Antidive/Antisquat: how much do I want?

[o_teunico]

I understand what antidive/antisquat is, but have no idea of how will more/less antidive/antisquat affect the car´s behaviour in real life. What are people´s experiences when changing suspensión arm´s lenght? I´m correct assuming that longer front arms will give less antidive?

[bill van snorkle]

I understand what antidive/antisquat is, but have no idea of how will more/less antidive/antisquat affect the car´s behaviour in real life. What are people´s experiences when changing suspensión arm´s lenght? I´m correct assuming that longer front arms will give less antidive?

If you are referring to your planned One Link or bent twin radius arm front suspension, then yes the longer and flatter the links orientation =less antidive. Similarly for rear suspension the long flat wishbone or radius arms will equal lower antisquat. As to what is too little, enough, or too much antidive/antisquat, would depend on the type of terrain you mostly drive on. Rock crawlers prefer low antisquat numbers, whereas mud racers like higher numbers. With portal axles, 36" tyres and a relatively short 3.2 metre wheelbase, I am basically stuck with what some would consider excessively high antisquat/antidive geometry unless I build a difficult to package multilink system front and rear.

[uninformed]

I wish there was one number to shoot for, but alas…. Here is a kicker, the lower the centre of gravity the higher the antis will be for a given set up. So for our lifted rigs that actually benefit from longer links the rate at which anits drop is quicker as the COG is higher. Plus wheelbase is a big factor. As Bill pointed out to me, what happens if say you make a front link long enough it passes the half way of wheelbase, or fore/aft COG??

as an example, my mate has a 118inch wheelbasse rig. His front radius arms are +500mm over stock (yes half a meter longer) but he has a lower COG than stock. I want to make my radius arms +300mm, I have a higher COG. Even though my arms will be shorter, I will probably have less Anit dive…...

Bill, typo on your wheelbase me thinks

[o_teunico]

Im not a mud racer, so I should then look for somrthing flat and long. But you know, its just vapour for the moment.

[Nigelw]

Im not a mud racer, so I should then look for somrthing flat and long. But you know, its just vapour for the moment.

Dammit I thought you had started building something now

[Bowie69]

One day it will all condense...

[discomark10]

depends what terrain you use it on, if you do any amount of hill climbing I'de keep the anti-squat to a reasonable level (I believe 50% is about right, right?) anti-squat on the rear causes death hop.

[bill van snorkle]

anti-squat on the rear causes death hop.

I guess there must be a lot of dead Unimogs and drivers in them thar hills then Mark !

[Bush65]

depends what terrain you use it on, if you do any amount of hill climbing I'de keep the anti-squat to a reasonable level (I believe 50% is about right, right?) anti-squat on the rear causes death hop.

You simply can't make a generalisation like Anti squat causes the suspension to hop. You clearly don't understand.

There should be little problem with 80% AS and good suspension geometry. What is bad, and is the actual cause of hopping, is anti squat rising as the rear suspension moves on a climb.

[discomark10]

Hmm, so if you had a fixed amount of anti-squat it wouldn't be an issue, but what causes the hop is that whe the geometry changes the anti-squat amount increases and causes a sort of loop of rising and falling anti-squat as it pushes back?

Is that how it works?

[bill van snorkle]

Have you ever seen a couple of blokes bouncing up and down on a stuck vehicles front or rear bumper in an effort to regain traction? I've seen 2wd cross country trials buggys designed with exaggerated antisquat geometry do the same thing.

[uninformed]

Have you ever seen a couple of blokes bouncing up and down on a stuck vehicles front or rear bumper in an effort to regain traction? I've seen 2wd cross country trials buggys designed with exaggerated antisquat geometry do the same thing.

Bill, I have seen this and a lot of us have done it. My question is, is the point of regained traction when the vehicle is at its most squated, and then as it rises with the men rising its not actually getting any more traction. My point being, while it may produce a result, I think there may be other things going on like the mass of the men and the force of them bouncing etc that may make the it seem like hop is a good thing, but I'm thinking lower AS (to a degree) will actually propel the vehicle forward more so…..

[bill van snorkle]

Well Serg, the bouncing practice isn't always successful of course,but when it does work, if it was merely the extra weight of the blokes on the bumper that caused the vehicle to regain traction then they wouldn't need to bounce. Back in the old days when 4wds were much rarer and everyone was making buggies from cut down VW's, it was the Kombi based vehicles with their portal axles and outrageously high antisquat geometry that proved to be the most capable.

