UNSPRUNG WEIGHT BENIFITS

Discussion in 'SRT Hellcat Wheels and Tires' started by rollieman, Oct 1, 2015.

  1. rollieman

    rollieman Silver Member

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    Lots of positive feedback available in this forum relative swapping out the factory steel DS to aluminum, with several members reporting shaving off .10 at the drag strip which is an excellent gain for a simple bolt in process.

    Some consider a DS as unsprung weight while others consider it rotating mass weight ---- the answerer may be a combination of both? Regardless, it has a much higher value compared to sprung weight or dead weight!

    My question is: what about tire weight which is considered unsprung weight as well?

    The reason I bring it up is because the standard 275-40-20 Pirelli PZero Nero tire specs out @ 35 lbs which compared to other ultra high performance all season tires such as the 275-40-20 General G-Max which weighs only 30 lbs. There may even be other quality tires out there that are just as light or lighter ???

    Not sure if lighter tires will yield better ET's ---- but it will most definitely improve handling.

    What do you guys think??
     
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  2. rollieman

    rollieman Silver Member

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    I just noticed the speed rating on the General G-Max which is only W, this may account for the lesser weight ?
     
  3. roostinyfz

    roostinyfz Hellcat Member

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    I would think this would make a significant difference down the track. I know the best mods we did dragging the hills of glamis was always lighter paddles. Grip and contact patch is also a major concern.
     
  4. Driver72

    Driver72 Gold Member

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    First:

    In a rolling acceleration test I did that took 8.8 seconds stock, the aluminum DS reduced .2 seconds off.
    So in the 1/4 mile an aluminum DS will shave at least .2 seconds off your time.
    This is probably more to do with the fact it now has less rotating mass (aluminum DS is about 8 lbs less in weight) in the drivetrain, which therefore allows more power to get to the wheels and too the ground. 8 lbs loss in the wheels and tires won't make anywhere near the benefit as it's much further down stream. The aluminum DS is also one piece instead of two, that probably helps as well as it's one less thing it has to "transfer" power through.

    Second:

    Unsprung and/or rotational mass definitely improves acceleration but there are exceptions to every rule.
    Rotational mass will improve acceleration in lower powered cars MUCH more than they will higher powered cars like the Hellcat.

    For instance, if you remove 5 lbs per corner on a car with 150 hp and 150 lb-ft of torque, it will make a noticeable difference.
    In our Hellcat's, probably not even measurable.

    I've also done tests comparing rotational mass acceleration differences in a few different cars of average power.
    What I learned in for the "average" powered car of say 200-250 hp for every 1 lbs of rotational mass you remove it's equal to about 3 lbs of "regular" weight. For instance, if you were to remove 20 lbs of rotational mass in aftermarket tires, it would be about the same as if you left the stock tires on but removed 60 lbs out of the interior of the car. In one of the car's I was able to drop 11 lbs per corner (stock wheels were heavy and had heavy run flat tires).

    So if you were to get those tires that were 5 lbs less each (or 20 lbs total) it would be similar to removing 60 lbs out of the car (or like taking out the passenger seat).
    Again, in our Hellcat's that isn't going to make much of a difference in acceleration.

    But you also have to weigh the other pros and cons of switching out tires for less weight.
    If the tires have a softer sidewall (which is highly likely if they are dropping 5 lbs per tire) you are going to have a car that will handles worse just because of that fact alone. It will probably also have worse braking distances as the softer sidewall won't allow the car to stop as fast. In a car that's 4450 lbs, having a softer sidewall is likely harming you more than doing good when it comes to overall performance. Sure you may have a bit less rotational mass, but in the 707 hp car, it will be so minimal, it won't be noticeable and probably barely even measurable.
    Next point: In a car that has as much power as the Hellcat's, having less rotational mass will also mean spinning the tires easier.
    So launching the car could be harder and if those lighter softer tires don't have as much grip, even at 40 or 50 mph, flooring the throttle may result in more wheel spin.

    Bottom line: I wouldn't sacrifice the performance of the tire just to lose 5 lbs in rotational mass on a car as powerful as the Hellcat's. If you have a daily driver that's an "average" car with an average 170-200 hp 4 banger, and you can lose 5 lbs per corner with an equal or better tire than what comes stock on the car, by all means swap it. It may feel a bit "peppier".
    For the Hellcat, if you want to change tires to improve acceleration you'd be better off going with 305/35/20 tires, as they are shorter tires and therefore it essentially lowers your final drive ratio and therefore improves acceleration, but you'd still have a stiffer sidewall tire for other performance benefits. Don't worry about trying to save a few pounds in weight in the wheels/tires in these beasts.
     
    Last edited: Oct 2, 2015
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  5. Meat

    Meat Hellcat Member

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    The only thing I would like to mention is that it is more about rotational inertia than rotational mass. So the farther from the center of rotation a mass is, the greater the impact on rotational inertial (aka polar moment of inertia) therefore the greater the impact on acceleration.
     
  6. rollieman

    rollieman Silver Member

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    Excellent technical response --------- THANK YOU ! Sure looks like the aluminum DS is the way to go !
    Reducing ET by up to .20 with such a simple bolt on is amazing ----- no codes to worry about !!
    Additionally as you pointed out this car is too heavy to compromise the integrity of the tire, thus it needs the Y speed rated tire. Both the standard PZero Nero all season and the optional summer tire are both Y rated with the summer tire being .40" wider and .10 shorter plus 1 lb lighter, so slightly better specs; but, the concerns about driving in temperatures under 45 degrees for me is still a concern, thus I remain still on the fence on which way to go ???
     
  7. Driver72

    Driver72 Gold Member

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    Yes this is true, I was thinking of this when I was writing my post, just forgot to put it in there. Thanks
     
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  8. Driver72

    Driver72 Gold Member

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    You are welcome.
    If you live in cold climates, definitely find a high rated all season tire to deal with the weather for those climates.
    The other 8-9 months of the year, go summer tire if you'd like.
    The Continental ContactExtreme DW is a very good highly rated summer tire and is usually a 1-3 pounds lighter than others in it's class, but since they are also 106Y rated, they have the same integrity as the Pirelli PZero summer tires.
    I've used the Conti's before and they do a fine job. They need a bit of heat in them before you hammer on them though, as when they are cold, they will, like most tires, be squirrely.
     

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