Unsprung Weight

[Moochman4life]

Hey guys, I still don't fully understand the term "un-sprung weight" with regards to lighter/heavier wheels and tires and how it applies on the track.

I have a set of Steeda Q1S's just hanging out in my garage that I'm thinking about mounting something like an NT05 on to take the place of my staggered street setup, as well as running them at the track if I don't feel like running my Conti scrubs, or if there's bad weather or whatever. I started doing the weight-to-weight comparison of my current street setup vice the above idea and it reminded me of a couple conversations I recall about un-sprung weight.

So, my current street setup weighs 100lbs up front and 108 out back for a total of 208.

The NT05 setup on Q1S's would be 108 front and back for a total of 216 evenly distributed to all 4 corners.

Other than going from a staggered setup, how would the mannerisms change on track by evening out the weight....but also loading up the front a little bit more? Does that extra 8 lbs actually become something greater?

 

[SoundGuyDave]

Oh, yes it does... PARTICULARLY with regards to the wheels. Mechanically, the wheel/tire assembly acts like a flywheel, in that it stores engergy (rotational inertia). The heavier the setup, particularly with regard to distance from hub-center, the more energy is required to accelerate it up to a given speed. This translates to reduced net vehicle acceleration. Next, once you have that wheel/tire combo spinning up to speed, it takes additonal energy to slow the rotation, which results in net reduced braking efficiency. Bottom line is that heavier wheels equates to reduced acceleration, and reduced braking efficiency. That means slower entry speed into a corner, and extended braking zones; in other words, slower lap times, all other things being equal. Rotational weight is a critical factor, which is why the one-piece aluminum driveshafts, aluminum flywheels, etc. are so popular. And yes, you can feel a difference just changing out the driveshaft from the 43lb(?!) stock two-piece to a 19lb one-piece.

Note that all that weight is unsprung. Net SPRUNG weight also has a factor, in that it takes more energy to get up to speed, change direction with, and slow, however I've heard it said that unsprung weight is roughly equivalent to four times the sprung weight. If you take two cars with identical weights, one running 50lbs less sprung weight and supa-bling 22" rims, and the other with lightweight track wheels but 50lbs more chassis (sprung) weight, the car with the lightweight wheels will have an edge both into the braking zone as well as coming off the corner. This is why pro race teams are paying uber-dollars for unobtanium wheels that are lighter by grams than another, cheaper one. The car still has to scale at a minimum weight, but WHERE you put that weight makes a huge difference.

Now, if you're talking about a rain setup, the heavier wheels are still a penalty, but resistance to acceleration may not be a bad thing in the wet. You'll still take a braking hit, but overall, I wouldn't worry so much about them. For a pointy-end dry setup, though, give me the lightweight wheels!

 

[Moochman4life]

My staggered setup is a set of GT4 replicas weighing in at 26lbs front and 28lbs out back.

My Q1S's are 24lbs.

However, the tires on the staggered setup are actually pretty light at 24lbs front, 26lbs rear, but they're a shitty street tire that I've run some HPDE's on out of necessity.

The NT05's are 29lbs.

Would I be trading off some acceleration and braking for better cornering by going to the NT05 (or similar) on the Q1S's?

 

[SoundGuyDave]

Based on what you've been saying, it sounds like you have three sets of wheels....

1) Lightest wheel for the Continental scrubs... Your pointy-end set.
2) Heaviest wheel for the pure street role, probably the staggered setup.
3) Mid-weight for the occasional HPDE use, and for use as rain tires at the track.

Will you be trading off accel/braking? Yes, you net weight will be higher. Will your lap times drop? Probably, since you are adding more mechanical grip from the tire compound than you are losing from inertia (assuming the NT05 has more grip than your current tire). FWIW, I think there are MUCH better tires out there than the NT05 for rain/HPDE/Street use, like the *Spec, Rival, XS, etc. I've not been impressed with the NT05; they seem to grease up quickly, and aren't all that grippy to begin with.

 

[Moochman4life]

I used the NT05 as an example of the range of tire I'm looking for. I'm also looking at the Ventus RS3 and Advan AD08 for the same role. I was also considering the RE11, but I'm trying to keep the price below $300. Basically, a street tire that I could occasionally use on the track if I don't feel like trailering my setup, which is my plan once I get back stateside in about a month or so. Previously, I've always crammed everything into my Stang for track days.

Is there another tire you've had success with filling both roles?

You definitely answered my original question, thanks.

I've only been doing this for a few years. I've had a pretty big layoff from the track recently due to "work", and I'm trying to get all my shit that I've been half-assing straightened out prior to me getting back.

By the way, I'm in the market for a trailer...4'x4' or 4'x6'. If you guys know anyone who wants to part with one, please let me know!

 

[Moochman4life]

Oh, sorry...and I actually have 3.5 sets, currently:

1. Dedicated track set with Conti scrubs.
2. Staggered setup for street and occasional track use.
3. OEM set for the winter...and to be used as spares for extenuating circumstances.
4. Fresh set of Q1S's with no rubber sitting in my garage.

I'm seriously thinking about dumping the staggered setup, or keeping and slapping a set of DRs on the rears.

 

[SoundGuyDave]

I started my open-track addiction with a staggered set of NT555's. 245/285, and it was, um, not good. GREAT wear, cycled them out (ran them long past prime), but no grip to speak of. After those, I went to Dunlops in the 275/35-18 size, and life was good. Then I got a whiff of the Purple Crack when I started Time Trials, and it's been R6 for dry, Star Specs for wet ever since.

Now that I'm running Enduros, though, I need to up my "wet" game a bit, most likely to Hoosier H2Os.

At the end of last year, I bought an enclosed trailer, so yes, I have a 48x48 tire trailer, hitch, etc. sitting in storage, but I think the shipping would kill any deal, unless you have somebody that could pick up locally.

 

[Norm Peterson]

At some point, you'd probably want to be able to put some numbers on any compromises in acceleration or braking to see if they're enough to matter for your use.

In wheel to wheel racing or even top-level autocrossing, you'll want as light as you can afford (and where that light weight "isn't" also being important) , simply because your finishing position ultimately depends on grabbing all of the little advantages that you can legally find.

For an 8/10ths or 9/10ths track day session I really doubt that 8 lbs is going to matter. Some testing done for autocross a few years ago had 5 lbs/corner x 4 corners on a Mazda Miata being worth on the order of 0.1 to 0.2 seconds on a 45-ish second course. 16 lbs instead of 20 and on a much more powerful car to boot would be worth even less. Finishing position here is important mainly to the extent that you finish unscathed and without going agricultural.

That's just the weight factor. I'm pretty sure that you'd gain significantly more from having the square setup than 16 lbs would cost, assuming that my understanding of running "rear-size" tires at all four corners is correct.


FWIW, I've run a few mass moment of inertia sims, and a lb of wheel or tire compares more closely to 1.5 lbs sprung as far as acceleration is concerned (larger ratios are for powertrain components further upstream such as flywheels and clutches). That 1.5-ish ratio should hold for braking as well. That's the rotational effect, the unsprung effect is separate, also relatively small, and gets involved with the proper bump damping settings (bump damping requirements might tend to be a SQRT function of unsprung mass).


Norm