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.