Brinkley

I have been racing my boxster (3.6L 996) with 17 inch wheels for many years. Recently I purchased a set of 18 inch wheels and have noticed that my top speed and rate at which I get to top speed has been adversely affected by the 18's. Now before you think gear effect take a look at the wheel and tire specs below.

17 inch wheels: Front is 7" wide 38lbs w/ 225 Hoosier R6
while the Rear is 9" wide 44lbs w/ 275 Hoosier R6.
Hoosier Outside Diameter Front 24.7"
Hoosier Outside Diameter Rear 25.5"

18 inch wheels: Front is 8.5" wide 40lbs w/ 245 Hoosier R6
while the Rear is 10" wide 47lbs w/ 315 Hoosier R6.
Hoosier Outside Diameter Front 24.7"
Hoosier Outside Diameter Rear 25.6"

So, overall diameter (gearing effect) between the two sets is not the answer, So I'm looking at the difference in weight: 2lbs front and 3lbs rear per tire. I can't believe that 10 lbs total can cause a difference of up to 5-6 mph at the top end of a straight? Now if you can image, as you look at 2 laps of data acquistion, the area under the speed curve as I accelerate out of a corner, the seventeen's quickly attain a higher speed vs. the 18's and that higher speed increases down the straight. This causes a large loss in comparative lap time. Another fact, I carry higher speeds all through the corner with the 18's and get on the throttle earlier with the eighteens, as you'd expect with more rubber on the road. You would think faster corners and earlier on the gas would equal faster down the straight. It doesn't. The 17's are always faster. Also, my shifting RPM's are the same to insure I am keeping the engine in the same HP band. All data acquired by Motec.

What do you guys think is the mathematical or driver related problem here?

claykos

The wider wheels create much more aero drag. That was the entire reason behind the 6 wheeled F1 car back int he day (less front wheel area for the same contact patch). As well, rotating weight is much more significant than just adding weight.

VERBOTN

Clearly driver, I roll on 24's and have no problem

mark kibort

tire weight will effect acceleration as much as if that weight was sitting in the car X 2.

I change with 335 toyos vs 305 toyos that have 2lbs difference and diameter differences and see no changes in lap times or straight line speeds. this weekend Im going to a smaller diameter rear tire with the hoosiers, and only expect time changes to be due to grip of the tire.

I also changed to a 305 front tire vs my old 275 front ,with a near 5lb a corner weight increase. times didnt change much. my best when pushing 100%, times went down about .5 seconds with the larger tire. (better grip in the corners and braking) speeds all were near the same down the main straight.

Just as dynos are calibrated, you can use the rate of change of speed (acceleration ) and mass (weight of the tire), to determine an exact hp difference.
Doing the work for you, it works out to be about 2x the weight as if it was in the car. so you can see if you have a 6lb rolling weight increase, it will effect net HP available by 12lbs as if it was in the car. that might only be 1.2hp and I dont think you can notice this kind of hp/weight change.

Other factors could be that day. was it hotter, were you not pushing the car as hard, was the car not running the same as before? lots of factors.

mdrums

Could it be the rolling resistance of the wider tires?

mark kibort

doubtful. I thought i was going to lose a lot due to putting the wide tires up front. 2" wider! But, in the end, no difference in straightline speed. still 130mph exactly, at the end of the main straight of t-hill, still right on the bumper of the Vipers as was the case before. however, it was much nicer having the meat to slow down and cornering was much better. pretty good trade off for me, even at 10lbs extra rolling weight.

mk

chrisp

If the car is reaching or close to max speed then the aero drag might be a bigger contributor than say a car that is still accelrating strongly.

Brinkley, is your car relatively topped out at that point on the track?

Mark, is your car still accelerating strongly at the 130mph you refer to?

Brinkley

Mark,
track was almost identical, first run 12 noon vs. 2pm sunny hot. I was pushing as hard as I could. My first thought, when looking at the data was fast in, fast at apex, then the two speed traces combine as one at corner exit. That is where the 17's pick up speed and out accelerate the 18's.
Christian could be right, the driver.... the old adage, fast in slow out. BUT I'm fast in, faster at apex and back to full throttle all before corner exit and it is at corner exit that the slower in, slower at apex 17's speed trace meet.

Brinkley

chrisp,
No my car is still accelerating at the end of each of these straights. Sebring I'm hitting 136 on back straight and still climbing. While at homestead, 125. Top end based on Daytona is 156 mph through the triOval.

mark kibort

sure, its still accelerating, and the fact that it gets to the same speed from the same speed, shows that the differnce is very slight. I also did this test on the same day with the two different sets of toyos. 275/305s and then 305/305s (f/r)

aero is drag is not a big factor, as the only part of the tire exposed to air is unde the air dam/splitter, 3" by the extra width of the tire. 2" wider. thats only 6square inches of frontal area greater. Its like sticking your hand out the window.

mk

mark kibort

It has to be other factors, because it is pretty basic math. You have only 2 things changed on the physical car. heavier and wider tire. aero drag is less than 5 square ". Like your hand out the window on each side. nominal, and weight is 2lb per tire, 4lb per tire effect because it is rotating. so, 16 lbs as if it was in the car with you. Less than a net acceleration HP of 2. So, I believe you saw a difference, but it has to be from something else.

VERBOTN

B just having fun, and I have been thinking about this since our chat last week. More to come

Larry Herman

It could be a combination of factors that add up to the loss of acceleration that you are noticing. Mark's experience not withstanding (as his car is heavier with more power and he only changed the front tires) there certainly will be more rolling resistance with the wider tires, especially in the back. How much toe-in do you run? Anything more than about 1/16 total is going to cause significantly more friction as your tires get wider.

Also, 10 lbs may not be much, but if it is all around the circumference of the wheels (which it probably is - and it is farther out with the 18s), it could add quiet a bit to the inertial forces that you must overcome while accelerating.

Finally, though you were running the same tires, how many heat cycles were on the 17s vs the 18s? Older tires, even though still quite grippy could be a little harder and so have a little less rolling resistance. All these factors, though each one small, could add up to the bigger differences that you see.

ew928

+100


Sit in office chair. Hold heavy weight out with straight arms from body.
Start to spin.
Pull weight towards body.
You spin lots faster. (good old physics experiment)

With 18" wheels, heavy (wider) tires and heavier rim mass away from rotational center is like spinning with weight further from body.


Did the 18" wheels and wider rubber net you faster lap times? (just curious. sorry if I missed it. Just read it was retarding terminal speeds in the straights)

Circuit Motorsports

The 130 at the end of the straight both times could be misleading. As you say, the cornering is better so one would expect your V-min in the mid corner and corner exit is a bit higher. So you may be seeing a hit from the extra weight and width but not realizing it.