For my 38 project, I am planning to Mustang 8.8 axle with multi-link but my Boling Brother frame is actually setup for Ford 9". On Mustang 8.8 axle, the lower link is about 18" long and the upper is about 9" long. In my frame, with same axle, the lower link will be about 25" and upper will be around 22" long. The upper link will also point down a little. I used a flimsy BMW upper link just for mockup purpose. Once I get everything where I want, I will have it made using 1" DOM. While I am no suspension geometry expert, I don't see why my setup would hot work. Any suggestions? Waid
That will work just fine. There will be almost no pinion angle change with the bars closer to equal length. The super short upper bars are to roll the pinion down during suspension compression.
Will work fine. I always try to maximize length....longer always better. Shorter contributes to a choppy ride. Sent from my XT1710-02 using The H.A.M.B. mobile app
There is lots of good info on youtube you should look at, the top bar needs to point down when the car is at ride height and the bottom bar should be parallel - google instant centre. As mentioned the longer length of the top bar in your case is a plus. Glen
Not quite! but reasonably correct [and won't cause issues] Having the lower bar horizontal will shorten the wheelbase under compression, which promotes desirable "roll understeer" when cornering. But Most manufacturers have the lower bar 1" higher than horizontal [allowing for 2" of compression during normal circumstances] This causes the wheelbase to lengthen for the 1st 1" of compression, then shorten back to the original wheel base for the next 1" of compression. Technically this is promoting undesirable roll oversteer for the initial travel .So the manufacturers point the top arm downward at the front. So the lower arm lengthens the WB but the upper arm shortens the WB effectively cancelling each other out [at the axle centreline] The reason the upper arm is usually shorter is to shorten the "instant centre" during suspension compression, creating a "dynamic anti-squat" [this has the same effect as having really short ladder bars and the pinion angle pointing down] All of the pinion torque reaction tries to prevent rear squat during acceleration. [the driveshaft pinion tries to straighten AND the axle housing can get more leverage]
This is lots to swallow My lower link is not completely horizonal either. What does it mean for my setup above?
Remember the rear wheels are always perpendicular to the axle, so any variation in wheelbase will cause rear "roll steer". The problem is caused by the links needing to move in an Arc When a car leans over [body roll] on a corner you want the outside wheelbase to shorten, to effectively steer towards the inside of the corner. This generally requires a bit more steering input. If the outside wheelbase lengthened, the driver would need to apply some opposite lock to catch it. In a straight line on uneven surfaces it will feel very uncomfortable [similar to having too much toe out]. Having the lower link arcing "longer", and the upper link arcing "shorter" ,they try and cancel each other out [similar to a watts link] This keeps the path of travel closer to vertical [especially during the 1st 1" of suspension travel] You need to sit your car at the correct ride height, to set up the suspension . If the ride height changes when the car is completed , you adjust the spring heights to correct this.
Here is a diagram explaining how a triangulated 4-link locates the roll centre: It is conventional on these setups to place the rear roll centre high, to enable a relatively soft rear spring rate without sacrificing rear roll resistance, at the cost of some jacking tendency. It's a fairly good compromise for most situations, but the keener you are on chasing down that last bit of dynamic performance the greater the chances are that you might deviate from this arrangement. There is more than one way to skin a cat. The angled links are usually the upper short ones but they needn't be. All four links are usually ahead of the axle, but they needn't be. The upper links are usually shorter in side-elevation length than the lower links, but they needn't be; and they are usually not parallel in side elevation, but they might be. There are a hundred ways to apply the triangulated 4-link principle, if you understand how it works — but you have to know what you're doing. Despite the conventional high placement there is a lot to be said for a very low rear roll centre. It depends on how you intend to deal with elastic roll moment distribution, and it has to satisfy a number of other requirements, including the roll-steer issue Mimi mentions, but there are numerous ways to achieve this.
It all looks good except I would raise the attachment point of the upper bar to change instant center to improve bite if you decide you ever want go to the drag strip to play. It won't hurt the ride or the handling. I have set up several daily driver Fox body mustangs that do a trip to the track once in a while and they hook very well with street tires . Plus their is no lack of driver control in the corners or loss of comfort in the ride. I have done the same with early Malibu's that have the same basic design.
ok. Lots of info! Will my current setup work or will I have any handling issues? I may be able to raise the front mount for upper control arm little bit. See picture below. All other points are fixed and the bottom control arm is going to be close to level. My 38 truck is a weekend cruiser and will never see any kind of track.
No! If you find setting this up overwhelming , see if you can get your hands on a Mustang and measure all the heights of the pivot points. Ford did all the design research for you! The Mustang "Fox" body up to the SN95 platform had one of the best rear suspensions available. For Drag racing all you needed was shocks and poly bushings, and it is great on a street car. If you study the top drawing that Dawie posted, you can visualize how the "instant centre" moves rearward [shortens] with squat. Which is desirable for "forward bite" On a side note [explanation here] We have a FR500C Grand-Am car. Which has very aggressive roll understeer caused by lowering the car [the front pivot is 3" lower than the rear] The front straight of "Pukekohe Raceway" in NZ is very bumpy [and fast]! At 150 mph the rear of the car is "tram-lining" over the bumps, but you can take your hands off the steering wheel. Roll understeer is self correcting , whereas Roll oversteer promotes swaying [like a trailer "fish-tailing"] The side effect of having aggressive roll understeer , is increased rear "side bite" [we can get on the gas earlier on corner exit] Just copy what Ford did! They got it right
Mimilan, I would have loved to just bolt on the Mustang suspension. Unfortunately, the due to the design of this truck frame, it would be very difficult to do that. As I said my hard points are all fixed and the only thing I may be able to do is to raise the front mount for the upper control arm little bit. What difference in handling will it make if I raise or lower the front mount of the upper control arm ? I would like to point it down and keep it close to the frame so its strong. Waid
Without all the dimension it is difficult to give you exact recommendations. Please understand suspension under compression will load the tyres more than suspension during rebound [eg: drooping into potholes] So it is best to control roll steer for the first 2" of compressed travel [called Jounce] If the lower link is placed 1" higher at the front the amount off wheelbase variation is minimal for the first 1" of travel. Then the wheelbase returns to original length for the 2nd 1" of travel [basic trigonometry] So place you lower link either 1" higher at the front [or 1" lower at the axle] AT RIDE HEIGHT Because your lower links are relatively long [compared to normal] so it would actually benefit from being lower at the front. [I'd recommend 1" to 1-1/2" lower at the front] Again at ride height. You will need to correctly set the pinion angle before any brackets are welded on. There is massive compression loads on those links [accelerating the vehicle], so don't go too flimsy on the lower links. The upper link has less compression loads during acceleration , but huge compression loads during braking.
I'm sorry if I did not explain my upper mount comment in my last post. I meant to say raise the rear mount of the top bar , not the front mount of the the top bar.
Shouldn't the bars travel so that they intersect at an imaginary point at the dampner? Sent from my SM-A015A using The H.A.M.B. mobile app
I know it's tempting to use the cast top mounts on the rears that have them. And in mid-30s and later cars it can usually be made to work. On earlier cars with flatter frames and limited vertical space the mounts just are not in the right place. Usually too high. Here is a 9" Ford with fabricated mounts to get them positioned down below the top of the housing and the correct angle to the frame.
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