H Dog

Can anyone enlighten me as to the factory suspension alignment settings for toe, caster, and camber both front and rear (no caster at rear 'course)?

FWIW, here they are for the 968 Turbo S (deg:min):
F Camber -0:45
Caster +3:15
Toe +0:05
R Camber -1:05
Toe +0:10

keith

Interesting.

On my 86 951 (with 89 front offset), I'm going to run the following this time around (as a street/autoX compromise):

Front camber - neg 1.50deg.
Front Toe In - 1/32" to 1/16" total
Front Caster - as much as I can get, evenly

Rear Camber - neg 1.25deg.
Rear Toe - as close to 0 as I can get it!

Luke

HI Keith,,

Im kinda stupid about this so i was hoping you could explain to me the benifits of having more neg. camber in the front than the back ( on an autox course)?

Danno

Several reasons for camber differences. Weight transfer and cornering is one and the other would be camber changes as the suspension compresses.

Cornering causes a decelerating force on the car. It basically converts forward momentum into centripedal force that makes the car go in a circle. This deceleration (braking) causes weight transfer to the front wheels and the front end compresses more than the back. This is in addition to the 60lbs difference between the front & rear. As the suspension compresses and leans, you want negative camber to keep the tire vertical and the contact patch flat on the ground. Since the McPherson strut design doesn't cause a lot of negative camber as it compresses, you have to dial in enough camber at resting height to account for the lean.

In the rear, the semi-trailing arm DOES cause more camber as it compresses. Combined with less compression under cornering, the rear doesn't need as much camber as the front.

Now if you had an ideal suspension designed from scratch, you would use double unequal-length wishbone suspension like in Ferraris, Corvettes, SupraTT, etc. This allows you to custom-tailor a camber-change curve that matches the car's lean in the corner and you won't really need much negative camber at all. These also allow you to have high-cornering limits (because the tires stay vertical and contact-patch flat) AND a comfortable ride.

That said, these were the alignment settings I used last time at the track:

FRONT: -2.5 degrees camber
3.5 degrees caster
0.15 degrees total toe-in

REAR: -1.6 degrees camber
0.05 degrees total toe-in

I would have preferred less front toe-in or even a little toe-out for faster transitions, but I had set the toe to counter-act some of the extra wear that would occur with driving with so much negative camber on the street.

C DeVito

You're really trying to balance the car for nuetral handling in most cases. Absolute numbers are somewhat dependant on the application and driving style; but for Solo and our cars you want more negative camber in the front to keep the front tires from losing grip sooner than the back. With a 50/50 weight the smaller tires in the front tend to lose grip first especially in tight turns. Simply put the forces on the tire cause it to roll over on the outer side wall and lifting the inside patch. The trade off is, more negative camber will create less tire patch when rolling straight ahead. But for the extreme turns (solo), the outside wheel feeling the most force will have more rubber flat on the road. Hence it will not lose grip as soon. A friend runs 245x16 at all 4 corners and says the car is nuetral.

Carmen
89 951

H Dog

Danno,

>Cornering causes a decelerating force on the >car. It basically converts forward momentum
>into centripedal force that makes the car go >in a circle. This deceleration (braking) >causes weight transfer to the front wheels >and the front end compresses more than the >back. This is in addition to the 60lbs >difference between the front & rear.

I'm not sure about this -- in steady state, you've got your foot on the gas, and you're going around a constant circle at constant speed.

>As the suspension compresses and leans, you >want negative camber to keep the tire >vertical and the contact patch flat on the >ground.

This depends upon the suspension design. Some don't add camber at all.

>Since the McPherson strut design doesn't >cause a lot of negative camber as it >compresses, you have to dial in enough >camber at resting height to account
>for the lean.

This is an understatement! In fact, the MacPherson strut causes a lot of positive camber in bump (compression). So you have to dial in TONS of negative camber to get the suspension to work right. Or better yet, make the springs very stiff.

>Now if you had an ideal suspension designed >from scratch, you would use double >unequal-length wishbone suspension like in >Ferraris, Corvettes, SupraTT, etc.

Actually, the hot setup is a five-link suspension front and rear.

>This allows you to custom-tailor a >camber-change curve that matches the car's >lean in the corner and
>you won't really need much negative camber >at all. These also allow you to have
>high-cornering limits (because the tires >stay vertical and contact-patch flat) AND a
>comfortable ride.

In Britain, a small volume car maker, Noble, makes a car which circles at over 1 g, without antiroll bars. That's what happens when you don't compromise on cost and mass-pruduction needs.

>That said, these were the alignment settings >I used last time at the track:

>FRONT: -2.5 degrees camber
>3.5 degrees caster
>0.15 degrees total toe-in

>REAR: -1.6 degrees camber
>0.05 degrees total toe-in

>I would have preferred less front toe-in or >even a little toe-out for faster >transitions, but I had set the toe to >counter-act some of the extra wear that >would occur with driving with so much >negative camber on the street.

