Help with 951 Alignment

[BoostBuggy]

Just had the 951 aligned but there was (I hope a minor) an issue. The rear is showing toe-in of .48 degrees and the right is showing -.58 degrees so the rear wheels are basically pointed slightly to the right. What/how does one adjust the rear toe angle? (the alignment place didn't seem to understand the alignment diagrams and I couldn't offer any help)

I appreciate any help/suggestions.

thanks

[krystar]

first, find a different alignment place.

the rear toe adjustment needs a special tool


most likely anyone but a porsche shop won't have that tool. but it's cheap enough to just buy one yourself. if an alignment tech can't figure out how to use the tool on the rear suspension, that's a sign you need to find a different alignment place.

[BoostBuggy]

Thx. Just ordered the tool. I wouldn't be so quick to fault the alignment shop. I saw the same photo he saw and all it shows was the rear spring plate with a yellow arrow pointing to something that was completely unclear. No explanation or tools needs listed. But hey, it's an old car and I don't think this should be unexpected when the alignment guy is barely the same age as the car (maybe less)

For a $50 alignment I don't mind doing a little research and purchasing a $27 tool so they can do it again (at no additional charge)

[krystar]

i had my alignment done at a local NTB that had a good rep with the local import crowd. he pulled it up on his computer and it showed him the spring plate with the hole that the tool goes into.

here's one of the diagrams. maybe more helpful than what he brought up on the computer. this is just in front of the rear axle

[BoostBuggy]

Thx. That diagram will be helpful

[JAXTURBO]

Great info!

[Jfrahm]

That diagram is mislabled. Camber and toe are swapped.

[BoostBuggy]

@Jfrahm - Is that to say the rear camber is adjusted in a manner similar to toe? i.e. there is a tool to adjust camber?

[Jfrahm]

No, but it's all related. If you change the ride height, the toe and camber are affected. When you change the toe you change the camber so you end up playing with both of those until it works out. The P221 tool mainly adjusts the toe and you mainly adjust the camber with an eccentric bolt but you have to play them both against each other, like tuning a guitar with a floating whammy bar.

I made a P221 tool out of a scrap 15mm socket but it's not super easy. I ground it down to have one tooth sticking out (made of material that was once the socket wall) and made sure the tooth had real sharp 90 degree transitions at the base, if it's curved the tool walks out when you try to rotate it. If overheated it'll probably break when used. The tooth has to be pretty small to work also, a cheap socket might be too soft. If I had planned ahead I'd have bought the P221.

[BoostBuggy]

Thx for the response... I know(knew) the two are related in that changing one will affect the other. (BTW I was a guitarist years ago and stayed away from whammy bars specifically for the tuning issue).

I was reviewing the graphic krystar has provided and from the that (having not looked at the car) prompted my question. I have ordered the P221 tool from Pelican (best price!)

With any luck the tool will arrive on schedule (Tues) and I will be able to get the car back to the shop to finish (re-do) the alignment job before the holiday.

[smudo]

Hi, Hope that someone will be able to help on specific question i.e what would be optimal toe and chamber settings for our cars? I have ordered some suspension parts (27 mm TBs, 250 Weltmeister springs, new Powerflex bushings). Next spring I will also put on semi slick tires (most probably 225/50 16'' all around on standard PD wheels). The car will be occasionally used on track, but 95% will be street driven. I manage only around 2 - 3 t miles per year thus the setting might be also little bit more aggressive to the tires if there would be benefits handling wise. Or should I stick with standard settings (can I see them in repair manual?)?

[DannoXYZ]

Settings in the manual are for street use. For track-use with your configuration, the following will be much, much better:

FRONT
3.5º caster (maximum)
1/16" toe-in
-3 to -3.5 camber

REAR
0" toe-in
-2 to -2.5 camber

These are close to the limit of adjustability with stock suspension. With those soft Welt springs, you'll actually get even better grip with -4 to -5 degrees camber in front and -3 to -4 in rear.

[flash968]

yikes. that's way too much camber. i see this all the time, but it's backwards thinking, and results in the car actually being slower. it "feels" better, but it's actually slower.

obviously springs and swaybars would be involved here, but for the purpose of alignment theory we can skip that.

what happens with too much negative camber is you're going to overload the outside tire. it will reduce the amount of contact area on the inside tire, putting more load on the outside tire, causing it to heat up much faster and lose grip. what should be done is to increase contact patch on the INSIDE tire, not the outside tire. brace the lower suspension and upper strut tower points to reduce camber change, and then corner balance the car to limit pitching under braking. if the tires are heating up, then increase tire pressure to firm them up, reduce roll, and increase grip on outside tires.

a little rear toe will also dramatically improve turn in. 3/16" would be good

[speedysprocket]

Originally Posted by flash968

yikes. that's way too much camber. i see this all the time, but it's backwards thinking, and results in the car actually being slower. it "feels" better, but it's actually slower.

what happens with too much negative camber is you're going to overload the outside tire. it will reduce the amount of contact area on the inside tire, putting more load on the outside tire, causing it to heat up much faster and lose grip. what should be done is to increase contact patch on the INSIDE tire, not the outside tire.

Huh? Negative camber moves the contact patch to the inside of the tire when the car is just sitting there, static. How is standing the wheels more upright shifting the patch further to the inside? Is there some crazy 944 geometry that takes over in a turn?
I'm not arguing that -3 to -3.5 is a lot and probably too much, just wondering where your explanation is coming from.

