W heel set back is a fundamental indicator of larger problems that are often missed when measuring a collision-damaged vehicle. Though it’s a very easy measurement to make, technicians don’t often check it. But why?
I imagine it’s because it requires a tech to think outside the box before putting a vehicle on the alignment machine or sending it to the alignment shop. Many frame technicians don’t do suspension alignment work, and because they think of wheel set back as an alignment measurement, they don’t bother with it.
Unfortunately, those technicians are passing up a valuable opportunity to find additional damage and to correctly repair the vehicle. And that’s what we’re supposed to do, right? I sure hope you said yes because that’s what I’ve been doing for the past 38 years. I shuttle my repaired vehicles to a nearby alignment shop that I’ve dealt with for several years, and I always enjoy hearing the alignment techs ask, “Why do we have so few problems aligning your jobs?” I usually answer, “Precise measurement and repair.” They look at me with a kind of glassy stare, but it’s true, and all the time I’m thinking, “set back.”
Oh, I know, right about now you’re thinking, “This dude is out of it.” An alignment guy asks him a nice, complimentary question and he’s thinking “set back”? Well, whenever my three grown Irish daughters get together, they discuss their old man (as if I were in a “nether world”) and always comment that I’m “all technicalled out.” Not true. I’m just curious. I love geometry and the natural order and symmetry of everything – whether it’s a flower blossom or the undercarriage of an automobile.
Measuring for wheel set back is one such geometrical element of the undercarriage that intrigues me. It’s also affords me a valuable opportunity to find additional damage and correctly repair the vehicle – something your technicians can achieve if they just start thinking outside the box.
Set Up for Set Back
When checking wheel set back, the first thing I do is check my wheelbase measurement for the vehicle I’m working on. I do this because it tells me if wheel set back is supposed to exist in the vehicle I’m repairing. This is rare, but you sure don’t want to remove any set back if it was built into the vehicle, so always check your wheelbase spec to see if it’s supposed to be the same, right to left.
For the purpose of this article, I’ll be writing about symmetrical vehicles with equal wheelbases and even set back, front and rear. We’re not going to discuss all of the measurements that can be taken on a vehicle, but let me say this: When it comes to measuring a vehicle, you’re only limited by your imagination.
“What does that mean?” you ask.
Only that there are many more measurements that you can take on a vehicle than those supplied by the information provider you’re using, whether it be your frame equipment supplier’s charts and data or another source.
“How can I measure something if I don’t have a measurement for it?” you wonder.
Good question … kind of. We do have measurements – comparative measurements on the other side of the car or in the back of the car on the other end of our damage.
I can smell the smoke burning from the wheels spinning in your brain! Oh boy, I can hear all the “yeah, buts” harmonizing. Whoa! Just take it easy and let that old mind wander outside the box and out here on the trail with me. There now. The smoke has almost all blown away. I can see the car again, there it is! It’s on the bench, yeah, can you see it?
The Wheel and This Guy Keep on Turnin’
OK, we’ve got a front end wreck on a front-wheel-drive car. The nose is swung to the left about 25 mm from spec. The rails have barely visible buckles at the crossmember/engine cradle. See ’em? One rail is 6 mm high and the other is 4 mm low. Hmm, slight twist in the core support area, but no biggie because we’re replacing the core support after we straighten the structure.
I’m going to fight the feeling I’ve got right now to grab on to this thing and let the hydraulic juices flow. Boy, do I want to grab some clamps, chains and counter supports and start doin’ some “man” things. Pulling that nose over, hitting it with my three-pounder and doin’ that cool stress relieving thing.
Hold it right there. We haven’t got the full story yet.
“We haven’t?” you ask?
No, we haven’t.
Put your hammer down and come to the rear straight axle with me. Let’s check it for wheel set back. We’re not positive what exactly happened in the wreck or prior to the wreck, for that matter. Did the vehicle run off the road?
In our shop, we use a laser to measure, but you could easily do this with most other measuring systems or a tram gauge. I shoot a laser beam at the perpendicular angle, across the center line of the car. I’ve hung a string with a large nut on each end around the rear brake drum, on the right and left sides. These act as very accurate plumb lines. I move my beam until I see it hit the string on the side I’m working on, and I squat down to see where it’s hitting on the other side.
What the heck! The beam isn’t visible on the other side! Uh, oh!
I move my laser beam rearward until it hits the other line. I measure the difference left to right and find a 10 mm split. Now I look for the cause. It can’t be from the front end hit. Ah, hah! Who’d have thought. The U-shaped straight axle took a hit on the front side about a foot from the right side wheel. It must have taken a shot from a rock or something. There are a lot of fresh weeds and dirt under the car and some scratches on the muffler, but no other damage. That straight axle is lower than anything else under here.
