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Sadly Woolly has passed away
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I've been asked to post my thoughts here - please feel free to add to them.

There are 4 measurements that you need to know when fitting wheels:-

1. Diameter - this is an easy one - the overall diameter of the rim. Bear in mind that the wheel has to fit over your brake callipers, so the diameter should be at least the same as the rims you have taken off.

On most cars, the more 'oversize' the diameter, the lower the 'profile' of the tyre will be. This is so the overall diameter of the wheel and tyre stays the same. This then means the 'gearing' of the car and other functions such as the speedometer are correct. A lot of people think that larger diameter rims should 'fill the wheel arch' better. If the rim & tyre choice is made properly, there should be no difference in 'combined' circumference of wheel & tyre.


Bear in mind, that low profile tyres do not necessarily always equate with better ride & handling. A fair amount of the suspension of the car comes from the 'compliance' of the tyre. The lower the profile, the less the sidewall can 'squish' resulting in a harder ride. The grip can also be less - initially, it will seem good, as there is no sidewall flex, but, a lower profile tyre can start to 'break away' (side slip or loose lateral grip) earlier.


2. Width. A little more variation is allowed here - the wider the rim, the wider the tyre that you can fit. Up to a point. The standard width tyres are specified so that the tyres will clear all parts of the bodywork, regardless of the steering angle or suspension compression. Don't be fooled however - wider tyres do not necessarily mean better grip, especially in rain/snow. One of the best handling cars used to be a Mk1 Lotus Elan, which used to have really skinny tyres.

3. Offset. This is the relationship of the 'fitting plane' of the wheel, in relation to the width of the rim. If you imagine a flat alloy disc, with the wheel mounting holes in it, that fits inside an alloy ring that is the circumference of the wheel, where the tyre fits. The 'disc' can be fitted anywhere in the ring, from the inside edge, through the middle, to the outside edge. The offset measurement is important, as it determines (along with the rim diameter) that the wheel will clear the brake assembly components etc.

4. PCD and Mounting Hole Diameter. This is effectively the position & size of the holes for the lug/wheel nuts/bolts in the 'fitting plane' portion of the rim. Basically, these dimensions cannot be changed, else the rim will not fit on the wheel hub of the car. There is one more difference associated with this - the profile of the bearing surface of the wheel bolts/lug nuts on the 'fitting plane' of the rim. These can vary from flat to hemispherical. It's imperative that the wheel bolts/lug nuts profile match those of the rim. Also make sure the wheel bolts/lug nuts are the correct length.

The standard rims fitted to a new car are often a compromise between cost, looks, handling & fit. The car designers usually spend a fair bit of time ensuring that these are correct, so, when choosing new rims, it's often either a 'good idea' or 'imperative' that the replacement rims are the same dimensions as the factory fitted ones. The other point to bear in mind is 'quality control' of new rims. Factory supplied rims confirm to strict standards, who knows what testing has been done on 'cheapy', aftermarket ones.

The bottom line - stick with the rim sizes given in your owners handbook, or if you feel the need - increase the rim size, but keep the other 3 dimensions the same.
 

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The only thing I would add to that Woolly is the offset also has implications on the vertical line through the tyre, wheel and hub. Moving that line too far outboard has implications on the stress increased on the wheel bolts, wheel bearings etc. As you rightly point out. Manufacturers dont put wheels on as an afterthought. They are designed to suit the car and its set up.
 

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Another point that has occurred to me is that most after market rims have tapered bolt hole seats. OEM ones are domed So you cant use the OEM bolts with most after market wheels and vice versa. They will fit but the bolts will not seat properly in the rims and would be very very dangerous
 

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Sadly Woolly has passed away
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Discussion Starter #5
Another point that has occurred to me is that most after market rims have tapered bolt hole seats. OEM ones are domed So you cant use the OEM bolts with most after market wheels and vice versa. They will fit but the bolts will not seat properly in the rims and would be very very dangerous
Re-read section 4 :D
 

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I have an oppty to purchase some lightly used Mandrus rims (the pictures look perfect but will have to look at). My SLK has the standard staggered 17" rims. The Mundrus according to the owner are 8" width. The tires he had on were front P225/45R17 and rear P245/40R17. It looks like these would work as a calculator seems to measure an approximate inch between front and rear. In your agust opinions, do you think this to be the case. He wants $1K but I am sure that is wishful thinking when money talks.... Thanks!
 

