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      01-12-2010, 05:19 PM   #1
craig2859
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Question Wheels & sizes - confused...

Help I am "confused" .... lol


Have been watching and looking at all the "wheel" threads over the last few months, and it seems such a minefield, ie spacers, clearance etc, etc.

I am clueless with what wheels I need myself, and do not want to steal anyone's thread, so am asking for some "clear" advice here.


In the next few months I will be looking at getting some new wheels - style not 100% on yet, and thats not the point of this post.

However what I would like to know is what wheels do I need to look for size wise - so I need 2 sets of figures really:

20" Wheels - what is the complete size I will need, and also what tyres should I be looking for to match, also will I need any spacers, etc, etc?


19" Wheels - what is the complete size I will need, and also what tyres should I be looking for to match, also will I need any spacers, etc, etc?


Like I say if someone can let me know in laymans terms exactly what size to look for and what additional bits I may need - with tyre sizes for both sizes that would be great - I can then start looking

My Car is a 06 330d M Sport - 4 door saloon, no suspension mods or anything.




Thank you
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      01-12-2010, 10:31 PM   #2
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Talking

So many wheel threads recently


Right, here goes...


There are three essential things when it comes to deciding what wheels to get -

1) Size in diameter (eg 17", 18", 19")
2) Size in width (8J, 9J, 10J etc... ps.. 8J = 8" wide)
3) Offset

For an E9x, the bolt pattern or PCD must be 5x120 - this never changes regardless of the alloy wheel you put on.


Point 1

The most important goal, ignoring the style of the wheel, is that the wheel fits underneath the arches without complication. The actual diameter of the wheel is not actually that important - there are other aspects that should be given greater consideration to. In theory, assuming other parameters of the wheel stay constant (ie. width and offset) you should be able to get even a 21" wheel to fit underneath the arches on a 3 series. The reason is that the entire diameter of wheel and tyre together can be maintained at a roughly constant figure by altering the sidewall profile of the tyre. In other words, if you have a 19" wheel with x mm of tyre sidewall, and want to increase to a 20" diameter wheel, you can reduce the sidewall dimensions of the tyre so that the overall size doesn't change. The downside is obviously less compliance in the tyre, amongst other things.

The diameter of the wheel is actually the least important aspect when it comes to choosing a wheel that will fit.



Points 2 and 3 above are connected and should be considered together.

The overall width of the OEM rear 19" wheel on a BMW 3 series is 9J, or 9 inches. The wheel itself sits in the space between the spring/damper and the inner rear wheel arch. There is a finite amount of space between the two items, within which the rear wheel must be able to turn freely under full load and compression. If you start increasing the size of the rear wheel in terms of its width, ie. go from a 9J to a 10J (or 10 inch) wheel, then you will use more of the available room between the strut/damper and the inner wheel arch. If you go to a 12J wheel, there probably isn't enough physical space.

That's the first, basic aspect of wheel size to consider.



Now look at this offset diagram below


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You can see that the offset effects the position of the mounting point of the wheel onto the hub. If the offset is ZERO, the mounting point to the hub is in the centre line of the wheel, and therefore the inner and outer wheel faces are equidistant either side of the hub surface.

A wheel with a positive offset (ET > 0) will move the mounting point to the right of the centre line, therefore moving the entire wheel inwards towards the strut/damper.



Let's take this example -
  • Assume a standard E92 has an 9J x 19" rear wheel, with an offset of ET39. That means the wheel is 9 inches wide, and is positively offset by 39mm. Let's assume there is 20mm of clearance when the suspension is uncompressed between the outside of the 9J OEM wheel and the inside of the rear wheel arch.
  • We now change the wheel to a 10J x 19" wheel, still with an offset of ET39. The overall width of the wheel has increased by 1 inch, or by 25.4mm. This is an overall increase, so effectively it is an increase of 12.7mm either side of the centre line. Because we have assumed the offset is still ET39, the relative position of the centre of the wheel in relation to the hub has not changed. Therefore the wheel has grown equally on both sides and now protrudes 12.7mm further into the arch, and 12.7mm closer to the strut.
  • Remember we assumed there was only 20mm of clearance with the original 9J OEM wheel? Now with the 10J wheel there is only 7.3mm of clearance left, when the suspension is uncompressed. When the suspension is compressed and the wheel pushed into the arch, that 7.3mm of clearance remaining becomes a serious issue.
  • To compensate for this, you can alter the offset of the wheel. Increase the offset from ET39 to ET44 gives an increase of 5mm. This means the entire wheel is shifted in towards the strut/damper by 5mm. Therefore this same 10J wheel now with an offset of ET44 has 12.3mm (7.3mm + 5mm) clearance between the wheel arch, but equally 5mm less clearance to the strut/damper. Remember also that the spokes of the wheel will also be 5mm closer to the hub/brake caliper/brake disc surface



You can hopefully see that it's a balancing act when choosing the right size of wheel.

