X5 30d - Strange Staggered Setup?
 

Hi All,

Sorting many many issues with the car at the moment but when I was starting to work on the electronic parking brake actuator the inside wall is down to threads on the rears. So I looked for some tyres but the expected combinations don't match what I have.

The front tyres are "Rotalla Setula s-race ru01" Size 255/45zR20 105y
The Rears are "Rotalla Setula S Race RS01+" Size 275/40zR20 106w

Alloys are Fox FX3

https://i.imgur.com/uZuG6TO.jpeg

https://i.imgur.com/PVwtkWE.jpeg

Holy resize them photos, Batman!


So the sizing looks OK-ish:


https://www.tacomaworld.com/tirecalc...5r20-275-40r20


The key is the 'revs per mile'.... THIS is the important thing for 'not damaging the differentials'


695 vs 704, or 9 dellta.... we like to see this closer to 1%...1,5% is close.


Your problem on the inner edge wear is too much toe. Set your toe to 0.01 to 0.02 toe in on EACH wheel. And set camber to 'mid spec', and BALANCED side to side.


https://xoutpost.com/bmw-sav-forums/...alignment.html


https://xoutpost.com/bmw-sav-forums/...alignment.html


Lots more too. GL!

PS. If you buy new tires, get the revs/mile to within 1%.


All OE tire sizes- no matter what the rims, 18, 19, 20, 21, the outer tire diameters are ALL the same...and front and rear are likewise usually nuts on: revs/mile within 0 to revs/mile

Ard, your a welcomed response to my thread! I always do quite a lot of research I'm sure I have one of your posts bookmarked (your wheel alignment specs that looks like a mathematical algorithm) but I was actually going to possibly DM you before I took my car to Chemix to be aligned after the tyres.. Low and behold you replied on here now anyway :D

So yes, I believe by revs/mile your talking about the revolutions the tyres do per mile, the smaller the difference the less the differential has to compensate?.. Right now your saying I'm at about 1.5% difference?

When I do go to get some new rear tyres (the 275 ones) what exactly can I do to get this back into 1%? Is it to do with the second number (aspect ratio?)

And when I go to Chemix in Stourbridge (I'm from UK) There's a chap there called Simon and he's so good at alignments, just has a huge amount of knowledge on different cars and setups and spends ages trying to do a good job, we worked for ages on my E90 but unfortunately I kinked the rear subframe and he adjusted everything to get it as best as possible before I spent weeks replacing the subframe to get the final alignment.

So yeah I'll probably show him your post when I go so we're on the same page, I'm all for your "upto 40k" with a good alignment!

Yes, ‘how many revolutions the tire makes in a mile of driving’…. The smaller the differnfece the less the ‘differential’ needs to ‘accomodate’. More difference = heat and abuse.


When you look up a tire on, say, tirerack, for that tire in that size they will list ‘revs per mile’

You match those.

You can also just look at the ‘outer diameter’ of the tire, and match them

You dont ‘build’ a ‘revs/mile’ by choosing the numbers- say 275 and 40 or 265/45… it can be VERY tire specific. A Michelin in a 275/40-20 may have a very different t revs/mile compared to a Pirelli 275/40-20.

Interesting, I thought I had it almost figured out as I used a calculator and matched the revs/mile which you quoted above so I assumed that calculator was right. I then changed to the next tyre width up (285 from 275) which gave me a crazy close revs/mile but I really struggled to find the manufacture specifications for the revs/mile and not only that, going to 285 from 275 had a pretty steep increase.

I was originally looking at the Avon ZX7 275/40/20 for £148.95 per tyre but oddly enough not even the Avon website appears to have the revs/mile details

https://www.avontyres.com/en-gb/tyre...pe=car&speed=K

In the meantime I will check tirerack like you said as I've almost got the new parking brake actuator on the car and with ribbons showing on the rears I need to get them ASAP.

Going from a 285 (say 40) to a 275/40...


the change is 10mm wider, but 4mm taller- on each side, so 8mm more in diameter...8mm is a lot.


There are some tire calculators that will give you revs/mile for a set of numbers, but each mfg has a bit differnet specs. Not every - for example- 275/40-18 will have the same revs/mile


GL


Oh, BMW OE fitments are pretty solid matches. What does BMW put on their 20 inch rims? That should be fine


edit...but if you already have an oddball set on the front you may never find a good match.

Because of the timing of things I've just went ahead with the Avon ZX7's with the same 275/40/20 as what's on their now giving me an approx 1.5% difference. I'm going to research what differential fluid to use and buy some transfer case fluid from BMW, its not the best outcome but my gosh this cars been running by somebody who doesn't give an absolute crap about it so with everything I'm doing I'm hoping it will be alright and if I'm not mistaken, 1.5% isn't ideal, but it's not really bad is it?

And, it's Chemix time very soon! So we're on the same page:

front toe negative 0.02 both sides
front camber balanced does that just mean within the machines/bmw' s camber specs

rear toe and camber just following the machines spec as good as possible?

Update

Went back through the links and member @JBF posted their before and after alignment which you said looked good. Both front and rear toe aiming for your target of negative -0.01 to -0.02 which is exactly what they managed and camber for both front and rear was as close to the machines spec

I think I'll just take that and ask Simon to match as close as possible.

That is a great diagram and good set of numbers.


DO keep in mind you want toe *in* so positive toe. Toe out is no fun.


Actually, with the slightly larger tire up front, it will be under more weight- (unladen, the front of the x5 is WAY heavier) hence the revs per mile, the ACTUAL rolling diameter will be tad smaller. It will 'self correct' the size mismatch. Same psi front and rear, your rear will 'roll a bit taller'. IMO


Unless you are driving 600lbs in the rear plus 4 people all the time... ;)

I thought when you mentioned it before it was just a mistake, but my fronts are 255 and tears are 275 so the bigger tyres are on the rear.

Also, I have the 7 seater x5 because there's 6 of us. Two bigger adults up front, two children in the very back (3rd row) and baby and child in middle row. So it's quite a change but the car will go from just me in it to 6 people.

And got it, positive toe! Negative would have my wheels trying to leave the car haha.