Why raise the differential?

I pose the question because I am trying ti understand this modification and can not seem to find an explanation of why using the search. Can anyone explain the purpose?

It looks like the two methods are install an 80-82 bat wing or modifying the cross member by removing the rubber bushings to raise or in more severe cases trim the frame and making your own crossmember.

I am closely studying the rear of my car to remove squat and build traction.

Any info would be very helpful.

Thanks

Only purpose is to lower the center of gravity without screwing up the suspension geometry.

Are you going to drag race?

pcf_mark said:

I pose the question because I am trying ti understand this modification and can not seem to find an explanation of why using the search. Can anyone explain the purpose?

It looks like the two methods are install an 80-82 bat wing or modifying the cross member by removing the rubber bushings to raise or in more severe cases trim the frame and making your own crossmember.

I am closely studying the rear of my car to remove squat and build traction.

Any info would be very helpful.

Thanks

Click to expand...

Think of it for what it really is. You are LOWERING the frame in the back. The differential stays the same height from the ground.

I do not plan on drag racing but I plan to run the car once or twice a year. On the street the rear squat is out of control and very annoying. I have the car lowered enough to get the half shafts in decent alignment. When I put the power down the car squats and squirts. The front bolt of the t-arm is pointing down hill looking at the car from the side which I think is contributing to my squat issues. I was hoping to raise that in the pocket. It made me think raising the diff may help also because that is the point where the torque is being managed.

"Only purpose is to lower the center of gravity without screwing up the suspension geometry." This makes sense if you lower the car enough to make the lower links and half shafts unhappy and start inducing crazy toe or camber changes. What is the cross over point and how do you measure?

Set up info:
68 Coupe BBC
Six speed manual
16" C4 rims with Gatorback tires
VB shocks for fiberglass spring
VB dual mount spring
VB heavy duty camber kit
Urethane bushings throughout
Lowered from stock but I am not sure the exact height at the fender lip.

Thanks again for any info.

Really your trailing arm is pointing downwards at the front????
Never seen before.... anyway this will add a lot of squat, of course!
Because of the IRS rearend the C3 have very little anti-squat..... may some 20% or less.
The reaction of the torque is applyed directely to the frame and can't help for theis issue... onlt the T/A geometry and some trick can help:
Of course (it's a vectors matter) the T/A must point upwards.... the more the better for anti-squat.
If you really wont more antisquat to plant bettere the tires in acceleration, you can try to modify a little the lower mounting of the differential:
Put some very soft rubber coushion between the front differential bracket and the frame... then install an arm connected to the lower flange of the leaf-spring and to the rear crossmember .
In this way you can have some force of reaction pushing up the front of the car during hard acceleration.

VBandP produce a KIT for this purpose.... but it help a little.

For a less radical modify.... a stiffer leaf-spring and a pair of good shocks are helpfull!

Sorry for my english....

Been through this one with a friend. Raising the differential will NOT change the "squat". It will lower the CG which is why people do it.

Raising the attaching point of the trailing arms to the frame WILL change the "squat".

The reason the car squats is because the trailing arm is driving downward when you apply power. If you change (raise) the attachment point of the trailing arm you change the geometry of how the power is applied to the frame.

You need to be careful because if you raise the attachment point of the trailing arm too far you will be picking up on the frame when you apply power and will cause the rear of the vehicle to raise instead of squat.

Prior to doing any changes get together with someone who truly understands chassis dynamics and geometry so that you don't FUBAR the whole deal.

Rich:beer:

When you raise the differential you raise the CG relative to the frame. Raising the diff is done when severely lowering the car, this way the suspension geometry stays about the same as a normal ride height, you're in effect lowering the car around it.

This has 2 major advantages, first you retain a proper roll center height and more importantly you can keep the half shafts from going past horizontal and resulting in horrible bump toe out.

As for anti squat, some racers when raising the diff also raise the a arm pivot point. Like Pier Paolo said, there's not a whole lot of anti squat you can get with an IRS because the torque effect is mostly concentrated in a "rigid" frame hung differential.