An 8J x 20" ET37 wheel will fit within the arch just as well as an 8J x 17" ET37 wheel. All you have to do is alter the tyre sizes so that the same overall diameter, and therefore rolling circumference, remains constant.



Example -
  • 8J x 17" ET34 with 225/45/17 tyres - has an overall diameter of 24.97 inches
  • 8J x 20" ET34 with 225/30/20 tyres - has an overall diameter of 25.31 inches
  • The difference between the two is 1.3%, which is perfectly acceptable.
  • Both sizes of wheel will fit, because overall they are the same size and width, and located or offset relative to the hub in the same dimension.



So, the bottom line is this -

You can choose any size of wheel, whether it be 17" or 20". If you keep the width dimensions and offsets the same as BMW OEM, then you won't have any problems. The ride will be the only thing that suffers as the sidewall profile decreases.

If you start increasing the width of the wheels, then the offsets will likely need to be altered a notch if the change is dramatic. Remember that the wider the wheel, the wider the tyre, and that also starts eating into available room under the arches.



As a rule of thumb -

1) Front wheel
  • OEM 8J front wheel is between ET34 and ET37 (depending on model)
  • 8.5J front can be installed, with an offset of approximately ET35 - ET40
  • 9J front can just be squeezed in, but with a high offset of about ET42

I wouldn't recommend a 9J front unless you plan on making your car a track car. 8.5J is the widest I would suggest, and ET40 will give the most clearance to the inner arch, without compromising turning circle


2) Rear wheel
  • 9J OEM rear wheel is between ET37 and ET39, depending on model
  • 9.5J rear can be installed, with an offset of ET40
  • 10J rear can be installed, with an offset of ET40 (but you have to go skinny on the tyres, or roll the rear arches to create more room)
  • 10.5J rear can be installed, but ideally with an offset of around ET44, and you'll have to roll the rear arches and probably dial in more negative camber (tilt the wheels in vertically) to give more room. You will be getting close to the strut/damper in this configuration.



For what it's worth, my suggestion is to use either an 8J front and 9J rear combination, or an 8.5J front and 9.5J rear together. 19" wheels is the largest I would be happy with personally, as the ride hasn't deteriorated too much, but more importantly the weight of the wheel hasn't increased drastically. A 20" wheel is typically much heavier in comparison to a 19" and adds a huge amount of unsprung weight to each corner of the car, making the handling much more heavy, more cumbersome, and the ride quite atrocious.



Tyres

Tyre sizes are chosen to match the wheels used, and depend largely on the width of the wheel. There are combinations of sizes front & rear that are used together to maintain the same relative rolling circumference.


For instance -
  • Example 1

    Front - 8J x 19" or 8.5J x 19" = 235/35/19 tyre
    with
    Rear - 9J x 19" or 9.5J x 19" = 265/30/19 tyre
  • Example 2

    Front - 8.5J x 19" = 235/35/19 tyre
    with
    Rear - 10J x 19" = 275/30/19 tyre (just about, might need arch surgery or increased offset to ET42 or so)

    or

    Front - 8.5J x 19" = 245/35/19 tyre
    with
    Rear - 10J x 19" = 285/30/19 tyre (will need arch surgery!)
  • Example 3

    Front - 8.5J x 20" = 235/30/20 tyre
    with
    Rear - 10.5J x 20" = 275/25/20 (will need major arch surgery!)

    or

    Front - 8.5J x 20" = 245/30/20 tyre
    with
    Rear - 10.5J x 20" = 285/25/20 tyre (will need major arch surgery!)



Finally... Spacers

Once you've chosen your wheel and tyre combination and mounted it on the car, you might suddenly realise that there is more room to the inner wheel arch than you expected, and you want to push the wheels out as far as possible to create an aggressive stance. To do this, you can use a hub-centric spacer which bolts between the hub surface and the mounting plate of the wheel. That's all a spacer does - it pushes the wheel further out to the side of the car by a predetermined amount. A 5mm spacer pushes the wheel out by 5mm, which is the same as reducing the offset of the wheel by 5mm (eg. going from ET39 --> ET34)



So... if you have a question along the lines of "Will E46 wheels fit and what size tyres do I need?", it's quite easy to work out for yourself whether they fit. If the bolt pattern is 5x120, that's a start. If the width of the wheel is an 8J and the offset is ET35-ish, then yes it will work. If the offset of the E46 wheel is ET20 for example, then you're 15mm too far out into the wheel arch, and it probably won't fit. If the offset of the E46 wheel is ET50 compared to the E9x standard ET34, then the wheel will sit 16mm too far inwards in comparison. However a 15mm spacer will compensate for that creating an effective offset of ET35, by pushing the wheel 15mm further out.

