Obtainable precision on rear alignment?

[Solamar]

I'm using the Scruggs method to align my car. Front was easy, rear has been a bit more challenging (as expected).

I'm using a digital level to set camber. The level displays tenths of a degree. If I'm shooting for 2* what would be the practical tolerance? +/- two tenths?

For rear toe I'd be happy with +1/32 to -1/16 per side, but I'd like both sides to be the same (either positive or negative). The adjustment method is quite crude, what's an acceptable tolerance left to right?

I'm on my fourth attempt at the rear. Each attempt has been getting me closer👍. And I am driving around the block between each attempt.

[dannobee]

Define this "Scruggs method."

When using a proper alignment machine, the vast majority of us were able to split the tenths. I.E., we would set up the car with a 0.4 camber split favoring the left side on the front to adjust for road crown. So if your spec was 0.0 deg camber, the left front would get +0.2 deg and the right front would get -0.2 deg. If you ignored this and set it up 0.0 on both sides, the crown in the road would cause the car to drift to the right.
As you know, toe is more critical than camber. I would always shoot for the preferred setting, within 0.01". If the toe is off side to side, but the total toe is correct, the vehicle's thrust angle will be off. You might see this in the steering wheel not being centered or the car crabbing while driving down the road. But in normal circumstances, even 1/8" off side to side wouldn't be noticeable . Total toe should be within spec though.

[uwanna]

I tried the Scruggs method several times and never was satisfied with the result.
Took it to my alignment shop and was surprised how far off the alignment was
after the Scruggs method that I did with great care. Spent $100 on shop
job and now it's perfect. My time is worth more than the $100.
I'm converted, lasers are more precise than my aging eyes.

[Solamar]

Originally Posted by dannobee

Define this "Scruggs method."

-Parallel strings
-Measure toe
-Lift corner
-Measure lifted corner toe for reference
-Take off tire and adjust desired change off reference toe.
-Reinstall tire
-Settle suspension
-Reset strings
-Remeasure toe
-Repeat
-Repeat
-Repeat
-Repeat

[Solamar]

Originally Posted by uwanna

I tried the Scruggs method several times and never was satisfied with the result.
Took it to my alignment shop and was surprised how far off the alignment was
after the Scruggs method that I did with great care. Spent $100 on shop
job and now it's perfect. My time is worth more than the $100.
I'm converted, lasers are more precise than my aging eyes.

I have three $100 dollar alignment shops nearby. One will not work on torsion bar Porsches. The other two say no problem, are totally confused when they see the rear suspension, then charge $100 to set only front toe.

I also have three Porsche specialty shops nearby that will do the alignment for $700 to $1000.

Time and money require me to do incremental suspension work which would require multiple alignment adjustments.

And I really enjoy spending time wrenching on the Porsche. I am starting to not love this alignment process however...

Getting closer.

[Solamar]

Done.

Four hrs yesterday, two more today. Not bad.

Front
Zero toe, left and right equal
L 1.3* R 1.4* Camber

Rear left
1/32" toe in
1.9* camber

Rear right
1/16" toe in
1.8* camber

[3 pedals]

Solomar, let me know how that drives at speed.

[dannobee]

Originally Posted by 3 pedals

Solomar, let me know how that drives at speed.

It might be a bit darty at freeway speeds with zero toe. And might drift to the right if there are crowned roads in your neck of the woods.

I've set/checked the toe hundreds of times at the track. If we were "stringing" the car at the track, we had more problems than where to park in the winners circle. The problem with stringing a car is obviously getting the strings accurately positioned. We had our own method which included carefully measuring the distance of the string from frame datum obtained from the frame diagrams. And of course, the car must be perfectly level, either by adjusting the stands, or a known perfectly flat piece of concrete (good luck with that one).

Setting toe without an alignment machine is easiest accomplished with a "toe bar" and a tape measure, not strings.

[stownsen914]

Not saying the OP did this, but one of the biggest mistakes people make when aligning at home is assuming their garage floor is flat and parallel. Usually they are neither. A floor can easily be slanted by .5-1.0 degrees and go unnoticed, but will make your camber adjustments off by that amount (which is a lot) if not accounted for.

[Solamar]

Originally Posted by 3 pedals

Solomar, let me know how that drives at speed.

The reason for this alignment is because I changed to-

20/27 torsion bars
Custom valved, digressive shocks
Poly Bronze front bushings
Mono ball upper strut mount
Turbo tie-rod ends

So identifying a change in driving as an effect from the alignment would be optimistic.

I only drove ~ 2 miles on the highway, accelerated to 80, slowed to exit. Then drove ~ 60 miles on twisty back roads. The suspension is everything I had hoped for, flat around corners and still a pleasure to drive on poorly paved roads. Front is lighter, planted and more responsive. Happy Camper

Soon I'll put in some hwy time and report back.

