Is "One tire replaced, must replace the other three" a scam???

[ard]

I hear of this alot.

I personally beleive it is a scam to sell tires. What is the truth?

Lets analyze this on two levels:

1. BMWs requirements, to comply with warranty. This is limited to DOCUMENTED information, not "what my SA said" or "What BMW said at the 800 number"

2. The technical truth.


OK, my two cents:

I have Michelin Latitude Touring ZP on my car 255/50-19. Tread depth new is ~10/32.

They rotate at 720 revolutions per mile, with an OD of 29.1"= per published specs

The KEY to these kinds of questions is the "rev/mile"... mismatches between one wheel and the other, in the wheel's revolution speed, is the direct cause of differential damage- hence looking at the rev/mile mismatch is what matters.

Lets calculate the change in rev/mile between a new tire and a fully used tire.

New= 720 at 29.1" diameter

Used = 8/32 of wear, or 0.25in or 6.35mm

The radius of rotation is 29.1 /2 = 14.55in

the used tire will have a radius of 14.55 - 0.25 = 14.30

To calculate the revs per mile that a 14.55/14.30*720 = 732

So, a fully used tire will rotate 12 times more, each mile, as compared to new. The differential will need to 'dissipate' this speed mismatch...

Importantly, if the other three were fully used, you'd get new anyway. So lets say your other three 'perfectly good' tires are at 50%. A 50% tire will have 0.125" of wear, and will come in at 726 rev/mile. So a total of 6 revs differnt from new, or just under ONE PERCENT.

In my opinion, the technical design specs for an AWD system should be able to tolerate SOME total rotational mismatch ..HOW much is the subject of discussion...

Thoughts?

A

[FSETH]

Don't have specific info or numbers to share, but I agree with you. I don't see the reason to replace 3 tires because you HAVE to replace one. The differences in diameter between two tires of the same size (one used and one new) should not be significant enough to cause any damage, IMO. Many people buy tires in pairs...

I would replace two at a time at minimum, but not all four because of one tire unless I was due up for four new shoes as you have already stated.

[TQ2K]

Welcome and nice to see you here Ard.

[rayxi]

If you are replacing with the same tire (brand, model and size) and the existing 3 have servicable tread then you are good to go.

[swissfrank]

your calculations are great, glad you're "thinking"

i think 12 rotations over a mile should well be within tolerance. Having a slightly under or slightly over inflated tire probably accounts for a much larger discrepancy than your thread wear.

[ard]

I've had one tire store INSIST they cannot replace jsut one- 'Insurance requirement: we had to replace a transmission once".

One recent thread here a fellow was stating he had to replace all 4 tires due to one being unrepairable, and wished he had the tire insurance (never mind the insurance would not cover the other three..)

You hear it a lot.

Personally, I' probably replace the two on one axle if the 'good' one was under 50% (and keep it as a spare)

I suspect the active control of torque front to rear will be more able to handle the minor difference- whereas the side to side mismatch has a direct impact on the mechanicals within in the differential

A

PS thanks for the welcome. Just added a 2010 X5 Diesel...

[anerbe]

^ purchase by axle, not corner

although there could be situations where one tire would get much more wear (alignment off on a corner), and running would be alright, you figured out it will ultimately cause more work for the diff being always in action.

it's unknown how much it will affect longevity of a diff, but usage is wear, and wear isn't good. perfect idea to use as a spare.

also, tires with two different wear and age will act differently. you don't want this left and right, as it will cause unpredictable handling ( turning left may understeer, but turning right may induce oversteer). when you buy a new tire, you aren't 100% certain if they kept the same compound/construction on the tire if it's 3 years newer than the rest of your tires.

[blktoptrvl]

Originally Posted by ard

I've had one tire store INSIST they cannot replace jsut one- 'Insurance requirement: we had to replace a transmission once".

One recent thread here a fellow was stating he had to replace all 4 tires due to one being unrepairable, and wished he had the tire insurance (never mind the insurance would not cover the other three..)

You hear it a lot.

Personally, I' probably replace the two on one axle if the 'good' one was under 50% (and keep it as a spare)

I suspect the active control of torque front to rear will be more able to handle the minor difference- whereas the side to side mismatch has a direct impact on the mechanicals within in the differential

A

PS thanks for the welcome. Just added a 2010 X5 Diesel...

I suspect you are speaking of my previous post...

If so, then, for me, I would have been glad to replace just one. But when it came right down too it, my other 3 had no more that 6-8 K miles left, and I needed to drive 3500 miles.

I did not want to drive across the country - especially in the rain soaked conditions on 3 worn out tires. As it was, in one section of the drive I could feel the car plane with 4 new tires so I was glad to replace them all.