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Rear Caster Measurement - Possible to do on Floor?
This question is for those of you who have the caster adapter tool and angle gauge to measure the rear caster yourself. Was there enough room for you to measure it while the car was on the ground or did you have to lift it in the air (wheels supported obviously) in order to get underneath? I'm a bigger guy and I'm unsure if I'd be able to sneak my way under the rear bumper to get to the rear knuckle as it sits inside the wheel. I could probably get my arm in there (blindly) but I don't know if I could finagle my body under to see what I'm actually doing.
I think you can get your head and one hand close enough to see the mounting holes in the knuckle for mounting a tool and measuring, but unless your tires are on turn plates or something to allow them to slide easily, you may not get an accurate measurement. So any decision you make regarding the measurement you see may not be the best. Here's what we use to measure caster just a piece of cold rolled steel that rests on two 1/4 bolts. The bolts fit in the caster measuring holes of the knuckle. Attach a digital level and you have caster.:
Yes, you can get under there if you work at it. You have room for one arm and your head. Remember you need to zero the angle gauge to the position the car sits on the floor. I do that by placing a 6 ft long carpenter's level lengthwise under the car next to the rear tire on each side. When doing the left side the level is zero'd on the carpenter's level to show 0 degrees when vertical to the plane of the floor on that side of the car. When I do the right side I place the carpenter's level alongside the right side rear tire and repeat the process for that side. This duplicates the instructions that come with the GM Gauge which tell the mechanic to zero the gauge to the alignment rack on each side of the car.
Yes, you can get under there if you work at it. You have room for one arm and your head. Remember you need to zero the angle gauge to the position the car sits on the floor. I do that by placing a 6 ft long carpenter's level lengthwise under the car next to the rear tire on each side. When doing the left side the level is zero'd on the carpenter's level to show 0 degrees when vertical to the plane of the floor on that side of the car. When I do the right side I place the carpenter's level alongside the right side rear tire and repeat the process for that side. This duplicates the instructions that come with the GM Gauge which tell the mechanic to zero the gauge to the alignment rack on each side of the car.
Bill
Bill,
Thanks for your reply. Regarding the carpenter's level laid alongside the inside of the tire, can you explain why that is necessary if one has a digital angle gauge that has the ability to measure relative angles? I was planning on placing the gauge on the ground at the center of the tire (so it's touching relatively the same point of the ground) and orientating it so that the magnetic base was perpendicular to the floor. Once I set that gauge position as my starting (zero) point, I would place it on the caster adapter tool and the difference in angle should be the caster angle. Where have I gone wrong in my assumptions?
I have the Granatelli's on my car. Not sure if they are like the LG's. Subie, the the Grannies are a bit of a PITA to adjust, not sure what you mean by a safety washer. If you don't have the Grannies or equivalent on your car, you will spend a lot of adjustment time because the range limitation of the OEM links makes you work to get camber, caster and toe all where you want them. Camber adjustment affects your caster angle, your toe angle and thrust angle. Toe adjustment affects toe and thrust angle.
I think all Bill is telling you is that he is using the level to give him a good horizontal reference plane to set 0 deg for the instrument he is using for caster. He is thus reducing measurement error from side to side. I do my alignments on a 4-post lift and i check level before starting with a 4-ft spirit (Brick Mason's} level. Bill is fast enough on the track that he may be able to see the effect of caster settings on the response of his car. I haven't tried experimenting, I just set to what the factory says wrt caster. I have found that I get pretty good result just resetting my digital gages before the alignment, then checking as a last step. If I see the gages are off, i re-zero and check all again, adjust if necessary. I change to track settings and then back to street after the track event, so I've done it quite a few times on my C7 and did as well on my C5 track car.
Garage floors are seldom level, in fact in our area, when I built my new garage, I wanted a level floor. The contractor said the county inspector would not pass a level floor. Had to have a slope for drainage. Which brings up a point. If your floor is not perfectly level, then when you park your car and put it in part (auto) or leave it in gear (manual), once you get out of the car, it will put a torque on the suspension and any measure you take will not be a true static reference measure of caster. Getting a true reference static caster will require the surface upon which the tires sit to be level, and allow the tires freedom to slide slightly to remove any torque on the suspension.
When using a digital angle gage, for your convenience, you need to know which way is positive WRT caster. Or for that matter, you could drop a plumb line, measure, than use a trig table to get the caster angle. But what a pain that would be.The little digital level I use has a small arrow but it does not zero, so you have to subtract the reading from 90 to get the caster angle. To change caster you will be using the camber adjustment eccentric bolts, and you will need to use both front and rear eccentrics. So in my pic above, the digital angle is giving me +0.3 deg caster. Yes, you can get your head in there just to measure, but I think getting to the front eccentric to adjust will require you to invite Stretch Armstrong to help (yes my children are in their mid 40's and had that toy in the day).
This is a timely post. I took my car to two local alignment shops and a Chevy dealership and all said that they couldn't measure caster, let alone adjust it...and GM dealers were supposed to have a speacialized tool.
They don't have to carry the tool but it can be loaned from GM. It gets overnights to the dealership requesting it and there are idiot proof (hopefully) instructions on its use.
This is a timely post. I took my car to two local alignment shops and a Chevy dealership and all said that they couldn't measure caster, let alone adjust it...and GM dealers were supposed to have a speacialized tool.
WOW. I just posted about an alignment shop here in NY that has the angle gauge and does Vette alignments right.
ALL my new Vettes have come from the factory with their wheel alignments out of spec and these guys at Little Bear in Buchanan NY do it right. Just brough in my 2019 ZR1 and the alignment was out of spec and the left rear caster was way off, it’s insane that dealerships don’t have the proper tools and to think that the alignments they do don’t include this adjustment if the customer is unaware of the need and the existance of this tool to do it.
Regarding adjustable toe arm, I remember being able to get to -1.8 or -1.9 without having to resort to the adjustable toe link for hitting my goal of -2.0. In other words, it only became necessary for the last bit of my goal (as well as that 0.75 degrees caster).
This time around, I'm thinking about just sticking with the OEM link, but replacing that last UCA washer with a much thinner one. My reasoning is that the UCA approach seems to have less effect on toe curve than those links since it's a symmetrical approach (like car getting lowered), and hopefully would not change the toe curve like those links would. Thoughts?
The washers are 1mm thick and worth about .25 deg of camber, if 4 are taken from a side (one per dogbone bolt). They also throw a toe change of about 3/16 inch each side which is close to the range of adjustment of the OEM link.
I think you have a challenge ahead if you are shooting for +0.7 caster and and close to -2.0 camber with the stock links. Maybe removing all washers from the back dogbones, and just removing one from each bolt of the front dogbones will get you closer, but if +0.7 caster is your target, the adjustable links may be your only option.
09-10-2018, 05:40 PM
Rear Caster Measurement - Possible to do on Floor?
