Weight and balance experts, I need your help
 

Guys,
I just got the oppprtunity to borrow a set of scales till tomorrow noon. I just put my car on them and got the following results with a full tank of gas and me in the car.

LF 740 RF 679
LR 668 RR 737

Total 2824

Total front 1419(50.2%)
Total rear 1405(49.7%)

Total left 1408(49.8%)
Total right 1417(50.1%)

LR to RF cross 1346(47.6%)

How good or bad is this and can I get closer moving my spring perches and if so how??? Maybe I could just gain a little weight and sit on the LR fender.
Thanks for all your help, Steve

Steve,

This is very approximate, but it should get you pretty close: I'm assuming 1 turn is roughly 1/6" collar height.

LF damper - unscrew the spring collar 1 turn (down on the shock)

RF damper - 1 turn up

With a Jag rear, you only have to adjust one damper per side:

LR damper - 1 turn up

RR damper - 1 turn down

Well, I'm assuming that you had you swaybars disconnected when you took the readings.

F to R looks pretty good.
Since the total F to R is 50.2%, then the left and right sides, each must also have the same distribution for the weights to be ideal.

The good news is that by my quick look at it, you are about 30 pounds low on the RF & LR,,,,,and 30 pounds high on the LF & RR.

I "rounded" to the nearest pound, and here is what I calculated your ideal corner weights should be,,,,according to the 50.2 front /49.7 rear total distribution you posted.

RF ideal weight= % on front tires X total weight on both right tires, divided by 100 (50.2 X 1417)/100= 711 pounds


LF ideal weight= % on front X total left, divide 100,
(50.2 X 1408)/100 = 707 pounds

RR ideal = % on rear X total right, divide 100,
(49.7 X1417)/100= 704 pounds

LR ideal= % on rear X total left, divide 100,
(49.7 X 1408)/100 = 700 pounds

For a great set up, you want to be within 10 pounds of the ideal weight.......
You are at around 30 off per corner, so you aren't too far off.

Remember, to increase the weight on a tire, you raise the ride height on it,,,,,,,,,,,,and to decrease the weight you lower it.

The fun is that any change in one corner will affect all four.
Usually you try to make a 1/4 adjustment on each corner to get your goal. ....which means you want to make roughly a 7-8 pound increase at the RF and LR,,,,,,,,,and a decrease in the LF and RR.

I hope this helps. I just dug up some of last years "note's" from my racecar and stuck in your numbers,,,,,,,,so I think I have you headed in the right direction.

I'm sure if I spewed & got it all wrong,,,,,,,,one of the chassis guru guys will be happy to point it out. :D :D

Steve, Have you checked the surface you are doing these measurements from ?? It must be perfectly level or you can't depend on these numbers at all. I am puzzeled with these numbers, why is the RR 69lbs heaver than the LR (maybe a battery) but usually the entire left side is heaver than the right due to the driver. Try weighing without the driver and send me some numbers. Your front to back numbers are not bad, but you need to work on the cross weights. This car will probably turn great one way , and understeer the other way. With the RR weighing 69 lbs heaver on one side it may have an strong oversteer in one direction and not in the other. If you are going to do any road course work I would like to see the numbers a lot closer. Also check the ride height of the entire vehicle. What is your weight ?? Corner weighting a car requires some trial and error, and to answer your question about spring perches and how much . You can move weight by raising and lowering the corners but only to fine tune the car. We need to move some major weight around before starting to jack with the springs. Maybe you left a big bag of money in the trunk on the RR.
Later, db

Bob,
Wow, Thanks. I'l play with the springs and report back.

Alan,
Thanks for the detailed analysis. It is amazing what you can learn on this forum.

Cobra 42,
I weigh 165. I checked the trunk, the big bag of money is gone.:CRY: :CRY: I think Bob may have it:3DSMILE: Here are the numbers with me out of the car:

LF 689 RF 683
LR 590 RR 698

Total front 1371(51.5%)
Total rear 1289(48.4%)

Total right 1381(51.9%)
Total left 1278(48%)

LR-RF cross 1271(47.7%)

Also, The engines are offset one inch to the passenger side in ERA'S.


