I'll never trust an alignment shop again.

[BurrowZ]

Thrust angle is all based off of the rear axle, going forward. An incorrect toe-in or toe-out reading will throw off this thrust angle.

When you align the front according to the rear, you are also aligning to the thrust angle which ensures the vehicle will not "dogtrack" if correct. Although this is more common in vehicles with a solid rear axle, independent rear axles can suffer from this too.

This is why you need to "sweep" the front end after every adjustment(at least on the Hunter), or when switching to the front axle after doing the rear. It then shows the correct, updated readings for the front after the rear adjustments have been made, taking into account the new thrust angle.

 

[91DSMdave]

And I forgot that I also need a couch..so smart strings could be even further off than I thought.

Here ya go!

best of craigslist: Big Nasty Couch

Its amazing what you can find on craigslist.

Why do you think that you have to do the rear first? It's a car, not a Kardashian.

On the alignment note, thats rediculous. If I would have made a mistake like that in school, my instructor would have cut my finger off!! Let alone an "experienced employee".

 

[jtmcinder]

Yeah, I know about using front toe to deal with cars with non-adjustable rear toe, which is why I've been careful to say "2G DSMs" in several of my posts. But cars with non-adjustable rear toe usually also don't have rear camber adjustments, either - mostly because a large number of cars with non-adjustable rear toe are like that because they're a form of live-axle. Thus, it doesn't make much sense to say that you have to start with the rear of these cars, since there is absolutely nothing to adjust in the rear of those cars.

In any event, all of this is irrelevant to 2Gs. Without a good reason - and I sure as heck haven't seen one yet - I would do the front first. Not only do you align the front before the rear, you usually corner-weight the front first, as well. Again, the reason is that it's a front-heavy car with higher front wheel-rates.

 

[DSMnoobsause]

So Since the rear is non adjustable you assume the rear will
Stay the same? Any pothole or corner can bend or damage a suspention component. If the back was checked and found to be out of spec with a non adjustable suspention wouldn't that indicate a component is bent, damaged or worn out?

 

[PieEyedPiper]

So Since the rear is non adjustable you assume the rear will
Stay the same? Any pothole or corner can bend or damage a suspention component. If the back was checked and found to be out of spec with a non adjustable suspention wouldn't that indicate a component is bent, damaged or worn out?

This is what I've been thinking to myself all along. To be honest I'm fascinated you're taking this position, Jtm. To assume the rear is exactly how it is supposed to be is like saying centerline/thrust angle is irrelevant all together and would always be perfect. It seems you're trading a car that drives straight for one that gains whatever benefit doing the front first apparantly gives a front heavy car. :shrug:

What would I need to do to convince you that the rear ought to be done first and that such things as thrust angle are relevant? Because you've not exactly convinced me (I'm assuming you care) of your position. But I'm all ears.
Oh, and I loved one of your supporting statements regarding doing the rear first on vehicles that don't have rear adjustments. You must still be getting used to that borrowed keyboard. Either that or it's a terrible joke.

mostly because a large number of cars with non-adjustable rear toe are like that because they're a form of live-axle. Thus, it doesn't make much sense to say that you have to start with the rear of these cars, since there is absolutely nothing to adjust in the rear of those cars.

 

[jtmcinder]

Jeebus. I never said that you don't align the rear (assuming that it's adjustable); I just said that there's no good reason to START with the rear when both ends are adjustable and there is a very good reason to start with the front.

 

[NWHTanK]

Ok the front and the rear are on seprate planes.If said rear was out of spec it has nothing to do with the front alignment EXEPT traking. U could have messed up toe and or camber in the rear but its only going to affect at most tracking.

 

[PieEyedPiper]

Jeebus. I never said that you don't align the rear (assuming that it's adjustable); I just said that there's no good reason to START with the rear when both ends are adjustable and there is a very good reason to start with the front.

I know you didn't. You said that you don't start at the rear of cars that don't have rear adjustment. The fact that you spelled that out and seemingly used that bit as supporting evidence that you don't start at the rear of any car is the joke. I happen to know that you are actually more qualified than I am to identify some odd form of red herring or straw man.
There's nothing actually wrong with your statement other than the part where it's seemingly being used to mislead/misdirect/derail/discredit the real issue. No shit you wouldn't start at the rear of a car with no rear adjustment. Jeebus.


Honestly, you ought to be a lot more forthcoming. This is ludicrous.

