Link Numbers

[06h3]

I am surprised there isn't a link # thread here yet. The old forum had some much info it was hard to sift through, with that said, the desirable numbers seemed to have changed from 2004 to today when going through those old threads. I guess we got smarter, technology got better, etc. These are newb questions but figured it could spark some discussion especially for guys wanting to learn.

What makes a bigger difference in off road performance doing general 4 wheeling? Front or rear suspension, from what you have seen/experienced? That could help someone decide where they want to start if they are doing links one end at a time.

Anti Squat in the rear: For desert high speed KOH Stuff, 40%-60%? General all purpose stuff 80-100%? What about moon buggy stuff, sand hollow type climbs and rock bouncing?

Anti dive: Disregarding the actual dive aspect of it when braking (anti-lift) what do you want to look for when moving foward, hitting a ledge on a climb, doing steep sand hollow type climbs, moon buggy climbs, rock bouncing or going fast in the whoops?

Roll Axis: Is it a general consensus that closer to zero is always better and a very high roll axis causes a rig to be unstable? Does near zero apply to everywhere?

Roll Center: It seems Roll center is linked to Roll axis but is there a number target or do you want it to have a specific relationship of COG or Roll axis?

I'll just throw my experience out there. Not much....I did a SAS with a 3 link. I have put almost 50k miles on the SAS in all kinds of terrain. First I had a very high roll axis and very soft springs, it worked awesome in the rocks and cruised down the highway just fine but lack of up travel and soft springs sucked in the desert. I then inboarded my links on the frame side and out boarded them on the axle end, it lowered my roll axis. Did a longer track bar, went from coil springs to coil overs and went from 3.5-4in of uptravel to 5.5in of uptravel. In the desert it is so much better then before. In the rocks it seems to get tippy quicker but that might be stiffer springs, and less droop rather then roll axis, unforunately I changed too much to definitively say, this is what changed this. Anyways, my link number testing experience is limited but figured guys who have redone suspensions 5 times on one rig or built 10 rigs over the years can chime in.

 

[azdirt3.0]

This would be a great thread. I need to update my numbers for where all my mounts ended up in real life to see where my numbers ended up. I tried to follow the information I found on the other site and for a front 3 link ended up with approximately 45.4% anti-dive, 35.5% Anti-lift, 0.5* roll axis, and 26.5" roll center. This is with a theoretical 4200 lb rig on 39s and belly height at 24" (guessed a 38" CG height).

Still building the rig so have no idea how it will perform but I was going for something that could be driven at speed in the desert as well as a crawler can. Spring under rear for now till a 4 link down the road.

 

[06h3]

This is the first I have heard that anti dive and anti lift are different. How does one calculate that? lol

 

[azdirt3.0]

That is set up in the latest version of the calculator that was posted here in the resources. I don't recall if it was on previous versions. Not sure how legit my number is for this looking into the background calculations. Like Anti-dive, this uses CG heights and I am probably off on those as my rig is no where near built and weighed. Anti lift takes into account Anti Squat CG rather than Anti Dive CG.

The Math behind the calculations are: Instant Center Z-Axis/(Wheelbase-Instant Center Y-Axis)) * (F Drive Bias*Wheelbase/Anti Squat Center of Gravity Height)

 

[Treefrog]

Both the front and rear suspensions have a roll center. Its a point in the vertical plane of the tire centerline, solid axle and IFS/IRS. The roll centers are the two points that define the roll axis.

My understanding, from reading forums, is that for roll axis flatter is better, slight understeer is preferred for general purpose rigs, where as faster rigs seem to prefer slight oversteer.
Regarding roll axis height, the farther the cg is from it, the more body roll when cornering or off camber. There are some downsides to a high roll center, but i can't remember them at the moment.

In general the lower the anti value the more the vehicle pitch changes during acceleration, both brakes and gas.
As far as the specific antis go:
Squat (gas) - Higher speed stuff has been tending towards lower numbers, 10-30%. Moon buggies are in the same range. Trail rigs seem to be a bit higher. Rock bounces might be higher, but their links are pretty flat so I doubt it.
Dive (braking) - Lower values are preferred for the fast stuff, because it rides smoother during braking.
Front Lift (gas) - Low values can be a problem on steep climbs when the it wants to lift the front. Often addressed via a suck down winch.
Rear Lift (Braking) - Not worried about all that much, same thing as the front with suck down winches.

I make no claim as to the accuracy of which groups use what values, but that is my understanding of where the numbers are these days. Hopefully someone with more experience chimes in.

As far as front or rear first, it is best to design the suspension as a whole not just each end individually. But it is probably wisest to do the rear first, it sees the majority of the weight and force when climbing, which is where bad suspension seems to show. Also you tend to not worry about steering in the rear.

 

[Lil'John]

Going to watch this so I can understand the numbers.

My fab guy did the calculator for my front three link and came up with:

This was for a general wheeling rig that is going to have 35" tires, 110" wheelbase, and ~19" frame height.

 

[Treefrog]

Going to watch this so I can understand the numbers.

My fab guy did the calculator for my front three link and came up with:


This was for a general wheeling rig that is going to have 35" tires, 110" wheelbase, and ~19" frame height.

