If you Google the question: “how much does unsprung weight matter” the overwhelming near-consensus is that matters a lot. Unsprung mass, we’re told, will adversely negatively effect acceleration, braking, steering feel (heavier feel if there’s more mass) and handling, and overall subjective ‘feel.’
Unsprung mass = weight of (from most unsprung to least) tires, wheels, rotors, calipers, cv axles, knuckles, dampers, control arms, springs, etc.
SO, why have OEMs been adding so much of unsprung mass especially in the form of bigger wheels and wider (often heavier) tires. The HOnda Accord and Toyota Camry both have 19 inch wheels weighing nearly 30 lbs. each, the low-profile very wide tires are quite heavy as well. As I pointed out above, wheels and tires are the worst unsprung weights because they are rotational and also further away from the center axis cv axle.
In spite of this, modern cars appear to handle quite well (without steering feel) and ride quite comfortably.
So, maybe unsprung mass does not matter all that much, hmmmm???
People like the looks of larger tires and don’t drive their cars in a way where unsprung mass is meaningful to them. If someone drives sedately everywhere unsprung mass effects aren’t perceptible.
Mustangman can give a better answer than mine!
My guess that other unsprung suspension components are lighter, offsetting some of the tire/wheel weight gain.
Me? I much prefer my larger, wider tires on my newer vehicles than a 6.95/14 inch tire on a 50/60s car.
I know that my 1976 spindles, control arms, rotors, calipers are much heavier than my 09’s strut, control arm, hub bearing, rotor and calipers…
And the 09 will out handle/corner the 76 any day of the week… the 76 is way quicker/faster, but doesn’t handle as good…
But agreed 100%, I was waiting on Mustangman’s reply also…
Cars have been getting heavier over the past few decades, ever-increasing safety standards and more mandated safety equipment (air bags, ABS, ESC, backup cameras, etc.) add weight and complexity. So tires get wider, brakes get bigger to accommodate the extra weight.
With that said bigger wheels aren’t always heavier. The OEM wheels for my Mustang weighed 32.5 pounds for the rears (19x9.5) and 32 pounds for the fronts (19x9). I went with 20x10’s for a square setup, they weigh 27 pounds each. The weight of the tire went up by two pounds up front, and the back stayed the same. Overall going to larger wheel/tire combo netted me a 17 pound loss in unsprung weight.
They claim to have to use lighter brake disks that can’t be reused and flimsy bumpers to save weight for fuel economy, then they install huge wheels that have a significant negative impact on fuel economy, and they try to sell bigger and heavier vehicles.
It doesn’t seem that they are trying to reduce fuel economy across the fleet average. In cases where they haven’t discontinued a car model completely in the US like they did with the station wagon, maybe they’ve been trying to make cars worse on fuel economy and other things so they don’t cut in to the more profitable truck sales.
The rest of the car is the mass supported by springs, that would be the ‘sprung mass’. The wheels/tires/etc are not supported by springs, so they’re the ‘unsprung mass’. A Radio Flyer is 100% ‘unsprung mass’.
Big wheels for big brakes and… styling. Design sells cars. Big wheels look better so even cars that are not performance models get these 19 inch wheels.
Aluminum kuckles are standard these days. Many cars also have aluminum control arms, caliper and wheels. All this to allow huge brakes and big wheels and not destroy the ride.
It is a challange for the suspension designers as well as the shock guys but extra stiff bodies really helps a lot. If the car feels like a bank vault, it is much easier to adjust the shocks to provide good wheel control over washboard surfaces with monster low profile tires. And the shock guys like cars that handle, too, so big wheels and short sidewall tires are the result.
You’re thinking about it backwards. Unsprung mass is below the springs, sprung mass is above the springs.
The axle under the rear of your truck is unsprung mass. The bed (and contents of it) is sprung mass.
Back when I was interested in getting a car to go fast, I was told that removing one pound of unsprung weight was like removing 10 pounds of sprung weight.
Unsprung weight often has a large effect on a vehicle because most (but not all) of it is rotational. Rotating mass requires more momentum to move, meaning that each additional pound has an exponential effect.
Rotational mass applies to the parts that must be accelerated or decelerated when the speed of the vehicle changes. This includes items that rotate, like the driveshaft, brake rotors, wheels, and tires. Rotating mass is roughly three times harder to accelerate than sprung weight.
This means that dropping just 10 pounds per wheel with the addition of a lighter set of racing-oriented wheels would equate to a reduction of almost 120 pounds of sprung weight. In most drag racing instances, 100 pounds lost equates to a tenth of a second or one car length in the quarter-mile. It’s a small amount on paper, but it adds up if you’re racing competitively.
Unsprung mass is actually sprung by the tire. The tire acts like a spring between the road and the axle. It acts a bit like a shock absorber, too. Bounces at roughly 10 to 18 cycles per second (hertz)
The shocks are mounted between the axle and the car body (or frame). Shocks can easily damp those motions but damping the axle is a bit more difficult. The body bounces rolls and pitches between 1.2 to 3.5 cycles per second. That seperation between body and wheel frequencies helps the choice in shock damping.
Since springs have two ends, I guess it partly depends on which of the two ends of a spring is said to be the “unsprung” side. If a spring was nailed to a tree in order to weigh a fish, and the fish was on the other end, I’d say the fish end was the “sprung” side. But by similar logic the tree end could also be the “sprung” side.
The main issue with unsprung weight is when you hit a bump or hole in the road. The sprung weight goes relatively smooth over the bump or hole because the spring absorbs the up and down motion. But the unsprung weight will tend to go airborne for a moment.
If you hit a bump in a curve, especially near the limits of adhesion, you don’t want that tire to be off the pavement for very long. Engineers, with the aid of computers have been able to tune the suspensions to quickly return the tire to the pavement, even with higher unsprung weight. This results with better handling and ride.
There is also the trade off of higher unsprung weight but with wider tires and better brakes. With the better suspension tuning, you can have it all, or at least most of it.
Mt Bike racers figured out early in the sport that spring-suspended bikes were considerably faster on bumpy trails than the original non-suspended type. The theory goes that the spring compresses when the front tire hits up against the bump, the bike slows down, but speeds up near the same amount as the spring expands and pushes on the bike on the other side of the bump.
On a car, the spring is not there to suspend the axle from the frame. The spring is there to hold the frame up from the axle. Any load borne by the spring is sprung weight.
Buicks through 1960 had an enclosed driveshaft called torque tube drive. This added to the unsprung mass. Consumer Reports was critical of this arrangement. I had a 1954 Buick some years back. Consumer Reports was correct in my opinion. That Buick wasn’t the best handling car on the road. Chevrolet used the torque tube drive through 1954. The 1955 with an open driveshaft and ball joint suspension handled much better than the 1954.