So far the known world disagrees with you and you're seemingly content with that. That doesn't mean you're incorrect though, that'd be an error in logic too large for even myself. I'm in no way trying to start something from nothing but If I may be frank: I'm frustrated.

I have professors who gets kicks out of watching me squirm. I don't need that here.

I'm asking. Will you tell us?

 

[DSMnoobsause]

I to would like to see your explanation. And an explanation why in technical school's they are feeding false info about the proper way to complete an alignment. As You are implying it is not nessisary to check the rear first.

 

[DSMnoobsause]

Ok the front and the rear are on seprate planes.If said rear was out of spec it has nothing to do with the front alignment EXEPT traking. U could have messed up toe and or camber in the rear but its only going to affect at most tracking.

Bad tracking would cause abnormal tire wear on all four wheels not just the affected corner. If the back tracks and the front is aligned straight you will still have to turn the wheel slightly to correct for the track. Then the front would not have proper settings due to suspention geometry. Cars are made to drive straight not to track. That's why suspention pivot points are set equal from side to side.

 

[jtmcinder]

I have no idea what more I can tell you guys. I already told you why you start at the front: because changes to the front are more likely to alter the rear than the other way around. If you do the rear first, then, after you've done the front, you have to do the rear again. If you do the front first, then, after you've done the rear, you're done.

I then explained why this occurs: because the wheel-rates in the front are almost always higher, such that small changes in one front corner's ride-height will alter the rear more than a small change in rear ride-height will alter the front.

As to the question of why tech schools teach you to do it the other way around: if you want my honest and blunt opinion it is because most tech schools treat students as mere objects to obey orders, as opposed to people that should know how things actually work and, therefore, why things should be done a certain way. Thus, because some cars don't have rear adjustments, they get that out of the way first and always end at the front (or some other equally silly reason). The key is that tech schools (and ASE cert, for that matter) has little to do with understanding a lot more to do with teaching people to follow the manual without asking (annoying) questions.

As to whether it bothers me when I disagree with most other people: only a little. I'm self-aware enough to know that being in the minority is correlated with being wrong. So this situation always makes me think it through a few more times. But when I keep coming back to the original position, it ceases to bother me and can often switch to being a point of pride. I'm one of those people who knows what the Beatles' "Fool on the Hill" was really about.

Now my fingers hurt (since I stink at typing) ... cheers

 

[jtmcinder]

Bad tracking would cause abnormal tire wear on all four wheels not just the affected corner. If the back tracks and the front is aligned straight you will still have to turn the wheel slightly to correct for the track. Then the front would not have proper settings due to suspention geometry.

You are seriously over-estimating the amount of Ackerman in most production cars, but are, at the core, correct.

 

[DSMnoobsause]

I now deal with racecar setups where small changes make big differences. The same applies to BMW's and other similar performance cars. I speek from experience not from a book.

 

[jtmcinder]

Then you already know that BMWs have more Ackerman than any car with powered front wheels and, thus, how the experience you "speek" of has little to do with DSMs.

 

[NWHTanK]

Bad tracking would cause abnormal tire wear on all four wheels not just the affected corner. If the back tracks and the front is aligned straight you will still have to turn the wheel slightly to correct for the track. Then the front would not have proper settings due to suspention geometry. Cars are made to drive straight not to track. That's why suspention pivot points are set equal from side to side.

Are u gonna ajust the front to compinsate for the rear NO. If u do then u are a crapy tech and not fixing the problem with the car or truck. Rember its called tracking for a reason.U can still track and and still not show wear on your tires.

 

[DSMnoobsause]

Are u gonna ajust the front to compinsate for the rear NO. If u do then u are a crapy tech and not fixing the problem with the car or truck. Rember its called tracking for a reason.U can still track and and still not show wear on your tires.

Jtmcinder is stating that. My statement was saying that a car was ment to drive straight and is not supose to track. That's why you check the back and then repair and or adjust where needed and then adjust the front.

 

[jtmcinder]

There are actually a bunch of cars out there with no way to adjust rear toe (e.g., most live-axles), so you are often stuck with a non-zero thrust angle unless you are willing to do some more serious work. It would not surprise me one bit if techs are taught to set the front to match the rear in this situation and, because of Ackerman issues, that actually makes sense. Even if the car is driving down the street with the body slightly rotated relative to the direction of travel, that's better than having to steer to the side to get the car to travel in a straight line.

