Is this a setup for street use, or will it see track time?
If your goal is to run 305/35-18 out back, then 275/35-18 would be the best match for the front. The two popular tire choices that I am aware of in those sizes are the Nitto NT01 and Toyo R888R, and we strongly prefer the NT01 for track use. I am not familiar with any street tires that are offered in these sizes.
18x10.5" Front Fitment Notes:
Fitting 18x10.5" wheels under the front fender arches is a balancing act between the following:
- Selecting wheels with an appropriate offset
- Dialing in the appropriate amount of negative camber
- Taking into consideration the intended tire size and model (not all are created equal)
- Taking into consideration your existing or future suspension silhouette and spring stack height
Even with all of the above dialed in, enthusiasts considering 10.5" wheels with a track-oriented compound should expect some light scrubbing around the liners under suspension compression or at steering angle - Par for the course for fast lap times.
The ideal offset for a 10.5" wheel on the front of an E9X M3 is typically between ET17 and ET22. The higher the offset within that range, the less negative camber you are forced to run to clear the fender arches. Most hardcore track guys running 10.5" wheels up front are already running somewhere around -3.5º, so dialing in negative camber shouldn't really be a barrier. We chose to produce our 10.5" wheels for the E9X M3 chassis with an offset of ET22, and since many will run these wheels in the rear as a part of a rotatable square setup or as a staggered setup, anything lower than an ET22 would limit rear tire width due to rubbing the rear fenders.
Beyond using the correct wheel offset, the silhouette and stack height of the suspension is equally important. Motorsport oriented coilovers like JRZ and MCS with 5" linear springs work very well, and depending on the stack height helpers springs may or may not be used. The idea is to keep the lower spring perch above the tire where it does not impede clearance. With a suspension setup like this and approx. -3.5º of camber, 18x10.5" ET22 (or lower) can accommodate 295/30-18 compounds without spacers under the front fenders.
Other popular coilovers like KW and Ohlins R&T are known for eating up inner clearance, and will require spacers or will need to be modified (replace spring configuration). I am not entirely sure where the BC Racing BR coilovers stand in that regard, but unless the configuration replicates that of JRZ or MCS, you'll likely need a minimum of 5mm spacers as you mentioned.
Of course, front tire size and model will heavily contribute to both inside clearance to the suspension and outside clearance to the fender edge.
18x11" Rear Fitment Notes:
The ideal offset for accommodating 305/35-18 tires like the models mentioned at the beginning of the post on an 18x11" wheel is around ET28-29. Note at full droop the wheel/tire will typically make light contact with the subframe, so you'll need to ensure you are securing the wheel to the hub evenly. Once under load, there is plenty of clearance (see pictures below). Negative camber and fender rolling is required to prevent rubbing under suspension compression.
Here are a couple of photos of my E92 M3 which is running a similar setup - 275/35-18 / 305/35-18 NT01. As a side note, I strongly prefer piloting my car with my other 18x10.5" rotatable square setup. I was also about ~3 seconds faster when I compared the two configurations back to back during my last outing at Thunderhill, so that reinforced the butt dyno.
Subframe contact under full droop 305/35-18 NT01's:
Clearance under load:
- Ryan
