Question about Suspension

[jj]

In the 1960's, I drove 2 Pontiac Catilinas, both BF cars, that drove very nicely because they had rather high-rate springs for their size, and they also had negative-roll-center design for the suspension. The 1969 ex-state patrol car was very nice in terms of sleeperhood, it looked like a grandma car, but would move right on out, freeway or US 219 north of Marlinton.

I notice that most cars seem to use positive roll centers nowadays.

I'm not an expert in the field, so why is that?

 

[jj]

Errr. Nobody seems to have anything to say. Hunh. No auto engineers, I guess, or did GM pension them all off?

 

[Monketi Ghost]

The "suspension" wordplay that has been going on is funny! I wish I knew more about cars, and that I could come up with something clever to add!

 

[ChuckieR]

jj, maybe you should start a new thread with the term "Vehicle Suspension" or "Car" or something in the title. I think "suspension" is too ambiguous. I took a peak, but I honestly thought it was going to be a question about forum rules and when an account gets suspended!

Maybe throw the word "physics" into the subject, to pique the interest of the really smart folks

i.e., "Question regarding physics of positive-roll vs. negative-roll vehicle suspension".

Sorry I don't have a good answer, not being a car expert. I can only guess that it might have something to do with how the vehicle will handle in a swerve maneuver? Or maybe the heavier and "higher center of mass" cars of the past had more body roll, and needed the negative roll of the tires to compensate? Just guesses.

 

[Luceiia]

Interesting read at this link.

...the sideways force acting on the car can be said to be acting at one point, the C of G. The forces resisting the car can be said to be acting at one point, the RC. The further these two points are apart, the greater effect the lateral forces will have on the suspension, or the more the car will roll. Theoretically, if the C of G was below the RC, the car would lean into corners like a motorbike!

I'll nose around later and see what else I can add to the suspense.


Luceiia

 

[jimlintott]

I did some looking around on the subject. I'm a car nut so it was fun. I couldn't find the terms positive or negative in reffering to roll center but for sure there is a difference between older cars and newer ones. Older ones like you mention generally used a double control arm up front where as most of todays cars are Mcpherson strut which only has a single, lower control arm. Tires are different too. Bias ply tires were still around then but all new cars are on radials and often low profile ones. I'm going to guess that they want to keep these radials flatter on the road so they engineer in a lot less camber change when the body rolls over and I know that they want much less body roll. Independent rear suspension is more common and live axles at the rear are less common. Way less body roll with independent rear which can actually make the car squat and resist rolling.

Handling is very subjective. Pickup trucks are a big seller as they give a very similar ride to the big cars from the 60s (which were essentially trucks with car bodies). Some people like that soft mushy ride. I personally can't stand it. To me a good handling car is one that you can tell whether the quarter you just ran over is heads or tails. I get into a Grand Marquis and start thinking. Whoa these seats are way to soft and padded, whoa this steering is way overboosted. The people who own those things are more interested in creature comfort than driving. That's fine that's their choice, it sure bugs me when they say their car handles well (it doesn't, period).

jj, just curious by what you mean by negative and positive roll center?

 

[Frostbite]

Once I put a stinkbomb in a my highschool teacher's desk. I got suspended.

 

[RedCoat]

It's been a while since I did chassis work, but here's my two cents:

When you say positive/negative roll center, I assume you're referring to roll center height. The roll center for vehicle's generally isn't the same in the front as it is in the rear, so to be totally accurate you have to refer to a "roll axis" that connects the roll centers of the front and rear suspensions.

I think it would be unusual for your 1960's era vehicle to have had a negative roll center height. Roll centers are typically at or around ground level, in my experience, because having the roll center below the ground causes weird handling quirks and so-called "jacking forces" on the chassis that tend to change trim height while cornering.

I would guess that the desireable handling and ride properties of your older car resulted from a) a huge wheelbase, which helps ride quality without requiring soft, smooshy springs, and b) a whole hell of a lot of mass. Massive 5000# cars just don't react to road loads like little vehicles do, and generally ride a little smoother than a lighter (i.e. modern) vehicle with a proportional spring rate.

-RedCoat

(Card carrying member, Society of Automotive Engineers)

 

[jimlintott]

I have a question for you RedCoat. You suggest mass as contributing to ride but isn't this mass sprung weight? If two different size vehicles are sprung proportionately wouldn't they feel roughly the same.

I suspect that the different feel might have more to do with the fact that modern cars are much stiffer in the chassis and body, are sprung somewhat stiffer (as more of a premium has been placed on handling and cornering) and are often on a low profile tire which again is like a stiffer spring.

I'm not saying you are wrong, I'm just trying to learn something new.

