4 Links by the Numbers

7-5-07

   
 
 

In previous link suspension articles I discussed the basics, then went more in depth on link materials. In this write up I want to discuss the initial design phase using the four bar link calculator available here. This program can help and confuse you if you don't know what the numbers mean.  To make matters worse, there is no correct numbers as some people prefer more anti-squat while others prefer less and then factors such as wheelbase and water in the tires aren't considered, etc, etc.. 

I think the best way to get a good baseline for a new suspension is to find one that you know works in the terrain you like and then plug the numbers into the 4 link calculator.  Hopefully the owner doesn't mind you rolling around under their rig with a tape measure :) What I did here was asked each member in the club to submit their measurements of their rear suspension which I then plugged into the calculator to create the graphs you see below.  Many people have seen these rigs in action on the videos I have made so you can see how they work first hand.  These rigs have also been used for years so any handling quirks will be noted. 

When I gathered the numbers I figured I would see very similar graphs since we as a club design our rear suspensions to climb well under throttle. To my surprise the graphs were not as similar as I would have thought.

Note - the hyperlinks that follow are links to videos which illustrate some of what I am talking about.


Scott's rear 4 link numbers.

Scott runs the highest anti-squat numbers of anyone in our group.  As you know his rig climbs very well but some of that can be attributed to his 112" wheelbase.  Most of the time Scott runs several hundred pounds of water in each front tire and with that extra weight his rig seems to defy gravity at times, how else can you explain him nearly driving the Asylum waterfall?

Scott on Asylum

When he took the water out for part of our spring road trip we discovered his rig liked to lift the front tires on even small obstacles and consequentially he had problems keeping the rig on all fours, rolling 5 times in a few days time. Scott's rig is also much more nose heavy than anyone else's in the club so going down stuff was can be sketchy, especially without water in the fronts. 

I think Scott's suspension setup really works well with water in the front to keep the nose down, that combined with sticky tires and the 112" wheelbase makes his rig a near perfect climber. For reference, Scott does not run any sort of sway bar and his axles are full width.

Scott's take:

 Part of the reason I lift front tires alot is because I almost always have the winch tight due to my crappy front links. My front lowers are too short so if I don't have the winch tight and I try to climb up a ledge the tire tries to go backwards kind of like a TJ. Hopefully that will be fixed when I redo my lowers on this next build. I'm not sure how much of the tire lifting can be contributed to the rear links?


Chris's rear 4 link numbers.

My rig was has been a good climber and I have enough confidence to be able to launch it at obstacles such as the big waterfall on S&M without fear of the rig becoming unstable (note, I was not running water).  I personally have found that a lower anti-squat number has proven to start less wheel hop and made the rig climb better although the fat sidewalls on the old 42" Iroks could still introduce some wheel hop. I typically run without water in the tires and I rarely lift the tires on smaller obstacles and when I do lift the tires on the technical obstacles the rig is very forgiving and you can really push the limits of off camber driving.  I do have to run a sway-bar otherwise body roll is more than I would like.  My axles are very narrow by D60 standards, my track with including the tires is only around 76".  For reference my wheelbase was 108"

My theoretical numbers for my 4 seater.

I am re-configuring my rear suspension for the 4 seater to move the instant center further out in front of the rig, lowering the anti-squat percentage and raising the roll center.  My goals are to eliminate the swaybar an make the rig less likely to hop on full throttle climbs. I will also be stretching the wheelbase so the suspension links get roughly 3 inches longer which makes the anti-squat numbers go up a bit slower when the rear suspension drops away (the link angles become greater). We'll see how this pans out.

In summary my setup worked very well and I got to the point where I was not afraid to point and shoot it at a steep obstacle like the big car wash in Moab. If anything I had to be more careful giving it a big bump to keep the supercharger from breaking the tires loose when the boost came on.  I guess this is one spot where the torque of a V8 would be nice. For strictly technical wheeling I was 100% happy with the suspension as it was very forgiving in extreme off camber situations.

Adding 150lbs water to my front tires was like icing on a cake and only made a good thing better.  I could side hill like no tomorrow and the rig did climb better.  The best example I can think of is the big wall on Upper collateral Damage.  Without water my front end gets really light and I have to bump it to make it but with water in the front tires I can crawl that obstacle easily. In fact, with water in the front tires I found myself not even bothering to winch down the front end on climbs.


Jack's rear 4 link numbers.

Jack has been running his setup for over 3 years now and I would have to say he has a great all around performer.  He usually runs no water in the tires, runs no swaybar and has managed to climb one obstacle that has thwarted all of my attempts (last fall on S&M) but still has 3 to go to match Scott (second fall on Saw, the "Mother-in-Law" fall on 7 Sisters and "Big Nasty" on Sckullcrusher). Jack's wheelbase is the same as my 3 seater at 108". Since adding the V8 Jack has made climbs even easier, with the instant and abundant torque on tap from his 5.0 he can modulate the throttle and power up stuff that would bog down lesser motors as seen on his climb on the Upper Helldorado waterfall taken from BTG Volume 3.


Jay's rear 4 link numbers.

Jay's rear 4 link is relatively new so I can't make a whole lot of comments about it.  From what I have seen it handles crawling well enough and he does not run a swaybar (although he still has leaf springs up front).  Since Jay is still running Toyota axles he is a bit more conservative than the rest of us so he doesn't hit the big falls with momentum but we are working on him :)


Brian's rear 4 link numbers.

Brian's chassis was built by Shaffer's Offroad (Diablo) and also uses a rear 4 link with no swaybar. His rig does fine crawling but has a tendency to lift the front tires unexpectedly which has caused one pretty big roll and a few other exciting moments (such as lifting the drivers side tire at the top of the S&M waterfall as seen on BTG Volume 1).   

We suspect we could greatly improve the performance of his chassis if we could get the instant center further out in front of his rig instead of near the center of the motor. Simply moving the upper link mount at the chassis up 2" should make a big difference but due to clearance issues it will require some extensive chassis modifications. Water in the front tires would also work but the tradeoff is having to lug all that weight around.


Barry's rear 4 link numbers

Barry runs around with our club a lot and has one of the best climbing rigs I have seen, he is typically the one that makes the impossible previously undriven obstacles.  Note the long wheelbase (for down here) and he has always run water in the tires since going to that wheelbase (Barry says he's scared to try it without). I am guessing the LS1 and nitrous don't hurt either when needed :)


Here are the actual numbers from my project Hellraiser 3 project.  No comment yet on the performance other than it seems stable in every situation I have put it in so far.

Hellraiser 3 Front Link Numbers

Hellraiser 3 Rear Link Numbers

Hellraiser 3 Rear Link Numbers, Lower Link in the Lower Position.


I plan on updating this page as I gather more info from the other rigs of people who run with the club. Hopefully this will serve as a good reference if not an interesting comparison.