Car alignment question
 

I have a 2015 4wd Jeep Grand Cherokee. 25,000 miles and I had pretty bad wear on the outside of both front tires, and inside of both rears. Took the car to the local jeep dealer (first mistake) to get aligned before getting new tires. Service manager comes out, looks at the tires and basically says: yes you need an alignment, but we can't align it until you get the new tires. Aligning the car with the present worn tires will be inaccurate.
I "thought" alignment was based on the wheel/chassis. On a race car I never changed alignment each time I replaced tires.
What am I misunderstanding here?
Thanks
gary

Don’t they mount the alignment tools to the outside of each wheel? I agree that I’ve never heard of tires impacting alignment.

The alignment is done to the rim. You should find a new shop.

When I do it at home I reference off the tire sometimes, but that is not the right way to do it.

I'm not an alignment expert, but I agree with the alignment shop. Yes the alignment machine is connected to the wheel but if the tire is worn uneven, it will show camber that really isn't there.

Very common on the Chrysler products, and on the dealer.

I went in for a drag link recall on my ram and the service advisor tried to sell me an alignment even though the recall did one. They're all money grabbing a holes.

They connect the laser sensors on the wheels not tires.... especially for toe....

Camber would change?
So theoretically we should get a new alignment with each new set of tires?
Just trying to learn/understand. I often know just enough to not understand the real picture
Gary

Now that I've thought about more, I'm changing my mind, a little. The hard points of the suspension shouldn't be affected by the tire wear. But a rubber bushing suspension could deflect differently by the tires being worn uneven vs new. So, does it matter a whole bunch if the tires are worn? Probably not. Does it matter? Yes.

I agree, but if I were getting new tires anyway, I'd do the alignment after, not before. They check tire pressure before they do an alignment (good shops do anyway). So if the tire makes no difference why bother checking the pressure?

Tire pressure affects ride height, which might marginally affect caster/camber?

I can see tire pressure being more important than tire wear.

Either way, for a daily driver it's above and beyond what I do.

Yup, probably not enough to notice on a Jeep though.

So unless there us something really crazy about the tires like different sizes on each corner- it makes no difference. Typical upsell.

Worn tires absolutely affects the alignment settings. Your car is not sitting straight/properly/correctly if they are worn. Think of it like getting it done if the car was sitting on a cobblestone road or some kind of unlevel surface.. Yes most alignment machines hook to the wheel/rims but if that rim is leaning one way of the other because of the tire that is mounted to it, then when the alignment is done, it will be set to that incorrect setting and when the new tires go on, your alignment is off again...

So does a passenger. And a full fuel tank.

^^^^agree to disagree^^^^^ assuming no loose, worn or bent components, the wear on tire is the result of 1 or more of 'planes' out of spec. Installing new tires only does not change the caster/ camber / thrust or toe as those are fixed mechanical settings even though incorrect. The tires having any effect would only be true if the tires were different diameters side to side or front to back.

^^ yes.. almost... Agree that a plane was wrong which caused the tire to wear unevenly in the first place. (we dont know if it came that way or if he hit a curb or etc.. )

The next point, is that the planes cannot be set correctly if the car is not sitting square to begin with - because of the worn tires. Need to get the car square again by replacing the tires and starting from square 1.

Bingo!

Makes sense- kind of.
Again I thought it was based on chassis and wheel- did not realize tires would make a difference.
I will give them the benefit of the doubt- although our local jeep dealer has the worst service department I have ever used.
thanks for helping me understand
gary

I'm with the Gambler here It's a mechanical measurement. If you have one or more tires that are dramatically different diameter then obviously it would affect it but with for fairly equally worn correct size tires no

Need to look at it from the other direction. It appears you're looking at from the suspension (alignment) outward but it doesnt work that way. You have to look at it from the tires inward. If a tire (any or all of them) is not sitting exactly square due to wear, (normal wear or uneven wear) then the suspension angles are altered. If there were no tires involved, then yes, once you set it, its a mechanical setting that doesnt change. But as a tire wears, the result of the angles changes.

In other words, lets say you measure a brand new car. Now drive it 50K miles and recheck the alignment. It will show off because of the tire wear. (even though no mechanical changes were made to the alignment settings. Now If you put new tires on it without making any mechanical changes to the settings, chances are you will be right back in spec. However if you make changes and THEN put new tires on, you will out of spec again as you have aligned the car the the worn tires.

Yes, chassis and wheel, but, imagine if you will, that you had 3 tires sitting on flat level ground, but the fourth tire was sitting in a 2" deep depression (or 2" tall platform, for the purposes of exaggerated illustration of 1 wonky tire). That corner of the vehicle's suspension would be compressed or in droop (and to a lesser extent, at least one of the other corners would be a little off). You couldn't reliably align the vehicle like that. It would be like aligning a vehicle with 3 good tires and a compact spare. You need all 4 tires to be in similar shape and with similar air pressures so that they load the suspension evenly. It would also be like getting an alignment with a car with a horribly wonky corner balance.