[Solamar]

Originally Posted by dannobee

It might be a bit darty at freeway speeds with zero toe. And might drift to the right if there are crowned roads in your neck of the woods.

I've set/checked the toe hundreds of times at the track. If we were "stringing" the car at the track, we had more problems than where to park in the winners circle. The problem with stringing a car is obviously getting the strings accurately positioned. We had our own method which included carefully measuring the distance of the string from frame datum obtained from the frame diagrams. And of course, the car must be perfectly level, either by adjusting the stands, or a known perfectly flat piece of concrete (good luck with that one).

Setting toe without an alignment machine is easiest accomplished with a "toe bar" and a tape measure, not strings.

I expect you will be right about darty at hwy speeds. I'm hoping the 6* Caster will stabilize things. And you are absolutely correct about drifting off the crown. What effect would you expect from a slight + or - rear toe?

One change I made to the Scruggs method is the strings are attached front and rear to holes drilled in an aluminum bar at equal distance, this keeps them parallel, then you measure equal spacing of the string off the front hub and rear hubs.

Originally Posted by stownsen914

Not saying the OP did this, but one of the biggest mistakes people make when aligning at home is assuming their garage floor is flat and parallel. Usually they are neither. A floor can easily be slanted by .5-1.0 degrees and go unnoticed, but will make your camber adjustments off by that amount (which is a lot) if not accounted for.

Luckily I recently poured a new garage floor and insisted that it did not have the typical 2% fall to the front. It is flat.

[Mahler9th]

Looks like you are on the right track.

The"Scruggs" method... that brings back memories.

We used to AX back about 20+ years ago. Great driver and his little booklet was a great contribution.

The Puhn book also has some great perspective. Notice that the car on the cover was built in Norcal and its race team was from our area.

The SmartCamber and SmartStrings tools from Smart Racing Products are also great, and their instructions are first rate. Of course the founder of SRP and Scruggs knew and lived not too far from each other, and both long term PCA members.

I use a SmartCamber device and my own custom string set up.

I like to measure from the machined surfaces of the front spindles and the rear stub axles. I use fishing line and can measure to 1/32". For rear toe I always go for 1/32" IN left and right. If you are careful, you can develop expertise to change just the rear toe without affecting the camber. Takes a lot of practice and some common sense.

For the front, a common number for cars used for performance driving is about 1/8" total toe out.

My application is racing, but I have used the same target numbers and processes for street and street/track Porsches with a variety of tires for ~25 years.

I have verified my settings against alignment racks many, many times and they are always spot on. It takes patience and practice.

[Solamar]

Thanks for the input Mike. Trying different toe settings is part of why I'd like to become proficient at setting my own alignment.

On Saturday I was adjusting rear camber and toe with the rear-most locking bolts (spring plate to trailing arm) loose, and attempt to adjust and tighten the cams as I went. My Aha moment on Sunday was to keep those bolts snug enough to hold your adjustments. Made all the difference.

I love the ideal of the smart strings staying attached to the car, eliminating the need to re-set after settling the suspension. But, I'm cheap

[GH85Carrera]

When I rebuilt my suspension with all new rubber and shocks and new torsion bars I got is "close enough" to drive to the alignment rack. I am lucky enough to have a dealership and the one "ole grey" that is factory trained on Porsches from the 911SC to the Taycan and lots of experience.

He let me watch as we set up the corner balance, and set it to Euro ride height.

And I know my fat pig Carrera weight 2741 pounds with all the carpet, fantastic AC and heat, a spare tire and 3/4 tank of gas.

If you want a good alignment, use a alignment rack, a corner balance scale and do it right.

[Mahler9th]

I will never forget visiting the home workshop of CW as he was finalizing the development of the SmartStrings product. Great product, great person. Great racer, and great at the technical details of racing as an engineer. High achievements at the highest level.

I have seen pro teams in IMSA use SmartStrings-- again, very convenient and easy to use.

SRP/CW have a great "coffee table" book available too: Kussmaul Chronicles.

Ask me some day how that book connects to the relatively recent World Series win by the Cubs!

SRP has some videos up on Youtube that can be very helpful to many folks.

Creating a custom SmartStrings-like set up for my car was pretty easy since it is a race car and has removable panels. And I can adjust toe and camber in the back absolutely independently since I have the ERP 935-style spring plates. I also have coilovers which makes it relatively easy to dial in the corner balance to small tolerances.

Before I installed those spring plates, I had to learn a few tricks for making tiny rear adjustments whilst minimizing iterations. You can get some idea from the picture below. The 0.429" number relates to camber, and the leading edge of the trailing arm relates to toe.

And of course you will see once again the "mechanism" used to lock the position of the camber eccentric.



I have logged many, many hours setting up various Porsches, including my own '75 race car. Sadly, it seems that fewer and fewer alignment shops can easily deal with 911 (and 944-family) trailing arm cars-- I have seen many struggles.