Will writes:
Hello, Don Sajeev! Over on Autoblog, I read Jonny Lieberman’s post on the Aston Martin 177 and it’s inboard shocks. I am not sure I will ever have to deal with this, but what is the deal with reducing unsprung weight as much as possible? I don’t understand: Is it just about greater control of the vehicle+motion through shocks & springs, or is it something greater? It would be nice to find out what El Mehtador + the Piston Slap community thinks about this.
Sajeev answers:
Yup, Lieberman’s right. Less unsprung weight (http://en.wikipedia.org/wiki/Unsprung_mass) means more grip on uneven surfaces and therefore better control, especially when you nail the throttle post-apex on a bumpy road. But . . .
There’s always a but: Aston didn’t grab the low hanging fruit from their gigantic wheels and outboard brakes. Inboard brakes (a la vintage Jaguars, http://www.cwiinc.com) and smaller wheels are probably a better way to lessen unsprung weight. Of course, that takes away from the Aston’s flash factor which is unbelievably important at this asking price and for this James Bond-associated brand.
But, still, again: the One-77’s twanky-inch rolling/stopping stock is complete overkill for its 3300lb curb weight. Even über-expensive BMW tuner Steve Dinan avoids wheels larger than 19″ for ideal tread grip and less unsprung mass. Proof positive is the current M3’s available 18″ wheels: the smaller wheels are faster around a road course. According to Dinan, that is.
Oversize wheels aside, increasing unsprung weight isn’t that bad. Take my Lego-like Fox-body Mercury Cougar: after installing some cheapo (sorry, Bertel) Chinese-casted 17×8.5″ wheels (at least 15 pounds heavier than the 15″x7″ stockers) and fully boxing the lower control arms (a common, low-buck Mustang upgrade), the car rode better and cornered with more solidity/confidence on the street. Which made my first interstate jaunt with the new parts an eye opener: putting the 6-speed stick into super cruise mode, the Cougar barreled down the highway, obliterating pavement joints like an E-Class Benz. A ghetto-engineered E-class, but, still, the car felt more confident and less darty.
[Send your queries to mehta@ttac.com]
Isn’t it more reducing unsprung mass?
You want the shocks and springs to control the motion of the wheel not the fact that the wheel is huge and therefore resists changing direction?
I’m thinking of an S-Class driving down a bumpy road. The wheels and moving up and down like crazy but the vehicle, being so massive, barely moves at all. You want the ratio of vehicle mass to wheel mass to be as high as possible.*
*Please note(on this issue) I barely know what I’m talking about.
I really need to find a way to test-drive an Aston Martin one of these days… maybe the “MBA” on my business card will get some salesman’s heart a-flutter, even if my income really doesn’t quite justify such a car :) .
My understanding is the same as jmo’s.
If the changes to the Cougar improved the ride despite increasing unsprung mass, something else also changed. Like the tires. And the rigidity of the upgraded control arms.
What I least understand is–how does that Fox-body Cougar happen to have a six-speed stick?
Meh, my old school E 28s’ fragile front suspension last longer because I keep the stock 195 70 14s. So do the wheels. On 16″ wheels, the low pro tires are one pothole away from damage. The ride suffers. Other than bling, the only reason to go go larger than 16″ is to run huge brakes.
jmo : You want the shocks and springs to control the motion of the wheel not the fact that the wheel is huge and therefore resists changing direction?
You make the car’s job easier with less unsprung weight. The shocks and springs are just one part. Again, this means more to a Elise than it does an S65 AMG. And S-class needs big heavy wheels to give the road presence of a Mercedes. The foxy Cougar needed modern rolling stock, and more modern driving dynamics that a 17×8.5″ wheel delivers…even if these particular castings are too heavy, IMO.
Michael Karesh : What I least understand is–how does that Fox-body Cougar happen to have a six-speed stick?
An overabundance of free time and some money lying around. That, and the belief that anybody can tune a Mustang…and that’s boring. :)
Andy D : Other than bling, the only reason to go go larger than 16″ is to run huge brakes.
And you only need so much braking power for certain weight classes. Even Baruth can’t cook a 13″ rotor behind a 17″ wheel on a 3500lb car without putting his own life in danger. (Just a guess.)