So..... I've performed alignments since 1981 professionally. I've used just about every type / brand of equipment and aligned just about everything that would fit on the rack. Further, I've studied this in depth and have attended a lot of training courses over the years. To the point that I taught other technicians 'the rope's' of the business of aligning vehicles. My specialty was problem vehicles that had been to other shops and end result was not satisfactory-or-where vehicles were heavily modified, like a 4x4 truck that was lifted with tall tires, which throws everything off from the manufacturer's original engineering of all the planes of angles working in unison. Specifically steering axis inclination along with axle centerline ( height) point of intersection with the center of the tire tread as it meets the road.

While the summation of this part of my career may or may not qualify me as an expert, at least possibly very experienced. One thing to remember is that when the vehicle is on the rack, all 4 wheels/ tires are sitting on 'floating' plates that allow the suspension to settle naturally.

Remember that the tire wear you see is a direct result of 1 or more angles out of spec, it is a reflection of what is wrong with the settings. Installing new tires w/o changing any mechanical settings will yield the exact same readings/ measurements as before as the vehicle is sitting on floating plates on the rack.

To the OP, depending on the amount and severity of the wear on the tires, the old tires CAN affect how the vehicle drives AFTER a proper alignment if not changed.

^^ Agree, it's just a question of degrees. F1 versus my daily driver.

^ this is why I ask questions on Pelican rather than just look them up on the internet. (also why I preface things I have an opinion on with "I'm not an expert" since there probably is one here)

I've always wondered just what can be adjusted on the suspension of your average daily driver with MacPherson strut front suspension, other than toe in? I've put brakes on my Infiniti FX35 a few times now and I don't recall ever seeing anything that could be shimmed or adjusted other than the tie rod ends.

Cool. Did not know that.

Assuming nothing bent, all part are good, you can easily add camber adjustment by slotting the upper bolt hole in the strut stem to knuckle attachment point. Some aftermarket manufacturers offer camber/ caster adjustment plates on top where the upper strut mount attaches to the tower.

These types of modifications are for the driver that wants more aggressive settings for better handling that are not attainable otherwise.

So when I take my daily driver in for an alignment, they're checking everything, but if anything other than the toe is out of spec. worn or bent parts need to be replaced. Or possibly some time on the Celette bench.

Have to agree to disagree with that last part.. I'm in the same boat though. Professional ASE Master Tech and have been wrenching since 1982.. (you got me beat by one year..lol.) I've also been to numerous classes etc and then taught at the manufacturer level for many years...etc.
I agree about the floating plates obviously as that accounts for the suspension to settle etc, but that doesnt account for a tire that is not sitting flat/straight/true because of wear. Even with the suspension settled, if the inside of your tire has no tread and the outside is full (for whatever reason) then the entire wheel is tilted inward, even with the suspension settled, - alignment angles with it. So if you align the car in that situation, when you put new tires on later, your angles will be incorrect...

It is rather diffcult to try to explain this over text!! I do remember watching a training video on this exact thing WAY back when though and I guess it just stuck with me..

Exactly. I had a F100 back in the '80's that had been hit in the front. Would get it aligned when I bought tires. Then one day the shop that had been aligning it for years told me that it was too badly damaged to align. Steering box was out of whack, yada yada yada. I said you have been aligning it for years. He said yeah, I know but we got this new computerized machine that won't let me. I drove it for few more years without alignment before trading it in. Tires never wore uneven.

Toe and camber for sure. Toe via tierod ends. Camber is usually either via an eccentric bolt where the upright bolts to the strut or via the strut top mount (like a 911).

I think caster is also usually adjustable on the front.

Modern vehicles are so much better engineered, tighter tolerances, tight manufacturing tolerances, tighter assembly tolerances. So unless something is worn out or taken a hit / damaged - camber / caster is usually within range

But, ugh, "within range" can be total crap. I've had plenty of alignments over the years that were "within range" but resulted in a steering wheel that was off (due to laziness, I assume), or required constant tension on the steering wheel to drive a straight line (we aligned it for crown in the road), or had each side of the vehicle at extremes of the range like positive camber on one side and negative camber on the other side.

I just try not to use places that are likely to give that sort of alignment any more.

Thanks again for explaining my question, so that even I can kind of understand.
You guys are great!!
Gary

My Cayman has always handled incredible but even when new it felt a little off in the rain. A few months ago I was driving in heavy rain and it felt like the back end wanted to step out in a scary way. So I took it in to check alignment. Turns out the rear alignment was just a little off. I wonder if it was like that from the factory? All good now but point is even new a car could be out of tolerance.