Rolling Resistance, Drag and Aero effects of changing rim spec

[CrisSilberGrau]

Currently trying to figure out where I lost 5/6kph down the back straight at Mosport.

Old setup: EC7R 18x9.5 ET35 265/35/18 TrofeoR 43.5lbs Total weight
New setup: E88 18x10 ET23(?) 265/35/18 CRS 48.5lbs Total weight

Some wisdom says that the overall weight of the tire setup doesn't have that big an effect on the accel but I'm a bit skeptical of that. Source is the TireReviews YT channel where the test involved an F80 with various weight of rim and tire setup overall weights.

The new setup does stick out of the front fenders more than the old setup, off set, width and CRS are fatbois. Perhaps there is a drag penalty there to also conincide with the much heavier rear rotational mass.

Last track day was done in October of 2025 and top speed was around 220km/h and was reached and exceeded numerous times.
This new setup was run Aug 27th this year on a mild day with temperatures in the mid 20s Celsius (74-77ish F). Top speed was max 217kph and on the PB of the day lap was 214/215kph. There is a distinct acceleration delta from day to day in 4th gear.

Car setup was almost identical save for rear cambre went up by 0.1 of a degree to -3.6deg.

For reference the alignment setup was/is:
Front Camber -3.5 Toe 3mm toe out
Rear Camber -3.6 Toe 2mm toe in

Weather on the most recent day was not ideal in the sense that it rained for a few days during the week leading up to the track day. However, the drive out of turn 5 all the way to turn 8 on the back straight is very similar in terms of min exit speed, so removing for some factors....

Am I right in thinking this loss is two fold (ignoring weather, driver etc.):
1. heavier rear rotational mass by at least 5.5lbs per axle
2. higher drag penalty of the front tires as they are protruding more

PS. I will post a speed over time graph from my AIM SoloDL in a few hours.

[OldRanger]

Cris,
I’m interested in this subject of drag related to the front wheel - eager to see where this discussion goes.

Looks like you’ve got a mild/moderate aero package with a front splitter; without canards or a tire blocker.

We know these cars suffer from poor aero around the front wheel…so it could be drag, but you’d have to go into the Aim data to prove that. That kind of analysis would also benefit from extremely consistent exit speeds from the prior turn, in both sessions you’re comparing.

Lots of variables in this problem. Hardware and human.

[CrookedCommie]

Did your overall wheel+tire diameter change? October vs August air makes a big difference in engine power.

[CrisSilberGrau]

I agree OldRanger the AIM Solo DL im using is not very consistent. I can tell you that the corner speed out of the last corner before the back straight at Mosport (Turn 5) I'm consistently bad in, on average I always have my min speed at around 75 or slightly above.

The main variables IMO are:

• Front rim offset E88s are 18x10 and I assume (not sure how to accurately verify) ET23 or 21.5, thus a much poorer tire aero profile.
• Weather Oct 13 at 13C and Aug 28 at around 24ish C

Tire diameter may have changed very slightly. A) From changing tire manufacturer TrofeoR to CRS B) From changing rim width. I can take a picture of the TrofeoR on the EC7, they looked quite bulgy.

The aero profile of the car remained the same, no changes to it at all.

[CrisSilberGrau]

Here is a graph of my two best times in October last year and this year. See the distinct accel difference in 4th gear. The average speed in some corners is up considerably as I've gotten used to the small aero components and learned to trust them (mkrennsport lip and GT4 wing). However, the discrepancy in "in gear" accel is just a bit off to me. This is also the first time I'm really trying to audit my driving.

[CrisSilberGrau]

In your experience, on cars like ours does rotational mass matter a lot, ie the wheel and tire package on terms of weight. Tire reviews yt channel seems to say it doesn't but the test was conducted with an f80 M3 with gobs of turbo torque... I'm leaning towards yes it does quite a lot for us.

[CrisSilberGrau]

I will also give my goal time another try. Anything in the 1:2x.xx at Mosport for a road car is seriously moving, usually anything that can reach north of 230kp/h on the back straight can reach that time with relative ease in terms of usual mods required. For this car it will be hard, and I also want to do it with 200TW tires to compare to most of the other cars running the same. Optimal times for both these days was 1:30.5xx need to first hit that, and then find .6xx

Changes I have coming are as follows:
Custom Canards - went old school and put cotton tufts all over the car and put a big fan in front of it and looked where I had movement to properly place and orientate them. Used PSD Designs carbon ones that I bought. However, those are to be mounted at a very steep angle and low on the OEM bumper which showed very little movement in air. Im trying to direct some of that high pressure air from the bumper to merge with low pressure air foil and "throw" it over the car on to the rear wing.

OEM rear sway bar - Ive been running Hotchkis bars for a long time. A complaint I've had is the car not having enough "drive" out of corners. Id have to be really egregious with the throttle in order to unsettle it at all. As well I can hear the OEM diff not locking up and spinning just the inside wheel at times for a second. The OEM diff was just rebuilt and put in with 3.91 gears last year (for the OCT day as well).

Optional - new 550cc injectors and fuel rail install plus tune. I've had these parts for about 2 years and not installed them. Currently running on an old tune that we did with Schrick cams, and now I have the CatCams with slightly different lift on exhaust, DLC rockers and the drilled rocker shafts.