Thread: 130 on portals

Originally Posted by uninformed

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Bushes: the portals will put more stress into the axle end bushes and making the RA's longer will compound this...

cheers,
Serg

IMHO longer radius arms will be slightly beneficial for the load on axle end bushes, but nowhere near enough to counteract the extra load from portals and/or larger tyres.

Originally Posted by slug_burner

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Effects on anti dive and anti squat. This is where I start to lose it a bit. the alignment of all the bits that join the axle assembly to the chassis do not change so why do we change the dive and squat characteristics of the vehicle?

I don't know what it is that determines roll centre but I would expect that if CoG has gone up so would roll centre. Although I can understand what CoG means and how you could determine it but not sure about roll centre. ...

IMHO it helps to remember what dive and squat is, and what causes it before getting into the anti's.

During acceleration and deceleration (deceleration is really an acceleration in the opposite direction to motion), the resulting inertia causes the force acting on the suspension springs to change. During braking the load on the front springs increase and that on the rears reduces - so the front of the vehicle dives down and the rear raises. During acceleration the reverse happens and the rear squats.

The force causing acceleration is applied at the tyre contact with the road and has to be transferred through the suspension links to the vehicle sprung mass. The geometry of the suspension links can be arranged so that the additional force that occurs in the links during acceleration will occur in a direction so that a component of it opposes the dive or squat - this component force is called anti dive or squat.

100% anti dive/squat results when the anti dive/squat force is equal but opposite to the dive/squat forces. Note it can exceed (greater than 100%), but is better if somewhat less than 100%.

Even if the geometry of the suspension remains the same, increasing the COG by larger tyres or portals will increase the dive/squat forces. The anti forces can only increase (to restore previous anti %) by changing the suspension geometry.

The location of the roll centre differs with suspension types and geometry. What is better, depends upon the use. For example a F1 vehicle the roll centre may be below the road surface, while for off road a high roll centre is best - F1 has very stiff suspension to reduce roll, where offroad we want suspension flex, and a higher roll centre reduces roll (no roll would occur if the roll centre was at the same height as the COG.

With a coil sprung RRC, Disco I, or defender, the front roll centre is near the midway point on the panhard rod, and the rear roll centre is at the A-frame ball joint - these are the points that don't move when the axle articulates, ie. the suspension rolls about these points (actually it is the sprung mass that rolls about these points).

Hi Brendan I'm pretty sure I went past you on the highway between Beresfield and Hexam yesterday? I was in a white 110 and snuck a late wave in.

Sam

John /Serg,with regards to body roll, the Roll axis is really the centre height above the spring saddles on the axle housings that the sprung mass rolls around, is it not? If the track width increase was sufficient to maintain original centre of gravity, then portals on their own,or even with larger tyres will not alter the vehicles roll centre and lateral rollover angle, would that be correct?
Scrub radius is still an aspect of fitting portals to standard axles that i'm not too happy about. Power steering and constant 4wd does tend to mask the bump steer effect of a large scrub radius. My own conversion on an old series 2a with manual steering and part time 4wd, although quite manageable if driven slowly over rough broken terrain, does give me a real indication of how potentially dangerous things could get with a sudden tyre failure on the road at speed. I've mentioned it before on an earlier post(#58) that i believe the track width increase should ideally occurr at the axle housing and the wheel rim back spacing increased to restore the relationship of the tyres contact patch and the point where the king pin axis meets the road.This modification, which I intend to do on my own vehicle before It resumes service as a daily driver, will require the larger internal diameter of a 16'' split rim which would have the appearance of the front wheels of a dual wheel truck. Not pretty but much safer IMO.

Wagoo.

Ill try to add a little more to what John and Bill have said.

Let it be known, im just a carpenter and have only a very little experience with this stuff. Bill and John's Knowledge AND experience is well beyond what I will probably ever have.

John, I was unsure about the length of the RA and how it affects the bush loading. But I was sure that the portals and or increase in tyres would increase load on them.

Basicly, anti-squat, in the style designed into rear suspension to resist squating during acceleration, is a combination of: Wheelbase, Cog, and in the case of the rear of a coil rover (RR,Def, D1), the relation of the rear trailing arms and the Aframe. When viewd from the side of the vehicle, if you draw a line from the center of the ball joint through the chassis end bush. Draw another line from the center of the axle end bush of trailing arm, through the chassis end bush....where these to meet is the Convergence point. Now draw a line from rear tyre contact patch through this convergence point and extend it....where it passes over the center line of the front axle determins the amount of squat....if it was to pass through the COG exactly at the center line of front axle it would be 100% anti-squat...above it would be greater than 100%

Anit-dive is similar but a bit different. (John or Bill may well correct me here??) It is a combination of: Wheelbase, COG, front to rear brake bias and link geometry....looking side on again, you have wheelbase, and COG. drawing a line from contact patch through the chassis end bush of radius arm (RA from here on in). Divinding the wheelbase into brake bias percent, ie if the front braking was 60%, then you would have a vertical line drawn 60% of the wheelbase from the front axle center line. Where the contact patch - RA bush line interesects this brake bias line will determine anti-dive. If it were to intersect directly at this line and also through the COG line, it would be 100%

Bill, when you say roll axis, I belive you mean roll center height. There are 2 types of Roll axis:

#1 Axle roll axis: this being front and rear. Usually expressed in understeer or oversteer...Front: in this case is determined by the angle of the RA....more specificly the axle center line, as the 2 RA bushes at axle end disect the axle centerline, and the center of chassis end bush....So if the axle center line and the chassis bush center are level, ie neither angling up or down from front to back, this would be 0° front axle roll axis.

