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Old 06-04-2007, 12:14 PM   #1
Slowsoul
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Default Rotational Weight and Shock Load.

In any high stress/shock situation be it downshifting on the freeway from 5th to 3rd, drag racing, etc...does rotational mass effect the amount of shock on the driveline?

Say, heavy tires and wheels for instance. (112lbs)

I would think so.

(for the record I do not drag race my car, but I do high RPM downshifts occasionally)
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Last edited by Slowsoul; 06-04-2007 at 12:19 PM.
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Old 06-04-2007, 01:09 PM   #2
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Yes...to what degree, we can't really compute that and say XXX wheels/tires are XXX worse than YYY wheels/tires.
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Old 06-04-2007, 01:23 PM   #3
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Oh c'mon I thought you knew EVERYTHING!

No, There is no reason to compare XXX tires/wheels, with YYY tires/wheels, but the weight totals are relevant. Like my current set up 20lb Rota Slipsteams paired with 28lb 255/40/17 tires...compared to the new wheels that I am getting. 25lb Melleniums paired with 21lb 235/40/18.

Weight removed from each corner is added up, correct? So if I loose 2lbs at each corner, that would be a total of 8lbs of rotational mass droped.

Last edited by Slowsoul; 06-04-2007 at 01:34 PM.
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Old 06-04-2007, 05:36 PM   #4
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The rotational weight that is really important is the flywheel... Heavy flywheel with extra grippy clutch is worse than a light flywheel with a stock clutch.
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Old 06-04-2007, 07:29 PM   #5
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I have an ACT streetlight flywheel with an organic Exedy Clutch paired with a standard RA gear set.

The reason that I posted was because I have just recently noticed a few "driveablitly issues" in regards to the way my car has been behaving lately. I don't know where to put the blame. My WRX has 82k on it, it doesn't burn excessive oil, and compression is still good.

When I bought my '02 WRX (6 months ago) it already had an ACT streetlight Flywheel and Exedy Clutch installed (less than 5k on the clutch/flywheel) it felt great. Every once and a while I found myself 30 yards ahead of everyone else at the light before I knew it. I attribute this to coming from a stock 89 toyota 4 runner clutch to an 02 Impreza aftermarket lightweight flywheel/clutch combo. I don't race off of lights, I just wasn't used to the car/clutch yet. At that point in time I was running 225/45/17, however those tires were cupped and a few months later and I decided I wanted to get some 255s. (because they looked so good .)

So, I installed the 255s and, at first, I didn't notice much of a difference as far as acceleration is concearned. However, just recently, I definately have noticed a difference. Specifically on hills.

A few weeks ago I was at a stop light...by myself...feeling a bit Randy...and decided to get on it, I slippped my clutch, and subsquently roasted my clutch...oops. Let me add that this was the first time I had attempted anything remotely close to a "drag style" lauch since getting the 255s. I was a little worried, but not too much, my clutch is fairly new and in a couple of days it was back to normal. However, just the other day I was on a really steep driveway and I slipped my clutch (because it was all I could do to keep the engine from stalling) and burnt the **** out of the clutch again. Ouch...

Now, I am not on REALLY steep driveways very often...so it really isn't THAT big of a deal to me yet, but it definately makes me wonder how much my 255s are effecting my performace.

The tires alone weigh in at 28lbs a piece.

A burnt clutch is one thing, but a broken tranny is another. I am just trying to get an idea of the "order of importance" as far as rotational mass is concearned.

Last edited by Slowsoul; 06-04-2007 at 07:40 PM.
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Old 06-29-2014, 12:22 AM   #6
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Quote:
Originally Posted by Slowsoul View Post
In any high stress/shock situation be it downshifting on the freeway from 5th to 3rd, drag racing, etc...does rotational mass effect the amount of shock on the driveline?

Say, heavy tires and wheels for instance. (112lbs)

I would think so.

(for the record I do not drag race my car, but I do high RPM downshifts occasionally)
This is a very old thread but I thought I would start here since I was just ready to post a similar point. I just calculated the rotational inertia of the stock 02 WRX wheels and an 18" Rota wheel with 235/40-18 tires. The 18" rota's weigh 6.5lbs each heavier than the stock 16" WRX wheels. I used simple I=Mr^2 to make comparisons. I was surprised to find the Rota wheel package has 31.8% more rotational inertia than the stock WRX wheels. Even if the wheels weighed the same there would be a 12.7% increase due to moving the mass farther from the center of rotation. The kinetic energy is going to increase proportionally with this increase in rotational inertia. 32% is a big increase and it will result in a significant increase in shock loading on the transmission.

Here is my question. How many people with stock wheels and tires have had transmission issues? How many have destroyed their stock transmission after upgrading the tires to a larger diameter low profile design? I could see a stock car handling a wheel upgrade without issues but if you are already pushing the limits of the transmission with engine upgrades a wheel package could push you over the edge.
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Old 06-29-2014, 07:08 AM   #7
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If you used the link in the FAQ for the inertia calculations, also reference my formula correction, go to the Excel formula and make the change.

As far as the rest, here's some of my data:
-I broke the 5 speed on a stock setup, with stock wheels and Pirelli All season tires. This was back in '04 and I was doing quarter mile passes, with the G Timer.
Later, I had many upgrades but stopped launching and never drag raced. Then supported the transmission with a lightweight flywheel and cf driveshaft.
It lasted 10 years, then eventually I made too much torque for it.
There is a failure mode that does not involve shock as well for these.
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