I better tell the parts person we need an axle assembly. That right side wheel will have some out-of-spec tow in. Boy, I’m glad I checked that because the parts will be delivered about the time I’m done with the nose, and we won’t have a big surprise or delay when we reach the end of the job. Better give the adjuster a call, but I think I’ll wait until I check the front set back too.
OK, let’s move up front. (Isn’t this exciting? I feel like Sherlock Holmes in search of clues to a mystery. No wonder my wife thinks I’ve gone off the deep end. Oh well, I’m having fun. Uh, oh, here she comes, better wipe that big smile off my face … don’t want her to think I’m having too much fun.)
I double-check the steering wheel. Yep! The wheel is centered, and the front rotors point straight ahead just like I set them up when I benched the car. I still like to double-check because my helper did some tear down after we benched the car, and he could’ve turned a wheel to gain access to something. I’m still trying to train him that it’s OK to take the wheels off once you bench the car as long as you mark the wheel as to its location. He likes to take the front bumper, fenders and skirts off and then pull the wheels, but that’s another story.
My plumb strings are hanging off the front hubs in the same location on the left and right hubs. Now I’m expecting the right wheel to be forward of the left one because the front is over to the left. But I’ve got enough experience to know that you can never be positive without measuring. Those tires act like anchors, and if the vehicle is heavy enough, the structure can actually move in the opposite direction from the hit in the crossmember area, almost as if it were hit on the opposite side of the impact. It happens.
Just as I thought. When I shoot my beam across the center line, the right side appears forward of the left by about 9 mm, but maybe that will come out when I pull for sway. At least I know it’s there, so I can watch what happens and probably learn a little, too. The lower control arms look good – at least they didn’t hit any boulders.
I make my pulls and correct my structure with no big problems. The structure is straight. I’ve measured all control points, including the bolts holding the engine cradle/crossmember. Everything looks beautiful. Almost time to call Mr. Adjuster, but let’s check that set back again and see if it’s equal now from side to side. It isn’t. How can this be? I measured the bolts that hold the crossmember/engine cradle, which is what the control arms are bolted to.
Yeah, that’s right, but remember that engine cradle has clearance holes where it’s bolted to the structure. I’ve got a case of cradle shift, and I’m not talking about a restless infant with sleeping problems. This engine cradle moved when those big rubber anchors dug into the pavement and slid to the side.
I get my porta-powers out and put a front-to-rear push on the right side of the cradle and a rear-to-front push on the left side. I loosen the bolts and tighten them. Release the porta-powers and check set back. Too far. I reverse my procedure. Wow, that cradle will move quite a bit.
OK, it’s dead on, but I want to check from the end of each lower control arm to the centerline. I’ve got to do this because if that cradle is to one side more than the other, I’m going to have a camber problem. I don’t like split camber. That’s a tire wear problem.
I check it, and the ends of the control arms are equidistant from the centerline. Life is good. Knowledge brings peace of mind.
Back in Place
Finally, it’s time to call the adjuster and give him the news. The rear axle is on its way and when it comes, I’ll be using my set back measurement to ensure a positive end to this little repair saga.
Kind of fun, wasn’t it? (OK, OK, maybe I have a strange sense of satisfaction.) But measuring for wheel set back is an important step in delivering a complete and timely repair job to your customers. Give it a try in your shop, and your techs will be sure to find their own sense of satisfaction in a job well-done.
Rear set back is a measurement referencing a vehicle’s rear wheels to an imaginary line running perpendicular to the centerline of the vehicle and is measured as an angle. If a vehicle has rear set back, one rear tire/wheel assembly sits further back from this imaginary line than the other. Causes of rear setback include frame, chassis and rear cradle misalignment due to a collision. If the vehicle has a set back condition, it may pull to the opposite side of the set back.
Front set back is a measurement referencing a vehicle’s front wheels to a line running perpendicular to the vehicle centerline and parallel to a line drawn through the centers of the spindle. If a vehicle has front set back, one front tire/wheel assembly sits farther back from this imaginary reference line than the other. Positive set back indicates that the right front wheel is set back further than the left. Negative set back indicates the left front wheel is further back than the right. Front set back can be checked during a normal alignment and is used to diagnose collision damage or cradle misadjustment. If the cradle is adjusted incorrectly or damage is present, it’s not unusual to also see a reduced positive caster reading on the side with set back. Excessive set back can cause an alignment pull to the side with set back.
Writer Mike West, a contributing editor to BodyShop Business, has been a shop owner for almost 30 years and a technician for almost 40 years. His shop in Seattle, Wash., has attained the I-CAR Gold Class distinction and the ASE Blue Seal of Excellence.