Adjusting Corner Weights

[Chris Duncan]

Got some scales and some floor tiles as slip pads and trying to adjust corner weights with the coilover height adjusters. Was wondering about the math.

It's a given that front to rear weight split is about 40/60. But what happens when the side to side weight split is not 50/50. Let's say you're tuning with driver weight with LHD, or you have the battery off center, or the steering column and wheel weight.

So starting with a 50/50 side to side, 40/60 frt/rear with a 2200 lbs car

LF 440 RF 440

LR 660 RR 660

and adding a 140 lbs driver on the left, should the correct weights be

LF 440 + 40% of 140 (56) = 496 RF 440

LR 660 + 60% of 140 (84) = 744 RR 660

or

LF 496 RF 440

LR 744 RR 660

so the end result is that the percentage difference front to rear is the same on each side

496/744 is a 40/60 split
and
440/660 is a 40/60 split

this is just for example because you know some of the driver weight load will go to the passenger side

I'm just thinking that all your trying to accomplish is to remove diagonal twist/loading or "wedging" as they say in the stock car world. And by the math that means each side should have the same percentage split even if they are different in total weight.

Hope this is a little clearer than mud.

 

[Jac Mac]

Not quite that simple Kalun as the drivers weight when seated is spread over 3 points ,for sake of argument- backrest- butt- feet , and as these are at their respective distances from c of g it will change your distribution a little in what you read on the scales as compared to what you calculate by simply adding your whole body weight at one point.

Jac Mac

 

[Dean Lampe]

My understanding is the diagonal balance is the primary concern. I could be wrong, but thats how we have always done it.

 

[Chris Duncan]

Think I just answered my question by looking at the different displays on the scale. It has one reading called "cross" and it's a percentage of the diagonal totals. I'm assuming for a neutral handling road course you want this to be 50/50?

so from my end figure

LF 496 RF 440

LR 744 RR 660

diagonal totals

744+440=1184
496+660=1156

this is not 50/50, so my first assumption of percentages left to right is wrong?

Okay, another question, are you supposed to do this with drivers weight?

 

[Dean Lampe]

You are on the right track. I would consider increasing the spring tension on the RR or lower the LR.

 

[Ron Earp]

We put a driver in the car and adjust cross weights. RR to LF, LR to RF. I don't ever worry about the left and right totals much. If you try and fix those your cross weights are going to be more than likely wrong. You can wedge the car for particular tracks to help on turning right or left, but we rarely fool with that at the tracks we drive.

Ron

 

[Bob MacDonald]

In the GTD 'forty', the driver's approx CG position is on centre between the wheels but 10 ins offset from the vehicle centre line. Hence with the 'forty' track and wheelbase, a 140 lb driver would add 96 lbs to the drivers side and 44 lbs to the other.

This would in turn add 48 to each driver side wheel and 22 to each of the otherside wheels.
To get back to an equal front to rear % on both sides an adjustment of approx 2.5 lbs per wheel ( two up / two down).

The initial weights set of 660/440 on both sides would firstly be
708/488--682/462 with the driver
but after adjustment these would change to
710/485--680/465
and the front to rear ratio on both sides would then be 59.4%/40.6%.

I believe a front/ rear ratio of 55/45 is generally closer to measured results on the 'forty'.
The fuel tanks can each hold 60 to 80 lbs of fuel when full and would also influence the weight distribution somewhat.

Hope this helps

 

[Bob MacDonald]

When making adjustments to the weights only one corner needs to be changed, the other three corners automatically adjust up and down by the same amounts.
However if the amount of adjustment is large it would be better to make the adjustment in halfs across a diagonal. This will lower or raise the vehicle ride height without tilting one corner.

Bob

 

[Dutton]

First, congrats again!

Just my 2 cents on the alignment: When aligning the FF years ago, I approached it with the mindset that the corners of the car were no different that the corners of a chair in the kitchen. When the front axle was balanced side to side, and the rear was balanced side to side, sweet things always seemed to happen on track.

