Hmm, quoting:
"Mil is a length measurement unit. Mil, also known as a thou is equal to one thousandth of an inch (0.001 inch). The mil or thou is still used in the United States to measure thickness of various materials.".

As you note, mm (millimeter) is metric or as it's called today, SI (for International Standard; Note many European's swap words, i.e Standard International!) It's about 0.040 inches.

 

The link he provided was a conversion for milliradians, or Mils. Super confusing, because machinists also call a millimeter a mil as you noted. https://www.google.com/search?q=milliradians

Either way, you cant convert the millimeters of toe in DSC spec to degrees using the calculator he provided. In order to convert mm or inches of toe to degrees, you need to know where the toe measurement originated from. The hub, barrel of wheel, or tire. In this car, DSC used the barrel of the wheel. (I called and asked a while back). Using a C7 Z as reference, the conversion is .056º in the front and .059º in the rear for DSC alignment specs.

 

atljar, Note the DSC spec notes -0.5 mm, mm is the abbreviation for millimeters. I never introduced or noted Mils, I think even mixing this into the thread only adds confusion for others.

My bad? I'll revisit my conversion from the search to convert MM to degrees.

This is another reason why the DSC spec is a bad spec as the required conversion for us, and most of us are reasonably well educated enough to understand the differences in measurements can produce different interpreted meanings. The DSC spec is not a good one to share unless it's provided with uniform measurements that are not open to conversion.

 

I'd also like to suggest toe should be measured in degrees as the possibility to measure toe could allow the desired measurement to rotate based on a circle from the rotation of the upper and lower ball joints allowing the spindle to rotate in a circle if there was nothing attached to the spindle and allowing it unrestricted movement where it could complete a 360 degree rotation if unimpeded.

Measuring toe in millimeters would depend entirely on the location of the measurement and the larger the arc the larger the toe measurement would be!

Measuring toe in degrees is uniform whether you are 2 inches from the center of the radius or 24 inches so no matter what .5° is .5°

Do we all agree?

 

Agreed. DSC is measuring in millimeters.

This is where I disagree. You posed this link and comment...
That link is converting milliradians to degrees so your conversion isnt correct. Correct numbers are -0.5mm = -0.056º on a 19" wheel and -0.059º on a 20" wheel
No big deal, just trying to get things square for people reading in the future. I agree that measuring toe in inches or mm isnt ideal, but its also the easiest method if your doing home alignments. You just need to know the reference point, which as I noted above is the outer circumference of the wheel face or barrel. I honestly think you have a really firm grasp of what is going on here, just the link was a bad one and therefore the numbers you came up with were incorrect.

 

Yep that is confusing! Angles are measured in degrees or radians so see how a milliradian can be used.

Yep to measure toe given in an angle into a length measure you have to measure from the center of the wheel to wherever you are going to make the distance measure at the tire. Recall I did mine for the street rod to the same groove in a tread on both tires.

 

When you are using strings you measure the distance of the wheel from a line passing by the wheel at the wheel centerline. If the rear of the wheel (rim) is 5/32 away from the string and the front of the rim is 6/32 away from the string you have 1/32 of toe in. Again it is simple trig to calculate the degrees. When you do the calculation and vary it by an inch or two one way or the other from your baseline you find the angle changes in the thousandths so you can settle on using the diameter of the rim. It really doesn't matter whether you use the barrel of the wheel or the very edge of the rim, there just isn't a big enough change to make a large difference in the angle calculation. You might get .056 Vs .059 but as far as anybody is concerned the number is still .06 degrees. As an example 1/32 of toe is ~.09 degrees after rounding for 19 inch (0.094236), 20 inch (0.089525) and 21 inch (0.085262) wheel diameters. That is plenty close enough for any alignment work given the tolerances that are applied to the numbers. You can get involved in the numbers and circle the drain worrying about them but the biggest thing is to get into the ball park and make sure you have toe in when you want toe in Vs having toe out and vice versa.

Bill

 

Does anyone know why DSC measures in degrees for caster and camber yet they use millimeters for the toe which in theory should also be in degrees for consistancy?

 

Because they are a race shop and I'm sure are doing lots of field alignments. Setting toe in the field is done with a tape measure and string. Degrees is "better" in the sense that it eliminates some potential sources of error but not something that can be easily done at the track

 

Yeah, but guys are posting the DSC specs as if they were generated from a machine without specifically noting where they should be measuring the toe from. Back in the day I set many a short track car's toe using a toe bar in late model, open comp, southwest tour, and winston west. It was quick and efficient and to set the camber we'd take tire temperatures accross the contact patch and if you didn't have enough camber it was easy enough to figure out which way the top of the tire needed to go.

 

subieworx,

Are you saying for best tire wear and spirited street driving that I should tell the Service Manager to tell the alignment tech to set the Camber to -1 (degrees?) for all 4 wheels, and the toe to 0 (degrees?) for all 4 wheels?

And then what do I tell them to set the Caster at for front? ...and Caster for the rear?

Also, do they have to have that special bracket mounting piece to align the rear? Is that a must have item to get it correct? What is that piece called or what is that item part number please?

Thank you subieworks



.

 

Well that depends. My "spirited" driving will likely be different than yours. I tend to set my cars up like they were going to the track for the street. But I own a performance shop and that's how I roll.

Those specs should work fine for the average user who wants the car to handle well. For caster front go wtih stock recommendations. For rear go with .7 as DCS mentions.