Clear?! Hopefully...



I think that covers pretty much all the basic aspects when it comes to choosing alloy wheels
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      01-13-2010, 04:10 AM   #3
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Quote:
Originally Posted by E92Fan View Post
Remember also that the spokes of the wheel will also be 5mm closer to the hub/brake caliper/brake disc surface
Great write up Tony

Just a point - changing the offset doesn't have to move the spokes in relation to the mounting face or change your brake clearances....

If a rim width goes up by 1inch (call it 25mm) then if they add 12.5mm to each rim lip the offset would be the same, however for example, if they added all 25mm to the inside lip, the offset would move by inwards 12.5mm, but the spokes would not move in relation to the mounting face, only the tyre/rim position would change

But I suppose with different offset versions of the same wheel a manufacturer would just machine more off the mounting face to increase the offset, thus moving the whole wheel and spokes toward the brake.

So it is not 100% that a wheel will fit based on offset, width and diameter. Brake clearance could still be compromised even if the other parameters are met - is that right?

Also,

As I'd guessed from looking, BMWs have a positive offset wheel, but that is not clear in the 'ET' value, or is positive assumed with no sign?,so negative offset wheels would be 'ET-xx'?

Cheers!
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      01-13-2010, 07:02 AM   #4
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Quote:
Originally Posted by doughboy View Post
Great write up Tony

Just a point - changing the offset doesn't have to move the spokes in relation to the mounting face or change your brake clearances...

If a rim width goes up by 1inch (call it 25mm) then if they add 12.5mm to each rim lip the offset would be the same, however for example, if they added all 25mm to the inside lip, the offset would move by inwards 12.5mm, but the spokes would not move in relation to the mounting face, only the tyre/rim position would change
That is correct. However in practice that isn't how a wheel is made and if the wheel design changes from a 9J to a 10J, the extra 1J isn't just added to the inside of the wheel as that would change the fundamental rotational balance of the wheel drastically. Going up a from a 9J to a 10J will in 99% of cases result in a equally distributed increase around the centre line.

Quote:
Originally Posted by doughboy View Post
But I suppose with different offset versions of the same wheel a manufacturer would just machine more off the mounting face to increase the offset, thus moving the whole wheel and spokes toward the brake.
Again this is correct. Most ET adjustments are in the range of +/- 5-10mm. A particular wheel design will generally only offer ET within a 10mm range, because any more than that and the balance of the wheel changes too much. The easiest way for a manufacturer to offer a range of ETs is to, as you have said, machine off the mounting point. They won't cast a separate wheel for each ET as it will cost too much money. Hence why the position of the spokes might change relative to the hub surface and brake calipers.

Quote:
Originally Posted by doughboy View Post
So it is not 100% that a wheel will fit based on offset, width and diameter. Brake clearance could still be compromised even if the other parameters are met - is that right?

Correct again. Even if you have the offset, width and diameter all sorted, you still need to make sure the spokes clear the brake calipers sufficiently. This is even more important if you have upgraded brakes, like APs, which take up more room than the standard OEM calipers.


Quote:
Originally Posted by doughboy View Post
Also,

As I'd guessed from looking, BMWs have a positive offset wheel, but that is not clear in the 'ET' value, or is positive assumed with no sign?,so negative offset wheels would be 'ET-xx'?

Cheers!
Positive is assumed with no sign - ET35 = ET +35mm.

If you have a negative offset wheel (very rare) then it will be signed -ET35 or ET-35
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      01-13-2010, 07:20 AM   #5
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Thanks Tony, very informative indeed, your post would be a good sticky actually?


So the biggest and widest I can practically go (not saying I will of course), without any work, ie Buy and fit would be this example you posted:

Example 3

Front - 8.5J x 20" = 235/30/20 tyre
with
Rear - 10.5J x 20" = 275/25/20 tyre


and if I where too stick with 19"

Example 1

Front - 8J x 19" or 8.5J x 19" = 235/35/19 tyre
with
Rear - 9J x 19" or 9.5J x 19" = 265/30/19 tyre


Both of these set ups would be a case of simply buying them and fitting them as normal (not to bothered about discussing ride quality etc - at this point, I will ask that question when I know what to go for) ?