Assuming he used the 3 link calc., the anti-dive value will be a bit over half of the anti-squat value shown. The anti lift will be around half as well, when in 4wd. This is because the front isn't doing all the braking or driving.

The roll center height is likely accurate, but the roll axis angle in that calculator is the one for the front suspension, not the entire vehicle.

I could be a bit off on the math, but does a ~36" sprung (no axles, wheels, tires) cg height seem correct for the rig?

 

[Lil'John]

Assuming he used the 3 link calc., the anti-dive value will be a bit over half of the anti-squat value shown. The anti lift will be around half as well, when in 4wd. This is because the front isn't doing all the braking or driving.

The roll center height is likely accurate, but the roll axis angle in that calculator is the one for the front suspension, not the entire vehicle.

I could be a bit off on the math, but does a ~36" sprung (no axles, wheels, tires) cg height seem correct for the rig?

The calculator used was three link from 6/17/2004 ;)

For CG, his advise/rule of thumb was to use my similar rig I have and use the cam button(height) as approximate CG. This is assuming no ability to get a real measurement. The number used for CG was 41".

The rear axle is going to be sprung under.

 

[gt1guy]

Both the front and rear suspensions have a roll center. Its a point in the vertical plane of the tire centerline, solid axle and IFS/IRS. The roll centers are the two points that define the roll axis.

My understanding, from reading forums, is that for roll axis flatter is better, slight understeer is preferred for general purpose rigs, where as faster rigs seem to prefer slight oversteer.
Regarding roll axis height, the farther the cg is from it, the more body roll when cornering or off camber. There are some downsides to a high roll center, but i can't remember them at the moment.

Roll axis, Roll couple axis, Roll axis inclination, are different names for the same thing.................the single connection between the front and rear suspensions. It's what makes the suspension "system" whole. It's what gives the vehicle it's "feel" during transients. Lateral load transfer through the RC’s happens almost immediately at each end, while lateral load transfer through the roll couple axis is based on suspension roll stiffness and has to wait for the chassis to attain some *'s of roll angle.

I probably murdered that description.

General rule is you want the roll couple axis to run slightly uphill to the rear. Or, you front RC should be lower than the rear.


A downside to super high roll centers is having zero warning before it tips over.

 

[Treefrog]

Roll axis, Roll couple axis, Roll axis inclination, are different names for the same thing.................the single connection between the front and rear suspensions. It's what makes the suspension "system" whole. It's what gives the vehicle it's "feel" during transients. Lateral load transfer through the RC’s happens almost immediately at each end, while lateral load transfer through the roll couple axis is based on suspension roll stiffness and has to wait for the chassis to attain some *'s of roll angle.

Pretty spot on from what I've read. The big issue is that most numbers threads look at the roll axis in the calculator. In the older versions, like the one pictured above by Lil'John, the roll axis is not the roll axis you are describing. In the older versions it is the roll axis of that end of the suspension, in the case of a 4-link it is the line defined by the two points created by where the upper links would intersect and where the lower links would intersect.

What I said about trying to keep it close to flat is in regards to the vehicle roll axis, not the suspension roll axis.

 

[gt1guy]

Told you I probably murdered it. Ya, you have RC's that each have a axis of their own, and the axis that couples them both together. I was speaking of the latter.

 

[06h3]

Both the front and rear suspensions have a roll center. Its a point in the vertical plane of the tire centerline, solid axle and IFS/IRS. The roll centers are the two points that define the roll axis.

My understanding, from reading forums, is that for roll axis flatter is better, slight understeer is preferred for general purpose rigs, where as faster rigs seem to prefer slight oversteer.
Regarding roll axis height, the farther the cg is from it, the more body roll when cornering or off camber. There are some downsides to a high roll center, but i can't remember them at the moment.

In general the lower the anti value the more the vehicle pitch changes during acceleration, both brakes and gas.
As far as the specific antis go:
Squat (gas) - Higher speed stuff has been tending towards lower numbers, 10-30%. Moon buggies are in the same range. Trail rigs seem to be a bit higher. Rock bounces might be higher, but their links are pretty flat so I doubt it.
Dive (braking) - Lower values are preferred for the fast stuff, because it rides smoother during braking.
Front Lift (gas) - Low values can be a problem on steep climbs when the it wants to lift the front. Often addressed via a suck down winch.
Rear Lift (Braking) - Not worried about all that much, same thing as the front with suck down winches.

I make no claim as to the accuracy of which groups use what values, but that is my understanding of where the numbers are these days. Hopefully someone with more experience chimes in.

As far as front or rear first, it is best to design the suspension as a whole not just each end individually. But it is probably wisest to do the rear first, it sees the majority of the weight and force when climbing, which is where bad suspension seems to show. Also you tend to not worry about steering in the rear.

How does one get an anti lift # from anti dive? I made some changes to my suspension to lower the roll axis and it feels more stable but I feel it unloads a little more on steep climbs then before.

 

[Treefrog]

How does one get an anti lift # from anti dive? I made some changes to my suspension to lower the roll axis and it feels more stable but I feel it unloads a little more on steep climbs then before.

It's the same concept, the difference is where the vertical line along which you are getting the values from is located. For anti-dive, the line is at wheelbase*brake bias. For anti lift it's wheelbase*drive bias. Measurements start at rear axle. For biases 1 is 100% at the front, 0 for 100% at rear.