 

[rwguinn]

I have a question for you RedCoat. You suggest mass as contributing to ride but isn't this mass sprung weight? If two different size vehicles are sprung proportionately wouldn't they feel roughly the same.

I suspect that the different feel might have more to do with the fact that modern cars are much stiffer in the chassis and body, are sprung somewhat stiffer (as more of a premium has been placed on handling and cornering) and are often on a low profile tire which again is like a stiffer spring.

I'm not saying you are wrong, I'm just trying to learn something new.

It takes more energy to move more mass. There is only a limited amount of energy available from the road surface at, say, 60MPH.
The stiffer suspensions require a stiffer chassis to maintain frequency separation between suspension and chassis.

RW

 

[jj]

jj, just curious by what you mean by negative and positive roll center?

Well, back then, the way it was explained to me was that a negative roll center describes the idea that if you push sideways on the car at the CM, the car will roll TOWARD you.

I don't think you can do that with struts, now that I think about it, at least in the way they did it with control arms.

NOTE: I could be wrong, that's just a thought based on how the old ones I looked at worked, it depends on the anchor points and lengths of the two control arms.

Said cars had very little lean going around corners, and fairly stiff suspensions, never mind that they were the size of medium-sized busses

This is roughly the opposite of the "caddilac ride".... that spongy "is there a road down there or not" wallow...

 

[jj]

I would guess that the desireable handling and ride properties of your older car resulted from a) a huge wheelbase, which helps ride quality without requiring soft, smooshy springs, and b) a whole hell of a lot of mass. Massive 5000# cars just don't react to road loads like little vehicles do, and generally ride a little smoother than a lighter (i.e. modern) vehicle with a proportional spring rate.

-RedCoat

(Card carrying member, Society of Automotive Engineers)

Well, yep, it had a wheelbase bigger than some apartments, both length and width, and it did weigh 4900 lbs. Had a 10.75:1 428 cu in engine, with two Rochester 4-barrels. (You know the joke about passing everything but an Amaco station? You got it!) (Well, let's be specific. Playing around you got 4-6 mpg. On the highway you got 19-20, running on 2 of those 8 barrels. Had to replace all the exhaust valves and most of the suspension when we got it (cheap), it was, well "used" to say the least.)

It also had (modded for pursuit cruiser use) high-rate springs (there were 6 grades, it had grade 5 in front, 4 in back, if I recall correctly) as opposed to the 'stock' 2 and 1. It was not an understeering car at all, it was neutral to slightly oversteering under stress or acceleration, and you COULD feel them bumps, man.

Some days I wish I had one of them, with a modern ignition and injection system, all polished up and looking like my gramma's car. They were SO "slow" looking but they did 0-60 under 8 seconds in a cloud of hydrocarbons, and they were geared for highway even then, not for accileration.

 

[jj]

because having the roll center below the ground causes weird handling quirks and so-called "jacking forces" on the chassis that tend to change trim height while cornering.

Hm, would that be sort of a "rise up and lean into the turn" sort of thing? Because if that's it, you got it.

 

[69dodge]

What you describe sounds like a high roll center, actually.

You got 20 mpg? Wow. I rarely get more than that, and I only have a 318 engine. On the other hand, my lower limit is 11-12 mpg. (I tend to drive rather slowly on city streets and insanely fast on the highway.)

 

[jj]

What you describe sounds like a high roll center, actually.

You got 20 mpg? Wow. I rarely get more than that, and I only have a 318 engine. On the other hand, my lower limit is 11-12 mpg. (I tend to drive rather slowly on city streets and insanely fast on the highway.)

]

Yeah, the thing was trimmed to run on 2 barrels if you weren't goofing off, and all 8 if you punched it. I htink the accilerator pumps by themselves dumped about a quart

I dunno, the car was advertised at the time to have a "negative roll center" but I never quite managed to get a good description of what that meant.

In any case, it leaned in, instead of out, of a turn. Lots of body up and down but handled quite nicely, predictably, and (old bias ply, remember) would drift like a champ.

 

[peptoabysmal]

jj, maybe you should start a new thread with the term "Vehicle Suspension" or "Car" or something in the title. I think "suspension" is too ambiguous. I took a peak, but I honestly thought it was going to be a question about forum rules and when an account gets suspended!

Maybe throw the word "physics" into the subject, to pique the interest of the really smart folks

i.e., "Question regarding physics of positive-roll vs. negative-roll vehicle suspension".

Sorry I don't have a good answer, not being a car expert. I can only guess that it might have something to do with how the vehicle will handle in a swerve maneuver? Or maybe the heavier and "higher center of mass" cars of the past had more body roll, and needed the negative roll of the tires to compensate? Just guesses.

LOL

I thought it was going to be on particles in suspension, say in a liquid or some such.