If high antisquat was the killer of offroad ability that some claim, then WildFing with its portal axles, short wheelbase and 44" long rear One Link should be no more capable than an old Morris Marina, yet the only conditions where it doesn't show most other built 4 wheel drives a clean pair of heals are where horsepower and wheel speed are required.

It is because of WildFings abilities that I take the Link calculator and the theories around it with a large grain of salt. If I lived somewhere where I frequented places with large areas of high traction rock, I might change my mind, but those sort of conditions are extremely rare in my state.

[discomark10]

If I lived somewhere where I frequented places with large areas of high traction rock, I might change my mind, but those sort of conditions are extremely rare in my state.

Thats a very good point, the ploblem with learning from the yanks on pirate is its not necessarily relevant for us in reguards to desired builds

[uninformed]

Throttle control plays a large roll. Some US guys build in certain amounts of antisquat so they get good "launch" or bite at take off. I certainly dont think we want to run low like 20%. John's comment on rising AS is quite important IMO.

[De Ranged]

Ive had a couple of goes at an explanation of the forces... and that is more important than any number and stuff has come up so instead of posting half of something I just delete it lol

High or low AS isnt bad, it is just part of an equation, you also have rollcenter height and roll axis angle (understeer/oversteer), now what alot of people forget is CoG, spring rate, compression and rebound damping, weight distribution, sprung vs unsprung weights, tyre size grip and mass, HP, wheel track & wheel base and there is more I can add lol

Your handling is a combination of these saying that because your truck drives well with a high or low AS and that is definitive proof that... "this is the best..." go join pirate you'll fit in lol you would also have to stop listening and work on the idea the more times you say it the more truth it has

Since you guys are so fond of radius arms and versions of this... the NZ number 2 trials truck, number 1 the yr before runs radius arms.... that point up from the chassis mounts.... work that one out, oh and here is a pic'y

For the books I tend to set up suspension with a starting point of 60% and modify from there..... my toy that is the kiwi build is setup with 86% from memory but does the opposite of normal suspension, as it extends my %AS gets lower the idea being that there is a point where the extension balances out the grip, I cant say Im right or wrong yet as the front end hasnt been done yet and the standard landy leaf spring rate is lol yea well...

The best bit of advice you will ever get (you will also find it on pirate lol) is to build in adjust-ability on mine there is additional mounting points, by changing the combination on these I can get over 100% down to 40 something %

Yes AS can give you an advantage, but in my books unless you are able to sort the other things to use this advantage worry about what made the guys above Number 1 a low CoG and a decent roll center height

lol be buggered 3rd times the charm only took me a few hrs a load of washing and some other cleaning up and didnt bother with a 3 page discription of the forces working on links etc

[Bush65]

The OP said:

I understand what antidive/antisquat is, but have no idea of how will more/less antidive/antisquat affect the car´s behaviour in real life.

Hmm, so if you had a fixed amount of anti-squat it wouldn't be an issue, but what causes the hop is that whe the geometry changes the anti-squat amount increases and causes a sort of loop of rising and falling anti-squat as it pushes back?

Is that how it works?

It is worth going back to the understanding of what dive and squat are, and what anti-dive and anti-squat are.

When a vehicle is stationary or traveling at a constant speed the downward force on the suspension springs is constant (sprung weight) and compresses the springs to their static ride height.

When the vehicle accelerates forward, inertia (force = mass x acceleration) increase the down force at the rear axle and reduces the down force at the front axle. In some suspension systems that will cause the height of the rear springs to compress and the front springs to extend. That action is called squat.

Dive is the opposite and occurs during deceleration causing the front springs to compress and the rear springs to extend.

Both these actions can, during high rates of acceleration or deceleration, feel very uncomfortable. So suspensions have been developed to reduce the amount of squat and dive. They do so by creating forces in the suspension arms/links, during acceleration and deceleration that counteract the squat or dive and this leads to the notion of anti-.....

If the uplift from the rear suspension arms/links during acceleration exactly counteracts the affects of squat, it is said that the suspension has 100% anti-squat. Similarly for dive at the front suspension.

If the rear suspension produces over 100% anti-squat the rear springs will extend. The ride height changes, and it may, or may not hop. As far as comfort of road vehicles is concerned, it is better that the anti's are a bit less than 100%, and over 100% feels uncomfortable, because the it is unexpected/abnormal.

