Maximize Performance: Benefits of Moving Rear Suspension Mounting Points Forward

[B565]

Hi , most rear suspension 4 link brackets mounted on the rear end housing are centered or slightly forward of the rear axle centerline . Why ? And what are the effects of moving the mounting points forward 2”,3”,4” forward ? Thanks for the help .

 

[jack action]

As you put the connecting points away from the centerline, you create a moment under lateral forces. This moment will contribute to increasing the stresses and deformations of the links, thus requiring stronger components and inducing more and more [undesired?] steering to your axle.

 

[B565]

What about weight transfer or how fast and hard the hit to the rear tires would be affected ? I’ve heard longer 4 link bars in general will soften the hit to the tire which makes sense to me . But still unclear how moving the hole 2” , 3” forward of axle centerline would affect the dynamics of the launch on a Drag Race car . I should have been more specific with my question. Thank you very much for the response.

 

[Ranger Mike]

Whoa up there ,Hoss…

In drag racing it is all about tires and traction. You have an engine lifting the left side of the car UP under severe acceleration and swinging force (most racers call this weigh transfer) to the right side of the car. At the same time you have the rear tires hooking up and you have the whole body/frame and body twisting under torque. What direction?? What side has more tire bite? Which way will the car want to shoot?

Now lets look at the rear end under acceleration. The top links are pulling down and the lower links are literally pushing up trying to lift the car. This is literally creating down force on the tires.

We have a major problem here. If everything was balanced 50-50% the car would hook up and go straight! It does not.

We have a much more proportional load going to the right side of the car adding down force to the right rear tire and what happens? The car wants to shoot to the left. We have a built in push condition to the left side. Not a winning strategy. How do you compensate for this?In stock car racing we run a 3 link suspension and control for this by figuring % left side weight and moving the top link so it is 50-50% proportional to the left side weight %

In drag racing , you can not really calculate with weight scales the true % weight on each rear tire under acceleration. So how do you adjust and tune in this factor?

You vary the amount of LIFT ( downward force on the tires) the lower arms put on the chassis to cure the left side push.

By varying the distance of the lower arm mounting holes you are changing the length of the lever acting on the car body. If you study vector and forces you find different length effect force, different angles effect force. Bottom line is if you change one side lower arm length you change the down load force on the tire and you change the traction.

 

[B565]

Agreed with everything you’ve stated . Again I should have been more specific . The upper control arm pivot point is not centered on the rear axle but rather 2” in front . What would be the effect of the upper control arm mounting point 2” 3” 4” forward of the rear axle launching a drag race car and assuming all I/C measurements and anti squat % is in reasonable range. Thanks for the response

 

[jack action]

But still unclear how moving the hole 2” , 3” forward of axle centerline would affect the dynamics of the launch on a Drag Race car .

It is still unclear how your connection point is moving with respect to the other connection points. Does it change the distance between those points? Does it change the link angles with respect to the chassis? All of that matters more in my opinion than simply the absolute position of a single point.

This will affect the anti-squat of the vehicle. And if the anti-squat is anything but 100%, the links will move and most likely change the amount of anti-squat. One important objective you want to achieve is smoothness in the motion, i.e. nothing suddenly stops or binds.

Some people will talk about "increasing" the downforce on the tires, but it doesn't happen. All you can do is move the front weight towards the rear wheels. Once the front wheels leave the ground, the whole weight of the car is on the rear wheels and you cannot put more than that. The only mechanism involved in that weight transfer is related to the wheelbase and the CG height, no matter the type of suspension you have (or whether you don't have one at all).

What can a suspension do during launch to increase that weight transfer? The only you can do is to raise the CG height.

You can raise the CG height by extending the rear wheel suspension as much as you can, i.e. anti-squat greater than 100%. The same thing goes with the front wheels: extend the suspension as much as possible to be able to smoothly come down afterward. This yellow Barracuda is the set-up I would aim for:

​

The entire car is raised up (CG at its highest) and you can see the front wheels slightly wobbling for a second, proving the entire weight is the rear wheels.

The rest of the video shows other setups with comments from the author. What is important to remember is how the smoothness of the delivered torque is more important than trying to get the highest CG, and this is why other recipes can work better even though the lift is not as high. The blue pickup truck in the same video is a good example of that: The suspension motion allowed the driveshaft to hit the chassis which destabilized the whole process, so less lift works better for that vehicle with that suspension.

​

The point of the whole thing is that trying to single out "How far do you need to move the rear end housing bracket?" is not the true question you need to ask yourself and there is certainly not a single answer for every possible vehicle and setup you can think of.

What would be the effect of the upper control arm mounting point 2” 3” 4” forward of the rear axle launching a drag race car and assuming all I/C measurements and anti squat % is in reasonable range.

If the anti-squat is 100% in both cases, the suspension will not move and you will probably won't see any difference. If the anti-squat is anything but 100% in both cases (but still the same value), the end result will depend on the resulting link length differences. A shorter link will most likely induce more radical changes as the suspension moves up or down which might upset the smoothness of the motion or allow for some binding or hitting a bump stop. But this might be compensated by different damping or anti-squat or I.C. lever arm length or ..., or ...., ... and in the end still obtain the same result or even a slightly better one. (You basically are trying to limit the suspension motion that is upsetting your grip by adding some exterior forces.)