.
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      01-13-2010, 08:20 AM   #6
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Quote:
Originally Posted by craig2859 View Post
Thanks Tony, very informative indeed, your post would be a good sticky actually?


So the biggest and widest I can practically go (not saying I will of course), without any work, ie Buy and fit would be this example you posted:

Example 3

Front - 8.5J x 20" = 235/30/20 tyre
with
Rear - 10.5J x 20" = 275/25/20 tyre


and if I where too stick with 19"

Example 1

Front - 8J x 19" or 8.5J x 19" = 235/35/19 tyre
with
Rear - 9J x 19" or 9.5J x 19" = 265/30/19 tyre


Both of these set ups would be a case of simply buying them and fitting them as normal (not to bothered about discussing ride quality etc - at this point, I will ask that question when I know what to go for) ?


.


No not quite, I think you misread - Example 3 in all configurations will need major work, because in that example the rear wheels are 10J or 10.5J wide.

The key is the width of the wheel, not the diameter. The wheels with the widths in Example 1 are the only ones that will definitely fit without need of, or with very little, thought, because they are either 8J or 8.5J on the front, and 9J or 9.5J on the rear.

So you look for wheels with that width, and choose a diameter thereafter. So you can use an 8J x 18" front wheel, or a 8.5J x 20" front wheel - it doesn't matter as long as the width is within the specified range. Same thing for the rears.

Remember, the diameter of the wheel is unimportant - the issue is the width
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      01-13-2010, 08:38 AM   #7
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275 tyre will fit a 9j rim. Close to rubbing though. 265 is fine.
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      01-13-2010, 10:05 AM   #8
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cheers for all the info everyone, at least I know what to be looking for now.

My plan is once I get some new wheels and tyres to replace my current OEM 19s I will change them to a winter set of tyres for next year (leaving my new ones for summer use), any suggestions on decent "winter" tyres, while its fresh in peoples minds with this current weather ?
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      01-13-2010, 10:19 AM   #9
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These be your fellas:

http://tyremen.myshopify.com/collect.../19inch-winter
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      02-19-2010, 04:14 PM   #10
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some great advice here!
Sorry for asking a stupid question, but why do BMW put 255wide on their 9J rears - that is whats on my current rear wheels - if 265 fits good as well. I imagine it would give better edge protection to the alloys too?? I only ask cause I was looking to change to a 230 style wheel and would consider the wider rear if you guys thought it was better (ie for grip/tyre choice etc). Also, would going from 255/30 to 265/30 be a problem as it would slightly increase the over rolling circumference of the wheel?
Cheers all.
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      02-19-2010, 05:04 PM   #11
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why do BMW put 255wide on their 9J rears - that is whats on my current rear wheels - if 265 fits good as well.

Cost.

Also, would going from 255/30 to 265/30 be a problem as it would slightly increase the over rolling circumference of the wheel?
Cheers all.
You have 255/35 as standard so an increase to 265/30 is hardly a change at all. Been there, no problems.

Yves
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      02-19-2010, 05:42 PM   #12
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On the fronts I have 225/35/19 and on the rear it is 255/"30"/19 not 35. Otherwise I take your point - the wider tyre isnt affected by going from a 35 to 30 profile as the overall circum is the same, but i'm already there on a 30 rear tyre......
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      02-19-2010, 05:51 PM   #13
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265/30 is no problem, slightly larger diameter but should be no problem. gets more tricky when you are lowered.
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      02-20-2010, 12:12 PM   #14
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My BBS rims that I'm selling were listed as 8.5 but I have just measured them and they are 9" - is this normal? Is there a special way of measuring the rim width?
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      02-21-2010, 08:24 AM   #15
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Quote:
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My BBS rims that I'm selling were listed as 8.5 but I have just measured them and they are 9" - is this normal? Is there a special way of measuring the rim width?
Are you measuring overall outside width?

The stated width will be from bead seat to bead set, i.e. where the tyre sits rather than the outside cosmetic face.

http://resources.coker.com/wheel-tec...rim-width.html
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      02-21-2010, 08:53 AM   #16
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Quote:
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Are you measuring overall outside width?

The stated width will be from bead seat to bead set, i.e. where the tyre sits rather than the outside cosmetic face.

http://resources.coker.com/wheel-tec...rim-width.html
- that's ok then.
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