How to do rear alignment?

[Alex]

I just spent a few hours today with my Fastrax and toe plates and finished my front alignment. Came out exactly as I hoped it would.

Now I'm onto the rear, and I don't have a fricken clue how to do caster/camber/toe in the rear.

I realize there are probably methods like taking a centerline of the chassis, etc..., but (a) I suck at geometry so measuring and stringing stuff up will do me NO good because I'll do it wrong and (b) I'm not tracking the car so I don't need perfect handling, I just don't want to kill my tires.

Paying for an alignment is out of the question - there are no decent shops around here (hell, it took me 3 times to get my Z06 done properly at one of the better shops).

So, how the heck do we do a rough (streetable) rear alignment????

I assume the first step is getting the tires centered and pointed straight? No idea how to do that, lol.

What about camber - can I use my fastrax and read the camber like I do in te front (adjust it to ground to make it level, then plop it on the wheel and read the camber).

No idea about caster at all.

 

[Alex]

Whelp, now I'm sorry I googled it because I'm more confused than ever, lol

it seems you can measure camber the same way as in the front (which is what I thought)

No idea about toe - I imagine first you want to center the tires (?how?), then run a small amount of toe in, like 1/16''

Caster is still a complete mystery. It seems to me from research rear caster is irrelevant other than in bumpsteer (?doesn't the angle of the toe bar deal with that?) and anti-squat. However, for some reason Vipers have rear caster specs and those guys feel that unless they use a special Viper specific alignment tool for getting rear caster their cars will crash. On the other hand, the solstice/sky has rear caster adjustment, but apparently that was just because GM wanted to re-use the front suspension components and marketed it as a special feature (that needs a special dealership tool in order to read/set)

 

[Jim Cowden]

Alex

As you stated rear caster if you have bump steer issues or wish to use roll steer if the geometry allows it.

My first bit of advice is use a flat area.
The best way is use a laser level that has a horizontal line.
Put the car where you wont to do the alignment and mark around the tyres with chalk or tape.
Roll the car back then put the laser in front of the tyre patches and measure in the centre of the patch to the beam, place sheet metal shims the size of the tyre patch at least.
place required shims to get all the same hieght, I place 2 more on all patches that have some grease between this allows the suspension to move when you make adjustments.
I dont use turntables because you cant roll the car straight up onto the level ground without ramps and things.
You wont need t/tables on the rear anyway.

Roll the car onto the pathes with all the shims in place, you know have the car level and you can do the alignment.
If your floor is out you will get incorrect settings on your camber and casters.

My rear caster I have a flat spot on the casting of the upright and I use this to get 0.caster using a spirit level.
Fran should be able to tell you a recomendation or just set it at 0.
If you want tyre life set the neg to 0 to 1 deg it will all depend on neg gain in the suspension geometry , 0 is a start and add to it slowly if you get curious.
I set my rear to to just enough to do the job, one method is to set it at about 3mm to in then keep winding it of till you fell it is lossing its stability in acceleration/braking or at high speed then go back.
Hope it helps to give you a starting point.
From memory slc has upper lower arms it may require moving the upper inners in and out on the front rear pick up points to adjust caster but it will bepend on the layout.

I use the laser beam down the side of my 40 to do toe but fishing line is ok.
You will need to work out a datum or method for your line but once you have it its done.
I made some bars that lock on the car when the clips are up and it only takes me about 1 min to set them up.
Whatever you make or come up with you dont want to pull bodywork off,trust me.

Jim

 

[Bill Musarra]

Do a search using various words about alignment. Just using alignment I came up with these. Tom's is the best as he has some good pics. There is an article that Dean Lampe wrote for one of the car mags that outlined the string method. Very good article in two parts. I have it at home but could not come up with it.

Bill



http://www.gt40s.com/forum/gt40-build-logs/24525-toms-rcr-40-trackracer.html#post223405

http://www.gt40s.com/forum/gt40-tech-chassis-brakes-tires-wheels/21773-performing-alignments.html

 

[Fran Hall RCR]

Alex.

The way that the rear upright is designed, by setting the upper and lower controls centered in their chassis mounts the caster is preset...no need for adjustment.

Camber is easy .....just adjust the rod ends
Toe....again just rod end adjustment.

 

[Alex]

Alex.

The way that the rear upright is designed, by setting the upper and lower controls centered in their chassis mounts the caster is preset...no need for adjustment.

Camber is easy .....just adjust the rod ends
Toe....again just rod end adjustment.