In short, I agree with Sauce on that one.

edit: my post crossed with the above .. I have no idea why you are saying that I said anything about this and I do not see this as providing any reason for aligning the rear first. In fact, given that we're now talking about cars that don't allow you to align the rear, it's incredibly silly to argue that these cars provide support for the idea that you should align the rear first. But if it makes you happy, then let me admit right now, in public, that you are correct that cars that do not allow you to align the rear should always have the rear aligned first.

 

[NWHTanK]

There are actually a bunch of cars out there with no way to adjust rear toe (e.g., most live-axles), so you are often stuck with a non-zero thrust angle unless you are willing to do some more serious work. It would not surprise me one bit if techs are taught to set the front to match the rear in this situation and, because of Ackerman issues, that actually makes sense. Even if the car is driving down the street with the body slightly rotated relative to the direction of travel, that's better than having to steer to the side to get the car to travel in a straight line.

In short, I agree with Sauce on that one.

edit: my post crossed with the above .. I have no idea why you are saying that I said anything about this and I do not see this as providing any reason for aligning the rear first. In fact, given that we're now talking about cars that don't allow you to align the rear, it's incredibly silly to argue that these cars provide support for the idea that you should align the rear first. But if it makes you happy, then let me admit right now, in public, that you are correct that cars that do not allow you to align the rear should always have the rear aligned first.

Im sorry but I diagree u can adjust toe and and tracking on the rear. Fixing frame issuses and replacing bent parts also the rear has adjustment including live axel cars by lossening the bolts and shifting said part.

 

[DSMnoobsause]

Then you already know that BMWs have more Ackerman than any car with powered front wheels and, thus, how the experience you "speek" of has little to do with DSMs.

Im Very familiar with BMW's and MINI's since they are BMW's now. so i do have a good idea on all platforms. I have personally tracked and raced many different platforms, AE86,240sx,350z,240z,300zx,Dsm,BMW and Honda.
I have had success with checking the rear first. then adjust if necessary then moving to the front and adjusting. there are big differences in suspension systems and how they work but you can take the same rules applying them to others and have success.

 

[jtmcinder]

I should also note (before logging off for the night) that I'm beginning to suspect that Sauce and I are not talking about the same thing, which might explain the difference of opinion on order of alignment. He seems to be talking about situations where you might change your mind about the target front alignment settings because you can't align the rear in the way that you wish. I've never really thought about this, probably because I've never had a car that wouldn't allow me to do whatever I wanted in the rear. I doubt that I'd change my mind about how I wanted to do the front even if I couldn't do what I wanted in the rear, but I suppose that if I knew I might not be able to get anywhere close to what I wanted in the rear, I might do it first to see if it was so bad that I'd change my mind about the front.

I've been talking about 2G DSMs, where you can always get what you want in the rear. For these cars, you do the front first for the reasons I've given: because adjustments to the front are much more likely to change the rear than v.v.

 

[jtmcinder]

Im Very familiar with BMW's and MINI's...

Then throw one of each on the rack and do the following: align the front to zero toe (both sides) and then steer left until the right front is at 5* and write down what the left front is at. This will show you something very important about well-designed rear- vs well-designed front-wheel drive. Both of those cars have excellent designs; they both can turn inside a 2G any day.

 

[NWHTanK]

When u set up the machine what order does it have u do? Top to bottom and front to rear.

 

[DSMnoobsause]

I should also note (before logging off for the night) that I'm beginning to suspect that Sauce and I are not talking about the same thing, which might explain the difference of opinion on order of alignment. He seems to be talking about situations where you might change your mind about the target front alignment settings because you can't align the rear in the way that you wish. I've never really thought about this, probably because I've never had a car that wouldn't allow me to do whatever I wanted in the rear. I doubt that I'd change my mind about how I wanted to do the front even if I couldn't do what I wanted in the rear, but I suppose that if I knew I might not be able to get anywhere close to what I wanted in the rear, I might do it first to see if it was so bad that I'd change my mind about the front.

I've been talking about 2G DSMs, where you can always get what you want in the rear. For these cars, you do the front first for the reasons I've given: because adjustments to the front are much more likely to change the rear than v.v.

I will agree to that. I to think we were not on the same Page. I Handle cars that cant be dealt with without changing something in the suspension. like lengthening or shortening control arms to accommodate camber gain. also adjusting the pivot points. shifting control arms to give caster etc.
If we will, I would like to end it with this statement from my Since passed redneck father. "there is more then one way to skin a cat" ha ha and i believe that could be true in this circumstance.