 

[Ranger Mike]

I suggest you read Race Car Suspension class in this mechanical engineering forum. pay attention to the index and look up 3rd link posts and Rear end Instant Center posts. I cover a little on 4 link too but is floater type for dirt cars. Also much on anti squat. itis in index too.

 

[Lnewqban]

Please, see:
https://www.motortrend.com/features/weird-science-may-1987-982-1404-56-1/

 

[Ranger Mike]

Instant Center - we want the angles of the stock 4 link to form an Instant Center (IC) as far forward as possible. This longer lever cushions the acceleration when the tires hook up. Too short an IC distance means we have a lot of angle change on both sides and the car will be real darty when you nail it. Longer is better and gives the driver more control because you don't have a lot of angle change as the shorter IC has.

So we have explored the long vs short top link location relative to the rear axel center line. We have not discussed the top link location relative to the rear wheel track width.

Remember, lower links PUSH up toward the car body, Top links are pulled downward by the rear end torque reaction.

In round track racing we use weight scales to measure static weight on each wheel. We seek left side weight of 55 to 58% total vehicle weight. This helps maintain tire contact in the turn. Road racing setups are strictly 50-50% side to side weights as the car is turning left and right and we want no bias to one side. Drag racing “should” be 50-50 but it is not. You can set the car up for STATIC 50-50% but this changes due to the dynamics of torque reaction.

Under acceleration you ae literally jacking up the left side of the car and adding downforce to the right rear tire. This is not a 50-50 weight % situation as you have more force than 50% on the right rear tire. One way to measure this is to put scales under the car and literally jack up the left side and read the right side scales. It helps if you know how much shock travel you have on the rear shocks. This is easy to measure next time at the drag strip. Most race shocks have a rubber o-ring on them that you can set at zero before the car launches. Then measure how much right side compression or left side extension you have. Bottom line is you will find the right rear has significantly more weight on this. You are inducing a hazardous push to the left but having too much right side tire bite.

How to solve this/ Simple. Once you know the additional amount of right side weight, calculate the % right side weight.

Lets say you have a 3000 pound car with 60” track width, 48% rear weight and 50-50% right side weight. So 48% rear weight 1,440 lbs. or 720 lbs. on each rear wheel. The factory designed the car for 50-50 side weigh distribution and chances are you have not cobbled and hacked on the top mounting points.

What you have discovered is the right rear tire when under acceleration has 850 lbs. This means the left rear wheel has 590 lbs. We have 41% weight on the left rear and 59% on the right rear tire. A slight change from 50-50% now guess what happens?

Let us look at a shipping pallet with a 426 Hemi strapped to it on the concrete garage floor. You hook a chain on the pallet, not at the center of the pallet, but a foot to the right of center. Now pull the chain, The pallet will cock to the right side.

The above pallet example is a way to show that you must hook up a locked rear end so both tires bite the same. This induced Push is dangerous and is so easy to cure and eliminates one huge rear end traction problem. The pallet example above shows that the right side moves first because force is not placed on the 3rd link equally. Remember down force! We do not really cock the rear end as in the above example. What really happens is more force is added the side of the car the link is closest to. In this case we have more force added to the right rear tire so we create a PUSH condition on launch.

The cure- on a 3rd link situation you have the stock 50-50% location of the top link located at 30” (center of the 60” track width). As we see this is not the best option. A better solution would be to locate the top link 24.6” from the right tire center line which is 59% of the 60” track width. Now the 3rd link pulls an equal amount for each tire. The 4-link system is a little more complicated but can be fixed. You want to measure the two top link center distance between the two. Find the center point. You want to keep the distance between the top link mount points the same but move the center POINT to the 24.6” location. It may not be possible but now you know why the car pushes on acceleration.

This is a scenario for education only and gives prospective into rear end traction under dynamic loading. I suggest you do your homework and proceed accordingly.

 

[B565]

Hi , most rear suspension 4 link brackets mounted on the rear end housing are centered or slightly forward of the rear axle centerline . Why ? And what are the effects of moving the mounting points forward 2”,3”,4” forward ? Thanks for the help .View attachment 334021

I’m fully aware of how the 4 link works Im asking what would the effects be with the diagram bellow . The pivot points on the rear end are always centered or slightly in front of the rear axle .

 

[jack action]

If the pivot points give the same I.C. location (links have the same angle), then there is no advantage initially. the shorter upper arm will impose more radical changes in the geometry as it moves, which you may want or not (most likely not).

But if you only set the pivot point forward as in your first image, assuming you keep the same locations for the I.C. and the other pivot points, it is pretty obvious that once extended as in your second image, the pinion angle will be much bigger - up to a point I doubt it would not cause problems. The shorter length of the upper link will most likely pull the axle forward as the suspension is extended, causing the driveshaft to bind.

Just redo the second image with the modified pivot location and you will see the differences as easily as one can explain here. Remember that the axle on your drawing can be replaced by a single link going from one pivot point to the other. Add a straight, perpendicular, line for the pinion and see how the angle and location change. If the angle between the driveshaft and the pinion is not the same as the one between the driveshaft and the transmission output shaft, or if the driveshaft length varies more than what is allowed, you will be in trouble, and the geometry of your suspension will be the least of your problem.

 

[Ranger Mike]

I am out of here.