There is no way you can get enough anti squat in an IRS to get pro squat, it just doesn't go beyond about 25%, and that's nowhere near enough.

Raising the differential will indeed not change the anti squat characteristics because you are not altering the trailing arm, however lowering the car does alter the trailing arm angle and as such affects anti squat.

Here's an extreme anti squat IRS geometry, this is about as far as you can go. Keep in mind that what really matters is the virtual swing arm that these 2 links offer (another advantage of the C4 setup, not only does it remove the trailing arm from toe control, you can use short links and have a virtual swing arm with the desired characteristics. Although this virtual arm has a non fixed virtual pivot, it moves with susp. travel)

any ideas on how much the trailing arms should be raised for general street/ strip use?

73c34me said:

any ideas on how much the trailing arms should be raised for general street/ strip use?

Click to expand...

NONE...if your going racing 1".....hardly worth the trouble.

body is off & have a rolling chassis. If raising 1", is there any down side to this? Also, is this a trick that the pure stock & fast class guys are using to run the unbelivably quick times on bias tires are using?

Stroker 427 - no need to apologize for your English. It is far better than my Italian!

The combination of wheels/tires and the lower ride height to get the half shaft at a decent angle and nearly parallel with the camber link has the T/A going down hill at the front at ride height.

Since there does not appear to be any down side I am going to try and raise 1" within the existing pockets of the frame. I can always go back!

Twin Turbo - I understand the concept of the virtual control arm or instant center of a pair four link bars. What I am confused about now is why the diff is the torque application point. If I have a set of four link bars like the car in your photo (damn! that looks cool) can't I set the instant center under the c/g and get rear lift versus squat? What are the dynamics of the diff in this suspension?

Thanks - this is very informative. It is cold in PA now but I am dedicating tomorrow to tuning my carb with my wide band O2 so I can move on to get this squat figured out as my winter project.

Yes...... you can set the instant center virtually where you like in relaction to the CG of the car... and this is the only thing you can do with IRS system.

Regarding the torque matter, try to imagine one thing :
When you make a hole with the dril in a thin shetmetal and you push too much.... the result is the bit sticking in the metal and the drill counter-rotating in your hands! (dangerous!).

This is the torque reaction.

In a IRS system the torque reaction is all in the differential (the wheel hub is basically only a bearing...) and the direct consequence is the impossibility to build antisquat!

In a Liveaxis system, with correct trailing arms, you can have more than 100% antisquat.... have you never seen drag cars rising the back when launching???
This means more than 100% antisquat.

By the way you must remember that the more antisquat designed in your car.... the more wheel-hop you will have under braking!

Wheel-hop is a veru bad thing when track-racing a car:

it is pratically impossible to brake very late.... unless you will accept a rearend jumping away at the begining of the corner.

In fact track-cars can leave with very low antisquat due to the fact that the grip of the tires can be enough to come out from the corner very fast.

The only limit is a loss in traction at the start of the race (unless the race will have a rolling start).

In a race, te must important thing is the elapsed time from the start to the finish.... the quicker will win!

Typically in an average race there is ONE start and HUNDREDS of corners..... I guess you can finish alone the phrase! ;-)

First of all you must define which kinf of use you will do of your car..... then you can start to think to the mods!

Just my two cents.....

Sticking with the torque reaction for one more post. Because the diff is bolted to the frame all torque reaction is at the ends where the diff cross member is bolted to the frame. As a result all the torque is acting on that member twisting. None of it can be used for anti squat or lift because it is not acting longitudinally along the axis. All of it is lost in twist.

The braking aspect I had over looked. I currently can lock up my rear brakes with little effort if I am on the brakes and down shifting. I do not want to aggitate that problem.

I think I am going to take your advice and think about how I use the car a little bit more. I would like some compromise on the squatting even if I only go as far as getting the t-arms level to neutralize squat.