It has been found that having a high amount of anti-squat helps traction during acceleration. This added traction comes from the vertical acceleration of the sprung mass. I.e. the rear sprung mass has some upward acceleration, and one of the laws of motion states that, "every action has an equal, but opposite reaction". It is this reaction to the anti-squat force in the suspension arms/links that momentarily push down harder on the rear axle.

The opposite is true if the anti-squat is low, i.e. momentary lowering of traction.

Understanding most of that leads to understanding the phenomenon of hopping.

In many/most rear suspension arms/links, during acceleration the lower arms are in compression and the upper arms are in tension. If the lower arms angle up to the chassis the compression in them pushes up on the chassis. If the upper arms angle down from the chassis the tension in them will pull down on the chassis. The net difference in the vertical components of these forces is the amount (not the %) of anti-squat, i.e. how much the load/force on the springs change.

If traction is high, the force in the rear lower arms will be high during hard acceleration, or climbing a steep grade. If during this event, traction becomes lower, the acceleration and the force in the lower arms suddenly reduce. So the rear rises and falls as the traction changes, and hopping may, or may not, occur.

If the suspension has a high % of anti-squat, and it becomes higher as the suspension moves during acceleration or climbing, the system is likely to become unstable (this is definite hopping). Because the suspension rises, the anti-squat increases, causing the suspension to rise further, which causes the anti-squat to increase, which causes ....... When there is a reduction in traction, the opposite occurs. This is an unstable system.

Recall also what was said above as to why anti-squat increase traction. when this hopping occurs the traction changes even if the friction from the surface is constant. Bumps also increase/decrease traction.

A very low anti-squat is not likely to cause hopping.

If the suspension is designed with higher anti-squat, and such that the anti-squat % reduces as the suspension droops, then you can obtain increased traction (from the higher anti-squat) with a stable suspension (because the anti-squat reduces with suspension droop) that avoids hopping.

[De Ranged]

If the suspension is designed with higher anti-squat, and such that the anti-squat % reduces as the suspension droops, then you can obtain increased traction (from the higher anti-squat) with a stable suspension (because the anti-squat reduces with suspension droop) that avoids hopping.

Im on desert race forums, 4wd and offroad racing forums and some automotive engineering forums and you are the first person who has had the same conclusion....

Now just a little bit he didnt add

and this is part of the 3 page dribble.... and a key thing, and exlplains why a high AS gives you grip, you understand its a weight transferance to the back yea... but how does weight move when its fixed lol it doesnt slide back on your chassis.... lol na bugger it Im gona simplify it

right the problem for us with too much AS is the weight is transferred from the springs to the links, this is what causes the back to lift due to the vectored force from acceleration, you fall prey to something we dont really think of in 4wding the ratio of sprung to unsprung mass.... half of the weight of the links is unsprung weight .... take for example bills example of mud drags, its basicly a big flat puddle of mud so this an't a problem, if your trying to climb a rutted potholed track at pace it is a problem that greater unsprung weight doesnt want to change direction into these ruts and potholes = less grip kinda defeats the purpose of that high AS lol

Now you can compensate for this the most obvious one is to drive slower... why worry about a high AS then just work on more grip and articulation to keep the tyres in contact

If you want to use it then get your CoG low this will reduce the climbing ratio as you tilt you nose uphill, increase your wheel base and run low as rebound damping

I hope you are able to see there is alot more to this than just one thing.... it is just one piece of a big puzzle

Like I said build in adjust-ability, I set my median at 60% and have options either way where possible when I build, this is because these other things effect handling, some times these are things that I cant assess in the build... for example as I joke with people the "muppet factor" how reackless you are, how you drive

And just a note if your stuck on radius arms they ramp quickly so I'd start them at a lower setting say 50%.... this is just a guesstamat none of the guys that get me to build there toys have pushed for radius arms, so Ive never played with them

[bill van snorkle]

I had a visit this morning from a journalist/photographer and long time offroad vehicle builder, who want to do a series of stories on Old School 4x4 enthusiasts who build their own stuff, as opposed to handing over a blank cheque to the ARB Corp for lots of shiney bells and whistles.