I have no idea if your dealer will have the tools. Not all do. You have to specifically ask if they have the tools to do it and then check that what they are telling you is accurate. I wouldn't be surprised if most dealers would immediately say yes, then back pedal when pushed on the issue.

 

FWIW, look at the allowed range that GM specifies for your suspension. The Dealer Service Manager may NOT set the suspension outside the GM range. I went just somewhat under the GM centerline specs favoring better tire wear but within the allowed range. Turned out my rear wheels on the Grand Sport were set from the factory near the max allowed camber, so it was well under that!

Second I would NOT have anyone mess with rear caster unless they know what they are doing. There was at least one post where the dealer tech messed it up. It is another set of adjustments. Frankly if NOT tracking not sure I would let a dealer touch rear caster. If Tracking or driving very aggressively find a shop that knows what they are doing, has done it before and understand yourself what you are achieving!

 

You can't do an alignment on the rear of a c7 and not "mess" with the caster though. Changing camber will change caster back there. It will have to be dealt with and can be done properly by anyone with the tool. It's not hard to do just has to be done.

 

But as you say it can be done IF they have the tool and KNOW what they are doing. Frankly is was similar to when I had the Grand Sport done at the local dealer. Had I just said "check alignment" they would have fixed the one toe that was in Red an slightly out and said, "All's fFne" and I would still have the rears at -1.6 degrees camber! I was watching as the two techs pulled the Grand Sport up on their Hunter machine ramps and it only had ~ 1/2 inch clearance to my Carbon Fiber OEM splitter. Then as I stayed and watched was pleased he had my spread sheet I gave the service writer. He then pugged in the car info and only one toe was in Red,

I understand it's iterative, as he adjusted toe to get close to the centerline I spec'd the camber changed and visa versa. Took him ~30 minutes!

 

Subie is 100% correct. There are two eccentrics on each lower control arm, and two shim washers under each upper control arm dogbone bolt. When you change camber, you are going to change caster, whether front or rear. Better to be informed of the consequence of any alignment setting being off. For best street driving getting all settings on the left close to those on the right will help driveability on the road. You need some camber in these cars,setting them to 0 then getting on the interstate where nominal speeds are often 85-90, then asking the car for some more mph is courting disaster.

The beauty of the C7 is that now you can change rear caster, and for cars with big fat tires get a setting that will reduce the chance the car to go into a snap oversteer when a mistake is made in a turn. What happens with this suspension design is when torque is added to wheels (pushing on the happy pedal) the rear knuckle is twisted forward. If you have negative rear camber, the knuckle acts as a lever and increases the force on the leaf spring, increasing suspension compression. Now, when this happens, the tie rod whose axes are different, pull on the toe setting, increasing rear toe (out). Picture your outside rear tire, in a turn, the one with all the grip, now pointing further away from the vector of the car. That big *** tire with lots of grip and a vector different than the car's vector and you have throttle-steer. People experienced with track days use this as an advantage to rotate the car so they have the optimum vector of the car at the finish of a turn. People not able to take advantage of this, or stab the throttle quickly observe panoramic views (spin). The usual response of the inexperienced is to lift the throttle or worse, hit the brake. This puts the grip in the front and none in the back to a car that is just starting to spin. Then the panoramic view is just a blur.

Regarding spirited driving, well, curves in highway roads are DIFFERENT than on the track. Almost every curve on the highway has negative camber. This is because a curve in the road is there to go around an obstacle, usually a hill or a gully next to the road. The DOT logic, especially where winter temperatures go below freezing is to pitch the camber of the road so water drainage is most efficient. They expect the driver to respect wet, or icy pavement. You are at a disadvantage as a "spirited driver" in these curves, because the car is put in a situation where snap oversteer is likely and the escape choice is the ditch or hitting oncoming traffic.

If you like speed, then go autoX or HPDE and learn how to drive a car. Your need to be a spirited driver may lessen and you won't be putting yourself or other innocent drivers at increased risk. We have plenty of folks willing to put us all at risk with their friggin' cell phone use. At least a spirited driver is paying attention to his behavior and therefore better able to manage chaos. Driving today's roads is a curious combination of ballet dancing and cage-fighting.

 

 

Now there's a term I've heard few drivers or pretty much anyone else ever mention... "polar moment of inertia"! I remember when I was a young greenhorn trying to understand stockcar chassis setups and flew from the west coast to Indiana for a Ray Dillon stock car chassis school put on for 3 days and I learned that term and others. I also had the honor of closing the bar on one of my nights there with Dick Trickle, those are great memories.

 

^^^
Yep, we were sponsors to the Petty Team toward the end of his driving career. More apt to hear "Plowing" and Lose" versus Over and Understeer from the NASCAR crowd! They understand what is was and during a race used tire pressure or the rear spring load jacking adjustment and "give me a full crank" on the right rear from a knowledgeable driver on the communication system - like Richard!

 

When I lived in UpState NY the Chevy Dealership also sold BMWs and had a BMW alignment machine in the shop. When they performed alignments on my Vettes they used the BMW machine which had toe read outs in millimeters. DSC could be using a similar machine since they have done a lot of work on German cars.

After my track incident and subsequent repair which required replacement the entire right rear suspension parts except the spring the body shop did a realignment on my car and I noticed their Hunter machine was set up to read toe in degrees. The mechanic said it might be possible to choose displaying the settings in inches or millimeters but wasn't sure.

When doing this in your own garage the only way to get close to 0.5 mm measurement is by accident. The measurement tools, parallax error and the human eye aren't sufficient to get a measurement less than 1/32 of an inch. The thickness of the string is part of the problem.

Bill