Roll-axis question

I want to go over a few things to make sure I understand them since the books are a little vague on the subject. This is based on a solid axle rearend set-up.

You get your set-up designed and let's say that based off the roll-axis, you have neutral roll steer.

1) Since the roll axis will probably change as the rear ride height chages, I take it that you have neutral roll steer at only this ride height?

2) If the above answer is yes, then ideally you would want a set-up that would add roll understeer during compression? I was thinking that when you accelerated out of a corner, without 100% anti-squat, the rear suspension is going to compress a little. Since the vehicle is leaning, then the roll axis will come into play, correct?

3) If both of those statements above are true, then lets say a set up that had some roll understeer at ride height, but moved to a neutral steer during compression, wouldn't be ideal? Would losing understeer be the same as gaining oversteer?

im pretty iffy on solid axle dynamics, but assuming ive gotten some of it through to my head,
it will be pretty hard to design a suspension without any roll steer throughout a given stroke... but i doubt its impossible.(think watts link in both directions)
but in 'generic' link setups, you will be neutral at only 1 point in the stroke.
how much roll steer per degree of body roll depends on the setup obviously.

as for 'neutral roll steer'... thats a hard-to-define-term imo.
since you will always gain or lose some steering angle due to roll, i dunno if you can call it 'neutral', at the changing point between over and under yawing due to roll.

there is no 'ideally' in this case, as in a tight low speed track (think autoX), roll overyaw might be beneficial to swing around the cones, especially at low speeds you may want a lot of this.
but on high speed road courses, obviously this may end up throwing you into the wall.
assuming this is what you want, then yes, underyawing will probably be the direction you want

anytime there is roll, the yaw angle of the tires vs the longitudinal centerline of the truck will come into play, but that does not guarentee over/under yaw.
depending on the tires, some will grip more at a higher slip angle vs another tire that will grip more at a smaller slip angle. this is why you see F1 cars running anti-ackerman and such weird-ass dynamics.

sadly, we do not have the tire data they have, so its mostly a guessing game/trial and error.



   s10cyncrvr wrote:
   
3) If both of those statements above are true, then lets say a set up that had some roll understeer at ride height, but moved to a neutral steer during compression, wouldn't be ideal? Would losing understeer be the same as gaining oversteer?



notice how i am replacing all the over/understeer by over/underyaw.

this is something i learned at Claude Roulle's VD class.

cornering is all about yaw. how much steering or grip you get is not the final goal.
the final goal is to get the vehicle to yaw faster.

so when you say its understeering, it means you want it to yaw faster than it is,
where when you say it is oversteering, you want it to yaw less than it is, because the tires are not keeping up.

it might take a while to remap that thought into your head, but once you do, it makes SO much sense and helps keep things straight.

as for your #3 question,

to picture it clearer... assuming you had irs, static toe in which bumps out during compression

again, this may be what you want in slow tight tracks, but is that what you want?

also, on paper, oversteer on entry and understeer on corner exit with neutral during the middle of the corner would seem ideal.

but it all comes back to whether the driver can drive such a dynamic change as well.

you can do so much with the suspension but dont forget the driver is a limiting factor as well.


and yea... not sure if i was able to help at all but i blabbed. lol

   greencactus3 wrote:
   im pretty iffy on solid axle dynamics, but assuming ive gotten some of it through to my head,
it will be pretty hard to design a suspension without any roll steer throughout a given stroke... but i doubt its impossible.(think watts link in both directions)
but in 'generic' link setups, you will be neutral at only 1 point in the stroke.
how much roll steer per degree of body roll depends on the setup obviously.


Cool, so we're agreed that the roll axis describes the behavior of the rear suspension at that one height only. With most set-ups the roll axis will change as the suspension height changes.

   greencactus3 wrote:
   as for 'neutral roll steer'... thats a hard-to-define-term imo.
since you will always gain or lose some steering angle due to roll, i dunno if you can call it 'neutral', at the changing point between over and under yawing due to roll.


I thought that was the idea of neutral roll steer. That the rear suspension wouldn't contribute anything to how the car steers when the body rolls on the suspension. Let's say you had zero bumpsteer in front and neutral roll steer in back, couldn't you drive the vehicle in a straight line with no steering input, even if it was rolling to the side?

   greencactus3 wrote:
   there is no 'ideally' in this case, as in a tight low speed track (think autoX), roll overyaw might be beneficial to swing around the cones, especially at low speeds you may want a lot of this.
but on high speed road courses, obviously this may end up throwing you into the wall.
assuming this is what you want, then yes, underyawing will probably be the direction you want


Yeah, I defiently don't want to have any oversteer. This will be a predominetly high speed handler and as a beginning driver I'd rather have the front end push at it's limits. Plus I'll have about 500 horses to get the rearend to come around when I need it.

   greencactus3 wrote:
   anytime there is roll, the yaw angle of the tires vs the longitudinal centerline of the truck will come into play, but that does not guarentee over/under yaw.
depending on the tires, some will grip more at a higher slip angle vs another tire that will grip more at a smaller slip angle. this is why you see F1 cars running anti-ackerman and such weird-ass dynamics.

sadly, we do not have the tire data they have, so its mostly a guessing game/trial and error.


I don't know what you just said there.


   greencactus3 wrote:
   notice how i am replacing all the over/understeer by over/underyaw.

this is something i learned at Claude Roulle's VD class.

cornering is all about yaw. how much steering or grip you get is not the final goal.
the final goal is to get the vehicle to yaw faster.

so when you say its understeering, it means you want it to yaw faster than it is,
where when you say it is oversteering, you want it to yaw less than it is, because the tires are not keeping up.

it might take a while to remap that thought into your head, but once you do, it makes SO much sense and helps keep things straight.


What's yaw? and as far as VD class, I've never had those problems, I always wear protection Razz

   greencactus3 wrote:
   as for your #3 question,

to picture it clearer... assuming you had irs, static toe in which bumps out during compression

again, this may be what you want in slow tight tracks, but is that what you want?

also, on paper, oversteer on entry and understeer on corner exit with neutral during the middle of the corner would seem ideal.

but it all comes back to whether the driver can drive such a dynamic change as well.

you can do so much with the suspension but dont forget the driver is a limiting factor as well.


and yea... not sure if i was able to help at all but i blabbed. lol

i don't know much about the subject, but i'd say that if you have adjustable rear links setup, you could adjust the rear roll steer at the track.

for instance, if you are racing at a mostly left hand turn track, you'd want the roll steer setup so that toe of the axle would be done for left turns and sacrifice some on right turns.

i think you'd have to keep the roll steer setup as neutral at ride hight.

i believe it's ideal to have the suspension setup so that the roll steer is more independent from the body roll of the truck. it's better to have it designed into the suspension and try to keep from relaying on the roll of the vehicle to create it.

you wanna see roll steer in action, check out nascar nowadays.

(http://motorsport.com/photos/nascar-cup/2008/as/nascarcup-2008-as-as-0193.jpg)

they toe the rear end to add "side-bite" nascar just recently limited the amount of toe to 1 deg. some teams were running over 2 deg, they were unable to pull onto nascar's inspection rigs and just about couldn't park into the garage stalls. lol