[flash968]

that's not what i was saying. increasing negative camber reduces the contact patch of the inside tire. as the car rolls, less inside tire makes contact. this is exacerbated by increasing negative camber. while the outside tire is making more contact, the net contact patch area (the sum of the inside and outside tires) is actually reduced. further, the outside tire is compressed more due to the increased load, and also heats up more. so, you have to increase pressure to fight that. it can quickly become a spiraling issue. i'm not saying that standing it up will increase the inside contact area, but rather that it won't reduce it as much in a turn. semantics maybe, but the same effect nonetheless.

that's why bracing helps so much on these cars. it's really pretty amazing at how soft the frame structure is, and much of a difference it makes . i'm frankly quite surprised that braces didn't come out much sooner.

let me be clear though that there is a certain amount of negative camber that will help, but too much actually results in less cornering. you can actually draw it out on paper, and i do that with every new car i start messing with.

[speedysprocket]

Maybe we call the inside and outside of the tire something different? To me, inside contact patch would be the portion of the tire touching the ground that is closests to the center of the car. Outside being furtherest from the center of the car. As you increase negative camber, the top of the tire if angle in towards the car, shifting wieght to the inside of the tire...

[flash968]

lol - yeah - i'm talking about the inside tire, not the inside of a tire.

in a turn, you have 2 tires on the inside of the turn, and 2 tires on the outside of the turn. that's what i'm talking about. by increasing negative camber too far you essentially lift the inside tires off the ground, reduce the net contact patch area, increase the load on the outside tires, and ultimately reduce the overall grip.

you really have to draw it out on paper to figure out at what point the negative camber becomes a losing proposition. suspension geometry, ride height, spring rate, damper setting, wheel size, offset, tire size and corner weight all play a part in this. you can't just use somebody else's settings, unless the car is set up exactly the same.

he problem with these cars is that the chassis itself is very soft. that's because it's a unibody car. what makes it so great at being a GT is exactly what hurts it when you try to drive the car hard. consequently you need to start bracing those points that move. the lower suspension mounting points move over 1/4" under anything more than moderate loads. that's a huge geometrical change that can really scrub speed.

[smudo]

Flash and Danno, thanks for the comments. I did not mention that I will instal also 30mm front and 19 mm (LR 5 point adjustable) sway bars. I have also already installed front strut brace (non-adjustible). Does it change anything in the above comments?

Flash, what would be approximate optimum chamber in your opinion for for maximum contact area of inside and outside tires while hard cornering? Also, as I mentioned, the car mainly will be street driven and I will participate some youngtimer events where the lap times are important, but the main purpose is just to have fun and enjoy fast driving on track.

[DannoXYZ]

Originally Posted by smudo

I did not mention that I will instal also 30mm front and 19 mm (LR 5 point adjustable) sway bars. I have also already installed front strut brace (non-adjustible). Does it change anything in the above comments?

That's the same configuration I had when I arrived at those settings; same Welt springs and 968 M030 bars. Those camber angles give the fastest times at Sears Point and Laguna Seca with a similar configuration to your car. It's actually less camber than the SpeedvisionGT cars were running the same year I used those settings. Ken's BMW was running -5 to -6.5 camber depending upon the track:




The problem is MacPherson struts lose camber as it compresses and the car leans. You have to compensate for that camber-loss with an initial negative setting such that the outside tyre is vertical at full-lean. Even then, you still want a little negative because the tyre-squirm pulls under the outside-edge. Here's picture of TurboS vs. 968 M030.



You want to figure out the balance where the increase in outside-grip no longer outpaces the loss of grip from the inside tyre. Lateral-weight transfer under cornering is (CorneringG * COGheight)/TrackWidth. As you make your car corner faster with lowering and wider, stickier tyres, the less and less the inside-tyre contributes to total grip. In many cases, the inside-tyre contributes zero percentage to cornering-grip, so you want to maximize outside-tyre camber for flat contact-patch:




Simple to log everything, suspension settings and pyrometer readings of tyre outside, middle and inside tread and correlate to your track-times at the same track.


And changing camber doesn't affect lateral-weight transfer in any way. It cannot overload the outside-tyre and underload the inside tyre because the weight-transfer is still the same. What you're trying to do is create even tyre-temps across the tread on the outside-tyre to maximize its grip for any given lateral-weight transfer. Again, keeping logs of your suspension settings, recording tyre-pressure, temps across the tread and your performance-numbers is the way to arrive at optimum settings.

You'll find that there's no one optimum setting that will work for both street and track. Of the adjustments, camber makes the most difference and camber-plates are an easy way to switch between street and track settings.

[flash968]

yup - that's exactly why i was saying the towers, and most importantly the lower points, need bracing. while the strut tower tops move, they don't move nearly as much as the lower points, which are generally ignored. braced properly, they stop compressing and the geometry holds. we have found that we can reduce the amount of camber previously needed by more than half. it can come down even more in a car with a space tube frame cage.

smudo - as i said, it really depends on the car and its setup. it's not "cut and paste". even something as simple as a shock change can affect the way the geometry works under compression. to determine what alignment settings to apply to achieve maximum cornering, i would need to set the ride height, corner balance the car, and then measure the car under compression and extension. i've gotten out of the business of doing that a long time ago, but when we did, it cost about $2000 and took 2 days. while we often did it at the track, it wasn't lap times that we focused on, but rather entry and exit speeds, as well as physical measurements of the suspension during driving. lap times can easily be affected by way too many things for that information to be truly useful. we had to do it manually with expensive tools that you had to read often. nowadays you can keep driving, log that stuff on a computer, and do it a lot faster. the tools are still expensive though.