Roll center and roll axis angle

De Ranged

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Gidday

Don't know if I'm allowed to start threads here or not yet so please bear with me if this double posts or something else lol

Right I'm in the process of building a long travel Winch Challange/Rock Crawler (1982 double cab Hilux ute) becouse this is gona see alot of road k's and possibly some offroad racing (forest tracks) I'd like to make certian I'm on the right track with a few things

I have a good understanding about what works on a club trucks and trials trucks and have setup several trucks now with good success (hope to have the latest one debute about mid season for the national trials series) I have learned most of what I know from books, playing round with Triaged calculators (awsome tools :D) and then appling these results

My issue is I'd rather not discover through trial and error what dosn't work for high speed suspension lol and alot of the guys that build locally don't really know the science behind what they are doing, and alot are ego's that need to be stroked :rolleyes: not my style lol
So I thought i'd pitch a few questions out to you guys

First off if you are running beam axle front and you have the links behind the axle pushing it, your roll axis angle is measured from the front

so in the example above the roll axis angle would be - not + so would be understeer not oversteer as pictured... just want this confermed :D

Now for stability at speed I'm working on keeping the Roll Center Height as static to the CoG as possible through its travel so the truck dosn't feel like its moving under you

Roll Center Height.... this is an interesting one... online I've read that you want this close to the CoG, as this stops body roll... yet all the books (which are about car suspensions) warn that this will cause jacking in corners, I've yet to experiance this so would anyone care to explain why (and the books just say it will happen :rolleyes: not why)
At this stage I'm more inclined to run closer to the CoG as this makes the truck handle better on sidelings, a common theme in compitition here

Roll Axis Angle, I'm trying to get the gain and loss as the suspension cycles as low as possible with about 5 deg understeer (front and rear) at rest, so its steering while a bit slow is nice and predictable

Am I on the right track??

Cheers Reece
 

Triaged

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No...that would still be roll oversteer in the front. Remember that roll oversteer (left turn) in the rear the tires point out of the turn (right) in roll while in the front roll oversteer will cause the tires to point into the turn (left).

You don't get jacking with solid axle suspensions like you do in independent suspensions. With a Panhard bar you can get jacking if it is not level to the ground, however it will jack up when turning one way and jack down in the other.

Give this link a read (even if you aren't a pirate fan). It is much the same topic as yours.
http://www.pirate4x4.com/forum/showthread.php?t=726469
 

De Ranged

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No...that would still be roll oversteer in the front. Remember that roll oversteer (left turn) in the rear the tires point out of the turn (right) in roll while in the front roll oversteer will cause the tires to point into the turn (left).
lol thank you for that, I was locked into thinking of the reversed arms so...:rolleyes: not something I've ever had to worry about, for what I've built I'ver always set it as close to 0 for tracking... trials is a slow technical sport ;)

You don't get jacking with solid axle suspensions like you do in independent suspensions.
Well considering all the books I've read are more concerned with independent its not surprising they didn't mention that

With a Panhard bar you can get jacking if it is not level to the ground, however it will jack up when turning one way and jack down in the other.
This effect also comes into play when you put a truck into an extreme sideling, tipping you over.... and is why I'm building a variation of the Watts link on the front of my trails truck ;)

Give this link a read (even if you aren't a pirate fan). It is much the same topic as yours.
http://www.pirate4x4.com/forum/showthread.php?t=726469
Thank you for that there is some good reading in there

Thank you for your help

Cheers Reece
 
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NoThrottle

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I hope you're still around and subscribed to this thread. How did you go with this? I'm at about the same point now as you were then. "a variation on a watt's link" would that be a mumford? I've been racking my brain (steering pun intended) on how to get the steering to work through either of those links without horror bump steer. I have a feeling the key is to hang an intermediate link of the steering off the top end of the watts link bar in the middle so it moves one way to shorten the lower (axle) half of the steering link in bump. Sorry for the ancient thread dig fellas, but this sort of information is still scarce.
 

FABRICATOR

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I hope you're still around and subscribed to this thread. How did you go with this? I'm at about the same point now as you were then. "a variation on a watt's link" would that be a mumford? I've been racking my brain (steering pun intended) on how to get the steering to work through either of those links without horror bump steer. I have a feeling the key is to hang an intermediate link of the steering off the top end of the watts link bar in the middle so it moves one way to shorten the lower (axle) half of the steering link in bump. Sorry for the ancient thread dig fellas, but this sort of information is still scarce.

If you are talking the same solid axle and forward running links setup, then another way to have smooth and fairly bumpsteer-free steering is to have a drag link that runs fairly parallel with and equal in length to the suspension control arms. One way to use the drag link is to connect its forward end directly to a two-arm knuckle in conjunction with a single cross tie rod. Another way is to run the drag link to a bellcrank idler off the axle, link to one knuckle and then across, or from the idler out to each wheel. The steering box setup takes either a radical relocation of the box, or more minor changes, an idler, and two drag links.
 

NoThrottle

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.......run the drag link to a bellcrank idler off the axle, link to one knuckle and then across, or from the idler out to each wheel.

