Unsprung Weight

[MagicMtnDan]

Unsprung Weight

This is one of the most critical factors affecting a vehicle's road holding ability. Unsprung weight is that portion of a vehicle that is not supported by the suspension (i.e. wheels, tires and brakes) and therefore most susceptible to road shock and cornering forces. Lighter weight wheels reduce unsprung weight and provide more precise steering input and improved "turning in" characteristics.


Unsprung weight
The following is from Wikipedia: http://en.wikipedia.org/wiki/Automobile_handling



Ignoring the flexing of other components, a car can be modeled as the sprung weight, carried by the springs, carried by the unsprung weight, carried by the tires, carried by the road. Unsprung weight is more properly regarded as a mass which has its own inherent inertia separate from the rest of the vehicle. When a wheel is pushed upwards by a bump in the road, the inertia of the wheel will cause it to be carried further upward above the height of the bump. If the force of the push is sufficiently large, the inertia of the wheel will cause the tire to completely lift off the road surface resulting in a loss of traction and control. Similarly when crossing into a sudden ground depression, the inertia of the wheel slows the rate at which it descends. If the wheel inertia is large enough, the wheel may be temporarily separated from the road surface before it has descended back into contact with the road surface.

This unsprung weight is cushioned from uneven road surfaces only by the compressive resilience of the tire (and wire wheels if fitted), and which aids the wheel in remaining in contact with the road surface when the wheel inertia prevents close-following of the ground surface. However, the compressive resilience of the tire results in rolling resistance which requires additional kinetic energy to overcome, and the rolling resistance is expended in the tire as heat due to the flexing of the rubber and steel bands in the sidewalls of the tires. To reduce rolling resistance for improved fuel economy and to avoid overheating and failure of tires at high speed, tires are designed to have limited internal damping.

So the "wheel bounce" due to wheel inertia, or resonant motion of the unsprung weight moving up and down on the springiness of the tire is only poorly damped, mainly by the dampers or Shock absorbers of the suspension. For these reasons, high unsprung weight reduces road holding and increases unpredictable changes in direction on rough surfaces (as well as degrading ride comfort and increasing mechanical loads).

This unsprung weight includes the wheels and tires, usually the brakes, plus some percentage of the suspension, depending on how much of the suspension moves with the body and how much with the wheels; for instance a solid axle is completely unsprung. The main factors that improve unsprung weight are a sprung differential (as opposed to live axle) and inboard brakes. (The De Dion tube suspension operates much as a live axle does, but represents an improvement because the diff is mounted to the body, thereby reducing the unsprung weight.) Aluminum wheels also help. Magnesium alloy wheels are even lighter but corrode easily.

 

[KaiserM715]

This unsprung weight includes the wheels and tires....(snip)...Aluminum wheels also help. Magnesium alloy wheels are even lighter but corrode easily.

Which is why 22"s and the like are not the best choice, unless looks are the only thing important to you. It also emphasizes selecting a strong, light wheel over Asanti's...

 

[frogslinger]

Subtle Dan...

go with these:

 

[MagicMtnDan]

I thought we'd get a thread going about unsprung weight so we can discuss it here instead of in somebody's wheel thread.

 

[BigJ]

Good knowledge, and good call starting a new thread for it.

At one of the **** events (click here for **** info) I tested what difference, if any, a 20" wheel made over a 17" wheel with all other things remaining equal. Both on the dyno and at the 1/4mile track, the difference was imperceptible; there just wasn't any.

Now that test involved cars that run mid to high 12sec quarters all day, lay down somewhere in the neighborhood of 350hp/350trq, and weigh in at almost 5000lbs with driver. NOT race cars, by any means. But none the less, they were cars we drive daily and occasionally take to the track to race. The larger wheel, in that application, just didn't matter.

How does that translate to the Raptor? I have no idea without testing it. But logically, when I consider the power numbers we're all at, and the weight of these trucks... well my hypothesis is we wouldn't see any real detrimental effect with larger wheels, in terms of speed performance.

