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.
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
goddammit i typed a 20 min response and forum failed to post it so im typing again...
s10cyncrvr wrote:
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?
i think i agree on this, but you must ignore tire deflection and its effects Razz
s10cyncrvr wrote:
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.
cliffs: we dont have any usable tire data so we have to go blindly and hope for the best.
s10cyncrvr wrote:
What's yaw? and as far as VD class, I've never had those problems, I always wear protection Razz
vehicle dynamics Sad and protection is good Very Happy
yaw/pitch/roll. the 3 rotational axes. what the rudder controls in an airplane
s-10driver wrote:
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.
agreed. most of us (with solid axle on linked suspension) will be able to adjust this by ride height
which works out imo.
for autox: higher ride height = more roll, more steering input from rear, more anti squat for the 'jerky' throttle inputs, etc
for road courses: lower ride height = opposites of those
s-10driver wrote:
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.
this has been discussed many times in depth on our formula team, but it has been ruled out due to the fact that us amatuer drivers will not be able to drive a car that handles noticably different on right and left turns, even if there are fewer of one direction, its not 'enough'
s-10driver wrote:
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.
agreed. too much and you lose consistency/predictability
s-10driver wrote:
you wanna see roll steer in action, check out nascar nowadays.
aside from the fact that nascar sucks, i wouldnt look too deeply into their suspension...
they run not only truely assymetrical suspension setups, but truely assymetrical cars.
and then there's the banked turns + aero.
the normal forces generated by going along a banked turn at those speeds is incredible.
we will not see those forces.
and aero. when a car handles differently depending on how dense the crowd is in the stands (wind block or lack of)
you know they are going damn fast
The only models I have seen with completely neutral roll steer have the lower links facing forward and the uppers facing reward, all links having equal length. That would make for some horrific pinion angle changes.
Using long links and setting it neutral at ride height is the best option, IMO, for a 3 or 4 link.
Supposedly a Satchel link produces neutral roll steer but I can't find anything supporting that.
Yeah, that looks like what I'll be doing.
assuming you end up having the forward links at top and bottom links going back,
the instant pitch center seems to fly forwards and back in sus travel.
with a quick sketch you can actually have the pic go across both sides of the axle.
I meant setting it up to be neutral and have longer links. It will be a forward facing 4-link.
oh ok. i was worried for a second lol