That was my plan-B
Yes it is a radius arm setup, quite long arms at about 55 inches, about as far as I am willing to go with a 117 inch wheelbase
Unfortunately I dont think I have space for that setup, the transfer kicks out to the drivers side.
Fully compressed the front differential pumpkin fits into a recess on the inside edge of the chassis rail and clears the chassis by 1/2 an inch and the timing cover on the motor by 1 inch. On the outside of the chassis rail is the coil spring and shock absorber. I suppose I could run the forward link on the navigators side of the vehicle where there is more space but things start getting ugly there - on the other hand the steering column taking a 180 degree turn across the car toward the relocated steering box is certainly easier than trying to move or link it back under the driver seat.
Everything is tight, I am building it for a low polar moment of inertia and trying to make it corner well in the tight stuff because with the weight of the cast iron block, head and a 300 pound gearbox along with the full chassis it will never be fast in the straights. The engine and gearbox are pushed back into the cab a foot.

The car has solid axles front and rear, is 4x4 and used for Australian desert racing. Rear is a 5 link and front is a radius arm with Panhard.

The front Panhard is causing me problems which is why I'm trying to move away from it, on sweeping fast corners turning toward the drivers side it oscillates because of the Panhard jacking which then upsets and unloads the front end and sends all drive to the rear putting it into oversteer, that combined with too short links on the rear giving it rear steer with body roll makes it a real dog to drive fast. Also the front panhard limits travel by binding everything up as it pulls the front axle sideways with big droop

If I can keep the steering box in the original position I would prefer it, but a self compensating steering linkage through a watts link on the chassis is going to be ugly and fragile.

Getting back to roll center: From the little I know fixing the roll center to the chassis should help the car handle more predictably. I have plenty of space in the rear to make the rear watts link pivot adjustable to fine tune the roll axis. I'm guessing the COG and I dont have access to half the technology you guys do so I don't think I will get it right the first time.

The more I think about it the more I am leaning toward relocating the power steering box under the navigators seat with an idler link on the back of the differential, it might even be easier to make it left hand drive if the steering box is on that side. Thanks Fabricator you have given me new insight, I've been stumped on this for months.

There are a few photos of the car under construction here: https://www.facebook.com/#!/media/set/?set=a.188434601233069.47400.187297714680091&type=3

general arrangement of the frontend, remember it's right hand drive. (Yeah it's a diesel, but about 350hp)


Roughly what the new radius arms will look like at full bump


full bump not much room.
 

De Ranged

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Gidday

Can't really help you much, my trials truck had the steering box relocated to the axle... trials is a slow speed sport and CoG to me is more important than reaction mass of the suspension... this has all been cut off (recently lol) so I could move my motor forward and being competition I didn't post a build thread
I asked and have asked again based on curiosity for people who have raced non-pan hard setups if it was noticeably worth the work to setup and complexity.... if there was a gain in times, speed.... and I have yet to have anyone produce any data worth looking at
If you are keen to do it the best way (easiest and least complexity) I found is convert to push pull steering using a steering box mounted near the chassis pivot point for your right hand side radius arm your drag link works along the side of the chassis then the steering arm on right hand side has mounts for draglink (to side) and tie-rod (to front)
Basicly the same as the beam axle hilux but with a draglink as long as the radius arm (there is a good sorce for the box)

Cheers Reece
 

GEspo

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Adding here some info on the "slip angle":

"As the tire rotates, the friction between the contact patch and the road results in individual tread 'elements' (finite sections of tread) remaining stationary with respect to the road. If a side-slip velocity u is introduced, the contact patch will be deformed. When a tread element enters the contact patch, the friction between the road and the tire causes the tread element to remain stationary, yet the tire continues to move laterally. Thus the tread element will be ‘deflected’ sideways. While it is equally valid to frame this as the tire/wheel being deflected away from the stationary tread element, convention is for the co-ordinate system to be fixed around the wheel mid-plane.

While the tread element moves through the contact patch it is deflected further from the wheel mid-plane. This deflection gives rise to the slip angle, and to the cornering force. The rate at which the cornering force builds up is described by the relaxation length."

Adding here some info on under and over steer w/ regard to front and rear slip angle:

"The ratios between the slip angles of the front and rear axles (a function of the slip angles of the front and rear tires respectively) will determine the vehicle's behavior in a given turn. If the ratio of front to rear slip angles is greater than 1:1, the vehicle will tend to understeer, while a ratio of less than 1:1 will produceoversteer.[2] Actual instantaneous slip angles depend on many factors, including the condition of the road surface, but a vehicle's suspension can be designed to promote specific dynamic characteristics. A principal means of adjusting developed slip angles is to alter the relative roll couple (the rate at which weight transfers from the inside to the outside wheel in a turn) front to rear by varying the relative amount of front and rear lateral load transfer. This can be achieved by modifying the height of the roll centers, or by adjusting roll stiffness, either through suspension changes or the addition of an anti-roll bar."
 

GEspo

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Also:

"Understeer can typically be understood as a condition where, while cornering, the front tires begin to slip first. Since the front tires are slipping and the rear tires have grip, the vehicle will turn less than if all tires had grip. Since the amount of turning is less than it would be if all tires had traction, this is known as under-steering.
The opposite is true if the rear tires break traction first(oversteer)."
 
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