Anyway, they asked to see Wildfing up on the articulation ramp to get a clearer view of the suspension ,transmission and portal axles. we got around to discussing Antisquat and why WildFing doesn't play by the rules. Now these guys are no dills, one in particular is at least my equal regards technical experience,It was surreal that the answer came to the three of us simultaneously. Looking at the chassis attachment point of the rear One Link showed that it was close to mid wheel base and as near as dammit to the for/aft location of the Centre of gravity. What that means is that the upward push from the end of the OneLink is fighting close to the whole of the trucks unsprung mass. Now WildFing is no lightweight at 2200KG's.Subtract say 750kg for axle assemblies, wheels /tyres and that leaves around 1450 kg or 3190 lbs of unsprung mass that the One link is trying to push upwards against. The effective length of my One Link is 4ft. If I was dragging an extremely load or climbing a very steep gradient in just rear wheel drive and was able to generate 8000 lb ft of torque at the rear axles, the lifting force at the end of the One Link would be 2000lbs. I don't think that 2000 lbs of upforce is going to make much impression on 3190 lbs of downforce, and that is why Wildfing doesn't jack its bum up when climbing.

BTW, 8000lb ft of torque at the axles is well in excess of what Rover type diff/axles could cope with. Even a standard 30 spline Dana 60 is rated at only 6000lb ft.

Also, very little serious 4wding is done in just rear wheel drive, and the 'squatting' effect of radius arm. OneLink or 3 link front end would also apply a downforce close to the for/aft centre of gravity position as well, which should probably further oppose that 2000lbs of upforce from the rear One Link.

[Bush65]

Im on desert race forums, 4wd and offroad racing forums and some automotive engineering forums and you are the first person who has had the same conclusion....

Snip ...

I know for sure that it has been discussed before on the Pirate 4x4 forum.

In the early years on Pirate, most were building suspensions with well in excess of 100% anti-squat, with the aim to increase traction. This came about from the experience of drag racers.

I believe I am correct in claiming that the credit for changing to lower AS and reducing change in AS so that they climb better, should go to Sam Overton (strangerover). Serg (uninformed) should remember back that far and has a good knowledge of some of what Sam has done.

[Bush65]

Bill, you have made a simple typo, where you have said unsprung mass, you would have meant sprung mass.

[bill van snorkle]

I thought the StrangeRover name belonged to Alvin Smith from the UK from 30 or so years back who plopped a triumph TR7 body on a RRC chassis?

Anyway, MogRover as I knew and remembered it, continued the rear trailing arms on the same plane as standard, only they were longer. Wouldn't that give the same instant centre as standard, but reduced rear axle roll steer?

[bill van snorkle]

Bill, you have made a simple typo, where you have said unsprung mass, you would have meant sprung mass.

Thanks for picking that up John. I'm not sure how I made that boo boo. Another example of the need to be able to edit posts after more than a couple of hours!Particularly for technical subjects where a simple typo may lead someone astray.

Deranged, I'm sorry once again, I occasionally have to take heavy medication too. But your medication obviously is affecting your comprehension ability.

I dont' think that too many contributors to these One Link, 3 Link, Triangulated 2 Link and Antisquat/Antidive threads are all that enamoured with radius arms, hence the recent proliferation of these threads.

[uninformed]

Bill, thanks for that post regarding your vehicle.

Maybe DeRanged has been following my plight of Radius arms on the various forums. I by no means think they are the be all and end all. Far from it. Everything is a compromise, and in my case engineering/legalities make it easier to play with radius arms than changing to say a 3 link. I also like that there is some roll resistance built in. I hope my planned longer RA don't loose to much of that, as the absence of a ARB in the front is handy packaging etc wise.

[De Ranged]

Deranged, I'm sorry once again, I occasionally have to take heavy medication too. But your medication obviously is affecting your comprehension ability.

I dont' think that too many contributors to these One Link, 3 Link, Triangulated 2 Link and Antisquat/Antidive threads are all that enamoured with radius arms, hence the recent proliferation of these threads

sorry about that the first reference the winning trials truck was to make a point that AS isnt as critical as most think and the fact it has radius arms was sort of sarcastic lol I tend to type how I speak

The second comment about setting AS lower for radius arms was more for similar systems like 1 links or 2 links

Just back to the thread been contemplating what to do with the front of my toy.... My whole idea with the reverse rate AS for the rear was to improve it when climbing... with the front end anti dive, do I have the same need, for me with an offroad toy (trailer queen)

when I need brakes Im not going to be going fast so its not going to be a long sustained brake, when Im really wanting them to work is most likely when Im pointing down hill... so given this a standard rate with a moderately high rate to start or at rest......

so whats your thinking on this ?