Wheel offsets / tyre sizes for 18" alloys

[Senninha]

Hi all,

Im sure someone more in tune with such info than I will know the answer ...

Im running my NSX on OEM 17" O2 alloys with 15mm front and 25mm rear spacers, dropped on to the lower perch of the Bilsteins. For me the car has the perfect stance for an NSX.

Im considering some options and would like to know what offset would be needed to maintain the stance without removing the spacers if I fitted 18" alloys all round as I would probably switch to the current set up for winter use and don't want the hassle (fronts) of swapping the spacers.

Do I simply stay with OEM despite the larger diameter? What tyre sizes would avoid any rubbing issues for rebound or lock?

Thanks in advance, Paul

[Nick Graves]

It would be very useful to know the width & offset of your current wheels and the proposed ones.

Convert the width to mm, halve it & subtract the offset. Do the same with the new ones.

IOW, if the new Rimz is 25.4 mm wider, you'll need to increase the offset by 12.7 mm to keep the face in the same place and hope the back end doesn't foul your suspension. And the new spokes still clear your brakes (backset)...

[havoc]

It all depends on the wheel width Paul rather than wheel diameter - going up an inch in diameter, if the wheel width (note: not tyre) remains the same, won't affect offset.

Looks like the fronts are 17" x 7", offset 55, while the rears are 17" x 9", not certain on offset (similar, I think).

So e.g. if you get 18" x 7" at the front, you need a similar offset. But if you go for 18" x 8" wheels at the front, you need to increase the offset by 1/2" / 12mm as Nick has said to preserve the visual look from the outside - but in that instance you'll lose an inch of clearance against the inner liner, so might experience scrub at full lock...

Edit: This is a decent calculator to see the effect of an example wheel...
http://www.1010tires.com/Tools/Wheel-Offset-Calculator#

[Kaz-kzukNA1]

Hi, Paul.
I'm afraid it's not that simple and best if you get the info from the owners using exactly the same selected wheel, tyre, rideheight, spacer, alignment or you just need to calculate it with some measurements taken based on your existing setup and tyre selection.

You must decide on the tyre first before selecting the wheel although you may be forced to select different tyre if you are looking for the specific wheel that may not have the desired spec for the tyre you selected.

First of all, I don't know this is the case even on the LHD model but as you saw on your RHD NSX, there is quite significant difference between the R & L front wing/suspension geometry with RHD models.
You won't notice it using the OEM setup but as soon as you go aggressive with the offset (or add the spacer), you will notice that the L side sticks out more than the R side.

Therefore, if you are aiming for the hellaflush looking, you need to take this into consideration on the RHD model.
Still, from what you wrote above, I think you are not that hellaflush at the moment and bit inward/inset at the front and just about flush (or even bit sticking out) at the rear.


OEM 17/17 wheel spec is F: 17 x 7JJ +55, R: 17 x 9JJ +56.



In theory, what others mentioned above is correct but for human eyes, because of the negative camber, if you increase the wheel diameter, the top of the tyre shifts inward even with the same offset. Thus, the appearance changes.
Also, changing the tyre or wheel width (J number) will stretch/square the edge of the tyre so again, this will change the appearance. You probably saw this at some of the events with massive hellaflush setup and over stretched tyre to catch the spectator's interest.


Please double check that there is enough clearance all the way against the Bilstein damper when the corner is loaded and at full droop.
Even when using the OEM wheel, I place the Bilstein damper spring seat at specific angle/rotation and then check the clearance.
Depending on how you place the seat, the clearance changes dramatically.
Please note I'm not talking about the mistake of placing the spring seat/perch upside down.


The wheel width (J number) and the tyre selection changes the appearance because it will affect the stretch/square shape.


Even if the tyre size numbers are the same (for example, 255/40/17), the actual size is different from one manufacture to the other and even within the same manufacture, different tyre model would result in different real measurements even for the same tyre size number.


Even with the same wheel/tyre setup, difference in rideheight will result in different appearance.
Because of the lever ratio effect, A-arms don't move linear/proportionally along with the changes in damper length.
It follows the arc shape. And obviously, this will change the camber angle mentioned above.


18" at the front….
Depending on how the tyre sits on the wheel (stretched/squared), you may struggle getting enough clearance without modifying the wheel arch liner or even the wing edge when the corner is loaded or when making sharp turn.


Also, please check the clearance against the brake caliper.
Depending on the spoke design, it may hit the caliper outer side face.


You could get basic idea by fixing fishing line or thin string with weight from the top of the wing and then measure the clearance against the top edge/sidewall of the tyre.
As you are using 17" at the moment, you need to aim for the virtual reference point where the edge of 18" tyre sits using your existing chassis setup.
This will sort of help you in getting some idea.
Presuming you haven't changed the alignment massively from the OEM setup, using AD-08R 255/35/18 on 9.5J +40 wheel will result in just about the tyre top sidewall touching the string from the rear wing panel.
About 81mm from the hub virtual plain.
Please note that AD-08R measured width is massively wider than other model/manufacture tyres even in the same size spec.
For the front, you really need to review the clearance when loaded or making sharp turn as it will rub.


Please use the following information as a reference only as it would be different on your NSX but hope close enough.

Many people doesn't like 18/18 combo and prefers 17/18 or 18/19 one instead but anything above 17" in the rear, you will notice the sluggishness on launch mainly because of the combined weight.
As you have 6MT, you will also notice this when in 6th gear and apply WOT due to effect of combined gearing effect. Though, 18" at the rear looks very nice…..

The popular combo is F: 18 x 8.0J +35 without spacer on 215/35/18, R: 18 x 9.0J +40 without spacer on 255/35/18 using Bilstein lower perch.
You can play with the above size depending on the availability and requirement of your selected tyre.
Tyre for the front 215/35/18..… May struggle finding it ….

Please note that at the Front, if you go beyond about +40 offset, it will significantly change the braking stability or pick up more noise from the road surface imperfection.


Kaz

[Nick Graves]

Indeed - the offset really should be determined by reference to the steering centre axis, unless you want increased kickback, jounce and braking instability. On many cars, the additional leverage also wears wheelbearings, etc.

Also, bigger, heavier wheels introduce gyroscopic effects and make the steering feel more sluggish. Which is why I prefer the 16" fronts, even over the OEM 17"s.

But for some people, optics are more important than haptics.

[Senninha]

Thanx for the detailed feedback Gents ...

By starting with tyre it becomes clear that even options at 18 are limited ... the only F&R match currently coming up is the Bridgestone RE050 for fronts, RE050A for the rears ... apart from loading is there anything else to consider for this option

Regards, Paul

[havoc]

Rolling radius change particularly if the car is lowered and also if the wheels are running a different effective offset to OEM (i.e. both lead to more risk of fouling under full lock or under compression).

Sidewall depth vs suspension stiffness vs UK potholes. 40-profiles are usually OK in the UK (going by mainstream cars with 225 / 235 / etc. width and typical modern saloon/hot hatch suspension...which is altogether probably pretty similar to us), but 35 and esp. 30-profiles seem to cause issues with punctures or (worse) bent rims.

Visual appearance if you combine both lowering (reduces physical and thus visual 'space' in the wheelarch) and bigger size (ditto, but visual only). Plenty of Yanks have gone to 18"s though, so have a hunt on Prime for some images / ideas.