F = ma. If you reduce the mass term the same force will allow for a higher acceleration and so result in faster suspension response.
And S-class needs big heavy wheels to give the road presence of a Mercedes.
The need to be big to look right. But, if they could make the wheels significantly lighter, having a giant Mercedes with superlight wheels would feel even more “Mercedes” like on the road.
jmo : having a giant Mercedes with superlight wheels would feel even more “Mercedes” like on the road.
Maybe more BMW like. Benzes have that feel good, obliterate the road kinda presence about them, and I attribute some of that to heavier wheels. (and they are indeed heavy, the 17″ SL500 wheel I last grabbed was heavier than the wider 17″ wheel on my Cougar)
Actually, this is a very complex subject, and can’t be explained in a paragraph or two.
Lots of factors influenced ride and handling. Unsprung weight is pnly one of them, and probably a minor one at that.
One of the most overlooked factors is the torsional stiffness of the body structure.
Tire construction and size is probably the most important ride factor, followed closely by shock damping, suspension compliance, and tire inflation.
Plus perception. There is no objective standard for “ride” and “handling”. One man’s BMW M3 is another man’s oxcart.
You are correct that a lower unsprung mass is better, although this is not always true also, but close enough.
But, people are incorrect that pushrod suspensions lower unsprung mass. As previously stated on this topic the goal is to reduce inertia of what ever is being moved when the suspension moves, to it is less unsprung mass and more unsprung inertia. So for a pushrod suspension, you are ADDING the inertia of the pushrod and the crank arm to suspension movements. Why people believe this reduces the interia of the system is beyond me, the damper/shock is only partly sprung anyway.
What inboard suspensions get you is, first reducing drag on open wheel cars. Second, you get better control keeping the mechanical advantage on the spring/damper either progressive or regressive. With a conventional spring/damper attached directly to the a-arm, you end to get a lower spring spring rate as the suspension compresses. This is basically the opposite of what you want. Sure you can run springs that increase stiffness as they are compress, but your damper still is regressive. You also can get away with running a smaller lighter damper, because you can better use the entire stroke over the suspension travel. Also the shorter damper has some advantages in reducing bending and some other small things. You can also reduce bending on the a-arms if you are avoiding putting the spring/damper in the middle of an a-arm, this would allow for possibly lighter a-arms.
But, mostly they just look cool.
The bonnet gaps are just terrible. “Hand built” indeed.
@Sajeev Mehta: HEY! -Thanks for posting my question!!!
@chaparral66: Awesome!
Inboard brakes do reduce unsprung weight a lot, but they also make brake jobs more of a hassle, and in hard driving, the brakes and differential can tend to cook one another, which is not good for either’s health. Furthermore, if the differential leaks (not an outlandish occurrence on a British sports car), it can contaminate the brakes, as well as your driveway.
As for unsprung weight, the simplest way to consider its importance is to ask yourself whether you’d rather be hit with a three-pound mallet or a ten-pound sledge…
It’s irrelevant. The One-77 is simply a rolling art and design showcase. Just stare at the photos in awe.
It’s not the bigger wheels or smaller wheels that make the difference in the ride… it’s the proper matching of the weight of those wheels and brakes to the suspension. A suspension designed with lightweight wheels in mind will give a very choppy ride with heavier, bigger, wheels… which tend to increase oscillation over bumps and overwork the dampers.
I’ve driven quite a few cars where the owners (or dealerships) tried to improve the handling with bigger wheels. Doesn’t always work. And sometimes it can lead to unintended consequences… like poor wheel control and axle-hop going sideways… never a nice thing to find out when you’re driving at the limit of grip.
Less weight is usually better… both sprung and unsprung. Renault’s work with the R26R allowed them to actually use softer dampers, because they had less weight to deal with. Heavy cars that handle well often have an awfully choppy ride… especially combined with the big, heavy wheels they need to fit to go around the massive brakes they attach to these things.
Which is why BMW’s switch to runflats was an epic fail… suspensions designed to work with regular tires were overwhelmed by the extra weight of the runflat rubber. Their newer cars ride a bit better on the standard runflats, but I doubt that they can find any solution to the incredibly jittery ride over small, low frequency bumps due to the uber-stiff sidewalls.