I agree, it is easy to do, but I'm struggling w/ how to measure rear toe. I can measure total toe, but in the rear you could have wheels turned like: / / and have 0 total toe

 

[mark b.]

in the past i have ran a string down the centerline of a chassis which should be your thrust line, take a level and put it across the rear tires across the axle horizontally and on the outside and use a bungy cord to hold it on, drop a plumb line off the level at the front and rear of the tire each side and take measurements from there. should get you real close, and i'm sure you know a little toe in is mucho better than toe out.

i have found on a machine that getting 0 toe is like shooting craps unless there is a threaded sleeve somewhere. a half turn of a heim joint will go from toe out to toe in, and turning both sides a half turn moves the thrust line about 7-9 degrees if i remember right. a good string job can be real close, but a machine is dead nuts exact and will use the front uprights also in the calculations and give you the setback and all sorts of little tidbits.

 

[Fran Hall RCR]

The toe is adjusted vie a threaded toe link that does not require a full half turn..its left/right hand threaded ...so minute variations are easily done..

 

[Jim Cowden]

You will need to find the centre line of the chassis Alex.
It may not be your thing but it is the way to do it.
It is not hard ,it is just time consuming.

If you dont have string lines how did you do the front toe
.
Did you get anything out of earlier post #3 and #4, because you seem to still be at the same point in post #6 ??.

Jim

 

[Alex]

You will need to find the centre line of the chassis Alex.
It may not be your thing but it is the way to do it.
It is not hard ,it is just time consuming.

If you dont have string lines how did you do the front toe
.
Did you get anything out of earlier post #3 and #4, because you seem to still be at the same point in post #6 ??.

Jim

Yes, i read it and am thinking. Always thinking before doing

I set front toe with toe plates. Easy peasy

[ame=http://www.amazon.com/Longacre-Toe-Plates-Includes-Tapes/dp/B000VAOHB2]Longacre Toe Plates (Set) Includes (2) Tapes : Amazon.com : Automotive[/ame]

 

[Howard Jones]

Given that caster can be set statically per Frans method, set first, then all you nee is a straight edge and a good steel tap that is accurate to 1/32".

1. Find exact center line of car, I would guess you can measure between the like a arms mount points on each side of car. Devise by two and mark on chassis using a plumb bob and string. Do both front and back.

2. Set car on ground. Most garage floor are flat enough. don't move it again.

3. Tranfer centerline to floor with plumb bob and string. Mark floor to match centerline of car. I use masking tape and a pilot point marker

4. Set rod ends at all a arm mounts to same number of threads exposed for each location. All top rears must be the same both sides and all bottom rears must be the same on both sides. fronts should be done in the same manner. Mark floor with tape and marker at centerline of each axle. Use string and plumb bob again. Now you have the cars datum points marked onto the floor. Center line of chassis, centerline of each axle , and all four wheels are set to be the same distance from the center line left to right. All of your measurements will be taken off these 4 points to centerline of car.

5. now you need a nice 90 degree carpenter's square with at least 24 inches on the long arm is best. Mark a line at each axle center onto floor at 90 degrees to centerline of car and at outboard edge of wheel. Now you have the wheel center point at the outboard edge marked on the ground.

6. front toe. use the carpenter's square. place against tire side at front where wheel and tire meet and at 90 degrees to vertical, drop the plumb bob to the ground from here and measure to centerline. set toe to 1/32 shorter distance at front of tire than at centerline of wheel mark on ground to centerline of car. Do other side the same way. You now have toe set to 1/16 total toe in, with each side set to 1/32. This is a good start point.

7. rear toe. do the same thing at the rear that you did at front. Now all four tires are set to 1/32 " toe in.

8 camber, gonna use a little trig here. the idea is to use the square, a bubble level, and a 6 inch steel rule to set the distance at the top of the wheel inboard a measured amount less than vertical.

place the square against the wheel rim with the short arm sticking outboard and horizontal on the centerline of the wheel and use bubble to set it vertical. If the wheel is perfectly straight up (0 degrees) then placing the bubble level on top of the horizontal arm will get you perfectly level.

Using this calculator

Right Triangle Trig Calculator

Put in side a and b to get angle example 18 inch vertical side and .3 inch horizontal side will get you about neg 1 camber.