While not dead center as in the FF, the drivers position in a 40 is far more adventageous than with nearly all other cars which will make for a somewhat easier alignment.

If it's any help, I did find that colorful language during the alignment usually helped the car come around to my way of thinking... have any left from the build?

Best,

T.

 

[Chris Duncan]

Okay, the previous figure of 2170 lbs. was with about 3 gal fuel, but oil and coolant full.

Here's the numbers with 3/4+ fuel and springs all at designed ride height

LF 474 RF 483
LR 655 RR 625

LEFT 50.4% 1129
REAR 57.2% 1280
CROSS 50.9% 1140
TOTAL 2239

And after 2-1/2 turns tighter on the RR (the front is already low enough) Which means that spring is wound up 3/16" shorter than the LR.

LF 485 RF 475
LR 644 RR 636

LEFT 50.4% 1129
REAR 57.1% 1280
CROSS 50.0% 1119
TOTAL 2240

And with 130 lbs in the driver seat

LF 532 RF 497
LR 692 RR 649

LEFT 51.6% 1224
REAR 56.6% 1341
CROSS 50.2% 1189
TOTAL 2370

So the suspension points are off by approx 3/16", the battery in front is getting the F/R away from 40/60, and the pedal box and steering make the left side heavier.

 

[Dutton]

Adding the weight can really change things.

You're darn close. A few more tweaks to the LR may well be close enough. I see that the springs are getting shorter but how's the ride height?

Best,

T.

 

[Randy V]

I've scaled a lot of cars and can only offer the following;

1) Make sure you disconnect your anti-sway/roll bars
2) If you have gas charged shocks, make sure they are all at the same pressure if possible.
3) Pick a fuel load and stick with it from here on out - I typically scale with 1/2 cell of fuel. In a 40, that would be 1/2 load in each side.
4) Make sure you have your alignment done ahead of time. Changing your caster and camber will jack the weight differently
5) After each change - Bounce / roll the car to settle out the suspension - even if only a few inches fore/aft on the scale pads.
6) Know that no two cars - even if built identically - will ever scale exactly the same.
7) Take your time... This is not the time to have the spouse or buddies hanging around as it can be very easy to lose your concentration.
8) Write down every change and notate the result.

 

[Malcolm]

From my experience with scales I would concur that you need to get your fuel loadings settled and consistant before you rely upon your work. Each litre of fuel = 0.75 kg which can screw things up pretty quickly if the next time you set you have different loads side to side. A suggestion would be to make your fuel cells easily drainable so that you can drain the tanks down in the garage or pit so set up is from a static base point each time.

I always go for the diagonal balance as the prime aim. You are very likely to get to a point where you will work out that unless you move physical mass about the car it is impossible to achieve a perfect set up. A 40 is not a single seater. So if you are still building consider how often you plan to have a passenger in the car when on track and also where to locate the battery and fire extinguisher systems to help offset a drivers weight. That is, if you want to be picky over this!

When I used to do sprints at Goodwood I used to play around with fuel loading so that whilst keeping it to a minimum, I used to carry it on the drivers side. Being predominantly right hand turns this weighted the car to that side and would perhaps help with cornering. An unproved theory that kept me happy at the time! I was happy with the potential downside for the two left hand turns. On hill climbs where there was no clear bias in the number of turns in any particular direction, I used to load the passenger tank to offset my wieght in the drivers seat.

 

[Bob MacDonald]

The numbers indicate your vehicle CG is 6.79 ins behine the front/rear centre and 0.22 ins towards the diver side from the centre line.

The change you made of +11lbs to RR produced +11 to LF
-11 to LR and -8 to RF ( a bit of sticktion somewhere?)

A further +1 to LR or RF OR -1 to LF or RR will give
RR 635 RF484
LR 645 LF 476
both at 57.1% / 42.9%
Diagonals L 1121 R 1119 Total 2240 ( interesting, exactly 1 ton )

This is as good as you can get (ie very good!) since the CG isn't on the centre line, if it were then an equal diagonal figure would be possible.

Bob