I'm going to run out in the garage and play a bit, I'll keep you all posted. Thanks again, Steve

Gang,
Well I set it up 8 different ways. The last six times I felt like I was chasing my tail. As I look back over my notes it's hard to say which is the better set up as some measurements are slightly better while others are slightly worse. I'm going to try and go back to one of my previous settings in the morning. Right now I'm here:
LF 720 Rf708
LR684 RR717

Will try and go back to here:
LF 710 RF 717
LR692 RR714

Thanks for all your help. Steve

Corner-weighting
 

Steve,
Corner-Weighting your Cobra is a VERY precise measurement. It is also VERY in depth. It should take about 4-5 hours for 2 people to tune the suspension properly. It is more than a matter of simply turning spring height to suit a certain "feel". But a balanced car is worlds above an unbalanced car. Ready to dive into information overload? Here we go...

I see your posts of your measurements. You want to have your cross/diagonal weights equal. For example (LF + RR) = (RF + LR). In your case (720 + 717) = (708 + 684). (In other words 1437 vs. 1392) You are 45lbs off diagonally. You are affecting how the weight transfer is applied, side to side. This will give you a balance upon L&R cornering. In other words, it will handle left turns the same way it handles right turns. Do not worry about Front to Rear balance for now.

To properly weight the car...
1) Your scales must be level to within 0.060 inches of each other. NO garage is level. Use laser leveling system to shim scale height.
2) You must have the anti-roll bars disconnected.
3) You must have EXACTLY the same air pressure in each tire
4) Must have 1/2 tank of fuel
5) Must have driver sitting in the seat, helmet in lap, with all the crap out of the trunk, (ie. be in race trim).

In conjunction with corner-weighting, you must have alignment tools to measure the camber, caster, and toe. Changing the ride height on a corner can change your camber gain by as much as 1-2 degrees. This would greatly affect handling.
So what you do is get "close" on the corner weights, then take suspension alignment measurements (camber, caster, toe). Write those down. Adjust the alignment to whatever your manufacturer recommends to be suitable for your intended application. Then check out the corner diagonal weights again. If you are within 10 lbs on the diagonals, that's close enough, leave the corner weights alone & mark the spring collars so you know not to move them from this position. As long as your alignment is where you want it AND your corner-weights are within 10lbs diagonally, you're perfect.

Now back to the Front to Rear balance thing...
When done, you may raise or lower the rear or front to suit your "front to rear" balance needs. BUT, you can only move them in pairs. In other words, move BOTH rears up, or BOTH rears down. Or both fronts up? Moving them in pairs will not change corner-weights because corner-weights are measured diagonally. Just remember that you might be messing up your alignment if you move the front pair. I would suggest (raising/lowering) the rear pair a bit to suit your needs. And set rear tire air pressure 2lbs less than the fronts.
And dont forget to put the anti-roll bars back on.
:)
With track experience, you will discover new alignment settings to fine tune your car for even more handling! But that comes in time...
I provide this explanation only from personal experience preparing my racecar.

Happy motoring!
Stephen
Cobra University
stephen@cobrauniversity.com

Stephen,

Thanks for the reply. I picked up some info from you at the Sandwich Factory Saturday and once I get my car on the road will definately look you up.
That was my first attempt with the scales. Thanks to Alan I had the sway bars disconected. The scales were not level with in sixty thousandths. I'll have another go at it in the future and this time I'll have a little clearer idea as to what I'm shooting for.
Once again thanks to all that took the time to reply, I really appreciate it and I learned quite a bit.
Steve D***ey

Stephen, (Cobra-U).

That was an absolutely perfect chassis balancing disertation, my hat is off to you there Burger boy! Very nicley stated, do you want a job as our chassis race set up engineer?

PS: Geno is coming to sell for me.

Rick

Rick,
Oh no you don't! Stephen is staying right here in SoCal where he belongs. Aren't you Stephen.

I am saying "sunny SoCal" since it is raining at the moment. At least it isn't snowing though.

Sorry I missed you guys at the Sandwich Factory. People been misbehavin' lately and there has been lots of work to do.

Steve Dickey,
I am glad you got your car done, running, and home. :D Can't wait to see it. Let me know how the paint turned out.

Hey Jim,
Sorry you did not make the Factory last weekend. I figure it's never good if you are busy at work!
The car is home and painted. Don't know if you could say it is running or not. I'm currently burning through dist. dive gears,so driving too far from home is out of the question. I may be tearing into it again to put in a new cam.:CRY: :CRY: One of these years I'm going to make it to the meeting and prove I've really got one and am not just a wanna-be.
Take care, Steve