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The guy came up with some additional data...
Here is the info:
Wheel Diameter: 17"
Wheel Width: 8"
Wheel Bolt Pattern:5x112
Wheel Offset: 42
Wheel HubBore: 74
 

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I think the offset on our stock wheels is 35.
Someone correct me if I'm wrong.
Don't know if the 42 offset will work or not. Someone else will have to answer that one also.
 

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Stock front is 7.5" ET 37.
New wheel 8" ET42 will stick 11mm more inwards. It might possibly rub the suspension.
You might be needing some spacers.

Stock rear is 8.5" ET 30.
So that will stick 6 mm more inwards. I think you'll have that play at the rear, but the wheel as a whole will go 18 mm inward compared to stock. I don't think it will look good, but this too can be resolved with spacers.

Furthermore, stock bore is 66,6 so you will need some adaption there too. Maybe the spacers can solve this too.

Personally I would only go for it if I really wanted them very bad.
But that's up to you ofcourse.
 

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Installed Mandrus Arrow Chrome 18x8.5" on my 2006 SLK55 AMG. Kept the same original tire sizes - the front tires 225's are a bit stretched - they are right at the minimum width recommended for this wheel size. Caution on the lug bolts - got a chrome set and a set of four security locking bolts from the wheel dealer, after installing wheels, lowered off of the rack and the car only moved about 3/4 of a tire revolution before making a hard stop. Determined that the locking stud was too long and hitting the emergency/parking brake inside the wheel hub. Swapped out the four locking studs with ones from the standard chrome set and the wheel would turn. While backing out of bay there was a slight taping noise but it stopped after a few feet. Drove car home with no problems. Next day applied and released the parking brake and the noise was back. Pulled the wheels, did some measuring of the OEM lug bolts and how far they extend through the OEM wheels. Then measured the Mandrus wheels and how far the chrome lug bolts extend through - they were at least 3/8" longer. If you look into the hub through the lug bolt holes you can see brake parts (shoes, springs ...). Mandrus dealer said he could get another set of chrome studs that were shorter but when I asked how much shorter he said about 3 mm. So I wrapped the chrome stud with painters tape, bought two M12x1.5 nuts at auto part store and screwed them both on to the stud, placed in vice and cut 3/8" off of every stud. Double nutted studs prior to cutting to make sure they would clean up the threads when removed. Everything good now!
 

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6007.us
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Is ET the same as Offset ? Near as I can tell you want a 36mm offset for a 7.5" rim and 30mm for an 8.5".


This is much more important on the front than the rear since too much/too little will cause bump steer.


My SLK350 came with the two different width 17" rims but suspect the tires were put on by a dealer since are 225x45x17" Hankooks all around and it is supposed to have a 255x40x17 in the rear. Feels just fine but will probably be an itch.


OTOH I have an extra set of wheels for my 170-clone, 18x7.5 front and 19x9 rear that use 225x40x18s on the front and 255x35x19 rear. Has Conti DWSs on it now and are great in our local frog stranglers. Prolly go for the new DWS06 though Would also be handy having the same wheels and tires on both cars.


Interesting is that both sets have the same tire diameters, 25" front and 26" rear so should be no fitment issues.


If anyone is interested they made over 70,000 Crossfires and they sold on both sides of the pond so wheels should be available at the knackers..


Mine is the 170 coupe Mercedes never built & they also built a soft top roadster. I just wanted a retractible. Odd part is my Crossie is an 07 but the SLK is a '06 so my 170 is newer than my 171. Such a life.


 

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Just thought I'd see what it looks like with 19x9's and 255x35x19s (26" diameter, 1.4" more than stock - edited to correct). Did find that the wheel bolts for the SLK are the same as my R170-clone.


Interesting what happens when you have a lot of oddball parts just sitting in the garage.

I have this problem: I give a lot of stuff away but it keeps coming back threefold.

ps why could I not just drag and drop the pic ?
 
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