Rear: In our case again, it is determined by the angle of rear trailing arm (because they are parrallel). That is to say, drawing a line through the axle end bush of TA through the chassis end bush center...Once again if it was level, no angle up or down from back to front, it would be 0° rear axle roll axis.

From what I have seen, most stock rovers are ABOUT 0°-2° front axle roll axis...as this is a positive 2° it is roll oversteer. In the rear on stock rovers it is somewhere about 4°-6° degrees rear axle roll axis, roll oversteer.

#2 Vehicle roll axis: this is determind by the front and rear Roll center heights: Front, is the mid point of the chassis (when viewed from the front) where it disects the panhard rod...so draw a vertical line through the mid point of chassis and where it intersects the panhard is your front Roll center height. Rear, as the convergence points for the upper links, that is the A frame or Y or wishbone, is the ball joint. So the center of the ball joint is the rear axle Roll center height. Draw a line through the front and rear Roll center heights(when viewd from the side) and that is the Vehicle Roll axis.... Generally speaking, a higher Roll center in the rear makes the rear let go first when traction is lost through a corner.

As John said regarding roll center heights: higher in 4wd's....but of coarse there is effects, that is a higher roll center makes a vehicle more stable say on side slopes but makes it harder for the suspension to work....look at desert racing trucks with floppy rolly bodies...they have a lower roll center but there suspension is very supple and keeps power to the ground...(this is a very basic idea of the concept I have given, just remember one small change can have alot of different effetcs)

things to consider...not only would portals OR larger tyres change dive and squat, but wheelbase alone also does, that is to say a defender 90,110 and 130 all have different AS/AD numbers even though they share the same suspension links, tyre size etc.

I hope I got all that right....my brains a bit fried now haha I will add more later...maybe if I havent got it all wrong hahahahah

thinking out loud about your question Bill....lets take a stcok defender and all its geometry etc....now if you just add portals AND make them wide enough to keep the COG at the same point as stock. You wont have change the lateral Roll over angle in theroy, BUT... you would have change the axle Roll center heights and therefore how the vehicle handled on and off road. You would have also change the AS and AD because you have change the height of the links and the interaction of contact patch, links and COG has changed within each other...Axle roll axis would remain the same.Veichle roll axis would also remain the same angle....

You are much more familiar with scrub radius and I believe what you say. The only info I can add is the vehicle I know runs 16x6 milartary rims with 750-16s on it. It seems fine and handles well......

A simple example:

to get an idea of what adding portals does to the front end with regards to any driving, whether it be on road of off.

take a long handle screwdriver: either flat or philips.....with the handle in your hand and the tip on the ground, push the tip along the ground over and cracks in the concrete....first try it at a very shallow angle, almost parrallel with the ground....now raise your hand to make this angle steeper, doing it again......imagine this screwdriver is the line drawn from contact patch of front wheel through the RA chassis end bush center.....

food for thought

Originally Posted by uninformed

A simple example:

to get an idea of what adding portals does to the front end with regards to any driving, whether it be on road of off.

take a long handle screwdriver: either flat or philips.....with the handle in your hand and the tip on the ground, push the tip along the ground over and cracks in the concrete....first try it at a very shallow angle, almost parrallel with the ground....now raise your hand to make this angle steeper, doing it again......imagine this screwdriver is the line drawn from contact patch of front wheel through the RA chassis end bush center.....

food for thought

You are correct Serg I did mean Roll centre not roll axis as I was merely looking at a single axle and trying to describe how body roll is not affected by an increase in axle to ground height.I am not good at analogies but here goes.Jack the vehicle up and support on 4 adjustable jack stands under the axles.Now bodily rock the vehicle laterally on its suspension. Next screw the jackstands up 4 or 5 inches to simulate the fitting of portals and rock the vehicle again. The roll centre height measured from the ground is now 4 or 5 inches greater, but measured from the axle is the same as before so the effort required to rock the vehicle is no more nor less than before.
I like your screwdriver analogy above, and it does illustrate why my front suspension is much more resistant to compression, particularly when striking double wheel bumps or obstacles simultaneously.The projected line from the front tyres contact patch to the RA chassis pivots or in my case the instant centre of the 3 control arms is so steep that the forces to compress the suspension are almost equalled by the forces trying to extend it, so that the suspension is on the verge of locking.
A phenominen (Sp) I discovered related to both portal offset and antisquat geometry may be of interest to some.
I got bogged up to the axle housings in thick deep muck a couple of months ago. If you do that in a non portalled rig, usually the tyre centre is still above ground level and the winch will pull the vehicles wheel forward, up and out of the mud holes.
In my case, because of the portals the tyre centre height was around 5 inches below ground level, so the pull from the winch only served to make the wheels plow even deeper into the mire, assisted by the more severe antisquat geometries willingness to allow the suspension to extend to its limits just from the force of the winch pulling forward.
I won't go into the gory details of how I eventually recovered the vehicle, suffice to say that it wasn't easy or pretty
Wagoo.

I think you are corect Bill, as the relationship between Roll center and axle and body has stayed the same, with ONLY portals fitted....but I just pictured in my head what you said.....

Now if you are to chnage the roll center height and not the axle or body it would change things.

regarding your getting stuck and the wheels plowing further in. In my mind this happens in any coils sprung rig in deep mud, even if the wheels arent coverd like yours...In my mind I see the winch being pulled at chassis, the chassis moving forward and the RA to contact patch line digging in untill something of greater resistance forces it up, like a rock and the chassis pulling up till the axle reaches full droop....now a 2m long RA would chnage this abit....kinda like how a dozer blade has nice long low arms that mount back at the rear drive on a track driver dozer?????

am I making nay sense....lol