yes, or do you mean what is done in reality?

No, I mean what is done in practice. This is a common way to use this term.

or in reality, they are more tippy, not seem more tippy?

It is not clear that in all cases the quad is more tippy. It depends on the suspension design. See below.

what about the body roll? I thought body roll was an undesireable transfer of weight that had an adverse effect on handling?

and if you get rid of the roll aren't you compromising the ride by making it stiffer than an SRA to prevent the roll? Do you think that's good on an ATV?[/quote]

Body roll is only bad if the roll does not occur around the center of gravity of the machine. Then you get weight shift. The practice of designing a suspension to roll around the center of gravity is well known. On the other hand, sports quad riders have learned to compensate for the force on their bodies and the quad by lying low on the inside corner of thier quad as they go around the corner. This is not related to roll.

Bryce

 

hmmmm...

doesn't roll on an off camber shift the center of gravity and transfer weight no matter where the COG is on flatlands?

 

The center of gravity may not change, even if you are on a hill. What changes is the force that acts on the center of gravity when you are on a hill. Once the force vector acting on the center of gravity lines up with the downhill tire patch, the quad is just about to tip over. The center of gravity can remain in the same spot depending on how the suspension is designed. Often times it does not. However, since the center of gravity may be higher on a IRS quad compared to a SRA quad, it will be more tippy on a hill. Typically IRS is used to get more ground clearance on a quad. On the other hand, it does not have to be designed like that. A sports quad could be designed with IRS so that the center of gravity is no higher than an SRA. And if the suspension rotates around the center of gravity, it will not cause a weight shift in the corner. Then you will see the benefit of the low unsprung weight of the IRS. Which is to assure maximum traction of the drive tires. This difference in design phylosophy can be seen by comparing a high performance design of a Porsche to the higher articulation design of Hummer I. They both have IRS, but the goal of the IRS is quite different.
Bryce

 

so the roll creates a moment working to the bad side of the COG on side hills? right?

bigger roll, softer ride, bigger moment, less stable (current ATV IRS)

less roll, stiffer ride, less moment, more stable (current ATV SRA)

right??

 

Roll does not create a moment. If you want to look at the moment, it is a force vector operating vertically on a lever arm going through the down hill side tire patch. When that moment starts acting in the opposite direction, the quad tips over. Now it often happens that the center of gravity changes because more weight is on the downhill side suspension. This causes it to be more "Tippy". Although this is more because of design and less because it is a SRA or IRS. Either suspension can be designed either way.
Bryce

 

Very good comparison.


I must also say that I hadn't thought about the Hummer 1 when I post my original question. The H1 is one of the most extreme off-road vehicle and it indeed has IRS that improve the groudn clearance a lot by creating an arch between the wheels and not a barrier like SRA does... However, I still tend to think that solid axle design is more rugged than independant suspension... I guess there is a trade-off here...

The more I think about it, the more I get interested in IRS... I'm more and off-roaders on very rough terrains than a high performance rider where speed, acceleration and stability are more important. For ME, and ATV is first a vehicle to go on very rough terrains, in mud and snow where a pick-up truck cannot go.

 

not sure about that comparison - I.S. is meant to do the same thing, improved the stability of the contact patch - Hummer or Porsche

There is no lateral shift of the CG during a body roll (or) in an off-camber maneuver on ATVs? And that body roll does not produce a roll moment as well? I think there are a lot of riders that would disagree based on seat of the pants alone.

The IRS ATVs I’ve driven obviously have a roll center that is not the same as the CG height, so there is a roll moment. With current ATV independent rear suspensions that do not limit the roll as much as an SRA (so they ride better) - there is less roll resistance or roll stiffness. So you get more roll and you come closer to your rollover threshold due to a higher roll moment, a lateral shift (and increase in height) of the CG, and suspension compression on the low side that leads to a further shift in the CG by moving the contact patch closer to the body reducing the track width. Shifting your weight can shift the CG back some, but there is still more roll and roll consequences with the current ATV IRS. SRA is more stable in off cambers and pushed to extremes the one with less roll will be more capable in off cambers.

As this roll moment is upsetting to the vehicle control system (driver), many find this an undesirable situation and feeling when riding off camber.

But they do ride cushy.

 

My point is that the roll is caused by the moment, not the other way around. But I agree that the center of gravity often does not always line up with the roll due to suspension movement. My other point is that the suspension can be designed for performance instead of articulation. And in that case the IRS will be superior to the SRA.
Bryce

 

yes - thank you, I stated that backwards - I thought you said there was no roll moment or shift in the CG on these ATVs in earlier posts

my point was that the current ATV IRS and SRAs both have their strong and weak points

I do think that IRS is superior in potential