But that's just a semi educated guess, and this really should be tested thoroughly.

 

[Madcowranch]

Cool J. IIRC, Car and Driver did the same test some time back and recorded some differences but I can't remember how much. I'll try to dig up that article.

 

[Madcowranch]

Here ya go...
http://www.caranddriver.com/features/10q1/effects_of_upsized_wheels_and_tires_tested-tech_dept

Acceleration and fuel econ suffered as wheel diameter increased. (more than I expected)

 

[BigJ]

Cool J. IIRC, Car and Driver did the same test some time back and recorded some differences but I can't remember how much. I'll try to dig up that article.

I recall that article too. If I remember, they got far more scientific that I did, measuring forces at the hub and what not?

The tests I ran are in no way that detailed. They are literally as simple as roll a car on the dyno, run it three times and regular intervals, average the results, jack it up, swap to bigger wheels, run it three times at the same regular intervals, average the results and compare. And a similar approach was taken at the track.

Physics is physics, and I have no doubt the bigger wheel shows a detrimental affect under the microscope. But in "the real world", which is what my test tries to replicate, there just was no statistical advantage of one over the other, given the power we were pushing and the weight of the cars.

Which data set is more "accurate"? That's up to the reader to decide

 

[BigJ]

Here ya go...
http://www.caranddriver.com/features/10q1/effects_of_upsized_wheels_and_tires_tested-tech_dept

Acceleration and fuel econ suffered as wheel diameter increased. (more than I expected)

Yeah thats it! And it totally figures, I think. The power to weight ratio of the vehicle they chose is far less than what I tested. But what I tested is far more powerful, in those terms, than the Raptor. In fact I'd bet the actual power to weight ratio of the Raptor is more in line with the car they tested, meaning maybe larger wheels do have a more noticeable effect.

That's why we'd need to test 'em

 

[MagicMtnDan]

Good knowledge, and good call starting a new thread for it.

At one of the **** events (click here for **** info) I tested what difference, if any, a 20" wheel made over a 17" wheel with all other things remaining equal. Both on the dyno and at the 1/4mile track, the difference was imperceptible; there just wasn't any.

Now that test involved cars that run mid to high 12sec quarters all day, lay down somewhere in the neighborhood of 350hp/350trq, and weigh in at almost 5000lbs with driver. NOT race cars, by any means. But none the less, they were cars we drive daily and occasionally take to the track to race. The larger wheel, in that application, just didn't matter.

How does that translate to the Raptor? I have no idea without testing it. But logically, when I consider the power numbers we're all at, and the weight of these trucks... well my hypothesis is we wouldn't see any real detrimental effect with larger wheels, in terms of speed performance.

But that's just a semi educated guess, and this really should be tested thoroughly.

Thanks for your into Jason. If I understand the testing that you did it was all straight line acceleration and had nothing to do with handling, braking and suspension performance. These are all real-world issues that Raptor owners have to deal with if/when they move to larger, heavier wheels. And that doesn't begin to address the benefits of having a taller sidewall vs. a shorter sidewall offroad.


And thanks to Madcowranch for posting the link to that excellent article by Car & Driver (I forgot about that one). Here are the last two paragraphs from that article:

We do know that the heavy, 19-inch setup suffered from the most impact harshness and seemed to tax the suspension the most. In contrast, the two smallest wheel-and-tire combos showed a propensity for more understeer on the skidpad but provided a more controlled and supple ride. And although it didn’t register on the dBA-meter, the 15- and 16-inch arrangements had a more pleasant sound quality than the larger tires.

The 19-inch wheels definitely look the coolest. But the 17- and 18-inch setups offer a better compromise of grip, acceleration, price, and ride harshness, so we’re not surprised VW uses 17- and 18-inch sizes on its hot Golf, the GTI. If it’s acceleration you’re after, stick with the smaller, lighter wheels and tires. And remember, unless you believe it is better to look good than to feel good, take our advice and stay away from extremely low-profile sidewalls and massively heavy wheels.