Weighing A car

Did a search didnt find anything - About to get to the track - Will need to be scaling the car in the near future - cant find my chassis book so thought I would ask the experienced

Car is MS tube chassis with 4 link and struts, coil overs all the way around - 2500 all in - probably 900HP +

You start by putting a scale on each corner under the tires and have the driver park his behind in the seat - my assumption is then you adjust the coil overs so that the left to right weight is the same, front and rear respectivley - that is what I assume - how about some real world input

Thanks
Ron

all liquids driver in seat holding helmit and suit.....

Here's the way we do it. We have a rigid front so it may be a little different for a suspended front car.

1) Make sure everything is square and true, both chassis and rear axle in the chassis. Check for twist/out of parallel. Rear axle square to chassis to start is very important.

2) Have your coil overs set at the same spring height side to side. If you can measure spring rate per inch of travel do that. (we use a scale and drill press)

3) Level scales relative to each other. Scales that are not level will give you crap measurements. We use a laser to level our scales.

4) Make sure your tires are very close on circumference and tire pressures side to side.

5) Put the car on the scales and adjust the rear roll bar to obtain equivalent rear corner weights. Front doesn't really matter is our case, although it shouldn't be wildly off.

Hope this is helpful

Yea, all the "race weight" needs to be in the car wether your using scales for setup, or are just setting the car up manually by hand without scales. No side-to-side staggering of any of the shock/strut spring pedestal spinner/adjuster heights. You set initial suspension preload with the 4-link bars, not the spring heights.

Along that same thinking I feel the anti-roll also shouldn't be used at all for any preload adjusting on a door car, again that's part of the 4-link's job. You unhook the anti-roll, set the desired preload with the 4-link bars while all the race weight is in the car, then adjust/reconnect the anti-roll so that it's "neutral" so that no weight/loading is on either of the two links (both links are loose & you can "rattle" the rod-ends).

With faster back half & full chassis cars that use both a 4-link & an anti-roll you might find that as the car gets faster & faster (through more & more added power) you will need less & less preload weight on the right side (passenger side) slick to launch straight. So because of this the 4-link's side-to-side preload weight bias might need to be set around "zero" (neutral), or towards the driver's side, or towards the passenger side........ it will depend on how much actual "at launch" engine power/torque you have to work with.

Thanks Guys - Appreciated

Need to do some more studying - dont have an anti roll bar in this car - need to read up on the 4 link adjustment for weight bias

I could tune the old foxbody with the anti roll bar, adjust the left side length for more or less bite while watching how straight it launched. 10 sec car but it was very particuliar, with 1/2 turn making a difference on driving out left or right instead of straight

If a 4-link car doesn't have an anti-roll, setting the 4-link bars up with a zero/neutral weight bias is about the best idea (for a baseline starting point) in a lot of cases. This is because you don't have to worry about the anti-roll exerting more & more rotational torque force/weight loading to the right (pas) tire as the power level increases.

So without an anti-roll you might find that you have a much larger "fudge" window on the 4-link preload settings. Of course the down side is tons of possible body roll depending on the amount of actual power you have to work with at the launch. You also sometimes see more people using spring height staggering to fudge the preload around (and trying to control body roll) when an anti-roll isn't used.

This is a very good book. http://www.jerrybickel.com/complete-guide-to-chassis-performance-manual-rt-2000.html Has a lot of real world info.

There was a chassis shop slowing down and I got a set of Longacre scales for $700 - complete with adjustable platforms and a Laser leveling device. I have sandbags in the seat to equal my weight. The mock up engine is short pistons, rods and cam - but that is all center weight, and the trans needs the gear set and no drive shaft, again all center weight - Yes will re scale when car is complete

The rear 4 link is neutral to start with as will as the locater bar

I got 667 LF 545 RF
I got 668 LR 670 RR
1338 Rear or 52%
Total 2545

So after looking at Dave Morgans book and Rick Jones book (less help of the 2) it would appear that I am close enough to try the car at the track.

My concern is the heavy LF compared to the right, I took 2 turns out of the left front coil over adjustment and the weight only went down 10 lbs. So put it back to where it was. It is adjusted the same as the right front when you measure the spring locaters.

I am thinking that this may be normal as the diver weight adds to the left side. But I told that you normally run 50 to 100 plus preload on the right rear which would add even more weight to the left front if i read the books correctly.

So I am hoping from experience someone can say this is typical or normal for a chassis car with a 4 link rear and strut front.

Appreciated
Ron in Houston

I took 2 turns out of the left front coil over adjustment and the weight only went down 10 lbs. So put it back to where it was. It is adjusted the same as the right front when you measure the spring locaters.


Leaving the spring seats the same height side to side is the way to go, takes that as a possible problem out of the picture in the future. If your car had an anti-roll I would be concerned about the higher static diagonal weights (driver front/passenger rear) since an anti-roll loads additional rotating weight to the passenger side slick during the launch. But since you don't have an anti-roll I agree the current weights could be close enough for at least a rolling burnout to get a ball park idea what direction it wants to point during the burnout/launch.

As a secondary "non scientific test" to go along with what the scales say, you could see what the top bars "feel" like. On a car without an anti-roll a ball park way to see how "even" (neutral) both the top bar's preload is (at full race weight) just involves crawling under the car & grabbing both top bars. You slowly rotate them both on the rod-ends at the same time & "feel" how much effort it takes to rotate them both. Surprisingly if both top bars "feel" close to the same in effort to rotate, the rear suspension should be damn close to being neutral side to side static weight bias (preload) in the rear for a lot of cars without an anti-roll.

Just remember that if there is any possible unwanted binding in the rear suspension, it will give you a false reading in what you "feel".