Right, static negative camber is often used with toe out to keep the tires from fighting each other.

Typically, camber should be set after taking tire temp measurements, so that the temps are equal across the face of the tread.

Note that the feel through the steering wheel is dependent upon 2 forces: what is known as mechanical trail, and pneumatic trail. Mechanical trail comes from caster, and this force is a function of the steering angle. Pneumatic trail comes from the interaction between the tire and the road, and as the tire approaches the limit of adhesion, pneumatic trail starts to fall off, leaving only the mechanical trail. If mechanical trail is very, very high with respect to pneumatic trail, then as pneumatic trail falls, the driver will not feel the front end washing away. So care must be taken in not setting the car up with too much caster.

Rear Toe out gives faster turn-in, since as the outside rear tire loads up, it dominates what's happening at the rear axle, and it wants to scoot around, in oversteer. Front toe out does the opposite -- gives the illusion of understeer -- plus it is fairly dangerous at high speeds with a rear wheel drive car -- it is unstable. So most rear wheel drive cars are set up with a small bit of front toe in.

Hope this helps.

keith

This is good stuff - let's talk about the caster some more in direct respect to the 951:

What is TOO MUCH?
What is too little? etc...

Toe in will even tire wear in relation to a negative camber setting? This is new info for me.

I need more!
(Getting ride heaight/corner balance/and re-aligned soon!)

Luke - I have a rollover problem with front tires on autox that no amount of inflation will cure - so I need more neg. camber. Interestingly, I cannot get the rears to roll over to the edge of the treadwidth, even, hence I am dialing out some negative in the rear.

The factory used the alignment settings as one more way of inducing understeer for "safety", I suspect, hence the more positive camber in the front...

RPG951S

>>Now if you had an ideal suspension
>>designed from scratch, you would use
>>double unequal-length wishbone suspension
>>like in Ferraris, Corvettes, SupraTT, etc.
>>This allows you to custom-tailor a camber-
>>change curve that matches the car's lean
>>in the corner and you won't really need
>>much negative camber at all. These also
>>allow you to have high-cornering limits
>>(because the tires stay vertical and
>>contact-patch flat) AND a comfortable ride.

Although I agree with you completely with regards to the theoretical advantages to using a double-wishbone suspension design... in the real world, there are SO many other variables involved that proper suspension design/tuning is still a bit of a magic art.

Case in point: If you ever seen any of these 'ultimate handling comparision' (done by various car 'rags), almost all of the current and historical 'winners' did NOT have the [supposedly] superiour double-wishbone setup. Some of the past winners that I can recall are: E36 M3 (Struts/trailing arms, I think), Subaru Impreze (Struts/Struts.. I think), E30 M3 (Struts/trailing arms, I think), and our lowly but wonderfull 951 ...
A NSX, Supra, RX7TT or Ferrari has never 'won' any of these street/track competitions.

Now I'm not try to say that double-wishbones are in some way 'inferior', but they don't automatically make a car handle 'better'. It's kind of like the mid / front engine debate... in theory, a mid-engine design is better, but in reality... it's a wash.(except at the very highest levels of competition)

That said, when designing a pure racing application, double-wishbones are definately the way to go. Minimizes camber changes, minimizes toe changes, better steering geometery...etc...etc.

Anyways... just my thoughts.

Raagi

Danno

Uh, I don't know what reviews you must be reading, but here's a few:

Ok, I agree that we're talking ultimate potential here and most street cars aren't taking their suspension designs to the limit. There variations of grey rather than absolute black&white superiority. What the double-wishbone usually allow is better ride-comfort for the same handling & grip (5-link is a variation of double-wishbone, just split the bones into two links). They don't have to crank up the spring-rate to prevent body-roll and the dreaded camber-roll because they have a camber-change curve that matches the body-roll rate.

IBRJA

As for the alignment specs, I don't know. But, (and this is the important part)my Mechanic said the adjustments should be made with a loaded 944. That is with a topped off gas tank, and (2) 160 lb. people in the car. He will load his workers on the car, in the driver and passenger side, or load sufficient weight in the bucket seats before adjusting the alignment. He did this with my '87 951 about 2K miles ago and all seems OK to date. He said that the bool calls for the car to be loaded when re-aligning the car. I can't say that I've seen the publication, but I am satisfied with the job.

Danno

Yes, that's right. Any decent alignment shop will check the alignment with weight in the car to simulate actual driving conditions. The gas should be 1/2 of a tank though because that's the average of all your driving conditions.

The shop that does my corner-balancing always loads the driver's seat with my 180lbs. That's enough to throw the corner-weights off by 5%.