Thread: 130 on portals

Originally Posted by uninformed

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

Yes Serg, except when a non portalled rigs tyres have sunk down to the point where the axle tubes are sitting on the deck there is still a sort of mud 'ramp' roughly in the shape of slightly less than half the tyre radius and leading upwards so that a front or rear pull would tend to assist the wheel to roll up the ramp, wheras with portals allowing the tyre to sink below centre height, the 'ramp' is effectively turned upside down so that the wheel prefers to roll ever downwards .
Wagoo.

Originally Posted by series3

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

Sam
It possibly would have been Brendan,as I believe that he is on his way to The Cape.

Wayne

Originally Posted by uninformed

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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.
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The load in the bushes depends upon the horizontal force between the road surface and the tyre, and the height from the application of that force and the bushes at the chassis. The rotation due to the height is combined with the horizontal force and is resisted by the RA. A longer RA needs a lower force at the chassis bush and so the load is slightly reduced at some bushes at the axle.

Originally Posted by uninformed

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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%

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Not strictly correct. You have described a graphical method that resolves the forces and reactions acting on the suspension during acceleration.

As per my previous post, the anti forces depend on the force at the tyre contact and the geometry of the links.

Wheelbase and COG height affect the squat (or dive), but not the anti's. As such they will affect the % of anti related to the squat or dive.

What you have called convergence point is more correctly called instant centre (IC). IC is the point that a swing arm, equivalent to the link system would pivot about. This point is important for resolving the multiple link forces to an equivalent force that would produce the same effect.

With a 4wd anti squat can be produced at the front end, but because front suspension should be set-up for anti dive the anti squat is negative, but generally small enough to ignore. Similarly during braking, the rear brakes produce forces in the rear links (in the opposite direction to acceleration) and produce negative anti dive there.

It is normal to only determine anti squat for the rear suspension and anti dive for the front suspension.

Originally Posted by uninformed

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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

Basically the suspension couples the sprung masses to the unsprung masses.

The front and rear suspensions each have a roll axis that passes through their roll centre.

The body rolls about another roll axis that passes through both the front and rear roll centres.

A flatter roll axis is preferred to reduce roll steer.

On a side slope, the body of a vehicle with low roll centres will flop over to the downhill side which shifts the COG further downhill increasing risk of rollover.

The disadvantage of high roll centres is the body shifts to the side when the axle articulates. At high speeds on uneven ground the inertia can give the effect of having stiffer suspension resisting articulation.

Thanks John, id figured youd set me straight.

I had a feeling that I had Instant Center IC and Convergence Point mixed up....Is the CP the ball Joint on our rovers, or where the 2 angled links on a 4 link (when viewed from above) would meet?

So what does it all mean.....IMO you only need so much anti-squat and anti-dive to perform well on road....Take the amount on a stock RangeRover classic, on its stock 29 inch tyres...lets say the amount on this vehicle is fine as a base line for both on and offroad....well adding bigger tyres/spring lift/portals makes these values higher....to the point where it can be detremental offroad. In the USA when they are wheeling on high traction rock, they tend to like low AS type suspension so they can get smoother power delivery to the ground and less induced wheel hop...on the East coast they prefer alittle more in the looser dirt stuff as this helps load the front and provide a bit more traction up there. As I described before, the front end going over bumps etc just gets worse the higher the rig goes....front axle roll axis would be better on a stock spring height portal rig, than a 5 inch spring lift rig none portal rig. But both still not haddling corrigations etc as well as stock. Every person has different wants and needs, for me I like the stock RR as a base line and would love to build a portal axled rig with this as my guideline, using longer RA's and TA-Aframe to get that power to the wheels smoothly and also handle road safetly and comfortably. The rear end would be able to flex even better than stock with longer arms and a well sorted shock angle...the front would also flexmore than stock with a longer RA....for me balance between the 2 is important...to the point where id be happy with a full 12 unch travel shock be used front and rear....keep them the same so a spare is easy....I like the idea of a higher ball joint to help with the rotation of the rear axle. The front for ease sake would use stock bush location...running poly bushes and keeping a good eye on them....funny thing is that these days the polys are flexing as well if not better than rubber oem...


Probably all a pipe dream as QLD transport wont give us any guideline to do the above

Serg, I haven't kept abreast of poly bush developments. I went off them about 8 years ago when fitting a set of ARB OME radius arm bushings to find they were sloppier than the worn out original equipment ones I was replacing. All of ARBs stock at the time were the same..The chassis end RA polys also permanently squashed flatter after only a couple of hundred kms service.
What brand of polys would you suggest is worth a try ?
Wagoo.

Bill, Rick and some others including myself are having good success with Superpro, sold through Fulcrum. Im running these poly at: chassis end of RA, TA and Aframe, with rubber at axle end of RA and TA and a MR Automotive high ball,greasbale adjustable ball joint. My portal'd mate has poly at axle end of RA only... rubber every where else. I quiz him everytime I see him how the custom arms are holding up and how the bushes are fairing.....so far all good. His rig gets used almost daily on his property and runs into town (1hr one way) every week or 2.

Thanks Serg. The way this thread has gone all technical like, I hope it's not in danger of Mods moving it to the 'Extreme ' forum. That would be a shame, as IMO that is where threads go to rest before dying
Wagoo.

Originally Posted by uninformed

...Superpro...

x2.