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Discussion Starter · #1 ·
I ran this weekend using two different tire / wheel combos.

A. 18 x 11.5 x 25.5 R370 goodyears in the factory 18" rims all the way around.

B. Nittro R2 on the factory 17/18" wheels 275/40/17 and 305/35/18

The physical size of the tires in the rear are the same dia. and very close on the width. The Nitto fronts have about 1/2" less rubber and 1/2" shorter. The slicks do not fit perfect on the stock rims in this size ( better options out there for the rim ) so the contact patch up front may only be about 1/4" wider with the slicks. and maybe even 1/4" less than the 305's.

First let me say the Nittos are pretty impressive and will make an excellent to and from the track tire. They do not give back much compared to the stickier tires in the grip department.

The big difference comes in braking and acceleration between the two.

Set A. weights about 8 lbs less up front in 18" vs. 17" with nittos and the rears 10 lbs less. Yes, 36 lbs less rotating mass with the slicks. They have no steel belts and are not safe for street duty by any means.

With the lighter set, the car stops faster and does not activate abs as easy, meaning you can brake even later. The car accelerates quicker by 5 mph in most of the the spots on the track where you can wind 3rd to redline ( say 100mph )or whatever you braking point is. Example : slicks 114 mph and brake, Nittos 109 - 110 and brake. It was very noticable in both departments everywhere on the track.

Some will attribute this to just the size of the tire itself, which is true. But the slicks had got greasy and were sliding all over, what got me was the MPH difference from one point to the next. I switched to the Nittos and the grip was far better than the greasy slicks. The Nittos are predictable and felt real good, very close to the GSCS I ran in the past. So thumbs up to Nitto for a great compromise tire.

Long story short, rotating mass in this case made a huge difference that could be felt. Many of us go out once every couple months and never get a chance to do a back to back comparison an hour or two apart.

No science here but I did see lap times with the Nittos increase about 1.5 - 2.0 secs with the Nittos over when the slicks were still pretty fresh. When the slicks got greasy the Nittos ran about 1.0 to 1.5 seconds faster.

So for those of you hoping to improve your braking and acceleration and wondering about all the theories out there. I got a chance to actually feel it in action this weekend.

1. Reduce the rotating mass.
Look not only at the weight of the wheel, but also the tire

2. Increase the surface area contact patch.
Stickier or wider and sticker tires will allow you to go
15 - 24 feet deeper before abs actives, assuming you accelerated to 114-115 mph and had to stop in the same spot to turn.


For those interested....the R370's were bought used and were run hard for (3 )30min sessions before they got greasy. The 4th session was wild half way through. I let them cool over night and tried them again the next day. They were good for about 4-5 very hard laps and they went away again. While they still have some rubber left (around 2/32 avg.) they are not recovering anymore. These tires are new to me so maybe someone else will chime in on how other compounds last or recover.
 

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The rotating wheel has inertia that is proportional to it's mass and the square of it's radius. When the car accelerates, a torque is applied to each wheel to make them accelerate along with the car (usually. turn off TC to see a difference to this rule). The torque required is proportional to the acceleration *and* the inertia of the wheel. Reduce the inertia of the wheel, you reduce the torque required to turn it. The torque that's concerved goes to pushing the mass of the entire car instead of just the wheels. The car accelerates faster.

It's not just the 36LBs of deadweight you took of the car, but the fact that the rotating inertia of the wheels was also reduced. That inertia has a "multiplying" effect that makes each pound worth much more than it's weight alone.

A lightweight flywheel/PP does the same thing. I did this mod a couple of months ago. The SOTP improvements were surprising.

That being said, some of your improvement was undoubtedly due to the slick's "tread" (or lack thereof), not just the mass itself. I would not expect braking to be substantively improved by minor reductions in mass. ABS operation is primarily function of tire and brake/rotor grip, not wheel inertia.
 

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Discussion Starter · #3 ·
Sled,

I agree lots of factors. But the big suprise was the acceleration. Something like 5 mph to 100 is pretty significant on a road course. I could measure it in 4 different parts of the track.

I too have done the flywheel program on other cars, they do rev quicker and get up to speed quicker. You would think it would work in a similar way. The more weight on the end of a string swung in a circle creates more pulling force to the outside of the circle. You would think that interia in the wheel would be similar, but maybe not as it is mounted at a fixed point and spread evenly over the diameter.

I guess what I was trying to say is that it took a bit longer to stop the car when braking at the same spot, this caused me do go deeper with the pedal, thus activating the ABS. It just did not slow down as fast, a few feet from 114-118 mph gets pretty noticable when you are about to make a 90 deg left turn or drive off the track. The ABS is a grip issue as you better described, but from the SOTP I feel the inertia played some role in this.
 

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How do you perceive the Nitto's to corner compared to the GSCs?

I've been considering those as replacements in another 5Kmiles or so when my current tires wear out. I care about cornering grip more than acceleration (I don't have any troubles with hooking up the GSCs).
 

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A reduction in unsprung weight and rotating mass should yield a nice improvement in handling, braking and accelleration.

Sal
 
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