If the long edge of the square is set vertically against the wheel and the short side is sticking out horizontally with the bubble level on top and then you adjust the square to make the bubble level horizontal. then you can measure the distance to the wheel edge from the square edge with a small rule. Our example would be 0.3 inches.

turn in rod ends at the top a arm chassis mounts to tip the top of the wheel inboard to make neg camber adjustments. do both the same number of turns. once you have one front then do the other the same way and move to the rear of the car. good start point would be about a .5 degree neg camber at all four corners.

If you really want to make this easy cut the long arm to the diameter of the wheel and it will fit right up against the wheel without the tire being in the way.

If you take your time this really works. I took my GT40 to a laser alignment shop afterwards and it was too close to make changes. You can also make camber changes at the track with a simple measurement and turning the top rod ends in or out the same amount.

 

[Chris]

Hey Alex,

Its a bit time consuming getting the string line method setup but its reasonably simple. Ive attached a couple images

This is just kind of a top view. You setup and find your centerline of the chassis (see photo below for plumbob etc) From there, you setup 2 jackstands with string tied between them outside of your rear tire, doesnt really matter how close to the rear tire, a few " should be fine, after that, you can drop a plumbob off the string from the jackstands and mark it on the floor on some tape...then measure from the marks from your centerline you got earlier and measure out to the new plumbob mark you made from the string on your jackstands, its time consuming and you really should measure from the centerline of the rear of the car and the front out to the string, you want the measurements to be the same from the center of the chassis to the jackstand string.

Once that is equal measurement. You can measure back from the string to the sidewall of your rear tire on the front side of the rim and the back side of the rim. If these measurements are equal you have 0 toe out. To set toe out you would want to see a measurement on the front side of the side wall less than the back...

Hopefully its not to confusing. There is a company in Kelowna that builds and designs some top end race car scales/scalepads/toe bars and so on and so on. Maybe they can help you out with a good method to setup your car or even if you are planning a trip into BC it might be worth while seeing if they can setup your car. Check out their website

MK Technologies - Race Support Equipment, Racing Products, Scale Platforms, Chassis Alignment Equipment, Pit Equipment, Industrial Design and Metal Fabrication in Kelowna British Columbia, Canada


If my string method makes a bit of sence and your thinking of this method and need to ask a few questions, PM me...Maybe you can get in contact with a top end race team in AB that has a good understanding of aligning a car. It will be the same steps as they would take on their race car to do your car.

Cheers,
Chris,

 

[Cliff Beer]

Here's a real rough and simple method I have used in the past for rear toe......

I use zero toe....which happens to make it easier as well.

This assumes the front wheels are centered on the centerline of the car by the way.

1. Get the car on as level of ground as you can, as suggested by others,
2. Take a couple of paint cans (full) and glue a piece of string or fishing line to the top of both at a spacing of about 15 feet.
3. measure your front and rear track (outside of wheel to outside of wheel). Compare the two, then divide the difference in half. The rear track will be wider than the front.
4. line up the string on the rear tires (half way up the tire is best) and see where the string falls relative to the front tires. In other words, the string will extend forward and clear the front tires by some amount.
5. for zero toe, you adjust the toe on the rears such that you have the 1/2 measured distance (in step 3 above) clearance between the string and the front tire.

Again, this is a rough and simple approach, not recommended for any seriously intense driving/track time.

 

[mark b.]

and the all important make sure the rack is centered with the frame, not necessarily with the rack itself if it is off centered.

 

[MCutter]

On the race car I use the string method used in the post above. The only difference is we use the face of the hub or spindle to measure from since a direct line to the center of the chassis is hard to measure. If you want to verify if its a good place to measure, the first time its done use the chassis centerline and then after the line is set up measure to the spindle or hub-face on the center of the wheel. If the suspension components are equal this number should be the same for left and right front and left and right rear. If they are then the chassis and suspension are square enough to use the hubs to set the strings up in the future.

Now to set up the strings mount a bar on jack stands in front and in back of the car. Run a string from the front to back bar and set the strings equal distance. Now set the measurements to the front and rear hub equal and the line should be square and you can measure toe off the string to the front and rear of each tire.

For street stability I would think a little toe in front would be good.
The most important toe measurement would be the rear however. In particular no toe out in the rear. Toe out in the rear will give rear steer. Given the power of these cars I would again say a little toe in would be good also.

The other question you need to have answered is bump steer, do these cars have inherent bump steer and a way to adjust it? If so then this is an important item to get set correctly again to avoid steering s the suspension travels in particular in the rear again.

Mat

 

[Fran Hall RCR]

Bump is totally adustable front and rear and can quite easily be set to near absolute zero...