Fixing Issue with Corner Weights

So here are the corner weights on my Datsun 1200. 

519 450

419 416

All of the suspension parts / set up is straight out of the Datsun Competition Suspension Manual.

Some background; the fronts are traditional coil overs but the rear is live axle with leaf springs. The shackles have adjustment holes but they are at 3/4" increments so make fine adjustments really isn't possible.

The last time I did the car it was as follows:

491 475

401 447

As you can see the cross weight percentage stays pretty much at 52/48 regardless of how you adjust the car. 

So my choices are coming up with a way to make fine adjustments to the rear of the car or physically moving parts around.

Moving Weight:

There is nothing substantial on the left front side of the car. In fact the 26lb battery is still located in standard location (front right) so there is nothing I can move around. In theory I can move the motor and transmission over to the right by as much as 1" (183lb motor 54lb trans) Not sure what effect moving 237lbs 1' to the right would have on the car (1804 w/driver). 

Making rear ride height adjustable:

As mentioned the shackle adjusting holes result in big changes plus the current position is such that it eliminates undesirable roll steer.

I'm sure I could making some shackles or some other method to make fine changes. 

With all that said the I'm turning the fastest lap times ever with the car and the handling is the best it's ever been, so I may well be obsessing over this unnecessarily.

Basically I'm trying to thing of a way of moving 34lbs from the left front to the right front, while keep the rear as is.

I don't know what you did between the two readings, but cross weights should be the only thing you can adjust without physically moving stuff around. I always use the mental picture of a table with uneven legs, they always rock on the diagonal.

Looking at those weights, it looks like there was a fair shift from right to left. Is something binding? Fuel load? The overall weight went up by 10lbs, so something changed.

Before: 51.8% cross, 53.7% front, 52.0% right

After: 51.7% cross, 53.3% front, 49.1% right

In reply to Keith Tanner :

So a couple fo things. Originally we put 140lbs in the drivers seat, but this time I actually sat in the car with all the gear on and I'm acutally 152 with gear on (I weigh 145lbs).

In the meantime I swapped to my back up motor; two things it's 12bs lighter (1200cc vs 1500cc) but the alternator (8.5lbs) is on the left side vs the right side on the 1500 motor. 

Additionally I pulled the passenger side race seat, harness and window net out of the car (33lbs total) out of the right side of the car. I can't remeber but the 4 speed may have been in the car when it was last corner wieghted. The current 5 speed is 16lbs heavier.

The car is actually lighter than it was before 1652lbs vs 1674lbs. The net weight difference is 10lbs due to the calculated driver weight of 140 vs the actually fully geared driver at 152lbs. 

What I'd do in this case would be to raise one front corner by, say, two turns and drop the other front corner by the same amount. You should see a change in the diagonal weights. Is that what happens?

In reply to Keith Tanner :

I'll have to get back on the scales and see. I wasn't in the shop when the initial part was done, I got there in the last 10 minutes that it was worked on.

You might be able to get both the ride hight and the cross weights where you want them using just the front spring adjusters but if not you could consider changing the rear shackles to sliders.

You can then use shims between the slider box and the frame to adjust spring height.  The sliders have the further advantage of having less lateral play than conventional shackles.  That's the setup I've got on my Camaro.

Now that the ~50lb weight transfer has been explained, it's not that your cross weights don't change, but rather that your combination of changes happened to result in similar cross weights.

I don't see why the cross weights shouldn't be able to be dialed in using just the fronts, but I'd be watching for things like excessive ride height irregularity that could affect the suspension geometry or wheel travel. If you can get the front to 505/465, the rears should naturally end up at 435/400. So you'll be raising the passengers front and/or lowering the drivers front.

Driven5 said:

Now that the ~50lb weight transfer has been explained, it's not that your cross weights don't change, but rather that your combination of changes happened to result in similar cross weights.

I don't see why the cross weights shouldn't be able to be dialed in using just the fronts, but I'd be watching for things like excessive ride height irregularity that could affect the suspension geometry or wheel travel. 

And therein lies one of the problems. I'd likely have to relocate pick up points on the lower control arms or raise the ride height of the car in order to avoid undesirable camber changes. 

In reply to Tom1200 :

Are you already as high on the front passenger side and as low on the front driver side as you're willing to go? If not, go further and see how close it gets you. If so, what about shimming the rear suspension?

Some additional thoughts:

1) Asymmetry isn't inherently bad, at least not in a racecar. The track is asymmetric too.  It is possible that technically 'better' corner balancing will turn slower overall times, as you'll have more and/or more critica turns one direction than the other. You may gain more than you lose by sacrificing some handling balance one direction in favor of the other. Your stated corner weights should be inclined towards more understeer on right handers and more oversteer on left handers.

2) Your live axle also behaves asymmetrically.  Power-on it will increase the load on the passenger front and driver rear, and decrease it on the driver front and passenger rear... Exactly what you'd need for more 'ideal' corner weights anyway. Of course, it will also do the opposite power-off and exaggerate the current corner weight imbalance.

In reply to Driven5 :

The front suspension is as low as it can go on both sides.

Most of the tracks I run are clockwise.

The car actually oversteers everywhere; as it is very underpowered I have the car set up really loose.

So you're unable to move the weight and unwilling to adjust the front springs.

In that case, I'd reweigh it after raising the drivers rear or dropping the passengers rear by one shackle hole. This would arm me with actual data for to use in figuring out how much shimming may need to occur to get to 435/400 across the rear... And bring the front to 505/465.

In reply to Driven5 :

I should clarify; I'm not unwilling to adjust the front springs or at least trying to adjust them. I was just pointing out they were as low as they could go. 

Looks like I'm going to have to make up some parts to get the fine adjustments I want.

Tom1200 said:

In reply to Driven5 :

Looks like I'm going to have to make up some parts to get the fine adjustments I want.

You currently don't have any idea how fine of adjustments you actually need... Why not start with the adjustments you already have available?

Driven5 said:

Tom1200 said:

In reply to Driven5 :

Looks like I'm going to have to make up some parts to get the fine adjustments I want.

You currently don't have any idea how fine of adjustments you actually need... Why not start with the adjustments you already have available?

Well said; I am indeed making assumptions that may or may not be true. When I first started running this car 31 years ago I'd work with whatever I had..................I need to get back to that mindset, thanks.

I think I would start by moving the engine trans 1" to the right and se what that does.

How much effort is transferring 4.75 pounds from left to right worth to you?