Thanks Dave

Leaving the spring seats the same height side to side is the way to go, takes that as a possible problem out of the picture in the future.

How do you measure this, just use a ruler and compare side to side? How close do you get the measurements to each other?
I was going to borrow a set of scales to use on my 92 GT, but I'm not sure where to start. I've got QA1 coilover adjustable struts in the front , QA1 coilover shocks in the rear with Wolfe Racecraft single adj. lowers and double adj. uppers, and a sway bar. The car goes straight, but it usually will put the tires up in smoke almost immediately. I've had the struts reworked to loosen them up and gone to 150# springs in the front trying to get some weight transfer, and it still smokes the tires (325x50x15 BFG Drag Radials). I'm at a loss to figure out whats wrong! Sorry, if I'm hijacking the thread.
Gary

How do you measure this, just use a ruler and compare side to side? How close do you get the measurements to each other?

On a rear coil overs just grab a dial caliper & measure the distance from the bottom of the spring seat/pedastal/adjuster to the bottom of the threads to see if they are the same. On front struts you might have to just rely on a tape measure to get the distance since you might not be able to get a dial caliper in there depending on the strut design. Playing with the side to side spring height stagger really only has an effect on side to side preload. If it's a car thats going straight now and you have some stagger, you might as well leave it in there. Removing the stagger could mess with a straight launch. But if your setting up a new car (or a reset on a running car) you might as well start with no stagger.


The car goes straight, but it usually will put the tires up in smoke almost immediately.


where is the I/C at? How tight/loose is the rear shock extension valving set at? How tight/loose is the front strut extension valving set at? Are you running enough air pressure for the drag radials?......yuck...drag radials.....:D

Some good up close launch video would help.

where is the I/C at? How tight/loose is the rear shock extension valving set at? How tight/loose is the front strut extension valving set at? Are you running enough air pressure for the drag radials?......yuck...drag radials.....:D
Some good up close launch video would help.

Rear shocks- 3 clicks from loose
Front Strut- Full loose
Air pressure- 12-15# cold
Where is I/C at?- Thats a good question! Apparently it's probably somewhere that it shouldn't be.:( The factory unequal 4 link makes it a little difficult to figure out. Where should it be? I'll have to get it up on stands and see if I can figure out where it is. Baseline Suspension has some graphs of the factory points online, I'll see what I can figure out. What kills me is that I've seen cars with the same suspension as I have running in the 8's and all I do is sit and spin:mad:. I'm baffled.
Gary

Rear shocks- 3 clicks from loose........Where is I/C at?- Thats a good question!

There are some online I/C calculators you can use. Don't really know how accurate they really are, but they are probably close enough. Here's a real basic one for finding the I/C. Use the I/C placement info, but ignore the useless anti-squat bull $hit info.........

http://inductionmotorsports.com/im/ic.html (http://inductionmotorsports.com/im/ic.html)

I might be wrong but having the rear extension valving only 3 clicks from full loose sounds too damn loose & violent to me, especially for big block torque numbers. Sounds like something you might use on a small block car, or a car with a really, really long/lazy I/C length.

Again some good up close launch video taken from the correct angle could go a long way toward helping everyone here on the site offer up some opinions.

There are some online I/C calculators you can use. Don't really know how accurate they really are, but they are probably close enough. Here's a real basic one for finding the I/C. Use the I/C placement info, but ignore the useless anti-squat bull $hit info.........

http://inductionmotorsports.com/im/ic.html (http://inductionmotorsports.com/im/ic.html)


Thanks, that looks like it's what I need to find the IC!
As far as the rear shock setting: If my car actually transferred weight violently, it would make thinks simple. All I would have to do is tighten up the front and rear settings to calm it down. Unfortunately, something is out of whack somewhere and I get very little weight transfer before it goes up in smoke. I'm going to scale the car and get the IC measurements as soon as I can to get a starting point. Hopefully I'll be able to find the problem.
Gary

As far as the rear shock setting: If my car actually transferred weight violently, it would make thinks simple.


You can't have front-to-rear weight transfer (violent or otherwise) without traction already being present......cart before the horse.:D

Weight transfer doesn't cause initial traction to happen, it's the other way around, initial traction & slick wind-up helps weight transfer to start happening. Initial traction actually happens from the rear suspension's separation (or initial "hit") smashing/screwing the slick into the track surface at the launch, & not from weight transfer.


So when I'm refering to it possibly being "too violent", I'm talking about the speed of the initial separation & not weight transfer. The initial separation could be hitting the tire/sidewall too violently overtaxing the sidewalls. Hitting the tire/sidewall harder & harder doesn't always mean the car will hook harder & harder. Sometimes a softer/calmer initial "hit" will produce better traction (and resulting weight transfer). Every combo is different, thats why track testing is so important.

You can't have front-to-rear weight transfer (violent or otherwise) without traction already being present......cart before the horse.:D

I guess that might explain why I've been battling with traction issues for 8 years:confused:. Adding more horsepower last year didn't help;)! Well maybe when I get around to weighing my car and calculating the IC we'll be able to figure something out. Also, if I change out the stock GT rear bumper cover for the 93 Cobra one I have, I'll have enough room to install my 28x13.50x15 ET Streets. The ET Streets are probably more suited to the HP level of my car. Thanks for taking the time to try and "educate" me.
Gary

Why mess with the ET streets or drag radials in the first place? Why not simplify things & just put on a set of conventional drag slicks from M/T or Hoosier?

Why mess with the ET streets or drag radials in the first place? Why not simplify things & just put on a set of conventional drag slicks from M/T or Hoosier?
Because it is a street car. If the ET Streets work out for street use, I'll probably get a set of 28x11.5x15 ET Drags (which are basically the same size) for track use & mount them on the rims from my drag radials.
Gary