger

High caster values will not cause the understeer

High caster values will contribute to understeer by reducing tire contact patch with additional negative camber at tight slow corners where the steering wheel is turned to it's maximum. At the same time steering effort is increased as the suspension struggles to bring the negative camber back to a more neutral position.

911st · 10-26-2010, 07:16 AM

That is true if one exceeds there ideal neg camber value. However, on a stock 911 we are so neg camber challenged up front we just can not get enough.

With tire deflection, springs, and bushings we see about 5-6 deg of lean. If we started close to stock height we could get about 1.5deg of static camber, about .5 from built in camber curve gain, and about .5 from the caster effect for a total at the wheel of at best around -3.5 deg which is short of what we need.

Most of us have been to a Porsche autoX event and may have noticed how much static neg camber they have dialed into the front of the dedicated race cars. It is usually pretty extreme.

With a purpose built race car that has its neg camber dialed in yes, a lot of caster can effect handling in different radius corners. I believe many purpose built race cars run about half the caster we do.

Wil Ferch · 10-26-2010, 07:47 AM

911st... I'd like to see your camber-curve graphs and I think the camber-change-with-suspension movement takes on different characterisitcs depending upon the amount of suspension movement UP or DOWN from static ride height...what the original static ride is....and if you have stock or raised spindles. PM me and we can identify email addresses that can be used if the information is transmitable that way. TIA.